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

Lee YJ, Suh HN, Han HJ. Effect of BSA-induced ER stress on SGLT protein expression levels and -MG uptake in renal proximal tubule cells. Am J Physiol Renal Physiol 296: F1405–F1416, 2009. First published February 11, 2009; doi:10.1152/ajprenal.90652.2008.—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 -methyl-D-glucopyranoside ( -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 -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 2 (eIF2 ) phosphorylation in a time-dependent manner. Furthermore, transfection with a GRP78-specific small interfering RNA (siRNA) inhibited BSA-stimulated SGLT expression and -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, eIF2 phosphorylation, SGLT expression, and -MG uptake. Moreover, the cells upregulated peroxisome proliferator-activated receptor(PPAR ) mRNA levels in response to BSA or troglitazone (a PPAR agonist), but BSA was ineffective in the presence of GW9662 (a PPAR antagonist). In addition, both BSA and troglitazone stimulated GRP78 and eIF2 activation, SGLT expression, and -MG uptake, whereas GW9662 inhibited the effects of BSA. BSA also stimulated phosphorylation of JNK and NFB, and GW9662 or GRP78 siRNA attenuated this response. Moreover, SP600125 or SN50 effectively blocked SGLT expression and -MG uptake in BSAor PPAR agonists (troglitazone or PGJ2)-treated PTCs. We conclude that BSA induces ER stress through ROS production and PPAR activation, which subsequently activates JNK/NFB signaling to enhance glucose uptake in renal PTCs.