Direct transcriptional activation of promyelocytic leukemia protein by IFN regulatory factor 3 induces the p53-dependent growth inhibition of cancer cells.

IFN regulatory factor 3 (IRF3) is a transcriptional factor that plays a crucial role in activation of innate immunity and inflammation in response to viral infection, and is also involved in p53-dependent inhibition of cell growth. Although functional activation of IRF3 by viral infection is relatively well documented, the biological role and regulatory mechanism underlying cell growth inhibition by IRF3 are poorly understood. Here, we show a novel regulatory pathway connecting IRF3-promyelocytic leukemia protein (PML)-p53 in primary and cancer cell lines. Overexpression of IRF3 induces p53-dependent cell growth inhibition in cancer cell lines with normal p53 activity. In addition, doxycycline-induced expression of IRF3 in U87MG cells inhibits tumor growth in nude mice in vivo. IRF3 is found to increase expression of PML by a direct transcriptional activation as determined by PML-promoter-luciferase and chromatin immunoprecipitation assays. When PML is depleted by RNA interference-mediated knockdown, IRF3 fails to increase p53 acetylation and its transcriptional activity. Taken together, the results of the present study indicate that direct transcriptional activation of PML by IRF3 results in the p53-dependent growth inhibition of normal and cancer cells in vitro and in vivo, which is suggestive of a novel regulatory network between the innate immune response and tumor suppression.