Ascorbic acid alters collagen integrins in bone culture.

The effects of ascorbic acid on collagen synthesis, mineralization, and integrins were investigated in a mineralizing organ culture system derived from 20-day fetal rat parietal bones. A significant dose-dependent decrease in calcification at 96 h was demonstrated with decreasing concentrations of ascorbic acid (100-0 microg/ml). No effect on DNA content, [3H]thymidine incorporation, or dry weight was found in control (100 microg/ml ascorbic acid) bones compared with bones treated with decreased ascorbic acid concentrations (10, 1, and 0 microg/ml). Collagen synthesis, measured by [3H]proline incorporation, and alpha1(I) procollagen messenger RNA levels were also unaffected. However, ascorbic acid produced a dose-dependent decrease in the hydroxyproline content, with a maximal 76.8% decrease in bones without ascorbic acid compared with the control bones with 100 microg/ml ascorbic acid. Light microscopy of the ascorbic acid-deficient bones revealed a disruption of the osteoblast layer with misshapen osteoblasts and a decrease in the osteoid seam. The loss of osteoblast organization was also confirmed by analyzing the integrins for collagen by Northern and Western blot and immunofluorescence microscopy. A dose-dependent decrease in alpha2 and beta1 integrin messenger RNA levels and in alpha1, alpha2, and beta1 protein were found in 96-h bone cultures deficient in ascorbic acid. These integrin subunits mediate the binding of osteoblasts to collagen. Immunofluorescence microscopy also demonstrated a dose-dependent decrease in alpha2 and beta1 staining of the osteoblast layer. However, the protein levels of alpha3 and alpha5 subunits were not affected. No beta5 was detected, whereas only bones cultured without ascorbic acid demonstrated a small decrease in alpha(v) and beta3 protein levels. The alpha3, alpha5, alpha(v), and beta3 subunits are involved in cell binding to extracellular matrix proteins other than collagen. Thus, the integrins for collagen are down-regulated, probably in response to the underhydroxylated collagen fibrils, which causes a disruption of osteoblast organization leading to a decrease in mineralization of bone. Integrin assays for specific extracellular proteins may be useful tools in detecting matrix defects in various metabolic bone diseases.