The proteasome controls levels of most cellular proteins, and its activity is regulated under stress, quiescence, and inflammation. However, factors determining the proteasomal degradation rate remain poorly understood. Proteasome substrates are conjugated with small proteins (tags) like ubiquitin and Fat10 to target them to the proteasome. It is unclear if the structural plasticity of proteasome-targeting tags can influence substrate degradation. Fat10 is upregulated during inflammation, and its substrates undergo rapid proteasomal degradation. We report that the degradation rate of Fat10 substrates critically depends on the structural plasticity of Fat10. While the ubiquitin tag is recycled at the proteasome, Fat10 is degraded with the substrate. Our results suggest significantly lower thermodynamic stability and faster mechanical unfolding in Fat10 compared to ubiquitin. Long-range salt bridges are absent in the Fat10 structure, creating a plastic protein with partially unstructured regions suitable for proteasome engagement. Fat10 plasticity destabilizes substrates significantly and creates partially unstructured regions in the substrate to enhance degradation. NMR-relaxation-derived order parameters and temperature dependence of chemical shifts identify the Fat10-induced partially unstructured regions in the substrate, which correlated excellently to Fat10-substrate contacts, suggesting that the tag-substrate collision destabilizes the substrate. These results highlight a strong dependence of proteasomal degradation on the structural plasticity and thermodynamic properties of the proteasome-targeting tags.

Aim: FAT10, a ubiquitin-like modifier protein, influences apoptosis, DNA damage response and tumor growth, with unclear effects on cancer prognosis. Methods: We reviewed FAT10 expression\'s impact on malignancy prognosis through a systematic review and meta-analysis, including studies up to September 2023 from PubMed, EMBASE and Web of Science. Results: From 18 studies involving 2513 patients, FAT10 overexpression significantly reduced overall and disease-free survival across various tumors, indicating correlations with advanced disease stage, poor differentiation, lymph node metastasis and larger tumor size. Conclusion: FAT10\'s overexpression suggests a negative prognostic value in cancer, meriting further investigation.PROSPERO Registration Number: CRD42023431287.
This study investigated a protein called FAT10, which is involved in how cells behave, including how cancer cells grow and survive. It analyzed previous research to see if high levels of FAT10 in patients with cancer can help predict how serious their cancer is and how it might progress. After reviewing 18 studies involving 2513 patients, we found that patients with more FAT10 in their cells often had a worse outlook, including a higher chance of the cancer returning and a shorter overall survival time. This pattern existed for different types of cancer. Our findings suggest that measuring FAT10 levels could be helpful for doctors to better understand a patient\'s cancer and choose the best treatment. However, more studies are needed to confirm our results.

The ubiquitin-like protein ISG15 (interferon-stimulated gene 15) regulates the host response to bacterial and viral infections through its conjugation to proteins (ISGylation) following interferon production. ISGylation is antagonized by the highly specific cysteine protease USP18, which is the major deISGylating enzyme. However, mechanisms underlying USP18\'s extraordinary specificity towards ISG15 remains elusive. Here, we show that USP18 interacts with its paralog USP41, whose catalytic domain shares 97% identity with USP18. However, USP41 does not act as a deISGylase, which led us to perform a comparative analysis to decipher the basis for this difference, revealing molecular determinants of USP18\'s specificity towards ISG15. We found that USP18 C-terminus, as well as a conserved Leucine at position 198, are essential for its enzymatic activity and likely act as functional surfaces based on AlphaFold predictions. Finally, we propose that USP41 antagonizes conjugation of the understudied ubiquitin-like protein FAT10 (HLA-F adjacent transcript 10) from substrates in a catalytic-independent manner. Altogether, our results offer new insights into USP18\'s specificity towards ISG15, while identifying USP41 as a negative regulator of FAT10 conjugation.

The upregulation of programmed death ligand 1 (PD-L1) plays a crucial role in facilitating cancer cells to evade immune surveillance through immunosuppression. However, the precise regulatory mechanisms of PD-L1 in hepatocellular carcinoma (HCC) remain undefined. The correlation between PD-L1 and ubiquitin-like molecules (UBLs) was studied using sequencing data from 20 HCC patients in our center, combined with TCGA data. Specifically, the association between FAT10 and PD-L1 was further validated at both the protein and mRNA levels in HCC tissues from our center. Subsequently, the effect of FAT10 on tumor progression and immune suppression was examined through both in vivo and in vitro experiments. Utilizing sequencing data, qPCR, and Western blotting assays, we confirmed that FAT10 was highly expressed in HCC tissues and positively correlated with PD-L1 expression. Additionally, in vitro experiments demonstrated that the overexpression of FAT10 fostered the proliferation, migration, and invasion of HCC cells. Furthermore, the overexpression of FAT10 in HCC cells led to an increase in PD-L1 expression, resulting in the inhibition of T cell proliferation and the enhancement of HCC cell resistance to T cell-mediated cytotoxicity. Moreover, in vivo experiments utilizing the C57BL/6 mouse model revealed that overexpression of FAT10 effectively suppressed the infiltration of CD8 + GZMB + and CD8 + Ki67 + T cells, as well as reduced serum levels of TNF-α and IFN-γ. Mechanistically, we further identified that FAT10 upregulates PD-L1 expression via activating the PI3K/AKT/mTOR pathway, but not in a ubiquitin-like modification. In conclusion, our findings indicate that FAT10 promotes immune evasion of HCC via upregulating PD-L1 expression, suggesting its potential as a novel target to enhance the efficiency of immunotherapy in HCC.

Although sorafenib is the first-line therapeutic agent for advanced hepatocellular carcinoma (HCC), the development of drug resistance in HCC cells limits its clinical efficacy. However, the key factors involved in mediating the sorafenib resistance of HCC cells and the underlying mechanisms have not been elucidated. In this study, we generated sorafenib-resistant HCC cell lines, and our data demonstrate that HLA-F locus-adjacent transcript 10 (FAT10), a ubiquitin-like protein, is markedly upregulated in sorafenib-resistant HCC cells and that reducing the expression of FAT10 in sorafenib-resistant HCC cells increases sensitivity to sorafenib. Mechanistically, FAT10 stabilizes the expression of the PTEN-specific E3 ubiquitin ligase NEDD4 that causes downregulation of PTEN, thereby inducing AKT-mediated autophagy and promoting the resistance of HCC cells to sorafenib. Moreover, we screened the small molecule Compound 7695-0983, which increases the sensitivity of sorafenib-resistant HCC cells to sorafenib by inhibiting the expression of FAT10 to inhibit NEDD4-PTEN/AKT axis-mediated autophagy. Collectively, our preclinical findings identify FAT10 as a key factor in the sorafenib resistance of HCC cells and elucidate its underlying mechanism. This study provides new mechanistic insight for the exploitation of novel sorafenib-based tyrosine kinase inhibitor (TKI)-targeted drugs for treating advanced HCC.

Endocrine therapy that blocks estrogen signaling is the most effective treatment for patients with estrogen receptor positive (ER+) breast cancer. However, the efficacy of agents such as tamoxifen (Tam) is often compromised by the development of resistance. Here we report that cytokines-activated nuclear IKKα confers Tam resistance to ER+ breast cancer by inducing the expression of FAT10, and that the expression of FAT10 and nuclear IKKα in primary ER+ human breast cancer was correlated with lymphotoxin β (LTB) expression and significantly associated with relapse and metastasis in patients treated with adjuvant mono-Tam. IKKα activation or enforced FAT10 expression promotes Tam-resistance while loss of IKKα or FAT10 augments Tam sensitivity. The induction of FAT10 by IKKα is mediated by the transcription factor Pax5, and coordinated via an IKKα-p53-miR-23a circuit in which activation of IKKα attenuates p53-directed repression of FAT10. Thus, our findings establish IKKα-to-FAT10 pathway as a new therapeutic target for the treatment of Tam-resistant ER+ breast cancer.

OBJECTIVE: Peroxisome Proliferator-Activated Receptor α (PPARα) is a key regulator of hepatic lipid metabolism and therefore a promising therapeutic target against Metabolic-dysfunction Associated Steatotic Liver Diseases (MASLD). However, its expression and activity decrease during disease progression and several of its agonists did not achieve sufficient efficiency in clinical trials with, surprisingly, a lack of steatosis improvement. Here, we identified the Human leukocyte antigen-F Adjacent Transcript 10 (FAT10) as an inhibitor of PPARα lipid metabolic activity during MASLD progression.
RESULTS: In vivo, the expression of FAT10 is upregulated in human and murine MASLD livers upon disease progression and correlates negatively with PPARα expression. The increase of FAT10 occurs in hepatocytes in which both proteins interact. FAT10 silencing in vitro in hepatocytes increases PPARα target gene expression, promotes fatty acid oxidation and decreases intra-cellular lipid droplet content. In line, FAT10 overexpression in hepatocytes in vivo inhibits the lipid regulatory activity of PPARα in response to fasting and agonist treatment in conditions of physiological and pathological hepatic lipid overload.
CONCLUSIONS: FAT10 is induced during MASLD development and interacts with PPARα resulting in a decreased lipid metabolic response of PPARα to fasting or agonist treatment. Inhibition of the FAT10-PPARα interaction may provide a means to design potential therapeutic strategies against MASLD.

The extracellular ubiquitin-proteasome system is involved in sperm binding to and/or penetration of the vitelline coat (VC), a proteinaceous egg coat, during fertilization of the ascidian (Urochordata) Halocynthia roretzi. It is also known that the sperm receptor on the VC, HrVC70, is ubiquitinated and degraded by the sperm proteasome during the sperm penetration of the VC and that a 700-kDa ubiquitin-conjugating enzyme complex is released upon sperm activation on the VC, which is designated the \"sperm reaction\". However, the de novo function of ubiquitin-activating enzyme (UBA/E1) during fertilization is poorly understood. Here, we show that PYR-41, a UBA inhibitor, strongly inhibited the fertilization of H. roretzi. cDNA cloning of UBA1 and UBA6 from H. roretzi gonads was carried out, and their 3D protein structures were predicted to be very similar to those of human UBA1 and UBA6, respectively, based on AlphaFold2. These two genes were transcribed in the ovary and testis and other organs, among which the expression of both was highest in the ovary. Immunocytochemistry showed that these enzymes are localized on the sperm head around a mitochondrial region and the follicle cells surrounding the VC. These results led us to propose that HrUBA1, HrUBA6, or both in the sperm head mitochondrial region and follicle cells may be involved in the ubiquitination of HrVC70, which is responsible for the fertilization of H. roretzi.

BACKGROUND: The efficacy of anti-vascular endothelial growth factor (VEGF) therapy is limited. However, the key factors involved in limiting the efficacy of anti-VEGF therapy and the underlying mechanisms remain unclear.
OBJECTIVE: To investigate the effects and mechanisms of human leukocyte antigen F locus-adjacent transcript 10 (FAT10), a ubiquitin-like protein, in limiting the efficacy of anti-VEGF therapy in hepatocellular carcinoma (HCC) cells.
METHODS: FAT10 was knocked out in HCC cells using the clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 technology. Bevacizumab (BV), an anti-VEGF monoclonal antibody, was used to evaluate the efficacy of anti-VEGF therapy in vivo. Mechanisms of FAT10 action were assessed by RNA sequencing, glutathione S-transferase pulldown assays and in vivo ubiquitination assays.
RESULTS: FAT10 accelerated VEGF-independent angiogenesis in HCC cells which limited BV efficacy and BV-aggravated hypoxia and inflammation promoted FAT10 expression. FAT10 overexpression increased levels of proteins involved in several signaling pathways in HCC cells, resulting in upregulation of VEGF and multiple non-VEGF proangiogenic factors. Upregulation of multiple FAT10-mediated non-VEGF signals compensated for the inhibition of VEGF signaling by BV, enhancing VEGF-independent angiogenesis and promoting HCC growth.
CONCLUSIONS: Our preclinical findings identify FAT10 in HCC cells as a key factor limiting the efficacy of anti-VEGF therapy and elucidate its underlying mechanisms. This study provides new mechanistic insights into the development of antiangiogenic therapies.

Emerging evidence showed that FAT10 is a vital regulator of tumor occurrence and development. The molecular mechanisms underlying the specific role of FAT10 in colorectal cancer (CRC) are not yet known.
To investigate whether FAT10 participates in the proliferation, invasion and metastasis of CRC.
This study investigated the function and clinical significance of FAT10 protein expression in CRC. Furthermore, over-expression and knockdown experiments of FAT10 were developed to explore their effects on CRC cell migration and proliferation. Moreover, a molecular mechanism of FAT10 regulate calpain small subunit 1(Capn4) was explored.
In this research, the FAT10 expression level was elevated in CRC tissues compared to corresponding normal tissues. In addition, the elevated FAT10 expression level is significantly linked to advanced clinical stage and poor CRC prognosis. Furthermore, a very high expression of FAT10 was observed in CRC cells, and FAT10 overexpression significantly enhanced the in vivo proliferation, invasion, and metastasis of the cells, whereas knockdown of FAT10 inhibited all these cellular factors in both in vivo and in vitro environments. Moreover, the outcomes of this study suggested that FAT10 enhances colorectal cancer progression through enhancement of Capn4 expression, leading to the progression of various human tumors, as reported by previous research. The mechanism via which FAT10 promotes CRC cells proliferation, invasion, and metastasis involves modification of the ubiquitination and degradation processes of Capn4.
FAT10 is a vital regulator of the tumorigenesis and advancement of CRC, thus serving as a promising pharmaceutical target for treating CRC patients.