A stable transfected line of human glomerular epithelial cells

Dexamethasone increases the phosphorylation of nephrin in cultured podocytes

BACKGROUND

We reported that nephrin is phosphorylated at Y1204 and Y1228 under normal conditions and that the phosphorylation is decreased in puromycin nephrosis and in human minimal change nephrosis. These results indicate that the phosphorylation of nephrin is important for maintaining normal podocyte function. However, little is known about the regulation of nephrin phosphorylation. Here, we investigated whether glucocorticoid, a drug used to treat glomerular diseases with proteinuria, might affect the phosphorylation of nephrin.

METHODS

Human cultured podocytes transiently expressing human nephrin were treated with dexamethasone (Dex), and the phosphorylation of nephrin was determined by immunoblot with the anti-pY1228 antibody.

RESULTS

Dex treatment for 24 h increased the phosphorylation of nephrin; this increased phosphorylation was inhibited by the glucocorticoid receptor antagonist but not by the mineral corticoid receptor antagonist. A shorter incubation time (30 min) did not increase the phosphorylation, and actinomycin D and cycloheximide treatments abolished the increased phosphorylation. The activation of Src-family kinases was correlated with nephrin phosphorylation, both of which were abolished by small interfering RNA (siRNA) treatment for serum/glucocorticoid-induced kinase 1 (SGK1).

CONCLUSIONS

These results clarify a novel action of glucocorticoid on nephrin phosphorylation through SGK1. Glucocorticoid treatment for human glomerulonephritis may exert its function by regulating the phosphorylation of nephrin.

Intracellular mislocalization of mutant podocin and correction by chemical chaperones

The NPHS2 gene encoding the podocin protein was causally linked to the autosomal recessive type of steroid-resistant nephrotic syndrome. In this study, we investigated the consequence of the R138Q mutation of podocin, one of the most common missense mutations in the NPHS2 gene, by examining the expression of the wild-type and R138Q mutant podocins in mammalian cells. Either myc- or FLAG-tagged wild-type podocin was strongly stained in plasma membrane, particularly in the fine processes wherein the protein was colocalized with actin stress fibers. On the other hand, the R138Q mutant podocin was completely retained intracellularly and colocalized with the endoplasmic reticulum (ER) marker, calnexin. These results suggest that the R138Q mutation affected podocin protein folding, thereby interfering with the mutant protein's departure from the ER. To determine if the ER retention of R138Q mutant is correctable, cells were incubated with the chemical chaperones glycerol, trimethylamine-N-oxide, and DMSO. Using these two methods, namely, cell surface labeling with sulfo-NHS-S-S-biotin and Alexa 488-streptavidin, and immunostaining to detect the podocin protein close to the plasma membrane, we confirmed that these chemical chaperone treatments elicit a cellular redistribution of R138Q podocin. Our results reveal defective cellular processing of the mutant podocin, and provide evidence for pharmacological correction of the processing defect.

Phenotypic profiles of cultured glomerular cells following repeated cycles of hydrocarbon injury

BACKGROUND

The glomerulus has been implicated as a target of hydrocarbon injury in vitro and in vivo. In the present studies, the phenotypic profiles of cultured rat glomerular cells (GCs) following repeated cycles of hydrocarbon injury were evaluated. Cultured GCs were incubated for 24 hours with benzo[a]pyrene (BaP; 3 micromol/L), a prototypical polycyclic aromatic hydrocarbon, and were allowed to recover overnight before two additional cycles of chemical challenge during serial propagation in vitro. At the end of this regimen, control cultures were characterized by predominance of fusiform cells that grew in "hills and valleys," while GCs subjected to hydrocarbon injury displayed an epithelial morphology characterized by a rounded, polygonal shape clearly distinct from that normally exhibited by glomerular mesangial cells (GMCs) in culture.

METHODS

Indirect immunofluorescent detection of cell markers was conducted to identify cells of mesenchymal or epithelial origin. Measurements of DNA synthesis and cell number were performed to determine proliferative capacities of the different cell types in response to hydrocarbon challenge.

RESULTS

Immunofluorescence studies revealed that control GC cultures contained mostly alpha-smooth muscle (SM) actin-positive cells, with a few (5.1% +/- 2.6) E-cadherin-positive cells occasionally identified. In contrast, BaP-treated cultures exhibited a mixed cell population in which E-cadherin-positive cells were predominant (66.6% +/- 4.1). Single-cell cloning of naive cultures of GCs yielded four clones, three of which exhibited a fusiform morphology and were alpha-SM actin positive (SCC 1 through SCC 3) and one (SCC 4E) that exhibited epithelial characteristics similar to those found in hydrocarbon-treated cultures. Immunofluorescence studies showed that epithelial cells in hydrocarbon-treated cultures, as well as SCC 4E-derived clones, were vimentin positive and cytokeratin negative, characteristics similar to glomerular visceral epithelial cells (GVECs). DNA synthesis and cell proliferation in clone SCC 1 were decreased following acute BaP challenge, while growth rates in SCC 4E-derived clones were unaffected by hydrocarbon injury. Repeated cycles of hydrocarbon challenge in clonal populations yielded different profiles of DNA synthesis, with significant decreases in SCC 1 and no changes in SCC 4E.

CONCLUSIONS

These observations suggest that hydrocarbon injury induces differential responses in cells of the glomerulus, resulting in inhibition of GMCs and selective growth advantage of GVECs. These alterations are reminiscent of critical events described in the pathogenesis of focal segmental glomerulosclerosis and raise important questions about the pathogenesis of hydrocarbon-induced nephropathies.