Host-virus interactions in KSHV-related malignancies: evaluating the role of STIP1 as a therapeutic target

Kaposi's sarcoma-associated herpesvirus (KSHV) is a virus that is linked to the development of a type of cancer known as Kaposi's sarcoma (KS) in individuals with compromised immune systems. KS is one of the top 10 cancers identified in men, women and children in South Africa, where it is the third most common cancer in African men. Despite this, there are no specific or effective treatments for KS that target the KSHV virus directly. As KS is an AIDS-defining disease, controlling HIV/AIDS and improving immune function using antiretroviral agents has been investigated as a possible treatment for KS. However, this is not always effective as many patients with well-controlled HIV infection still develop KS, and some individuals can experience life-threatening side-effects once they start antiretroviral therapy. Consequently, focused, specific and effective anti-KSHV therapies are urgently needed. KSHV is a member of the herpesvirus family and has two distinct life cycles in cells, a persistent life-long infection where the virus is mainly dormant (known as latency) and an infectious cycle that produces new viruses from the host cells (known as lytic replication). Uniquely for KSHV, both the latent and lytic replication cycles contribute to the development of KSHV-associated cancers. Therefore, it is essential to study the virus-host cell interactions which regulate both latent and lytic phases to fully understand KSHV-related disease. Moreover, inhibiting either or both phases may provide an opportunity to develop novel antiviral strategies to inhibit KS formation. This project focuses on a family of proteins found in host cells known as molecular chaperones. Molecular chaperones are needed for KSHV to undergo both latent and lytic replication cycles, acting as broad host cell factors for viral function. Molecular chaperones are themselves regulated by a family of host proteins known as co-chaperones, which are accessory proteins that fine-tune the function of chaperone systems. We have exciting preliminary data implicating the host co-chaperone STIP1 in multiple aspects of KSHV biology. This proposal will investigate how STIP1 functions during the KSHV latent and lytic phases and develop new ways to inhibit STIP1's function for use as KSHV-targeted therapeutics. We will apply a combination of molecular virology and drug discovery to describe in detail the viral and human cell processes controlled by STIP1 during KSHV latency and lytic replication. We will then use that information to design molecules capable of inhibiting STIP1 function in KSHV, which could subsequently be developed into KSHV-specific antivirals in future.

Molecular chaperones are essential for protein homeostasis. Hsp90 and Hsp70 function as broad host factors for viral protein folding and are essential for both KSHV latent and lytic replication cycles. As such, we believe KSHV is exquisitely sensitive to perturbations in molecular chaperone systems and inhibition of these pathways is a viable therapeutic strategy. However, targeting molecular chaperones directly is not without issue, due to cells having the ability to circumvent chaperone inhibition by activating alternative stress pathways. For example, resistance to the cellular stress response triggered by Hsp90 inhibition can be overcome by upregulating other chaperones, including Hsp70. To circumvent this issue, we will target alternative components of the stress complexes. Co-chaperones are accessory proteins that fine-tune the function of Hsp70-Hsp90 complexes. Specifically, the co-chaperone STIP1 (also known as HOP) couples the de novo and stress-related protein folding pathways of Hsp70, to the conformational regulation cycle of Hsp90. Therefore, inhibiting STIP1 function is a potential mechanism to simultaneously perturb both Hsp70 and Hsp90 function in viral infection. Our preliminary data suggest that STIP1 is required for survival of latently KSHV infected PEL cells and production of lytic virions. During lytic KSHV replication we determined that STIP1 become enriched in host networks regulating translation and ribosome biogenesis, cytoskeletal remodelling, and metabolism, all of which are necessary for KSHV infection. In addition, four KSHV-encoded proteins were associated with STIP1 during lytic replication. Based on this, this project will aim to understand the mechanisms by which STIP1 regulates the identified host and viral pathways, while concurrently extending the analysis of STIP1 as a therapeutic target in KSHV-infected cells and identifying hit compounds that inhibit STIP1 which could be developed into antiviral agents in future.