SPHINGOSINE KINASE MEDIATES CYCLIC-AMP SUPPRESSION OF APOPTOSIS IN RAT PERIOSTEAL CELLS

Prostaglandin E stimulates bone formation in humans and animals, and i ncreases intracellular cAMP in osteoblastic cells. We found that cAMP inhibits apoptosis in osteoblastic cells, and examined the mechanism o f this effect. We report that the cAMP elevating agent, forskolin, inc reases cell number in the rat periosteal cell line (RP-II), by suppres sing apoptosis in a cell type-specific manner. In RP-II, forskolin tra nsiently upregulates extracellular signal-regulated kinase activity, a known suppressor of apoptosis. PD98059, a selective inhibitor of the extracellular signal-regulated kinase pathway, only partially reverses the antiapoptotic effect of forskolin, which suggests an additional m echanism for cAMP action. We found that forskolin stimulates cytosolic sphingosine kinase (SPK) activity in these cells; in two other osteob lastic cell lines, however, forskolin does not suppress apoptosis. In contrast to the partial opposing effect of PD98059 to forskolin action , N,N-dimethylsphin-gosine, a specific inhibitor of SPK, completely re verses the antiapoptotic effect of forskolin, and has no effect on apo ptosis in the absence of forskolin. These findings show for the first time that cAMP activates SPK in a cell-type-specific manner, and sugge st that cAMP suppression of apoptosis in RP-11 periosteal cells is med iated by its stimulation of SPK.