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PRIMARY RESEARCH DIRECTIVES
EBV mediated cell cycle arrest during lytic replication
Herpesviruses are ubiquitous viruses that are responsible for numerous disorders in humans. The most well known of these pathologies are those associated with reactivation of viral genetic programs in immunocompromised hosts [e.g. Herpes Simplex virus (HSV), Cytomegalovirus (CMV), and the Epstein Barr virus (EBV)]. While diseases associated with herpes simplex virus and cytomegalovirus are primarily associated with their viral replication programs, Epstein Barr virus latency associated genes play an important cell cycle promoting role in the life cycle of the virus. Consequently, in immuno-competent individuals, EBV is becoming recognized as a likely etiologic agent in a growing number of cancers including African Burkitt's lymphoma, Nasopharyngeal carcinoma, and the invasive (estrogen receptor negative) form of breast cancer.
Although CMV and EBV are well known as human tumor viruses, older literature indicated that herpes viral replication occurs primarily in growth arrested tissues. Unlike small DNA tumor viruses such as SV40, Adenovirus, and human papilloma virus, herpes viruses encode their own DNA polymerase as well as enzymes involved in nucleotide metabolism. Consequently, they don't require S phase entry for viral DNA replication. Our earlier studies showed that EBV itself blocks cell cycle progression during this phase of the viral life cycle. This work demonstrated that the EBV encoded transcription factor, Zta, alters the regulation of multiple cell cycle control factors, and efficiently blocks proliferation of tumor cells. Together, these studies indicate that although EBV encodes genes that contribute to tumorgenesis (i.e. the latency associated genes), it also encodes the capacity to reverse the growth of such tumor cells (although this capacity is normally kept shut down) (Figures 1 & 2). Since this time, numerous studies (from other groups as well as our own) have firmly established that lytic genes encoded by HSV, CMV, and EBV elicit a cell cycle arrest. Drawing on studies from all three of these viruses, a conceptual understanding of the role of viral growth inhibitory genes in herpes viral life cycles is emerging.
We have been particularly interested in addressing the molecular basis of Zta's cell growth arrest function (Figure 2) so that we can better understand the role that this activity plays in the viral life cycle. In addition, we are also developing methods that exploit this important property in anti-tumor strategies to target EBV associated cancers. More specifically, we are developing agents that specifically induce the expression of this anti-proliferative gene in tumor cells (see Figure 1 & 3).
One concern regarding the safety of such an approach is the concommitant production of infectious virions. Although it is not clear at present whether this is a significant safety issue, we have discovered a means through which all of the anti-tumor potential of Zta expression can be manifested without progression into the lytic cycle. As shown in Figure 3, phosphorylation of Serine 186 by Protein Kinase C (PKC) is essential for induction of lytic viral production, but phosphorylation of Serine 186 is not required for any of the indicated anti-tumor properties of Zta. Moreover, phosphorylation of Serine 186 mitigates Zta's growth arrest activity (unpublished). As the result of these studies, we think that a potent anti-tumor approach can be developed inwhich Zta expression is induced in the absence of PKC signaling (Figure 3).
EBV is present in tumor cells and only a very low percentage of the normal B-cell population (1 in a million). Importantly, this means that the Zta gene is essentially present only in the tumor cells and agents that induce Zta expression will offer exquisite specificity in anti-tumor approaches.
Related Publications:
Role of the E2F-1 transcription factor in cell cycle regulation.
Another research effort being carried out in our laboratory focuses on key transcriptional regulatory programs involved in cell cycle control. The regulation of the E2F family of transcription factors by the tumor suppressor protein, pRb, is central to controlling the expression of E2F target genes and consequently, cell cycle progression. Our studies have focused primarily on how the interaction between pRb and E2F affects E2F DNA binding activity, its stability, and its ability to communicate with basal transcription machinery. In addition, we have been addressing how viral oncogenes affect the interactions between E2F and pRb, and between E2F and transcriptional co-activators, in an effort to understand the mechanisms utilized by viral oncogenes to elicit cell cycle progression.
Related Publications: