When Do Lasses (Longevity Assurance Genes) Become CerS (Ceramide Synthases)?

Ceramide is the key intermediate in the pathway of sphingolipid (SL) 2The abbreviations used are: SL, sphingolipid; CerS, ceramide synthase; ER, endoplasmic reticulum; TM, trans-membrane. biosynthesis (1Merrill Jr., A.H. J. Biol. Chem. 2002; 277: 25843-25846Abstract Full Text Full Text PDF PubMed Scopus (506) Google Scholar) and an important intracellular signaling molecule (2Kolesnick R. J. Clin. Invest. 2002; 110: 3-8Crossref PubMed Scopus (389) Google Scholar, 3Hannun Y.A. Obeid L.M. J. Biol. Chem. 2002; 277: 25847-25850Abstract Full Text Full Text PDF PubMed Scopus (770) Google Scholar). Ceramide consists of a sphingoid long chain base to which a fatty acid is attached via an amide bond (Fig. 1). When the chemical composition of SLs was first determined in the 1940s (4Klenk E.Z. Physiol. Chem. 1942; 273: 76-86Crossref Scopus (146) Google Scholar), stearic acid (C18:0) was identified as the major fatty acid attached to the sphingoid base. Later, with the development of more sensitive techniques (5Trams E.G. Giuffrida L.E. Karmen A. Nature. 1962; 193: 680-681Crossref PubMed Scopus (15) Google Scholar, 6O'Brien J.S. Fillerup D.L. Mead J.F. J. Lipid Res. 1964; 15: 109-116Abstract Full Text PDF Google Scholar), it became clear that mammalian SLs contain a wide variety of fatty acids, ranging in length from C14 to C32, that are predominantly saturated and can contain α- or ω-hydroxyl groups (7van Meer G. EMBO J. 2005; 24: 3159-3165Crossref PubMed Scopus (423) Google Scholar, 8Merrill Jr., A.H. Sullards M.C. Allegood J.C. Kelly S. Wang E. Methods. 2005; 36: 207-224Crossref PubMed Scopus (484) Google Scholar). Within the past few years, there has been renewed interest in the functional significance of this fatty acid variability, and this minireview will focus on current advances in our understanding of how the fatty acid composition of ceramide is regulated. In particular, we will discuss a recently discovered family of mammalian ceramide synthase (CerS) genes and emerging evidence that specific ceramides containing distinct fatty acids play important roles in cell growth and apoptosis. CerS acylates sphinganine (dihydrosphingosine) to form dihydroceramide and sphingosine to form ceramide; sphinganine is produced through the de novo biosynthetic pathway and sphingosine mainly through SL degradation (1Merrill Jr., A.H. J. Biol. Chem. 2002; 277: 25843-25846Abstract Full Text Full Text PDF PubMed Scopus (506) Google Scholar, 9Futerman A.H. Riezman H. Trends Cell Biol. 2005; 15: 312-318Abstract Full Text Full Text PDF PubMed Scopus (270) Google Scholar). Early studies detected CerS activity in microsomal fractions, and the enzyme was shown to utilize a variety of fatty acyl-CoAs although some specificity toward certain CoAs was recognized (10Morell P. Radin N.S. J. Biol. Chem. 1970; 245: 342-350Abstract Full Text PDF PubMed Google Scholar). CerS activity was later localized to the cytoplasmic leaflet of the endoplasmic reticulum (ER) (11Mandon E.C. Ehses I. Rother J. Van Echten G. Sandhoff K. J. Biol. Chem. 1992; 267: 11144-11148Abstract Full Text PDF PubMed Google Scholar, 12Hirschberg K. Rodger J. Futerman A.H. Biochem. J. 1993; 290: 751-757Crossref PubMed Scopus (164) Google Scholar). The first clue to the molecular identity of CerS was obtained in yeast, when it was shown that Lag1p and Lac1p were required for the synthesis of C26-ceramide (13Guillas I. Kirchman P.A. Chuard R. Pfefferli M. Jiang J.C. Jazwinski S.M. Conzelmann A. EMBO J. 2001; 20: 2655-2665Crossref PubMed Scopus (228) Google Scholar, 14Schorling S. Vallee B. Barz W.P. Riezman H. Oesterhelt D. Mol. Biol. Cell. 2001; 12: 3417-3427Crossref PubMed Scopus (230) Google Scholar). 3In contrast to mammalian cells, yeast SLs contain mainly C26-fatty acid. Lag1 was originally isolated in a screen for longevity-related genes (15D'Mello N.P. Childress A.M. Franklin D.S. Kale S.P. Pinswasdi C. Jazwinski S.M. J. Biol. Chem. 1994; 269: 15451-15459Abstract Full Text PDF PubMed Google Scholar). A paralog, 4Paralogs are parallel genes in the same species, and an ortholog (see below) is the same gene in different species; homolog is a term encompassing both ortholog and paralog. Lac1, was found in a data base screen (16Jiang J.C. Kirchman P.A. Zagulski M. Hunt J. Jazwinski S.M. Genome Res. 1998; 8: 1259-1272Crossref PubMed Scopus (29) Google Scholar) along with homologs in human, mouse, and Caenorhabditis elegans. The Asc1 gene in tomato was subsequently isolated as a toxin-resistant gene (17Brandwagt B.F. Mesbah L.A. Takken F.L. Laurent P.L. Kneppers T.J. Hille J. Nijkamp H.J. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 4961-4966Crossref PubMed Scopus (182) Google Scholar); among the toxins tested was fumonisin B1, a CerS inhibitor (18Wang E. Norred W.P. Bacon C.W. Riley R.T. Merrill A.H. J. Biol. Chem. 1991; 266: 14486-14490Abstract Full Text PDF PubMed Google Scholar). Homologs of Lag1 and Lac1 have now been found in most species. Six paralogs are known in human and mouse, and were named Lass (longevity assurance) genes; in all species studied to date, at least two genes have been found in every organism. All of the Lass genes have a distinguishing motif in the C-terminal portion of the protein. This motif has been defined in a narrow but specific manner as the Lag domain, a region of ∼50 amino acid residues (19Venkataraman K. Futerman A. FEBS Lett. 2002; 528: 3-4Crossref PubMed Scopus (57) Google Scholar), and in a broader manner as the Tram-Lag-CLN8 (TLC) domain, a region of ∼200 residues also found in other proteins (20Winter E. Ponting C.P. Trends Biochem. Sci. 2002; 27: 381-383Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar). The broader definition is based on two additional proteins, TRAM1 and CLN8; TRAM1 was found in a search for homologs of Lag1 in humans. The exact function of TRAM proteins is not known, but they are thought to be involved in polypeptide transition through the ER (21Gorlich D. Hartmann E. Prehn S. Rapoport T.A. Nature. 1992; 357: 47-52Crossref PubMed Scopus (241) Google Scholar, 22Rapoport T.A. Science. 1992; 258: 931-936Crossref PubMed Scopus (279) Google Scholar). CLN8 was found by screening for genes causing neuronal ceroid-lipofuscinosis (23Ranta S. Zhang Y. Ross B. Lonka L. Takkunen E. Messer A. Sharp J. Wheeler R. Kusumi K. Mole S. Liu W. Soares M.B. Bonaldo M.F. Hirvasniemi A. de la Chapelle A. Gilliam T.C. Lehesjoki A.E. Nat. Genet. 1999; 23: 233-236Crossref PubMed Scopus (258) Google Scholar), an inherited metabolic disorder (24Futerman A.H. van Meer G. Nat. Rev. Mol. Cell Biol. 2004; 5: 554-565Crossref PubMed Scopus (655) Google Scholar). Both TRAM1 (13Guillas I. Kirchman P.A. Chuard R. Pfefferli M. Jiang J.C. Jazwinski S.M. Conzelmann A. EMBO J. 2001; 20: 2655-2665Crossref PubMed Scopus (228) Google Scholar) and CLN8 (25Guillas I. Jiang J.C. Vionnet C. Roubaty C. Uldry D. Chuard R. Wang J. Jazwinski S.M. Conzelmann A. J. Biol. Chem. 2003; 278: 37083-37091Abstract Full Text Full Text PDF PubMed Scopus (88) Google Scholar) failed to complement the yeast Lag1/Lac1 knock-out, suggesting that the TLC domain is not sufficient for CerS activity, whereas some human LASS proteins tested were able to complement CerS activity (25Guillas I. Jiang J.C. Vionnet C. Roubaty C. Uldry D. Chuard R. Wang J. Jazwinski S.M. Conzelmann A. J. Biol. Chem. 2003; 278: 37083-37091Abstract Full Text Full Text PDF PubMed Scopus (88) Google Scholar, 26Yu Y. Lu H. Pan H. Ma J.H. Ding Z.J. Li Y.Y. Microbiol. Res. 2006; 161: 203-211Crossref PubMed Scopus (4) Google Scholar). In humans there are 16 TLC domain-containing genes (Fig. 2): six LASSes, three TRAMs, CLN8, and six other novel genes. All 16 genes are conserved in mouse. When the 16 human proteins are aligned, four distinct branches appear. FAM57A and -B are one subgroup and are closer to CLN8 and the other novel proteins than to LASSes and TRAMs. The TRAM proteins form their own subgroup but are relatively close to the LASSes. The LASSes subdivide into two distinct groups, with LASS1 in its own category and LASSes 2–6 on a separate branch. This is consistent with the fact that LASS1 is much closer to the yeast proteins than the others (27Mizutani Y. Kihara A. Igarashi Y. Biochem. J. 2005; 390: 263-271Crossref PubMed Scopus (321) Google Scholar). A subset of Lass genes is predicted to contain a Homeobox (Hox) domain (19Venkataraman K. Futerman A. FEBS Lett. 2002; 528: 3-4Crossref PubMed Scopus (57) Google Scholar). In humans and mice, all but Lass1 have a Hox domain, in Drosophila only one of the four TLC-containing genes have a Hox domain, and no Hox domains are found in yeast and plant Lass genes. The Hox domain is derived from homeobox proteins, which are sequence-specific transcription factors important in development (28Gehring W.J. Gene (Amst.). 1993; 135: 215-221Crossref PubMed Scopus (97) Google Scholar), but there is no evidence that the Hox domain in Lass genes acts as a transcription factor. The fact that many family members lack the Hox domain suggest that it is unlikely to be involved in the catalytic mechanism of CerS. The six human LASS genes are located on different chromosomes (with the exception of LASS1 and -4, which are located on the same chromosome but at distant locations). Human and mouse orthologs 4Paralogs are parallel genes in the same species, and an ortholog (see below) is the same gene in different species; homolog is a term encompassing both ortholog and paralog. show strong similarity both in sequence and genomic structure, with the two major domains (i.e. Hox and TLC) encoded by the same relative exon in each of the LASS genes. All six genes in human (except, to date, LASS3) encode at least two isoforms, one of which lacks the homeobox domain and part of the TLC domain; the relevance of these short isoforms is currently unknown but may imply similar mechanisms of transcriptional regulation. Fig. 3 shows the genomic organization and protein isoforms of one of the LASS genes, namely LASS2. Recently, a splice variant of LASS5 has been shown to be expressed in lymphoma and other tumor cells and may be involved in tumor recognition by the immune system (29van Hall T. Wolpert E.Z. van Veelen P. Laban S. van der Veer M. Roseboom M. Bres S. Grufman P. de Ru A. Meiring H. de Jong A. Franken K. Teixeira A. Valentijn R. Drijfhout J.W. Koning F. Camps M. Ossendorp F. Karre K. Ljunggren H.G. Melief C.J. Offringa R. Nat. Med. 2006; 12: 417-424Crossref PubMed Scopus (132) Google Scholar). The Lass genes appear to encode multi-transmembrane (TM) spanning proteins. The exact number of TM domains, and their topology, has not been resolved experimentally (9Futerman A.H. Riezman H. Trends Cell Biol. 2005; 15: 312-318Abstract Full Text Full Text PDF PubMed Scopus (270) Google Scholar, 20Winter E. Ponting C.P. Trends Biochem. Sci. 2002; 27: 381-383Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar, 25Guillas I. Jiang J.C. Vionnet C. Roubaty C. Uldry D. Chuard R. Wang J. Jazwinski S.M. Conzelmann A. J. Biol. Chem. 2003; 278: 37083-37091Abstract Full Text Full Text PDF PubMed Scopus (88) Google Scholar), although a recent study suggested that the yeast proteins, Lag1 and Lac1, contain eight putative TM domains with the N and C termini of the proteins facing the cytoplasm (30Kageyama-Yahara N. Riezman H. Biochem. J. 2006; (in press)PubMed Google Scholar). The subcellular location of the CerS proteins (at least those for which experimental data is available) is the ER (27Mizutani Y. Kihara A. Igarashi Y. Biochem. J. 2005; 390: 263-271Crossref PubMed Scopus (321) Google Scholar, 31Venkataraman K. Riebeling C. Bodennec J. Riezman H. Allegood J.C. Sullards M.C. Merrill Jr., A.H. Futerman A.H. J. Biol. Chem. 2002; 277: 35642-35649Abstract Full Text Full Text PDF PubMed Scopus (240) Google Scholar, 32Riebeling C. Allegood J.C. Wang E. Merrill Jr., A.H. Futerman A.H. J. Biol. Chem. 2003; 278: 43452-43459Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar), consistent with earlier observations (11Mandon E.C. Ehses I. Rother J. Van Echten G. Sandhoff K. J. Biol. Chem. 1992; 267: 11144-11148Abstract Full Text PDF PubMed Google Scholar, 12Hirschberg K. Rodger J. Futerman A.H. Biochem. J. 1993; 290: 751-757Crossref PubMed Scopus (164) Google Scholar) and similar to the location of Tram proteins (22Rapoport T.A. Science. 1992; 258: 931-936Crossref PubMed Scopus (279) Google Scholar). The yeast genes, Lag1 and Lac1, act in an obligate complex with an additional protein, Lip1 (33Vallee B. Riezman H. EMBO J. 2005; 24: 730-741Crossref PubMed Scopus (122) Google Scholar), an integral ER membrane protein with one predicted TM domain. The Lip1 regions required for CerS activity may be in the membrane or in the lumen of the ER. Mammalian homologs of Lip1 have not been found in data base searches. The activity of mammalian Lass proteins might conceivably be regulated by other TLC family members (34Schulz A. Mousallem T. Venkataramani M. Persaud-Sawin D.A. Zucker A. Luberto C. Bielawska A. Bielawski J. Holthuis J.C. Jazwinski S.M. Kozhaya L. Dbaibo G.S. Boustany R.M. J. Biol. Chem. 2006; 281: 2784-2794Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar). The first evidence for specific functional roles of mammalian Lass genes was obtained upon overexpression of LASS1 (formerly known as UOG1), which resulted in a selective increase in C18-ceramide in mammalian cells (31Venkataraman K. Riebeling C. Bodennec J. Riezman H. Allegood J.C. Sullards M.C. Merrill Jr., A.H. Futerman A.H. J. Biol. Chem. 2002; 277: 35642-35649Abstract Full Text Full Text PDF PubMed Scopus (240) Google Scholar). LASS4 (TRH1) and LASS5 (TRH4) were subsequently shown to selectively utilize C18/20 and C16 acyl-CoAs, respectively (32Riebeling C. Allegood J.C. Wang E. Merrill Jr., A.H. Futerman A.H. J. Biol. Chem. 2003; 278: 43452-43459Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar), LASS6 to produce shorter acyl chain ceramides (C14 and C16) (27Mizutani Y. Kihara A. Igarashi Y. Biochem. J. 2005; 390: 263-271Crossref PubMed Scopus (321) Google Scholar), and LASS3 to produce C18- and C24-ceramides (35Mizutani Y. Kihara A. Igarashi Y. Biochem. J. 2006; (in press)PubMed Google Scholar), although the surprisingly high levels of C18-ceramide synthesis are at variance with other analyses. 5I. Pankova-Kholmyansky, S. Epstein, E. Wang, J. C. Allegood, S. Kelly, A. H. Merrill, Jr., and A. H. Futerman, unpublished observations. Verification that mammalian LASS proteins are bona fide ceramide synthases, rather than regulators of endogenous ceramide synthases, was obtained when purified LASS5 was shown to possess CerS activity (36Lahiri S. Futerman A.H. J. Biol. 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Chem. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar). at it is to the role of ceramides in defined and the of the Lass genes and the possibility to experimentally their will of the roles of specific ceramide species. might ceramides containing different fatty acids upon cell or more of the membrane be by the fatty acid composition of ceramide J. A. J. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). specific ceramides with in signaling In both of the activity of the Lass genes by transcriptional or be an important and novel of the fatty acid composition of is known the levels of specific ceramides and the activity of Lass genes in different Early of CerS M. E. Y. Y. J. Lipid Res. 1998; Full Text Full Text PDF PubMed Google Scholar, R. M. A. K. R. Cell. Full Text PDF PubMed Scopus Google Scholar), and the restricted of Lass genes (32Riebeling C. Allegood J.C. Wang E. Merrill Jr., A.H. Futerman A.H. J. Biol. Chem. 2003; 278: 43452-43459Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar, J. J. Lipid Res. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar) is consistent with multiple levels of of CerS. the of is expressed at high levels in (32Riebeling C. Allegood J.C. Wang E. Merrill Jr., A.H. Futerman A.H. J. Biol. Chem. 2003; 278: 43452-43459Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar), which long acyl chain ceramides that are involved in the function of L. de la A. J. Clin. 2003; PubMed Scopus Google Scholar). The of a family of genes involved in the fatty acid composition of ceramides is to to a new of understanding of the of ceramide Y.A. Obeid L.M. J. Biol. Chem. 2002; 277: 25847-25850Abstract Full Text Full Text PDF PubMed Scopus (770) Google Scholar, A.H. Hannun Y.A. EMBO 2004; 5: PubMed Scopus Google Scholar). the function and of of the Lass genes is the role of the TLC domain and are there many TLC domain-containing proteins (Fig. with only some to the to be as genuine ceramide are the of the other family they act as regulators of the CerS proteins or they have are the of the splice (Fig. and might they act in a to the activity of Lass when CerS (Fig. are the regions in the LASS proteins that ceramide synthase are the regions for and sphingoid base and how are these regions conserved or how they the different LASS of the to these and other all the evidence implies that the key of Lass genes is their ceramide synthase activity rather than their role in This the we the six Lass genes as (Fig. to more their known