pp60c-src is required for the induction of a quiescent mesangial cell phenotype

The tyrosine kinase c-src associates with growth factor receptors, focal contacts and cytoskeletal proteins and is involved in signaling events. The aim of this study was to investigate the role of src in the regulation of mesangial cell (MC) proliferation and differentiation in three-dimensional (3D) culture in collagen gels. Using retroviral gene transfer we have overexpressed wild-type c-src, a kinase-negative c-src mutant (c-src295) and transforming v-src in MC. The MC differentiation in 3D culture was characterized by the formation of a nonproliferating multicellular network in control cells and in cells expressing wild-type c-src. Immunoblotting demonstrated a rapid down-regulation of the alpha-smooth muscle actin expression. The kinase-negative MC (c-src295) failed to differentiate, maintained a significant proliferative rate, and the alpha-smooth muscle actin expression remained stable during 3D culture. MC transformed with v-src showed a high level of tyrosine phosphorylation and proliferation in 3D culture. Analyses of proteins involved in cell cycle regulation demonstrated dephosphorylation of the retinoblastoma protein (Rb) during 3D culture in control and c-src transfected cells. Expression of v-src resulted in sustained Rb phosphorylation. Zymographic analysis of plasminogen activator (u-PA) revealed an inhibition of u-PA secretion in MC transfected with c-src295. These results indicate that c-src exerts regulatory effects on MC proliferation, cytoskeletal organization, matrix proteases and differentiation. Targeted manipulation of the c-src kinase may be useful in modulating MC behavior in vivo.