Cadmium impairs albumin reabsorption by down-regulating megalin and ClC5 channels in renal proximal tubule cells.
BACKGROUND
Cadmium (Cd) is a potent nephrotoxicant that impairs the reabsorptive and secretory functions of the renal proximal tubule, leading to albuminuria.


OBJECTIVE
To gain insights into the mechanisms of Cd-induced albuminuria, we investigated effects of Cd on the expression of megalin and chloride channel 5 (ClC5), two key players in albumin-receptor-mediated endocytosis.


METHODS
We used quantitative polymerase chain reaction, Western blotting, the albumin endocytosis assay, and confocal microscopy to evaluate effects of Cd on the expression and regulation of megalin and ClC5 in cultured LLC-PK1 cells, a pig proximal tubular cell model.


RESULTS
Ten micromolar cadmium chloride (CdCl2) caused a significant time- and dose-dependent decrease in both mRNA and protein levels of megalin and ClC5, whereas no changes resulted from exposure to other divalent metals (zinc chloride, manganese chloride, magnesium chloride, and nickel chloride). After inhibiting protein synthesis using cycloheximide (CHX), we found that levels of both megalin and ClC5 were lower in Cd-challenged cells than in cells treated with Cd or CHX only, which is consistent with reduced translation and/or posttranslational down-regulation. Moreover, Cd-induced degradation of megalin and ClC5 was abolished by the lysosomal pathway inhibitor bafilomycin A-1 but not by the proteasome system blocker MG-132, suggesting that the enhanced proteolysis was occurring via lysosomes. Using confocal microscopy, we observed a remarkable reduction of fluoroisothiocyanate (FITC)-labeled albumin uptake after Cd exposure.


CONCLUSIONS
We found that Cd reduced the transcriptional expression of megalin and ClC5 and, at the same time, increased the degradation of megalin and ClC5 proteins via the lysosomal pathway in an in vitro model of renal proximal tubular cells. Overall, these results provide valuable insights into the molecular mechanisms by which Cd impairs luminal protein reabsorption by renal proximal tubules.
Gena
Patrizia
P
Department of Physiology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.
Calamita
Giuseppe
G
Guggino
William B
WB
D000418:Albumins / Q000378:metabolism*; D000419:Albuminuria / Q000139:chemically induced / Q000378:metabolism; D000818:Animals; D002104:Cadmium / Q000633:toxicity*; D002478:Cells, Cultured; D018118:Chloride Channels / Q000235:genetics / Q000378:metabolism*; D015536:Down-Regulation / Q000187:drug effects; D004785:Environmental Pollutants / Q000633:toxicity*; D007687:Kidney Tubules, Proximal / Q000187:drug effects* / Q000378:metabolism; D026561:Low Density Lipoprotein Receptor-Related Protein-2 / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism
D000418:Albumins; C097503:CLC-5 chloride channel; D018118:Chloride Channels; D004785:Environmental Pollutants; D026561:Low Density Lipoprotein Receptor-Related Protein-2; D012333:RNA, Messenger; D002104:Cadmium