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

Abstract

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 beta-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.

De Novo Sphingolipid Synthesis Is Essential for Viability, but Not for Transport of Glycosylphosphatidylinositol-Anchored Proteins, in African Trypanosomes

“…Studies of both yeast and mammalian cells have shown that ongoing sphingolipid synthesis is essential for cell proliferation (14). However, recent work with the kinetoplastid parasite Leishmania demonstrate that log-phase promastigote- 456 SUTTERWALA ET AL.…”

Modulation of Sphingolipid Metabolism by the Phosphatidylinositol-4-phosphate Phosphatase Sac1p through Regulation of Phosphatidylinositol in Saccharomyces cerevisiae

“…This concept is supported by the literature, which indicates that sphingolipid metabolism represents a highly interconnected network. In previous studies, treatment with aureobasidin A, which inhibits the IPC synthase Aur1p (38), increased ceramides while reducing levels of diacylglycerol, the other product of the IPC synthesis reaction (23). Furthermore, the addition of inositol to the culture medium markedly increased cellular PtdIns (39), resulting in increased complex sphingolipid synthesis with concomitant decreases in ceramides (22).…”

“…The addition of inositol to the culture media decreased ceramide levels as IPC and, it was inferred, PtdIns increased (22); however, these effects have not been demonstrated in cells with perturbed PtdIns metabolic pathways. Furthermore, it has also been shown that inhibition of IPC synthesis increased the immediate metabolic precursor, ceramides (23), but the effect on other sphingolipids, including signaling sphingolipids, remains unclear. Together, these results raise tantalizing possibilities about the potential for turnover between PtdIns(4)P and PtdIns to regulate sphingolipid synthesis through nonsignaling mechanisms.…”

Sphingolipid Homeostasis in the Endoplasmic Reticulum and Beyond

“…In mammalian cells, SM production is similarly coupled to conversion of phosphatidylcholine (PC) to DAG. Thus, glycerolipid sensors such as the DAG-binding domain in protein kinase C could provide a mechanism to monitor the rate of sphingolipid biosynthesis (Cerbon and del Carmen Lopez-Sanchez 2003;Cerbon et al 2005).…”