Evidence for the uptake of a vitamin D analogue (OCT) by a human carcinoma and its effect of suppressing the transcription of parathyroid hormone-related peptide gene in vivo.

The present study was undertaken to clarify the pharmacokinetics of 22-oxa-1,25-dihydroxyvitamin D3 (22-oxa-1,25-(OH)2D3, OCT), a vitamin D3 analogue with little calcemic activity, and its effect on the transcription of parathyroid hormone-related peptide (PTHRP) gene in nude mice bearing a human carcinoma (FA-6) associated with humoral hypercalcemia. FA-6 tumor expressed vitamin D receptor (VDR) mRNA, and its nuclear extract contained a specific and saturable 1,25-(OH)2D3 binding activity. Although [3H]OCT administered intravenously into FA-6 tumor-bearing nude mice was cleared from the circulation more rapidly than [3H]1,25-(OH)2D3, the uptake of [3H]OCT into the tumor tissue, relative to the radioactivity in the circulation, was greater than that of [3H]1,25-(OH)2D3. Intravenous or oral administration of OCT reduced the steady-state levels of PTHRP mRNA in FA-6 tumor, and nuclear run-off assays demonstrated that the effect of OCT on PTHRP gene expression occurred at a transcriptional level. RNase mapping analysis revealed that both upstream and downstream promoters of the human PTHRP gene were down-regulated by OCT. Finally, OCT exerted a preventive as well as therapeutic effect on cancer-associated hypercalcemia with a marked prolongation of the survival time in tumor-bearing animals. These results suggest that OCT is effectively taken up by a VDR-positive human carcinoma in vivo and has a therapeutic potential for cancer-associated hypercalcemia through suppression of PTHRP gene transcription.