Swainsonine modulation of protein kinase C activity in murine peritoneal macrophages

The level of Ca2+, phospholipid-dependent, protein kinase C (PKC) activity in murine peritoneal macrophages treated with swainsonine, an indolizidine alkaloid, has been investigated. The present studies are based on our recent report that murine peritoneal macrophages are activated by swainsonine (Grzegorzewski, K.; Newton, S.A.; Akiyama, S.K.; Sharrow, S.; Olden, K.; White, S.L., Cancer Commun. 1:373-379, 1989). Presently, we have demonstrated that macrophages treated with swainsonine exhibited a substantial increase in PKC activity. The activity was enhanced as much as 4- to 5-fold over that obtained in untreated macrophages and was inhibited by H-7 (1-[5-isoquinoline sulphonyl]-2-methylpiperazine), D-sphingosine, or a monoclonal antibody specific for the active site of PKC. This represents the first report to demonstrate an effect of swainsonine on a second messenger system known to be involved in tumor promotion and macrophage activation. Elevation of PKC activity occurred much more slowly in swainsonine-treated cells than in cells treated with agents known to activate PKC directly, e.g., PMA (4-beta-phorbol-12-beta-myristate-13-gamma-acetate) or gamma-interferon (IFN-gamma). Furthermore the increase in PKC activity was inhibited by alpha-amanitine and cycloheximide, inhibitors of RNA and protein synthesis, respectively. These results suggest that swainsonine enhancement of PKC activity occurred by an indirect and possibly protein-synthesis-dependent mechanism. Whatever its precise mechanism of action, swainsonine provides a potentially important new probe to evaluate PKC mediated events. Selective enhancement of PKC activity may be important not only in elucidating the role of PKC in tumor promotion or macrophage activation but, also, in contributing to development of therapeutic regimens.