Dose-dependent effects of aluminum on osteocalcin synthesis in osteoblast- like ROS 17/2 cells in culture

This in vitro study evaluates the effect of aluminum (Al³⁺) on osteocalcin, a small protein that is produced by the osteoblast. After stimulation with various doses of 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃; 10^-11 to 10^-9 M], osteocalcin was consistently lower in the culture medium of ROS 17/2 osteoblastic cells conditioned with 5 μM Al³⁺-saturated transferrin (AlTR) than in apotransferrin (ApoTR)-treated controls. In a second experiment, cultures were conditioned with various doses of AlTR or ApoTR (1.6-8.0 μM) and stimulated with 10^-9 M 1,25(OH)₂D₃. High doses of AlTR (4.8-8.0 μM) resulted in lower medium and unchanged intracellular content of osteocalcin than treatment with equal amounts of ApoTR. However, in the same experiment, lower doses of AlTR or ApoTR (1.6 and 3.2 μM) yielded different results, i.e., increased medium and intracellular contents of osteocalcin in the Al³⁺-treated cells. Expression of osteocalcin mRNA was not altered in cultures conditioned with low (1.6 μM) or high (8.0 μM) concentrations of AlTR or ApoTR. Similarly, no effect of Al³⁺ was observed on total protein content, the rate of total protein synthesis, and the degradation of secreted osteocalcin in cultures conditioned with various doses of AlTR or ApoTR. These findings suggest that AlTR affects osteocalcin synthesis in a specific manner, without concomitant effects on the rate of total protein synthesis or on the rate of degradation of osteocalcin. This effect is dose dependent, i.e., low doses of AlTR stimulate and high doses suppress osteocalcin synthesis and/or secretion, and it appears to be posttranscriptional, since the expression of osteocalcin mRNA is not affected.