Caenorhabditis elegans mboa-7, a Member of the MBOAT Family, Is Required for Selective Incorporation of Polyunsaturated Fatty Acids into Phosphatidylinositol

Abstract

Phosphatidylinositol (PI) is a component of membrane phospholipids, and it functions both as a signaling molecule and as a compartment-specific localization signal in the form of polyphosphoinositides. Arachidonic acid (AA) is the predominant fatty acid in the sn-2 position of PI in mammals. LysoPI acyltransferase (LPIAT) is thought to catalyze formation of AA-containing PI; however, the gene that encodes this enzyme has not yet been identified. In this study, we established a screening system to identify genes required for use of exogenous polyunsaturated fatty acids (PUFAs) in Caenorhabditis elegans. In C. elegans, eicosapentaenoic acid (EPA) instead of AA is the predominant fatty acid in PI. We showed that an uncharacterized gene, which we named mboa-7, is required for incorporation of PUFAs into PI. Incorporation of exogenous PUFA into PI of the living worms and LPIAT activity in the microsomes were greatly reduced in mboa-7 mutants. Furthermore, the membrane fractions of transgenic worms expressing recombinant MBOA-7 and its human homologue exhibited remarkably increased LPIAT activity. mboa-7 encodes a member of the membrane-bound O-acyltransferase family, suggesting that mboa-7 is LPIAT. Finally, mboa-7 mutants had significantly lower EPA levels in PI, and they exhibited larval arrest and egg-laying defects. INTRODUCTIONVarious kinds of fatty acids are distributed in membrane phospholipids in mammalian cells and tissues (Lands and Crawford, 1976;Holub and Kuksis, 1978;MacDonald and Sprecher, 1991). The fatty acyl residues of individual phospholipids seem to be under strict metabolic regulation. In general, saturated fatty acids are esterified at the sn-1 position, whereas polyunsaturated fatty acids (PUFAs), such as arachidonic acid (AA), are commonly found at the sn-2 position. Three-fourths or more of the phosphatidylinositol (PI) fraction in rat liver and brain constitutes the 1-stearoyl-2-arachidonoyl species (Holub and Kuksis, 1971;Baker and Thompson, 1972). In contrast, the total pool of precursor phosphatidic acid (PA) in rat liver and brain has a fatty acid composition that does not resemble that of PI, showing a low AA content (Possmayer et al., 1969;Akesson et al., 1970;Baker and Thompson, 1972). Selectivity could be expressed during de novo synthesis at the level of formation of cytidine 5Ј-diphosphate (CDP)-diacylglycerol from PA and cytidine 5Ј-triphosphate or in the use of CDP-diacylglycerol in the final reaction. In fact, CDP-diacylglycerol synthase, which prefers 1-stearoyl-2-arachidonoyl PA as a substrate in vitro, has been cloned, although expression is restricted to testis, retina, and brain (Saito et al., 1997).An alternative mechanism for species selection has been proposed on the basis of turnover studies in rat brain in vivo (Baker and Thompson, 1972). [ 3 H]AA and [ 14 C]glycerol injected intracerebrally were incorporated almost exclusively into brain phospholipids. Comparison of PI and PA radioactivity suggest that the initial flux of AA into PI was independent of de novo synthesi...

Signaling Role for Lysophosphatidylcholine Acyltransferase 3 in Receptor-Regulated Arachidonic Acid Reacylation Reactions in Human Monocytes

“…The absence of an increased activity toward phosphatidic acid (PA) was not unexpected, given our data showing that zymosan-induced PL AA incorporation does not proceed via the de novo biosynthetic pathway. However, our inability to detect increased acyltransferase activity using either ethanolamine lysoglycerophospholipid or lysoPI is difficult to explain in view of our own data showing that AA incorporates not only into PC but also into ethanolamine glycerophospholipid and PI during zymosan stimulation of the cells and that acyl transferase enzymes showing clear preference for ethanolamine lysoglycerophospholipid and lysoPI have been described previously (48,64,65). It is possible that a significant portion of the AA reincorporated into these PLs in activated cells, particularly ethanolamine glycerophospholipids, enters indirectly via transacylation reactions using AA-containing PC as an AA donor.…”

The microsomal cardiolipin remodeling enzyme acyl-CoA lysocardiolipin acyltransferase is an acyltransferase of multiple anionic lysophospholipids

“…Significant reduction in LPIAT activity was also observed when ALCAT1 expression was reduced by either RNAi oligo. We also used two different RNAi oligos to knock down the human ortholog of Caenorhabditis elegans LPIAT1 (also named MBOAT 7 or Leng4) (19) and these two oligos managed to reduce LPIAT1 expression by 80% and 70%, respectively. Accordingly, LPIAT activity was reduced by 60% and 40%, respectively, by these two oligos (Fig.…”

“…The FA at the sn-2 position in PLs is often remodeled through the Lands cycle, i.e., deacylation through a member of the phospholipase A 2 family and reacylation through lysophospholipid (lysoPL) acyltransferases (lysoPLATs) (8)(9)(10). In recent years, two families of lysoPLATs have been discovered owing primarily to genomic efforts (11)(12)(13)(14)(15)(16)(17)(18)(19). These enzymes exhibit two distinct features.…”

Recent progress on acyl CoA: lysophospholipid acyltransferase research

“…Lyso-PI acyltransferase (LPIAT)1 (also called MBOA-7, MBOAT7, and LRC4) is the first PI synthetic enzyme in the remodeling pathway to be identified (33) (Fig. 2).…”