Chemosensitization of cancer in vitro and in vivo by nitric oxide signaling.

PURPOSE: Hypoxia contributes to drug resistance in solid cancers, and studies have revealed that low concentrations of nitric oxide (NO) mimetics attenuate hypoxia-induced drug resistance in tumor cells in vitro. Classic NO signaling involves activation of soluble guanylyl cyclase, generation of cyclic GMP (cGMP), and activation of cGMP-dependent protein kinase. Here, we determined whether chemosensitization by NO mimetics requires cGMP-dependent signaling and whether low concentrations of NO mimetics can chemosensitize tumors in vivo. EXPERIMENTAL DESIGN: Survival of human prostate and breast cancer cells was assessed by clonogenic assays following exposure to chemotherapeutic agents. The effect of NO mimetics on tumor chemosensitivity in vivo was determined using a mouse xenograft model of human prostate cancer. Drug efflux in vitro was assessed by measuring intracellular doxorubicin-associated fluorescence. RESULTS: Low concentrations of the NO mimetics glyceryl trinitrate (GTN) and isosorbide dinitrate attenuated hypoxia-induced resistance to doxorubicin and paclitaxel. Similar to hypoxia-induced drug resistance, inhibition of various components of the NO signaling pathway increased resistance to doxorubicin, whereas activation of the pathway with 8-bromo-cGMP attenuated hypoxia-induced resistance. Drug efflux was unaffected by hypoxia and inhibitors of drug efflux did not significantly attenuate hypoxia-induced chemoresistance. Compared with mice treated with doxorubicin alone, tumor growth was decreased in mice treated with doxorubicin and a transdermal GTN patch. The presence of GTN and GTN metabolites in plasma samples was confirmed by gas chromatography. CONCLUSION: Tumor hypoxia induces resistance to anticancer drugs by interfering with endogenous NO signaling and reactivation of NO signaling represents a novel approach to enhance chemotherapy.

Hypoxia induced resistance to doxorubicin in prostate cancer cells is inhibited by low concentrations of glyceryl trinitrate.

PURPOSE: Tumor hypoxia has been correlated with metastasis and resistance to chemotherapy. Hypoxia is also associated with human prostate cancers, which are highly resistant to chemotherapy. We hypothesized that hypoxia contributes to chemoresistance in prostate cancer cells and this hypoxia induced chemoresistance can be inhibited by low concentrations of nitric oxide (NO) mimetics. MATERIALS AND METHODS: Human PC-3 and mouse TRAMP-C2 prostatic adenocarcinoma cells were incubated in 20% or 0.5% O(2) for 12 hours with or without glyceryl trinitrate (GTN) (0.1 nM). This treatment was followed by a 1-hour incubation with doxorubicin and survival was assessed by clonogenic assays. Western blot analysis was used to measure NO synthase levels. The effect of hypoxia and GTN on cell cycle distribution was determined by flow cytometry. RESULTS: Hypoxic pre-incubation of the 2 cell lines resulted in increased survival following exposure to doxorubicin. Co-incubation of PC-3 and TRAMP-C2 cells with GTN (0.1 nM) inhibited the hypoxia induced resistance to doxorubicin. Each cell line expressed all 3 NO synthase isoforms at levels that were not significantly affected by O(2) concentrations. Cell cycle analysis revealed that there was no significant difference in the distribution of PC-3 cells at each stage of the cycle. However, incubation under hypoxia resulted in a small decrease in the number of TRAMP-C2 cells in S-phase. CONCLUSIONS: These findings indicate that NO may have an important role in the regulation of chemosensitivity in prostate cancer cells. Furthermore, the results suggest that GTN administration may represent a means of chemosensitizing prostatic carcinomas.