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

Yvonne Rellmann,Isabel Gronau,Rita Dreier,Uwe Hansen

doi:10.1155/2019/6404035

Yvonne Rellmann, Isabel Gronau + Show 2 more

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https://doi.org/10.1155/2019/6404035

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Abstract

Objective The integrity of cartilage depends on the correct synthesis of extracellular matrix (ECM) components. In case of insufficient folding of proteins in the endoplasmic reticulum (ER) of chondrocytes, ECM proteins aggregate, ER stress evolves, and the unfolded protein response (UPR) is initiated. By this mechanism, chondrocytes relieve the stress condition or initiate cell death by apoptosis. Especially persistent ER stress has emerged as a pathogenic mechanism in cartilage diseases, such as chondrodysplasias and osteoarthritis. As pharmacological intervention is not available yet, it is of great interest to understand cartilage ER stress in detail and to develop therapeutics to intervene. Methods ERp57-deficient chondrocytes were generated by CRISPR/Cas9-induced KO. ER stress and autophagy were studied on mRNA and protein level as well as by transmission electron microscopy (TEM) in chondrocyte micromass or cartilage explant cultures of ERp57 KO mice. Thapsigargin (Tg), an inhibitor of the ER-residing Ca2+-ATPase, and 4-Phenylbutyric acid (4-PBA), a small molecular chemical chaperone, were applied to induce or inhibit ER stress. Results Our data reveal that the loss of the protein disulfide isomerase ERp57 is sufficient to induce ER stress in chondrocytes. 4-PBA efficiently diffuses into cartilage explant cultures and diminishes excessive ER stress in chondrocytes dose dependently, no matter if it is induced by ERp57 KO or stimulation with Tg. Conclusion ER-stress-related diseases have different sources; therefore, various targets for therapeutic treatment exist. In the future, 4-PBA may be used alone or in combination with other drugs for the treatment of ER-stress-related skeletal disorders in patients.

Highlights

During endochondral ossification, chondrocytes produce large amounts of extracellular matrix (ECM) components [1]
The present study focuses on the low molecular weight chemical chaperone 4-Phenylbutyric acid (4-PBA). 4-PBA is approved for the treatment of urea cycle disorders, but the majority of investigations suggest that it acts as a chemical chaperone that attenuates endoplasmic reticulum (ER) stress in different cell types [22]. 4PBA supports folding processes by interacting with hydrophobic regions in unfolded proteins, preventing their aggregation
It is of great interest to understand ER stress and unfolded protein response (UPR) signaling in chondrocytes in detail

Summary

Introduction

Chondrocytes produce large amounts of extracellular matrix (ECM) components [1]. ECM proteins need to undergo posttranslational modification and folding in the endoplasmic reticulum (ER). ECM protein folding often begins with glycosylation and subsequent trimming of the N-linked glycans. These processes enhance the solubility and allow the interaction with calnexin and calreticulin to promote folding [5, 6]. Both lectins bind ERp57, a PDI which is in the focus of this study. Folded proteins move to the Golgi apparatus where posttranslational modification is completed and sorting into vesicles for the secretion into the ECM is established

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Conclusion