DIFFERENTIAL REGULATION OF SPHINGOMYELINASE AND CERAMIDASE ACTIVITIES BY GROWTH-FACTORS AND CYTOKINES - IMPLICATIONS FOR CELLULAR PROLIFERATION AND DIFFERENTIATION

Sphingosine is a product of sphingolipid metabolism that has been linked to a protein kinase C-independent mitogenic response. In previously published data, utilizing an in vitro model system for platelet-derived growth factor (PDGF)-induced vascular smooth muscle proliferation, we have demonstrated that sphingosine is increased at the expense of a concomitant decrease in ceramide formation, implicating an altered ceramidase activity. To explore mechanisms of growth factor stimulated sphingosine formation, we have developed and investigated a cell free model system assessing ceramidase activity. We now report that an alkaline, membrane-associated, ceramidase activity in the rat glomerular mesangial cell, a smooth muscle-like pericyte, is up-regulated by growth factors, apparently via a tyrosine kinase phosphorylation mechanism. PDGF also stimulated sphingomyelinase activity which generates sufficient substrate to drive the subsequent ceramidase reaction, Inflammatory cytokines, including interleukin-1, and tumor necrosis factor-alpha, stimulated sphingomyelinase but not ceramidase activity, a result consistent with the cellular accumulation of the ceramide, apoptidic, differentiating second messenger, Mitogenic vasoconstrictor peptides such as endothelin-1 stimulated neither sphingomyelinase nor ceramidase activities, An inhibitor of ceramidase activity, N-oleoylethanolamine, reduced PDGF- but not endothelin-1-stimulated proliferation. Thus, we conclude that, in mesangial cells, growth factors but not vasoconstrictor peptides or cytokines induce mitogenesis, in part, through ceramidase-mediated sphingosine formation.