The putative transcriptional corepressor ETO/MTG8 has been extensively studied due to its involvement in a chromosomal translocation causing the t(8;21) form of acute myeloid leukemia. Despite this, the role of ETO in normal physiology has remained obscure. Here we show that ETO is highly expressed in preadipocytes and acts as an inhibitor of C/EBP␤ during early adipogenesis, contributing to its characteristically delayed activation. ETO prevents both the transcriptional activation of the C/EBP␣ promoter by C/EBP␤ and its concurrent accumulation in centromeric sites during early adipogenesis. ETO expression rapidly reduces after the initiation of adipogenesis, and this is essential to the normal induction of adipogenic gene expression. These findings define, for the first time, a molecular role for ETO in normal physiology as an inhibitor of C/EBP␤ and a novel regulator of early adipogenesis.Adipose tissue is a key depot for the storage of energy as triglycerides and also plays a dynamic role in the regulation of metabolism (30). Studies of obese and lipodystrophic humans and rodents demonstrate that both increased and decreased adipose tissue mass are associated with insulin resistance and abnormal glucose and lipid metabolism (17,24,29). Thus, tight control of adipocyte development, size and insulin-sensitivity appears to be of critical importance in maintaining whole body energy homeostasis. The process of adipogenesis requires highly organized and precisely controlled expression of a cascade of transcription factors within the preadipocyte (25,32,35). The rapid and transient induction of the C/CAAT-enhancer binding proteins C/EBP␤ and C/EBP␦ is one of the earliest steps in this process (35). These transcription factors bind to specific sequences in the promoters of C/EBP␣ and the nuclear hormone receptor PPAR␥,

The zinc finger-containing transcription factors Egr1 (Krox24) and Egr2 (Krox20) have been implicated in the proliferation and differentiation of many cell types. Egr2 has previously been shown to play a positive role in adipocyte differentiation but the function of Egr1 in this context is unknown. We compared the roles of Egr1 and Egr2 in the differentiation of murine 3T3-L1 adipocytes. Egr1 protein was rapidly induced after addition of differentiation cocktail while Egr2 protein initially remained low before increasing on days 1 and 2, concomitant with the disappearance of Egr1. In marked contrast to the effects of Egr2, differentiation was inhibited by ectopic expression of Egr1 and potentiated by knockdown of Egr1. The pro-adipogenic effects of Egr1 knockdown were particularly notable when IBMX was omitted from the differentiation medium. However, knockdown of Egr1 did not affect C/EBPβ protein expression or phosphorylation of CREB Ser133. Further, Egr1 did not directly affect the activity of promoters for the master adipogenic transcription factors, C/EBPα or PPARγ2, as assessed in luciferase reporter assays. These data indicate that Egr1 and Egr2 exert opposing influences on adipocyte differentiation and that the careful regulation of both is required for maintaining appropriate levels of adipogenesis. Further, the pro-differentiation effects of IBMX involve suppression of the inhibitory influence of Egr1.