Abstract
The anti-neoplastic effects of 1,25-dihydroxyvitamin D3 (1,25D) are well documented in numerous tumor cell systems and animal models of cancer. However, despite this pre-clinical success, the clinical use of 1,25D is currently impeded by the dose-limiting hypercalcemia, and the risk of development of resistance to 1,25D. In this study, we investigated the mechanism of resistance to 1,25D of HL60-40AF cells, a model of drug-resistant acute myeloid leukemia, derived from HL60 cells by cultivation in the presence of 1,25D. The data indicate that transcriptional activity of vitamin D receptor (VDR) in 40AF cells increases only briefly when the cells are treated with 1,25D, despite greater basal cellular levels of VDR protein in the resistant than in the 1,25D-sensitive cells. Analysis of the 40AF VDR mRNA sequence indicated alterations in the 5' untranslated region (UTR), but coding domain variations were not observed. When resistance to 1,25D-induced differentiation of 40AF cells was reversed by a combination of 1,25D with potentiators of differentiation (plant derived antioxidants and a p38MAPK inhibitor), an increase in the level of nuclear VDR, as well as an increase in CYP24 mRNA expression was observed. These data suggest that decreased ability of 1,25D to induce VDR nuclear localization and the consequent VDR target gene transcription may be an important reason for the resistance of 40AF cells to 1,25D. Further, our data show that VDR localization and phosphorylation can be increased by combining 1,25D with potentiators of differentiation. Analysis of the mechanisms that underlie the reduction and potentiation of 1,25D-mediated changes in VDR activity may lead to the identification of new cellular targets that enhance 1,25D-induced monocytic differentiation.