Ruijuan Xu

Cloning of an alkaline ceramidase from Saccharomyces cerevisiae. An enzyme with reverse (CoA-independent) ceramide synthase activity.

Ceramide is not only a core intermediate of sphingolipids but also an important modulator of many cellular events including apoptosis, cell cycle arrest, senescence, differentiation, and stress responses. Its turnover may be tightly regulated. However, little is known about the regulation of its metabolism because most enzymes responsible for its synthesis and breakdown have yet to be cloned. Here we report the cloning and characterization of the yeast gene YPC1 (YBR183w) by screeningSaccharomyces cerevisiae genes whose overexpression bestows resistance to fumonisin B1. We demonstrate that the yeast geneYPC1 encodes an alkaline ceramidase activity responsible for the breakdown of dihydroceramide and phytoceramide but not unsaturated ceramide. YPC1 ceramidase activity was confirmed byin vitro studies using an Escherichia coliexpression system. Importantly, YPC1p also has reverse activity, catalyzing synthesis of phytoceramide from palmitic acid and phytosphingosine. This ceramide synthase activity is CoA-independent and is resistant to fumonisin B1, thus explaining why YPC1was cloned as a fumonisin B1-resistant gene.

Cloning and Characterization of a Saccharomyces cerevisiae Alkaline Ceramidase with Specificity for Dihydroceramide

In a previous study, we reported that theSaccharomyces cerevisiae gene YPC1 encodes an alkaline ceramidase with a dual activity, catalyzing both hydrolysis and synthesis of yeast ceramide (Mao, C., Xu, R., Bielawska, A., and Obeid, L. M. (2000) J. Biol. Chem. 275, 6876–6884). In this study, we have identified a YPC1homologue in S. cerevisiae that also encodes an alkaline ceramidase. We show that these two ceramidases have different substrate specificity, such that YPC1p preferentially hydrolyzes phytoceramide, whereas the new ceramidase YDC1p hydrolyzes dihydroceramide preferentially and phytoceramide only slightly. Neither enzyme hydrolyzes unsaturated mammalian-type ceramide. In contrast to YPC1p, YDC1p had only minor in vitro reverse activity of catalyzing dihydroceramide formation from a free fatty acid and dihydrosphingosine and no activity with phytosphingosine. Overexpression of YDC1p had no reverse activity in non-stressed yeast cells, but like YPC1p suppressed the inhibition of growth by fumonisin B1 albeit more modestly. Deletion of YDC1 andYPC1 or both did not apparently affect growth, suggesting neither gene is essential. However, the Δydc1 deletion mutant but not the Δypc1 deletion mutant was sensitive to heat stress, indicating a role for dihydroceramide but not phytoceramide in heat stress responses, and suggesting that the two enzymes have distinct physiological functions.

Golgi alkaline ceramidase regulates cell proliferation and survival by controlling levels of sphingosine and S1P

Sphingosine‐1‐phosphate (S1P), a sphingolipid metabolite, promotes cell proliferation and survival whereas its precursor, sphingosine, has the opposite effects. However, much remains unknown about their regulation. Here we identify a novel human ceramidase (haCER2) that regulates the levels of both sphingosine and S1P by controlling the hydrolysis of ceramides. haCER2 is localized to the Golgi complex and is highly expressed in the placenta. High ectopic expression of haCER2 caused fragmentation of the Golgi complex and growth arrest in HeLa cells due to sphingosine accumulation. Low ectopic expression of haCER2 increased S1P without sphingosine accumulation, promoting cell proliferation in serum‐free medium. This proliferative effect was suppressed by dimethylsphingosine, an inhibitor of the S1P formation, or by the RNA interference (RNAi) ‐mediated inhibition of S1P1, a G‐protein‐coupled receptor for S1P. The RNAi‐mediated down‐regulation of haCER2 enhanced the serum deprivation‐induced growth arrest and apoptosis of HeLa cells, which was inhibited by addition of exogenous S1P. Serum deprivation up‐regulated both haCER2 mRNA and activity in HeLa cells. haCER2 mRNA is also up‐regulated in some tumors. Taken together, these results suggest that haCER2 is important for the generation of S1P and S1P‐mediated cell proliferation and survival, but that its overexpression may cause cell growth arrest due to an accumulation of sphingosine.—Xu, R., Jin, J., Hu, W., Sun, W., Bielawski, J., Szulc, Z., Taha, T., Obeid, L. M., Mao, C. Golgi alkaline ceramidase regulates cell proliferation and survival by controlling levels of sphingosine and S1P FASEB J. 20, 1813–1825 (2006)

Alkaline Ceramidase 3 (ACER3) Hydrolyzes Unsaturated Long-chain Ceramides, and Its Down-regulation Inhibits Both Cell Proliferation and Apoptosis

Ceramides with different fatty acyl chains may vary in their physiological or pathological roles; however, it remains unclear how cellular levels of individual ceramide species are regulated. Here, we demonstrate that our previously cloned human alkaline ceramidase 3 (ACER3) specifically controls the hydrolysis of ceramides carrying unsaturated long acyl chains, unsaturated long-chain (ULC) ceramides. In vitro, ACER3 only hydrolyzed C18:1-, C20:1-, C20:4-ceramides, dihydroceramides, and phytoceramides. In cells, ACER3 overexpression decreased C18:1- and C20:1-ceramides and dihydroceramides, whereas ACER3 knockdown by RNA interference had the opposite effect, suggesting that ACER3 controls the catabolism of ULC ceramides and dihydroceramides. ACER3 knockdown inhibited cell proliferation and up-regulated the cyclin-dependent kinase inhibitor p21CIP1/WAF1. Blocking p21CIP1/WAF1 up-regulation attenuated the inhibitory effect of ACER3 knockdown on cell proliferation, suggesting that ACER3 knockdown inhibits cell proliferation because of p21CIP1/WAF1 up-regulation. ACER3 knockdown inhibited cell apoptosis in response to serum deprivation. ACER3 knockdown up-regulated the expression of the alkaline ceramidase 2 (ACER2), and the ACER2 up-regulation decreased non-ULC ceramide species while increasing both sphingosine and its phosphate. Collectively, these data suggest that ACER3 catalyzes the hydrolysis of ULC ceramides and dihydroceramides and that ACER3 coordinates with ACER2 to regulate cell proliferation and survival.

Alkaline Ceramidase 2 (ACER2) and Its Product Dihydrosphingosine Mediate the Cytotoxicity of N-(4-Hydroxyphenyl)retinamide in Tumor Cells

Increased generation of dihydrosphingosine (DHS), a bioactive sphingolipid, has been implicated in the cytotoxicity of the synthetic retinoid N-(4-hydroxyphenyl)retinamide (4-HPR) in tumor cells. However, how 4-HPR increases DHS remains unclear. Here we demonstrate that 4-HPR increases the expression of ACER2, which catalyzes the hydrolysis of dihydroceramides to generate DHS, and that ACER2 up-regulation plays a key role in mediating the 4-HPR-induced generation of DHS as well as the cytotoxicity of 4-HPR in tumor cells. Treatment with 4-HPR induced the accumulation of dihydroceramides (DHCs) in tumor cells by inhibiting dihydroceramide desaturase (DES) activity, which catalyzes the conversion of DHCs to ceramides. Treatment with 4-HPR also increased ACER2 expression through a retinoic acid receptor-independent and caspase-dependent manner. Overexpression of ACER2 augmented the 4-HPR-induced generation of DHS as well as 4-HPR cytotoxicity, and 4-HPR-induced death in tumor cells, whereas knocking down ACER2 had the opposite effects. ACER2 overexpression, along with treatment with GT11, another DES inhibitor, markedly increased cellular DHS, leading to tumor cell death, whereas ACER2 overexpression or GT11 treatment alone failed to do so, suggesting that both ACER2 up-regulation and DES inhibition are necessary and sufficient to mediate 4-HPR-induced DHS accumulation, cytotoxicity, and death in tumor cells. Taken together, these results suggest that up-regulation of the ACER2/DHS pathway mediates the cytotoxicity of 4-HPR in tumor cells and that up-regulating or activating ACER2 may improve the anti-cancer activity of 4-HRR and other DHC-inducing agents.

Substrate Specificity, Membrane Topology, and Activity Regulation of Human Alkaline Ceramidase 2 (ACER2)

Human alkaline ceramidase 2 (ACER2) plays an important role in cellular responses by regulating the hydrolysis of ceramides in cells. Here we report its biochemical characterization, membrane topology, and activity regulation. Recombinant ACER2 was expressed in yeast mutant cells (Δypc1Δydc1) that lack endogenous ceramidase activity, and microsomes from ACER2-expressiong yeast cells were used to biochemically characterize ACER2. ACER2 catalyzed the hydrolysis of various ceramides and followed Michaelis-Menten kinetics. ACER2 required Ca2+ for both its in vitro and cellular activities. ACER2 has 7 putative transmembrane domains, and its amino (N) and carboxyl (C) termini were found to be oriented in the lumen of the Golgi complex and cytosol, respectively. ACER2 mutant (ACER2ΔN36) lacking the N-terminal tail (the first 36 amino acid residues) exhibited undetectable activity and was mislocalized to the endoplasmic reticulum, suggesting that the N-terminal tail is necessary for both ACER2 activity and Golgi localization. ACER2 mutant (ACER2ΔN13) lacking the first 13 residues was also mislocalized to the endoplasmic reticulum although it retained ceramidase activity. Overexpression of ACER2, ACER2ΔN13, but not ACER2ΔN36 increased the release of sphingosine 1-phosphate from cells, suggesting that its mislocalization does not affect the ability of ACER2 to regulate sphingosine 1-phosphate secretion. However, overexpression of ACER2 but not ACER2ΔN13 or ACER2ΔN36 inhibited the glycosylation of integrin β1 subunit and Lamp1, suggesting that its mistargeting abolishes the ability of ACER2 to regulation protein glycosylation. These data suggest that ACER2 has broad substrate specificity and requires Ca2+ for its activity and that ACER2 has the cytosolic C terminus and luminal N terminus, which are essential for its activity, correct cellular localization, and regulation for protein glycosylation.

Role of alkaline ceramidases in the generation of sphingosine and its phosphate in erythrocytes

Plasma sphingosine‐1‐phosphate (S1P) has been suggested to mainly originate from erythrocytes; however, within the erythrocyte, how sphingosine (SPH) generation—the precursor to S1P—is controlled is unknown. SPH is only generated from the hydrolysis of ceramides via ceramidases. Five human ceramidases have been identified: 1 acid, 1 neutral, and 3 alkaline ceramidases (ACER1, ACER2, and ACER3). Here, we demonstrate that only alkaline ceramidase activity is expressed in erythrocytes and that it is instrumental for SPH generation. Erythrocytes have alkaline but not acid or neutral ceramidase activity on D‐e‐C18:1‐ceramide, a common substrate of ceramidases. Not only alkaline ceramidase activity but also the generation of SPH and S1P are increased during erythroid differentiation in K562 erythroleukemic cells. Such SPH and S1P increases were inhibited by the alkaline ceramidase inhibitor D‐e‐MAPP, suggesting that alkaline ceramidases have a role in the generation of SPH and S1P in erythroid cells. Alkaline ceramidase activity is highly expressed in mouse erythrocytes, and intravenous administration of D‐e‐MAPP decreased both SPH and S1P in erythrocytes and plasma. Collectively, these results suggest that alkaline ceramidase activity is important for the generation of SPH, the S1P precursor in erythrocytes.—Xu, R., Sun, W., Jin, J., Obeid, L. M., Mao, C. Role of alkaline ceramidases in the generation of sphingosine and its phosphate in erythrocytes. FASEBJ. 24, 2507–2515 (2010). www.fasebj.org

Alkaline ceramidase 2 regulates β1 integrin maturation and cell adhesion

The polypeptide core of the integrin βl subunit (βl) is glycosylated sequentially in the endoplasmic reticulum and the Golgi complex to form β1 precursor and mature βl, respectively. The βl precursor to mature βl conversion, termed βl maturation, regulates the cell surface levels and function of βl‐containing integrins, βl integrins. Here we demonstrate that the human alkaline ceramidase 2 (ACER2), a Golgi enzyme, regulates βl maturation by controlling the generation of sphingosine. ACER2 overexpression inhibited βl maturation, thus leading to a decrease in the levels of mature βl in T‐REx HeLa cells, whereas RNA interference‐mediated knockdown of ACER2 enhanced βl maturation in MCF‐7 cells. ACER2 overexpression decreased the cell surface levels of βl integrins, thus inhibiting cell adhesion to fibronectin or collagen, whereas ACER2 knockdown has the opposite effects. Treatment with all‐trans retinoic acid (ATRA) increased both the expression of ACER2 and the generation of sphingosine in HeLa cells and inhibited β l maturation. ACER2 knockdown attenuated the inhibitory effects of ATRA on both βl maturation and cell adhesion. In contrast, treatment with phorbol myristate acetate (PMA), a protein kinase C activator, decreased the expression of ACER2 and sphingosine in T‐REx HeLa cells, thus enhancing βl maturation. ACER2 overexpression inhibited the stimulatory effects of PMA on both βl maturation and cell adhesion. These results suggest that the ACER2/sphingosine pathway plays an important role in regulating βl maturation and cell adhesion mediated by βl integrins.— Sun, W., Hu, W., Xu, R., Jin, J., Szulc, Z. M., Zhang, G., Galadari, S. H., Obeid, L. M, Mao, C. Alkaline ceramidase 2 regulates βl integrin maturation and cell adhesion. FASEB J. 23, 656‐666 (2009)

Targeting (cellular) lysosomal acid ceramidase by B13: design, synthesis and evaluation of novel DMG-B13 ester prodrugs.

Acid ceramidase (ACDase) is being recognized as a therapeutic target for cancer. B13 represents a moderate inhibitor of ACDase. The present study concentrates on the lysosomal targeting of B13 via its N,N-dimethylglycine (DMG) esters (DMG-B13 prodrugs). Novel analogs, the isomeric mono-DMG-B13, LCL522 (3-O-DMG-B13·HCl) and LCL596 (1-O-DMG-B13·HCl) and di-DMG-B13, LCL521 (1,3-O, O-DMG-B13·2HCl) conjugates, were designed and synthesized through N,N-dimethyl glycine (DMG) esterification of the hydroxyl groups of B13. In MCF7 cells, DMG-B13 prodrugs were efficiently metabolized to B13. The early inhibitory effect of DMG-B13 prodrugs on cellular ceramidases was ACDase specific by their lysosomal targeting. The corresponding dramatic decrease of cellular Sph (80-97% Control/1h) by DMG-B13 prodrugs was mainly from the inhibition of the lysosomal ACDase.

Aging-related elevation of sphingoid bases shortens yeast chronological life span by compromising mitochondrial function.

Sphingoid bases (SBs) as bioactive sphingolipids, have been implicated in aging in yeast. However, we know neither how SBs are regulated during yeast aging nor how they, in turn, regulate it. Herein, we demonstrate that the yeast alkaline ceramidases (YPC1 and YDC1) and SB kinases (LCB4 and LCB5) cooperate in regulating SBs during the aging process and that SBs shortens chronological life span (CLS) by compromising mitochondrial functions. With a lipidomics approach, we found that SBs were increased in a time-dependent manner during yeast aging. We also demonstrated that among the enzymes known for being responsible for the metabolism of SBs, YPC1 was upregulated whereas LCB4/5 were downregulated in the course of aging. This inverse regulation of YPC1 and LCB4/5 led to the aging-related upregulation of SBs in yeast and a reduction in CLS. With the proteomics-based approach (SILAC), we revealed that increased SBs altered the levels of proteins related to mitochondria. Further mechanistic studies demonstrated that increased SBs inhibited mitochondrial fusion and caused fragmentation, resulting in decreases in mtDNA copy numbers, ATP levels, mitochondrial membrane potentials, and oxygen consumption. Taken together, these results suggest that increased SBs mediate the aging process by impairing mitochondrial structural integrity and functions.