SIGNALING PATHWAYS FOR SPHINGOSYLPHOSPHORYLCHOLINE-MEDIATED MITOGENESIS IN SWISS 3T3 FIBROBLASTS

Sphingosylphosphorylcholine (SPC), or lysophingomyelin, a wide-spectru m growth promoting agent for a variety of cell types (Desai, N. N., an d S. Spiegel. 1991. Biochem. Biophys. Res. Comm. 181: 361-366), stimul ates cellular proliferation of quiescent Swiss 3T3 fibroblasts to a gr eater extent than other known growth factors or than the structurally related molecules, sphingosine and sphingosine-1-phosphate. SPC potent iated the mitogenic effect of an activator of protein kinase C, 12-O-t etradecanoylphorbol 13-acetate, and did not compete with phorbol ester s for binding to protein kinase C in intact Swiss 3T3 fibroblasts. How ever, downregulation of protein kinase C, by prolonged treatment with phorbol ester, reduced, but did not eliminate, the ability of SPC to s timulate DNA synthesis, indicating that SPC may act via both protein k inase C-dependent and -independent signaling pathways. SPC induced a r apid rise in intracellular free calcium ([Ca2+]i) in viable 3T3 fibrob lasts determined with a digital imaging system. Although the increases in [Ca2+]i were observed even in the absence of calcium in the extern al medium, no increase in the levels of inositol phosphates could be d etected in response to mitogenic concentrations of SPC. Furthermore, i n contrast to sphingosine or sphingosine-1-phosphate, the mitogenic ef fect of SPC was not accompanied by increases in phosphatidic acid leve ls or changes in cAMP levels. SPC, but not sphingosine or sphingosine- 1-phosphate, stimulates the release of arachidonic acid. Therefore, th e ability of SPC to act as an extremely potent mitogen may be due to a ctivation of signaling pathway(s) distinct from those used by sphingos ine or sphingosine-1-phosphate.