Effect of BSA-induced ER stress on SGLT protein expression levels and α-MG uptake in renal proximal tubule cells

Abstract

Recent studies demonstrated that endoplasmic reticulum (ER) stress regulates glucose homeostasis and that ER stress preconditioning which induces an adaptive, protective unfolded protein response (UPR) offers cytoprotection against nephrotoxins. Thus the aim of the present study was to use renal proximal tubule cells (PTCs) to further elucidate the link between the BSA-induced ER stress and alpha-methyl-d-glucopyranoside (alpha-MG) uptake and to identify related signaling pathways. Among ER stress inducers such as high glucose, BSA, H2O2, or tumicamycin, BSA pretreatment ameliorated the reduction of Na(+)-glucose cotransporter (SGLT) expression and alpha-MG uptake by gentamicin or cyclosporine A. Immunofluorescence studies revealed that BSA (10 mg/ml) stimulated the expression of glucose-regulated protein 78 (GRP78), an ER stress biomarker. In addition, BSA increased levels of GRP78 protein expression and eukaryotic initiation factor 2alpha (eIF2alpha) phosphorylation in a time-dependent manner. Furthermore, transfection with a GRP78-specific small interfering RNA (siRNA) inhibited BSA-stimulated SGLT expression and alpha-MG uptake. In experiments designed to unravel the mechanisms underlying BSA-induced ER stress, BSA stimulated the production of cellular reactive oxygen species (ROS), and antioxidants such as ascorbic acid or N-acetylcysteine (NAC) blocked BSA-induced increases in GRP78 activation, eIF2alpha phosphorylation, SGLT expression, and alpha-MG uptake. Moreover, the cells upregulated peroxisome proliferator-activated receptor-gamma (PPARgamma) mRNA levels in response to BSA or troglitazone (a PPARgamma agonist), but BSA was ineffective in the presence of GW9662 (a PPARgamma antagonist). In addition, both BSA and troglitazone stimulated GRP78 and eIF2alpha activation, SGLT expression, and alpha-MG uptake, whereas GW9662 inhibited the effects of BSA. BSA also stimulated phosphorylation of JNK and NF-kappaB, and GW9662 or GRP78 siRNA attenuated this response. Moreover, SP600125 or SN50 effectively blocked SGLT expression and alpha-MG uptake in BSA- or PPARgamma agonists (troglitazone or PGJ2)-treated PTCs. We conclude that BSA induces ER stress through ROS production and PPARgamma activation, which subsequently activates JNK/NF-kappaB signaling to enhance glucose uptake in renal PTCs.

Tumor necrosis factor-α suppresses angiotensinogen expression through formation of a p50/p50 homodimer in human renal proximal tubular cells

“…A slight activation of NF-B was observed in control groups. It has been reported that serum albumin activates NF-B in RPTCs (18). Therefore, the minor activation of NF-B in the control groups might have been caused by the replacement of medium containing 1% FBS, which was required to prevent the loss of RPTCs character Fig.…”

Activation of AMP-Activated Protein Kinase Inhibits Albumin-Induced Endoplasmic Reticulum Stress and Apoptosis through Inhibition of Reactive Oxygen Species

“…Urinary albumin is also known to cause ER stress in tubular epithelial cells [2,4,5]. However, signaling pathways have not been revealed completely.…”

15-Deoxy-Δ12,14-prostaglandin J2 prevents inflammatory response and endothelial cell damage in rats with acute obstructive cholangitis

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