Modulation of phospholipase A2 activity by the tumour promoters phorbol esters and teleocidin

The effects of tumour promoters, namely phorbol esters and teleocidin, on the activity of porcine pancreatic phospholipase A2 (PLA2) was investigated by using a system of small unilamellar vesicles composed of dipalmitoyl-phosphatidylcholine (DPPC). DPPC vesicles encapsulating Quin 2 (Quin 2/DPPC vesicles) were suspended in a medium containing Ca2+. The addition of PLA2 to Quin 2/DPPC vesicles increased the fluorescence intensity of Quin 2. This increase was due to chelation of Quin 2 with Ca2+, which resulted from an increase in the permeability of the phospholipid bilayer caused by the hydrolytic activity of PLA2. The tumour promoters phorbol 12-myristate 13-acetate (PMA) and teleocidin, at low concentrations, enhanced PLA2 activity at temperatures below the phase-transition temperature of the membrane, but, in contrast, high concentrations of the tumour promoters suppressed PLA2 activity. Phorbol 12-myristate (PM) also had a similar effect on PLA2 activity. PMA and PM disturbed the membrane structure markedly, which was indicated by the enhanced leakage of carboxyfluorescein (CF) from DPPC vesicles encapsulating CF. On the other hand, phorbol 12,13-didecanoate and 4 alpha-phorbol 12,13-didecanoate, which did not disturb the membrane structure to the same extent, had an insignificant effect on PLA2 activity. It is therefore concluded that PLA2 catalyses the hydrolysis of phospholipids in bilayer vesicles which contain a moderate degree of structural defects. However, the effects of tumour promoters on PLA2 activity was not related to their potencies as inflammatory and tumour-promoting agents.