Androgen Receptor Regulation of Vitamin D Receptor in Response of Castration-Resistant Prostate Cancer Cells to 1 -Hydroxyvitamin D5: A Calcitriol Analog

Abstract

Calcitriol (1,25(OH)2D3) is cytostatic for prostate cancer (CaP), but had limited therapeutic utility due to hypercalcemia-related toxicities, leading to the development of low-calcemic calcitriol analogs. We show that one analog, 1-α-Hydroxyvitamin-D5 (1α(OH)D5), induced apoptosis in castration-sensitive LNCaP prostate cancer cells, but unlike calcitriol, did not increase androgen receptor (AR) transcriptional activity. LNCaP-AI, a castrate-resistant (CRCaP) LNCaP subline, was resistant to 1α(OH)D5 in the presence of androgens; however, androgen withdrawal (AWD), although ineffective by itself, sensitized LNCaP-AI cells to 1α(OH)D5. Investigation of the mechanism revealed that the vitamin D receptor (VDR), which mediates the effects of 1α(OH)D5, is downregulated in LNCaP-AI cells compared to LNCaP in the presence of androgens, whereas AWD restored VDR expression. Since LNCaP-AI cells expressed higher AR compared to LNCaP and AWD decreased AR, this indicated an inverse relationship between VDR and AR. Further, AR stimulation (by increased androgen) suppressed VDR, while AR downregulation (by ARsiRNA) stimulated VDR levels and sensitized LNCaP-AI cells to 1α(OH)D5 similar to AWD. Another cell line, pRNS-1-1, although isolated from a normal prostate, had lost AR expression in culture and adapted to androgen-independent growth. These cells expressed the VDR and were sensitive to 1α(OH)D5, but restoration of AR expression suppressed VDR levels and induced resistance to 1α(OH)D5 treatment. Taken together, these results demonstrate negative regulation of VDR by AR in CRCaP cells. This effect is likely mediated by prohibitin (PHB), which was inhibited by AR transcriptional activity and stimulated VDR in CRCaP, but not castrate-sensitive cells. Therefore, in castration sensitive cells, although the AR negatively regulates PHB, this does not affect VDR expression, whereas in CRCaP cells, negative regulation of PHB by the AR results in concomitant negative regulation of the VDR by the AR. These data demonstrate a novel mechanism by which 1α(OH)D5 prolong the effectiveness of AWD in CaP cells.

Down-Regulation of Ca2+-Activated K+ Channel KCa1.1 in Human Breast Cancer MDA-MB-453 Cells Treated with Vitamin D Receptor Agonists

“…Calcitriol and/or its analogs induce the up-regulation of VDR [33] and AR [34] as well as the down-regulation of ERα [12] in prostate and/or breast cancer cells. We examined the effects of the treatment with VDR agonists on the expression levels of their transcripts in MDA-MB-453 cells.…”

Novel strategies in the treatment of castration-resistant prostate cancer (Review)

“…Vitamin D receptor is implicated in several pathways in prostate cancer cells, principally in the regulations of cell differentiation and proliferation. Phase I studies have demonstrated that the use of vitamin D analogues is able to induce inhibition of proliferation and stimulates apoptosis in prostate cancer cells (61). A single-institution phase II study of docetaxel plus calcitriol in patients with metastatic CRPC demonstrated ≥50% PSA reduction in 81% of patients, with a median time to progression and median survival of 11.4 and 19.5 months, respectively (62).…”

Role of androgen and vitamin D receptors in endothelial cells from benign and malignant human prostate

“…The interdependence of AR and VDR may vary between different prostate cancer cell lines, and the cross-talk between AR and VDR has not been investigated thoroughly in prostate tumor endothelial cells, especially those from castration-resistant prostate tumors. Therefore, the effects of varying AR and VDR agonists or analogs that activate these receptors may result in differential modulation of nuclear receptor cross-talk and provide new approaches for selective inhibition of growth of prostate cancers (66).…”