Characterisation of polyunsaturated fatty acid synthesis in peripheral blood mononuclear cells.

Conversion of the essential n-3 (18:3n-3) and n-6 (18:2n-6) fatty acids to longer chain polyunsaturated fatty acids (PUFA) involves sequential desaturation and elongation reactions. Previous studies have reported gender differences in n-3 PUFA synthesis, whereas the effect of age is less clear. n-3 PUFAs are reported to have important effects on immune cell function. A previous study reported long chain PUFA synthesis in mitogen stimulated but not quiescent peripheral blood mononuclear cells (PBMCs). However, the underlying mechanism is not known. PUFA synthesis was investigated in PBMCs incubated with [1-13C]18:3n-3 for 48 h. Activation with the T-lymphocyte mitogen concanavalin A (Con A) increased PUFA synthesis. 22:6n-3 synthesis was not detected. [1-13C] incorporation was greatest for 20:3n-3 suggesting initial chain elongation is an important fate for 18:3n-3. Con A increased expression of three key genes (FADS2, FADS1 and ELOVL5) involved in PUFA synthesis, suggesting upregulation of the pathway is controlled at the transcriptional level. ELOVL2 expression was negligible, possibly explaining the lack of 22:6n-3 synthesis. Con A increased methylation of 12 CpGs in the FADS2 promoter contradicting the general view that DNA methylation represses transcription. Subsequent 5’RACE analysis verified that activated PBMCs were not using an alternative promoter for FADS2 transcription. Contrary to expectation, 18:3n-3 conversion in activated PBMCs was not affected by gender or menopausal status and there was no clear age effect. PUFA synthesis was constitutive in the Jurkat T-lymphocyte leukaemic cell-line and was higher than in PBMCs. FADS2, FADS1 and ELOVL5 mRNA expression was also higher in Jurkat cells and was associated with 50% lower methylation of 17 CpGs in the FADS2 promoter, suggesting transcriptional dysregulation of PUFA synthesis in Jurkat cells involves altered DNA methylation. These findings have provided novel insights into the regulation of PUFA biosynthesis in PBMCs and upregulation of the pathway in activated PBMCs suggests that newly synthesised PUFAs may be important for cell function