Malignant melanoma is a highly aggressive and
drug-resistant
cancer. Virotherapy is a novel therapeutic strategy based on
cancer cell lysis through selective virus replication. However, its clinical efficacy is modest, apparently related to poor virus replication within the
tumors. We report that the growth compromised herpes simplex virus type 2 (HSV-2) mutant, DeltaPK, has strong oncolytic activity for
melanoma largely caused by a mechanism other than replication-induced cell lysis. The ratio of dead cells (determined by
trypan blue or
ethidium homodimer staining) to cells that
stain with antibody to the major
capsid protein VP5 (indicative of productive
infection) was 1.8-4.1 for different
melanoma cultures at 24-72 h post-
infection. Cell death was due to activation of
calpain as well as caspases-7 and -3 and it was abolished by the combination of
calpain (
PD150606) and pancaspase (
benzyloxycarbonyl-Val-Ala-Asp-fluormethyl
ketone,
z-VAD-fmk) inhibitors. Upregulation of the autopahgy
protein Beclin-1 and the
pro-apoptotic protein H11/HspB8 accompanied DeltaPK-induced
melanoma oncolysis. Intratumoral DeltaPK injection (10(6)-10(7) plaque-forming unit (pfu)) significantly reduced
melanoma tumor burden associated with
calpain and caspases-7 and -3 activation,
Beclin-1 and H11/HspB8 upregulation and activation of caspase-1-related
inflammation. Complete remission was seen for 87.5% of the LM
melanoma xenografts at 5 months
after treatment termination. The data indicate that DeltaPK is a promising virotherapy for
melanoma that functions through virus-induced programmed cell death pathways.