Submitted on October 31, 2008
Revised on February 9, 2009
Accepted on February 9, 2009
Effect of BSA-induced ER stress on SGLTs protein expression levels and 
-MG uptake in renal proximal tubule cells
Yu Jin Lee1, Han Na Suh2, and Ho Jae Han1*
1 College of Veterinary Medicine, Chonnam National University
2 Biotherapy Human Resources Center
* To whom correspondence should be addressed. E-mail: hjhan{at}chonnam.ac.kr.
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 bovine serum albumin (BSA)-induced ER stress and 
-MG uptake, and to identify related signaling pathways. Among ER stress inducers such as high glucose, bovine serum albumin (BSA), H2O2, or tumicamycin, BSA pretreatment ameliorated the reduction of Na+/glucose co-transporter (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 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 ROS, and antioxidants blocked BSA-induced increases in GRP78 activation, eIF2 phosphorylation, SGLT expression, and -MG uptake. Moreover, the cells upregulated 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 NF-
B, and GW9662 or GRP78 siRNA attenuated this response. Moreover, SP600125 or SN50 effectively blocked SGLT expression and -MG uptake in BSA or PPAR agonists (troglitazone or PGJ2)-treated PTCs. We conclude that BSA induces ER stress through ROS production and PPAR activation, which subsequently activates JNK/NF-B signaling to enhance glucose uptake in renal PTCs.
