Extracellular matrix induced by TGFbeta impairs insulin signal transduction in 3T3-L1 preadipose cells

When 3T3-L1 preadipose cells are exposed to transforming growth factor beta (TGFbeta), they synthesize more extracellular matrix (ECM) and resist differentiation-inducing stimuli. The mechanism by which ECM suppresses adipose cell differentiation (adipogenesis) remains unknown. Since adipogenesis is an insulin/insulin-like growth factor-1 (IGF-1)-dependent process, we investigated whether TGFbeta-induced ECM inhibits insulin signaling. When preadipose cells were pretreated overnight with TGFbeta, we observed a 75% decrease in insulin-stimulated tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) compared to that in control cells. Culturing 3T3-L1 preadipose cells on fibronectin, a component of the ECM induced by TGFbeta, also inhibited insulin-dependent IRS-1 tyrosine phosphorylation and adipogenesis, supporting a role for ECM in mediating TGFbeta's inhibitory effect on insulin signaling. Since the insulin-stimulated association of phosphoinositide (PI) 3-kinase with IRS-1 depends on IRS-1 tyrosine phosphorylation, we measured the presence of the PI 3-kinase 85 kDa regulatory subunit in anti-IRS-1 immunoprecipitates. Following insulin stimulation, PI 3-kinase-IRS-1 association was reduced by 70% in TGFbeta pretreated vs. control preadipose cells. However, insulin-stimulated cellular production of PI(3,4,5)P3 was unaltered by TGFbeta pretreatment. This suggests that IRS-1-associated p85-type PI 3-kinase may represent a particular subset of total cellular PI 3-kinase that is specifically inhibited by TGFbeta. Reduction of insulin-stimulated association of IRS-1 with p85-type PI 3-kinase by TGFbeta may be one potential mechanism through which TGFbeta blocks 3T3-L1 adipose cell differentiation.