The addition of inositol to actively growing yeast cultures causes a rapid increase in the rate of synthesis of phosphatidylinositol and, simultaneously, triggers changes in the expression of hundreds of genes. We now demonstrate that the addition of inositol to yeast cells growing in the presence of choline leads to a dramatic reprogramming of cellular lipid synthesis and turnover. The response to inositol includes a 5-6-fold increase in cellular phosphatidylinositol content within a period of 30 min. The increase in phosphatidylinositol content appears to be dependent upon fatty acid synthesis. Phosphatidylcholine turnover increased rapidly following inositol addition, a response that requires the participation of Nte1p, an endoplasmic reticulum-localized phospholipase B. Mass spectrometry revealed that the acyl species composition of phosphatidylinositol is relatively constant regardless of supplementation with inositol or choline, whereas phosphatidylcholine acyl species composition is influenced by both inositol and choline. In medium containing inositol, but lacking choline, high levels of dimyristoylphosphatidylcholine were detected. Within 60 min following the addition of inositol, dimyristoylphosphatidylcholine levels had decreased from ϳ40% of total phosphatidylcholine to a basal level of less than 5%. nte1⌬ cells grown in the absence of inositol and in the presence of choline exhibited lower levels of dimyristoylphosphatidylcholine than wild type cells grown under these same conditions, but these levels remained largely constant after the addition of inositol. These results are discussed in relationship to transcriptional regulation known to be linked to lipid metabolism in yeast.

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The protein kinase C (PKC)-MAPK signaling cascade is activated and is essential for viability when cells are starved for the phospholipid precursor inositol. In this study, we report that inhibiting inositol-containing sphingolipid metabolism, either by inositol starvation or treatment with agents that block sphingolipid synthesis, triggers PKC signaling independent of sphingoid base accumulation. Under these same growth conditions, a fluorescent biosensor that detects the necessary PKC signaling intermediate, phosphatidylinositol (PI)-4-phosphate (PI4P), is enriched on the plasma membrane. The appearance of the PI4P biosensor on the plasma membrane correlates with PKC activation and requires the PI 4-kinase Stt4p. Like other mutations in the PKC-MAPK pathway, mutants defective in Stt4p and the PI4P 5-kinase Mss4p, which generates phosphatidylinositol 4,5-bisphosphate, exhibit inositol auxotrophy, yet fully derepress INO1, encoding inositol-3-phosphate synthase. These observations suggest that inositol-containing sphingolipid metabolism controls PKC signaling by regulating access of the signaling lipids PI4P and phosphatidylinositol 4,5-bisphosphate to effector proteins on the plasma membrane.The protein kinase C (PKC) pathway, also known in yeast as the cell wall integrity pathway, is a highly conserved signal transduction pathway that is activated in Saccharomyces cerevisiae during periods of polarized cell growth (1, 2) as well as by numerous environmental stresses, including elevated temperature (3), entry into stationary growth phase (4), and treatment with agents that interfere with cell wall biogenesis (5, 6). Signals produced on the cell surface are amplified and relayed by PKC to downstream targets through a three-component MAPK phosphorylation cascade composed of the MEK kinase Bck1p, the redundant MEKs Mkk1p and Mkk2p, and the MAPK Slt2p (Fig. 1A). Pkc1p homologs in mammals are directly regulated by lipids, including diacylglycerol (DAG) 2 produced by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P 2 ), anionic phospholipids, and sphingolipids (7,8). Nevertheless, these lipid metabolites do not appear to play identical roles in yeast PKC activation.In yeast, the phosphorylated derivatives of phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PI4P) and PI(4,5)P 2 , are essential for PKC signaling during heat stress (9). PI4P is produced on the plasma membrane by the PI 4-kinase Stt4p and is subsequently phosphorylated to PI(4,5)P 2 by the PI4P 5-kinase Mss4p. Stt4p was originally identified in a genetic screen for mutants that are hypersensitive to staurosporine, a specific inhibitor of PKC (10). Mutations in both STT4 and MSS4 cause cell lysis phenotypes associated with defects in cell wall integrity signaling, and these defects are suppressed by overexpression of PKC1 (10, 11). Stt4p-dependent pools of PI4P and PI(4,5)P 2 are proposed to regulate PKC signaling by plasma membrane recruitment of the guanine nucleotide exchange factor Rom2p, where it carries out multiple function...

Although antioxidants are used to treat an overdose of the analgaesic/antipyretic drug APAP (acetaminophen), roles of antioxidant enzymes in APAP-induced hepatotoxicity remain controversial. Our objective was to determine impacts of knockout of SOD1 (superoxide dismutase; Cu,Zn-SOD) alone or in combination with selenium-dependent GPX1 (glutathione peroxidase-1) on APAP-induced hepatotoxicity. All SOD1-null (SOD1-/-) and SOD1- and GPX1-double-knockout mice survived an intraperitoneal injection of 600 mg of APAP per kg of body mass, whereas 75% of WT (wild-type) and GPX1-null mice died within 20 h. Survival time of SOD1-/- mice injected with 1200 mg of APAP per kg of body mass was longer than that of the WT mice (934 compared with 315 min, P<0.05). The APAP-treated SOD1-/- mice had less (P<0.05) plasma ALT (alanine aminotransferase) activity increase and attenuated (P<0.05) hepatic glutathione depletion than the WT mice. The protection conferred by SOD1 deletion was associated with a block of the APAP-mediated hepatic protein nitration and a 50% reduction (P<0.05) in activity of a key APAP metabolism enzyme CYP2E1 (cytochrome P450 2E1) in liver. The SOD1 deletion also caused moderate shifts in the APAP metabolism profiles. In conclusion, deletion of SOD1 alone or in combination with GPX1 greatly enhanced mouse resistance to APAP overdose. Our results suggest a possible pro-oxidant role for the physiological level of SOD1 activity in APAP-mediated hepatotoxicity.

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