Studies on viral interferon regulatory factors of Kaposi's sarcoma-associated herpesvirus

Abstract

Kaposi`s sarcoma-associated herpesvirus (KSHV) plays a significant role in the development of Kaposi`s sarcoma, primary effusion lymphoma and some forms of multicentric Castleman`s disease. The KSHV open reading frame K9 encodes the viral interferon factor 1 (vIRF1), which down-regulates interferon (IFN)- and IRF-mediated transcriptional activation, and leads to cellular transformation in rodent fibroblasts and induction of tumors in nude mice. Here we show that vIRF1 directly associates with the tumor suppressor p53 and represses its functions. The vIRF1 interaction domains of p53 are DNA binding domain (a.a. 100-300) and tetramerization domain (a.a. 300-393) of p53. p53 interacts with the central region (a.a. 152-360) of vIRF1. vIRF1 suppresses p53-dependent transcription and deregulates its apoptotic activity. These results suggest that vIRF1 may regulate cellular function by inhibiting p53. In addition, using the yeast two-hybrid assay, we identified genes associated with retinoid-IFN-induced mortality-19 (GRIM19), which interacts directly with vIRF1, both in vivo and in vitro. The N-terminal region of vIRF1 is required for binding GRIM19. Colocalization of vIRF1 and GRIM19 was observed in 293T cells and BCBL-1 cells. The vIRF1 protein deregulates GRIM19-induced apoptosis in the presence of IFN/all-trans-retinoic acid (RA) and inhibits IFN/RA-induced cell death. Other DNA tumor viral protein, human papillomavirus type 16 E6, also bind GRIM19, suggesting that this is a general target of viral proteins. Our results collectively indicate that vIRF1 modulates IFN/RA-cell death signals via interactions with GRIM19. vIRF1 also inhibits TGF- β-induced transcription and recovers TGF- β-induced growth inhibition. vIRF1 directly interacts with Smad3 and Smad4. vIRF1 disrupts Smad3-Smad4 complex formation and its DNA biniding. These results suggest that suppression of Smad-induced signaling by vIRF1 may contribute to KSHV-associated carcinogenesis. Nuclear factor κB (...