Liver health is integral to overall human well-being and the pathogenesis of various diseases. In recent years, kynurenine and its derivatives have gradually been recognized for their involvement in various pathophysiological processes, especially in the regulation of liver diseases, such as acute liver injury, non-alcoholic fatty liver disease, cirrhosis, and liver cancer. Kynurenine and its derivatives are derived from tryptophan, which is broken down by the enzymes indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO), converting the essential amino acid tryptophan into kynurenine (KYN) and other downstream metabolites, such as kynurenic acid (KYNA), 3-hydroxykynurenine (3-HK), xanthurenic acid (XA), and quinolinic acid (QA). In liver diseases, kynurenine and its derivatives can promote the activity of the transcription factor aryl hydrocarbon receptor (AhR), suppress T cell activity for immune modulation, inhibit the activation of inflammatory signaling pathways, such as NF-κB for anti-inflammatory effects, and inhibit the activation of hepatic stellate cells to slow down fibrosis progression. Additionally, kynurenine and other downstream metabolites can influence the progression of liver diseases by modulating the gut microbiota. Therefore, in this review, we summarize and explore the mechanisms by which kynurenine and its derivatives regulate liver diseases to help develop new diagnostic or prognostic biomarkers and effective therapies targeting the kynurenine pathway for liver disease treatment.

The intratumoral microbiota, as an important component of the tumor microenvironment, is increasingly recognized as a key factor in regulating responses to cancer immunotherapy. Recent studies have revealed that the intratumoral microbiota is not uniformly distributed but instead exhibits significant spatial heterogeneity, with its distribution patterns influenced by factors such as tumor anatomy, local immune status, and therapeutic interventions. This spatial heterogeneity not only alters the interactions between microbes and the host immune system but may also reshape the immunogenic and immunosuppressive landscapes of tumors. The enrichment or depletion of microbiota in different tumor regions can influence immune cell infiltration patterns, metabolic pathway activities, and immune checkpoint molecule expression, thereby driving the development of resistance to immunotherapy. Moreover, certain bacterial metabolites form concentration gradients between the tumor core and margins, thereby regulating immune cell function. Therefore, understanding and manipulating the spatial distribution of intratumoral microbiota, particularly in resistant patients, holds promise for developing new strategies to overcome immunotherapy resistance. In the future, precise modulation strategies targeting microbial spatial heterogeneity, such as engineered bacterial vectors, probiotic combinations, and phage therapy, may open new avenues for immunotherapy.

The global burden of liver diseases continues to rise, encompassing diverse pathologies such as viral hepatitis, alcohol-associated liver disease (ALD), metabolic dysfunction-associated steatotic liver disease (MASLD), and hepatocellular carcinoma (HCC). In recent years, TNFR1-associated death domain protein (TRADD), a pivotal adaptor molecule in the TNF signaling pathway, has been found to play a dual regulatory role in the pathogenesis of liver diseases. Through its death domain, TRADD binds to TNFR1 and dynamically recruits downstream factors (e.g., TRAF2, RIPK1, FADD) to form Complex I or IIa, thereby activating pro-survival or pro-apoptotic signals that dictate hepatocyte fate and modulate the inflammatory microenvironment. This review systematically summarizes the molecular structure and functional networks of TRADD, along with its mechanistic roles in liver diseases: in HCC, TRADD expression correlates with tumor differentiation and is regulated by miRNA targeting; in ALD and MASLD, TRADD-mediated apoptosis is closely linked to fibrotic progression; and in acute liver injury, TRADD signaling is modulated by factors such as HO-1 to mitigate damage. Furthermore, TRADD inhibitors and antisense oligonucleotides demonstrate therapeutic potential. This review highlights the clinical translational value of TRADD as a diagnostic, therapeutic, and prognostic biomarker for liver diseases, providing a theoretical foundation for future precision medicine strategies.