Associations between vitamin D-binding protein isotypes, circulating 25(OH)D levels, and vitamin D metabolite uptake in colon cancer cells

Vitamin D metabolites have been extensively studied as cancer chemopreventive agents. Gc-globulin (GC) isotypes, based on rs7041 and rs4588 diplotypes, have varying affinities for 1α,25-dihydroxyvitamin D (1,25(OH)₂D) and 25-hydroxyvitamin D (25(OH)D), which may affect circulating metabolite concentration as well as delivery at the cellular level. We evaluated associations between GC isotype and circulating vitamin D metabolite concentrations in 403 ursodeoxycholic acid (UDCA) clinical trial participants. Metabolite uptake was evaluated in human colon cancer (HCT-116) cells treated with ethanol vehicle, 1,25(OH)₂D, or 25(OH)D, and with plasma from individuals with known GC isotype. Mammalian-2-hybrid and vitamin D-responsive element-based luciferase assays were used to measure the vitamin D receptor pathway activation as a marker for metabolite uptake. Regression analysis demonstrated significantly lower serum 25(OH)D concentration for clinical trial participants with 1F-2, 1S-2, or 2-2 isotypes (P < 0.01) compared with 1S-1S. Consistent with these in vivo observations, cellular data revealed that 25 (OH)D uptake varied less by GC isotype only at the higher concentration tested (P = 0.05), while 1,25 (OH)₂D uptake differed markedly by GC isotype across concentration and assay (P < 0.01). The 1F-1S and 1F-2 isotypes produced the greatest reporter gene induction with 1,25(OH)₂D treatment and, while activation varied less with 25(OH)D, the 2-2 isotype demonstrated increased induction at the lower concentration. These results suggest that vitamin D metabolite concentration and delivery to colon cells may vary not only by GC isotype, but also that certain isotypes may more effectively deliver 1,25(OH)₂D versus 25(OH)D. Overall, these results may help identify populations at risk for cancer and potential recipients of targeted chemoprevention.