Dynamics of Connexin 43 Down Modulation in Human Articular Chondrocytes Stimulated by Tumor Necrosis Factor Alpha

Abstract

Connexin 43 (Cx43) exerts pivotal functions in articular chondrocytes (CH). It is involved in the communication among cells and between cells and the extracellular environment, and it contributes to the maintenance of the correct cell phenotype. The pro-inflammatory cytokine TNFα induces a reduction in Cx43 expression in CH. Here, we studied the dynamics of this decrease in expression. We evaluated Cx43 protein and gene expression and the involvement of C-terminal domain (CTD) cleavage and proteasomal degradation. Treatments able to counteract TNFα action were also examined, together with Gap Junction (GJ) functionality and Cx43 localization. TNFα induced a significant reduction in Cx43 expression already at day 1, and the down modulation reached a peak at day 3 (−46%). The decrease was linked to neither gene expression modulation nor CTD cleavage. Differently, the proteasome inhibitor MG132 reverted TNFα effect, indicating the involvement of proteasomal degradation in Cx43 reduction. In addition, the co-treatment with the anabolic factor TGF-β1 restored Cx43 levels. Cx43 decrease occurred both at the membrane level, where it partially influenced GJ communication, and in the nucleus. In conclusion, TNFα induced a rapid and lasting reduction in Cx43 expression mostly via the proteasome. The down modulation could be reverted by cartilage-protective factors such as MG132 and TGF-β1. These findings suggest a possible involvement of Cx43 perturbation during joint inflammation.

SARS-CoV-2 Structural Proteins Modulated Blood-Testis Barrier-Related Proteins through Autophagy in the Primary Sertoli Cells

“…CX43 did not have alternative splicing but had transcriptional factor activity to directly regulate the transcription of N-cadherin [ 37 ]. The protein in the nucleus was shown as two distinct bands [ 38 ], and the expression of N-cadherin was significantly reduced in S-transfected cells. Hence, we suggest another hypothesis that the S reduced the expression of CX43 in the membrane, enhanced CX43 phosphorylation and transfer into the nucleus, and interfered with the structure of the gap junction.…”

Diversity in connexin biology

“…Here, GJA1-20k was proposed to be recruited to the nucleus by basic transcription factor 3 where it forms a complex with polymerase II to regulate N-cadherin transcription ( 127 ) directly implicating a connexin in regulating cellular adhesion at the transcriptional level. Nuclear localization of Cx43 or possibly a C-terminal Cx43 fragment was seen in Wnt-5A– and Wnt-9B–treated cells, whereas TNFα decreased Cx43 nuclear localization prompting the speculation that nuclear Cx43 may be involved in various signaling cascades ( 128 , 129 ). Another study suggests Cx43 is translocated to the nucleus during late G1 phase of the cell cycle by A-kinase anchoring protein 95 ( 130 ).…”

Ablation of Gap Junction Protein Improves the Efficiency of Nanozyme‐Mediated Catalytic/Starvation/Mild‐Temperature Photothermal Therapy

“…[ 20 ] Therefore, blocking Cx43 channels is expected to improve the effectiveness of tumor treatment. [ 21 ] Previous reports have suggested that these Cx43 channels protect tumor cells from ROS damage, thereby facilitating the resistance of tumors to high‐ROS‐mediated therapy. [ 22 ] Given that the innate character of the “bystander effect” of Cx43 in ROS‐mediated tumor catalytic therapy remains controversial, it requires further investigation.…”