INFLUENCE OF LIPOPROTEINS ON INTRACELLULA R SIGNAL-TRANSDUCTION IN HUMAN FIBROBLASTS

Lipoproteins induce signal transduction in human fibroblasts. Stimulat ion with HDL3 or LDL result in a rapid activation of phosphatidylinosi tol specific phospholipase C. This enzyme catalyses the degradation of phosphatidylinositol-diphosphate (PIP2) to inositoltriphosphate (IP3) and 1,2 diacylglycerol (1,2-DAG). Generation of IP3 leads to the rele ase of calcium from intracellular stores and therefore causes a rise i n cellular calcium concentration. The maximal effect was seen with con centrations around 15 mug/ml HDL3 or LDL. The maximum increase of [Ca+]i was 480 nM after stimulation with LDL and 400 nM with HDL3. 1,2-DA G as well as calcium are potent activators of protein kinase C. Accord ingly, after stimulation with both lipoproteins we observed a transloc ation of PKC(alpha) and PKC(epsilon) from the cytosol to the cell memb rane, which reflect the activation of these kinases. In addition to th e PLC pathway, both lipoproteins induce the adenylate cyclase signalli ng pathway and cause an increase in intracellular cAMP concentration. 10 s after stimulation cAMP concentration increased from 1 pmol/mg cel l protein to 10 pmol. The concentration returned to basal levels withi n 30 s after stimulation with LDL and 120 s after incubation with HDL3 . A rise in cGMP concentration was only seen after stimulation with LD L and not with HDL3. In summary the investigation of lipoprotein induc ed signal transduction indicated potential mechanisms, by which HDL3 a nd LDL might mediate their influence on various cell functions.