Molecular mechanisms of oncolysis by the herpes simplex virus type 2 mutant ΔPK

Abstract

Cancer is a largely incurable and fatal disease that is resistant to standard therapeutics. This resistance is mediated by enhanced proliferative and prosurvival signals caused by activating mutations in the Ras/Raf/MEK/ERK and PI3K/AKT pathways, and the contribution of highly resilient cancer stem cells (CSC). Oncolytic virotherapy is based upon cancer specific virus replication and cell lysis. The specificity of engineered oncolytic viruses (OV) is accomplished through the deletion of one or more genes that are critical to virus replication. The mutations responsible for uncontrolled proliferation of cancer cells compensate for these deletions, thereby allowing OV to target cancer cells. HSV-2 encodes a unique serine/threonine protein kinase (PK)--known as ICP10PK--that activates the Ras and PI3K survival pathways. The mutant virus ΔPK, which lacks this PK domain, exhibits severely reduced levels of virus replication and latency reactivation while simultaneously inducing programmed cell death (PCD) in neurons. This dissertation work investigated the ability of ΔPK to selectively replicate and induce PCD in cancer cells and to define the molecular mechanisms involved therein. The data demonstrate that ΔPK eradicates cancer cultures and melanoma xenografts while sparing normal cells, and is well tolerated in treated mice. ΔPK-induced lysis of melanoma monolayer cultures is dominated by caspase- and calpain-mediated PCD, but also involves autophagy induction and JNK/cJun activation. Importantly, ΔPK eradication of breast and melanoma CSC requires autophagy and/or calpain but not caspase activation, and the contribution of calpain-mediated facilitation of autophagic flux represents a novel node of cross-talk between these two death pathways. In addition, ΔPK infection of melanoma cultures concomitantly inhibits anti-inflammatory IL-10 secretion while inducing the expression and secretion of numerous pro-inflammatory cytokines. Melanoma xenografts treated with ΔPK also exhibit a marked innate immune response. Significantly, ΔPK induces its own cancer specific replication through the activation of JNK, whereas the induction of PCD has no effect on virus growth. Collectively, the data indicate that ΔPK differentially induces a spectrum of PCD and pro-inflammatory programs that preclude the selection of resistant subpopulations in both quiescent CSC and proliferative tumor cells. These mechanisms uniquely potentiate its oncolytic activity and highlight its clinical promise.