The effect of EGF on electrolyte transport is mediated by tyrosine kinases in the rabbit cortical collecting duct

Expression and Function of the Epidermal Growth Factor Receptor in Physiology and Disease

The epidermal growth factor receptor (EGFR) is the prototypical member of a family of membrane-associated intrinsic tyrosine kinase receptors, the ErbB family. EGFR is activated by multiple ligands, including EGF, transforming growth factor (TGF)-α, HB-EGF, betacellulin, amphiregulin, epiregulin, and epigen. EGFR is expressed in multiple organs and plays important roles in proliferation, survival, and differentiation in both development and normal physiology, as well as in pathophysiological conditions. In addition, EGFR transactivation underlies some important biologic consequences in response to many G protein-coupled receptor (GPCR) agonists. Aberrant EGFR activation is a significant factor in development and progression of multiple cancers, which has led to development of mechanism-based therapies with specific receptor antibodies and tyrosine kinase inhibitors. This review highlights the current knowledge about mechanisms and roles of EGFR in physiology and disease.

Mechanisms underlying rapid aldosterone effects in the kidney

The steroid hormone aldosterone is a key regulator of electrolyte transport in the kidney and contributes to both homeostatic whole-body electrolyte balance and the development of renal and cardiovascular pathologies. Aldosterone exerts its action principally through the mineralocorticoid receptor (MR), which acts as a ligand-dependent transcription factor in target tissues. Aldosterone also stimulates the activation of protein kinases and secondary messenger signaling cascades that act independently on specific molecular targets in the cell membrane and also modulate the transcriptional action of aldosterone through MR. This review describes current knowledge regarding the mechanisms and targets of rapid aldosterone action in the nephron and how aldosterone integrates these responses into the regulation of renal physiology.

Biphasic regulation of ENaC by TGF-{alpha} and EGF in renal epithelial cells

The epithelial sodium channel (ENaC) is regulated by epidermal growth factor (EGF). We investigate whether ENaC is regulated by another EGF receptor (EGFR) ligand, transforming growth factor-alpha (TGF-alpha). We show that chronic (24 h) treatment with TGF-alpha inhibits ENaC in Xenopus laevis kidney cells 20 times more strongly than EGF. By using single-channel measurements, we show that TGF-alpha significantly reduces the number of ENaC per patch. The open probability (P(o)) is unchanged by 24-h treatment with TGF-alpha. alpha-, beta-, and gamma-ENaC mRNA levels are significantly reduced by TGF-alpha or EGF. TGF-alpha or EGF reduces alpha- and gamma-ENaC proteins in the membrane; however, beta-ENaC is unchanged. TGF-alpha or EGF inhibits ENaC by activating EGFR since the EGFR inhibitor AG1478 blocks the effects of both. The MAPK 1/2 inhibitor U0126 also blocks the effect of TGF-alpha or EGF on ENaC, indicating that the MAPK1/2 pathway is involved in the TGF-alpha- or EGF-induced inhibition of ENaC. Interestingly, acute treatment (<1 h) with TGF-alpha or EGF does not inhibit ENaC current; it enhances ENaC activity by increasing P(o). Pretreatment of the cells with U0126 potentiates the acute TGF-alpha- or EGF-induced stimulation of ENaC. This TGF-alpha- or EGF-induced increase in sodium current is abolished by a phosphatidylinositol 3-kinase (PI-3 kinase) inhibitor, LY294002, suggesting that PI-3 kinase is involved in the activation of sodium transport. In conclusion, chronic treatment with TGF-alpha or EGF inhibits ENaC by decreasing the number of channels in the membrane transcriptionally through MAPK1/2 pathways, but acute treatment with TGF-alpha or EGF activates ENaC by increasing P(o) via PI-3 kinase.

Aldosterone stimulates activity and surface expression of NHE3 in human primary proximal tubule epithelial cells (RPTEC)

The steroid hormone aldosterone is a major regulator of extracellular volume and blood pressure. Aldosterone effectors are for example the epithelial Na(+) channel (ENaC), the Na(+)-K(+)-ATPase and the proximal tubule Na(+)/H(+) exchanger isoform 3 (NHE3). The aim of this study was to investigate whether aldosterone acts directly on proximal tubule cells to stimulate NHE3 and if so whether the EGF-receptor (EGFR) is involved. For this purpose, primary human renal proximal tubule cells were exposed to aldosterone. NHE3 activity was determined from Na(+)- dependent pH-recovery, NHE3 surface expression was determined by biotinylation and immunoblotting. EGFR-expression was assessed by ELISA. pH(i)- measurements revealed an aldosterone-induced increase in NHE3 activity, which was inhibited by the mineralocorticoid receptor blocker spironolactone and by the EGFR-kinase inhibitor AG1478. Immunoprecipitation and immunoblot analysis showed an aldosterone-induced increase in NHE3 surface expression, which was also inhibited by spironolactone and AG1478. Furthermore, aldosterone enhanced EGFR-expression. In conclusion, aldosterone stimulates NHE3 in human proximal tubule cells. The underlying mechanisms include AG1478 inhibitable kinase and are paralleled by enhanced EGFR expression, which could be compatible with EGF-receptor-pathway-dependent surface expression and activity of NHE3 in human primary renal proximal tubule epithelial cells.

Hydrogen peroxide and epidermal growth factor activate phosphatidylinositol 3-kinase and increase sodium transport in A6 cell monolayers

Activation of phosphatidylinositol 3-kinase (PI 3-kinase) is required for insulin stimulation of sodium transport in A6 cell monolayers. In this study, we investigate whether stimulation of the PI 3-kinase by other agents also provoked an increase in sodium transport. Both epidermal growth factor (EGF) and H2O2provoked a rise in sodium transport that was inhibited by LY-294002, an inhibitor of PI 3-kinase activity. PI 3-kinase activity was estimated in extracts from A6 cell monolayers directly by performance of a PI 3-kinase assay. We also estimated the relative importance of the PI 3-kinase pathway by two different methods: 1) coprecipitation of the p85 regulatory subunit with anti-phosphotyrosine antibodies and 2) phosphorylation of PKB on both Ser 473 and Thr 308 residues observed by Western blotting. Since the mitogen-activated protein kinase (MAPK) pathway has also been implicated in the regulation of sodium transport, we also investigated whether this pathway is turned on by insulin, H2O2, or EGF. Phosphorylation of ERK1/2 was increased only transiently by insulin and H2O2but quite sustainedly by EGF. Inhibitors of this pathway (U-0126 and PD-98059) failed to affect the insulin and H2O2stimulation of sodium transport but increased substantially the stimulation induced by EGF. The latter effect was associated with an increase in PKB phosphorylation, thus suggesting that the stimulation of the MAPK pathway prevents, in part, the stimulation of the PI 3-kinase pathway in the transport of sodium stimulated by EGF.

Chronic exposure to EGF affects trafficking and function of ENaC channel in cystic fibrosis cells

Using the whole-cell patch-clamp technique, we identified an amiloride (AMI)-sensitive Na(+) current in cystic fibrosis cells, JME/CF15, growing in standard medium. The reversal potential of this current depended on Na(+) concentrations and the cation selectivity was much higher for Na(+) than for K(+), indicating that the current is through ENaC channels. In contrast, cells from EGF-containing medium lacked AMI-sensitive Na(+) currents. In permeabilized cells growing in EGF-containing medium, alphaENaC was mainly detected in a perinuclear region, while in cells from standard medium it was distributed over the cell body. Western-blot analysis showed that in standard medium cells expressed fast-migrating EndoH-insensitive and slow-migrating EndoH-sensitive alphaENaC fractions, while in cells growing in the presence of EGF, alphaENaC was only detected as the fast-migrating EndoH-insensitive fraction. Long-term incubation of cells with EGF resulted in an increased basal Ca(2+) level, [Ca(2+)](i). A similar increase of [Ca(2+)](i) was also observed in the presence of 2muM thapsigargin, resulting in inhibition of ENaC function. Thus, in JME/CF15 cells inhibition of the ENaC function by chronic incubation with EGF is a Ca(2+)-mediated process that affects trafficking and surface expression of ENaC channels.

Receptor tyrosine kinases mediate epithelial Na(+) channel inhibition by epidermal growth factor

Epidermal growth factor (EGF) decreases Na(+) reabsorption across distal nephron epithelia. Activity of the epithelial Na(+) channel (ENaC) is limiting for Na(+) transport in this portion of the nephron. Abnormal ENaC activity and EGF signaling are both associated with polycystic kidney disease localized to the distal nephron. We tested here whether EGF and other ligands for receptor tyrosine kinases (RTK) decrease ENaC activity. EGF markedly and quickly decreased ENaC activity. The RTK inhibitor erbstatin blocked EGF actions on ENaC and when added alone increased channel activity, uncovering basal suppression by endogenous RTK. The protein tyrosine phosphatase inhibitor vanadate, similar to EGF, decreased ENaC activity. Growth factors and vanadate decreased ENaC activity by decreasing open probability. ENaC was not phosphorylated in response to EGF, indicating that intermediary proteins transduce the inhibitory signal from the EGF receptor (EGFR) to ENaC. We find that neither MAPK 1/2 nor c-Src is signaling intermediaries between EGFR and ENaC. Inhibition of ENaC paralleled decreases in plasma membrane phosphatidylinositol 4,5-bisphosphate levels [PtdIns(4,5)P(2)] and was abolished by clamping PtdIns(4,5)P(2). We conclude that EGF and other ligands for RTK decrease ENaC open probability by decreasing membrane PtdIns(4,5)P(2) levels.

Aldosterone rapidly activates Src kinase in M-1 cells involving the mineralocorticoid receptor and HSP84

We investigated the effect of aldosterone on Src kinase. In the kidney cell line, M-1 aldosterone leads to a >2-fold transient activation of Src kinase seen as early as 2 min after aldosterone administration. Maximal Src kinase activation was measured at an aldosterone concentration of 1 nM. In parallel to activation, autophosphorylation at Tyr-416 of Src kinase increased. Src kinase activation was blocked by spironolactone. Aldosterone led to increased association of Src with HSP84. Furthermore, rapamycin blocked aldosterone-induced Src activation. We conclude that Src activation by aldosterone is mediated through the mineralocorticoid receptor and HSP84.

Evidence for epidermal growth factor receptor as negative-feedback control in aldosterone-induced Na+ reabsorption

Aldosterone enhances Na+ reabsorption via epithelial Na+ channels (ENaC). Aldosterone also stimulates the protein kinase ERK1/2- and the epidermal growth factor (EGF) receptor (EGFR)-signaling pathway. Yet EGF and ERK1/2 are known inhibitors of ENaC-mediated Na+ reabsorption. In the present study, using the well-established Madin-Darby canine kidney C7 cell line, we tested the hypothesis that EGFR represents a negative-feedback control for chronic aldosterone-induced Na+ reabsorption [amiloride-inhibitable short-circuit current ( Isc)]. Mineralocorticoid receptor expression was confirmed by RT-PCR and Western blot analysis. Aldosterone enhanced ERK1/2 phosphorylation in an EGFR-dependent way. Furthermore, aldosterone stimulated EGFR expression. Aldosterone (10 nmol/l) induced a small transient increase in Isc under control conditions. Inhibition of ERK1/2 phosphorylation with U-0126 (10 μmol/l) stimulated Isc, indicating constitutive ENaC inhibition. Aldosterone exerted a significantly larger effect in the presence of U-0126 than without U-0126. EGF (10 μg/l) inhibited Isc, whereas inhibition of EGFR kinase by tyrphostin AG-1478 (100 nmol/l) enhanced Isc. Aldosterone was more effective in the presence of AG-1478 than without AG-1478. In summary, we propose that the EGFR-signaling cascade can serve as a negative-feedback control to limit the effect of aldosterone-induced Na+ reabsorption.

Epidermal growth factor inhibits14C-?-methyl-d-glucopyranoside uptake in renal proximal tubule cells: Involvement of PLC/PKC, p44/42 MAPK, and cPLA2

The effect of EGF on (14)C-alpha-methyl-D-glucopyranoside (alpha-MG) uptake and its related signaling pathways were examined in primary cultured rabbit renal proximal tubule cells (PTCs). Epidermal growth factor (EGF) (50 ng/ml) was found to inhibit alpha-MG uptake, a distinctive proximal tubule marker. The EGF effect was blocked by AG1478 (an EGF receptor antagonist) or genistein and herbimycin (tyrosine kinase inhibitors), respectively. In addition, the EGF-induced inhibition of alpha-MG uptake was blocked by neomycin and U73122 (phospholipase C inhibitors) as well as staurosporine, H-7, and bisindolylmaleimide I (protein kinase C inhibitors). EGF was also observed to increase inositol phosphate formation. Furthermore, both the EGF-induced inhibition of alpha-MG uptake and increase of arachidonic acid (AA) release were blocked by AACOCF(3) (a cytosolic phospholipase A(2) inhibitor), indomethacin (a cyclooxygenase inhibitor), and econazole (a cytochrome P-450 epoxygenase inhibitor). We examined the involvement of mitogen-activated protein kinases (MAPKs) in mediating the effect of EGF on alpha-MG uptake. Indeed, EGF increased phosphorylation of p44/p42 MAPK and the EGF-induced inhibition of alpha-MG uptake as well as the stimulatory effect of EGF on AA release was blocked by PD 98059 (a p44/42 MAPK inhibitor), suggesting a causal relationship. However, inhibitors of PKC also prevented the EGF-induced increase of AA release. In conclusion, EGF partially inhibited alpha-MG uptake via PLC/PKC, p44/42 MAPK, and PLA(2) signaling pathways.

Epidermal growth factor inhibits amiloride-sensitive sodium absorption in renal collecting duct cells

The effects of the ERK pathway on electrogenic transepithelial Na(+) absorption by renal collecting duct cells were determined. Approximately 90% of the unstimulated short-circuit current (15 +/- 1 microA/cm(2), n = 10) across conditionally immortalized murine collecting duct epithelial cells (mCT1) is amiloride sensitive and is likely mediated by apical epithelial Na(+) channels. Chronic exposure (24 h) of the epithelial monolayers to either EGF (50 ng/ml) or transforming growth factor-alpha (TGF-alpha; 20 ng/ml) reduced amiloride-sensitive short-circuit current by >60%. The inhibitory effect of EGF on Na(+) absorption was not due to inhibition of basolateral Na(+)-K(+)-ATPase, because the pump current elicited by permeabilization of apical membrane with nystatin was not reduced by EGF. Chronic exposure of the mCT1 cells to EGF (20 ng/ml, 24 h) elicited a 70-85% decrease in epithelial Na(+) channel subunit mRNA levels. Exposure of mCT1 cells to either EGF (20 ng/ml) or PMA (150 nM) induced rapid phosphorylation of p42/p44 (ERK1/2) and pretreatment of the monolayers with PD-98059 (an ERK kinase inhibitor; 30 microM) prevented phosphorylation of p42/p44. Similarly, pretreatment of mCT1 monolayers with PD-98059 prevented the EGF- and PMA-induced inhibition of amiloride-sensitive Na(+) absorption. The results of these studies demonstrate that amiloride-sensitive Na(+) absorption by renal collecting duct cells is regulated by the ERK pathway. This pathway may play a role in alterations in ion transport that occur in polycystic kidney disease.

Human Epidermal Growth Factor Receptor-1 Expression Renders Chinese Hamster Ovary Cells Sensitive to Alternative Aldosterone Signaling

The epidermal growth factor (EGF) regulates cell proliferation, differentiation, and ion transport using ERK1/2 as a downstream effector. Furthermore, the EGF receptor (EGFR) is involved in signaling by G-protein-coupled receptors, growth hormone, and cytokines via transactivation. It has been suggested that steroids interact with peptide hormones. Previously, we have shown that aldosterone modulates EGF responses in Madin-Darby canine kidney cells (Gekle, M., Freudinger, R., Mildenberger, S., and Silbernagl, S. (2002) Am. J. Physiol. 282, F669-F679). Here, we tested the hypothesis that human EGFR-1 can confer alternative aldosterone responsiveness with respect to ERK1/2 phosphorylation to Chinese hamster ovary cells, which do not express EGFR. Wild-type Chinese hamster ovary cells did not respond to EGF or aldosterone. After transfection of human EGFR-1, the cells responded to EGF, but not to aldosterone. However, when submaximal concentrations of EGF were used, nanomolar concentrations of aldosterone potentiated the action of EGF within minutes, resulting in a leftward shift of the EGF dose-response curve. This was not the case in mock-transfected cells. The EGFR kinase inhibitor tyrphostin AG1478 or the MEK1/2 inhibitor U0126 completely prevented the effect. Furthermore, aldosterone enhanced Tyr phosphorylation of c-Src and EGFR, and an inhibitor of cytosolic tyrosine kinases (4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyriociaine) prevented the action of aldosterone. Our data show that aldosterone uses the EGF-EGFR-MEK1/2-ERK1/2 signaling cascade to elicit its alternative effects. In the presence of EGF, aldosterone leads to EGFR transactivation via cytosolic tyrosine kinases of the Src family.

Rapid actions of aldosterone on cells from renal epithelium: the possible role of EGF-receptor signaling

It has been suggested that steroids interact with peptide hormones in part by rapid, potentially non-genomic, mechanisms. The peptide hormone epidermal growth factor (EGF) regulates cell proliferation and ion transport using ERK1/2 as downstream signal. Furthermore, the EGF-receptor (EGF-R) is involved in signaling by G-protein-coupled receptors, growth hormone and cytokines via transactivation. We show that aldosterone modulates Na(+)/H(+)-exchange in renal collecting duct-derived Madin-Darby canine kidney (MDCK) cells via ERK1/2 in a similar way as compared to growth factors. Furthermore, we tested the hypothesis that aldosterone uses the EGF-R as heterologous signal transducer in MDCK cells. Aldosterone induces a rapid increase of ERK1/2 phosphorylation and cytosolic Ca(2+)-concentration of similar extend as compared to EGF. Furthermore, aldosterone stimulates EGF-R Tyr-phosphorylation. Inhibition of EGF-R kinase abolished aldosterone-induced signaling. Aldosterone-induced Ca(2+)-influx seems to be mediated by the activation of ERK1/2, whereas ERK1/2 activation does not depend on Ca(2+)-influx. Our data show that aldosterone uses the EGF-R-ERK1/2 signaling cascade to elicit its rapid effects in MDCK cells.

Aldosterone interaction with epidermal growth factor receptor signaling in MDCK cells

Epidermal growth factor (EGF) regulates cell proliferation, differentiation, and ion transport by using extracellular signal-regulated kinase (ERK)1/2 as a downstream signal. Furthermore, the EGF-receptor (EGF-R) is involved in signaling by G protein-coupled receptors, growth hormone, and cytokines by means of transactivation. It has been suggested that steroids interact with peptide hormones, in part, by rapid, potentially nongenomic, mechanisms. Previously, we have shown that aldosterone modulates Na(+)/H(+) exchange in Madin-Darby canine kidney (MDCK) cells by means of ERK1/2 in a way similar to growth factors. Here, we tested the hypothesis that aldosterone uses the EGF-R as a heterologous signal transducer in MDCK cells. Nanomolar concentrations of aldosterone induce a rapid increase in ERK1/2 phosphorylation, cellular Ca(2+) concentration, and Na(+)/H(+) exchange activity similar to increases induced by EGF. Furthermore, aldosterone induced a rapid increase in EGF-R-Tyr phosphorylation, and inhibition of EGF-R kinase abolished aldosterone-induced signaling. Inhibition of ERK1/2 phosphorylation reduced the Ca(2+) response, whereas prevention of Ca(2+) influx did not abolish ERK1/2 phosphorylation. Our data show that aldosterone uses the EGF-R-ERK1/2 signaling cascade to elicit its rapid effects in MDCK cells.

Sodium reabsorption in aldosterone-sensitive distal nephron: news and contributions from genetically engineered animals

The precise adaptation of renal sodium excretion to systemic needs is to a large extent achieved by the regulation of sodium re-absorption in the aldosterone-sensitive distal nephron. Transcellular sodium re-absorption by the segment-specific cells of the aldosterone-sensitive distal nephron (often called principal cells) is mainly controlled at the level of the expression and activity levels of the epithelial sodium channel, the apical amiloride-sensitive sodium influx pathway. Recent investigations have identified the first early aldosterone-induced proteins that act on epithelial sodium channel function in expression systems. Indirect evidence suggests that one of these aldosterone-induced proteins, the serum- and glucocorticoid-inducible protein kinase SGK1, plays a central integratory role in the control of epithelial sodium channel surface expression and activity, also in the mammalian kidney. Gene-modified animals lacking epithelial sodium channel subunits or expressing mutant subunits have substantiated the central role of the epithelial sodium channel in sodium re-absorption and blood pressure control, as well as for neonatal lung liquid clearance. Mice overexpressing or lacking specific hormones or their receptors have been used to study their role in sodium transport regulation, but the study of mouse physiology appears to lag behind the generation of gene-modified mice. Nonetheless, these new animal models have had a strong impact on research, by stimulating the integration of knowledge and techniques learned from reductionistic molecular approaches into tissue and animal studies, thus breaking down barriers and stimulating collaborations.