4-Phenylbutyric Acid Reduces Endoplasmic Reticulum Stress in Chondrocytes That Is Caused by Loss of the Protein Disulfide Isomerase ERp57

Figure 1

CRISPR/Cas9-induced knockout of ERp57 in C28/I2 cells results in ER stress and UPR signaling with subsequent induction of autophagy and apoptosis. (a) Immunoblot analysis of ERp57 in transfected and control C28/I2 cells with GAPDH as an internal control. ERp57 is entirely absent in C28/I2 cells after CRISPR/Cas9-induced knockout of ERp57 and puromycin selection of the transfected cells. Accordingly, the transfected cells are referred to as KO cells. (b) Immunoblot analysis of ER stress and autophagy marker proteins with GAPDH as an internal control. Compared to control cells set as 1, the investigated marker proteins BiP, Chop, and LC3 II are increased in ERp57 KO cells relative to the GAPDH levels. This demonstrates that the loss of ERp57 activity in the ER leads to a reduced protein folding capacity. Accumulating proteins in the ER induce ER stress with subsequent UPR signaling and induction of autophagy and apoptosis. , , and . (c) Transmission electron microscopic analysis of ERp57 KO and control cells. Nontransfected C28/I2 cells (control) reveal regular stacks of rough ER (>) and low amounts of tiny autophagic vesicles () in the cytoplasm. In contrast, ERp57 KO cells show large autophagic structures including bulk cytoplasm and entire organelles. Particularly striking are the autophagosomes including ER whorls (presented enlarged in the right panel). All .

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