Selective Elimination of Malignant Melanoma Using the Novel Anti-tumor Agents, OSW-1 AND PEITC

Metastatic melanoma is amongst the most refractory of cancers. Drug resistance and lack of therapeutic selectivity are two main challenges to successful melanoma therapy. Herein, we investigated the mechanims of anticancer activity and therapeutic selectivity of two novel agents, 3β, 16β, 17α-trihydroxycholest-5-en-22-one 16-O-[2-O-4-methoxybenzoyl-β-D-xylopyranosyl]- [1→3]-2-O-acetyl-α-l-arabinopyranoside (OSW-1) and β-Phenylethyl Isothiocyanate (PEITC) in melanoma. OSW-1 inhibited melanoma cell viability at nanomolar concentrations with minimal toxicity to normal melanocytes. Mechanistic studies revealed that OSW-1 suppressed Disialoganglioside 3 Synthase (GD3S) gene expression in melanoma cells, leading to inhibition of gangliosides GD3 and GD2. GD3 is an abundantly expressed melanoma cell surface antigen with pivotal roles in cancer progression and invasion. OSW-1 promoted interaction between GD3 and mitochondrial voltage-dependent gating protein 1 (VDAC1). Subsequent VDAC1 activation and autophagic cell death was observed. Downregulation of VDAC1 ameliorated the cytotoxic effect of OSW-1 in melanoma, indicating that VDAC1 was critical to mediating OSW-1’s activity. Studies with OSW-1 resistant melanoma cells revealed that elevated ganglioside expression may form the mechanistic basis for the selectivity of OSW-1 towards melanoma. Melanoma cells are characterized by mitochondrial dysfunction and intrinsic oxidative stress compared to normal melanocytes. Reactive oxygen species (ROS) activate pro-oncogenic signaling pathways that promote cancer progression and metastasis. However, an excessive overload of ROS, beyond the cellular tolerance threshold may trigger oxidative damage and cell death. The natural product PEITC induced apoptosis in malignant melanoma. Biochemical studies revealed that PEITC depleted cellular glutathione leading to ROS accumulation, mitochondrial damage and cell death. Glutathione inhibition was crucial to mediating PEITC’s activity since the glutathione precursor, N-Acetyl Cysteine reversed ROS elevation and PEITC induced death. Normal melanocytes with a much lower basal ROS capacity were resistant to PEITC. Drug combination effect of PEITC with Carmustine and Temozolomide was synergistic, suggesting that redox modulating strategies may be effective in attenuating chemoresistance to these agents. My study outlines novel therapeutic strategies to effectively eliminate malignant melanomas. The intrinsic differences in ganglioside expression and redox status between normal and cancer cells may be exploited to selectively kill melanoma cells using OSW-1 and PEITC, thus making these compounds worthy of further pre-clinical and clinical investigation