Aim: 

To characterize the subcellular localization and transcriptional activity of mineralocorticoid and glucocorticoid receptors (MR and GR) in the mouse heart under various corticosteroid circulating levels.

Methods: 

The localization of MR in cardiac myocytes was studied by subcellular fractionation followed by western blots or by immunolocalization in fixed tissue. Physical interaction of MR with specific DNA promoters was studied by chromatin immunoprecipitaton (ChIP) using samples from fixed mouse heart. ChIP products were analyzed by quantitative PCR. Manipulation of circulating corticosteroid levels was performed by bilateral adrenalectomy (ADX) and/or pharmacological treatment using osmotic minipumps. All animal protocols were approved by the University Ethics Committee and followed international regulations.

Results: 

MR was mainly found in nuclear fractions of control tissue. ADX did not change subcellular localization, but the use of MR antagonist spironolactone caused a release of MR from chromatin. ChIP experiments demonstrated specific interactions of both MR and GR with DNA hormone response elements in the promoters of several corticosteroid-regulated genes.

Conclusions: 

Our results show that MR is mainly a nuclear factor in the normal mouse heart, where it interacts with specific gene promoters independently of corticosteroids.