Albumin is the most abundant protein in serum and contributes to the maintenance of oncotic pressure as well as to transport of hydrophobic molecules. Although albumin is a large anionic protein, it is not completely retained by the glomerular filtration barrier. In order to prevent proteinuria, albumin is reabsorbed along the proximal tubules by receptor-mediated endocytosis, which involves the binding proteins megalin and cubilin. Endocytosis depends on proper vesicle acidification. Disturbance of endosomal acidification or loss of the binding proteins leads to tubular proteinuria. Furthermore, endocytosis is subject to modulation by different signaling systems, such as protein kinase A (PKA), protein kinase C (PKC), phosphatidylinositol 3-kinase (PI3-K) and transforming growth factor beta (TGF-beta). In addition to being reabsorbed in the proximal tubule, albumin can also act as a profibrotic and proinflammatory stimulus, thereby initiating or promoting tubulo-interstitial diseases.

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Madin-Darby canine kidney (MDCK) cells originate from the renal collecting duct and consist of different cell subtypes. We cloned two MDCK cell subtypes denominated as C7 and C11 with different morphology and different function. The two clones maintained their functional differences after cloning. C7 monolayers exhibit a high transepithelial resistance (Rte = 5648 +/- 206 omega.cm2, n = 20) and secrete K+ (delta K+ = 1.31 +/- 0.08 mmol/l, n = 10) into the apical medium. C11 monolayers display a low Rte (330 +/- 52 omega.cm2, n = 20) and secrete Cl- (delta Cl- = 16.9 +/- 1.8 mmol/l, n = 10) into the apical medium. Aldosterone (1 mumol/l) stimulates K+ secretion (delta K+ of 3.58 +/- 0.11 mmol/l, n = 7) in C7 cells and H+ secretion in C11 cells (delta pH = 0.060 +/- 0.007, n = 10). Aldosterone-induced stimulation of K+ secretion is inhibited by apical application of amiloride (1 mumol/l). cAMP stimulates H+ secretion in C11 cells (delta pH = -0.068 +/- 0.004, n = 10). Furthermore, C7 cells are peanut-lectin(PNA)-negative and exhibit an intracellular pH of 7.39 +/- 0.05 (n = 7), whereas C11 cells maintain intracellular pH at 7.16 +/- 0.05 (n = 8) and a major fraction of cells is PNA positive. We conclude that we have cloned two subtypes of MDCK cells which stably express different functional characteristics. The C7 subtype resembles principal cells (PC) of the renal collecting duct, whereas the C11 subtype resembles intercalated cells (ICC) of the renal collecting duct.(ABSTRACT TRUNCATED AT 250 WORDS)

The present observations suggest that the two receptors cubilin and megalin are both involved in the endocytic uptake of albumin in renal proximal tubule cells.

The steroid hormone aldosterone is important for salt and water homeostasis as well as for pathological tissue modifications in the cardiovascular system and the kidney. The mechanisms of action include a classical genomic pathway, but physiological relevant nongenotropic effects have also been described. Unlike for estrogens or progesterone, the mechanisms for these nongenotropic effects are not well understood, although pharmacological studies suggest a role for the mineralocorticoid receptor (MR). Here we investigated whether the MR contributes to nongenotropic effects. After transfection with human MR, aldosterone induced a rapid and dose-dependent phosphorylation of ERK1/2 and c-Jun NH2-terminal kinase (JNK) 1/2 kinases in Chinese hamster ovary or human embryonic kidney cells, which was reduced by the MR-antagonist spironolactone and involved cSrc kinase as well as the epidermal growth factor receptor. In primary human aortic endothelial cells, similar results were obtained for ERK1/2 and JNK1/2. Inhibition of MAPK kinase (MEK) kinase but not of protein kinase C prevented the rapid action of aldosterone and also reduced aldosterone-induced transactivation, most probably due to impaired nuclear-cytoplasmic shuttling of MR. Cytosolic Ca2+ was increased by aldosterone in mock- and in human MR-transfected cells to the same extend due to Ca2+ influx, whereas dexamethasone had virtually no effect. Spironolactone did not prevent the Ca2+ response. We conclude that some nongenotropic effects of aldosterone are MR dependent and others are MR independent (e.g. Ca2+), indicating a higher degree of complexity of rapid aldosterone signaling. According to this model, we have to distinguish three aldosterone signaling pathways: 1) genomic via MR, 2) nongenotropic via MR, and 3) nongenotropic MR independent.