Mineralocorticoid hormone signaling regulates the 'epithelial sodium channel' in fibroblasts from human cornea

Altered Plasma Membrane Lipid Composition in Hypertensive Neutrophils Impacts Epithelial Sodium Channel (ENaC) Endocytosis

Biological membranes are composed of a lipid bilayer with embedded proteins, including ion channels like the epithelial sodium channel (ENaC), which are critical for sodium homeostasis and implicated in arterial hypertension (HTN). Changes in the lipid composition of the plasma membrane can significantly impact cellular processes related to physiological functions. We hypothesized that the observed overexpression of ENaC in neutrophils from HTN patients might result from alterations in the structuring domains within the plasma membrane, disrupting the endocytic processes responsible for ENaC retrieval. This study assessed the structural lipid composition of neutrophil plasma membranes from HTN patients along with the expression patterns of key elements regulating ENaC at the plasma membrane. Our findings suggest alterations in microdomain structure and SGK1 kinase activity, which could prolong ENaC presence on the plasma membrane. Additionally, we propose that the proteasomal and lysosomal degradation pathways are insufficient to diminish ENaC presence at the plasma membrane in HTN. These results highlight the importance of understanding ENaC retrieval mechanisms and suggest that targeting these mechanisms could provide insights for developing drugs to prevent and treat HTN.

Microenvironment mesenchymal cells protect ovarian cancer cell lines from apoptosis by inhibiting XIAP inactivation

Epithelial ovarian carcinoma is characterized by high frequency of recurrence (70% of patients) and carboplatin resistance acquisition. Carcinoma-associated mesenchymal stem cells (CA-MSC) have been shown to induce ovarian cancer chemoresistance through trogocytosis. Here we examined CA-MSC properties to protect ovarian cancer cells from carboplatin-induced apoptosis. Apoptosis was determined by Propidium Iodide and Annexin-V-FITC labelling and poly-ADP-ribose polymerase cleavage analysis. We showed a significant increase of inhibitory concentration 50 and a 30% decrease of carboplatin-induced apoptosis in ovarian cancer cells incubated in the presence of CA-MSC-conditioned medium (CM). A molecular analysis of apoptosis signalling pathway in response to carboplatin revealed that the presence of CA-MSC CM induced a 30% decrease of effector caspases-3 and -7 activation and proteolysis activity. CA-MSC secretions promoted Akt and X-linked inhibitor of apoptosis protein (XIAP; caspase inhibitor from inhibitor of apoptosis protein (IAP) family) phosphorylation. XIAP depletion by siRNA strategy permitted to restore apoptosis in ovarian cancer cells stimulated by CA-MSC CM. The factors secreted by CA-MSC are able to confer chemoresistance to carboplatin in ovarian cancer cells through the inhibition of effector caspases activation and apoptosis blockade. Activation of the phosphatidylinositol 3-kinase (PI3K)/Akt signalling pathway and the phosphorylation of its downstream target XIAP underlined the implication of this signalling pathway in ovarian cancer chemoresistance. This study reveals the potentialities of targeting XIAP in ovarian cancer therapy.

Ovarian ascites-derived Hospicells promote angiogenesis via activation of macrophages

Within the microenvironment, Carcinoma-associated mesenchymal stem cells (Hospicells) are able to influence ovarian tumor development via, among others, the facilitation of angiogenesis in the tumor site allowing an accelerated tumor growth. We demonstrate the presence of a chemotactism between endothelial cells and Hospicells, and a cell line specific increased secretion of pro-angiogenic cytokines such as IL-6, IL-8 and VEGF from ovarian adenocarcinoma cells. Hospicells are also able to attract and activate macrophages to a M2 phenotype and allow them to secrete a huge quantity of pro-angiogenic cytokines, favorable to tumor progression of all the associated ovarian adenocarcinoma cells tested.

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.

Characterization of active ion transport across primary rabbit corneal epithelial cell layers (RCrECL) cultured at an air-interface

Previously, we reported the development of a primary culture model of tight rabbit corneal epithelial cell layers (RCrECL) characterizing bioelectric parameters, morphology, cytokeratin, and passive permeability. In the present study, we specifically evaluated the active ion transport processes of RCrECL cultured from either pigmented or albino rabbits. Primary cultured RCrECL were grown at an air-interface on Clear-Snapwells precoated with collagen/fibronectin/laminin and mounted in a modified Ussing-type chamber for the evaluation of their active ion transport processes under short-circuited conditions. Contribution of active Na(+) and Cl(-) transport to overall short-circuit current (I(sc)) was evaluated by removing Na(+) and Cl(-), respectively, from bathing fluids of RCrECL and measurements of net fluxes of Na(+) and Cl(-) using (22)Na and (36)Cl, respectively. Amiloride and benzamil were used to determine the role of apical Na(+)-channel activities to net Na(+) fluxes. N-phenylanthranilic acid (NPAA), ouabain, BaCl(2) and bumetanide were used to determine the role of basolateral Na,K-ATPase, apical Cl(-)-channel, and basolateral K(+)-channel and Na(+)(K(+))2Cl(-)-cotransporter activities, respectively, in active ion transport across RCrECL. I(sc) of RCrECL derived from pigmented rabbits was comprised of 64+/-2% and 44+/-5% for active Na(+) and Cl(-) transport, respectively, consistent with net Na(+) absorption and Cl(-) secretion of 0.062+/-0.006 and 0.046+/-0.008 muEq/cm(2)/hr estimated from radionuclide fluxes. Apical amiloride and benzamil inhibited I(sc) by up to approximately 50% with an IC(50) of 1 and 0.1 microm, respectively, consistent with participation of apical epithelial Na(+)-channels to net Na(+) absorption across RCrECL cultured from pigmented rabbits. Addition of ouabain to the basolateral, NPAA to the apical, BaCl(2) to the basolateral and bumetanide to basolateral fluid decreased I(sc) by 86+/-1.5%, 53+/-3%, 18+/-1.8% and 13+/-1.9% in RCrECL cultured from pigmented rabbits, while 85+/-0.7%, 36+/-1.6%, 38+/-1.8% and 15+/-3.5% decreases are observed for RCrECL from albino rabbits, respectively. Air-interface cultured RCrECL from either pigmented or albino rabbits exhibited active ion transport properties similar to those present in excised tissues. This primary culture system may be a reliable in-vitro model for mechanistic characterization of corneal epithelial function and regulation of transport properties.

11beta-hydroxysteroid dehydrogenase type 1: a tissue-specific regulator of glucocorticoid response

11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) interconverts inactive cortisone and active cortisol. Although bidirectional, in vivo it is believed to function as a reductase generating active glucocorticoid at a prereceptor level, enhancing glucocorticoid receptor activation. In this review, we discuss both the genetic and enzymatic characterization of 11beta-HSD1, as well as describing its role in physiology and pathology in a tissue-specific manner. The molecular basis of cortisone reductase deficiency, the putative "11beta-HSD1 knockout state" in humans, has been defined and is caused by intronic mutations in HSD11B1 that decrease gene transcription together with mutations in hexose-6-phosphate dehydrogenase, an endoluminal enzyme that provides reduced nicotinamide-adenine dinucleotide phosphate as cofactor to 11beta-HSD1 to permit reductase activity. We speculate that hexose-6-phosphate dehydrogenase activity and therefore reduced nicotinamide-adenine dinucleotide phosphate supply may be crucial in determining the directionality of 11beta-HSD1 activity. Therapeutic inhibition of 11beta-HSD1 reductase activity in patients with obesity and the metabolic syndrome, as well as in glaucoma and osteoporosis, remains an exciting prospect.

On the mechanism of fluid transport across corneal endothelium and epithelia in general

The mechanism by which fluid is transported across epithelial layers is still unclear. The prevalent idea is that fluid traverses these layers transcellularly, driven by local osmotic gradients secondary to electrolyte transport and utilizing the high osmotic permeability of aquaporins. However, recent findings that some aquaporin knockout mice epithelia transport fluid sow doubts on local osmosis. This review discusses recent evidence in corneal endothelium that points instead to electro-osmosis as the mechanism underlying fluid transport. In this concept, a local recirculating electrical current would result in electro-osmotic coupling at the level of the intercellular junctions, dragging fluid via the paracellular route. The text also mentions possible mechanisms for apical bicarbonate exit from endothelial cells, and discusses whether electro-osmosis could be a general mechanism.

Regulation of the epithelial Na+ channel by cytosolic ATP

The epithelial Na+ channel (ENaC), composed of three subunits (alphabetagamma), is expressed in various Na(+)-absorbing epithelia and plays a critical role in salt and water balance and in the regulation of blood pressure. By using patch clamp techniques, we have examined the effect of cytosolic ATP on the activity of the rat alphabetagammaENaC (rENaC) stably expressed in NIH-3T3 cells and in Madin-Darby canine kidney epithelial cells. The inward whole-cell current attributable to rENaC activity ran down when these cells were dialyzed with an ATP-free pipette solution in the conventional whole-cell voltage-clamping technique. This run down was prevented by 2 mM ATP (but not by AMP or ADP) in the pipette solution or by the poorly or non-hydrolyzable analogues of ATP (adenosine 5'-O-(thiotriphosphate) and adenosine 5'-(beta,gamma-imino)triphosphate) in both cell lines, suggesting that protection from run down was mediated through non-hydrolytic nucleotide binding. Accordingly, we demonstrate binding of ATP (but not AMP) to alpharENaC expressed in Madin-Darby canine kidney cells, which was inhibited upon mutation of the two putative nucleotide-binding motifs of alpharENaC. Single channel analyses indicated that the run down of currents observed in the whole-cell recording was attributable to run down of channel activity, defined as NPo (the product of the number of channels and open probability). We propose that this novel ATP regulation of ENaC may be, at least in part, involved in the fine-tuning of ENaC activity under physiologic and pathophysiologic conditions.

Mineralocorticoid hormones exert dramatic effects on pluripotent human stem cell progeny

The authors studied mineralocorticoid receptor (MCR)-mediated effects of steroids on CD34(+) progenitor cells. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis showed the presence of mRNA for both the MCR and the alpha subunit of the epithelial sodium channel, a member of the amiloride-sensitive sodium channel (ASSC) superfamily, in human CD41(+) megacaryoblastic cells derived from cultured bone marrow CD34(+) isolates, as well as in the human erythromegakaryoblastic leukemia (HEL) cell line. Immunofluorescence also revealed the presence of both the MCR and ASSC in circulating CD34(+) and medullar CD41(+) megacaryoblastic cells, the former as a nucleocytoplasmic protein and the latter as a membrane-bound protein, as expected from earlier studies using MCR-specific targets. In a selective medium, the formation of erythrocyte burst-forming units, and of the granulocyte-macrophage colony-forming units, by circulating CD34(+) cells was influenced by the agonists deoxycorticosterone and aldosterone, as well as by the antagonists RU 26752 and ZK 91587, targeted for the MCR. The multiplication of the leukemic HEL progeny, derived from CD41(+) cells, was similarly altered by these steroids targeted for the MCR. In contrast, in the optimal growth medium, the multiplication, and colony formation by bone marrow CD34(+) progenitor cells were not altered by either aldosterone or ZK 91587. These and other studies reveal that the receptor-mediated action of mineralocorticoids may influence the functional maturation of the hematopoietic progenitor lineage, contrary to the classical notion where the mineralotropic effect would be a unique feature of the epithelial cell.

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.

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.