Inhibition of adipocyte differentiation by mechanical stretching through ERK-mediated downregulation of PPARγ2

This study investigated the effects of cyclic stretching on adipocyte differentiation of mouse preadipocyte 3T3-L1 cells. Confluent 3T3-L1 cells were treated with dexamethasone, 3-isobutyl-1-methylxanthine and insulin for 45 hours (induction period), followed by incubation with insulin for 9 additional days (maturation period). A transient burst of CCAAT/enhancer-binding protein (C/EBP) P and C/EBPdelta at an early stage (similar to3 hours) and a delayed induction (similar to45 hours) of C/EBPalpha and PPARgamma(2) were sequentially provoked during the induction period. Application of cyclic stretching during the entire induction period or only during the final 15 hours of the induction period significantly retarded the induction of glycerol-3-phosphate dehydrogenase (GPDH) activity and the accumulation of intracellular triglycerides by the end of the maturation period. Cyclic stretching for the entire induction period, as well as that applied during the final 15 hours of the induction period, significantly reduced the expression of PPARgamma(2) mRNA, whereas reduction in the expression of C/EBPdelta mRNA was only observed in response to stretching that had been applied during the entire induction period. The expression of C/EBPalpha and C/EBPbeta mRNA did not change in response to stretching. Stretching induced the phosphorylation of extracellular-signal-regulated protein kinases 1 and 2 (ERK1/2), which are members of the mitogen-activated-protein kinase (MAPK) family, during the induction period. PD98,059, a MAPK/ERK kinase inhibitor, reversed the stretch-induced reduction of PPARgamma(2) at both mRNA and protein levels achieved during the induction period. PD98,059 also restored GPDH activity and lipid droplet accumulation. Furthermore, the differentiation inhibited by the stretching was also restored by synthetic PPARgamma ligand. Collectively, these results suggest that the inhibition of adipocyte differentiation in response to stretching is mainly attributable to the reduced expression of PPARgamma(2), which is mediated by activation of the ERK/MAPK system.