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

Abstract

Sphingolipids and phosphoinositides both play signaling roles in Saccharomyces cerevisiae. Although previous data indicate independent functions for these two classes of lipids, recent genetic studies have suggested interactions between phosphatidylinositol (PtdIns) phosphate effectors and sphingolipid biosynthetic enzymes. The present study was undertaken to further define the effects of phosphatidylinositol 4-phosphate (PtdIns(4)P) metabolism on cell sphingolipid metabolism. The data presented indicate that deletion of SAC1, a gene encoding a PtdIns(4)P phosphatase, increased levels of most sphingolipid species, including sphingoid bases, sphingoid base phosphates, and phytoceramide. In contrast, sac1⌬ dramatically reduced inositol phosphosphingolipids, which result from the addition of a PtdIns-derived phosphoinositol head group to ceramides through Aur1p. Deletion of SAC1 decreased PtdIns dramatically in both steady-state and pulse labeling studies, suggesting that the observed effects on sphingolipids may result from modulation of the availability of PtdIns as a substrate for Aur1p. Supporting this hypothesis, acute attenuation of PtdIns(4)P production through Stt4p immediately increased PtdIns and subsequently reduced sphingoid bases. This reduction was overcome by the inhibition of Aur1p. Moreover, modulation of sphingoid bases through perturbation of PtdIns(4)P metabolism initiated sphingolipid-dependent biological effects, supporting the biological relevance for this route of regulating sphingolipids. These findings suggest that, in addition to potential signaling effects of PtdInsP effectors on sphingolipid metabolism, PtdIns kinases may exert substantial effects on cell sphingolipid profiles at a metabolic level through modulation of PtdIns available as a substrate for complex sphingolipid synthesis.Sphingolipids play vital roles in Saccharomyces cerevisiae, including regulation of translation, cell cycle, sporulation, ubiquitin-dependent proteolysis, actin cytoskeleton rearrangements, endocytosis, stress responses, and numerous other processes (see Refs. 1-3 for review). Phosphoinositides comprise another group of key lipid mediators that regulate numerous cellular functions, including organization of the actin cytoskeleton, endocytosis, cytokinesis, vacuolar morphology, and translation initiation (4 -7). The apparent overlap in biological roles of these distinct classes of lipids implies interaction between them. Indeed, recent studies support regulation of sphingolipid metabolism by phosphoinositides (8 -11).Sphingolipid metabolism has been reviewed extensively, but, in brief, sphingolipid synthesis begins with the condensation of palmitoyl-coenzyme A and serine by serine-palmitoyl transferase to form 3-ketodihydrosphingosine (12) (Fig. 1). This shortlived intermediate is quickly converted to the sphingoid bases dihydrosphingosine (DHS) 2 and phytosphingosine (PHS), which undergo phosphorylation to form sphingoid base phosphates or N-acylation to form ceramides (13, 14). Phytoceramide and phosphati...

Phosphoinositide-bis-phosphate is required for Saccharomyces cerevisiae invasive growth

“…Furthermore, the calcium sensitive phenotype of the csg2 complex sphingolipid biosynthesis mutant was suppressed by a mss4 ts mutation (Dunn et al, 1998). In contrast, inhibition of the plasma membrane PI4-kinase Stt4p by wortmannin resulted in an increase in complex sphingolipids (Brice et al, 2009). Similarly in the mss4-f12 mutant we observed reduced levels of PtdIns(4,5)P 2 and an accumulation of the complex sphingolipid, M(IP) 2 C. Furthermore, inhibition of complex sphingolipid biosynthesis results in an increase in plasma membrane PtdIns(4)P and PtdIns(4,5)P 2 (Jesch et al, 2010), suggesting that the increased PI levels observed in the mss4-f12 mutant are likely to contribute to increased complex sphingolipid levels.…”

Regulation of sphingolipid synthesis via Orm1 and Orm2 in yeast

“…3B, lane 1). Phosphatidylinositol is a component of complex sphingolipids in yeast, and it has been reported that sphingolipid metabolism is modulated by phosphatidylinositol levels regulated by Sac1 (Brice et al, 2009). Similarly to the response to myriocin, increased phosphorylation of Orm1 in sac1D cells might reflect a compensatory response to inhibited flux through the sphingolipid synthesis pathway.…”

Sphingolipid Homeostasis in the Endoplasmic Reticulum and Beyond

“…Additionally, they share headgroup modifications, with glycerolipids often providing the headgroup moieties for sphingolipids. For example, the production of inositolphosphorylceramide (IPC) in yeast relies on inositol derived from phosphatidylinositol (PI), with the latter being converted to diacylglycerol (DAG) (Brice et al 2009). In mammalian cells, SM production is similarly coupled to conversion of phosphatidylcholine (PC) to DAG.…”