Calcium modulates the cyclin D1 expression in a rat parathyroid cell line.

We have used a rat epithelial parathyroid cell line (PT-r) to study the expression and regulation of D-type cyclins. In PT-r cells the cyclin D1 gene is the most abundantly expressed, being transcribed in at least two mRNAs whose levels oscillate during the cell cycle. We also screened a cDNA library prepared from PT-r cells with the human cyclin D1 probe and isolated its rat homologue. Cyclin D2 and D3 mRNAs are both represented in PT-r cells but the former one is only barely detectable. Moreover, the oscillation of cyclin D3 transcript is slightly delayed when compared to cyclin D1 and D2. Since extracellular calcium inhibits parathyroid cell proliferation, we looked for the effect of the ion on the expression of cyclin D genes in PT-r cells. Increasing amounts of calcium in the incubation medium reduced the expression of rat cyclin D1 and D2. The effect appears to be cell-specific and probably mediated through the inhibition of mitogenic signalling pathways.