Vitamin D3 potentiates the antitumorigenic effects of arsenic trioxide in human leukemia (HL-60) cells

BACKGROUND: Arsenic trioxide (ATO) is a novel form of therapy that has been found to aid acute promyelocytic leukemia (APL) patients. Our laboratory has demonstrated that ATO-induced cytotoxicity in human leukemia (HL-60) cells is mediated by oxidative stress. Pro-oxidants have been known to play a role in free radical-mediated oxidative stress. Vitamin D(3), (Vit D(3)) an active metabolite of vitamin D has been reported to inhibit the growth of number neoplasms such as prostate, breast, colorectal, leukemia, and skin cancers. The goal of the present research was to use (HL-60) cells as an in vitro test model to evaluate whether low doses of Vit D(3) potentiate the toxicity of ATO and whether this toxic action is mediated via apoptotic mechanisms. METHOD: HL-60 cells were treated either with a pharmacologic dose of ATO alone and with several low doses of Vit D(3). Cell survival was determined by MTT assay. Cell apoptosis was measured both by flow cytometry assessment, and DNA laddering assay. RESULTS: MTT assay indicated that Vit D(3) co-treatment potentiates ATO toxicity in HL-60 cells in a dose dependent manner. A statistically significant and dose-dependent increase (p <0.05) was recorded in annexin V positive cells (apoptotic cells) with increasing doses of Vit D(3) in ATO-treated cells. This finding was confirmed by the result of DNA laddering assay showing clear evidence of nucleosomal DNA fragmentation in vitamin and ATO co-treated cells. CONCLUSION: The present study indicates that Vit D(3) potentiates the antitumor effects of ATO. This potentiation is mediated at least in part, through induction of phosphatidylserine externalization and nucleosomal DNA fragmentation. These findings highlight the potential impact of Vit D(3) in promoting the pharmacological effect of ATO, suggesting a possible future role of Vit D(3)/ATO combination therapy in patients with acute promyelocytic leukemia (APL).