The fatty acid desaturase 2 (FADS2) gene product catalyzes Δ4 desaturation to yield n-3 docosahexaenoic acid and n-6 docosapentaenoic acid in human cells

Abstract

Docosahexaenoic acid (DHA) is a Δ4-desaturated C22 fatty acid and the limiting highly unsaturated fatty acid (HUFA) in neural tissue. The biosynthesis of Δ4-desaturated docosanoid fatty acids 22:6n-3 and 22:5n-6 are believed to proceed via a circuitous biochemical pathway requiring repeated use of a fatty acid desaturase 2 (FADS2) protein to perform Δ6 desaturation on C24 fatty acids in the endoplasmic reticulum followed by 1 round of β-oxidation in the peroxisomes. We demonstrate here that the FADS2 gene product can directly Δ4-desaturate 22:5n-3→22:6n-3 (DHA) and 22:4n-6→22:5n-6. Human MCF-7 cells lacking functional FADS2-mediated Δ6-desaturase were stably transformed with FADS2, FADS1, or empty vector. When incubated with 22:5n-3 or 22:4n-6, FADS2 stable cells produce 22:6n-3 or 22:5n-6, respectively. Similarly, FADS2 stable cells when incubated with d5-18:3n-3 show synthesis of d5-22:6n-3 with no labeling of 24:5n-3 or 24:6n-3 at 24 h. Further, both C24 fatty acids are shown to be products of the respective C22 fatty acids via elongation. Our results demonstrate that the FADS2 classical transcript mediates direct Δ4 desaturation to yield 22:6n-3 and 22:5n-6 in human cells, as has been widely shown previously for desaturation by fish and many other organisms.

Desaturase and elongase-limiting endogenous long-chain polyunsaturated fatty acid biosynthesis

“…The final steps were long thought to be by direct Δ4-desaturation via 22:5n-3→22:6n-3. Biochemical data in rat liver developed in the 1990s established an alternative coupled microsomal-peroxisomal pathway via 22:5n-3→24:5n-3→24:6n-3→22:6n-3, where the last step is one round of β-oxidation in the peroxisomes [3]. Molecular studies since 2001 established that Δ4-desaturase (FADS2) is the final step in marine microorganisms (e.g.…”

“…Molecular studies since 2001 established that Δ4-desaturase (FADS2) is the final step in marine microorganisms (e.g. Thraustochytrium), marine teleost fish, and mammals, and very recently in humans [3]. …”

“…In humans, membrane-bound PUFA desaturases known as “front-end” desaturases introduce a nascent double bond between an existing double bond usually located between the carboxyl group and the 9 th carbon atoms from the terminal methyl (n-9) [5]. Front-end desaturation proceeds at the Δ4, Δ5, Δ6 and Δ8 positions and is responsible for endogenous biosynthesis of LCPUFA [3, 6, 7]. …”

“…In addition to humans, Fads2 has been cloned from mouse, rat and Caenorhabditis elegans [10]. It introduces a double bond at the Δ6 position (between carbons 6 and 7) by acting on at least six substrates (18:2n-6 → 18:3n-6, 18:3n-3 → 18:4n-3, 24:5n-3 → 24:6n-3, 24:4n-6 → 24:5n-6, 16:0 →16:1n-10, and 18:1n-9 →18:2n-9), Δ8-desaturation by acting on 20:1n-9 → 20:2n-9, 20:2n-6 → 20:3n-6, and 20:3n-3 → 20:4n-3, and Δ4-desaturation by acting on 22:4n-6 → 22:5n-6 and 22:5n-3 → 22:6n-3 [3, 10, 11]. The Δ8-desaturase activity provides an alternative pathway to LCPUFA biosynthesis, possibly available when Δ6-desaturase activity is compromised.…”

Both maternal and offspring Elovl2 genotypes determine systemic DHA levels in perinatal mice

“…Recently, Park et al (54) reported that Fads2 transcript in human cells can catalyze 4-desaturation and be actively involved in direct synthesis of DHA from 22:5n-3. While we did not detect any effects on the DHA level in Elovl2 / pups fed by Elovl2 / mothers, the maternal genotype appeared to have a significant impact on the expression levels of the desaturases, but not on Elovl5.…”