Decoding epithelial signals: critical role for the epidermal growth factor receptor in controlling intestinal transport function

Abstract

The intestinal epithelium engages in bidirectional transport of fluid and electrolytes to subserve the physiological processes of nutrient digestion and absorption, as well as the elimination of wastes, without excessive losses of bodily fluids that would lead to dehydration. The overall processes of intestinal ion transport, which in turn drive the secretion or absorption of water, are accordingly carefully regulated. We and others have identified the epidermal growth factor receptor (EGFr) as a critical regulator of mammalian intestinal ion transport. In this article, we focus on our studies that have uncovered the intricate signalling mechanisms downstream of EGFr that regulate both chloride secretion and sodium absorption by colonocytes. Emphasis will be placed on the EGFr-associated regulatory pathways that dictate the precise outcome to receptor activation in response to signals that may seem, on their face, to be quite similar if not identical. The concepts to be discussed underlie the ability of the intestinal epithelium to utilize a limited set of signalling effectors to produce a variety of outcomes suitable for varying physiological and pathophysiological demands. Our findings therefore are relevant not only to basic biological principles, but also may ultimately point to new therapeutic targets in intestinal diseases where ion transport is abnormal.

Figure 1

Pathways centering on the receptor for epidermal growth factor (EGFr) that regulate chloride secretion in colonic epithelial cells. Pathways that are activated by binding of EGF to EGFr are shown on the right-hand side and indicated by broken lines; those activated via EGFr transactivation are shown on the left-hand side of the figure, with solid lines. Ligands for receptors coupled to G_q, such as the m₃ muscarinic receptor for the agonist, carbachol (CCh), initially cause calcium mobilization that can stimulate chloride secretion. However, they subsequently activate a negative signaling cascade via EGFr to limit further secretion. EGF can also inhibit secretion via its receptor, but via distinct signals that inhibit basolateral potassium channels. Please note that other components of the chloride secretory mechanism (Na⁺,K⁺ ATPase, Na⁺/K⁺/2Cl⁻ cotransporter) are not shown in the figure for simplicity. MMP, matrix metalloproteinase; TFG-α, transforming growth factor α; ERK, extracellular signal-regulated kinase; PI3K, phosphatidylinositol 3-kinase; PKC, protein kinase C. For additional details, see text.

Figure 2

Different ligands that activate EGFr via either direct binding or transactivation elicit differing patterns of tyrosine residue phosphorylation in the cytoplasmic tail of the receptor. EGF and TGF-α both induce phosphorylation of six specific residues, albeit to different extents (as indicated by size of the circled letter P). Carbachol activates both the release of TGF-α as well as tyrosine phosphatases such as protein tyrosine phosphatase 1B (PTP1B) that serve to dephosphorylate specific residues.

Figure 3

Model for colonic fluid and ion transport in health. Surface cells absorb sodium ions, NaCl and short chain fatty acids (SCFA). Crypt cells secrete chloride ions. The active absorption and secretion of these solutes drives fluid recycling from the crypt lumen followed by reabsorption across the surface epithelium. Absorption exceeds secretion, particularly in the distal colon, so little fluid is lost to the stool.

Figure 4

Model for colonic fluid and ion transport in the setting of colitis. Both surface and crypt transport processes are compromised compared to the situation in healthy tissues (compare with Figure 3). The lack of crypt chloride and fluid secretion results in crypt stagnation, the accumulation of toxins, and reduced barrier function in this site. The lack of adequate absorption across surface cells results in a failure to salvage luminal fluid, and the clinical finding of diarrhea.

Figure 5

Mechanisms that regulate electrogenic absorption of sodium across epithelial cells. Sodium ions are taken up via the apical epithelial sodium channel (ENaC) and exit the cell via the basolateral Na⁺, K⁺ ATPase. The activity of ENaC can be reduced by agonists binding to receptors that stimulate mobilization of intracellular calcium stores. Elevations in intracellular sodium activate the ubiquitin ligase, Nedd4-2, which in turn triggers the internalization and degradation of ENaC. Agonists binding to receptors that elevate intracellular cAMP stimulate ENaC activity both directly, and by inhibiting Nedd4-2 activity. For additional details, see text.

Figure 6

Differential effects of EGF on colonic ion transport in normal and inflamed tissues.

Figure 7

Hypothesis as to how conflicting data in the literature showing both positive and negative effects of EGF on electrogenic sodium absorption can be reconciled. We hypothesize that in health (1), EGF predominantly activates ERK-dependent pathways that stimulate Nedd4-2 and thereby limit ENaC activity and sodium absorption. In the setting of inflammation (2), on the other hand, EGF signaling via PI3K-dependent pathways is favored, leading to sequential activation of phosphoinositide-dependent kinase 1 (PDK1), serum and glucocorticoid responsive kinase 1 (SGK1) and the ultimate inhibition of Nedd4-2, thus enhancing ENaC activity.