**Background:** Hepatocellular carcinoma (HCC) is the most common type of liver cancer, accounting for approximately 90% of all liver cancer cases, and is the fourth leading cause of cancer-related deaths worldwide. The 5-year relative survival rate for liver cancer was 20% from the mid-1970s to 2017. Risk factors include chronic hepatitis B (HBV, about 56% of cases), hepatitis C (HCV, about 20%), alcohol and smoking (about 20%), and increasingly non-alcoholic fatty liver disease (NAFLD). The TGF-β/SMAD signaling pathway is highly conserved and regulates hepatobiliary development, liver homeostasis, and repair. It plays a dual role in tumors: in early stages it induces apoptosis and inhibits proliferation, while in late stages it promotes progression through epithelial-mesenchymal transition (EMT), angiogenesis, and immune escape. This review focuses on the regulatory mechanisms of TGF-β/SMAD in HCC, including its role in different pathogenic factors, disease stages, cell populations, microenvironments, and interactions with microRNAs, as well as therapeutic strategies targeting this pathway.
**Methods:** This is a narrative review that synthesizes findings from numerous studies on the TGF-β/SMAD signaling pathway in HCC. The authors discuss the structure and function of TGF-β family members (TGF-β1-5) and SMAD proteins (R-SMADs, Co-SMAD, I-SMADs), the classical signaling cascade, and crosstalk with other pathways (e.g., MAPK, PI3K/AKT, WNT/β-catenin). They review evidence from in vitro and in vivo studies, including mouse models (e.g., SMAD3 HT mice with C-terminal mutations, SMAD4^Δhep^ hepatocyte-specific knockout mice, DEN-induced HCC models), human HCC cell lines (e.g., HepG2, Hep3B, HuH7, MHCC-97L), and clinical data from The Cancer Genome Atlas (TCGA) database and patient survival analyses. The review also covers studies on microRNA interactions and therapeutic agents, including small molecule inhibitors (e.g., Galunisertib, LY2109761), therapeutic vaccines (e.g., Belagenpumatucel-L), and traditional Chinese medicine extracts (e.g., Salvianolic acid B, Astragaloside IV).
**Key Results:** The TGF-β/SMAD pathway exhibits context-dependent dual roles in HCC. In normal or precancerous hepatocytes, TGF-β signaling induces cell cycle arrest by increasing p15^INK4b^ and p21^CIP1^ levels, and promotes apoptosis by reducing Bcl-2, Bcl-xL, and XIAP expression. In contrast, during HCC development and metastasis, TGF-β promotes proliferation, EMT, and invasion by activating Snail and Gli-1, stimulating MMP-2, MMP-8, and MMP-9 expression, and inducing angiogenesis and immune escape. SMAD2/3 phosphorylation patterns are critical: C-terminal phosphorylated SMAD3 (pSMAD3C) is protective against liver injury and tumor development, while linker region phosphorylated SMAD3 (pSMAD3L) and bisphosphorylated SMAD2/3 (pSMAD2L/C) have higher carcinogenic potential and are associated with poor prognosis. SMAD4 is upregulated in human HCC tissue and associated with poor differentiation and lower postoperative survival; its knockdown in Huh7 and Huh6 cells reduces colony formation and migration. SMAD7 acts as a tumor suppressor; low expression is linked to reduced survival in HCC patients, and its deficiency enhances cell proliferation via TGF-β and NF-κB signaling. MicroRNAs such as miR-122, miR-4458, and miR-542-3p inhibit HCC cell migration and invasion by targeting TGF-β or TβRI, while miR-21-3p downregulates SMAD7 to exert carcinogenic effects. Therapeutic agents targeting the pathway include Galunisertib (TβRI inhibitor), which reduces HCC aggressiveness and enhances stereotactic radiotherapy; LY2109761 (TβRI/II dual inhibitor), which suppresses cell migration; and traditional Chinese medicines like Salvianolic acid B, which promotes the switch from oncogenic pSMAD3L to tumor-suppressive pSMAD3C.
**Clinical Implications:** The TGF-β/SMAD signaling pathway is a promising therapeutic target for HCC, but its complexity and dual role pose challenges. Targeting this pathway with small molecule inhibitors (e.g., Galunisertib, LY3200882), therapeutic vaccines (e.g., Belagenpumatucel-L), and traditional Chinese medicine extracts (e.g., Songyou Yin, Echinacoside) shows potential in preclinical and clinical studies. However, side effects such as reversible cutaneous keratoacanthoma with Fresolimumab and cardiovascular effects with TβRI inhibitors highlight the need for careful development. The pathway's role in different cell types and disease stages requires further elucidation to optimize personalized medicine approaches. Combination therapies, such as Galunisertib with stereotactic radiotherapy or PD-1 blockade with anti-TGF-β/VEGF bispecific antibodies, may improve treatment outcomes. Overall, targeting TGF-β/SMAD signaling represents a new opportunity for HCC therapy, but ongoing research is needed to address its pleiotropic effects and minimize adverse events.