Thyroid hormone receptor alpha modulates fibrogenesis in hepatic stellate cells

Actions of thyroid hormones and thyromimetics on the liver

Thyroid hormones (triiodothyronine and thyroxine) are pivotal for metabolic balance in the liver and entire body. Dysregulation of the hypothalamus-pituitary-thyroid axis can contribute to hepatic metabolic disturbances, affecting lipid metabolism, glucose regulation and protein synthesis. In addition, reductions in circulating and intrahepatic thyroid hormone concentrations increase the risk of metabolic dysfunction-associated steatotic liver disease by inducing lipotoxicity, inflammation and fibrosis. Amelioration of hepatic metabolic disease by thyroid hormones in preclinical and clinical studies has spurred the development of thyromimetics that target THRB (the predominant thyroid hormone receptor isoform in the liver) and/or the liver itself to provide more selective activation of hepatic thyroid hormone-regulated metabolic pathways while reducing thyrotoxic side effects in tissues that predominantly express THRA such as the heart and bone. Resmetirom, a liver and THRB-selective thyromimetic, recently became the first FDA-approved drug for metabolic dysfunction-associated steatohepatitis (MASH). Thus, a better understanding of the metabolic actions of thyroid hormones and thyromimetics in the liver is timely and clinically relevant. Here, we describe the roles of thyroid hormones in normal liver function and pathogenesis of MASH, as well as some potential clinical issues that might arise when treating patients with MASH with thyroid hormone supplementation or thyromimetics.

Thyroid hormone receptor-β analogs for the treatment of Metabolic Dysfunction-Associated Steatohepatitis (MASH)

The association between suboptimal thyroid function ((sub)clinical hypothyroidism or low normal thyroid function) and the metabolic syndrome and metabolic dysfunction-associated steatotic liver disease (MASLD) is clearly established. Furthermore, in MASLD, thyroid hormones have low intracellular concentrations and the activation of the thyroid hormone receptor (THR) is reduced. Administration of thyroid hormone has been shown to reduce liver triglycerides by stimulating fatty acid disposal through lipophagy and beta-oxidation, and to lower LDL-cholesterol. As thyroid hormone exerts it's effects in many different organs, including heart and bone, several drug candidates have been developed acting as selective thyromimetics for the THR-β nuclear receptor with potent and targeted liver actions. Importantly, these compounds have reduced affinity for the THR-α nuclear receptor and tissue distribution profiles that differ from endogenous thyroid hormones thereby reducing unwanted cardiovascular side effects. The most advanced compound, resmetirom, is an oral drug that demonstrated, in a large phase 3 trial in MASH patients, the ability to remove liver fat, reduce aminotransferase levels and improve atherogenic dyslipidemia with a good tolerability profile. This translated into histological improvement that led to accelerated approval of this drug for active fibrotic steatohepatitis, a milestone achievement as a first MASH drug.

Nuclear receptors: pathophysiological mechanisms and drug targets in liver disease

Nuclear receptors (NRs) are ligand-dependent transcription factors required for liver development and function. As a consequence, NRs have emerged as attractive drug targets in a wide range of liver diseases. However, liver dysfunction and failure are linked to loss of hepatocyte identity characterised by deficient NR expression and activities. This might at least partly explain why several pharmacological NR modulators have proven insufficiently efficient to improve liver functionality in advanced stages of diseases such as metabolic dysfunction-associated steatotic liver disease (MASLD). In this perspective, we review the most recent advances in the hepatic NR field and discuss the contribution of multiomic approaches to our understanding of their role in the molecular organisation of an intricated transcriptional regulatory network, as well as in liver intercellular dialogues and interorgan cross-talks. We discuss the potential benefit of novel therapeutic approaches simultaneously targeting multiple NRs, which would not only reactivate the hepatic NR network and restore hepatocyte identity but also impact intercellular and interorgan interplays whose importance to control liver functions is further defined. Finally, we highlight the need of considering individual parameters such as sex and disease stage in the development of NR-based clinical strategies.

Metabolic reprogramming in liver fibrosis

Chronic liver diseases, primarily metabolic dysfunction-associated steatotic liver disease (MASLD), harmful use of alcohol, or viral hepatitis, may result in liver fibrosis, cirrhosis, and cancer. Hepatic fibrogenesis is a complex process with interactions between different resident and non-resident heterogeneous liver cell populations, ultimately leading to deposition of extracellular matrix and organ failure. Shifts in cell phenotypes and functions involve pronounced transcriptional and protein synthesis changes that require metabolic adaptations in cellular substrate metabolism, including glucose and lipid metabolism, resembling changes associated with the Warburg effect in cancer cells. Cell activation and metabolic changes are regulated by metabolic stress responses, including the unfolded protein response, endoplasmic reticulum stress, autophagy, ferroptosis, and nuclear receptor signaling. These metabolic adaptations are crucial for inflammatory and fibrogenic activation of macrophages, lymphoid cells, and hepatic stellate cells. Modulation of these pathways, therefore, offers opportunities for novel therapeutic approaches to halt or even reverse liver fibrosis progression.

The first MASH drug therapy on the horizon: Current perspectives of resmetirom

The rising prevalence of metabolic dysfunction-associated steatotic liver disease (MASLD) poses a significant global health challenge, affecting over 30% of adults worldwide. MASLD is linked to increased mortality rates and substantial healthcare costs, primarily driven by its progression to metabolic dysfunction-associated steatohepatitis (MASH), which can lead to severe liver complications including cirrhosis and hepatocellular carcinoma. Despite its growing burden, effective pharmacotherapy for MASLD/MASH has been lacking until the recent conditional approval of resmetirom by the FDA. Resmetirom, a liver-targeted thyroid hormone receptor-β selective drug, has shown promise in clinical trials for treating non-cirrhotic MASH with moderate to advanced fibrosis. It has demonstrated efficacy in reducing hepatic fat content, improving liver histology (both MASH resolution and fibrosis improvement), and ameliorating biomarkers of liver damage without significant effects on body weight or glucose metabolism. Notably, resmetirom also exhibits favourable effects on circulating lipids, potentially reducing cardiovascular risk in MASLD/MASH patients. The safety profile of resmetirom appears acceptable, with gastrointestinal adverse events being the most common, though generally mild or moderate. However, long-term surveillance is warranted to monitor for potential risks related to thyroid, gonadal, or bone diseases. Clinical implementation of resmetirom faces challenges in patient selection and monitoring treatment response, and will heavily rely on non-invasive tests for liver fibrosis assessment. Nonetheless, resmetirom represents a landmark breakthrough in MASLD/MASH treatment, paving the way for future therapeutic strategies aiming to mitigate the multifaceted risks associated with this complex metabolic liver disease.

Casual associations of thyroid function with inflammatory bowel disease and the mediating role of cytokines

Background

Previous observational epidemiological studies have suggested a potential association between thyroid function and inflammatory bowel disease (IBD). However, the findings remain inconclusive, and whether this association is causal remains uncertain. The objective of this study is to investigate the causal association between thyroid function and IBD.

Methods

Genome-wide association studies (GWAS) involving seven indicators of thyroid function, IBD, and 41 cytokines were analyzed. Bidirectional two-sample Mendelian randomization (MR) and multivariable MR were conducted to examine the causal relationship between thyroid function and IBD and to explore the potential mechanisms underlying the associations.

Results

Genetically determined hypothyroidism significantly reduced the risk of CD (odds ratio [OR] = 0.761, 95% CI: 0.655-0.882, p < 0.001). Genetically determined reference-range TSH was found to have a suggestive causal effect on IBD (OR = 0.931, 95% CI: 0.888-0.976, p = 0.003), (Crohn disease) CD (OR = 0.915, 95% CI: 0.857-0.977, p = 0.008), and ulcerative colitis (UC) (OR =0.910, 95% CI: 0.830-0.997, p = 0.043). In reverse MR analysis, both IBD and CD appeared to have a suggestive causal effect on the fT3/fT4 ratio (OR = 1.002, p = 0.013 and OR = 1.001, p = 0.015, respectively). Among 41 cytokines, hypothyroidism had a significant impact on interferon-inducible protein-10 (IP-10) (OR = 1.465, 95% CI: 1.094-1.962, p = 0.010). The results of multivariable MR showed that IP-10 may mediate the causal effects of hypothyroidism with CD.

Conclusion

Our results suggest that an elevated TSH level reduces the risk of CD, with IP-10 potentially mediating this association. This highlights the pituitary-thyroid axis could serve as a potential therapeutic strategy for CD.