High density lipoproteins stimulate molecular weight 80K protein phosphorylation in human trophoblast cells: evidence for a protein kinase-C-dependent pathway in human placental lactogen release.

Previous studies from our laboratory have demonstrated that high density lipoprotein (HDL) HDL and apolipoprotein-AI (apoAI) stimulate human placental lactogen (hPL) release from human trophoblast cells. To determine whether protein kinase-C (PKC) activation is involved in the mechanism of HDL- and apoAI-mediated hPL release, we examined the effects of these factors on the phosphorylation of cytosolic proteins known to be phosphorylated in response to PKC activation by phorbol myristate acetate (PMA). HDL and apoAI each caused a dose- and time-dependent increase in phosphorylation of a PMA-inducible 80K mol wt acidic cytosolic protein in a manner similar to that observed in many other cell types. Stimulation of 80K protein phosphorylation was apparent 5 min after the addition of HDL, apoAI, or PMA and was maximal at 15 min. Maximal 80K protein phosphorylation in cells exposed to PMA (1.6 microM), HDL (1500 micrograms/ml), and apoAI (600 micrograms/ml) was 284%, 206%, and 239% that in untreated cells, respectively. The increase in both 80K protein phosphorylation and hPL release in response to apoAI was prevented by pretreatment of the cells with the PKC inhibitor staurosporine (10 microM) or by down-regulation of PKC after extended preincubation of the cells with 16 microM PMA. (Bu)2cAMP and the adenylate cyclase activator forskolin, which stimulate hPL release, had no effect on 80K protein phosphorylation. These results strongly suggest that HDL- and apoAI-stimulated hPL release involves a PKC-dependent pathway. Since earlier studies also implicate a cAMP-mediated pathway in the stimulation of hPL release by these agents, it appears that multiple intracellular pathways are involved in the stimulation of hPL release.