Antioxidant therapy against TGF‐β/SMAD pathway involved in organ fibrosis

**Background:** Fibrotic diseases account for 45% of mortality in the United States and approximately one third of natural deaths worldwide. Fibrosis is characterized by excessive accumulation of extracellular matrix (ECM) components, driven largely by the transforming growth factor beta (TGF-β)/SMAD signaling pathway. Oxidative stress, resulting from an imbalance between reactive oxygen species (ROS) and antioxidant defenses, promotes fibrosis by activating fibroblasts, stimulating pro-inflammatory cytokines, and impairing ECM turnover. Antioxidants—classified as primary (free radical scavengers), secondary (chain initiation retarders), or tertiary (repair agents)—can oppose oxidative damage and may modulate the TGF-β/SMAD pathway. This narrative review aims to summarize the role of various antioxidants as therapeutic agents in organ fibrosis, with a focus on the TGF-β/SMAD pathway as the main target.

**Methods:** The authors conducted a narrative review of the literature, compiling studies that investigated the effects of antioxidants on TGF-β/SMAD signaling in models of organ fibrosis. The review covers gastrointestinal (especially liver), lung, kidney, cardiac, skin, and other fibrotic conditions. Data were extracted from published in vitro and in vivo studies, including those using cell lines (e.g., LX-2, HSC-T6, A549, HK-2) and animal models (e.g., C57BL/6 mice, Sprague-Dawley rats). The primary outcome was the effect of antioxidant treatment on TGF-β/SMAD pathway components and fibrosis markers such as α-smooth muscle actin (α-SMA), collagen I/III, fibronectin, and Smad proteins.

**Key Results:** The review presents extensive tables listing dozens of antioxidants and their effects across organ systems. In liver fibrosis, agents such as ferulic acid (FA), honokiol, isorhamnetin, and many others inhibited TGF-β/SMAD signaling, reducing α-SMA, collagen, and Smad2/3 phosphorylation. For example, FA (30 μM) reversed TGF-β-induced overexpression of α-SMA, FN, Col-I, Smad-2/3, p38, and JNK in LX-2 cells. Honokiol (10 mg/kg) reduced α-SMA expression in concanavalin-A-treated rats. In lung fibrosis, bleomycin (BLM) induced TGF-β/SMAD and EMT, while compounds like tanshinone IIA, ferulic acid, honokiol, and polydatin inhibited the pathway. Vitamin D3 (100 ng) inhibited TGF-β/SMAD in an ovalbumin-induced asthma model. In renal fibrosis, oleanolic acid (6 mg/kg) reduced TGF-β and its receptors, and losartan increased inhibitory Smad7. Melatonin (1 μM) blocked TGF-β expression and Smad phosphorylation in high-glucose-treated TH1 cells. In cardiac fibrosis, matrine (300 mg/kg), dapagliflozin (1 mg/kg), and empagliflozin (10 mg/kg) inhibited TGF-β/SMAD signaling. In skin fibrosis, LG283 (40-80 mg/kg) and DZ2002 (50-100 mg/kg) reduced α-SMA and phosphorylated Smad3. Notably, copper nanoparticles (100-400 mg/kg) and high-dose baicalin (up to 1600 mg/kg) induced TGF-β/SMAD and promoted fibrosis, indicating that some antioxidants can have pro-fibrotic effects depending on dose and context.

**Clinical Implications:** The review suggests that antioxidant therapy may provide a non-invasive, multi-target approach to prevent or treat organ fibrosis by inhibiting the TGF-β/SMAD pathway. Antioxidants are relatively safe and can disrupt multiple fibrotic pathways simultaneously, unlike more invasive or specific treatments. However, the evidence is primarily from animal models and cell lines; human clinical trials are lacking. The authors emphasize the need for further research to optimize dosing, evaluate efficacy across different fibrosis stages and etiologies, and confirm safety and effectiveness in humans. The potential for antioxidants to both inhibit and, in some cases, promote fibrosis (e.g., copper nanoparticles, high-dose baicalin) underscores the importance of careful dose selection and context-specific evaluation.