The Bcl-2 protein family is characterized by the ability to modulate cell death, and members of this family share two highly conserved domains called Bcl-2 homology 1 (BH1) and 2 (BH2) which have been shown to be critical for the death-repressor activity of Bcl-2 and Bcl-x L . Through sequence analysis we identified a novel viral Bcl-2 homolog, designated KSbcl-2, from human herpesvirus 8 (HHV8) or Kaposi sarcoma-associated herpesvirus. The overall amino acid sequence identity between KSbcl-2 and other Bcl-2 homologs is low (15-20%) but concentrated within the BH1 and BH2 regions. Overexpression of KSbcl-2 blocked apoptosis as efficiently as Bcl-2, Bcl-x L , 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 any negative regulatory effects of the cellular Bax and Bak proteins. Furthermore, the herpesvirus Bcl-2 homologs including KSbcl-2, BHRF1, and ORF16 of herpesvirus saimiri contain poorly conserved Bcl-2 homology 3 (BH3) domains compared with other mammalian Bcl-2 homologs, implying that BH3 may not be essential for anti-apoptotic function. This is consistent with our observation that amino acid substitutions within the BH3 domain of Bcl-x L had no effect on its death-suppressor activity.DNA sequences of the eighth human herpesvirus were recognized in Kaposi sarcoma (KS) tissues of AIDS patients through the application of representational difference analysis to discern DNA sequences in KS that were absent from normal DNA (1). This virus, known as human herpesvirus 8 (HHV8) or KS-associated herpesvirus, is more closely related to the ␥-2 herpesvirus, herpesvirus saimiri (HVS) than to Epstein-Barr virus (EBV) (a ␥-1 herpesvirus) based on DNA sequence analysis (1, 2). The preferential detection of HHV8 in KS lesions together with epidemiologic evidence implicate HHV8 in the etiology of KS (1). Recent serologic data suggest that HHV8 infections are acquired in the months to years prior to the onset of KS (3-5). However, HHV8 may be endemic in some geographic locations (5). Several cell lines derived from body cavity-based lymphomas (BCBL) are latently infected with HHV8 and productive lytic infection can be triggered in some of these lines upon addition of phorbol ester or butyrate (4, 6), which is similar to the lymphomagenic viruses EBV and HVS. The role of HHV8 in the etiology of KS or BCBL is not known. bcl-2 was first identified at the t(14;18) chromosomal breakpoint of follicular B cell lymphomas (7,8). It is a potent cell death-suppressor (9) and represents an unique type of protooncogene that extends cell survival by inhibiting apoptosis rather than promoting cell proliferation (10). Both EBV and HVS encode bcl-2 homologs called BHRF1 and ORF16, respectively (7, 11). Gene transfer studies have shown that BHRF1 can block apoptosis induced by various stimuli such as withdrawal of serum (12) or growth factors (13), treatment with antic...

The identification of self-renewing and multipotent neural stem cells (NSCs) in the mammalian brain holds promise for the treatment of neurological diseases and has yielded new insight into brain cancer. However, the complete repertoire of signaling pathways that governs the proliferation and self-renewal of NSCs, which we refer to as the 'ground state', remains largely uncharacterized. Although the candidate gene approach has uncovered vital pathways in NSC biology, so far only a few highly studied pathways have been investigated. Based on the intimate relationship between NSC self-renewal and neurosphere proliferation, we undertook a chemical genetic screen for inhibitors of neurosphere proliferation in order to probe the operational circuitry of the NSC. The screen recovered small molecules known to affect neurotransmission pathways previously thought to operate primarily in the mature central nervous system; these compounds also had potent inhibitory effects on cultures enriched for brain cancer stem cells. These results suggest that clinically approved neuromodulators may remodel the mature central nervous system and find application in the treatment of brain cancer.

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, Proc. Natl. Acad. Sci. USA 95:554-559, 1998). Gamma herpesviruses also encode homologs of the Bcl-2 family. All tested herpesvirus Bcl-2 homologs possess antiapoptotic activity, including the more distantly related homologs encoded by murine gammaherpesvirus 68 (␥HV68) and bovine herpesvirus 4 (BHV4), as described here. To determine if viral Bcl-2 proteins can be converted into death factors, similar to their cellular counterparts, five herpesvirus Bcl-2 homologs from five different viruses were tested for their susceptibility to caspases. Only the viral Bcl-2 protein encoded by ␥HV68 was susceptible to caspase digestion. However, unlike the caspase cleavage products of cellular Bcl-2, Bcl-x L , and Bid, which are potent inducers of apoptosis, the cleavage product of ␥HV68 Bcl-2 lacked proapoptotic activity. KSBcl-2, encoded by the Kaposi's sarcoma-associated herpesvirus, was the only viral Bcl-2 homolog that was capable of killing cells when expressed as an N-terminal truncation. However, because KSBcl-2 was not cleavable by caspases, the latent proapoptotic activity of KSBcl-2 apparently cannot be released. The Bcl-2 homologs encoded by herpesvirus saimiri, Epstein-Barr virus, and BHV4 were not cleaved by apoptotic cell extracts and did not possess latent proapoptotic activities. Thus, herpesvirus Bcl-2 homologs escape negative regulation by retaining their antiapoptotic activities and/or failing to be converted into proapoptotic proteins by caspases during programmed cell death.