Sphingomyelinase but not ceramide induces nitric oxide synthase expression in rat brain microglia

We have asked whether treatment of PC12 cells with cyclic adenosine monophosphate (cAMP) and epidermal growth factor (EGF) results, like treatment with cAMP and nerve growth factor (NGF), in irreversible neuronal differentiation characterized by irreversible neurite extension, loss of serum-dependence, and death by apoptosis after trophic factor withdrawal. Although EGF alone, unlike NGF, did not cause morphological differentiation or prevent cell death, synergy between a cAMP-mediated signal transduction pathway and a pathway activated by the EGF receptor tyrosine kinase resulted in the same irreversible differentiation. EGF/cAMP-differentiated cells required cAMP to survive, but NGF, through a TrkA-dependent mechanism, could substitute for cAMP. The cyclin-dependent kinase inhibitors olomoucine and roscovitine also promoted survival of the irreversibly differentiated cells, by a mechanism that must be determined, since cell death was not associated with nuclear (3)H-thymidine accumulation, an index of mitotic activity.