Synergistic cooperation between thapsigargin and phorbol ester for induction of nitric oxide synthesis in murine peritoneal macrophages

Abstract

The biochemical transductional events involved in NO synthesis are not fully understood. These studies, therefore, were undertaken to elucidate the role of intracellular calcium and protein kinase C (PKC) in the induction of nitric oxide (NO) synthesis in murine peritoneal macrophages. Thapsigargin (TG), Ca 2+ -ATPase inhibitor of endoplasmic reticulum, had modest activity on NO synthesis by itself, whereas phorbol ester, PKC activator, alone had no effect. When TG was used in combination with phorbol ester, there was a marked cooperative induction of NO synthesis in a dose-dependent manner. The optimal effect of phorbol ester was shown in the first 6 h after TG treatment. In addition, the ability of TG with phorbol ester on NO synthesis could be mimicked by another chemically unrelated inhibitor of Ca 2+ -ATPase, 2,5-di-( t-butyl)-1,4-benzohydroquinone, and Ca 2+ ionophore, A23187. This increase of NO synthesis was reflected as increased amount of NO synthase (NOS) mRNA, as determined by Northern blotting. Intracellular Ca 2+ transient by TG was not affected in the presence or absence of extracellular Ca 2+, indicating that TG must be effective on cytosolic Ca 2+ pool. In addition, chelation of intracellular Ca 2+ by acetoxymethyl ester of 1,2- bis-(2-aminophenoxy)-ethane- N,N,N′,N′-tetraacetic acid (BAPTA/AM), an intracellular Ca 2+ chelating agent, blocked TG- or TG + PMA-induced NO production. PKC inhibitors such as staurosporine or polymyxin B reduced only the synergistic cooperative effect of TG with phorbol ester without affecting TG-induced NO production. In addition, when the cells were pretreated with phorbol ester before TG treatment, there was no synergy between TG and phorbol ester, indicating that PKC is not directly involved in the expression of NOS but involved in “triggering” signal. Secretion of NO corresponded with tumor cell killing, but TG plus phorbol ester-activated macrophages failed to kill tumor cell targets in the presence of N G-monomethyl- l-arginine. Collectively, these data illustrate that mobilization of intracellular Ca 2+ provides a “priming” signal for induction of NOS gene expression by itself and it also requires PKC as a “triggering” signal for macrophage tumoricidal activity.