Adipogenesis or Myogenesis: A Rho Choice

Abstract

How can different cell fates arise from the same stimulation? Rho activity might be one key, at least for adipogenesis and myogenesis. Loss of p190-B RhoGAP, a guanosine triphosphatase-activating protein that inhibits Rho activity, in mice leads to reduced cell size and animal size. Sordella et al . show that newborn mice deficient for p190-B RhoGAP have decreased lipid staining, indicative of defective adipogenesis. Using mouse embryo fibroblasts from the mutant mice, the authors demonstrated that, in culture, these MEFs did not undergo adipogenic conversion in response to treatment with agents that activate glucocorticoid receptors (dexamethasone), adenosine 3′,5′-monophosphate (cAMP) (3-isobutyl-1-methylxanthine or IBMX), and insulin or insulin-like growth factor (IGF) receptors (insulin or IGF-1); instead, the cells underwent a myogenic conversion. Adipogenic differentiation was restored by pharmacological inhibition of Rho kinase (ROK); ROK phosphorylates insulin receptor substrate (IRS) inhibiting IGF-1 signaling. Expression of a constitutively active mutant of Rho (RhoV14) also blocked adipogenic conversion in wild-type MEFs and NIH 3T3-L1 cells. IGF-1 treatment of wild-type MEFs stimulated phosphorylation of p190-B RhoGAP, and in vitro, the purified insulin receptor directly phosphorylated p190-B RhoGAP. Mutation of the phosphorylated tyrosine (Y 306 ) to a nonphosphorylatable residue blocked IGF-1-stimulated redistribution of p190-B RhoGAP to lipid rafts (cholesterol and sphingolipid-enriched domains of the plasma membrane that are detergent-resistant). Mutation to a phosphorylation-mimicking residue promoted lipid raft accumulation of p190-B RhoGAP in the absence of IGF-1. Thus, phosphorylation of p190-B RhoGAP by the insulin or IGF-1 receptor causes redistribution of p190-B RhoGAP, allowing it to be localized in proximity to Rho and to inhibit Rho activity. When Rho activity was high (in the p190-B RhoGAP-deficient cells), the cells differentiated into myocytes instead of adipocytes. Application of the ROK inhibitor in the presence of IGF-1 to C2C12 cells, a pluripotent mesenchymal cell line that differentiates and forms myotubes, causes adipogenesis instead of myogenesis. In C2C12 cells, p190-B RhoGAP is expressed; however, during myogenic differentiation, phosphorylation of p190-B RhoGAP decreased. SHP-2, a protein phosphatase that can promote myocyte formation and also localizes to lipid rafts, increased during C2C12 differentiation and associated with p190-B RhoGAP. Thus, these results show how differentiation of two cell fates can be controlled through the Rho by pathways that counter-regulate p190-B RhoGAP. R. Sordella, W. Jiang, G.-C. Chen, M. Curto, J. Settleman, Modulation of Rho GTPase signaling regulates a switch between adipogenesis and myogenesis. Cell 113 , 147-158 (2003). [Online Journal]