Tumor necrosis factor-[Alpha] inhibits store-mediated [Ca.sup.2+] entry in the human hepatocellular carcinoma cell line HepG2

Tumor necrosis factor-[Alpha] (TNF-[Alpha]) is an important component of the early signaling pathways leading to liver regeneration and proliferation, but it is also responsible for several hepatotoxic effects. We have investigated the effect of TNF-[Alpha] on thapsigargin (TG)-induced store-mediated [Ca.sup.2+] entry (SMCE) in the human hepatocellular carcinoma cell line HepG2. In these cells, short-term (10 min) exposure to TNF-[Alpha] slightly increased SMCE. In contrast, long-term (12 h) exposure to TNF-[Alpha] significantly reduced SMCE. This effect was reversed by coincubation with atrial natriuretic peptide (ANP), which itself had no effect on SMCE. Cytochalasin D and latrunculin A, inhibitors of actin polymerization, abolished SMCE. Long-term exposure of HepG2 cells to TNF-[Alpha] abolished TG-induced actin polymerization and membrane association of Ras proteins. When TNF-[Alpha] was added in combination with ANP, these effects were reduced. These findings suggest that in HepG2 cells, TNF-[Alpha] inhibits SMCE by affecting reorganization of the actin cytoskeleton, probably by interfering with the activation of Ras proteins, and that ANP protects against these inhibitory effects of TNF-[Alpha]. calcium influx; actin cytoskeleton; Ras proteins; atrial natriuretic peptide