title: The mineralocorticoid receptor is a constitutive nuclear factor in cardiomyocytes due to hyperactive nuclear localization signals.
abstract: The mineralocorticoid receptor (MR), a member of the nuclear receptor family, mediates the action of aldosterone in target epithelia, enhancing sodium reabsorption. In addition, MR may have other physiological functions in nonepithelial tissues. Altered expression or inappropriate activation of cardiac MR is directly linked to the development of cardiac fibrosis, and MR blockade is beneficial for the treatment of heart failure. However, the physiological role, activation status, and target genes of MR in the heart are poorly known. Because ligand-free steroid receptors are typically cytoplasmic and translocate to the nucleus upon ligand binding, we examined the subcellular localization of MR under different corticosteroid levels using subcellular fractionation and immunostaining. Our results demonstrate that MR is a chromatin-bound factor in mouse left ventricle and in a cultured model of cardiomyocytes, HL-1 cells, regardless of circulating corticosteroid levels. Immunohistochemical localization of MR in human heart confirms the subcellular localization pattern. Mutation of nuclear localization signals (NLSs) demonstrates that MR constitutive nuclear localization mainly depends on the synergistic contribution of NLS0 and NLS1. Constitutive nuclear localization in HL-1 cells can be reverted by cotransfection of heat shock protein 90. Heat shock protein 90 expression levels in the mouse heart and HL-1 cells are lower than those found in other tissues, suggesting that low levels of cochaperones render MR NLSs hyperactive in cardiomyocytes. Even though MR is constitutively nuclear, corticosteroids still control the transactivation properties of the receptor in a model promoter, although other MR ligand-independent activities cannot be excluded.
authors.affiliation: Departamento de Fisiología, Facultad de Medicina, Universidad de La Laguna, La Laguna 38071, Spain.
authors.forename: Iván
authors.initials: I
authors.lastname: Hernández-Díaz
authors.forename: Teresa
authors.initials: T
authors.lastname: Giraldez
authors.forename: María Rosa
authors.initials: MR
authors.lastname: Arnau
authors.forename: Veronique A J
authors.initials: VA
authors.lastname: Smits
authors.forename: Frederic
authors.initials: F
authors.lastname: Jaisser
authors.forename: Nicolette
authors.initials: N
authors.lastname: Farman
authors.forename: Diego
authors.initials: D
authors.lastname: Alvarez de la Rosa
mesh_terms: D000305:Adrenal Cortex Hormones / Q000494:pharmacology; D000818:Animals; D019556:COS Cells; D002467:Cell Nucleus / Q000187:drug effects / Q000378:metabolism*; D002478:Cells, Cultured; D002522:Chlorocebus aethiops; D047369:Chromatin Immunoprecipitation; D006801:Humans; D051379:Mice; D008810:Mice, Inbred C57BL; D009206:Myocardium / Q000378:metabolism; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D019913:Nuclear Localization Signals / Q000378:metabolism / Q000502:physiology*; D021381:Protein Transport; D018161:Receptors, Mineralocorticoid / Q000737:chemistry / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D015533:Transcriptional Activation / Q000187:drug effects; D014162:Transfection
chemical_list: D000305:Adrenal Cortex Hormones; D019913:Nuclear Localization Signals; D018161:Receptors, Mineralocorticoid