Modulatory effects of PKC activity on increased 92-kDa gelatinase secretion by neonatal alveolar macrophages

We previously demonstrated that alveolar macrophages (AMs) from neonatal rats can secrete more 92-kDa gelatinase than AMs from adult rats. In this study, we investigated the role of the protein kinase C (PKC) pathway in the transductional regulation of 92-kDa gelatinase secretion by rat AMs, and we also evaluated maturational changes in this role with increasing postnatal age. After AM stimulation by phorbol 12-myristate 13-acetate (PMA), we observed a dose-dependent increase in gelatinase secretion that was significantly more marked in AMs from 6-day-old rats than in AMs from adult rats and that was inhibited by the PKC inhibitor calphostin C. Adenosine 3',5'-cyclic monophosphate mimetics or concanavalin A failed to induce an increase in gelatinase secretion by AMs. Time-dependent variations in PKC activity after PMA stimulation differed significantly between 6-day-old rats and adult rats; PKC activity decreased in adult AMs (50%) but remained stable in 6-day-old AMs. We therefore investigated age-related differences in the intracellular proteolytic degradation of PKC, which is thought to be mediated by calpains. Leupeptin, used as a calpain inhibitor, inhibited the decrease in PKC activity after exposure of adult AMs to PMA and induced a greater than threefold increase in PMA-induced gelatinase secretion. Calpain activity was significantly lower in AM extracts from 6-day-old than from adult rats. The physiological implication of these developmental changes in 92-kDa gelatinase regulation was demonstrated by investigation of AMs from 1-day-old rats that showed a high level of spontaneous PKC-dependent gelatinase secretion coexisting with very low calpain activity. We conclude that sustained PKC activity is a key factor in the increased gelatinase secretion by AMs seen during the postnatal period and is due, at least in part, to reduced PKC degradation.