Anti-apoptotic bcl-2 homologs encoded by Kaposi's sarcoma-associated virus and herpesvirus saimiri; a role during virus infections

Abstract

Background: Cellular bcl-2 was identified at t(14;18) translocation breakpoints in follicular lymphomas and is a potent inhibitor of programmed cell death induced by a variety of death stimuli. Bcl-2 is a 26kD mitochondrial membrane protein that contributes to tumorigenesis by preventing cell death rather than inducing cell proliferation and can complement c-myc in cell transformation. We have identified homologs of the cellular bcl-2 oncogene encoded by the gamma herpesviruses Kaposi's sarcoma-associated herpesvirus (KSHV) and herpesvirus saimiri(HVS). HVS is a T-cell tropic virus that causes rapid fatal lymphomas in monkeys. However, the role of HVS and KSHV viral bcl-2 homologs during infection or tumorigenesis is not known. Methods: The function of viral bcl-2 homologs was assessed by using a Sindbis virus vector system to express viral homologs of bcl-2. Recombinant HVS deleted forbcl-2 was constructed by homologous recombination. Results: Overexpression of KSbcl-2 of KSHV and ORF 16 (bcl-2 homolog) of HVS blocked apoptosis as efficiently as Bcl-2, Bcl-xL or another viral Bcl-2 homolog encoded by Epstein-Barr virus (BHRF1). Interestingly, KSbcl-2 neither homodimerizes nor heterodimerizes with other Bcl-2 family members, suggesting that KSbcl-2 may have evolved to escape the negative regulatory effects of the proapoptotic cellular homologs Bax and Bak. Furthermore, the herpesvirus Bcl-2 homologs including KSbcl-2, BHRF1, and ORF16 contain poorly conserved Bcl-2 homology 3 (BH3) domains compared to mammalian Bcl-2 homologs, implying that BH3 may not be essential for anti-apoptotic function. Consistent with the biology of Epstein-Barr virus infections, KSbcl-2 appears to be expressed during the lytic phase of the viral replication cycle. To begin to assess the role of herpesvirus bcl-2 homologs in viral replication and tumorigenesis, the bcl-w homolog of HVS was replaced with ßGAL. The resulting recombinant viruses exhibited a null plaque phenotype. Conclusions: Although bcl-2 homolog-deficient Epstein-Barr virus lacks an observable phenotype, bcl-2 homolog-deficient HVS was unable to produce plaques on Owl Monkey Kidney cells, suggesting that the anti-apoptotic function was required for efficient virus replication. KSbcl-2 of KSHV may have a similar role.