EXTRACELLULAR-MATRIX INDUCED BY TGF-BETA IMPAIRS INSULIN SIGNAL-TRANSDUCTION IN 3T3-L1 PREADIPOSE CELLS

When 3T3-L1 preadipose cells are exposed to transforming growth factor beta (TGF beta), they synthesize more extracellular matrix (ECM) and resist differentiation-inducing stimuli. The mechanism by which ECM su ppresses adipose cell differentiation (adipogenesis) remains unknown. Since adipogenesis is an insulin/insulin-like growth factor-1 (IGF-l)- dependent process, we investigated whether TGF beta-induced ECM inhibi ts insulin signaling. When preadipose cells were pretreated overnight with TGF beta, we observed a 75% decrease in insulin-stimulated tyrosi ne phosphorylation of insulin receptor substrate-1 (IRS-1) compared to that in control cells. Culturing 3T3-L1 preadipose cells on fibronect in, a component of the ECM induced by TGF beta, also inhibited insulin -dependent IRS-1 tyrosine phosphorylation and adipogenesis, supporting a role for ECM in mediating TGF beta's inhibitory effect on insulin s ignaling. Since the insulin-stimulated association of phosphoinositide (PI) 3-kinase with IRS-1 depends on IRS-1 tyrosine phosphorylation, w e measured the presence of the PI 3-kinase 85 kDa regulatory subunit i n anti-IRS-1 immunoprecipitates. Following insulin stimulation, PI 3-k inase-IRS-l association was reduced by 70% in TGF beta pretreated vs, control preadipose cells. However, insulin-stimulated cellular product ion of P1(3,4,5)P3 was unaltered by TGF beta pretreatment. This sugges ts that IRS-l-associated p85-type PI 3-kinase may represent a particul ar subset of total cellular PI 3-kinase that is specifically inhibited by TGF beta. Reduction of insulin-stimulated association of IRS-1 wit h p85-type PI 3-kinase by TGF beta may be one potential mechanism thro ugh which TGF beta blocks 3T3-L1 adipose cell differentiation. (C) 199 8 Wiley-Liss, Inc.