Negative association of melanoma differentiation-associated gene (mda-7) and inducible nitric oxide synthase (iNOS) in human melanoma: MDA-7 regulates iNOS expression in melanoma cells.

The melanoma differentiation association gene-7 (mda-7) is a novel tumor suppressor gene of which the protein expression decreases to nearly undetectable levels in metastatic melanoma. In contrast, expression of inducible nitric oxide synthase (iNOS) is increased in advanced stages of melanoma, and iNOS expression has been proposed as a potential prognostic marker in this disease. Thus, expression of these molecules in the same tumor appears to exhibit reciprocal characteristics. We hypothesize that the relative ratios of these melanoma progression molecules may define either tumor progression or tumor suppression in human melanoma. The first goal of this study was to determine whether MDA-7 expression in melanoma negatively correlates with iNOS expression. The second goal was to determine whether iNOS expression could be regulated by MDA-7 expression in melanoma cells. Expression of MDA-7 and iNOS proteins were evaluated by immunohistochemistry in a total of 81 tumor samples: 38 primary melanomas and 43 metastatic melanomas. Evaluation of these melanoma patient samples showed that expression of MDA-7 and iNOS exhibits a significant negative correlation (P = 0.03). In vitro studies revealed that iNOS expression in melanoma cell lines is lost in a dose-dependent fashion after treatment with an adenoviral vector encoding the mda-7 gene (Ad-mda7) or with rhMDA-7 protein, demonstrating that MDA-7 down-regulates iNOS expression. Furthermore, we demonstrate that the STAT-3-modulated expression of IFN regulatory factors 1 and 2 is regulated by MDA-7, which may alter iNOS gene expression. These studies demonstrate that expression of the MDA-7 tumor suppressor can negatively regulate iNOS expression in malignant melanoma cell lines.