Biomedical subjects

Michael Gekle

Publications and source records attributed to Michael Gekle.

41 records · Page 3Linked to original sources

PubMed2002

Short-term regulation of basolateral organic anion uptake in proximal tubular OK cells: EGF acts via MAPK, PLA(2), and COX1.

The organic anion transport system of the kidney is of major importance for the excretion of a variety of endogenous compounds, drugs, and potentially toxic substances. The basolateral uptake into proximal tubular cells is mediated by a tertiary active transport system. Epidermal growth factor (EGF) leads to an increase in the basolateral uptake rate of the model substrate para-aminohippuric acid (PAH) in opossum kidney (OK) cells. This stimulation is mediated by successive activation of the mitogen-activated protein kinases,mitogen-activated/extracellular signal-regulated kinase kinase (MEK) and extracellular regulated kinase isoforms 1 and 2 (ERK1/2). This study investigates the regulatory network of EGF action on PAH uptake downstream ERK1/2 in more detail. EGF stimulation of the basolateral uptake rate of [(14)C]PAH was abolished by the phospholipase A(2) inhibitor AACOCF3.[(14)C]PAH uptake was enhanced by arachidonic acid. Furthermore, EGF led to an increase in arachidonic acid release and to the generation of prostaglandins. AACOCF3 did not influence EGF-induced ERK1/2 activation, indicating that ERK1/2 is upstream of PLA(2). In addition, EGF stimulated the influx of extracellular Ca(2+). However, Ca(2+)-influx was not required for the stimulatory action of EGF on [(14)C]PAH uptake. Inhibitors of COX and lipoxygenases reduced [(14)C]PAH uptake dose-dependently, whereas inhibition of cytochrome P450 did not. In the presence of indomethacin, EGF had no stimulatory effect on [(14)C]PAH uptake. The inhibitory effect of indomethacin was not due to competitive action on PAH uptake. Furthermore, prostaglandin E(2) (PGE(2)) increased basolateral [(14)C]PAH uptake rate dose-dependently, and this increase was also observed in the presence of indomethacin. Selective inhibition of COX2 by indomethacin amid or indomethacin n-heptyl ester did not inhibit [(14)C]PAH uptake, whereas selective inhibition of COX1 dose-dependently inhibited [(14)C]PAH uptake. This and previous data lead to the conclusion that EGF successively activates MEK, ERK1/2, and PLA(2), leading to an increased release of arachidonic acid. Subsequently, arachidonic acid is metabolized to prostaglandins via COX1, which then mediate EGF-induced stimulation of basolateral organic anion uptake rate.

Animals↗

PubMed2002

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.

Aldosterone↗

PubMed2002

NHE3 serves as a molecular tool for cAMP-mediated regulation of receptor-mediated endocytosis.

Receptor-mediated, clathrin-dependent endocytosis (RME) is important for macromolecular transport and regulation of cell-surface protein expression. Pharmacological studies have shown that the plasma membrane transport protein Na(+)/H(+) exchanger 3 (NHE3), which shuttles between the plasma membrane and the early endosomal compartment by means of clathrin-mediated endocytosis, contributes to endosomal pH homeostasis and endocytic fusion events. Furthermore, it is known that NHE3 is phosphorylated and inhibited by cAMP-dependent kinase (protein kinase A). Here, we show, in a cellular knockout/retransfection approach, that NHE3 supports RME and confers cAMP sensitivity to RME, using megalin/cubilin-mediated albumin uptake in opossum kidney cells. RME, but not fluid-phase endocytosis, was dependent on NHE3 activity and expression. Furthermore, NHE3 deficiency or inhibition reduced the relative surface expression of megalin without altering total expression. In wild-type cells, cAMP inhibits NHE3 activity, leads to endosomal alkalinization, and reduces RME. In NHE3-deficient cells, endosomal pH is not sensitive to NHE3 inhibition, and cAMP does not affect endosomal pH or RME. NHE3 transfection into deficient cells restores RME and the effects of cAMP. Thus our data show that NHE3 is important for cAMP sensitivity of clathrin-dependent RME.

Albumins↗

PubMed2002

Albumin induces NF-kappaB expression in human proximal tubule-derived cells (IHKE-1).

BACKGROUND: Chronic renal diseases with enhanced glomerular protein filtration are accompanied by tubulointerstitial inflammation and progression to renal function deterioration. Filtered proteins, like albumin, seem to be a pathogenic factor per se in the progression of renal diseases. There is evidence that the nuclear factor kappaB (NF-kappaB) is involved in protein-overload stimulated renal inflammatory pathomechanisms. The aim of this study was to investigate albumin-induced NF-kappaB expression as well as NF-kappaB activity upon long long term exposure to albumin in human proximal tubular cells as only acute albumin-induced NF-kappaB activity has been reported so far. METHODS: To investigate the hypothesis, that NF-kappaB may be involved in protein-induced renal inflammatory pathomechanisms, we exposed human renal proximal tubule-derived cells (IHKE-1) to bovine serum albumin (BSA: 50 and 500 microg/ml). The NF-kappaB and TNF-alpha specific mRNA expression was detected by RT-PCR. NF-kappaB specific protein expression was analysed by Western blot. Reporter gene assays were performed to determine the NF-kappaB specific activity. RESULTS: Albumin-exposure induced an increase in NF-kappaB specific mRNA expression, NF-kappaB protein expression and activity. These effects are decreased by the protein kinase C (PKC) inhibitor bisindolylmaleimide I (BIM) and the tyrosine kinase inhibitor herbimycin A. An albumin-induced increase in TNF-alpha specific mRNA expression as biological, inflammatory parameter associated with the albumin-induced NF-kappaB activity was detectable. CONCLUSION: We suggest, that albumin-exposure induces an increase in NF-kappaB and TNF-alpha specific mRNA expression, NF-kappaB specific protein expression and protein activity in renal proximal tubule cells in culture, which is at least in part PKC and tyrosine kinase dependent.

Animals↗

PubMed1998

Renal Proximal Tubular Albumin Reabsorption: Daily Prevention of Albuminuria.

Although the glomerular filtration coefficient of albumin is small, the daily filtered load can be as much as 8 g. To prevent such massive losses of albumin, quantitative reabsorption along the proximal tubules is accomplished by "receptor"-mediated endocytosis. Albumin reaches the lysosomes where it is degraded to amino acids.

Journal Article↗