Nitric oxide mediates natural polyphenol-induced Bcl-2 down-regulation and activation of cell death in metastatic B16 melanoma

Intravenous administration to mice of trans-pterostilbene (t-PTER; 3,5-dimethoxy-4'-hydroxystilbene) and quercetin (QUER; 3,3',4',5,6-pentahydroxyflavone), two structurally related and naturally occurring small polyphenols, inhibits metastatic growth of highly malignant B16 melanoma F10 (B16M-F10) cells. t-PTER and QUER inhibit bcl-2 expression in metastatic cells, which sensitizes them to vascular endothelium-induced cytotoxicity. However, the molecular mechanism(s) linking polyphenol signaling and bcl-2 expression are unknown. NO is a potential bioregulator of apoptosis with controversial effects on Bcl-2 regulation. Polyphenols may affect NO generation. Short-term exposure (60 min/day) to t-PTER (40 microM) and QUER (20 microM) (approximate mean values of the plasma concentrations measured within the first hour after intravenous administration of 20 mg of each polyphenol/kg) down-regulated inducible NO synthetase in B16M-F10 cells and up-regulated endothelial NO synthetase in the vascular endothelium and thereby facilitated endothelium-induced tumor cytotoxicity. Very low and high NO levels down-regulated bcl-2 expression in B16M-F10 cells. t-PTER and QUER induced a NO shortage-dependent decrease in cAMP-response element-binding protein phosphorylation, a positive regulator of bcl-2 expression, in B16M-F10 cells. On the other hand, during cancer and endothelial cell interaction, t-PTER- and QUER-induced NO release from the vascular endothelium up-regulated neutral sphingomyelinase activity and ceramide generation in B16M-F10 cells. Direct NO-induced cytotoxicity and ceramide-induced mitochondrial permeability transition and apoptosis activation can explain the increased endothelium-induced death of Bcl-2-depleted B16M-F10 cells.