Transcription factor HMG box‐containing protein 1 (HBP1) modulates mitotic clonal expansion (MCE) during adipocyte differentiation

Abstract

Adipocyte differentiation is delicately controlled by a series of transcription factors, including CCAAT/enhancer‐binding protein (C/EBP)β. Previously, we demonstrated that suppression of transcription factor HMG box‐containing protein 1 (HBP1) by either HBP1 siRNA or shRNA in 3T3‐L1 preadipocytes accelerated cell cycle progression in the mitotic clonal expansion (MCE) stage, but impaired adipogenesis, indicating that HBP1 might be involved in the regulation of adipocyte differentiation. To further gain insight into the role of HBP1 in the MCE stage, we examined the expression pattern of HBP1 upon hormonal stimulation in growth‐arrested 3T3‐L1 cells, and found that, immediately following stimulation, HBP1 expression was down‐regulated, whereas (C/EBP)β was up‐regulated. Further, ectopic expression of HBP1 dose‐dependently suppressed C/EBPβ expression. In addition, administration of methylisobutylxanthine (MIX), a C/EBPβ activator, inhibited both the expression of HBP1 and its promoter activity as determined by a 2 kb HBP1 promoter‐reporter gene assay. These data indicate that HBP1 functions as a negative regulator of MCE, in which down‐regulation of HBP1 ensures the expression of C/EBPβ and the reentry of growth‐arrested 3T3‐L1 into the proliferation mode. In contrast, HBP1 expression was gradually elevated, along with a concomitant induction of p27, at the end of the MCE. Overexpression of HBP1 activated a p27 promoter‐reporter system, suggesting that HBP1‐mediated p27 expression may be needed for the termination of cell cycle as well as the transition of growth arrest into terminal differentiation at the end of MCE, whereas HBP1 knockdown may delay the progression. Taken together, our findings add HBP1 as a key transcription factor in the complicated regulatory cascade during adipocyte differentiation.Support or Funding InformationThis work was supported by the grant to NSC 102‐2320‐B‐039‐036 C‐Y Huang.