The mineralocorticoid receptor (MR) is a ligand-activated transcription factor that belongs to the nuclear receptor family, which arose in multicellular animals along with other vertebrate steroid receptors: the glucocorticoid receptor (GR), progesterone receptor (PR), androgen receptor (AR) and estrogen receptor (ER) [1–6]. The classical function of the MR in humans and other terrestrial vertebrates is to maintain electrolyte balance by regulating sodium and potassium transport in epithelial cells in the kidney and colon [7–12]. In addition, the MR also has important physiological functions in many other tissues, including brain, heart, skin and lungs [11, 13–22].
The human MR sequence reported in 1987 by Arriza et al [1] contains 984 amino acids (MR-984). Inspection of the MR sequence revealed that, like other steroid receptors, the human MR is composed of four modular functional domains: a large amino-terminal domain (NTD) of about 600 amino acids, followed in the center by a DNA-binding domain (DBD) of about 65 amino acids, followed by a small hinge domain of about 60 amino acids that connected to the ligand-binding domain of about 250 amino acids at the C-terminus, where aldosterone, cortisol and other corticosteroids bind to activate transcription [1, 2, 23–25].
Analysis of the human MR sequence by Arriza et al. [1] revealed that the MR was closely related to the glucocorticoid receptor (GR), which was consistent with evidence that some corticosteroids, such as cortisol, corticosterone and 11-deoxycorticosterone were ligands for both the MR and GR [13, 15, 21, 24, 26] and that aldosterone, cortisol, corticosterone and 11-deoxycorticosterone have similar binding affinity for human MR [1, 21, 26–28]. Activation of the MR by cortisol and corticosterone, two steroids that are ligands for the GR [28–30], is consistent with the evolution of the GR and MR from a common ancestral corticoid receptor (CR) in a cyclostome (jawless fish) that evolved about 550 million years ago at the base of the vertebrate line [24, 31–37].
Knowledge about the human MR family expanded in 1995, when Bloem et al. [38] cloned a human MR with 988 amino acids (MR-988). This human MR had four additional amino acids in the DBD due to alternative splicing between exons 3 and 4 of an in-frame insertion of 12 bp encoding Lys, Cys, Ser, Trp (KCSW) [39, 40] (Fig. 1). As shown in Fig. 1, the DBDs in terrestrial vertebrate MRs are highly conserved.
Thus, humans were found to contain two almost identical MR transcripts, in which their DBDs differ by only four amino acids. The number of distinct human MR genes expanded after a BLAST analysis of GenBank with the human MR sequence identified differences at codons 180 and 241 in the NTD of MR-984 and MR-988 in humans [41] (Fig. 2).
Human MR-984a, cloned by Arriza et al [1], has been studied for transcriptional activation by aldosterone and other corticosteroids [1, 26, 27, 29, 30]. However, it is not known if the KCSW insert in the DBD of human MR affects its transcriptional activation by corticosteroids, and if so, how? To answer these questions, we compared transcriptional activation by corticosteroids of human MR-984 to human MR-988 containing KCSW in the DBD transfected into in HEK293 cells in the presence of the TAT3 promoter. Here we report that compared to human MR-984, human MR-988, has about 50–90% increased transcriptional activation by aldosterone, cortisol, and other corticosteroids, when these MRs are transfected into HEK293 cells. The physiological responses to corticosteroids in humans with MR genes containing KCSW in the DBD and with differences at codons 180 and 241 in their NTD warrant investigation.