Inhibition of Proliferation and Induction of Apoptosis by 25- Hydroxyvitamin D3-3 -(2)-Bromoacetate, a Nontoxic and Vitamin D Receptor-Alkylating Analog of 25- Hydroxyvitamin D3 in Prostate Cancer Cells

The 25-hydroxyvitamin D3 (25-OH-D3) is a nontoxic and low-affinity vitamin D receptor (VDR)-binding metabolic precursor of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. We hypothesized that covalent attachment of a 25-OH-D3 analog to the hormone-binding pocket of VDR might convert the latter into transcriptionally active holo-form, making 25-OH-D3 biologically active. Furthermore, it might be possible to translate the nontoxic nature of 25-OH-D3 into its analog. We showed earlier that 25-hydroxyvitamin D3-3bromoacetate (25-OH-D3-3-BE) alkylated the hormonebinding pocket of VDR. In this communication we describe that 10 6 mol/L of 25-OH-D3-3-BE inhibited the growth of keratinocytes, LNCaP, and LAPC-4 androgen-sensitive and PC-3 and DU145 androgen-refractory prostate cancer cells, and PZ-HPV-7 immortalized normal prostate cells with similar or stronger efficacy as 1,25(OH)2D3. But its effect was strongest in LNCaP, PC-3, LAPC-4, and DU145 cells. Furthermore, 25-OH-D3-3-BE was toxic to these prostate cancer cells and caused these cells to undergo apoptosis as shown by DNA-fragmentation and caspase-activation assays. In a reporter assay with COS-7 cells, transfected with a 1 ,25dihydroxyvitamin D3-24-hydroxylase (24-OHase)-construct and VDR-expression vector, 25-OH-D3-3-BE induced 24OHase promoter activity. In a “pull down assay” with PC-3 cells, 25-OH-D3-3-BE induced strong interaction between VDR and general transcription factors, retinoid X receptor, and GRIP-1. Collectively, these results strongly suggested that the cellular effects of 25-OH-D3-3-BE were manifested via 1,25(OH)2D3/VDR signaling pathway. A toxicity study in CD-1 mice showed that 166 g/kg of 25-OH-D3-3-BE did not raise serum-calcium beyond vehicle control. Collectively, these results strongly suggested that 25-OH-D3-3-BE has a strong potential as a therapeutic agent for androgen-sensitive and androgen-refractory prostate cancer.