Cell-specific glucocorticoid repression of calcitonin/calcitonin gene-related peptide transcription. Localization to an 18-base pair basal enhancer element

Abstract

We have investigated the mechanisms underlying cell-specific glucocorticoid repression of calcitonin/calcitonin gene-related peptide (CGRP) gene expression. Treatment with the synthetic glucocorticoid dexamethasone has been shown to decrease mRNA levels in the 44-2C thyroid C cell line. Nuclear run-on assays showed that dexamethasone repressed transcription 2-3-fold in 44-2C cells. In contrast, dexamethasone stimulated calcitonin/CGRP transcription 4-6-fold in the CA77 thyroid C cell line. Transient transfection assays were used to map repression of reporter gene activity in 44-2C cells to a neuroendocrine cell-specific enhancer located between -920 and -1125 base pairs (bp). Within this region, an 18-bp element was found that conferred both full basal enhancer activity and dexamethasone-dependent repression in 44-2C cells. The 18-bp region contains possible binding sites for AP-1 and helix-loop-helix transcription factors as well as a glucocorticoid receptor half-site. Colocalization of repression and enhancer activity was then investigated in other cell lines. In CA77 cells, while the 920-1125 region strongly enhanced transcription, the 18-bp region conferred only partial activation and dexamethasone had little effect on reporter gene activity. Dexamethasone did not repress the calcitonin/CGRP activity in the heterologous HeLa and Rat1 fibroblast cell lines. These results suggest that glucocorticoids repress transcription of the calcitonin/CGRP gene by inhibiting cell-specific transcription factor activity.