Abstract
Endoplasmic reticulum (ER) stress-associated apoptosis plays a role in organ remodeling after insult. The effect of ER stress on renal tubular damage and fibrosis remains controversial. This study aims to investigate whether ER stress is involved in tubular destruction and interstitial fibrosis in vivo. Renal cell apoptosis was proven by terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) stain and poly-ADP ribose polymerase expression in the unilateral ureteral obstruction (UUO) kidney. ER stress was evoked and confirmed by the upregulation of glucose-regulated protein 78 (GRP78) and the common Lys-Asp-Glu-Leu (KDEL) motif of ER retention proteins after UUO. ER stress-associated proapoptotic signals, including B-cell chronic lymphocytic leukemia (CLL)/lymphoma 2-associated × protein (BAX) expression, caspase-12 and c-Jun N-terminal kinase (JNK) phosphorylation, were activated in the UUO kidney. Prolonged ER stress attenuated both unsplicing and splicing X-box binding protein 1 (XBP-1) protein expression, but continued to activate inositol-requiring 1α (IRE1α)-JNK phosphorylation, protein kinase RNA-like endoplasmic reticulum kinase (PERK), eukaryotic translation initiation factor 2α subunit (eIF2α), activating transcription factor (ATF)-4, CCAAT/enhancer binding protein (C/EBP) homologous protein (CHOP) and cleavage activating transcription factor 6 (cATF6)-CHOP signals, which induce ER stress-related apoptosis but attenuate adaptive unfolded protein responses in UUO kidneys. However, renal apoptosis and fibrosis were attenuated in candesartan-treated UUO kidney. Candesartan was associated with maintenance of XBP-1 expression and attenuated ATF4, cATF6 and CHOP protein expression. Taken together, results show that overwhelming ER stress leads to renal cell apoptosis and subsequent fibrosis; and candesartan, at least in part, restores renal integrity by blocking ER stress-related apoptosis. Reducing ER stress may present a way to attenuate renal fibrosis.
Highlights
Renal fibrosis, considered the common consequence of progressive renal disease, involves glomerulosclerosis and/or tubulointerstitial fibrosis [1,2]
This study demonstrated that activation of endoplasmic reticulum (ER) stress is critical for tubular cell loss and renal fibrosis in the ureteral obstruction (UUO) fibrosis model
The ER-luminal domain of protein kinase RNA-like endoplasmic reticulum kinase (PERK), inositol-requiring 1α (IRE1α) and ATF6 proteins interacts with the ER retention protein glucose-regulated protein 78 (GRP78) in the resting state
Summary
Renal fibrosis, considered the common consequence of progressive renal disease, involves glomerulosclerosis and/or tubulointerstitial fibrosis [1,2]. A number of stressors, including ischemia, hypoxia and oxidative injury, contribute to renal cell loss as renal fibrosis progresses [1,2,4,5]. The endoplasmic reticulum (ER) is involved in the folding of secretory and membrane proteins and in the apoptosis pathway [8]. Many conditions disturb the ER function and result in ER stress, including ischemia, hypoxia, glucose starvation, elevated protein synthesis, gene mutation and failure in protein folding, transport or degradation [9,10,11]. ER stress initiates three major signaling pathways, called the unfolded protein response (UPR) [12]. Three sensors along the ER stress pathway are activating transcription factor (ATF)-6, inositol-requiring 1α (IRE1α) and protein kinase RNA-like endoplasmic reticulum kinase (PERK) [8]. Unstressed, the three sensors are linked with the ER chaperone, glucose-regulated protein 78 (GRP78), but these UPR initiators are released from
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