INTERACTIONS OF 1,25(OH)(2)D-3 AND RETINOIC ACID IN THE REGULATION OFIEC-6 CELL ALKALINE-PHOSPHATASE ACTIVITY

The goal of the present work was to use IEC-6 cells to investigate the possible mechanisms underlying the regulation of alkaline phosphatase (ALP) activity (ALPA) by 1 alpha,25-dihydroxyvitamin D-3 (1,25(OH)(2) D-3) and retinoids. Here we demonstrate that the vitamin D analogs, 25 (OH)(2)-16-ene-23-yne-D-3 and 1 alpha,24S-(OH)(2)-22-en-26,27- dehydro vitamin D-3, which have been shown by others to bind to the intracellu lar vitamin D receptor (VDR), have similar effects to 1,25(OH)(2)D-3 i n increasing ALPA of IEC-6 cells. A third vitamin D analog, 25-(OH)-16 -ene-23-yne-D-3 (AT), which activates membrane 1,25(OH)(2)D-3 effects, but binds poorly to the intracellular VDR, did not stimulate ALPA of IEC-6 cells. These data suggest that the effects of 1,25(OH)(2)D-3 to increase ALPA are mediated by intracellular VDR rather than by membran e actions of the hormone. The all-trans and 9-cis retinoic acids alone each caused increased ALPA of IEC-6 cells without altering steady-sta te levels of ALP mRNA, suggesting that retinoic acids may regulate ALP A of IEC-6 cells at a posttranscriptional level. Vitamin D analogs whi ch bind intracellular receptors showed synergistic effects with either retinoid to increase ALPA, but there was no interaction with AT. Alth ough the retinoids alone did not alter ALP mRNA levels, addition of 1, 25(OH)(2)D-3 in combination with either retinoid increased ALP mRNA mo re than did 1,25(OH)(2)D-3 alone. These data suggest that the synergis tic effects of 1,25(OH)(2)D-3 and retinoids on IEC-6 cell ALPA are med iated by intracellular VDR. The results of these experiments indicate that 1,25(OH)(2)D-3 alters IEC-6 cell ALPA via increased mRNA levels, while retinoids appear to both have post-transcriptional effects and t he capacity to interact with 1,25(OH)(2)D-3 in altering ALP mRNA level s.