Received for publication June 5, 2006.
Revised July 12, 2006.
Accepted for publication August 4, 2006.

Calpain mediates the dioxin-induced activation and down-regulation of the aryl hydrocarbon receptor

Abstract

The aryl hydrocarbon receptor (AhR) is a ligand-activated basic-helix-loop-helix (bHLH) transcription factor that binds polyaromatic hydrocarbons (PAH), such as dioxin, and mediates their toxicity. Binding of PAH to AhR in the cytoplasm triggers a poorly defined transformation step of the receptor into a nuclear transcription factor. In this study, we show that the calcium-dependent cysteine protease, calpain, plays a major role in the ligand-induced transformation and signaling of AhR. Fluorescence imaging measurements showed that TCDD treatment elevates intracellular calcium, providing the trigger for calpain activation, as measured towards BOC-LM-CMAC, a calpain-specific substrate. Inhibition of calpain activity by the MDL28170 blocked the TCDD-induced nuclear translocation of AhR in Hepa1c1c7 mouse hepatoma cell line. Treatment of the human metastatic breast carcinoma cell line MT-2 with MDL28170 and PD 150606, two calpain-selective inhibitors, completely abolished the TCDD-induced transactivation of AhR as assessed by transcription of CYP1A1 gene. Previous studies have established that following TCDD-induced transactivation, the AhR undergoes a massive depletion; we show here that selective calpain inhibitors can block this step, suggesting that the ligand-induced down-regulation of the AhR is calpain-dependent. The data presented support a major role for calpain in the AhR transformation, transactivation and subsequent down-regulation, and provide a possible explanation for many of the reported phenomena of ligand-independent activation of AhR.

Key words: Regulation of gene expression, Regulation - transcriptional, Ah receptor

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