Downregulation of protein kinase C by phorbol ester increases expression of epidermal growth factor receptors in transformed trophoblasts and amplifies human chorionic gonadotropin production

Epidermal growth factor (EGF) and its homologue, transforming growth factor-α (TGF-α), regulate human chorionic gonadotropin (hCG) synthesis in the human placenta. The current study was designed to investigate the involvement of the protein kinase C pathway in EGF-mediated hCG-β production by JAr choriocarcinoma cells. Downregulation of protein kinase C activity by chronic exposure to the phorbol ester, phorbol 12,13-dibutyrate (PDB), produced a greater increase in hCG-β secretion than did activation of protein kinase C activity by short-term exposure to PDB. Pretreatment with the protein kinase C inhibitors calphostin and chelerythrine also resulted in enhanced basal and EGF-stimulated hCG-β production. Individual concentrations (5 nM EGF and 500 nM PDB) that maximally stimulated hCG production, were additive in combination. The additive effect of PDB on EGF-induced hCG-β secretion was mediated in part by increased JAr cell EGF-receptor concentrations detected by Western blot and Scatchard analyses. The results suggest that EGF and PDB stimulate hCG production in JAr cells by different but interactive mechanisms. It is speculated that downregulation of protein kinase C stimulates basal and EGF-mediated hCG-β production by uninhibiting other signalling pathways that regulate hCG-β secretion in trophoblasts.