Dioxin-induced up-regulation of the active form of vitamin D is the main cause for its inhibitory action on osteoblast activities, leading to developmental bone toxicity.

Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD) is known to cause bone toxicity, particularly during animal development, although its action mechanism to cause this toxicity has yet to be elucidated. Mouse pups were exposed to TCDD via dam's milk that were administered orally with 15 microg TCDD/kg b.w. on postnatal day 1. Here we report that TCDD causes up-regulation of vitamin D 1alpha-hydroxylase in kidney, resulting in a 2-fold increase in the active form of vitamin D, 1,25-dihydroxyvitamin D3, in serum. This action of TCDD is not caused by changes in parathyroid hormone, a decrease in vitamin D degrading enzyme, vitamin D 24-hydroxylase, or alterations in serum Ca2+ concentration. Vitamin D is known to affect bone mineralization. Our data clearly show that TCDD-exposed mice exhibit a marked decrease in osteocalcin and collagen type 1 as well as alkaline phosphatase gene expression in tibia by postnatal day 21, which is accompanied with a mineralization defect in the tibia, lowered activity of osteoblastic bone formation, and an increase in fibroblastic growth factor-23, a sign of increased vitamin D effect. Despite these significant effects of TCDD on osteoblast activities, none of the markers of osteoclast activities was found to be affected. Histomorphometry confirmed that osteoblastic activity, but not bone resorption activity, was altered by TCDD. A prominent lesion commonly observed in these TCDD-treated mice was impaired bone mineralization that is characterized by an increased volume and thickness of osteoids lining both the endosteum of the cortical bone and trabeculae. Together, these data suggest that the impaired mineralization resulting from reduction of the osteoblastic activity, which is caused by TCDD-induced up-regulation of vitamin D, is responsible for its bone developmental toxicity.