Effects of increased doses of 1,25 dihydroxyvitamin D3 on matrix and DNA synthesis in condylar cartilage of suckling mice

Abstract

The in vivo effects of high doses of 1,25(OH)2D3 were studied in condylar cartilage of suckling mice. Seven-day-old animals were treated with 20 ng of the hormone for 7 consecutive days. Biochemical assays on collagen content and synthesis were complemented by structural studies using light and electron microscopy. Indirect immunofluorescent methods were used for the localization of type I and II collagens and for fibronectin. This study revealed that the protein content of the condyle decreased substantially following the administration of the hormone. Protein synthesis increased in hormone-treated animals during the first 4 days but was significantly inhibited thereafter. Collagen synthesis, however, was inhibited instantaneously, followed by a decrease in the percentage of cold hydroxyproline of the total protein. Hormone-treated condyles showed a marked decrease in the distribution of type I collagen, no apparent change in the distribution of type II collagen, but an enhanced reactivity for fibronectin especially around hypertrophic chondrocytes. SDS-gel electrophoresis of collagen chains suggested that the hormone did not induce a significant change in the ratios of type I and II collagen chains, yet additional peaks became evident in 1,25(OH)2D3-treated specimens. The decrease in collagen synthesis was accompanied by ultrastructural changes in the appearance of the extracellular collagen bundles. They later appeared as a dense meshwork of collagen fibrils, a feature that was lacking in control tissues. The changes in collagen fibrillogenesis could be explained by our in vitro studies indicating a marked depression of 35S-sulfate incorporation secondary to treatment with 1,25(OH)2D3. The hormone was also found to suppress the incorporation of 3H-thymidine, hence it may be concluded that 1,25(OH)2D3, when used in high concentrations, possesses an inhibitory effect upon both the proliferative activity of the cartilage progenitor cells as well as upon the metabolic activity of the condylar cells as related to collagen and glycosaminoglycans synthesis.