Protein kinase C and intracellular pH regulate zymosan-induced lysosomal enzyme secretion in macrophages

Binding of zymosan particles to macrophage beta-glucan receptors has previously been shown to trigger exocytosis of preformed lysosomal contents. In the present study, the involvement of Ca(2+)-, PKC-, and pH-dependent processes in the signaling to macrophage lysosomal secretion by zymosan was investigated. Also, the PKC dependence of lysosomal secretion in response to some soluble agents that directly alters intracellular pH was considered. Signaling to macrophage lysosomal secretion differs from that of many other secretory systems, because an elevation of cytosolic Ca2+ did not trigger a large secretory response, nor did attempts to reduce cytosolic Ca2+ affect the lysosomal secretory response to other stimuli. PKC activation by phorbol diester was also a poor stimulus of lysosomal secretion. However, when triggered by zymosan or by soluble stimuli raising lysosomal pH, the secretory response could be down-regulated by a prior prolonged incubation with phorbol diester. Such treatment also had marked effects on the binding and uptake of zymosan particles, the study of which was made possible by a novel approach. Furthermore, a synergistic effect on lysosomal secretion was obtained when stimuli that elevated lysosomal pH and stimuli that activated PKC were combined. This is of likely relevance for the secretory response to zymosan particles, a stimulus that both activates PKC and elevates lysosomal pH. The secretory response to zymosan was furthermore shown to be inhibited by a reduction of extracellular pH or [Na+], conditions that impair macrophage extrusion of acid equivalents. Earlier studies using soluble stimuli have shown a sensitivity of the secretory response to changes in cytosolic pH. We suggest a model in which the lysosomal secretory response to an elevation of lysosomal pH (1) is dependent on basal PKC activity and (2) can be enhanced further by activation of PKC. We consider PKC activity and elevation of lysosomal pH as independent and necessary signals, while cytosolic pH has a modulatory effect on some component(s) in the signal transduction pathway or in the secretory apparatus itself.