Effects of Dexamethasone,Calcium and 1,25-Dihydroxycholecalciferol on Calcitonin and Calcitonin Gene-Related Peptide mRNA Levels from the CA-77 C Cell Line

We used the CA-77 cell, a murine C-cell line derived from a medullary thyroid carcinoma, to study the effects of glucocorticoids, calcium, and vitamin D metabolites on calcitonin (CT) gene expression. Total RNA was isolated, and CT and CT gene-related peptide (CGRP) mRNAs were measured by Northern hybridizations using specific probes. A control mRNA probe (cyclophilin) was used to quantitate the specificity of the changes in CT and CGRP mRNAs. The CA-77 C cell line cultured in basal conditions with a medium deprived of fetal calf serum, but was supplemented by insulin, expressed mainly the CGRP mRNA. Dexamethasone was found to increase both CT and CGRP mRNAs in a time- and dose-dependent way without changing the alternative splicing. A slight but significant increase in the steady-state CT mRNA level was found 3 days after addition of 10^-10 M dexamethasone; the same dose slightly decreased the CGRP mRNA level; concentrations of dexamethasone ≥ 10 ^-9 M elevated both mRNAs. A twelvefold increase for CT mRNA, and an eightfold increase in CGRP mRNA occurred 3 days after administration of 10 ^-6 M dexamethasone. Kinetic data revealed inductions of both mRNAs 24 hours after exposure to 10^-7 M dexamethasone, and highest CT and CGRP mRNAs levels were observed after 72 hours of treatment. Calcium from 1-4 mM in short-term (1 hour and 4 hour) or long-term stimulations (1 day and 4 days), with or without dexamethasone cotreatment was ineffective. CT and CGRP mRNAs levels were both half-reduced 48 hours after addition of 10^-7 M 1,25-dihydroxycholecalciferol; this effect was transient, as it disappeared 2 days later.