Inositol phosphoceramide synthase is a regulator of intracellular levels of diacylglycerol and ceramide during the G₁ to S transition in Saccharomyces cerevisiae

We recently reported that DAG (diacylglycerol) generated during sphingomyelin synthesis plays an important role in protein kinase C activation and cell proliferation in Madin–Darby canine kidney cells [Cerbon and Lopez-Sanchez (2003) Biochem. J. 373, 917–924]. In yeast cells, IPC (inositol phosphoceramide) synthase catalyses the transfer of phosphoinositol from phosphatidylinositol to ceramide to form IPC and generates DAG. In the present study, we found that, during the G₁ to S transition after N₂-starvation, there was a significant increase in the synthesis of IPC accompanied by a progressive increase (up to 6-fold) in the level of DAG. The increased DAG levels coincided with decrements in ceramide and sphingoid base levels, conditions that are adequate for the activation of putative protein kinase C required for the G₁ to S transition and proliferation of yeast cells. To separate the role of DAG generated during IPC synthesis from that originating from other sources, we utilized β-chloroalanine and myriocin, inhibitors of serine:palmitoyl-CoA transferase, the first committed step in sphingolipid synthesis, to avoid accumulation of sphingolipid intermediates. When the synthesis of sphingolipids was inhibited, DAG accumulation was significantly decreased and the G₁ to S transition was blocked; such blockage was avoided by metabolic complementation with phytosphingosine. The DAG/ceramide ratio was 0.27 and it changed to 2.0 during growth re-initiation, suggesting that the synthesis of phosphosphingolipids could act to switch growth arrest (increased ceramide) to a mitogenic signal (increased DAG), and that this signalling process is preserved in yeast and mammalian cells.