Identification of the inhibitory activity of walnut extract on the E3 ligase Syvn1

Synoviolin (Syvn1), an E3 ubiquitin ligase in endoplasmic reticulum‑associated protein degradation, is involved in rheumatoid arthritis, fibrosis, liver cirrhosis and obesity. We previously demonstrated that Syvn1 negatively regulates the function of peroxisome proliferator‑activated receptor gamma coactivator‑1β (PGC‑1β). In addition, treatment with a Syvn1 inhibitor suppressed weight gain in a mouse model of obesity by activating PGC‑1β via Syvn1 inhibition. It has been suggested that the Syvn1 inhibitors may have therapeutic benefits in obese patients. The present study tested the inhibitory activity of walnut extract, a natural product, on Syvn1 activity. Walnut extract inhibited the effect of Syvn1 on the cell proliferation of rheumatoid synovial cells and repressed the interaction between PGC‑1β and Syvn1 in an in vitro binding assay. Polyubiquitination of PGC‑1β by Syvn1 was suppressed by walnut extract in a concentration‑dependent manner, but walnut extract did not have an inhibitory effect on the autoubiquitination of Syvn1. Treatment with walnut extract in mouse embryonic fibroblasts increased the number of mitochondria, suggesting that exposure to the extract recovered PGC‑1β function. These results demonstrated that constituents of walnut extract may serve as lead compounds in drug development efforts aiming to produce drugs to treat patients with obesity and obesity‑associated metabolic diseases.