Role of intracellular calcium mobilization in the regulation of protein kinase C-mediated membrane processes

Abstract

Phorbol esters are potent tumour-promoting agents that exert pleiotropic effects on cells^1–3. Among these are the control of growth⁴, stimulation of release of stored bioactive constituents^5–8 and regulation of growth-factor surface receptors^9–13. Phorbol esters bind to and activate protein kinase C, leading to the phosphorylation of specific protein substrates presumed to be necessary for eliciting the full response^{6,8–12,14,15}. Strong evidence exists that specific binding of tumour promoter occurs at the membrane level in intact cells, resulting in activation of protein kinase C^16,17. Recent evidence concerning the release of bioactive constituents from platelets^5,8,14,18,19 and neutrophils^5,6 has linked agonist-induced protein kinase C activation and Ca²⁺ mobilization in a synergistic mechanism^8,20,21. Here we present a novel model of synergism between Ca²⁺ and phorbol esters that leads to transferrin receptor phosphorylation and down-regulation in HL-60 human leukaemic cells. Raising intracellular Ca²⁺, although ineffective by itself, increases the potency and rate of action of phorbol ester for activating protein kinase C and mediating transferrin receptor phosphorylation and down-regulation. We propose a molecular model in which increased intracellular Ca²⁺ recruits protein kinase C to the plasma membrane, thus ‘priming’ the system for activation by phorbol ester.