1,25-Dihydroxyvitamin D3 down-regulates aggrecan proteoglycan expression in immortalized rat chondrocytes through a post-transcriptional mechanism.

We have examined the effects of various analogs of vitamin D on the expression of the aggrecan proteoglycan by an immortalized rat chondrocyte cell line. The active metabolite of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), produced a concentration-dependent reduction in the synthesis of aggrecan as monitored by histochemical staining of the matrix, incorporation of [35S]sulfate, and the level of aggrecan core protein. Other analogs of vitamin D were much less potent or had no activity whatsoever. The reduced expression of aggrecan was caused by a dramatic decrease in the steady-state level of the mRNA coding for the aggrecan core protein. A nuclear run-off analysis revealed that the rate of transcription of the aggrecan gene was not significantly altered by 1,25(OH)2D3 treatment, suggesting that the metabolite was acting through a post-transcriptional mechanism. Experiments using the transcriptional inhibitor actinomycin D also supported a nondirect effect of 1,25(OH)2D3 on the expression of the aggrecan gene. These results suggest that the vitamin D metabolite activates a new pattern of gene expression which results in a more rapid turnover of the aggrecan mRNA. This system should be useful for characterizing the regulation of chondrocyte gene expression by vitamin D.