Background:The integrated stress response (ISR) maintains cellular homeostasis during aberrant protein folding (ER stress). Results:The ISR enhances glutathione synthesis through up-regulation of cystathionine ␥-lyase via the eIF2␣-ATF4 pathway. Conclusion: Cells undergoing the ISR induce cystathionine ␥-lyase, thereby maintaining cellular homeostasis. Significance: These findings link the cells response to ER stress and redox homeostasis through the ISR.

Epithelial-to-mesenchymal transition (EMT) contributes to renal fibrosis in chronic kidney disease. Endoplasmic reticulum (ER) stress, a feature of many forms of kidney disease, results from the accumulation of misfolded proteins in the ER and leads to the unfolded protein response (UPR). We hypothesized that ER stress mediates EMT in human renal proximal tubules. ER stress is induced by a variety of stressors differing in their mechanism of action, including tunicamycin, thapsigargin, and the calcineurin inhibitor cyclosporine A. These ER stressors increased the UPR markers GRP78, GRP94, and phospho-eIF2α in human proximal tubular cells. Thapsigargin and cyclosporine A also increased cytosolic Ca(2+) concentration and T cell death-associated gene 51 (TDAG51) expression, whereas tunicamycin did not. Thapsigargin was also shown to increase levels of active transforming growth factor (TGF)-β1 in the media of cultured human proximal tubular cells. Thapsigargin induced cytoskeletal rearrangement, β-catenin nuclear translocation, and α-smooth muscle actin and vinculin expression in proximal tubular cells, indicating an EMT response. Subconfluent primary human proximal tubular cells were induced to undergo EMT by TGF-β1 treatment. In contrast, tunicamycin treatment did not produce an EMT response. Plasmid-mediated overexpression of TDAG51 resulted in cell shape change and β-catenin nuclear translocation. These results allowed us to develop a two-hit model of ER stress-induced EMT, where Ca(2+) dysregulation-mediated TDAG51 upregulation primes the cell for mesenchymal transformation via Wnt signaling and then TGF-β1 activation leads to a complete EMT response. Thus the release of Ca(2+) from ER stores mediates EMT in human proximal tubular epithelium via the induction of TDAG51.

Hypertension and proteinuria are associated with the progression of chronic kidney disease (CKD). CKD occurs via a process of kidney fibrosis and nephron loss. The Dahl salt sensitive hypertensive rat (Dahl SS) is a model of hypertension caused by high salt, resulting in proteinuria and CKD. We hypothesized that hypertension‐induced proteinuria in the Dahl SS leads to renal fibrosis through the initiation of ER stress and the unfolded protein response (UPR). The UPR is an important modulator in the differentiation of various cell types, including myofibroblasts. We tested this using two models of hypertension, the Dahl SS rat and the spontaneously hypertensive rat (SHR) treated with a high salt (8% NaCl) or maintained on a low salt (0.4% NaCl) diet for 14 days. Kidneys from these animals were processed for histology and immunohistochemistry for markers of both ER stress and fibrosis. In the high salt treated Dahl SS rat, intratubular protein casts were observed. These rats also displayed renal fibrosis as shown by trichrome and α‐smooth‐muscle actin staining, and ER stress, shown by GRP78 and CHOP staining. Contrarily, the SHR developed no significant proteinuria or interstitial fibrosis in response to high salt despite malignant hypertension. These results suggest that hypertension‐induced proteinuria is associated with an ER stress response and renal interstitial fibrosis. Supported by CIHR – OSO‐115895.