Phosphorylation of Elovl5 changes its substrate preference to synthesize Mead acid in response to essential fatty acid deficiency
Polyunsaturated fatty acids (PUFAs) of the n-6 and n-3 series cannot be synthesized in mammals and therefore are called essential fatty acids (EFAs). Mead acid (20:3n-9) is an unusual n-9 PUFA, endogenously synthesized from oleic acid (18:1n-9) in an EFA-deficient state. Although Elovl5, a fatty acid elongase, has long been known to selectively elongate C18 and C20 PUFAs, it can use 18:1n-9 as a substrate for the synthesis of Mead acid under C20 PUFA-deficient, but not-sufficient, conditions. We found, by an in vitro enzyme assay, that the microsomal fraction obtained from PUFA-deficient, but not -sufficient, cells showed significant Elovl5 activity toward 18:1n-9, with no effect on its constitutive activity toward 18:3n-6, implying that Elovl5 acquires the activity toward 18:1n-9 under the PUFA-deficient conditions at the enzyme level. Further biochemical analysis revealed that Elovl5 was phosphorylated in the C20 PUFA-supplemented cells, and that treatment with an inhibitor of glycogen synthase kinase 3 (GSK3) completely abolished the phosphorylation of Elovl5 and retained the Elovl5 activity toward 18:1n-9, even in the presence of C20 PUFA. Finally, mutation of putative phosphorylation sites (T281A/S283A/S285A) on Elovl5 did not decrease the activity of Elovl5 toward 18:1n-9 by supplementation with C20 PUFA, suggesting that the phosphorylation of Elovl5 contributed to a change in substrate preference. Thus, by changing its substrate specificity in an EFA-deficient state, Elovl5 is able to regulate the synthesis of Mead acid to maintain levels of long-chain PUFAs.