Adipose differentiation is a complex process associated with coordinated changes in gene expression, cell morphology, and hormone sensitivity.1) Several transcription factors influence these processes, among which peroxisome proliferator activated receptor g (PPARg) and CCAAT/enhancerbinding protein (C/EBP) family have been extensively studied.2,3) C/EBPb and C/EBPd are induced in the early stage of adipocyte differentiation and then mediate the expression of PPARg and C/EBPa. [4][5][6] In contrast to C/EBPd, C/EBPb is able to induce spontaneous differentiation in 3T3-L1 preadipocytes and enhance the adipogenic potential in NIH-3T3 fibroblasts. 5,6) Highly specific for adipose tissues, PPARg plays a critical role in the expression of most adipocyte-specific genes 7) and is able to convert nonadipogenic mesenchymal cells, such as fibroblasts and myoblasts, to adipocytes. 8,9) On the other hand, although developmentally necessary for adipogenesis, 10) C/EBPa is not always expressed during adipose differentiation and is critical in the establishment of insulin sensitivity.
11)Several recent studies have shown that hypoxia-inducible factor (HIF) is also involved in the regulation of adipose differentiation. [12][13][14] HIF is a transcriptional factor that plays a central role in oxygen homeostasis in mammalian cells by stimulating expression of oxygen-regulated genes related to erythropoiesis, angiogenesis, glucose uptake, and glycolysis. 15,16) HIF is a heterodimer composed of HIF-a and -b, both of which belong to basic helix-loop-helix (bHLH)/Per-ARNT-Sim (PAS) domain transcriptional factors.17) The HIFb subunit is identical to the arylhydrocarbon receptor nuclear translocator (ARNT) that also serves as a heterodimeric partner with the arylhydrocarbon receptor (AhR).18) In contrast, it appears that the HIF-a subunit's sole but critical function is to mediate the response to hypoxia. Under normoxic conditions, HIF-a subunits are hydroxylated at two conserved proline (Pro564 and Pro402 in HIF-1a) residues and one asparagine (Asn803 in HIF-1a) residue by HIF-a prolyl hydroxylases (PHD1, PHD2, and PHD3) and asaparagine hydroxylase FIH-1 (factor inhibiting HIF-1), respectively, and the hydroxylated HIF-a subunit proteins are destabilized and transcriptionally suppressed.19) Deprivation of oxygen by exposing cells to hypoxia prevents HIF hydroxylase activity and then stabilizes and transcriptionally activates HIF-a subunits. The first identified isoform of HIF-a, HIF-1a, was originally discovered as a high-affinity DNA-binding protein localized to the 3Ј hypoxia-responsive element (HRE) on the erythropoietin (EPO) gene. Using a subtraction cloning approach, Imagawa et al. found that HIF-1a mRNA is transiently induced in 3T3-L1 preadipocytes upon treatment with the adipogenic hormone cocktail containing insulin, dexamethasone, and 3-isobutyl-1-methylxanthine.12) Yun et al. found that HIF-1a represses PPARg2 gene expression and then inhibits adipogenesis through the induction of expression of transcriptional repressor DE...