Src family tyrosine kinases participate in insulin-like growth factor I mitogenic signaling in 3T3-L1 cells

Insulin-like growth factor-I (IGF-I) stimulates proliferation and different iation of many cell types, including preadipocytes. We have previously show n that IGF-I stimulates proliferation of 3T3-L1 preadipocytes through activ ation of the extracellular regulated kinase (ERK)-1 and -2 mitogen-activate d protein kinase (MAPK) pathway, and that IGF-I-stimulated MAPK is predomin antly downstream of She, not IRS-1 phosphorylation. The Src family of nonre ceptor tyrosine kinases has been shown to mediate the mitogenic effects of other growth factors that also activate She and the ERK-1 and -2 MAPKs, Alt hough Src family kinases (SFK) have been implicated in IGF-I action, no spe cific role for SFKs in IGF-I regulation of mitogenesis has been previously demonstrated. We studied the role of SFKs in IGF-I mitogenic signaling in 3 T3-L1 preadipocytes, The SFK-selective inhibitor PP1 completely inhibited b oth IGF-I-stimulated DNA synthesis and MAPK activation in proliferating 3T3 -L1 cells. PP1 inhibited IGF-I phosphorylation of She but not of IRS-1. In addition, IGF-I activation of MAPK was inhibited in proliferating cells tra nsiently transfected with a dominant-negative c-Src, Finally, the kinetics of SFK and MAPK activation by IGF-I suggest that SFKs may act upstream of M APK. IGF-I activation of SFK members c-Src and Fyn occurred within 1 min of treatment, and activity was back to baseline by 10 min. Our previous studi es found that IGF-I activation of MAPK peaked at 5 min and was also back to baseline by 10 min. Our results are the first to demonstrate that SFKs med iate IGF-I mitogenic signaling in 3T3-L1 cells and add to the growing body of evidence that SFKs play a crucial role in IGF-I action.