The role of the de novo synthetic pathway in forming molecular species of phospholipids in resting lymphocytes from human tonsils

Correlation between low-temperature immunosuppression and the absence of unsaturated fatty acid synthesis in murine T cells

1. Studies were performed to determine if the inability of murine T cells to provide primary helper function at low temperature (27 degrees C) could be correlated with their inability to synthesize unsaturated fatty acids (UFAs). 2. In the absence of exogenous oleic acid (18:1), splenocytes responded to a T-dependent (TD) Ag (trinitrophenyl-keyhole limpet hemocyanin, TNP-KLH) at 37 degrees C but not at 27 degrees C. The addition of 150 microM 18:1 almost completely restored plaque-forming cell (PFC) responses to TNP-KLH at 27 degrees C but markedly suppressed PFC responses to the TD Ag at 37 degrees C. 3. During incubation at 27 degrees C, B cells converted 3- to 5-fold more stearic acid (18:0) to 18:1 and showed a greater accumulation of monounsaturated phospholipid molecular species than did T cells. 4. Following incubation in the presence of a rescuing dose of 18:1 (150 microM), both B and T cells accumulated large amounts of dioleoyl PC. 5. It is proposed that the absence of 18:1 synthesis in T cells is responsible for the unique low temperature susceptibility of this lymphocyte population.

Absence of unsaturated fatty acid synthesis in murine T lymphocytes

Stearic acid is toxic for T lymphocytes in vitro but has little effect on B lymphocytes. To investigate the molecular basis for this difference, purified murine T and B lymphocytes were compared for their abilities to incorporate and metabolize stearic acid. Unstimulated T and B cells incorporated identical amounts of stearic acid into six different phospholipids and four neutral lipids. After mitogen stimulation, fatty acid uptake was increased in both lymphocyte types, but cell-specific differences were seen in the distribution of stearic acid among the various cellular lipids. Doses of stearic acid that selectively inhibited T-cell proliferation resulted in a 5-fold greater accumulation of distearoylphosphatidylcholine in T cells than in B cells. Whereas T cells did not desaturate the exogenously derived stearic acid, up to 25% of the saturated fatty acid was converted to oleic acid in B cells. These findings suggested a deficiency of stearoyl-CoA desaturase (acyl-CoA, hydrogen-donor:oxygen oxidoreductase, EC 1.14.99.5) activity in T cells, which was confirmed by subsequent studies. Cell-free extracts from B cells displayed nearly 20-fold more stearoyl-CoA desaturase activity than T-cell extracts, and the level of stearoyl-CoA desaturase mRNA was 30-fold higher in B cells. Collectively, our data indicate that murine T cells are deficient in unsaturated fatty acid synthesis. The deficiency of stearoyl-CoA desaturase in T cells may represent the basis for the differing sensitivities of T and B lymphocytes to inhibition by saturated fatty acids.

Formation of diacylglycerol and degradation of phosphatidylinositol induced in rat lymphocytes by non-esterified oleic or linoleic acid

Rat spleen lymphocytes were incubated for 3 h with [14C]arachidonic acid in foetal calf serum. It was found that arachidonic acid distributed into phospholipids in the order phosphatidylcholine greater than phosphatidylethanolamine greater than phosphatidylinositol. After labelling with arachidonic acid the lymphocytes were washed, and incubated for up to 2 h with non-radioactive palmitic, oleic or linoleic acid dissolved in ethanol. The presence of ethanol or palmitic acid during a 2 h post-incubation had little effect on the amount of radioactivity found in different lipid fractions. Both oleic acid and linoleic acid, however, brought about an accumulation (up to 8-fold) of radioactivity in the diacylglycerol fraction. These fatty acids also brought about a change of radioactivity in several phospholipids, notably in phosphatidylinositol, which lost more than 50% of its counts during the 2 h incubation. Although maximum effects were seen at 2 h, diacylglycerol radioactivity was increased by 100% within 5 min after adding the fatty acids. The minimum concentration of fatty acids used (50 microM) gave an almost maximum response. The results indicate that unsaturated fatty acids may activate phosphatidylinositol phosphodiesterase in lymphocytes, as they do in brain. The possibility that a phospholipase A is activated is discussed. Possible implications for any experiments in which cells are incubated with fatty acids are pointed out.

Characterization of biochemical properties of melanosomes for structural and functional differentiation: analysis of the compositions of lipids and proteins in melanosomes and their subfractions

Two types of melanosomes were isolated from B16 and Harding Passey (HP) mouse melanomas to elucidate whether there are any features of melanosomal lipids and proteins related to the differences in morphology of melanosomes and in the biological activities of melanoma cells. Biochemical analyses were made to clarify (a) the lipid components of the 2 melanosomes and (b) the lipid and polypeptide compositions of their subfractions, i.e., the outer surface and inner core, resolved by a detergent, Brij-35, separately. We found (a) that the lipid contents in B16 melanosomes were much higher than those in HP, (b) that the B16 and HP melanosomes could be fractionated into a phospholipid-rich outer surface and phospholipid-poor core, (c) that both outer surface and core subfractions of HP were distinct from the corresponding subfractions of B16 with respect to phospholipid contents, (d) that the outer surface of B16 and HP revealed a polypeptide composition similar to each other, although the protein contents of the outer surface were much lower than those of core and (e) that the total melanosomes showed a marked difference in polypeptides between B16 and HP. In addition, both B16 and HP melanosomes revealed the alteration of lipid compositions, e.g., fatty acid acyl chain, similar to that observed during malignant transformation. There was not, however, any significant difference in acyl compositions between B16 and HP. It is likely that melanosomal lipid and protein not only affect the different morphogenesis of melanosomes but also reflect the different biological activities of whole cells, and that lipids, primarily located in the outer surface, regulate the functional aspects of melanosomes.