Regulation by calcitonin and glucocorticoids of calcitonin receptor gene expression in mouse osteoclasts

We previously studied regulation of the calcitonin (CT) receptor (CTR) by glucocorticoid (GC) and CT in cultures of mature mouse osteoclast-like cells (OCLs). The present studies were designed to examine the interaction of CT and GC in regulation of the CTR in osteoclasts and the molecular mechanisms involved. Treatment of OCLs with 10(-7) M dexamethasone (Dex) increased the CTR number in a time-dependent manner, whereas treatment with 10(-9) M salmon CT (sCT) reduced CTR number; neither treatment changed receptor affinity. Dex pretreatment somewhat antagonized the CT-induced reduction in [125I]sCT specific binding. Dex increased, and sCT pretreatment decreased, the sCT-responsive adenylate cyclase activity in parallel with the change in receptor binding. Dex treatment resulted in an increase in CTR messenger RNA (mRNA) levels, as assessed by reverse transcription-PCR, indicating that the increased CTR number was mediated by de novo CTR synthesis. This effect was specific to GCs and was not reproduced by mineralocorticoids or sex steroids. Treatment with sCT resulted in a rapid and profound reduction in CTR mRNA expression, and this reductions was somewhat delayed by Dex pretreatment. OCLs were treated with 5,6-dichloro-1 beta-D-ribofuranosyl benzimidazole to enable estimation of the mRNA decay rates in the absence of ongoing transcription. The stability of CTR mRNA was similar to the control value in Dex-treated OCLs, suggesting that the effect of Dex may be due to changes in transcriptional activity. Interestingly, transcriptional inhibition by 5,6-dichloro-1 beta-D-ribofuranosyl benzimidazole abolished the ability of CT to reduce CTR mRNA levels, suggesting that CT may act by increasing the rate of CTR mRNA decay, and that this effect requires ongoing transcription. The 3'-untranslated region of the mouse CTR mRNA contains four copies of the AUUUA motif, as well as other A/U-rich sequences, which have been shown to determine the stability of other mRNA transcripts. The stability results were consistent with the results of the nuclear transcript run-on assay, which indicated that treatment with Dex enhanced the rate of transcription, whereas CT had no effect. These results show that GC and CT influence CTR expression by distinct mechanisms and provide the basis for identification of the cellular factors involved.