Investigation of the role of protein kinase C (PKC) in cytostasis induced by tumour promoting phorbol esters and related compounds

Abstract

Protein kinase C (PKC) is considered to be the major receptor for tumour promoting phorbol esters such as 12-0- tetradecanoylphorbol-13-acetate (TPA). These agents evoke a plethora of biological effects on cells in culture. The growth of A549 human lung carcinoma cells maintained in medium fortified with 10% foetal calf serum (FCS) is arrested for 6 days by TPA and other biologically active phorbol esters. In the work described in this thesis, the hypothesis was tested that modulation of PKC activity is closely related to events pivotal for cytostasis to occur. The effect of several phorbol esters, of newly synthesized analogues of diacylglycerols (DAG) and of bryostatins (bryos) on cell growth and ability to modulate activity of PKC has been investigated.Determination of the subcellular distribution of PKC following treatment of cells with TPA and partial enzyme purification by non-denaturing poly-acrylamide gel electrophoresis revealed translocation of enzyme activity from cytosoUc to paniculate fraction. Chronic exposure of cells to TPA resulted in a time and concentration dependent degradation of enzyme activity. Synthetic DAG and DAG analogues, unable to arrest the growth of cells at non-toxic concentrations, were neither able to affect subcellular PKC distribution nor compete effectively for phorbol ester binding sites at physiologically relevant concentrations. Bryos 1,2,4 and 5, natural products, possessing antineoplastic activity in mice, elicited transient arrest of A549 cell growth in vitro. They successfully competed for phorbol ester receptors in A549 cells with exquisite affinity and induced a shift in sub-cellular PKC distribution, though not to the same extent as PTA. Enzyme down-regulation resulted from prolonged exposure of cells to nanomolar concentrations of bryos. In vivo studies demonstrated that neither PDBu nor bryo 1 was able to inhibit A549 xenograft growth in athymic mice. The growth of A549 cell populations cultured under conditions of serum-deprivation was inhibited only transiently by biologically active phorbol esters. Fortification of serum-free medium with EGF or fetuin was able to partially restore sensitivity to maintained growth arrest by PTA. PKC translocation to the paniculate cellular fraction and subsequent enzyme down-regulation, induced by TPA, occurred in a manner similar to that observed in serum-supplemented cells. However, total PKC activity and cytosolic phorbol ester binding potential were greatly reduced in the serum-deprived cell population. Western blot analysis using monospecific monoclonal antibodies revealed the presence of PKC-a in both A549 cell populations, with significantly reduced protein levels in serum- deprived cells. PKC-/9 was not detected in either cell population.