Hop2 interacts with the transcription factor CEBPα and suppresses adipocyte differentiation

CCAAT enhancer binding protein (CEBP) transcription factors (TFs) are known to promote adipocyte differentiation; however, suppressors of CEBP TFs have not been reported thus far. Here, we find that homologous chromosome pairing pro-tein 2 (Hop2) functions as an inhibitor for the TF CEBP alpha. We found that Hop2 mRNA is highly and specifically expressed in adipose tissue, and that ectopic Hop2 expression suppresses reporter activity induced by CEBP as revealed by DNA transfection. Recombinant and ectopically expressed Hop2 was shown to interact with CEBP alpha in pull-down and coimmuno-precipitation assays, and interaction between endogenous Hop2 and CEBP alpha was observed in the nuclei of 3T3 pre-adipocytes and adipocytes by immunofluorescence and coimmuno-precipitation of nuclear extracts. In addition, Hop2 stable overexpression in 3T3 preadipocytes inhibited adipocyte differentiation and adipocyte marker gene expression. These in vitro data suggest that Hop2 inhibits adipogenesis by sup-pressing CEBP-mediated transactivation. Consistent with a negative role for Hop2 in adipogenesis, ablation of Hop2 (Hop2(-/-)) in mice led to increased body weight, adipose volume, adipocyte size, and adipogenic marker gene expression. Adipogenic differentiation of isolated adipose-derived mesen-chymal stem cells showed a greater number of lipid droplet-containing colonies formed in Hop2(-/-) adipose-derived mesenchymal stem cell cultures than in wt controls, which is associated with the increased expression of adipogenic marker genes. Finally, chromatin immunoprecipitation revealed a higher binding activity of endogenous CEBP alpha to peroxisome proliferator-activated receptor gamma, a master adipogenic TF, and a known CEBP alpha target gene. Therefore, our study identifies for the first time that Hop2 is an intrinsic suppressor of CEBP alpha and thus adipogenesis in adipocytes.