Abstract

It is well known that the efficiency of intestinal active calcium transport is regulated by the Vitamin D receptor pathway and Vitamin D analogs seem to exhibit differential effects on intestinal active calcium transport. To investigate the molecular basis for the difference among Vitamin D analogs, we tested three Vitamin D analogs: 1,25-dihydroxyvitamin D₃, 19-nor-1,25-dihydroxyvitamin D₂, and 1α-hydroxyvitamin D₂ ex vivo and in vitro.

In 5/6 nephrectomized rat intestinal active calcium transport, 19-nor-1,25-dihydroxyvitamin D₂ did not show a significant effects on intestinal active calcium transport at all the concentrations tested, while 1α-hydroxyvitamin D₂ at 0.33 and 0.67 μg/kg and 1,25-dihydroxyvitamin D₃ at 1 μg/kg significantly stimulated calcium transport.

In Caco-2 cells, 19-nor-1,25-dihydroxyvitamin D₂ did not show a significant effect on calcium transport, while 1,25-dihydroxyvitamin D₃ and 1,25-dihydroxyvitamin D₂ (the active form of 1α-hydroxyvitamin D₂) stimulated calcium transport by 934 and 501% at 0.1 μM, respectively.

1,25-Dihydroxyvitamin D₂ potently induced the expression of CALB3 and TRPV6 mRNA with an EC₅₀ of 0.3 and 1.0 nM, whereas 19-nor-1,25-dihydroxyvitamin D₂ was 10-fold less potent than 1,25-dihydroxyvitamin D₂ in inducing CALB3 and TRPV6 mRNA. The three Vitamin D analogs had no significant effect on the expression of PMCA1 mRNA. These Vitamin D analogs did not change the expression of Vitamin D receptor (VDR) up to 10 nM, but stimulated CYP24A1 expression in a dose-dependent manner with the potency in the order of 1,25-dihydroxyvitamin D₃ > 1,25-dihydroxyvitamin D₂ = 19-nor-1,25-dihydroxyvitamin D₂.

These results suggest that the differential effect of Vitamin D analogs on stimulating intestinal and Caco-2 calcium transport may be in part due to its different effect on stimulating CALB3 and TRPV6 mRNA expression.

Keywords: Vitamin D analogs; Paricalcitol; Calcitriol; VDR; Secondary hyperparathyroidism; Intestinal calcium transport; Caco-2; CALB3; TRPV6

Abbreviations: 1,25-dihydroxyvitamin D₃, calcitriol; 19-nor-1,25-dihydroxyvitamin D₂, paricalcitol; 1α-hydroxyvitamin D₂, doxercalciferol; 1,25-dihydroxyvitamin D₂, 25-hydroxy-doxercalciferol; CALB3, calbindin 3, Vitamin D-dependent calcium-binding protein, CABP9K, CABP1, calbindin D9K; TRPV6, transient receptor potential cation channel subfamily V member 6, epitherial calcium channel 2 (ECAC2), calcium transporter 1 (CAT1); CYP24A1, 1,25-dihydroxyvitamin D₃ 24-hydroxylase

Article Outline

1. Introduction

2. Materials and methods

2.1. Materials

2.2. 5/6 nephrectomized rats

2.3. Measurement of gastrointestinal transport

2.4. Cell culture

2.5. Calcium transport in Caco-2 cells

2.6. Real-time reverse transcription-PCR

3. Results

4. Discussion

Acknowledgements

References