Evidence for modulation of pericryptal sheath myofibroblasts in rat descending colon by transforming growth factor beta and angiotensin II

Genomic and rapid effects of aldosterone: what we know and do not know thus far

Aldosterone is the most known mineralocorticoid hormone synthesized by the adrenal cortex. The genomic pathway displayed by aldosterone is attributed to the mineralocorticoid receptor (MR) signaling. Even though the rapid effects displayed by aldosterone are long known, our knowledge regarding the receptor responsible for such event is still poor. It is intense that the debate whether the MR or another receptor-the "unknown receptor"-is the receptor responsible for the rapid effects of aldosterone. Recently, G protein-coupled estrogen receptor-1 (GPER-1) was elegantly shown to mediate some aldosterone-induced rapid effects in several tissues, a fact that strongly places GPER-1 as the unknown receptor. It has also been suggested that angiotensin receptor type 1 (AT1) also participates in the aldosterone-induced rapid effects. Despite this open question, the relevance of the beneficial effects of aldosterone is clear in the kidneys, colon, and CNS as aldosterone controls the important water reabsorption process; on the other hand, detrimental effects displayed by aldosterone have been reported in the cardiovascular system and in the kidneys. In this line, the MR antagonists are well-known drugs that display beneficial effects in patients with heart failure and hypertension; it has been proposed that MR antagonists could also play an important role in vascular disease, obesity, obesity-related hypertension, and metabolic syndrome. Taken altogether, our goal here was to (1) bring a historical perspective of both genomic and rapid effects of aldosterone in several tissues, and the receptors and signaling pathways involved in such processes; and (2) critically address the controversial points within the literature as regarding which receptor participates in the rapid pathway display by aldosterone.

Osteopontin knockout does not influence the severity of rectal damage in a preclinical model of radiation proctitis in mice

BACKGROUND

Radiation damage to the normal gut is a dose-limiting factor in the application of radiation therapy to treat abdominal and pelvic cancers. All tissue cell types react in concert to orchestrate an acute inflammatory reaction followed by a delayed chronic scarring process. Osteopontin (OPN) is a matricellular protein known to be involved in various physiological but also pathological processes such as tissue inflammation and fibrosis.

AIMS

The aim of our study was to determine whether OPN knockout influences the severity of radiation proctitis and to investigate the role of OPN in the development of radiation-induced gut damage.

RESULTS

Here we show that human radiation proctitis is associated with increased immunostaining of the intracellular and extracellular/matrix-linked isoforms of OPN. Moreover, endothelial cells in vitro and rectal tissue in a preclinical model of radiation proctitis in mice both respond to radiation exposure by a sustained increase in OPN mRNA and protein levels. Genetic deficiency of OPN did not influence radiation-induced rectal damage and was associated with significantly decreased animal survival. The acute and late radiation injury scores were similar in OPN-null mice compared with their control littermates.

CONCLUSION

This study shows that in our model and given the pleiotropic actions of OPN in tissue inflammation and fibrosis, further studies are necessary to understand the precise roles of OPN in radiation-induced proctitis and to determine whether OPN is a useful therapeutic tool in prevention of radiation-induced intestinal tissue injury.

Vasopressin regulation of epithelial colonic proliferation and permeability is mediated by pericryptal platelet-derived growth factor A

Arginine vasopressin (AVP) has trophic effects on the rat distal colon, increasing the growth of pericryptal myofibroblasts and reducing the colonic crypt wall permeability. This study aimed to reproduce in vitro the effects of AVP observed in vivo using cultures of human CCD-18Co myofibroblasts and T84 colonic epithelial cells. Proliferation of myofibroblasts was quantified by bromodeoxyuridine incorporation; the expression of platelet-derived growth factor A (PDGFA), platelet-derived growth factor B, epidermal growth factor, transforming growth factor-β and vascular endothelial growth factor was measured by PCR and the expression of epithelial junction proteins by Western blot. Arginine vasopressin stimulated myofibroblast proliferation and the expression of PDGFA without affecting the expression of platelet-derived growth factor B, epidermal growth factor, transforming growth factor-β or vascular endothelial growth factor. These effects were prevented when AVP receptor inhibitors were present in the medium. Pre-incubation of CCD-18Co cells with anti-PDGF antibody or with an inhibitor of the PDGF receptor abolished the effects of AVP. When colonocytes were incubated with medium obtained from myofibroblasts incubated with AVP, both cell proliferation and the expression of epithelial junction proteins increased; however, direct incubation of colonocytes with AVP did not modify these variables. These results demonstrate that AVP stimulates myofibroblast proliferation and induces PDGFA secretion, implying that PDGFA mediates local myofibroblast proliferation by an autocrine feedback loop and regulates epithelial proliferation and permeability by a paracrine mechanism.

Aldosterone induces myofibroblast EGF secretion to regulate epithelial colonic permeability

In vivo studies show that raised aldosterone (Aldo) during low-Na adaptation regulates the growth of pericryptal myofibroblasts and reduces the permeability of the colonic epithelium. The aim of this study was to reproduce in vitro the in vivo condition of increased Aldo using human CCD-18Co myofibroblasts and T84 colonic epithelial cells to measure myofibroblast and epithelial proliferation and the expression of intercellular junction proteins. Proliferation was quantified by measuring 5-bromo-2′-deoxyuridine incorporation. The myofibroblast expression of EGF, VEGFa, and transforming growth factor-β1 (TGF-β1) was measured by real-time PCR and the expression of junctional complex proteins by Western blot. Aldo stimulated the proliferation of myofibroblasts by 70% ( P < 0.05) and increased EGF mRNA expression by 30% ( P < 0.05) without affecting VEGFa and TGF-β1. EGF concentration in the incubation medium increased by 30% ( P < 0.05) 24 h after Aldo addition, and these effects were prevented by the addition of spironolactone. Myofibroblast proliferation in response to Aldo was mediated by EGF receptor (EGFR) and involved both MAPKK and phosphatidylinositol 3-kinase pathways. When T84 cells were incubated with medium from myofibroblasts stimulated with Aldo (conditioned medium), the expression of β-catenin and claudin IV was increased by 30% ( P < 0.05) and proliferation by 40% ( P < 0.05). T84 proliferation decreased when α-EGF, or the EGFR antagonist AG1478, was present. Results in vivo indicate that rats fed a low-salt diet showed an increased expression of EGF and EGFR in the colonic mucosa. These results support the view that changes in colonic permeability during low-Na adaptation are mediated by the EGF secreted by myofibroblasts in response to raised Aldo.

Mesenchymal cells of the intestinal lamina propria

The mesenchymal elements of the intestinal lamina propria reviewed here are the myofibroblasts, fibroblasts, mural cells (pericytes) of the vasculature, bone marrow-derived stromal stem cells, smooth muscle of the muscularis mucosae, and smooth muscle surrounding the lymphatic lacteals. These cells share similar marker molecules, origins, and coordinated biological functions previously ascribed solely to subepithelial myofibroblasts. We review the functional anatomy of intestinal mesenchymal cells and describe what is known about their origin in the embryo and their replacement in adults. As part of their putative role in intestinal mucosal morphogenesis, we consider the intestinal stem cell niche. Lastly, we review emerging information about myofibroblasts as nonprofessional immune cells that may be important as an alarm system for the gut and as a participant in peripheral immune tolerance.

Arterial hypertension due to fructose ingestion: model based on intermittent osmotic fluid trapping in the small bowel

Based on recently reported data that fructose ingestion is linked to arterial hypertension, a model of regulatory loops involving the colon role in maintenance of fluid and sodium homeostasis is proposed.In normal digestion of hyperosmolar fluids, also in cases of postprandial hypotension and in patients having the "dumping" syndrome after gastric surgery, any hyperosmolar intestinal content is diluted by water taken from circulation and being trapped in the bowel until reabsorption. High fructose corn sirup (HFCS) soft drinks are among common hyperosmolar drinks. Fructose is slowly absorbed through passive carrier-mediated facilitated diffusion, along the entire small bowel, thus preventing absorption of the trapped water for several hours.Here presented interpretation is that ingestion of hyperosmolar HFCS drinks due to a transient fluid shift into the small bowel increases renin secretion and sympathetic activity, leading to rise in ADH and aldosterone secretions. Their actions spare water and sodium in the large bowel and kidneys. Alteration of colon absorption due to hormone exposure depends on cell renewal and takes days to develop, so the momentary capacity of sodium absorption in the colon depends on the average aldosterone and ADH exposure during few previous days. This inertia in modulation of the colon function can make an individual that often takes HFCS drinks prone to sodium retention, until a new balance is reached with an expanded ECF pool and arterial hypertension. In individuals with impaired fructose absorption, even a higher risk of arterial hypertension can be expected.

Role of vasopressin in rat distal colon function

The specific role of vasopressin in colonic crypt function and its possible synergistic action with aldosterone were studied. Sprague-Dawley rats fed a high-Na+ (HS; 150 mM NaCl) or a low-Na+ (LS; 150 microM NaCl) diet were deprived of water or infused with vasopressin, and some animals were treated with specific vasopressin receptor subtype V1 and V2 antagonists. The expression of the epithelial Na+ channel (ENaC), alpha-smooth muscle actin (alpha-SMA) and aquaporin-2 (AQP-2) were determined by immunolocalization in distal colonic mucosa. The pericryptal Na+ concentration was determined by confocal microscopy, using a low-affinity Na+-sensitive fluorescent dye (sodium red) and crypt permeability was measured by the rate of escape of fluorescein isothiocyanate-labelled dextran (10 kDa) from the crypt lumen into the pericryptal space in isolated rat distal colonic mucosa. A high plasma concentration of vasopressin raised alpha-SMA expression in the pericryptal sheath (P < 0.05), increased the pericryptal Na+ accumulation in this space (P < 0.01) and caused a reduction of crypt wall permeability (P < 0.01). All these effects were reversed by selective blockade of V1 and V2 receptors. No synergistic effects with aldosterone were observed. Dehydration and vasopressin infusion increased AQP-2 expression in distal colonic mucosa (P < 0.05). This action of vasopressin was prevented by tolvaptan, a specific V2 receptor antagonist (P < 0.05). It is concluded that vasopressin has trophic effects in the rat distal colon, increasing pericryptal myofibroblast growth which affects crypt absorption, and these effects are independent of the presence of aldosterone.

Pericryptal myofibroblast growth in rat descending colon induced by low-sodium diets is mediated by aldosterone and not by angiotensin II

Pericryptal myofibroblast growth in descending colonic crypts correlates with the activation of the renin-angiotensin-aldosterone system. Earlier work showed that during the transition from a high-Na(+) (HS) to low-Na(+) (LS) diet there are changes in the colonic crypt wall and pericryptal sheath. As LS diet increases both aldosterone and angiotensin II, the aim here was to determine their individual contributions to the trophic changes in colonic crypts. Experiments were conducted on control and adrenalectomized Sprague-Dawley rats fed an HS diet and then switched to LS diet for 3 days and supplemented with aldosterone or angiotensin II. The actions of the angiotensin-converting enzyme inhibitor captopril, the angiotensin receptor antagonist losartan and the aldosterone antagonist spironolactone on extracellular matrix proteins, claudin 4 and E-cadherin myofibroblast proteins, alpha-smooth muscle actin (alpha-SMA) and OB-cadherin (cadherin 11), angiotensin type 1 and TGFbetar1 membrane receptors were determined by immunolocalization in fixed distal colonic mucosa. The LS diet or aldosterone supplementation following ADX in HS or LS increased extracellular matrix, membrane receptors and myofibroblast proteins, but angiotensin alone had no trophic effect on alpha-SMA. These results show that aldosterone stimulates myofibroblast growth in the distal colon independently of dietary Na(+) intake and of angiotensin levels. This stimulus could be a genomic response or secondary to stretch of the pericryptal sheath myofibroblasts accompanying enhanced rates of crypt fluid absorption.

Aldosterone reduces crypt colon permeability during low-sodium adaptation

Fluid and electrolyte absorption by colonic crypts depends on the transport properties of crypt cellular and paracellular routes and of the pericryptal sheath. As a low-Na(+) diet increases aldosterone and angiotensin II secretion, either hormone could affect absorption. Control and adrenalectomized (ADX) Sprague-Dawley rats were kept at a high-NaCl (HS) diet and then switched to low-NaCl (LS) diet for 3 days. Aldosterone or angiotensin II plasma concentrations were maintained using implanted osmotic mini-pumps. The extracellular Na(+) concentration in isolated rat distal colonic mucosa was determined by confocal microscopy using a low-affinity Na(+) -sensitive fluorescent dye (Sodium red, and Na(+) -insensitive BODIPY) bound to polystyrene beads. Crypt permeability to FITC-labelled dextran (10 kDa) was monitored by its rate of escape from the crypt lumen into the pericryptal space. Mucosal ion permeability was estimated by transepithelial electrical resistance (TER) and amiloride-sensitive short-circuit current (SCC). The epithelial Na(+) channel, ENaC, was determined by immunolocalization. LS diet decreased crypt wall permeability to dextran by 10-fold and doubled TER. Following ADX, aldosterone decreased crypt wall dextran permeability, increased TER, increased Na(+) accumulation in the pericryptal sheath and ENaC expression even in HS. Infusion of angiotensin II to ADX rats did not reverse the effects of aldosterone deprivation. These findings indicate that aldosterone alone is responsible for both the increase in Na(+) absorption and the decreased paracellular and pericryptal sheath permeability.