[Characterization of novel genes regulating adipocyte differentiation]

The mid- and late stages of adipocyte differentiation are known to be regulated by transcription factors such as peroxisome proliferator-activated receptor (PPAR)gamma and CCAAT-box/enhancer binder protein (C/EBP) families. However, events in the early stage of adipocyte differentiation remain largely unknown. To gain insights into the molecular mechanisms underlying the beginning of adipocyte differentiation, we have isolated 102 genes, which are induced at the beginning of the differentiation of mouse 3T3-L1 preadipocytes, using the polymerase chain reaction (PCR)-subtraction method. Of these, 46 appear to be unknown genes. Since rapid amplification of cDNA end (RACE), cDNA library screening, and a genome database search have revealed that two of these genes are novel, we have named them factor for adipocyte differentiation (fad) 24 and fad158. The database research of amino acid sequences revealed that fad24 has a basic leucine zipper motif and an NOC domain, and fad158 has four transmembrane domains and eight leucine-rich repeats. The expression of fad24 and fad158 transiently increased after the addition of adipogenic inducers [insulin, dexamethasone, 3-isobutyl-1-methylxanthine, fetal bovine serum (FBS)]. RNAi-mediated knockdown of fad24 or antisense fad158 inhibited adipogenesis of 3T3-L1 preadipocytes and decreased expressions of PPARgamma and C/EBPalpha. Furthermore, the constitutive overexpression of fad24 or fad158 in the mouse fibroblast cell line NIH-3T3 resulted in adipocyte conversion when stimulated with adipogenic inducers and PPARgamma ligand BRL49653. Moreover, it was found that FAD24 localizes in the nucleus, especially within nuclear speckles and nucleolus, and FAD158 localizes to the endoplasmic reticulum (ER). Taken together, fad24 and fad158 appear to regulate adipocyte differentiation by activating the PPARgamma pathway.