Interferons (IFN) have broad cell-type specific effects, including growth inhibition, antiviral activity, modulation of differentiation, induction or inhibition of programmed cell death (apoptosis) and regulation of immune system genes and expression. The growth inhibitory and/or apoptotic effects of type I IFNs, which include IFN-alpha and IFN-beta, are mediated by protein kinase R, 2'-5' oligoadenylate syntase and additional IFN-inducible RNases. The RNA helicases comprise a family of proteins implicated in diverse processes of RNA metabolism, including degradation, translation and editing. The relevance of IFN-induced RNA helicases in mediating changes in cellular physiology is not understood. Subtraction hybridization identified melanoma differentiation associated gene-5 (mda-5) as a novel IFN-beta induced gene whose encoded protein contains a caspase recruitment domain (CARD) and RNA helicase motif. Purified MDA-5 fusion protein exhibits dsRNA-dependent ATPase activity and because of the presence of both CARD and RNA helicase domains represents a unique member of the RNA helicase gene family. Moreover, the RNA helicase motif of MDA-5 shares certain peculiarities with growth inhibitory and/or RNase III helicase proteins. These findings, including the growth suppressing activity of mda-5 in mammalian cells, support the hypothesis that MDA-5 is a unique RNA helicase involved in growth inhibitory/apoptotic RNA metabolism elicited by IFN treatment. Since a potential involvement of RNA metabolism in the apoptotic process is unique, analysis of MDA-5, which may represent a molecule that for the first time links these important processes, provides an extraordinary opportunity to define the role of apoptotic RNA metabolism and IFN action. This information could prove relevant in understanding the mechanism of cellular defense conferred by IFN against viral attack as well as the role of interferons in mediating antitumor responses. Experiments will be performed to define the biological role of mda-5 in IFN-induced growth inhibition and/or apoptosis, identify potential protein partners of MDA-5 involved in mediating mda-5 action, and identify RNA substrates of MDA-5 and characterize the ATPase activity of MDA-5. Understanding MDA-5 function could provide significant insights into RNA with novel apoptotic processing potential defining therapeutics delimiting infectivity and pathogenesis. Additionally, based on its chromosomal location, 2q24, the present studies could also shed light on a potential role of mda-5 in specific cancer etiologies as well as enhance our appreciation of the carcinogenic process.
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