1,25-Dihydroxycholecalciferol regulates chromogranin-A translatability in bovine parathyroid cells

Our previous studies indicated that regulation of bovine parathyroid chromogranin-A (CgA) by 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] may occur at the level of CgA turnover or mRNA translation. In the present study, immunoprecipitation of extracts of bovine parathyroid cells that had been pulse chased with [35S]methionine revealed that 1,25-(OH)2D3 had no effect on the disappearance time of intracellular CgA. Therefore, we examined the effect of 1,25-(OH)2D3 on the polyribosome profile of CgA mRNA analyzed by sucrose density gradients. In the presence of 1,25-(OH)2D3, there was a dose-dependent recruitment of CgA mRNA into the denser polyribosomal fractions by 24 h. To determine whether this increased ribosome loading represents increased or decreased efficiency of mRNA translation, ribosome transit time experiments were conducted. The average ribosome transit time and the specific PTH ribosome transit time were not altered by 1,25-(OH)2D3. However, that for CgA was doubled in the presence of 1,25-(OH)2D3. Thus, parathyroid CgA synthesis is regulated by the vitamin D sterol at the level of peptide chain elongation. These studies, therefore, explain the lack of quantitative correspondence between 1,25-(OH)2D3-induced CgA gene transcription and CgA protein levels by revealing a previously unsuspected level of regulation of mRNA translation in the parathyroid cell.