The Critical Role of Shc in Insulin-Like Growth Factor-I-Mediated Mitogenesis and Differentiation in 3T3-L1 Preadipocytes

• Published in: Molecular endocrinology (Baltimore, Md.) Md.), 2000-06, Vol.14 (6), p.805-813
• Main Authors: Boney, Charlotte M, Gruppuso, Philip A, Faris, Ronald A, Frackelton, A. Raymond
• Format: Article
• Language: English
• ISSN: 0888-8809; 1944-9917
• DOI: 10.1210/mend.14.6.0487

Insulin-like growth factor-I (IGF-I) stimulates mitogenesis in proliferating preadipocytes, but when cells reach confluence and become growth arrested, IGF-I stimulates differentiation into adipocytes. IGF-I induces signaling pathways that involve IGF-I receptor-mediated tyrosine phosphorylation of Shc and insulin receptor substrate 1 (IRS-1). Either of these adaptor proteins can lead to activation of the three-kinase cascade ending in activation of the extracellular signal-regulated kinase 1 and -2 (ERK-1 and -2) mitogen-activated protein kinases (MAPKs). Several lines of evidence suggest that activation of MAPK inhibits 3T3-L1 preadipocyte differentiation. We have shown that IGF-I stimulation of MAPK activity is lost as 3T3-L1 preadipocytes begin to differentiate. This change in MAPK signaling coincides with loss of IGF-I-mediated Shc, but not IRS-1, tyrosine phosphorylation. We hypothesized that down-regulation of MAPK via loss of proximal signaling through Shc is an early component in the IGF-I switch from mitogenesis to differentiation in 3T3-L1 preadipocytes. Treatment of subconfluent cells with the MEK inhibitor PD098059 inhibited both IGF-I-activation of MAPK as well as 3H-thymidine incorporation. PD098059, in the presence of differentiation-inducing media, accelerated differentiation in subconfluent cells as measured by expression of adipocyte protein-2 (aP-2), peroxisome proliferator-activated receptor γ (PPARγ) and lipoprotein lipase (LPL). Transient transfection of subconfluent cells with Shc-Y317F, a dominant-negative mutant, attenuated IGF-I-mediated MAPK activation, inhibited DNA synthesis, and accelerated expression of differentiation markers aP-2, PPARγ, and LPL. We conclude that signaling through Shc to MAPK plays a critical role in mediating IGF-I-stimulated 3T3-L1 mitogenesis. Our results suggest that loss of the ability of IGF-I to activate Shc signaling to MAPK may be an early component of adipogenesis in 3T3-L1 cells. plays an important role in preadipocyte growth and differentiation. IGF-I stimulates mitogenesis in many cell types in culture, including preadipocytes (1), and IGF-I (or pharmacological doses of insulin) is clearly required for preadipocyte differentiation in vitro (2, 3). This dual role of IGF-I, stimulation of both mitogenesis and differentiation, indicates that these responses are not necessarily mutually exclusive. In vitro, IGF-I stimulates differentiation of preadipocytes once density-induced growth arrest has oc