Human Herpesvirus 8 K1 Suppresses Fas-Mediated Apoptosis and Binds to the Fas Complex.

HHV-8 is associated with the development of primary effusion lymphoma (PEL) and Kaposi’s sarcoma. We have previously reported that PEL-derived cell lines constitutively express the HHV-8 K1 gene, which encodes for a transmembrane protein with an immunoreceptor tyrosine-based activation motif (ITAM). ITAMs are present in receptors regulating cell activation and survival. K1 in transgenic mice, driven by a constitutive promoter, is associated with development of lymph node hyperplasia and lymphoma. Given its NF-kB activation inducing properties, we hypothesized that K1 contributes to transformation, in part, by suppression of apoptosis. To study the role of HHV8 K1 gene product, we generated K1 stably expressing cell line BJABK1 and vector alone cell line BJABXS, with retrovirus vector pLXSN, and packaging cell line PT67 NIH 3T3. We also expressed a truncated K1 lacking the ITAM (K1m). Transient transfections were used to expressed epitope tagged K1. Anti-Fas antibody (CH-11) was used to induce apoptosis and cells were monitored for membrane protein Annexin-V, DNA integrity, and morphological features of apoptosis. Caspase 8, cFLIP and Fas were monitored by immunoblot analysis and flow cytometry. K1 was expressed in mouse hepatocytes by intravenous inoculation of plasmids, expressing K1 and green fluorescent protein. K1-Fas complex interactions were analyzed in immunoprecipitation-immunobloting analysis. K1 is expressed in PEL cells and PEL primary cells. Anti-Fas antibody induced apoptosis was suppressed in BJABK1 cells compared with BJABK1m and BJABXS cells. K1 also suppressed apoptosis, which had been induced by Fas ligand but not apoptosis induced by TRAIL or radiation. K1 expressing cells showed consistent lower levels of procaspase 8 cleavage products. Immunoprecipitation-immunoblotting analysis indicated Fas and K1myc were bound to the same complex. Taken together, the data indicate that expression of HHV8 K1 can suppress Fas-mediated apoptosis. K1 and Fas bind to the same complex in several cell types and in vivo. We anticipate K1 regulates apoptosis through binding of the Fas complex and represents a novel form of apoptosis regulation mediated by cellular proteins designated to modulated Fas.