Vitamin D increases tight-junction conductance and paracellular Ca2+ transport in Caco-2 cell cultures.

We investigated the effects of 1α,25-dihydroxyvitamin D3[1,25(OH)2D3] on paracellular intestinal Ca2+absorption by determination of transepithelial electric resistance (TEER), as a measure of tight-junction ion permeability and bidirectional transepithelial45Ca2+fluxes in confluent Caco-2 cell cultures. The rise of TEER to steady-state levels of ∼2,000 Ω ⋅ cm2 was significantly attenuated by 1,25(OH)2D3(by up to 50%) in a dose-dependent fashion between 10-11 and 10-8 M. Synthetic analogs of 1,25(OH)2D3, namely, 1α,25-dihydroxy-16-ene,23-yne-vitamin D3 and 1α,25-dihydroxy-26,27-hexafluoro-16-ene,23-yne-vitamin D3, exhibited similar biopotency, whereas their genomically inactive 1-deoxy congeners were only marginally effective. Enhancement of transepithelial conductance of Caco-2 cell monolayers by vitamin D was accompanied by a significant increase in bidirectional transepithelial45Ca2+fluxes. Although 1,25(OH)2D3also induced cellular45Ca2+uptake from the apical aspect of Caco-2 cell layers and upregulated the expression of calbindin-9kDa mRNA, no significant contribution of the Ca2+-adenosinetriphosphatase-mediated transcellular pathway to transepithelial Ca2+ transport could be detected. Therefore stimulation of Ca2+fluxes across confluent Caco-2 cells very likely results from a genomic effect of vitamin D sterols on assembly and permeability of tight-junctional complexes.