Involvement of the protein kinase C substrate, SSeCKS, in the actin-based stellate morphology of mesangial cells.

Activation of protein kinase C is a key signal transduction event in mesangial cell dedifferentiation and proliferation, yet little is known about downstream substrates or their roles in normal or diseased glomeruli. SSeCKS, a novel protein kinase C substrate originally isolated as a src-suppressed negative mitogenic regulator in fibroblasts, controls actin-based cytoskeletal architecture and scaffolds key signaling kinases such as protein kinase C and protein kinase A. Based on the morphologic similarity between SSeCKS-overexpressing fibroblasts and stellate mesangial cells, we hypothesized that SSeCKS might play a role in mesangial cell morphology in a protein kinase C-dependent manner. Immunoblotting, in situ staining and northern blotting detected abundant expression of SSeCKS in human and rodent mesangial cells and glomerular parietal cells but not in renal tubular epithelia. Immunofluorescence analysis showed enrichment of SSeCKS in mesangial cell podosomes and along a cytoskeletal network distinct from F-actin. Activation of protein kinase C by phorbol ester resulted in a rapid serine phosphorylation of SSeCKS and its subsequent translocation to perinuclear sites, coincident with the retraction of stellate processes. These effects were blocked by concentrations of bis-indolylmaleimide that selectively inhibit protein kinase C. Finally, ablation of SSeCKS expression using retroviral anti-sense vectors induced (1) an elongated, fibroblastic cell morphology, (2) production of thick, longitudinal stress fibers and (3) repositioning of vinculin-associated focal complexes away from the cell edges. These data suggest a role for SSeCKS as a downstream mediator of protein kinase C-controlled, actin-based mesangial cell cytoskeletal architecture.