Stimulatory Action of Polyunsaturated Fatty Acids on Pyruvate Oxidation
In a previous report, we have shown that short-term exposure of cultured infant skin fibroblasts (ISF) to various radioactive long-chain fatty acids gave different rates of labeled acetyl-CoA incorporation into cholesterol. The present work was extended to infant liver cells (ILC). We also examined whether nonradioactive unsaturated and polyunsaturated fatty acids (PUFAs) could individually influence the level of acetyl-CoA and/or HMG-CoA, substrates for cholesterogenesis: any modification in the flow of acetyl-CoA, a product of both ß-oxidation and pyruvate dehydrogenase (PDH) activity. would affect HMG-CoA level, HMG-CoA reductase (HMGR) activity, and then cholesterol synthesis.
The lipoprotein-deprived medium currently used to stimulate HMGR activity in cultured cells 24-36 hr before determination also provoked a moderate PUFA deficiency, which was progressively cancelled by addition of linoleic (LI) or arachidonic (AR) acid to the medium. During the 6-hr period of rehabilitation, the level of acetyl-CoA from LI and AR ß-oxidation was rather low and without effect on PDH deactivation compared to cells incubated with oleic acid (OL). In fact, adding 0.25 mM PUFA to the lipoprotein-poor medium proved more effective in increasing PDH activity than incubation of control cells with 5 mM dichloroacetate, an inhibitor of PDH kinase.
Addition of LI, and especially of AR, had a stimulatory effect on PDH and HMGR activity. The PDH activity was more responsive to the increase in PUFA concentration in ISF (up to 47% stimulation) than in ILC (10%). The HMGR activity was very sensitive to LI and AR concentration, and the increase in activity reached 39% and 70%, respectively. In contrast, the addition of 0.25 mM OL had inhibitory effect on PDH and HMGR activity (7–19% and 30–35% lower than controls, respectively). These differences in PDH and HMGR activity between the groups had repercussion on cholesterol specific radioactivity, determined after cell incubation with labeled fatty acids. When cells were kept in the medium supplemented with fetal bovine serum and not transferred to lipoprotein-deprived medium, addition of OL again provoked reduction of PDH activity (20% lower than controls), whereas adding LI or AR had only a slight (if any) inhibitory effect (7–13%).
Therefore, the slight inhibitory effect of the added PUFAs on pyruvate (and then on glucose) oxidation—contrasting with the significant inhibitory effect of OL, considered a risk factor of cardiovascular disease—suggests one of the mechanisms of their beneficial function. This interpretation also takes into account the possible impact on cholesterol turnover.
KeywordsPyruvate Dehydrogenase Cellular Lipid Cellular Cholesterol Label Fatty Acid Pyruvate Oxidation
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