Background: Cardiac fibrosis, characterized by aberrant cardiac fibroblast (CFs) activity and extracellular matrix deposition, is a fundamental pathological process underlying multiple heart diseases. Tripartite motif 34 (TRIM34), a K63-linked ubiquitin ligase, is upregulated in failing human hearts, yet its function in cardiac fibrosis remains unclear. This study aims to investigate the role of TRIM34 in cardiac fibrosis.

Methods: In vivo, fibrosis was induced by myocardial infarction (MI) via left anterior descending coronary artery ligation in mice, while TRIM34 knockdown was achieved using recombinant adenovirus-mediated shRNA. Fibrosis area and related markers were evaluated. In vitro, cardiac fibroblasts were treated with transforming growth factor beta 1 (TGF-β1) and transfected with shTRIM34; proliferation, migration, invasion, and fibroblast activation markers were assessed. Protein interaction, ubiquitination, and promoter activity were examined. Rescue experiments were performed using SMAD family member 7 (SMAD7) or heterogeneous nuclear ribonucleoprotein L (HNRNPL) knockdown.

Results: Upregulation of TRIM34 was found in fibrotic tissue of mice and TGF-β1-exposed CFs. Knockdown of TRIM34 reduced the cardiac fibrosis area and the expression of fibrosis markers in MI mice. Also, TRIM34 knockdown decreased the cell viability, numbers of migrated and invaded cells, and the level of myofibroblast marker, with the increased level of fibroblast markers in TGF-β1-induced CFs. Moreover, TRIM34 directly interacted and promoted ubiquitination degradation of HNRNPL. Furthermore, TRIM34 suppressed the transcription of the SMAD7 promoter by inhibiting the expression of HNRNPL. Rescue experiments showed that knockdown of HNRNPL or SMAD7 counteracted the antifibrotic role of shTRIM34 in MI mice and TGF-β1-exposed CFs.

Conclusion: TRIM34 exacerbated cardiac fibrosis by ubiquitinating HNRNPL and regulating SMAD7 expression, highlighting TRIM34 as a potential therapeutic target for fibrotic heart disease.