The mineralocorticoid hormone receptor and action in the eye

Local ocular renin-angiotensin-aldosterone system: any connection with intraocular pressure? A comprehensive review

The renin-angiotensin system (RAS) is one of the oldest and most extensively studied human peptide cascades, well-known for its role in regulating blood pressure. When aldosterone is included, RAAS is involved also in fluid and electrolyte homeostasis. There are two main axes of RAAS: (1) Angiotensin (1-7), angiotensin converting enzyme 2 and Mas receptor (ACE2-Ang(1-7)-MasR), (2) Angiotensin II, angiotensin converting enzyme 1 and angiotensin II type 1 receptor (ACE1-AngII-AT1R). In its entirety, RAAS comprises dozens of angiotensin peptides, peptidases and seven receptors. The first mentioned axis is known to counterbalance the deleterious effects of the latter axis. In addition to the systemic RAAS, tissue-specific regulatory systems have been described in various organs, evidence that RAAS is both an endocrine and an autocrine system. These local regulatory systems, such as the one present in the vascular endothelium, are responsible for long-term regional changes. A local RAAS and its components have been detected in many structures of the human eye. This review focuses on the local ocular RAAS in the anterior part of the eye, its possible role in aqueous humour dynamics and intraocular pressure as well as RAAS as a potential target for anti-glaucomatous drugs.KEY MESSAGESComponents of renin-angiotensin-aldosterone system have been detected in different structures of the human eye, introducing the concept of a local intraocular renin-angiotensin-aldosterone system (RAAS).Evidence is accumulating that the local ocular RAAS is involved in aqueous humour dynamics, regulation of intraocular pressure, neuroprotection and ocular pathology making components of RAAS attractive candidates when developing new effective ways to treat glaucoma.

Vascular mineralocorticoid receptor activation and disease

Mineralocorticoid receptor activation in endothelial and smooth muscle cells can promote vascular disease by increasing oxidative stress, promoting inflammation, accelerating vascular stiffness, remodeling, and calcification, altering vessel responsiveness to various vasoactive factors, thus altering vascular tone and blood pressure, and by altering angiogenesis. Here, we review the recent evidence highlighting the impact of vascular mineralocorticoid receptor activation in pathological situations, including kidney injury, vascular injury associated with metabolic diseases, atherosclerosis, cerebral vascular injury during hypertension, vascular stiffening and aging, pulmonary hypertension, vascular calcification, cardiac remodeling, wound healing, inflammation, thrombosis, and disorders related to angiogenic defects in the eye. The possible mechanisms implicating mineralocorticoid receptor activation in various vascular disorders are discussed. Altogether, recent evidence points towards pharmacological mineralocorticoid receptor inhibition as a strategy to treat diseases in which overactivation of the mineralocorticoid receptor in endothelial and/or smooth muscle cells may play a pivotal role.

Irvine-Gass Macular Edema Responding to the Combination of Oral Mineralocorticoid-Receptor Antagonist With Dexamethasone Drops

Long-lasting postoperative macular edema is a therapeutic challenge. The authors report an efficient combination therapy of oral mineralocorticoid receptor antagonists (eplerenone [Inspra; Pfizer, New York City, NY] or spironolactone, 25 mg/day to 50 mg/day) and topical dexamethasone (four times/day and progressive dose tapering) in three refractory cases following complex cataract or retinal detachment surgery. In Case 1, central macular thickness (CMT) decreased from 523 μm to 214 μm and visual acuity (VA) improved from 20/200 to 20/50 during a 6-month period. In Cases 2 and 3, CMT improved from 505 μm to 333 μm and from 438 μm to 316 μm during 5- and 3-month periods, respectively; however, VA remained unchanged (20/100 and 20/200) due to photoreceptor damage. [Ophthalmic Surg Lasers Imaging Retina. 2017;48:936-942.].

The estrogenic retina: The potential contribution to healthy aging and age-related neurodegenerative diseases of the retina

These last two decades have seen an explosion of clinical and epidemiological research, and basic research devoted to envisage the influence of gender and hormonal fluctuations in the retina/ocular diseases. Particular attention has been paid to age-related disorders because of the overlap of endocrine and neuronal dysfunction with aging. Hormonal withdrawal has been considered among risk factors for diseases such as glaucoma, diabetic retinopathy and age-related macular disease (AMD), as well as, for Alzheimer's disease, Parkinson's disease, or other neurodegenerative disorders. Sex hormones and aging have been also suggested to drive the incidence of ocular surface diseases such as dry eye and cataract. Hormone therapy has been approached in several clinical trials. The discovery that the retina is another CNS tissue synthesizing neurosteroids, among which neuroactive steroids, has favored these studies. However, the puzzling data emerged from clinical, epidemiological and experimental studies have added several dimensions of complexity; the current landscape is inherently limited to the weak information on the influence and interdependence of endocrine, paracrine and autocrine regulation in the retina, but also in the brain. Focusing on the estrogenic retina, we here review our knowledge on local 17β-oestradiol (E2) synthesis from cholesterol-based neurosteroidogenic path and testosterone aromatization, and presence of estrogen receptors (ERα and ERβ). The first cholesterol-limiting step and the final aromatase-limiting step are discussed as possible check-points of retinal functional/dysfunctional E2. Possible E2 neuroprotection is commented as a group of experimental evidence on excitotoxic and oxidative retinal paradigms, and models of retinal neurodegenerative diseases, such as glaucoma, diabetic retinopathy and AMD. These findings may provide a framework to support clinical studies, although further basic research is needed.

Hydrocortisone stimulates neurite outgrowth from mouse retinal explants by modulating macroglial activity

PURPOSE

There is mounting evidence that retinal ganglion cells (RGCs) require a complex milieu of trophic factors to enhance cell survival and axon regeneration after optic nerve injury. The authors' goal was to examine the contribution of components of a combination of hormones, growth factors, steroids, and small molecules to creating a regenerative environment and to determine if any of these components modulated macroglial behavior to aid in regeneration.

METHODS

Postnatal day 7 mouse retinal explants embedded in collagen were used as an in vitro model of neurite regeneration. Explants were treated with the culture supplements fetal bovine serum, N2, and G5 and a mixture of G5 and N2 components, designated enhanced N2 (EN2). Explants were evaluated for neurite outgrowth over 7 days in culture. The effects of each treatment were also evaluated on cultured RGCs purified by Thy1 immunopanning. Immunohistochemistry and qPCR analysis were used to evaluate differences in gene expression in the explants due to different treatments.

RESULTS

EN2 stimulated significant neurite outgrowth from explants but not from purified RGCs. Elimination of hydrocortisone (HC) from EN2 reduced the mean neurites per explant by 37%. EN2-treated explants demonstrated increased expression of Gfap, Glul, Glt1, Cntf, Pedf, and VegfA compared with explants treated with EN2 without HC. Subsequent experiments showed that increased expression of Cntf and Glul was critical to the trophic effect of HC.

CONCLUSIONS

These data suggest that the HC in EN2 indirectly contributed to neurite outgrowth by activating macroglia to produce neurotrophic and neuroprotective molecules.

Could mineralocorticoids play a role in the pathophysiology of open angle glaucoma?

Since the pathomechanisms of primary open angle glaucoma are still not defined, different aspects related to this topic have to be discussed and further investigated. Possible candidates are the mineralocorticoids, which are known to lower intraocular pressure. A data search and personal investigations assume a limited role of mineralocorticoids for the development of glaucoma. Specific experiments for a final conclusion are, however, not yet performed.

[Retinal drug targets]

Retinal effects of systemically administered drugs are rare due to the hematoretinal barriers that protect the retina from circulating active principles. However, some compounds may have direct or indirect toxic effects on the retina through direct interaction with a specific receptor or due to their accumulation within pigment of uveal cells. In the latter case, toxicity is dose-dependent and may be observed years after cessation of medication, as observed with antimalarial drugs. Anti-infective and anti-inflammatory agents, particularly glucocorticoids, are currently injected peri- or intraocularly. The mechanisms and the exact toxicity of glucocorticoids on the retina remain poorly understood. More recently, anti-VEGF has been specifically developed for the treatment of retinal diseases. However, the long-term blockade of VEGF on normal retinal physiology should be determined taking into account VEGF and VEGF receptors expression in the normal and pathologic retina. Whilst enormous advances are made in the treatment of retinal diseases, basic research is still required to define more accurately the molecular targets of drugs to improve their benefits and reduce their potential side effects.

Effects of haemodialysis on circulating endothelial progenitor cell count

During haemodialysis (HD) the endothelium is the first organ to sense and to be impaired by mechanical and immunological stimuli. We hypothesized that a single HD session induces mobilization of endothelial progenitor cells (EPCs) and that cardiovascular risk factors may influence this process. We quantified EPCs at different maturational stages (CD34+, CD133+/VEGFR2+) in blood samples from 30 patients, during HD and on the interdialytic day, and in 10 healthy volunteers. Samples were drawn at the start of HD, 1, 2 and 3 h after, at the end of HD and at 24 h on the interdialytic day. Patients were divided into two groups based on a recent risk scoring system (SCORE project): low-risk (LR) and high-risk groups (HR). HD patients showed a significantly reduced basal number of EPCs with respect to healthy volunteers. In contrast, we observed increasing EPCs during HD whereas they diminished on the interdialytic day. The EPC number was directly correlated with HD time progression. The EPC number during HD was increased in the HR group with respect to the LR group. We had a direct correlation between risk score and number of EPCs. Cardiovascular risk factors influenced the mobilization of stem cells from the bone marrow. This feature could be the direct consequence of an augmented request of stem cells to respond to the most important endothelial impairment but could also show a defective capacity of EPCs to home in and repair the sites of vascular injury.

Systemic hypertension and glaucoma: mechanisms in common and co-occurrence

AIMS

To determine whether systemic hypertension and glaucoma might coexist more often than expected, with possible implications for treatment.

METHODS

Case-control study using general practitioner database of patients with glaucoma matched with controls for age and sex.

RESULTS

Hypertension was significantly more common in the 27,080 patients with glaucoma (odds ratio 1.29, 95% confidence intervals 1.23 to 1.36, p<0.001) than in controls. Treatment by oral beta blockade appeared to protect from risk (odds ratio 0.77, 95% CI 0.73 to 0.83, p<0.0001), but oral calcium channel antagonists or angiotensin converting enzyme (ACE) inhibitors did not (odds ratios 1.34, 1.24 to 1.44 and 1.16 1.09-1.24, respectively, p<0.0001 in each case). Oral corticosteroid treatment was associated with enhanced risk (odds ratio 1.78, 1.61 to 1.96).

CONCLUSION

Common pathogenetic mechanisms in ciliary and renal tubular epithelia may explain coincidence of glaucoma and systemic hypertension. The choice of cardiovascular treatment, could substantially influence glaucoma incidence, with beta blockade protecting and ACE inhibitors or calcium channel blockers not affecting underlying risk.

Expression and distribution of the serum and glucocorticoid regulated kinase and the epithelial sodium channel subunits in the human cornea

The sodium transporting capacity of the corneal endothelium is vital for preserving corneal transparency, and has traditionally been attributed to the endothelial pump transporting sodium and bicarbonate across the corneal endothelium, maintaining the cornea in a dehydrated state. Recent studies have shown that the enzyme, serum and glucocorticoid regulated kinase isoform 1 (SGK1), plays a pivotal role in the corticosteroid induction of epithelial sodium transport in tissues such as the distal nephron, through activation of the epithelial sodium channels (ENaC). This study was designed to identify whether these elements were present within the human cornea. In situ hybridisation studies were conducted on paraffin embedded sections from six human eyes, using in-house generated cRNA antisense probes for human SGK1 and ENaC subunits (alpha, beta, gamma), and confirmed expression of SGK1 and all ENaC subunits in the corneal endothelial cytoplasm. Although ENaC subunits were not demonstrated in the corneal epithelium, SGK1 mRNA was identified in the nuclear region of central basal cells of the corneal epithelium, and limbal epithelial cells. Minimal chromagen precipitation was seen in the Bowman's membrane, corneal stroma, or Descemet's membrane. Control experiments consisted of no antisense probe, competition of the labelled antisense cRNA probe by a 60-fold excess unlabelled antisense cRNA, and use of labelled sense cRNA probes, revealing minimal or no hybridisation signal throughout the corneal layers. These data define components of the mineralocorticoid regulatory pathways of sodium transport in human corneal endothelium, and provide evidence for an additional mechanism contributing to corneal transparency and the 'metabolic' sodium pump.

Inhibition of 11beta-hydroxysteroid dehydrogenase type 1 lowers intraocular pressure in patients with ocular hypertension

BACKGROUND

Intraocular pressure (IOP) is maintained by a balance between aqueous humour (AH) production (dependent on sodium transport across a ciliary epithelial bi-layer) and drainage (predominantly through the trabecular meshwork). In peripheral epithelial tissues, sodium and water transport is regulated by corticosteroids and the 11beta-hydroxysteroid dehydrogenase (11beta-HSD) isozymes (11beta-HSD1 activating cortisol from cortisone, 11beta-HSD2 inactivating cortisol to cortisone).

AIM

To analyse expression of 11beta-HSD in the human eye and investigate its putative role in AH formation.

DESIGN

Multipart prospective study, including a randomized controlled clinical trial.

METHODS

The expression of 11beta-HSD1 in normal human anterior segments was evaluated by in situ hybridization (ISH). RT-PCR for 11beta-HSDs, glucocorticoid and mineralocorticoid receptors (GR, MR) was performed on human ciliary body tissue. AH cortisol and cortisone concentrations were measured by radioimmunoassay on specimens taken from patients with primary open-angle glaucoma (POAG) and age-matched controls. Randomized, placebo-controlled studies of healthy volunteers and patients with ocular hypertension (OHT, raised IOP but no optic neuropathy) assessed the effect of oral carbenoxolone (CBX, an inhibitor of 11beta-HSD) on IOP.

RESULTS

ISH defined expression of 11beta-HSD1 in the ciliary epithelium, while RT-PCR analysis of ciliary body tissue confirmed expression of 11beta-HSD1, with additional GR and MR, but not 11beta-HSD2 expression. In both POAG patients and controls, AH concentrations of cortisol exceeded those of cortisone. The CBX-treated healthy volunteers who demonstrated the largest change in urinary cortisol metabolites, indicative of 11beta-HSD1 inhibition, had the greatest fall in IOP. Patients with OHT showed an overall reduction of IOP by 10% following CBX administration, compared to baseline (p<0.0001).

DISCUSSION

CBX lowers IOP in patients with ocular hypertension. Our data suggest that this is mediated through inhibition of 11beta-HSD1 in the ciliary epithelium. Selective and topical inhibitors of 11beta-HSD1 could provide a novel treatment for patients with glaucoma.

Receptor-mediated adrenocorticoid hormone signaling in ocular tissues

Sodium-water balance is causally linked to the functional expression of a number of important ocular tissues, viz. corneal deturgescence, aqueous humor secretion by the iris, hydration of the lens, retinal photoreception, and choriocapillary angiogenesis. The regulation of sodium absorption in the eye is generally believed to be under the control of Na(+),K(+)-activated adenosine triphosphatase, although evidence for this view is at best circumstantial. Contemporary work has shown widespread distribution of the mineralocorticoid hormone receptor and its colocalization with the amiloride-sensitive sodium channel in cells of diverse embryological origins. All available evidence favors the idea that the transcriptional regulation of the apical sodium channel by adrenocorticoids, and not the basolateral sodium pump, is critically important to sodium-water homeostasis in various ocular tissues, in a manner previously believed to be limited exclusively to the epithelial cell in various peripheral organs. Based upon these parameters, models are presented to help in understanding the direction of sodium absorption in a number of ocular tissues. Thus, the regulation of the sodium channel by steroid hormones seems to be a universal feature of the living cell that may have important implications in the understanding and management of normal ocular functions and their modification in human pathology.

Oxysterol 7alpha-hydroxylase (CYP39A1) in the ciliary nonpigmented epithelium of bovine eye

The CYP39A1 oxysterol 7alpha-hydroxylase preferentially catalyzes the 7alpha-hydroxylation of 24-hydroxycholesterol and has been suggested to play a role in the alternative bile acid synthesis pathway in the liver. The presence of CYP39A1 oxysterol 7alpha-hydroxylase has been reported only in the liver. To investigate the physiologic characteristics of the ciliary processes in bovine ocular tissues, we raised a mAb, 42C, against nonpigmented epithelial (NPE) cells, which have tight junctions that act as a blood-aqueous barrier and are involved in producing aqueous humor and maintaining ocular homeostasis. Immunohistochemical analysis showed that 42C antibody reacted intensely with an antigen in the NPE cells of the ciliary processes but not with other ocular tissues. The SDS-PAGE profile of immunoaffinity-purified antigens from bovine ciliary processes showed a predominant protein of molecular mass of 44.0 kDa. The amino acid sequence of this antigenic protein was identical to human CYP39A1 oxysterol 7alpha-hydroxylase. Immunoreactivity with 42C antibody was found only in hepatocytes and ocular tissues. These data suggest a new physiologic function for the CYP39A1 oxysterol 7alpha-hydroxylase in addition to the production of bile acids and provide new insight into the physiologic role of the ciliary NPE cells concerning the metabolism of sterols in the eye.

Ocular disposition and tolerance of ganciclovir-loaded albumin nanoparticles after intravitreal injection in rats

Cytomegalovirus (CMV) infection mainly affects endothelial cells of ocular vessels, optic nerve and the retina, resulting in direct or autoimmune damages, uveoretinitis and disturbed vision. The use of colloidal carriers for the intravitreal delivery of ganciclovir may prolong its residence in the eye, minimizing the opacification observed for macroscopic implants. The aim of this work was to evaluate the ocular toxicity induced by the prolonged presence of ganciclovir-loaded bovine serum albumin nanoparticles after their intravitreal injection. The intraocular disposition of these carriers was also studied by immunochemistry. Two weeks post-injection, a significant amount of nanoparticles remained in the vitreous cavity, mainly in a thin layer overlying the retina and in the area close to the blood aqueoUs barrier. Their prolonged residence in the eve seemed to be well tolerated and the histological evaluation of the retina, mainly the photoreceptor layer, and adjacent tissues revealed the absence of inflammatory reactions or alterations in the tissue architecture (i.e. cellular infiltrations or vascular inflammation). In addition, nanoparticles neither alter the expression and distribution of arrestin and rhodopsin autoantigens nor the mineralocorticoid receptor. In summary, the vision was not affected by autoimmune phenomena or alterations in the behavior of ophthalmic cells due to the intravitreal injection of these nanoparticles.

Immunohistochemical distribution of 11beta-hydroxysteroid dehydrogenase in human eye

11beta-hydroxysteroid dehydrogenase (11beta-HSD) regulates local actions of corticosteroids at glucocorticoid and mineralocorticoid receptors. Corticosteroids are thought to play important roles in ocular function. However, mechanisms of intraocular corticosteroid action are still unclear. Therefore, in this study, we examined the immunohistochemical localization of 11beta-HSD type 1 (11beta-HSD1), 11beta-HSD type 2 (11beta-HSD2), mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) in human ocular tissues from patients (6 months to 78 years of age; n = 10) retrieved from surgical pathology files. Both 11beta-HSD2 and MR immunoreactivity was detected only in non-pigmented epithelium of the ciliary body, but was undetectable in cornea, lens, iris, retina, choroid and sclera, in all the cases examined. GR was detected in all cell types in the human eye. 11beta-HSD1 immunoreactivity was not detected in the human eye in this study. These results suggest that 11beta-HSD2 play an important role in human ocular mineralocorticoid action, such as the production of aqueous humor, in the ciliary body. The widespread expression of GR suggests that glucocorticoids may play an important role in the function and homeostasis of the human eye.

Mineralocorticoid receptor-mediated signaling regulates the ion gated sodium channel in vascular endothelial cells and requires an intact cytoskeleton

The PCR analysis followed by sequence alignment showed that both the mineralocorticoid receptor (MCR) and the epithelial sodium channel (ENaC) genes were expressed in the human vascular endothelial cell line (ECV). The growth and multiplication of the ECV in culture were influenced by both aldosterone and the MCR-specific antagonist ZK 91587. Following double labelled immunofluorescence recorded by confocal microscopy, both the MCR and the ENaC were found to colocalize with the tubulin filaments in ECV cells in situ; no association was observed with cellular actin. ZK 91587 not only eliminated the basal expression, but it also impaired the transactivation of the ENaC gene by aldosterone. The disruption of actin and tubulin by cytochalasin D and colchicine, respectively, resulted in the total elimination of ENaC induction by aldosterone. These studies suggest that (i) the transcriptional regulation of the ENaC gene by the MCR-mediated signalling is not restricted to epithelial cells and requires cytoskeleton integrity in ECV cells in situ, (ii) tubulin may form a new and novel mediator in cell regulation, and (iii) the vascular tone may actually be regulated via transactivation of the ion gated sodium channel in the endothelial cell of the blood vessels under direct, receptor-mediated action of aldosterone.

Epithelial sodium channel and the mineralocorticoid receptor in cultured rat Müller glial cells

Müller glial cells are the major non-neuronal cells of the retina. They are involved in retinal function and exert a profound influence on the function of retinal neurons. We present an in vitro study of the localization of the mineralocorticoid receptor (MCR) and of the epithelial sodium channel (ENaC) in rat Müller glial cells isolated from rat retina, using respectively, a polyclonal antiserum raised against the rat purified MCR, and a rabbit polyclonal antibody against the 14-amino acid (aa) peptide QGLGKGDKREEQGL, which corresponds to the N-terminal region (44-58aa) of the alpha-subunit of the ENaC. In an immunocytochemical study using anti-MCR and anti-ENaC antibodies, the MCR was detected as a protein present in the cytoplasm and in the nucleus, whereas ENaC was detected as a membrane-bound protein. Reverse transcription-polymerase chain reaction (RT-PCR) analysis using specific primers, 5'-CTGCCTTTATGGATGATGGT-3' (sense), 5'-GTTCAGCTCGAAGAAGA-3' (antisense) for ENaC and 5'-AGGCTACCACAGTCTCCCTG-3' (sense) and 5'-GCAGTGTAAAATCTCCAGTC-3' (antisense) for MCR, showed expression of the ENaC and MCR genes in Müller cells. The presence of ENaC and MCR was detected as the predicted bands of 520 bp and 843 bp, respectively. In both cases, 100% identity was observed between the sequences of rat Müller cell (RMC) PCR products and rat kidney. Interestingly, the basal levels of ENaC were increased in vitro by the MCR-specific hormone, aldosterone. Thus, our results strongly suggest that the Müller glial cells may play a role in the regulation of extracellular Na+ concentration, which could be regulated by steroid-mediated sodium uptake.

Demonstration of the mineralocorticoid hormone receptor and action in human leukemic cell lines

We studied the expression of the mineralocorticoid receptor (MCR), and of the amiloride-sensitive sodium channel (ASSC) regulated by the MCR, in human leukemic cell lines. Cell extracts from TF1 (proerythroblastic), HEL (human erythroblastic leukemia) and U937 (myeloblastic) cell line were positive for the ASSC, as a 82 kDa band in Western blots developed with the aid of a polyclonal antibody raised against the peptide QGLGKGDKREEQGL, corresponding to the region 44-58 of the alpha subunit of the epithelial sodium channel (ENaC) cloned from rat colon, linked to KLH. The polyclonal antibody against the MCR revealed a single band of about 102 kDa in extracts from HEL and TF1 cells. The immunofluorescent labelling of the MCR in all cell lines showed a nucleocytoplasmic localization of the receptor but the ASSC was exclusively membrane-bound and these results were confirmed by confocal microscopy. The expression of the MCR in the HEL cells was evident as a predicted band of 843 bp (234 amino acids) in electrophoresis of the PCR product obtained after total RNA had been reverse transcribed and then amplified using the primers 5'-AGGCTACCACAGTCTCCCTG-3' and 5'-GCAGTGTAAAATCTCCAGTC-3' (sense and antisense, respectively). The ENaC was similarly evident with the aid of the primers 5'-CTGCCmATG GATGATGGT-3' (sense) and 5'-GTTCAGCTCGAAGAAGA-3' (antisense) as a predicted band of 520 bp. In both cases, 100% identity was observed between the sequences of the PCR products compared to those from known human sources. The multiplication of the HEL cells was influenced by antagonists (RU 26752, ZK 91587) targeted for specificity to the MCR and this was selectively reversed by the natural hormone aldosterone. These steroids also provoked chromatin condensation in the HEL population. These permit new and novel possibilities to understand the pathobiology of human leukemia and to delineate sodium-water homeostasis in nonepithelial cells.

Paradoxical effects of mineralocorticoids on the ion gated sodium channel in embryologically diverse cells

PCR analysis and Western blotting revealed the expression of the mineralocorticoid receptor (MCR) and the epithelial sodium channel (ENaC) genes at the level of RNA, DNA, and protein in several leukemic cell lines, fibroblasts from human cornea, and epithelial cells from ocular tissues. Following immunofluorescence, the MCR appeared to be primarily nuclear whereas the ENaC was almost exclusively membrane-bound. Paradoxically, the MCR-specific antagonist ZK 91587 actually stimulated the multiplication of human erythroblastic leukemia cells, contrary to the inhibitory effect of the antagonist RU 26752 on the multiplication of corneal fibroblasts; both effects were opposed by aldosterone. In quantitative PCR, both basal and aldosterone-induced levels of ENaC were diminished by ZK 91587 in the corneal fibroblast, in contrast to the stimulation observed in the retinal pigmentary epithelium. Thus, contrary to the existing notions, (a) antimineralocorticoids can act both as agonists and antagonists, and (b) the receptor-mediated action of mineralocorticoids on the sodium channel is not restricted to the epithelial cell.

General overview of mineralocorticoid hormone action

The adrenal cortex elaborates two major groups of steroids that have been arbitrarily classified as glucocorticoids and mineralocorticoids, despite the fact that carbohydrate metabolism is intimately linked to mineral balance in mammals. In fact, glucocorticoids assured both of these functions in all living cells, animal and photosynthetic, prior to the appearance of aldosterone in teleosts at the dawn of terrestrial colonization. The evolutionary drive for a hormone specifically designed for hydromineral regulation led to zonation for the conversion of 18-hydroxycorticosterone into aldosterone through the catalytic action of a synthase in the secluded compartment of the adrenal zona glomerulosa. Corticoid hormones exert their physiological action by binding to receptors that belong to a transcription factor superfamily, which also includes some of the proteins regulating steroid synthesis. Steroids stimulate sodium absorption by the activation and/or de novo synthesis of the ion-gated, amiloride-sensitive sodium channel in the apical membrane and that of the Na+/K+-ATPase in the basolateral membrane. Receptors, channels, and pumps apparently are linked to the cytoskeleton and are further regulated variously by methylation, phosphorylation, ubiquination, and glycosylation, suggesting a complex system of control at multiple checkpoints. Mutations in genes for many of these different proteins have been described and are known to cause clinical disease.

Effects of glucocorticoids and mineralocorticoids on proliferation and maturation of human peripheral blood stem cells

It has been shown that hematopoietic progenitors can be expanded ex vivo in the presence of various cytokine combinations. Glucocorticoids (GC) are involved in the self-renewal of erythroid progenitors in chicken. To see whether GC have a similar effect on hematopoiesis in humans, CD34(+) peripheral blood stem cells were cultured in serum free medium in the presence of a GC, triamcinolone acetonide. However, our results demonstrate an inhibition of both erythroid and granulocyte-macrophage (GM) proliferation and a modification of erythroid colony morphology. Furthermore, RU38486 (Mifepristone), a potent GC antagonist, was unable to reverse the inhibitory effect of triamcinolone acetonide. We also identified and characterized another steroid subfamily, the mineralocorticoid (MC) subfamily, in human PB CD34(+) cells. The MC, aldosterone, significantly enhanced GM colony formation and diminished the erythroid colony number. Neither of effects were inhibited by ZK91587, an antagonist specific to the MC receptor (MCR). In contrast, ZK91587 reversed the stimulatory effect of deoxycorticosterone on GM colony formation. Cytoplasmic staining for MCR was observed in CD34(+) cells incubated with a polyclonal antiserum raised against human MCR. To our knowledge, this is the first demonstration of the presence of MCR in human PB CD34(+) cells.

Immunochemical analysis of the sodium channel in rodent and human eye

The presence of the amiloride-sensitive sodium channel (ASSC) in ocular tissues was studied with the aid of a polyclonal antiserum raised against the 14 amino acid peptide QGLGKGDKREEQGL. This sequence corresponds to the region 44-58 of the alpha subunit of the channel, termed ENaC, cloned from rat colon. The antibody titers, measured by the ELISA technique, rose to 1&ratio;2560 4 weeks after immunization, and this bleed was used in all subsequent experiments. Immunoblotting with the polyclonal anti-alphaENaC serum, revealed a major band of 82-86 kDa in extracts prepared from whole bovine or rat retina; a minor component of 92 kDa in the extract from bovine ciliary body may represent a glycosylated species. Immunohistochemistry, using the alphaENaC-specific antiserum, revealed strong fluorescence in specific areas of the rat and human eye. Pronounced labelling was observed in the epithelial cell layer of the retina, the lens, as well as both the pigmented and the nonpigmented epithelium of the ciliary body and the iris. All of the cell layers (epithelium, endothelium and fibroblasts) in the cornea, the blood vessels in the iris, and iris epithelium, were also strongly immunopositive. The somatic body of the photoreceptor cells (cones and rods) in the inner and outer segments could be traced to forming a synapse in both the internal and external portions of the internal nuclear layer. The bipolar cells and ganglia in the neuronal compartment also exhibited occasional immunofluorescence. The method of fixation and the source of the tissue were important parameters for the immunochemical localization of the ENaC. The resolution was very poor when rat eye was fixed in Bouin's solution but this method was satisfactory for human tissues. For rat eye, optimum resolution was obtained with AMeX fixation. This widespread distribution of the ENaC generally colocalizes with the previously observed immunopositivity for the mineralocorticoid receptor such that steroid hormone-mediated ion regulation would appear to add a new parameter to the functional expression of ocular tissues.

Enhanced activation of the mineralocorticoid receptor in genetically hypertensive rats

The relative abundance and availability of the mineralocorticoid receptor (MCR) appeared to be similar in the heart, kidney and ocular tissues of the genetically hypertensive SHR and normotensive WKY rats by a number of criteria including Western blotting, immunoprecipitation, dot blot analysis, and immunohistochemistry. On the other hand, the activation of the MCR, as judged by binding to DNA cellulose, was significantly enhanced in the hearts and kidneys of 14 week-old, hypertensive, SHR rats compared to the normotensive WKY animals. The activation of the renal MCR was elevated in the SHR strain even at the age of six weeks when the tail arterial pressure was statistically identical to that of the WKY strain. Thus, precocious receptor activation may represent a primary lesion leading to hypertension in the SHR strain, thereby providing a new model to elucidate the hypertensive state.

The antiglucocorticoid action of mifepristone

Glucocorticoid hormones influence the physiological activity of almost all cell types in the mammal. This is accomplished via a soluble receptor that, in the presence of an appropriate steroid, modifies the activity of RNA polymerase by binding to the site where different factors assemble for the initiation of cell transcription. The development of antiglucocorticoids has permitted the molecular elucidation of a number of underlying events. Contrary to the classical view, it is now clear that the affinity, stability and activability of the glucocorticoid receptor in the presence of a steroid are cell- and/or tissue-dependent events. The antiglucocorticoid RU 38486 can even activate transcription by binding to sites distinct from those that process transactivation by the agonist. Furthermore, glucocorticoids can sometimes activate the mineralocorticoid receptor, whereas mineralocorticoids can bind the glucocorticoid receptor. Since mifepristone is devoid of adverse toxicity, it has been used for the paraclinical diagnosis of the hypothalamus-pituitary-adrenal axis in normal volunteers, subjects with disorders of the behaviour, and the treatment of Cushing's disease. However, the whole spectrum of cell-specific processes that are antagonized by RU 38486 suggests wide ranging possibilities in the eventual application of antigluco-corticoids.