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Ceramide is emerging as a potential physiologic regulator of growth and differentiation in mammalian cells. This regulation may be mediated through the action of a serine/threonine ceramide-activated protein phosphatase (CAPP). In this study, the existence of a ceramide-mediated pathway of cell regulation in Saccharomyces cerevisiae was investigated. Incubating exponentially growing S. cerevisiae cells with 1-20 microM cell-permeable ceramide (C2-ceramide) produced a dose-dependent inhibition of proliferation. A number of other lipids and detergents, such as arachidonate, oleate, Triton X-100, dioctanoylglycerol, and phenylaminoalcohol ceramide analogs, were largely ineffective, demonstrating the specificity of the response. Stereospecificity was demonstrated, in that the D enantiomer of erythro-C2-ceramide was more potent than the L enantiomer. More dramatically, a highly specific structural requirement for C2-ceramide was demonstrated, in that 1-12 microM C2-dihydroceramide was completely ineffective at inhibiting growth. Since C2-dihydroceramide lacks the 4-5 trans double bond present in C2-ceramide, this suggests that the antiproliferative properties of C2-ceramide depend upon the presence of the double bond. This raises an interesting possibility; the dehydrogenase responsible for introduction of the double bond during endogenous ceramide synthesis may regulate cell growth by controlling the cellular concentrations of dihydroceramide and ceramide. The oxygenase responsible for introduction of the final hydroxyl group in phytoceramide could provide a similar regulatory function in yeast. The potential role of CAPP in ceramide action in yeast was investigated next. Crude extracts of S. cerevisiae also contained a ceramide-dependent serine/threonine phosphatase activity, which was sensitive to inhibition by okadaic acid. This enzyme exhibited stereospecificity and structural requirements identical to that of the ceramide-induced growth inhibition. We conclude that both the growth-inhibitory response to ceramide and CAPP activity are conserved in S. cerevisiae. The identical stereochemical and structural requirements in both the biological and phosphatase assays suggest that the anti-proliferative effects of ceramide in yeast may be mediated in part through the action of CAPP.
Fishbein et al. (Sat,) studied this question.