Previously, we showed that soluble guanylyl cyclase (sGC), via its ability to synthesize cGMP which in turn stimulates protein kinase G (PKG) activity, plays a critical role in reducing spontaneous apoptosis in neuroblastoma cells and human ovarian cancer cells. In the ovarian cancer cells, we found that basal activation of the sGC/cGMP/PKG signaling pathway caused anti-apoptotic/cytoprotective effects that were mediated, in part, by suppression of p53 accumulation, assessed by p53 siRNA gene-knockdown. The data suggested that basal sGC/cGMP/PKG activation in certain cancer cells may counteract the pro-apoptotic/therapeutic effects of chemotherapeutic agents (often involving p53 accumulation), potentially resulting in reduced effectiveness of the chemotherapy. The present study determines if a human mesothelioma cell line, NCI-H2452 cells, expresses PKG and if the inhibition of the upstream enzyme sGC (and subsequent depletion of cGMP) affects the induction of apoptosis caused by cisplatin. Using Western blot analysis, NCI-H2452 mesothelioma cells were found to express PKG, predominately the PKG-Ialpha isoform. Apoptosis was quantified using an ELISA that measures the levels of cytosolic histone-associated apoptotic DNA fragments. ODQ, a selective sGC inhibitor, was used at concentrations (10, 30 and 50 micromolar) that have been shown in previous studies to effectively suppress the sGC enzymatic activity in cancer cells and cause depletion of cGMP levels down to approximately 75%, 55% and 35%, respectively, of the control basal levels. At 30 and 50 micromolar, ODQ alone caused significant induction of apoptosis in the NCI-H2452 cells, elevating apoptotic levels by 12 fold and 15 fold, respectively, above control (non-ODQ) levels. At 10 micromolar, a concentration below the threshold for induction of apoptosis by ODQ alone, ODQ in combination with cisplatin enhanced (in fact, doubled) the pro-apoptotic effects of cisplatin at 1 micromolar. These results have uncovered a novel signaling pathway (sGC/cGMP/PKG) in mesothelioma cells that regulates apoptosis and the susceptibility of these cells to cisplatin-induced apoptotic cell death. This new information may be useful for developing new therapeutic agents that can sensitize mesothelioma cells to the cancer-killing effects of conventional chemotherapeutic agents.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA