Down-regulation of cystic fibrosis transmembrane conductance regulator gene expression by agents that modulate intracellular divalent cations

In cystic fibrosis (CF), epithelial cells are unable to normally up-regulate apical membrane Cl^- secretion in response to agents which increase cyclic AMP, but they do increase Cl^- secretion in response to increases in intracellular Ca²⁺. Since intracellular divalent cations regulate the expression of many genes, we hypothesized that mobilization of intracellular Ca²⁺ and/or other divalent cations might modulate not only Ca²⁺-dependent Cl^- channels but also cystic fibrosis transmembrane conductance regulator (CFTR) gene expression. To evaluate this concept, HT-29 human colon carcinoma cells were cultured under various conditions designed to manipulate intracellular divalent cation concentrations and CFTR gene expression was quantified at the levels of transcription, mRNA accumulation, mRNA half-life, and protein. Exposure to the divalent cation ionophores A23187 and ionomycin (agents which increase intracellular divalent cation concentrations) caused dose- and time-dependent reductions of CFTR mRNA levels, which could be blocked by the use of Ca²⁺- and Mg²⁺-free media. Ionophore-induced CFTR gene modulation was also observed with T84 human colon carcinoma cells and freshly isolated normal human bronchial epithelial cells. Incubation of HT-29 cells with thapsigargin, an agent that releases Ca²⁺ from intracellular stores, or in medium containing increased extracellular concentrations of Ca²⁺ or Mg²⁺ also caused down-regulation of CFTR mRNA levels. Transcription run-on analysis showed that, parallel with the decrease in CFTR mRNA levels, A23187 reduced the rate of transcription of the CFTR gene, while CFTR mRNA transcript half-life was unaffected. Consistent with the down-regulation of CFTR gene expression, CFTR protein levels also decreased after exposure to A23187. Thus, despite the independence of Ca²⁺-dependent Cl^- channels and cyclic AMP-dependent CFTR-related Cl^- channels in epithelial cells, increases in intracellular divalent cation concentrations down-regulate the expression of the CFTR gene at the transcriptional level, with consequent decreases in CFTR mRNA and protein.