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

Sphingosine is a product of sphingolipid metabolism that has been link ed to a protein kinase C-independent mitogenic response. In previously published data, utilizing an in vitro model system for platelet-deriv ed 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 ce ramidase activity. To explore mechanisms of growth factor stimulated s phingosine formation, we have developed and investigated a cell free m odel system assessing ceramidase activity. We now report that an alkal ine, membrane-associated, ceramidase activity in the rat glomerular me sangial 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 suffici ent substrate to drive the subsequent ceramidase reaction, Inflammator y cytokines, including interleukin-1, and tumor necrosis factor-alpha, stimulated sphingomyelinase but not ceramidase activity, a result con sistent with the cellular accumulation of the ceramide, apoptidic, dif ferentiating second messenger, Mitogenic vasoconstrictor peptides such as endothelin-1 stimulated neither sphingomyelinase nor ceramidase ac tivities, An inhibitor of ceramidase activity, N-oleoylethanolamine, r educed PDGF- but not endothelin-1-stimulated proliferation. Thus, we c onclude that, in mesangial cells, growth factors but not vasoconstrict or peptides or cytokines induce mitogenesis, in part, through ceramida se-mediated sphingosine formation.