Insulin and IGF-I inhibit calcium-dependent chloride secretion by T84 human colonic epithelial cells

Intestinal secretory mechanisms and diarrhea

One of the primary functions of the intestinal epithelium is to transport fluid and electrolytes to and from the luminal contents. Under normal circumstances, absorptive and secretory processes are tightly regulated such that absorption predominates, thereby enabling conservation of the large volumes of water that pass through the intestine each day. However, in conditions of secretory diarrhea, this balance becomes dysregulated, so that fluid secretion, driven primarily by Cl- secretion, overwhelms absorptive capacity, leading to increased loss of water in the stool. Secretory diarrheas are common and include those induced by pathogenic bacteria and viruses, allergens, and disruptions to bile acid homeostasis, or as a side effect of many drugs. Here, we review the cellular and molecular mechanisms by which Cl- and fluid secretion in the intestine are regulated, how these mechanisms become dysregulated in conditions of secretory diarrhea, currently available and emerging therapeutic approaches, and how new strategies to exploit intestinal secretory mechanisms are successfully being used in the treatment of constipation.

Ileal apical Na+-dependent bile acid transporter ASBT is upregulated in rats with diabetes mellitus induced by low doses of streptozotocin

Increased intestinal bile acid absorption and expansion of the bile acid pool has been implicated in the hypercholesterolemia associated with diabetes mellitus. However, the molecular basis of the increase in bile acid absorption in diabetes mellitus is not fully understood. The ileal apical Na(+)-dependent bile acid transporter (ASBT) is primarily responsible for active reabsorption of the majority of bile acids. Current studies were designed to investigate the modulation of ASBT function and expression in streptozotocin (STZ)-induced diabetes mellitus in rats and to examine the effect of insulin on rat ASBT promoter by insulin. Diabetes mellitus was induced in Sprague-Dawley rats by intraperitoneal injection of low doses of STZ (20 mg/kg body wt) on five consecutive days. Human insulin (10 U/day) was given to a group of diabetic rats for 3 days before euthanasia. RNA and protein were extracted from mucosa isolated from the small intestine and ASBT expression was assessed by real-time quantitative RT-PCR and Western blotting. Our data showed that ASBT mRNA and protein expression were significantly elevated in diabetic rats. Insulin treatment of diabetic rats reversed the increase in ASBT protein expression to control levels. Consistently, ileal Na(+)-dependent [(3)H]taurocholic uptake in isolated intestinal epithelial cells was significantly increased in diabetic rats. In vitro studies utilizing intestinal epithelial Caco-2 cells demonstrated that ASBT expression and promoter activity were significantly decreased by insulin. These studies demonstrated that insulin directly influences ASBT expression and promoter activity and that ASBT function and expression are increased in rats with STZ-induced diabetes mellitus. The increase in ASBT expression may contribute to disturbances in cholesterol homeostasis associated with diabetes mellitus.

p38 mitogen-activated protein kinase inhibits calcium-dependent chloride secretion in T84 colonic epithelial cells

We have previously shown that Ca(2+)-dependent Cl(-) secretion across intestinal epithelial cells is limited by a signaling pathway involving transactivation of the epidermal growth factor receptor (EGFR) and activation of ERK mitogen-activated protein kinase (MAPK). Here, we have investigated a possible role for p38 MAPK in regulation of Ca(2+)-dependent Cl(-) secretion. Western blot analysis of T(84) colonic epithelial cells revealed that the muscarinic agonist carbachol (CCh; 100 microM) stimulated phosphorylation and activation of p38 MAPK. The p38 inhibitor SB-203580 (10 microM) potentiated and prolonged short-circuit current (I(sc)) responses to CCh across voltage-clamped T(84) cells to 157.4 +/- 6.9% of those in control cells (n = 21; P < 0.001). CCh-induced p38 phosphorylation was attenuated by the EGFR inhibitor tyrphostin AG-1478 (0.1 nM-10 microM) and by the Src family kinase inhibitor PP2 (20 nM-2 microM). The effects of CCh on p38 phosphorylation were mimicked by thapsigargin (TG; 2 microM), which specifically elevates intracellular Ca(2+), and were abolished by the Ca(2+) chelator BAPTA-AM (20 microM), implying a role for intracellular Ca(2+) in mediating p38 activation. SB-203580 (10 microM) potentiated I(sc) responses to TG to 172.4 +/- 18.1% of those in control cells (n = 18; P < 0.001). When cells were pretreated with SB-203580 and PD-98059 to simultaneously inhibit p38 and ERK MAPKs, respectively, I(sc) responses to TG and CCh were significantly greater than those observed with either inhibitor alone. We conclude that Ca(2+)-dependent agonists stimulate p38 MAPK in T(84) cells by a mechanism involving intracellular Ca(2+), Src family kinases, and the EGFR. CCh-stimulated p38 activation constitutes a similar, but distinct and complementary, antisecretory signaling pathway to that of ERK MAPK.

TGF-beta effects on epithelial ion transport and barrier: reduced Cl- secretion blocked by a p38 MAPK inhibitor

Growth factors affect a variety of epithelial functions. We examined the ability of TGF-beta to modulate epithelial ion transport and permeability. Filter-grown monolayers of human colonic epithelia, T84 and HT-29 cells, were treated with TGF-beta (0.1-100 ng/ml, 15 min-72 h) or infected with an adenoviral vector encoding TGF-beta (Ad-TGF beta) for 144 h. Ion transport (i.e., short-circuit current, I(sc)) and transepithelial resistance (TER) were assessed in Ussing chambers. Neither recombinant TGF-beta nor Ad-TGF beta infection affected baseline I(sc); however, exposure to > or = 1 ng/ml TGF-beta led to a significant (30-50%) reduction in the I(sc) responses to forskolin, vasoactive intestinal peptide, and cholera toxin (agents that evoke Cl(-) secretion via cAMP mobilization) and to the cell-permeant dibutyryl cAMP. Pharmacological analysis of signaling pathways revealed that the inhibition of cAMP-driven epithelial Cl(-) secretion by TGF-beta was blocked by pretreatment with SB-203580, a specific inhibitor of p38 MAPK, but not by inhibitors of JNK, ERK1/2 MAPK, or phosphatidylinositol 3'-kinase. TGF-beta enhanced the barrier function of the treated monolayers by up to threefold as assessed by TER; however, this event was temporally displaced from the altered I(sc) response, being statistically significant only at 72 h posttreatment. Thus, in addition to TGF-beta promotion of epithelial barrier function, we show that this growth factor also reduces responsiveness to cAMP-dependent secretagogues in a chronic manner and speculate that this serves as a braking mechanism to limit secretory enteropathies.

Transactivation of the epidermal growth factor receptor in colonic epithelial cells by carbachol requires extracellular release of transforming growth factor-alpha

We have shown previously that the muscarinic agonist, carbachol (CCh), transactivates the epidermal growth factor receptor (EGFr) via calmodulin, Pyk-2, and Src kinase activation. EGFr phosphorylation causes extracellular signal-regulated kinase (ERK) activation and inhibits CCh-stimulated chloride secretion across intestinal epithelial cells. Here we investigated whether CCh-stimulated EGFr transactivation involves EGFr ligand release. Pre-incubation of T(84) cell monolayers with a neutralizing antibody to the EGFr ligand binding domain decreased CCh-induced phosphorylation of EGFr and ERK. CCh-stimulated efflux of (86)Rb+ from T(84) cell monolayers, which parallels changes in chloride secretion, was potentiated by anti-EGFr pre-incubation. Anti-EGFr did not reduce CCh-stimulated Pyk-2 phosphorylation. Co-incubation with the Src kinase inhibitor PP2 and anti-EGFr had an additive inhibitory effect on CCh-induced ERK phosphorylation greater than either inhibitor alone. CCh caused the basolateral release of transforming growth factor alpha (TGF-alpha) into T(84) cell bathing media. A metalloproteinase inhibitor, WAY171318, reduced CCh-induced phosphorylation of ERK and completely blocked EGFr phosphorylation and TGF-alpha release. We conclude that CCh-stimulated EGFr transactivation and subsequent ERK activation, a pathway that limits CCh-induced chloride secretion, is mediated by metalloproteinase-dependent extracellular release of TGF-alpha and intracellular Src activation. These findings have important implications for our understanding of the role of growth factors in regulating epithelial ion secretion.

Fluid and electrolyte transport in the small intestine

The small intestine is in a dynamic state of secretion and absorption, the sum of which results in net absorption. Secretion is principally the result of chloride and bicarbonate extrusion through apical chloride channels after the activation of the second messengers cAMP, cGMP, and calcium. In addition to the cystic fibrosis transmembrane conductance regulator, several other candidate chloride channels have been identified and proposed to play a role in intestinal secretion, including the calcium-dependent chloride channel hCLCA1. Pathways leading to the negative control of secretion have been described that use cellular messengers, including inositol (3,4,5,6) tetrakisphosphate and phosphatidylinositol 3-kinase, which may act via basolateral potassium channels. The control of ion transport can also be viewed in terms of the enteric nervous system. The reflex neural pathways involved in enterotoxin-induced secretion have been substantiated and shown to involve 5-hydroxytryptamine, substance P, and the neurokinin 1 and 2 receptors in the sensory arm, and vasoactive intestinal peptide in the secretomotor efferents. Absorption of glucose in addition to active cotransport with sodium via the Na/glucose cotransporter protein has also been shown to occur passively through a carrier-mediated mechanism, using the membrane protein glucose transporter protein 2.