[Hormone-like action of blood plasma lipoproteins on human platelets and smooth vascular muscle cells].

It has been found that blood lipoproteins are capable of inducing rapid and reversible elevations of [Ca2+]i in human blood platelets and vascular smooth muscle cells (VSMC) loaded with Ca(2+)-sensitive fluorescent probes. The effects of LDL and HDL3 were dose-dependent and reached saturation at physiological concentrations of these lipoproteins. Both lipoproteins activated the phosphoinositide turnover, producing elevated levels of diacylglycerol and inositol mono-, bis- and triphosphates. Analysis of isomers of inositol phosphates in lipoprotein-treated VSMC by anion-exchange HPLC supported the view that LDL and HDL3 activate polyphosphoinositide-specific phospholipase C. These data demonstrate that lipoproteins, similarly to aggregation inducers and vasoactive hormones, stimulate second messenger systems in platelets and VSMC. It was shown that pretreatment of cells with protein kinase C activators, cAMP- and cGMP-dependent protein kinases, significantly decreased the hormone-like effects of the lipoproteins. Preincubation of VSMC with pertussis toxin also attenuated the effects of LDL and HDL3. In contrast, adrenaline potentiated the 2-3-fold LDL-induced elevation of [Ca2+]i in platelets. Considering that increase in the intracellular cAMP and cGMP and the activation of protein kinase C are known to inhibit the effects of Ca(2+)-mobilizing hormones, the results obtained demonstrate a similarity between the mechanisms of activation of cell-signalling systems by hormones and lipoproteins, suggesting also that lipoprotein-induced activation may be transduced by G-proteins.