Protein kinase C in cultured human placental trophoblasts: identification of isoforms and role in cAMP signalling.

Abstract

The protein kinase C (PKC) superfamily is a family of serine/threonine kinases involved in the regulation of many cell functions. The objective of the present work was to identify the PKC isoenzymes present in human placental trophoblasts and to compare their relative responses to acute and chronic phorbol 12-myristate 13-acetate (PMA) treatment. In addition, the effect of PMA treatment on ligand-stimulated cAMP production was determined. In total extracts prepared from cultured or freshly purified trophoblasts, PKC isoforms alpha, epsilon and zeta were detected. Following acute treatment with PMA, PKC alpha and PKC epsilon were translocated from the cytosol to the particulate fraction. Prolonged treatment (up to 36 h) with PMA resulted in the temporal down-regulation of PKC alpha and PKC epsilon. PKC zeta did not respond to either acute or chronic treatment with PMA. An acute 10 min treatment of the cells with PMA (10(-10)-10(-6) M) enhanced isoprenaline- and adrenaline-stimulated cAMP production. An enhanced response was observed at all concentrations of isoprenaline tested (10(-9)-10(-4) M), suggesting an increased capacity to respond. The acute PMA effect was evident within 2 min and near maximal by 5 min. The acute response to PMA was lost in cells where PKC was down-regulated by prior PMA treatment. Epidermal growth factor, a potential ligand for the activation of PKC in trophoblasts, enhanced isoprenaline-stimulated cAMP production. In summary, activation of PMA-responsive PKCs (PKC alpha or PKC epsilon) appears to enhance ligand-stimulated cAMP production in trophoblasts. This may be a physiologically important example of 'cross-talk' between various signalling pathways in human placental trophoblasts.