The Extracellular SignaleCRegulated Kinase Isoform ERK1 Is Specifically Required for In Vitro and In Vivo Adipogenesis
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糖尿病学杂志 2005年第2期
1 Institut National de la Santee et de la Recherche Meedicale (INSERM) Unitee 568, IFR 50, Facultee de Meedecine, Universitee de Nice Sophia Antipolis, Nice, France
2 Unitee Mixte de Recherche (UMR) INRA/INA Physiologie de la Nutrition et du Comportement, Paris, France
3 Centre National de la Recherche Scientifique (CNRS) Unitee Mixte de Recherche (UMR) 6543, Facultee des Sciences, Universitee de Nice Sophia Antipolis, Nice, France
4 Institut National de la Santee et de la Recherche Meedicale (INSERM) E 0215 and Laboratory of Clinical and Experimental Pathology, Facultee de Meedecine and Pasteur Hospital, Universitee de Nice Sophia Antipolis, Nice, France
ABSTRACT
Hyperplasia of adipose tissue is critical for the development of obesity, but molecular mechanisms governing normal or pathological recruitment of new adipocytes remain unclear. The extracellular signaleCregulated kinase (ERK) pathway plays a pivotal role in many essential cellular functions, such as proliferation and differentiation. Using ERK1eC/eC mice, we investigated the role of this isoform in adipose tissue development. Mice lacking ERK1 have decreased adiposity and fewer adipocytes than wild-type animals. Furthermore, ERK1eC/eC mice challenged with high-fat diet are resistant to obesity, are protected from insulin resistance, and have a higher postprandial metabolic rate. To get insights into cellular mechanisms implicated in reduced adiposity in ERK1eC/eC animals, we analyzed adipocyte differentiation in ERK1eC/eC cells. Compared with wild-type control cells, mouse embryo fibroblasts and cultures of adult preadipocytes isolated from ERK1eC/eC adult animals exhibit impaired adipogenesis. An inhibitor of the ERK pathway does not affect the residual adipogenesis of the ERK1eC/eC cells, suggesting that ERK2 is not implicated in adipocyte differentiation. Our results clearly link ERK1 to the regulation of adipocyte differentiation, adiposity, and high-fat dieteCinduced obesity. This suggests that a therapeutic approach of obesity targeting specifically the ERK1 isoform and not ERK2 would be of particular interest.
Obesity represents a major health hazard and is a high-risk factor for type 2 diabetes and cardiovascular diseases. One of the main functions of adipocytes is to store surplus calories as triglycerides for subsequent retrieval during periods of need. Easy access to food associated with a reduction in caloric expenditure results in both the hypertrophy of adipocytes and the formation of new adipocytes from precursor cells. These processes account for adipose tissue expansion and obesity. Much of the knowledge concerning molecular mechanisms and signal transduction pathways involved in adipocyte differentiation or adipogenesis originates from investigations of preadipocyte cell lines. By contrast, little is known concerning signaling pathways implicated in the recruitment of new adipocytes from precursor cells in vivo. The extracellular signaleCregulated kinases (ERKs), a subfamily of the mitogen-activated protein kinases, are involved in signaling cascades that regulate a number of major cellular functions, such as cell proliferation and differentiation (1). Preadipocyte cell lines such as 3T3-L1 or 3T3-F442A were extensively used to investigate the role of the ERK pathway in adipogenesis, leading to controversial results. On the one hand, some studies indicate that ERK is not required for adipogenesis or blocks adipocyte differentiation (2eC6). On the other hand, specific inhibitors of mitogen-activated protein/ERK kinase (MEK), the upstream activator of ERK, inhibit adipogenesis (6eC8). In addition, we showed that activation of the ERK signaling pathway is required during early stages of adipocyte differentiation in embryonic stem cells (9).
So far no in vitro evidence has shown a differential role for the two main isoforms of the ERK pathway (ERK1 and -2), and they are activated by the same stimuli. However, in vivo ERK1 or -2 invalidation leads to different phenotypes, demonstrating different roles for the two isoforms. Mouse embryos lacking ERK2 die in utero before day 8.5 because of a defect in trophoblast and mesoderm development (10,11). On the contrary, ERK1eC/eC mice are viable and fertile; they have defective thymocyte maturation (12) and enhanced long-term memory (13).
Using the ERK1eC/eC animals (12), we studied the involvement of this isoform in adipogenesis in vivo and in vitro. The disruption of the ERK1 gene alters the development of adipose tissue in mice on normal diet and leads to resistance to high-fat dieteCinduced obesity. We show that mouse embryo fibroblasts and preadipocytes isolated from adult adipose tissue of ERK1eC/eC mice had impaired adipogenesis. We present evidence that this defect can be attributed to the specific lack of ERK1 because residual adipogenesis of ERK1eC/eC cells is not affected by U0126, a very potent and specific inhibitor of MEK. Therefore, analysis of ERK1eC/eC animals and cells clearly links impaired adipocyte differentiation to reduced adiposity and resistance to high-fat dieteCinduced obesity.
RESEARCH DESIGN AND METHODS
We generated ERK1eC/eC mice from heterozygotes issued from six backcrosses with C57BL/6 animals, as previously described (12), and control came from the same backcross. Mice were housed on a 12-h light/dark schedule and had free access to water and food. High-fat (45% fat, 35% carbohydrates, and 20% proteins) and standard diet (10% fat, 70% carbohydrates, and 20% proteins) were purchased from SAFE (Epinay/Orge, France). The experiments were conducted following standard ethical guidelines (European Union guidelines on animal laboratory care) and were approved by the Faculty of Medicine, University of Nice-Sophia Antipolis ethical committee.
Cells and adipocyte differentiation.
Mouse embryonic fibroblasts were prepared on day 14 postcoitum as described previously (14). Preadipocytes from adult tissue were isolated from subcutaneous fat pads of 8-week-old mice (15). Next, 2 days’ postconfluent cells were treated for 48 h with the adipocyte differentiation cocktail containing 5 e/ml insulin, 0.25 eol/l dexamethasone, 0.5 mmol/l IBMX (3-isobutyl-1-methylxanthine), and 10 eol/l thiazolidinedione. The medium was then replaced for 6 days by Dulbecco’s modified Eagle’s medium with insulin and thiazolidinedione only. Triglyceride accumulation has been measured using a kit (Triglyceride 100; ABX Diagnostics, Montpellier, France). Glycerol-6-phosphate dehydrogenase activity was measured as described (16).
Real-time quantitative PCR.
Gene expression analysis was performed with an ABI Prism 7000 (Applied Biosystems) and SYBR Green reagents (Eurogentec, Seraing, Belgium). cDNAs were synthesized from 2 e of total RNA using Superscript II reverse transcriptase (Invitrogen). The set of primers was designed according to the manufacturer software. Samples contained 1 x SYBR Green Master Mix, 0.5 eol/l primers, and 1/50 synthesized cDNA in a 25 e蘬 volume. PCR conditions were as follows: 10 min at 95°C, then 40 cycles of 15 s at 94°C, 30 s at 60°C, and 1 min at 72°C. We used 36B4 as an internal control.
Metabolic studies.
Glucose and insulin tolerance tests were performed on 13- to 14-week-old animals at 7 weeks after the beginning of the diet. Glucose (2 g/kg) and insulin (0.75 IU/kg) were administrated by intraperitoneal injection in awake mice. Blood samples were withdrawn from the tail vein at the indicated time, and glycemia was determined using a biochemical assay (Glucose PAP 250; ABX Diagnostics) and Accu-Check active bands (Roche Diagnostics, Mannheim, Germany). Quantification of triglycerides and free fatty acids was performed using, respectively, Triglycerides 100 (ABX Diagnostics) and NEFA-C (Wako Chemicals, Neuss, Germany) biochemical kits.
Energy expenditure, respiratory quotient, and locomotor activity.
Mice were placed for 22 h in a metabolic cage connected to an open-circuit, indirect calorimetry system, with the air flow adjusted to 0.5 l/min. The temperature in the metabolic cage (24 ± 1°C) was stable and controlled throughout the experiment. Oxygen consumption and carbon dioxide production were recorded at 10-s intervals, using a computer-assisted data acquisition program. Computer-assisted processing of respiratory exchanges and spontaneous activity signals made it possible to compute that part of the total metabolic rate devoted to fueling the energy cost of activity and thus (by continuously extracting the energy expended with activity) to compute the resting metabolism of these free-moving mice (17). The mice were housed in the metabolic cage at 1000h without food but with water. Postabsorptive resting energy expenditure (equivalent to basal metabolism) was measured between 1600 and 1800 h. At 1800 h, 1 g of the usual high-fat diet was given to the mice, and the thermogenic response to feeding (metabolism and respiratory quotient [RQ]) was computed for 8 h as the increase of metabolism and RQ above premeal baseline levels.
Measurement of ERK activation.
Tissue extracts were prepared using lysis buffer (9) and were analyzed by Western blot using antibodies against ERK phosphorylated on Thr202/Tyr204 (Cell Signaling Technology) and total ERK (Santa Cruz).
RESULTS
ERK1eC/eC mice have reduced adiposity compared with wild-type animals.
Independently of the defect in thymocyte maturation and enhanced long-term memory (12,13), no obvious phenotype has been described for ERK1eC/eC mice. To determine whether ERK1 invalidation had consequences on adiposity in mice, total carcass lipid content was measured and found to be reduced by 33% in ERK1eC/eC mice compared with ERK1+/+ animals at 3 and 10 weeks of age (Fig. 1A). Most organs were roughly similar in size between the genotypes, and no difference in brown adipose tissue and liver weight was noticed (data not shown). However, fat depots were significantly smaller in ERK1eC/eC animals compared with wild-type animals (Fig. 1BeCE). As important differences in adipose tissue were observed, we analyzed adipocyte size and number. Adipocytes from knockout animals were slightly smaller (data not shown), and, more importantly, a diminution of 40eC50% of the number of adipocytes from subcutaneous depots was observed in ERK1eC/eC mice (Fig. 1F). Food intake may explain the reduction of fat depots. Therefore, we evaluated this parameter in ERK1eC/eC and ERK1+/+ mice. Food intake was similar for each group of mice (knockout animals versus control animals: 28.8 ± 2.4 vs. 31.7 ± 2.5 g · weekeC1 · animaleC1, P = 0.76). Finally, our results indicate that the lack of ERK1 gene correlates in vivo with decreased adiposity and adipocyte hypocellularity.
High-fat diet stimulates ERK activity in white adipose tissue.
To investigate the role of the ERK pathway in the development of obesity, mice were subjected to high-fat diet (45% of total calories come from fat). We asked whether high-fat diet induced the activation of the ERK pathway in white adipose tissue, liver, and muscle. As shown in Fig. 2 (and data not shown), high-fat diet had no effect on ERK1 and -2 expression in the three tissues. Furthermore, ERK activity is not modified in liver (Fig. 2A) and muscle (data not shown) on hypercaloric regimen. By contrast, total ERK activity was more than three times higher in white adipose tissue from high-fat-diet mice compared with animals on standard diet (Fig. 2B). Interestingly, adipocytes from obese patients with type 2 diabetes have higher ERK activity than control patients (18), suggesting an important correlation between elevated ERK activity and obesity.
ERK1-deficient mice are resistant to high-fat dieteCinduced obesity and more sensitive to insulin.
To determine whether the invalidation of ERK1 prevents the occurrence of obesity, we analyzed the incidence of high-fat diet on wild-type and knockout mice. As expected, control mice became markedly obese on high-fat diet compared with their littermates on standard diet (10% of total calories from fat). In contrast, ERK1eC/eC mice did not develop obesity (Fig. 3A). Indeed, on high-fat diet, the average weight gain per week was significantly higher in control mice than in ERK1eC/eC mice (2.9 ± 0.4 vs. 1.7 ± 0.3 g/week, respectively; P < 0.008). In ERK1eC/eC animals under high-fat diet, inguinal axilliary fat pads and subcutaneous fat thickness were reduced by 51 and 30%, respectively (Fig. 3B). Interestingly, we noticed no difference in epididymal fat pad weight under high-fat diet, suggesting that depending on the location of the depot, the lack of ERK1 may affect adipose tissue development differently. Alternatively, because ERK1 invalidation did not completely block the development of adipose tissues, we cannot exclude that epididymal fat pads but no other depots have reached their maximal development in both wild-type and ERK1eC/eC mice. As observed for mice on standard diet, no difference of food intake was noticed between ERK1eC/eC and ERK1+/+ mice on high-fat diet (data not shown).
High-fat diet is frequently associated with insulin resistance. As expected, high-fat diet led to hyperglycemia in fed wild-type animals (246 ± 43 vs. 171 ± 15 mg/dl on high-fat vs. normal diet, respectively; P = 0.01). In contrast, fed ERK1eC/eC mice did not develop hyperglycemia (168 ± 13 vs. 196 ± 25 mg/dl for high-fat vs. normal diet, respectively; P = 0.1). Fasted mutant and control mice did not develop hyperglycemia, and no difference was found concerning triglycerides and free fatty acids in both groups under normal or high-fat diet (Table 1). We then determined the insulin sensitivity of the animals, performed a glucose tolerance test, and performed an insulin tolerance test. Wild-type mice on high-fat diet became glucose intolerant, a defect that did not appear in ERK1eC/eC animals (Fig. 3C). Likewise, the insulin tolerance test demonstrated severe insulin resistance in only wild-type mice on high-fat diet (Fig. 3D). Altogether, our results show that ERK1 invalidation protects from high-fat dieteCinduced obesity and glucose intolerance.
ERK1eC/eC mice have higher postprandial thermogenesis.
We then explored activity and energy expenditure in ERK1eC/eC mice. No difference in spontaneous activity was noticed in ERK1eC/eC compared with ERK1 wild-type mice (data not shown). Under fasted conditions, we observed no significant difference in the basal metabolic rate in ERK1eC/eC mice compared with wild-type animals (Fig. 4A). Basal RQ was identical in both groups (Fig. 4B), indicating that the ratio of glucose to lipid used to fuel basal metabolism was the same in mutant and control mice. We then investigated the thermogenic response to a calibrated test meal (1 g) in mice. The ERK1eC/eC mice displayed a higher postprandial increase in metabolic rate as well as in RQ (Fig. 4A and B), the higher RQ indicating that the increased thermogenic response to feeding was fueled by an increased rate of glucose oxidation (data not shown). Therefore, it can be extrapolated that under standard conditions (ad libitum feeding), energy expenditure may be significantly higher in ERK1eC/eC mice, which may contribute to the resistance to high-fat dieteCinduced obesity.
ERK1eC/eC cells have impaired adipogenesis.
To gain insight into cellular mechanisms involved in reduced adiposity and resistance to high-fat dieteCinduced obesity, we analyzed adipocyte differentiation in ERK1eC/eC cells. Mouse embryo fibroblasts isolated from ERK1eC/eC and wild-type embryos were induced to differentiate, as described in RESEARCH DESIGN AND METHODS. After differentiation, the accumulation of cytoplasmic triacylglycerol as lipid droplets, a marker of adipocyte differentiation, was strongly reduced in ERK1eC/eC cells (Fig. 5A). Concomitantly, the activity of glycerol-6-phosphate dehydrogenase, a lipogenic enzyme expressed in adipocyte, was reduced by half (Fig. 5B). Furthermore, the expression of specific adipocyte markers such as aP2, peroxisome proliferatoreCactivated receptor- (PPAR-), adiponectin, and leptin was diminished by >50% in knockout mouse embryo fibroblasts (Fig. 5C). On the contrary, myogenin, a marker of the myocyte lineage, was identically expressed in wild-type and ERK1eC/eC cells (Fig. 5D). This result clearly shows that ERK1 gene invalidation specifically impairs adipocyte differentiation and does not seem to interfere with myogenesis. This is in agreement with our data obtained in embryonic stem cells showing that inhibition of the ERK pathway interferes specifically with adipogenesis and not with myogenesis (9). The recruitment of new adipocytes from precursor cells takes place all life long. To determine whether adipocyte differentiation is affected in ERK1eC/eC adult tissues, cells from the stromal vascular fraction of white adipose tissue were isolated and induced to differentiate into adipocytes. As found in embryonic cells, after Oil Red O staining, we observed a strong reduction of adipocyte formation in ERK1eC/eC cells (Fig. 5E). Indeed, the total amount of triglyceride in ERK1eC/eC cells was reduced by 80% compared with wild-type cells (Fig. 5F). Our results demonstrate that ERK1 is required for adipocyte differentiation and plays a critical role in adipogenesis, from embryonic stages through adulthood.
Impaired adipogenesis may be caused either by a decrease in the number of adipocyte precursors or by a defect in terminal differentiation. Pref-1 is an inhibitor of differentiation and a specific preadipocyte marker not expressed in mature adipocytes (19eC21). Therefore, we assume that the level of Pref-1 expression, investigated in mouse embryo fibroblasts and preadipocytes before differentiation, is a direct reflection of the pool of adipogenic precursors. We analyzed its expression in mouse embryo fibroblasts and cells of the stromal vascular fraction before differentiation and also in white adipose tissue. Interestingly, compared with control, Pref-1 expression was markedly decreased in ERK1eC/eC cells and tissues (Fig. 5G). This result suggests that impaired adipogenesis observed in ERK1eC/eC cells may result from a reduction of the pool of preadipocytes.
ERK1 is not necessary for cell proliferation but is specifically required for adipocyte differentiation.
To get insights into molecular mechanisms underlying the defect of adipogenesis in ERK1eC/eC cells, we measured ERK activation upon addition of the adipocyte differentiation media to mouse embryo fibroblasts (Fig. 6A). As in standard growing conditions (12), the ERK activity of mouse embryo fibroblasts induced to differentiate was mainly caused by ERK2. Indeed, ERK1 activity represents an average of only 23 ± 11% of the total ERK activity throughout the time course of activation (quantification of Fig. 6A). Of note, the activity of ERK2 was not significantly changed in ERK1eC/eC cells (Fig. 6A), showing that there is no compensation in gene expression or kinase activation in these cells. We then analyzed the contribution of the ERK pathway to cell proliferation during adipocyte differentiation. Indeed, during the early stages of adipocyte differentiation, opposite results have been found concerning the requirement of cell division after the cells have reached confluence (6,22,23). Therefore, we tested the hypothesis that the reduction of the adipocyte precursor cell number in ERK1eC/eC cell cultures is caused by a slower growth rate of these cells. During the adipocyte differentiation protocol, we assessed cell proliferation by counting the cells after confluence (Fig. 6B). In this model, proliferation of wild-type cells was inhibited by the addition of differentiation medium, showing that cell division is not necessary for adipogenesis in these cells (Fig. 6B). The ERK1eC/eC mouse embryo fibroblasts had growth rates identical to ERK1+/+ cells in standard or differentiation media (Fig. 6B). This result demonstrates that ERK1 gene deficiency does not affect cell growth. Interestingly, the addition of U0126 inhibited cell growth similarly in both knockout and wild-type cells (Fig. 6B). Therefore, ERK2 seems to play an important role in mouse embryo fibroblast proliferation independently of ERK1.
We then investigated the role of the ERK2 isoform in adipocyte differentiation. Because ERK2 invalidation results in early embryonic death (embryonic day 6.5eC8.5), thus avoiding the isolation of mouse embryo fibroblasts (10,11), we analyzed the effect of the addition of U0126 on wild-type mouse embryo fibroblasts and on residual adipocyte differentiation in ERK1eC/eC cells. U0126 led to a 45% inhibition of adipogenesis in wild-type mouse embryo fibroblasts, which is identical to the decreased adipogenesis observed in knockout cells. Importantly, the inhibitor of the ERK pathway did not significantly affect residual adipocyte formation of ERK1eC/eC cells (Fig. 6C). Therefore, while still presenting 80% of total ERK activity, ERK1eC/eC cells had impaired adipogenesis, and the addition of U0126 had no effect on this adipogenesis. These results suggest that ERK1 plays a specific role in adipogenesis, whereas the ERK2 isoform is not involved. The latter is required for cell proliferation after confluence, a process not involved in the adipocyte differentiation of these cells (Fig. 6B).
Because c-Jun NH2-terminal kinase 1 (JNK1)-deficient mice are resistant to the development of obesity on high-fat diet (24), we determined whether the JNK pathway interferes with adipocyte differentiation by treating cells with the specific JNK inhibitor SP600125. We found that the inhibitor did not affect adipocyte formation in both wild-type and ERK1eC/eC mouse embryo fibroblasts (Fig. 6C). Thus, this result suggests that the JNK pathway is not required for optimal adipocyte differentiation. Indeed, no evidence for impaired adipogenesis was described in JNK1eC/eC animals (24). We also found that JNK is not involved in the early stages of adipocyte differentiation (9).
DISCUSSION
Many studies performed in preadipocyte cell lines showed that the ERK/mitogen-activated protein kinase pathway is implicated in adipocyte differentiation (2eC9,25). However, these studies lead to controversial results and were carried out using specific inhibitors of the ERK pathway such as the MEK inhibitors (PD98059 or U0126) or antisense oligonucleotides. Importantly, these approaches do not discriminate between ERK1 and ERK2. Here we show, using a genetic approach, that disruption of ERK1 in embryonic fibroblasts and adult preadipocytes leads to impaired adipocyte differentiation. We measured the expression of Pref-1 in two cell types before differentiation, at the preadipocyte stage. Pref-1 mRNA level is low in ERK1eC/eC cells, which differentiate poorly in adipocytes. We therefore suggest that the low level of Pref-1 in mouse embryo fibroblasts and stromal vascular cells is caused by a reduced number of adipocyte precursors. Compared with wild-type cells, Pref-1eCreduced expression suggests that this defect appears early in adipogenesis and could be caused by a reduced pool of preadipocytes. This result is in good agreement with our previous observation in embryonic stem cells (9). The positive role for the ERK pathway early in adipocyte differentiation can be compatible and may precede an opposed negative function later on in terminal differentiation. This hypothesis reconciles a positive role of ERK1 in adipogenesis (our results and others) to the late inhibitory effect of ERK activation on the adipogenic transcription factor PPAR- (2). This latter study demonstrates that phosphorylation of PPAR- by ERK inhibits adipocyte differentiation. We observed no difference in PPAR- transcriptional activities between ERK1eC/eC and wild-type cells (data not shown), indicating that invalidation of ERK1 has no effect on PPAR- transactivation.
ERK1 and -2 share an overall 75% identity at the amino acid level and are activated by the same stimuli. However, unlike ERK1eC/eC mice, ERK2 knockout animals are not viable, suggesting that the two isoforms have distinct biological functions and are not redundant (10eC12). Regarding adipocyte differentiation, exposure of mouse embryo fibroblasts, to the differentiating medium equally activates both isoforms, as observed in 3T3-L1 preadipocyte cell lines (8). Here, we show that ERK1 invalidation does not alter cell growth, whereas ERK2 inhibition blocks ERK1eC/eC cell proliferation. By contrast, ERK2 inhibition does not further affect the residual adipogenesis observed in ERK1-deficient cells. Therefore, by revealing that ERK1 deficiency affects specifically adipocyte differentiation, whereas ERK2 is required for proliferation, our results extend to adipogenesis the notion of distinct biological functions for the two genes. The molecular basis for such divergent activities is not yet understood. One hypothesis is that ERK1 might have preferential substrates implicated in adipogenesis.
We show here that the defect observed in vitro correlates with the reduced adiposity observed in ERK1eC/eC mice. Interestingly, as opposed to other tissues examined where ERK1 activity is weaker than ERK2, in white adipose tissues ERK1 and -2 are activated at the same level, suggesting an important role for this isoform. According to this observation, ERK1-deficient mice have reduced adiposity and fewer adipocytes than wild-type animals. Furthermore, as for preadipocyte and mouse embryo fibroblasts, we observed no compensatory expression of ERK2 in ERK1eC/eC adipose tissue (data not shown). Recently, several studies have shown that the adipose tissue contains pluripotent stem cells (26eC31), and they validate the concept that recruitment of new adipocytes from stem cells occurs from birth to the adult stage. White adipose tissues and cells from the stroma vascular fraction of ERK1eC/eC animals have reduced expression of the preadipocyte marker Pref-1. This suggests that, as observed in vitro, the pool of preadipocytes is diminished and that ERK1 is required during early stages of in vivo adipogenesis.
Obesity is characterized by the hypertrophy and the hyperplasia of adipocytes, and new cells are recruited through adipocyte differentiation. High-fat diet induces a strong activation of the ERK pathway, specifically in white adipose tissue and not in other tissues. This activation is required for the development of obesity because we found that ERK1eC/eC animals are markedly resistant to high-fat dieteCinduced obesity. Therefore the defect observed in the development of the white adipose tissue of these animals may be sufficient to partly protect them from obesity.
We showed in this study that basal metabolism on a per-mouse basis was not different between mutant and control mice. Furthermore, we found that the thermogenic response to ingestion of a meal was higher in ERK1eC/eC mice, which may contribute, in addition to impaired adipogenesis, to a resistance to obesity. We also demonstrated that the increased thermogenic response to feeding was fueled by an increased rate of glucose oxidation. It has been shown that meal-induced elevated thermogenesis may result from a higher insulin sensitivity in peripheral tissues (32eC34), leading to increased glucose oxidation. These data are thus in agreement with the better insulin sensitivity observed in mutant mice.
In conclusion, our results clearly link the invalidation of ERK1 to reduced adiposity and resistance to high-fat dieteCinduced obesity due to impaired adipocyte differentiation and higher postprandial metabolism. Furthermore, we have identified specialized and separate functions for the two isoforms: ERK1 in adipocyte differentiation and ERK2 in mouse embryo fibroblast proliferation. Further studies will be required to understand, at the molecular level, the diverging pathways between the two kinases at the level of their substrates. Our data suggest that targeting specifically the ERK1 isoform and not ERK2 would be of particular interest to fight obesity and insulin resistance.
ACKNOWLEDGMENTS
This work was funded by grants from INSERM, the Bettencourt-Schueller foundation, and the Association pour la Recherche pour le Cancer (ARC 4525). M.A. and L.C. were supported by a fellowship from INSERM-Reegion Provence Alpes Cte d’Azur and La lique nationale centre le cancer.
We thank Myriam Bost for her support. We are indebted to Jean Franois Tanti, Phillippe Gual, and Thierry Greemeaux for their helpful contribution to this work. We also thank the Animal Facility of the Facultee de Medicine of Nice.
ERK, extracellular signaleCregulated kinase; JNK, c-Jun NH2-terminal kinase; MEK, mitogen-activated protein/ERK kinase; PPAR, peroxisome proliferatoreCactivated receptor; RQ, respiratory quotient
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2 Unitee Mixte de Recherche (UMR) INRA/INA Physiologie de la Nutrition et du Comportement, Paris, France
3 Centre National de la Recherche Scientifique (CNRS) Unitee Mixte de Recherche (UMR) 6543, Facultee des Sciences, Universitee de Nice Sophia Antipolis, Nice, France
4 Institut National de la Santee et de la Recherche Meedicale (INSERM) E 0215 and Laboratory of Clinical and Experimental Pathology, Facultee de Meedecine and Pasteur Hospital, Universitee de Nice Sophia Antipolis, Nice, France
ABSTRACT
Hyperplasia of adipose tissue is critical for the development of obesity, but molecular mechanisms governing normal or pathological recruitment of new adipocytes remain unclear. The extracellular signaleCregulated kinase (ERK) pathway plays a pivotal role in many essential cellular functions, such as proliferation and differentiation. Using ERK1eC/eC mice, we investigated the role of this isoform in adipose tissue development. Mice lacking ERK1 have decreased adiposity and fewer adipocytes than wild-type animals. Furthermore, ERK1eC/eC mice challenged with high-fat diet are resistant to obesity, are protected from insulin resistance, and have a higher postprandial metabolic rate. To get insights into cellular mechanisms implicated in reduced adiposity in ERK1eC/eC animals, we analyzed adipocyte differentiation in ERK1eC/eC cells. Compared with wild-type control cells, mouse embryo fibroblasts and cultures of adult preadipocytes isolated from ERK1eC/eC adult animals exhibit impaired adipogenesis. An inhibitor of the ERK pathway does not affect the residual adipogenesis of the ERK1eC/eC cells, suggesting that ERK2 is not implicated in adipocyte differentiation. Our results clearly link ERK1 to the regulation of adipocyte differentiation, adiposity, and high-fat dieteCinduced obesity. This suggests that a therapeutic approach of obesity targeting specifically the ERK1 isoform and not ERK2 would be of particular interest.
Obesity represents a major health hazard and is a high-risk factor for type 2 diabetes and cardiovascular diseases. One of the main functions of adipocytes is to store surplus calories as triglycerides for subsequent retrieval during periods of need. Easy access to food associated with a reduction in caloric expenditure results in both the hypertrophy of adipocytes and the formation of new adipocytes from precursor cells. These processes account for adipose tissue expansion and obesity. Much of the knowledge concerning molecular mechanisms and signal transduction pathways involved in adipocyte differentiation or adipogenesis originates from investigations of preadipocyte cell lines. By contrast, little is known concerning signaling pathways implicated in the recruitment of new adipocytes from precursor cells in vivo. The extracellular signaleCregulated kinases (ERKs), a subfamily of the mitogen-activated protein kinases, are involved in signaling cascades that regulate a number of major cellular functions, such as cell proliferation and differentiation (1). Preadipocyte cell lines such as 3T3-L1 or 3T3-F442A were extensively used to investigate the role of the ERK pathway in adipogenesis, leading to controversial results. On the one hand, some studies indicate that ERK is not required for adipogenesis or blocks adipocyte differentiation (2eC6). On the other hand, specific inhibitors of mitogen-activated protein/ERK kinase (MEK), the upstream activator of ERK, inhibit adipogenesis (6eC8). In addition, we showed that activation of the ERK signaling pathway is required during early stages of adipocyte differentiation in embryonic stem cells (9).
So far no in vitro evidence has shown a differential role for the two main isoforms of the ERK pathway (ERK1 and -2), and they are activated by the same stimuli. However, in vivo ERK1 or -2 invalidation leads to different phenotypes, demonstrating different roles for the two isoforms. Mouse embryos lacking ERK2 die in utero before day 8.5 because of a defect in trophoblast and mesoderm development (10,11). On the contrary, ERK1eC/eC mice are viable and fertile; they have defective thymocyte maturation (12) and enhanced long-term memory (13).
Using the ERK1eC/eC animals (12), we studied the involvement of this isoform in adipogenesis in vivo and in vitro. The disruption of the ERK1 gene alters the development of adipose tissue in mice on normal diet and leads to resistance to high-fat dieteCinduced obesity. We show that mouse embryo fibroblasts and preadipocytes isolated from adult adipose tissue of ERK1eC/eC mice had impaired adipogenesis. We present evidence that this defect can be attributed to the specific lack of ERK1 because residual adipogenesis of ERK1eC/eC cells is not affected by U0126, a very potent and specific inhibitor of MEK. Therefore, analysis of ERK1eC/eC animals and cells clearly links impaired adipocyte differentiation to reduced adiposity and resistance to high-fat dieteCinduced obesity.
RESEARCH DESIGN AND METHODS
We generated ERK1eC/eC mice from heterozygotes issued from six backcrosses with C57BL/6 animals, as previously described (12), and control came from the same backcross. Mice were housed on a 12-h light/dark schedule and had free access to water and food. High-fat (45% fat, 35% carbohydrates, and 20% proteins) and standard diet (10% fat, 70% carbohydrates, and 20% proteins) were purchased from SAFE (Epinay/Orge, France). The experiments were conducted following standard ethical guidelines (European Union guidelines on animal laboratory care) and were approved by the Faculty of Medicine, University of Nice-Sophia Antipolis ethical committee.
Cells and adipocyte differentiation.
Mouse embryonic fibroblasts were prepared on day 14 postcoitum as described previously (14). Preadipocytes from adult tissue were isolated from subcutaneous fat pads of 8-week-old mice (15). Next, 2 days’ postconfluent cells were treated for 48 h with the adipocyte differentiation cocktail containing 5 e/ml insulin, 0.25 eol/l dexamethasone, 0.5 mmol/l IBMX (3-isobutyl-1-methylxanthine), and 10 eol/l thiazolidinedione. The medium was then replaced for 6 days by Dulbecco’s modified Eagle’s medium with insulin and thiazolidinedione only. Triglyceride accumulation has been measured using a kit (Triglyceride 100; ABX Diagnostics, Montpellier, France). Glycerol-6-phosphate dehydrogenase activity was measured as described (16).
Real-time quantitative PCR.
Gene expression analysis was performed with an ABI Prism 7000 (Applied Biosystems) and SYBR Green reagents (Eurogentec, Seraing, Belgium). cDNAs were synthesized from 2 e of total RNA using Superscript II reverse transcriptase (Invitrogen). The set of primers was designed according to the manufacturer software. Samples contained 1 x SYBR Green Master Mix, 0.5 eol/l primers, and 1/50 synthesized cDNA in a 25 e蘬 volume. PCR conditions were as follows: 10 min at 95°C, then 40 cycles of 15 s at 94°C, 30 s at 60°C, and 1 min at 72°C. We used 36B4 as an internal control.
Metabolic studies.
Glucose and insulin tolerance tests were performed on 13- to 14-week-old animals at 7 weeks after the beginning of the diet. Glucose (2 g/kg) and insulin (0.75 IU/kg) were administrated by intraperitoneal injection in awake mice. Blood samples were withdrawn from the tail vein at the indicated time, and glycemia was determined using a biochemical assay (Glucose PAP 250; ABX Diagnostics) and Accu-Check active bands (Roche Diagnostics, Mannheim, Germany). Quantification of triglycerides and free fatty acids was performed using, respectively, Triglycerides 100 (ABX Diagnostics) and NEFA-C (Wako Chemicals, Neuss, Germany) biochemical kits.
Energy expenditure, respiratory quotient, and locomotor activity.
Mice were placed for 22 h in a metabolic cage connected to an open-circuit, indirect calorimetry system, with the air flow adjusted to 0.5 l/min. The temperature in the metabolic cage (24 ± 1°C) was stable and controlled throughout the experiment. Oxygen consumption and carbon dioxide production were recorded at 10-s intervals, using a computer-assisted data acquisition program. Computer-assisted processing of respiratory exchanges and spontaneous activity signals made it possible to compute that part of the total metabolic rate devoted to fueling the energy cost of activity and thus (by continuously extracting the energy expended with activity) to compute the resting metabolism of these free-moving mice (17). The mice were housed in the metabolic cage at 1000h without food but with water. Postabsorptive resting energy expenditure (equivalent to basal metabolism) was measured between 1600 and 1800 h. At 1800 h, 1 g of the usual high-fat diet was given to the mice, and the thermogenic response to feeding (metabolism and respiratory quotient [RQ]) was computed for 8 h as the increase of metabolism and RQ above premeal baseline levels.
Measurement of ERK activation.
Tissue extracts were prepared using lysis buffer (9) and were analyzed by Western blot using antibodies against ERK phosphorylated on Thr202/Tyr204 (Cell Signaling Technology) and total ERK (Santa Cruz).
RESULTS
ERK1eC/eC mice have reduced adiposity compared with wild-type animals.
Independently of the defect in thymocyte maturation and enhanced long-term memory (12,13), no obvious phenotype has been described for ERK1eC/eC mice. To determine whether ERK1 invalidation had consequences on adiposity in mice, total carcass lipid content was measured and found to be reduced by 33% in ERK1eC/eC mice compared with ERK1+/+ animals at 3 and 10 weeks of age (Fig. 1A). Most organs were roughly similar in size between the genotypes, and no difference in brown adipose tissue and liver weight was noticed (data not shown). However, fat depots were significantly smaller in ERK1eC/eC animals compared with wild-type animals (Fig. 1BeCE). As important differences in adipose tissue were observed, we analyzed adipocyte size and number. Adipocytes from knockout animals were slightly smaller (data not shown), and, more importantly, a diminution of 40eC50% of the number of adipocytes from subcutaneous depots was observed in ERK1eC/eC mice (Fig. 1F). Food intake may explain the reduction of fat depots. Therefore, we evaluated this parameter in ERK1eC/eC and ERK1+/+ mice. Food intake was similar for each group of mice (knockout animals versus control animals: 28.8 ± 2.4 vs. 31.7 ± 2.5 g · weekeC1 · animaleC1, P = 0.76). Finally, our results indicate that the lack of ERK1 gene correlates in vivo with decreased adiposity and adipocyte hypocellularity.
High-fat diet stimulates ERK activity in white adipose tissue.
To investigate the role of the ERK pathway in the development of obesity, mice were subjected to high-fat diet (45% of total calories come from fat). We asked whether high-fat diet induced the activation of the ERK pathway in white adipose tissue, liver, and muscle. As shown in Fig. 2 (and data not shown), high-fat diet had no effect on ERK1 and -2 expression in the three tissues. Furthermore, ERK activity is not modified in liver (Fig. 2A) and muscle (data not shown) on hypercaloric regimen. By contrast, total ERK activity was more than three times higher in white adipose tissue from high-fat-diet mice compared with animals on standard diet (Fig. 2B). Interestingly, adipocytes from obese patients with type 2 diabetes have higher ERK activity than control patients (18), suggesting an important correlation between elevated ERK activity and obesity.
ERK1-deficient mice are resistant to high-fat dieteCinduced obesity and more sensitive to insulin.
To determine whether the invalidation of ERK1 prevents the occurrence of obesity, we analyzed the incidence of high-fat diet on wild-type and knockout mice. As expected, control mice became markedly obese on high-fat diet compared with their littermates on standard diet (10% of total calories from fat). In contrast, ERK1eC/eC mice did not develop obesity (Fig. 3A). Indeed, on high-fat diet, the average weight gain per week was significantly higher in control mice than in ERK1eC/eC mice (2.9 ± 0.4 vs. 1.7 ± 0.3 g/week, respectively; P < 0.008). In ERK1eC/eC animals under high-fat diet, inguinal axilliary fat pads and subcutaneous fat thickness were reduced by 51 and 30%, respectively (Fig. 3B). Interestingly, we noticed no difference in epididymal fat pad weight under high-fat diet, suggesting that depending on the location of the depot, the lack of ERK1 may affect adipose tissue development differently. Alternatively, because ERK1 invalidation did not completely block the development of adipose tissues, we cannot exclude that epididymal fat pads but no other depots have reached their maximal development in both wild-type and ERK1eC/eC mice. As observed for mice on standard diet, no difference of food intake was noticed between ERK1eC/eC and ERK1+/+ mice on high-fat diet (data not shown).
High-fat diet is frequently associated with insulin resistance. As expected, high-fat diet led to hyperglycemia in fed wild-type animals (246 ± 43 vs. 171 ± 15 mg/dl on high-fat vs. normal diet, respectively; P = 0.01). In contrast, fed ERK1eC/eC mice did not develop hyperglycemia (168 ± 13 vs. 196 ± 25 mg/dl for high-fat vs. normal diet, respectively; P = 0.1). Fasted mutant and control mice did not develop hyperglycemia, and no difference was found concerning triglycerides and free fatty acids in both groups under normal or high-fat diet (Table 1). We then determined the insulin sensitivity of the animals, performed a glucose tolerance test, and performed an insulin tolerance test. Wild-type mice on high-fat diet became glucose intolerant, a defect that did not appear in ERK1eC/eC animals (Fig. 3C). Likewise, the insulin tolerance test demonstrated severe insulin resistance in only wild-type mice on high-fat diet (Fig. 3D). Altogether, our results show that ERK1 invalidation protects from high-fat dieteCinduced obesity and glucose intolerance.
ERK1eC/eC mice have higher postprandial thermogenesis.
We then explored activity and energy expenditure in ERK1eC/eC mice. No difference in spontaneous activity was noticed in ERK1eC/eC compared with ERK1 wild-type mice (data not shown). Under fasted conditions, we observed no significant difference in the basal metabolic rate in ERK1eC/eC mice compared with wild-type animals (Fig. 4A). Basal RQ was identical in both groups (Fig. 4B), indicating that the ratio of glucose to lipid used to fuel basal metabolism was the same in mutant and control mice. We then investigated the thermogenic response to a calibrated test meal (1 g) in mice. The ERK1eC/eC mice displayed a higher postprandial increase in metabolic rate as well as in RQ (Fig. 4A and B), the higher RQ indicating that the increased thermogenic response to feeding was fueled by an increased rate of glucose oxidation (data not shown). Therefore, it can be extrapolated that under standard conditions (ad libitum feeding), energy expenditure may be significantly higher in ERK1eC/eC mice, which may contribute to the resistance to high-fat dieteCinduced obesity.
ERK1eC/eC cells have impaired adipogenesis.
To gain insight into cellular mechanisms involved in reduced adiposity and resistance to high-fat dieteCinduced obesity, we analyzed adipocyte differentiation in ERK1eC/eC cells. Mouse embryo fibroblasts isolated from ERK1eC/eC and wild-type embryos were induced to differentiate, as described in RESEARCH DESIGN AND METHODS. After differentiation, the accumulation of cytoplasmic triacylglycerol as lipid droplets, a marker of adipocyte differentiation, was strongly reduced in ERK1eC/eC cells (Fig. 5A). Concomitantly, the activity of glycerol-6-phosphate dehydrogenase, a lipogenic enzyme expressed in adipocyte, was reduced by half (Fig. 5B). Furthermore, the expression of specific adipocyte markers such as aP2, peroxisome proliferatoreCactivated receptor- (PPAR-), adiponectin, and leptin was diminished by >50% in knockout mouse embryo fibroblasts (Fig. 5C). On the contrary, myogenin, a marker of the myocyte lineage, was identically expressed in wild-type and ERK1eC/eC cells (Fig. 5D). This result clearly shows that ERK1 gene invalidation specifically impairs adipocyte differentiation and does not seem to interfere with myogenesis. This is in agreement with our data obtained in embryonic stem cells showing that inhibition of the ERK pathway interferes specifically with adipogenesis and not with myogenesis (9). The recruitment of new adipocytes from precursor cells takes place all life long. To determine whether adipocyte differentiation is affected in ERK1eC/eC adult tissues, cells from the stromal vascular fraction of white adipose tissue were isolated and induced to differentiate into adipocytes. As found in embryonic cells, after Oil Red O staining, we observed a strong reduction of adipocyte formation in ERK1eC/eC cells (Fig. 5E). Indeed, the total amount of triglyceride in ERK1eC/eC cells was reduced by 80% compared with wild-type cells (Fig. 5F). Our results demonstrate that ERK1 is required for adipocyte differentiation and plays a critical role in adipogenesis, from embryonic stages through adulthood.
Impaired adipogenesis may be caused either by a decrease in the number of adipocyte precursors or by a defect in terminal differentiation. Pref-1 is an inhibitor of differentiation and a specific preadipocyte marker not expressed in mature adipocytes (19eC21). Therefore, we assume that the level of Pref-1 expression, investigated in mouse embryo fibroblasts and preadipocytes before differentiation, is a direct reflection of the pool of adipogenic precursors. We analyzed its expression in mouse embryo fibroblasts and cells of the stromal vascular fraction before differentiation and also in white adipose tissue. Interestingly, compared with control, Pref-1 expression was markedly decreased in ERK1eC/eC cells and tissues (Fig. 5G). This result suggests that impaired adipogenesis observed in ERK1eC/eC cells may result from a reduction of the pool of preadipocytes.
ERK1 is not necessary for cell proliferation but is specifically required for adipocyte differentiation.
To get insights into molecular mechanisms underlying the defect of adipogenesis in ERK1eC/eC cells, we measured ERK activation upon addition of the adipocyte differentiation media to mouse embryo fibroblasts (Fig. 6A). As in standard growing conditions (12), the ERK activity of mouse embryo fibroblasts induced to differentiate was mainly caused by ERK2. Indeed, ERK1 activity represents an average of only 23 ± 11% of the total ERK activity throughout the time course of activation (quantification of Fig. 6A). Of note, the activity of ERK2 was not significantly changed in ERK1eC/eC cells (Fig. 6A), showing that there is no compensation in gene expression or kinase activation in these cells. We then analyzed the contribution of the ERK pathway to cell proliferation during adipocyte differentiation. Indeed, during the early stages of adipocyte differentiation, opposite results have been found concerning the requirement of cell division after the cells have reached confluence (6,22,23). Therefore, we tested the hypothesis that the reduction of the adipocyte precursor cell number in ERK1eC/eC cell cultures is caused by a slower growth rate of these cells. During the adipocyte differentiation protocol, we assessed cell proliferation by counting the cells after confluence (Fig. 6B). In this model, proliferation of wild-type cells was inhibited by the addition of differentiation medium, showing that cell division is not necessary for adipogenesis in these cells (Fig. 6B). The ERK1eC/eC mouse embryo fibroblasts had growth rates identical to ERK1+/+ cells in standard or differentiation media (Fig. 6B). This result demonstrates that ERK1 gene deficiency does not affect cell growth. Interestingly, the addition of U0126 inhibited cell growth similarly in both knockout and wild-type cells (Fig. 6B). Therefore, ERK2 seems to play an important role in mouse embryo fibroblast proliferation independently of ERK1.
We then investigated the role of the ERK2 isoform in adipocyte differentiation. Because ERK2 invalidation results in early embryonic death (embryonic day 6.5eC8.5), thus avoiding the isolation of mouse embryo fibroblasts (10,11), we analyzed the effect of the addition of U0126 on wild-type mouse embryo fibroblasts and on residual adipocyte differentiation in ERK1eC/eC cells. U0126 led to a 45% inhibition of adipogenesis in wild-type mouse embryo fibroblasts, which is identical to the decreased adipogenesis observed in knockout cells. Importantly, the inhibitor of the ERK pathway did not significantly affect residual adipocyte formation of ERK1eC/eC cells (Fig. 6C). Therefore, while still presenting 80% of total ERK activity, ERK1eC/eC cells had impaired adipogenesis, and the addition of U0126 had no effect on this adipogenesis. These results suggest that ERK1 plays a specific role in adipogenesis, whereas the ERK2 isoform is not involved. The latter is required for cell proliferation after confluence, a process not involved in the adipocyte differentiation of these cells (Fig. 6B).
Because c-Jun NH2-terminal kinase 1 (JNK1)-deficient mice are resistant to the development of obesity on high-fat diet (24), we determined whether the JNK pathway interferes with adipocyte differentiation by treating cells with the specific JNK inhibitor SP600125. We found that the inhibitor did not affect adipocyte formation in both wild-type and ERK1eC/eC mouse embryo fibroblasts (Fig. 6C). Thus, this result suggests that the JNK pathway is not required for optimal adipocyte differentiation. Indeed, no evidence for impaired adipogenesis was described in JNK1eC/eC animals (24). We also found that JNK is not involved in the early stages of adipocyte differentiation (9).
DISCUSSION
Many studies performed in preadipocyte cell lines showed that the ERK/mitogen-activated protein kinase pathway is implicated in adipocyte differentiation (2eC9,25). However, these studies lead to controversial results and were carried out using specific inhibitors of the ERK pathway such as the MEK inhibitors (PD98059 or U0126) or antisense oligonucleotides. Importantly, these approaches do not discriminate between ERK1 and ERK2. Here we show, using a genetic approach, that disruption of ERK1 in embryonic fibroblasts and adult preadipocytes leads to impaired adipocyte differentiation. We measured the expression of Pref-1 in two cell types before differentiation, at the preadipocyte stage. Pref-1 mRNA level is low in ERK1eC/eC cells, which differentiate poorly in adipocytes. We therefore suggest that the low level of Pref-1 in mouse embryo fibroblasts and stromal vascular cells is caused by a reduced number of adipocyte precursors. Compared with wild-type cells, Pref-1eCreduced expression suggests that this defect appears early in adipogenesis and could be caused by a reduced pool of preadipocytes. This result is in good agreement with our previous observation in embryonic stem cells (9). The positive role for the ERK pathway early in adipocyte differentiation can be compatible and may precede an opposed negative function later on in terminal differentiation. This hypothesis reconciles a positive role of ERK1 in adipogenesis (our results and others) to the late inhibitory effect of ERK activation on the adipogenic transcription factor PPAR- (2). This latter study demonstrates that phosphorylation of PPAR- by ERK inhibits adipocyte differentiation. We observed no difference in PPAR- transcriptional activities between ERK1eC/eC and wild-type cells (data not shown), indicating that invalidation of ERK1 has no effect on PPAR- transactivation.
ERK1 and -2 share an overall 75% identity at the amino acid level and are activated by the same stimuli. However, unlike ERK1eC/eC mice, ERK2 knockout animals are not viable, suggesting that the two isoforms have distinct biological functions and are not redundant (10eC12). Regarding adipocyte differentiation, exposure of mouse embryo fibroblasts, to the differentiating medium equally activates both isoforms, as observed in 3T3-L1 preadipocyte cell lines (8). Here, we show that ERK1 invalidation does not alter cell growth, whereas ERK2 inhibition blocks ERK1eC/eC cell proliferation. By contrast, ERK2 inhibition does not further affect the residual adipogenesis observed in ERK1-deficient cells. Therefore, by revealing that ERK1 deficiency affects specifically adipocyte differentiation, whereas ERK2 is required for proliferation, our results extend to adipogenesis the notion of distinct biological functions for the two genes. The molecular basis for such divergent activities is not yet understood. One hypothesis is that ERK1 might have preferential substrates implicated in adipogenesis.
We show here that the defect observed in vitro correlates with the reduced adiposity observed in ERK1eC/eC mice. Interestingly, as opposed to other tissues examined where ERK1 activity is weaker than ERK2, in white adipose tissues ERK1 and -2 are activated at the same level, suggesting an important role for this isoform. According to this observation, ERK1-deficient mice have reduced adiposity and fewer adipocytes than wild-type animals. Furthermore, as for preadipocyte and mouse embryo fibroblasts, we observed no compensatory expression of ERK2 in ERK1eC/eC adipose tissue (data not shown). Recently, several studies have shown that the adipose tissue contains pluripotent stem cells (26eC31), and they validate the concept that recruitment of new adipocytes from stem cells occurs from birth to the adult stage. White adipose tissues and cells from the stroma vascular fraction of ERK1eC/eC animals have reduced expression of the preadipocyte marker Pref-1. This suggests that, as observed in vitro, the pool of preadipocytes is diminished and that ERK1 is required during early stages of in vivo adipogenesis.
Obesity is characterized by the hypertrophy and the hyperplasia of adipocytes, and new cells are recruited through adipocyte differentiation. High-fat diet induces a strong activation of the ERK pathway, specifically in white adipose tissue and not in other tissues. This activation is required for the development of obesity because we found that ERK1eC/eC animals are markedly resistant to high-fat dieteCinduced obesity. Therefore the defect observed in the development of the white adipose tissue of these animals may be sufficient to partly protect them from obesity.
We showed in this study that basal metabolism on a per-mouse basis was not different between mutant and control mice. Furthermore, we found that the thermogenic response to ingestion of a meal was higher in ERK1eC/eC mice, which may contribute, in addition to impaired adipogenesis, to a resistance to obesity. We also demonstrated that the increased thermogenic response to feeding was fueled by an increased rate of glucose oxidation. It has been shown that meal-induced elevated thermogenesis may result from a higher insulin sensitivity in peripheral tissues (32eC34), leading to increased glucose oxidation. These data are thus in agreement with the better insulin sensitivity observed in mutant mice.
In conclusion, our results clearly link the invalidation of ERK1 to reduced adiposity and resistance to high-fat dieteCinduced obesity due to impaired adipocyte differentiation and higher postprandial metabolism. Furthermore, we have identified specialized and separate functions for the two isoforms: ERK1 in adipocyte differentiation and ERK2 in mouse embryo fibroblast proliferation. Further studies will be required to understand, at the molecular level, the diverging pathways between the two kinases at the level of their substrates. Our data suggest that targeting specifically the ERK1 isoform and not ERK2 would be of particular interest to fight obesity and insulin resistance.
ACKNOWLEDGMENTS
This work was funded by grants from INSERM, the Bettencourt-Schueller foundation, and the Association pour la Recherche pour le Cancer (ARC 4525). M.A. and L.C. were supported by a fellowship from INSERM-Reegion Provence Alpes Cte d’Azur and La lique nationale centre le cancer.
We thank Myriam Bost for her support. We are indebted to Jean Franois Tanti, Phillippe Gual, and Thierry Greemeaux for their helpful contribution to this work. We also thank the Animal Facility of the Facultee de Medicine of Nice.
ERK, extracellular signaleCregulated kinase; JNK, c-Jun NH2-terminal kinase; MEK, mitogen-activated protein/ERK kinase; PPAR, peroxisome proliferatoreCactivated receptor; RQ, respiratory quotient
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