Decrease in CGRP and CT levels either contained in or released by CA-77 C cells after combined treatments with 1,25-dihydroxyvitamin D3 analogues and 9-cis retinoic acid.

This study examined the action of 9-cis retinoic acid and 1,25-dihydroxyvitamin D3 analogues (KH 1060, EB 1089 and MC 903) on the release of calcitonin (CT) and calcitonin gene-related peptide (CGRP) in the rat C cell line CA-77. This cell line mainly secretes CGRP. Using radioimmunoassays (RIAs) for CT and CGRP, we measured the release of both peptides in the culture medium as well as the amount of these proteins contained in the CA-77 C cells. 9-cis retinoic acid decreased the release of both CGRP and CT dose-dependently in the range between 1 nM and 1 microM. The half-effective dose was 10 nM. The treatment of CA-77 C cells with 0.1 microM calcitriol alone only slightly decreased the release of both CT and CGRP. The increase in the amount of CT and CGRP released by the action of 1 microM dexamethasone was reduced by 1 microM 9-cis retinoic acid, and this effect was enhanced by the addition of 0.1 microM calcitriol or KH 1060, EB 1089 and MC 903. When the C cells were continuously stimulated by dexamethasone, after 6 days of exposure to the combined treatment with calcitriol analogues + 9-cis retinoic acid, there was a greater decrease in the amount of CGRP contained in the C cells than after treatment with 9-cis retinoic alone. Our data suggested that combined treatment with retinoic acid and calcitriol analogues exerted a stronger inhibition on the amounts of the two peptides either contained in the cells or released in the medium than each hormone alone.

Expression of glucagon-like peptide 1 receptor in a murine C cell line: regulation of calcitonin gene by glucagon-like peptide 1.

We have characterized, by RT-PCR amplification using specific primers, the presence of glucagon-like peptide-1 (GLP-I) receptor mRNA in CA-77 cells, a C cell line derived from a rat medullary thyroid carcinoma. Down-regulation of the GLP-1 receptor mRNA was observed after exposure of CA-77 C cells with GLP-1 (7-37). Increased secretion of both calcitonin gene-related peptide (CGRP) and calcitonin (CT) occurred after treatment with GLP-1 (7-37) associated with elevated steady-state levels of CGRP and CT mRNA. GLP-1 (7-37) increased cAMP formation in CA-77 cells in a dose-dependent manner; exendin (9-39), a GLP-1 receptor antagonist, inhibited cAMP production. The GLP-1 peptide which is produced by intestinal cells could be involved in the control of CT secretion through an entero-thyroidal axis implying GLP-1 receptor and increased CT gene expression.

Effects of 17 beta-estradiol on calcitonin and calcitonin-gene-related peptide secretions and contents in a murine medullary thyroid carcinoma C-cell line (CA-77).

The effect of 17 beta-estradiol on calcitonin (CT) and calcitonin-gene-related peptide (CGRP) secretions in the murine CA-77 C cell line was studied after 1, 3, 5 and 6 d of treatment. The release of both CT and CGRP significantly increased 1, 3, 5 and 6 d after addition of 0.1 mumol/l estradiol alone to the culture medium. The C cell content of both peptides also increased after d of treatment with the same dose of estrogen. The enhanced CT and CGRP secretions induced by 17 beta-estradiol were not inhibited by the simultaneous addition of 5 mumol/l of all-trans-retinoic acid. Dexamethasone alone increased the release of both peptides within 6 d. However, when cells were treated simultaneously with estradiol and 1 mumol/l dexamethasone, the addition of retinoic acid blunted both the CT and CGRP secretions induced by dexamethasone. These results showed that the positive effects of 17 beta-estradiol on both CT and CGRP secretions were modulated by dexamethasone and retinoic acid.

Steroid hormones and retinoic acid interact in the regulation of calcitonin and calcitonin gene-related peptide secretion and messenger ribonucleic acid levels in CA-77 C cells.

Northern hybridizations were used to evaluate the modulated action of retinoic acid (R.A.) in presence of dexamethasone (Dex) and/or calcitriol (1,25-(OH)2D3) on calcitonin (CT) and calcitonin gene-related peptide (CGRP) mRNA steady state levels in the murine CA-77 C cell line. Dex 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 Dex; the same dose slightly decreased the CGRP mRNA level; concentrations of Dex > or = 10(-9) M elevated both mRNAs. Calcium from 1-4 mM in short-term (1 hr. and 4 hrs.) or long-term stimulations (1 day and 4 days), with or without Dex cotreatment was ineffective. Dex alone (10(-6) M) elicited a 2-fold increase in CGRP mRNA and a 9-fold increase in CT mRNA steady state levels after 6 days of treatment whereas addition of 5.10(-5) M R.A. alone for 6 days decreased both the CGRP and the CT mRNA steady state level (12- and 4-fold decreases, respectively). Our results showed that 5.10(-7) M R.A. blunted in part (-30%) the rise of CT and CGRP mRNA induced under Dex; whereas doses > or = 5.10(-6) M maximally decreased both CT and CGRP mRNA (2- and 9-fold decreases, respectively). The fall under R.A. alone was enhanced when CA-77 cells were cotreated during 6 days with 10(-7) M 1,25-(OH)2D3 (-68% versus -37%). Moreover, the fall in CGRP mRNA (18-fold) of CA-77 cells treated simultaneously with 10(-6) M Dex, 5.10(-6) M R.A. and 10(-7) M 1,25-(OH)2D3 was greater than the decrease (9-fold) observed when the same dose of R.A. blunted the Dex induction. The results obtained by RIA for the CT and CGRP C cell content and release in the culture medium strengthened those observed on both CT and CGRP mRNAs, since a good parallelism was observed between the peptide biosynthesis, secretion and the mRNA levels. Our data suggest that R.A. and 1,25-(OH)2D3 exert a stronger inhibition of the CT gene by a likely coupled action of the two compounds probably via the formation of an heterodimer receptor.