Involvement of protein kinases in the potentiation of lipopolysaccharide-induced inflammatory mediator formation by thapsigargin in peritoneal macrophages

We have explored the regulatory roles played by Ca²⁺-dependent signaling on lipopolysaccharide (LPS)-induced nitric oxide (NO), prostaglandin E₂ (PGE₂), tumor necrosis factor α (TNF-α), and interleukin-6 (IL-6) release in mouse peritoneal macrophages. To elevate intracellular Ca²⁺, we used thapsigargin (TG) and UTP. Although LPS alone cannot stimulate NO synthesis, co-addition with TG, which sustainably increased [Ca²⁺]_i, resulted in NO release. UTP, via acting on P2Y₆ receptors, can stimulate phosphoinositide (PI) turnover and transient [Ca²⁺]_2+i increase, however, it did not possess the NO priming effect. LPS alone triggered the release of PGE₂, TNF-α, and IL-6; all of which were potentiated by the presence of TG, but not of UTP. The stimulatory effect of LPS plus TG on NO release was inhibited by the presence of Ro 31-8220, Go6976, KN-93, PD 098059, or SB 203580, and abolished by BAPTA/AM and nuclear factor κB (NF-κB) inhibitor, PDTC. PGE₂, TNF-α, and IL-6 release by LPS alone were attenuated by Ro 31-8220, Go6976, PD 098059, SB 203580, and PDTC. Using L-NAME, soluble TNF-α receptor, IL-6 antibody, NS-398, and indomethacin, we performed experiments to understand the cross-regulation by the four mediators. The results revealed that TNF-α up-regulated NO, PGE₂, and IL-6 synthesis; PGE₂ up-regulated NO, but down-regulated TNF-α synthesis; and PGE₂ and IL-6 mutually up-regulated reciprocally. Taken together, murine peritoneal macrophages required a sustained [Ca²⁺]_i increase, which proceeds after TG, but not UTP, stimulation, to enhance LPS-mediated release of inflammatory mediators, particularly for NO induction. Activation of PKC-, ERK-, and p38 MAPK-dependent signaling also are essential for LPS action. The positive regulatory interactions among these mediators might amplify the inflammatory response caused by endotoxin.