Estrogen-related receptor alpha (ERR(x) is an orphan nuclear receptor that regulates cellular energy metabolism by modulating gene expression involved in fatty acid oxidation and mitochondrial biogenesis in brown adipose tissue. However, the physiological role of ERR alpha in adipogenesis and white adipose tissue development has not been well studied. Here, we show that ERR alpha and ERR alpha-related transcriptional coactivators, peroxisome proliferator-activated receptor gamma (PPAR gamma) coactivator-1 alpha (PGC-1 alpha) and PGC-1 beta, can be upregulated in 3T3-L1 preadipocytes at mRNA levels under the adipogenic differentiation condition including the inducer of cAMP, glucocorticoid, and insulin. Gene knockdown by ERR alpha-specific siRNA results in mRNA down-regulation of fatty acid binding protein 4, PPAR gamma, and PGC-1 alpha ot in 3T3-L1 cells in the adipogenesis medium. ERR alpha and PGC-1 beta mRNA expression can be also up-regulated in another preadipocyte lineage DFAT-D1 cells and a pluripotent mesenchymal cell line C3H10T1/2 under the differentiation condition. Furthermore, stable expression of ERRa in 3T3-L1 cells up-regulates adipogenic marker genes and promotes triglyceride accumulation during 3T3-L1 differentiation. These results suggest that ERR alpha may play a critical role in adipocyte differentiation by modulating the expression of various adipogenesis-related genes. (c) 2007 Elsevier Inc. All rights reserved.