Polyunsaturated fatty acid metabolism in a cell culture model of essential fatty acid deficiency in a freshwater fish, carp (Cyprinus carpio)

Proliferation of an essential fatty acid deficient cell line from carp (EPC-EFAD; epithelioma papillosum carp-essential fatty acid deficient) is stimulated by supplementing the cells with C₂₀, but not C₁₈ polyunsaturated fatty acids (PUFA). It is hypothesized that the differential ability of the PUFA to stimulate proliferation of the EPC-EFAD cells may be related to the extent of the cells' ability to desaturate and elongate C₁₈ PUFA. In the present study, the metabolism of ¹⁴C-labeled C₁₈ and C₂₀ PUFA was investigated in EPC-EFAD cells in comparison with normal EPC cells. The incorporation of all the PUFA was significantly greater in EPC-EFAD cells but the rank order, 20:5n-3 > 18:3n-3 = 18:2n-6 >20:4n-6 was the same in both cell lines. The proportion of radioactivity from all labeled PUFA recovered in phosphatidylethanolamine and total polar lipids was significantly lower in EPC-EFAD cells compared to EPC cells, whereas the proportion of radioactivity recovered in all the other phospholipid classes and total neutral lipid was greater in EPC-EFAD cells. Both cell lines desaturated1-¹⁴C]18:3n-3 and 1-¹⁴C]20:5n-3 to a greater extent than the corresponding (n-6) substrates but the desaturation of all the ¹⁴ C-labeled PUFA was significantly greater in EPC-EFAD cells compared to EPC cells. The results showed that, although essential fatty acid deficiency had several significant effects on PUFA metabolism in EPC cells, the fatty acid desaturation/elongation pathway was not impaired in EPC-EFAD cells and so they can desaturate 18:3n-3 to 20:5n-3 and 22:6n-3, and 18:2n-6 to 20:4n-6. However, 20:4n-3 and 20:3n-6, and not 20:4n-6 and 20:5n-3, were the predominant C₂₀ PUFA produced by the elongation and desaturation of 1-¹⁴C]18:3n-3 and 1-¹⁴C]18:2n-6, respectively. Therefore, the previously reported inability of 18:3n-3 and 18:2n-6, compared to 20:5n-3 and 20:4n-6, to stimulate proliferation of the cells is apparently not due to a general deficiency in the fatty acid desaturation pathway in EPC-EFAD cells but may be related to potential differences in eicosanoid profiles in cells supplemented with C₁₈ PUFA compared to C₂₀ PUFA.