The regulation of preadipocyte differentiation by glucocorticoids

Abstract

The adipogenic stimulus provided by glucocorticoids is most obvious in the central obesity of individuals with Cushing's syndrome. Glucocorticoids promote preadipocyte differentiation in culture, in part by enhancing the transcriptional potential of a key adipogenic factor, C/EBPbeta. They do so by promoting proteasome dependent degradation of a pool of C/EBPbeta-associated HDAC1, a transcriptional repressor protein. This potentiates C/EBPbeta-mediated transcription of C/EBPalpha, a master regulator of differentiation. Analysis of the mechanisms by which HDAC1 is targeted for degradation has led to the identification of TIF1beta as a cofactor of C/EBPbeta-mediated transcription. TIF1beta expression modulates the association of C/EBPbeta with HDAC1 and promotes the ubiquitylation and proteasome dependent turnover of HDAC1 that appears to be required for the transactivation potential of TIF1beta. Furthermore, TIF1beta promotes 3T3 L1 preadipocyte differentiation through both HDAC1-dependent and independent mechanisms. The elucidation of the molecular mechanisms by which glucocorticoids potentiate adipogenesis has been largely limited to murine model systems. In this work, I defined glucocorticoid requirements and their effects on the early transcriptional events that drive human primary preadipocyte differentiation. I identified striking similarities and some differences between the 3T3 L1 and human preadipocyte systems. Notably, accumulation of C/EBPbeta was enhanced by glucocorticoids and C/EBPalpha was differentially expressed upon induction of differentiation in human preadipocytes. Furthermore, glucocorticoids provided a survival signal that was required for differentiating 3T3 Ll cells to progress through clonal expansion when cultured in chemically defined medium. Clonal expansion is a specific property of the 3T3 Ll cell line that is not shared by human preadipocytes, which did not require glucocorticoids for survival of the initial stages of differentiation. Lastly, I have shown that exposure of preadipocytes to glucocorticoids primes the cells with increased differentiation capacity. Microarray analysis of gene expression following pretreatment in human preadipocytes yielded factors that represent (1) previously unidentified, potential adipogenic targets of glucocorticoids and (2) potential mediators of this priming effect. Notably, pretreatment of human preadipocytes with physiological concentrations of steroid promoted increased insulin responsiveness. This response was specific to the human preadipocytes and was not observed in either the 3T3 Ll preadipocytes or human adipocytes.