Background: Currently, dacarbazine (DTIC) is the only approved systemic treatment for metastatic malignant melanoma. However, the modest treatment effect encourages studies on novel therapeutic molecules, delivery systems and combination therapies. Full-length TRAIL, delivered from an adenoviral vector (Ad-hTRAIL), was studied in combination with DTIC or the histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA) in human melanoma cell lines.
Methods: The cytotoxic potential of the combination treatments was assessed by cell viability measurements and CalcuSyn analysis. Involvement of apoptosis was analyzed by TUNEL staining, mitochondrial membrane potential measurements, and activation and expression levels of caspases and other mediators of apoptosis.
Results: Ad-hTRAIL in combination with DTIC or SAHA resulted in additive or synergistic growth inhibition compared to each treatment used as single agent. Both combinations augmented apoptosis, which was mediated through the death receptor (DR) pathway by enhanced activation of caspase-8, and through increased loss of mitochondrial integrity. Provoked cleavage of Bid, which bridges the extrinsic and intrinsic apoptosis pathways, and downregulation of the anti-apoptotic mediators Bcl-X(L), Mcl-1 and XIAP (but not Bcl-2) were critical contributing factors. Increased levels of DR4 and DR5 were not a common underlying mechanism as DTIC did not affect the levels of either of the receptors. However, SAHA-induced expression of DR4 may have reduced the TRAIL resistance in the SKMEL-28 cell line.
Conclusion: Administration of Ad-hTRAIL in combination with DTIC or SAHA enhances apoptosis in human melanoma cell lines, and suggests that the therapeutic potential of such treatment strategies should be further evaluated for possible clinical use.