Sphingoid base 1-phosphate phosphatase: A key regulator of sphingolipid metabolism and stress response

Abstract

The sphingolipid metabolites ceramide and sphingosine-1-phosphate are second messengers with opposing roles in mammalian cell growth arrest and survival; their relative cellular level has been proposed to be a rheostat that determines the fate of cells. This report demonstrates that this rheostat is an evolutionarily conserved stress-regulatory mechanism that inf luences growth and survival of yeast. Although the role of sphingosine-1-phosphate in yeast was not previously examined, accumulation of ceramide has been shown to induce G 1 arrest and cell death. We now have identified a gene in Saccharomyces cerevisiae, LBP1, that regulates the levels of phosphorylated sphingoid bases and ceramide. LBP1 was cloned from a yeast mutant that accumulated phosphorylated long-chain sphingoid bases and diverted sphingoid base intermediates from sphingolipid pathways to glycerophospholipid biosynthesis. LBP1 and its homolog, LBP2, encode very hydrophobic proteins that contain a novel-conserved sequence motif for lipid phosphatases, and both have long-chain sphingoid base phosphate phosphatase activity. In vitro characterization of Lbp1p shows that this phosphatase is Mg 2؉ -independent with high specificity for phosphorylated long-chain bases, phytosphingosine and sphingosine. The deletion of LBP1 results in the accumulation of phosphorylated long-chain sphingoid bases and reduced ceramide levels. Moreover, deletion of LBP1 and LBP2 results in dramatically enhanced survival upon severe heat shock. Thus, these phosphatases play a previously unappreciated role in regulating ceramide and phosphorylated sphingoid base levels in yeast, and they modulate stress responses through sphingolipid metabolites in a manner that is reminiscent of their effects on mammalian cells.Branching pathways of sphingolipid metabolism may mediate growth arrest, stress, or proliferative responses depending on the cell type and the nature of the stimulus. Ceramide is emerging as an important regulatory component of stress responses and programmed cell death, known as apoptosis (1-5). In contrast, another sphingolipid metabolite, sphingosine-1-phosphate (SPP), has been implicated as a second messenger in cellular proliferation (6) and antagonizes ceramide-mediated apoptosis (7). Thus, it has been suggested that the relative intracellular levels of ceramide and SPP are a critical factor for cell survival. Although the ceramide͞SPP rheostat may be an inherent characteristic of mammalian cells, external stimuli can reset this ratio (7-9). A variety of stress stimuli, including Fas ligand, TNF-␣, IL-1, growth factor withdrawal, anticancer drugs, oxidative stress, heat shock, and ionizing radiation, increase ceramide levels (1, 2, 10, 11), whereas platelet-derived growth factor and other growth factors stimulate rapid, transient elevations in SPP levels (6). The mechanisms that regulate the levels of these sphingolipid second messengers are under intense investigation with most of the attention focused on degradative pathways: sphingomyelinase, which...

Lag1p and Lac1p Are Essential for the Acyl-CoA–dependent Ceramide Synthase Reaction inSaccharomyces cerevisae

“…Moreover, mutant lag1⌬lac1⌬ cells also have problems surviving at 37°C or after heat shock (unpublished results). Until now, it was claimed that only the sphingoid bases and their phosphorylated counterparts are required for the full heat shock response in yeast (Mandala et al, 1998;Skrzypek et al, 1999). From our results it is conceivable that ceramide is also necessary for survival under these stress conditions.…”

“…Moreover, an accumulation of PHS and DHS-1P was observed. It has been shown that exogenous DHS is first phosphorylated upon uptake by the long-chain base kinases Lcb4p and Lcb5p (Nagiec et al, 1998) and then dephosphorylated by Ysr2p or Ysr3p before it can be efficiently converted to ceramide or PHS (Mao et al, 1997;Mandala et al, 1998; see also Figure 1). From our [ 3 H]DHS labeling, the phosphorylation of sphingoid bases and the subsequent dephosphorylation appears to be unaltered because lag1⌬lac1⌬ cells exhibited wild-type amounts of DHS-1P and accumulated PHS.…”

Stress signals for apoptosis: ceramide and c-Jun kinase

“…This e ect is stereospeci®c as an analog of the natural precursor for ceramide, dihydroceramide, fails to induce apoptosis. Recent work has also implicated a role for sphingolipids including ceramides in signaling the heat shock response in yeast (Dickson et al, 1997;Jenkins et al, 1997;Mandala et al, 1998;Wells et al, 1998). In this regard, mutants incapable of rapid synthesis of sphingolipids in response to heat (Figure 2) fail to develop the capacity to grow at elevated temperatures.…”

Sphingosine-1-phosphate phosphohydrolase-1 regulates ER stress-induced autophagy

“…Interestingly, in Saccharomyces cerevisiae, the homolog of SPP1 has been shown to be a key regulator of stress responses. 12,13 Because previous studies have shown that SPP1 also has a key role in the regulation of the relative levels of S1P and ceramide and thus cell fate, 14 it was important to examine its function in regulating autophagic responses leading to cell survival or cell death. Our results suggest that SPP1, by its influence on intracellular S1P levels, regulates the unfolded protein response (UPR) and ER stress-induced autophagy.…”