BACKGROUNDPodocyte apoptosis is recently indicated as an early phenomenon of diabetic nephropathy. Pancreatic β-cells exposed to saturated free fatty acid palmitate undergo irreversible endoplasmic reticulum (ER) stress and consequent apoptosis, contributing to the onset of diabetes. We hypothesized that palmitate could induce podocyte apoptosis via ER stress, which initiates or aggravates proteinuria in diabetic nephropathy.

METHODSPodocyte apoptosis was detected by 4',6-diamidio-2-phenylindole (DAPI) stained apoptotic cell count and Annexin V-PI stain. The expressions of ER molecule chaperone glucose-regulated protein 78 (GRP78), indicators of ER-associated apoptosis C/EBP homologous protein (CHOP), and Bcl-2 were assayed by Western blotting and real-time PCR. GRP78 and synaptopodin were co-localized by immunofluorescence stain.

RESULTSPalmitate significantly increased the percentage of cultured apoptotic murine podocytes time-dependently when loading 0.75 mmol/L (10 hours, 13 hours, and 15 hours compared with 0 hour, P < 0.001) and dose-dependently when loading palmitate ranging from 0.25 to 1.00 mmol/L for 15 hours (compared to control, P < 0.001). Palmitate time-dependently and dose-dependently increased the protein expression of GRP78 and CHOP, and decreased that of Bcl-2. Palmitate loading ranging from 0.5 to 1.0 mmol/L for 12 hours significantly increased mRNA of GRP78 and CHOP, and decreased that of Bcl-2 compared to control (P < 0.001), with the maximum concentration being 0.75 mmol/L. Palmitate 0.5 mmol/L loading for 3 hours, 8 hours, and 12 hours significantly increased mRNA of GRP78 and CHOP, and decreased that of Bcl-2 compared to 0 hour (P < 0.001), with the maximum effect at 3 hours. Confocal microscopy demonstrated that GRP78 expression was significantly increased when exposed to 0.5 mmol/L of palmitate for 8 hours compared to control.

CONCLUSIONPalmitate could induce podocyte apoptosis via ER stress, suggesting podocyte apoptosis and consequent proteinuria caused by lipotoxic free fatty acid could be ameliorated by relief of ER stress.

Apoptosis ; drug effects ; Cells, Cultured ; Endoplasmic Reticulum Stress ; physiology ; Heat-Shock Proteins ; analysis ; physiology ; Humans ; Insulin Resistance ; Palmitic Acid ; pharmacology ; Podocytes ; drug effects ; pathology

Early diabetic nephropathy is characterized by glomerular hyperpermeability as a result of impaired glomerular filtration structure caused by hyperglycemia, glycated proteins or irreversible advanced glycosylation endproducts (AGE). To investigate the effect of ginseng total saponin (GTS) on the pathologic changes of podocyte ZO (zonula occludens)-1 protein and podocyte permeability induced by diabetic conditions, we cultured mouse podocytes under: 1) normal glucose (5 mM, = control); 2) high glucose (HG, 30 mM); 3) AGE-added; or 4) HG plus AGE-added conditions and treated with GTS. HG and AGE increased the dextran filtration of monolayered podocytes at early stage (2-8 hr) in permeability assay. In confocal imaging, ZO-1 colocalized with actin filaments and beta-catenin at cell contact areas, forming intercellular filtration gaps. However, these diabetic conditions suppressed ZO-1 immunostainings and disrupted the linearity of ZO-1. In Western blotting, diabetic conditions also decreased cellular ZO-1 protein levels at 6 hr and 24 hr. GTS improved such quantitative and qualitative changes. These findings imply that HG and AGE have an influence on the redistribution and amount of ZO-1 protein of podocytes thereby causing hyperpermeability at early stage, which can be reversed by GTS.
Actin Cytoskeleton/metabolism ; Animals ; Cell Line ; Diabetic Nephropathies/physiopathology ; Glomerular Filtration Rate ; Glucose/*pharmacology ; Glycosylation End Products, Advanced/*pharmacology ; Hyperglycemia/physiopathology ; Membrane Proteins/*metabolism ; Mice ; *Panax ; Permeability/drug effects ; Phosphoproteins/*metabolism ; Plant Preparations/*pharmacology ; Podocytes/drug effects/pathology/physiology ; Saponins/*pharmacology ; beta Catenin/metabolism