Background/Purpose: In rheumatoid arthritis (RA), the pro-inflammatory cytokine interleukin-6 (IL-6) induces systemic inflammation by activating hepatocytes through IL-6 receptor (IL-6R)- mediated JAK/STAT3 signaling pathway. The present study was undertaken to determine the role of epigallocatechin 3-gallate (EGCG), an active compound found in green tea, in regulating IL-6-induced JAK/STAT3 pathway in Hep3B cells and in a rat adjuvant-induced arthritis (AIA) model.

Methods: Hep3B cells were starved, pretreated overnight with EGCG (5-20 µM), and then stimulated with IL-6 (50 ng/ml). Cells were harvested after 30 minutes to evaluate the effect on different phosphorylation sites on JAK/STAT3 using Western immunoblotting, or after 24 hours of IL-6 stimulation for C-reactive protein (CRP) expression using Western blotting and qRT-PCR methods. Nuclear and cytoplasmic extracts were prepared to study nuclear translocation and DNA binding activity using ELISA kit. Liver homogenates and nuclear extracts were prepared from naïve, AIA, and AIA+EGCG (50 mg/kg, i.p.) treated rats. A molecular docking study of EGCG to STAT3 was performed using an X-ray crystal model of mouse STAT3 (PDB ID: 1BG1) as a template and EGCG as a ligand.

Results: Pretreatment of Hep3B cells with EGCG inhibited IL-6-induced STAT3^Ser727 and STAT3^Tyr705 activation in a dose-dependent manner, which may partially be attributed to the inhibition of pJAK1^Tyr1022/1023 and pJAK2^Tyr1007/1008 expression by roughly 37% and 27%, respectively (p<0.05). Inhibition of pSTAT3^Ser727 was more prominent than pSTAT3^Tyr705 inhibition with EGCG pretreatment. Evaluation of the nuclear fractions from the treated Hep3B cells showed that EGCG significantly inhibited the nuclear translocation of IL-6-induced STAT3 (p<0.05). This resulted in ~25% decrease in STAT3 DNA binding activity by EGCG treatment and a dose-dependent decrease in CRP expression (p<0.05). Surprisingly, EGCG exhibited no effect on the endogenous STAT3 regulator, SOCS3. Administration of EGCG (50 mg/kg, i.p.) daily for 10 days significantly reduced the severity and incidence of arthritis compared to the untreated rats (p<0.05; n=6/group). Evaluation of the liver homogenates from these rats showed that EGCG inhibited pSTAT3^Ser727 by ~70% and reduced its nuclear translocation. Molecular docking studies showed that EGCG can bind to R382, R417, R423, S465, and N469 residues on STAT3, which may potentially inhibit the DNA binding activity of STAT3.

Conclusion: Our results suggest that the EGCG may elicit its beneficial effects in part through the suppression of IL-6-induced systemic inflammation.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/epigallocatechin-3-gallate-egcg-suppresses-systemic-inflammation-by-inhibiting-il-6-induced-stat3-activation-in-cultured-hepatocytes-and-in-liver-tissue-of-adjuvant-induced-arthritis-aia-rats/