PubMed1995
It is largely admitted nowadays that the early stage of the atherosclerotic lesion involves formation of oxidized (and minimally oxidized) low-density lipoprotein. Their properties are briefly reviewed. It is recalled that a lipolytic process also takes place both at the lumenal surface and in the subendothelial space of the vessels implying lipoprotein lipase (LpL) activity. Recent studies emphasize the role of LpL in accumulating LDL in the vascular tissue (Rutledge & Golberg, J. Lipid Res., 1994, 35, 1152-1160), but the role of LpL-generated unesterified fatty acids (UEFA) in these two locations and their possible implication in atherogenesis are largely neglected. Physiological and pathophysiological significance of UEFA in the human adherent monocyte modulation of the superoxide anion (O2.-) production has been examined by our group, leading to a possible mechanism of modulation of LDL oxidative modification. The O2.- production-modulating effect of a 30-min UEFA preincubation has been studied in intact human adherent monocytes (HAM) after stimulation by a direct effector of protein kinase C (PKC). It has been established that UEFA alone (in the absence of PKC effectors) were not able to modulate the O2.- production of HAM whereas they had such a capacity in the presence of PKC effectors, phorbol myristate acetate (PMA) or diacylglycerol (DAG). In this case inhibitors of PKC such as GF 109203 X suppressed the modulating effect. UEFA have also been shown to possess a bimodal action in the presence of PKC effectors: they depressed or enhanced O2.- production at micromolar or nanomolar concentrations, respectively. All these results contrasted with others obtained in neutrophils or nonadherent monocytes, suggesting an absolute requirement of PKC for the phagocyte-NADPH oxydase (PHOX) activation especially in the case of HAM. In HAM, the maximal enhancing effects were obtained with monomethyl ramified saturated (MMRS) and linear unsaturated (LU) FAs such as arachidonic, eicosapentaenoic and docosahexaenoic acids (with exception of oleic, linoleic and linolenic acids which were without effect), whereas the maximal depressing effects were obtained with MMRS-FAs and LU-FAS such as oleic, linoleic and docosahexaenoic acids. Further investigations in HAM led us to examine the UEFA capacity at modulating the translocation of PKC, on the one hand, and the endogenous phosphorylation and membrane translocation of p47phox, on the other, in the presence of PMA or DAG. Using 13-methyl myristic (iso15:0) as FA model, it has been established that i) it was able to amplify or diminish PKC translocation at nanomolar and micromolar concentrations, respectively (this was also the case with arachidonic acid) ii) it enhanced and depressed the endogenous phosphorylation and the membrane translocation of p47phox at nanomolar/micromolar concentrations and iii) it was inactive in the absence of PMA or DAG. Taken together, our results strongly suggest that the active UEFA act directly on the monocyte PKC, modifying its kinase activity through interactions with PMA/DAG binding site of the regulatory domain of the protein. This leads to modulate the phosphorylation and translocation of p47phox, which in turn allows the assembling of the active PHOX complex and triggers the O2.- production. The direct action of UEFA on the PKC regulatory-domain known to strongly interact with the membrane lipids was also supported by the fact that linear saturated FAs that have already been reported to be unable to penetrate a lipid layer were devoided of effect on monocytic O2.- production. The free form of oleic and linoleic acids and, to a lesser extent, docosahexaenoic acid (in the case of oral administration of fish oil) are present at micromolar concentrations in the plasma.(ABSTRACT TRUNCATED AT 400 WORDS)