Hassan Baig

Background:  Metabolic Dysfunction-Associated Steatohepatitis (MASH), previously known as NASH, the progressive inflammatory subtype of nonalcoholic fatty liver disease (NAFLD), affects up to 25% of the global population and is projected to become the leading cause of liver transplantation by 2030¹. MASH is driven by metabolic dysfunction and sustained liver injury, leading to activation of hepatic stellate cells (HSCs) and fibrotic remodeling. Transforming Growth Factor-β1 (TGF-β1) is a key cytokine in this process, signaling through both Smad3-mediated transcriptional pathways and PI3K/Akt/mTOR-dependent translational mechanisms²⁻⁴. Current treatments for MASH lack efficacy against advanced fibrosis, necessitating new therapeutic strategies targeting these pathways.

Objective:  This narrative review examines how TGF-β1 promotes HSC-driven fibrosis in MASH through canonical and non-canonical pathways, evaluates upstream regulators, and explores translational therapeutic implications.

Search Methods:  A comprehensive PubMed search was conducted from 2017 to 2024 using the keywords: “TGF-β1,” “hepatic stellate cells,” “MASH,” “fibrosis,” “Smad3,” “PI3K/Akt,” and “ghrelin.” Five peer-reviewed studies were selected based on mechanistic depth, relevance to NAFLD/MASH progression, and inclusion of in vitro, animal, and/or human clinical data.

Results:  In early MASH, hepatocytes under metabolic stress secrete latent TGF-β1, which is activated by inflammatory cytokines (TNF-α, IL-1β) released by infiltrating macrophages⁵. Ishiyama et al. visualized this process using a novel SS3D reconstruction technique, showing colocalization of macrophages and collagen in fibrotic liver zones⁵.  Activated TGF-β1 initiates two major fibrotic signaling cascades. In the canonical Smad3 pathway, phosphorylated Smad3 translocates to the nucleus and upregulates genes like TLR2, promoting macrophage recruitment and hepatic inflammation². Chen et al. showed that Smad3 knockdown attenuated TLR2 expression and proinflammatory cytokines, confirming Smad3’s role in immune crosstalk². Smad3 also represses metabolic regulators. Briansó-Llort et al. found that TGF-β1 suppresses SHBG expression via Smad3 and STAT3, impairing hepatocyte function and contributing to metabolic dysregulation³. This aligns with clinical findings linking reduced SHBG levels to advanced NAFLD¹.  The non-canonical PI3K/Akt/mTOR pathway complements this response by promoting myofibroblast transformation. Ezquerro et al. showed that TGF-β1 stimulation of LX-2 HSCs increased phosphorylation of Akt/mTOR and expression of fibrotic markers (COL1A1, α-SMA, TIMP-1)⁴. Acylated ghrelin attenuated this response, while its antagonist LEAP-2 intensified fibrosis, both in vitro and in bariatric patients⁴.  Additionally, Sonic Hedgehog (Shh) signaling in injured hepatocytes was shown to amplify Smad3 activity and fibrogenic gene expression. Zhou et al. found that Shh⁺ hepatocytes expressed elevated TGF-β1 and recruited more HSCs, suggesting a paracrine amplification loop⁶. These mechanisms collectively form a feed-forward circuit in which inflammation, transcriptional activation, and ECM production sustain fibrosis even after the initial injury resolves.

Conclusions:  TGF-β1 drives hepatic fibrosis in MASH via coordinated activation of Smad3 and PI3K/Akt/mTOR pathways. Inflammatory cytokines, Shh signaling, and the ghrelin/LEAP-2 hormonal axis enhance this signaling. Targeting both arms of this dual pathway may be necessary to disrupt fibrogenesis. Smad3 inhibitors, ghrelin analogs, and mTOR blockade represent promising therapeutic strategies. Biomarkers such as SHBG and ghrelin/LEAP-2 ratios may help monitor disease progression and therapeutic response.

Works Cited:

1. Xu X, Poulsen KL, Wu L, et al. Targeted therapeutics and novel signaling pathways in non-alcohol-associated fatty liver/steatohepatitis (NAFL/NASH). Signal Transduct Target Ther. 2022;7(1):287. doi:10.1038/s41392-022-01119-3
2. Chen Q, Yi J, Liu F, et al. TGF-β1 contributes to hepatic inflammation in animal models with nonalcoholic steatohepatitis by Smad3/TLR2 signaling pathway. Mol Immunol. 2022;152:129–139. doi:10.1016/j.molimm.2022.10.014
3. Briansó-Llort L, Fuertes-Rioja L, Ramos-Perez L, et al. Transforming growth factor-beta 1: A new factor reducing hepatic SHBG production in liver fibrosis. J Cell Physiol. 2022;237(9):3598–3613. doi:10.1002/jcp.30818
4. Ezquerro S, Tuero C, Becerril S, et al. Antagonic effect of ghrelin and LEAP-2 on hepatic stellate cell activation and liver fibrosis in obesity-associated nonalcoholic fatty liver disease. Eur J Endocrinol. 2023;188(7):564–577. doi:10.1093/ejendo/lvad071
5. Ishiyama S, Hayatsu M, Toriumi T, et al. Assessing the combined impact of fatty liver-induced TGF-β1 and LPS-activated macrophages in fibrosis through a novel 3D serial section methodology. Sci Rep. 2024;14(1):11404. doi:10.1038/s41598-024-60845-6
6. Zhou X, Wang P, Ma Z, et al. Novel interplay between Sonic Hedgehog and Transforming Growth Factor-β1 in human nonalcoholic steatohepatitis. Appl Immunohistochem Mol Morphol. 2020;28(2):154–160. doi:10.1097/PAI.0000000000000724