Expression of thymoproteasome subunit β5t in type AB thymoma

The role of proteasomes in tumorigenesis

Protein homeostasis is the basis of normal life activities, and the proteasome family plays an extremely important function in this process. The proteasome 20S is a concentric circle structure with two α rings and two β rings overlapped. The proteasome 20S can perform both ATP-dependent and non-ATP-dependent ubiquitination proteasome degradation by binding to various subunits (such as 19S, 11S, and 200 PA), which is performed by its active subunit β1, β2, and β5. The proteasome can degrade misfolded, excess proteins to maintain homeostasis. At the same time, it can be utilized by tumors to degrade over-proliferate and unwanted proteins to support their growth. Proteasomes can affect the development of tumors from several aspects including tumor signaling pathways such as NF-κB and p53, cell cycle, immune regulation, and drug resistance. Proteasome-encoding genes have been found to be overexpressed in a variety of tumors, providing a potential novel target for cancer therapy. In addition, proteasome inhibitors such as bortezomib, carfilzomib, and ixazomib have been put into clinical application as the first-line treatment of multiple myeloma. More and more studies have shown that it also has different therapeutic effects in other tumors such as hepatocellular carcinoma, non-small cell lung cancer, glioblastoma, and neuroblastoma. However, proteasome inhibitors are not much effective due to their tolerance and singleness in other tumors. Therefore, further studies on their mechanisms of action and drug interactions are needed to investigate their therapeutic potential.

Thymic Carcinoma: Unraveling Neuroendocrine Differentiation and Epithelial Cell Identity Loss

BACKGROUND

The histogenesis of thymic epithelial tumors (TETs) has been a subject of debate. Recent technological advancements have revealed that thymic carcinomas often exhibit a phenotype akin to tuft cells, which is a subset of medullary TECs. Here, we further explored the gene expression signatures of thymic carcinomas in relation to tuft cells and their kinships-ionocytes and neuroendocrine cells (neuroendocrine group).

METHODS

We analyzed a single-cell RNA sequencing dataset from the normal human thymus. Concurrently, we examined publicly available datasets on the mRNA expression and methylation status of TECs and lung cancers. Real-time quantitative PCR was also conducted with our tissue samples.

RESULTS

Thymic carcinomas displayed a neuroendocrine phenotype biased toward tuft cells and ionocytes. When exploring the possible regulators of this phenotype, we discovered that HDAC9 and NFATC1 were characteristically expressed in the neuroendocrine group in adult TECs and thymic carcinomas. Additionally, the pan-thymic epithelium markers, exemplified by PAX9 and SIX1, were significantly suppressed in thymic carcinomas.

CONCLUSIONS

Thymic carcinomas might be characterized by unique neuroendocrine differentiation and loss of identity as thymic epithelial cells. Future studies investigating the role of HDAC9 and NFATC1 in thymic epithelium are warranted to explore their potential as therapeutic targets in TETs.

Histogenetic and disease-relevant phenotypes in thymic epithelial tumors (TETs): The potential significance for future TET classification

Thymic epithelial tumors (TETs) encompass morphologically various subtypes. Thus, it would be meaningful to explore the expression phenotypes that delineate each TET subtype or overarching multiple subtypes. If these profiles are related to thymic physiology, they will improve our biological understanding of TETs and may contribute to the establishment of a more rational TET classification. Against this background, pathologists have attempted to identify histogenetic features in TETs for a long time. As part of this work, our group has reported several TET expression profiles that are histotype-dependent and related to the nature of thymic epithelial cells (TECs). For example, we found that beta5t, a constituent of thymoproteasome unique to cortical TECs, is expressed mainly in type B thymomas, for which the nomenclature of cortical thymoma was once considered. Another example is the discovery that most thymic carcinomas, especially thymic squamous cell carcinomas, exhibit expression profiles similar to tuft cells, a recently discovered special type of medullary TEC. This review outlines the currently reported histogenetic phenotypes of TETs, including those related to thymoma-associated myasthenia gravis, summarizes their genetic signatures, and provides a perspective for the future direction of TET classification.

The clinicopathological significance of thymic epithelial markers expression in thymoma and thymic carcinoma

BACKGROUND

The classification of thymomas is based on the morphology of epithelial tumor cells and the proportion of lymphocytes. Type A thymomas are composed of the spindle or oval tumor epithelial cells. Tumor cells of B thymomas are epithelioid-shaped with increasing atypia. Type AB thymomas have the features of epithelial tumor cells of A and B thymomas. The diagnosis can be difficult because of the complex morphology. Some novel thymic epithelial markers have been reported in several preclinical studies, but they have not been applied to clinical practice. Here, we investigated the expression of 3 cortical and 3 medullary markers, which are thymoproteasome-specific subunit β5t (β5t), thymus-specific serine protease 16 (PRSS16), cathepsin V, autoimmune regulator (AIRE), CD40 and claudin-4.

METHODS

Immunohistochemistry was used to analyze 53 cases of thymomas and thymic squamous cell carcinomas (TSCC), aiming to explore the expression of cortical and medullary epithelial markers and their correlation with histological classification, Masaoka-Koga stage, and prognosis.

RESULTS

Our results found that for cortical epithelial markers the expression of β5t, PRSS16, and cathepsin V was higher in type AB and B thymomas than in micronodular thymoma with lymphoid stroma (MNT), and we observed a dramatic increase of β5t and PRSS16 expression in type AB compared to type A thymomas. In medullary epithelial markers, the expression of AIRE was higher in type A than in B3 thymomas. CD40 and β5t expression were associated with the Masaoka-Koga stage. High cathepsin V expression was related to a good prognosis and a longer progression-free survival.

CONCLUSION

This is the first comprehensive analysis of the role of thymic cortical and medullary epithelial markers as biomarkers for differential diagnosis and prognosis in thymic tumors. Thymic medullary epithelial immunophenotype was found to exhibit in type A, MNT, and TSCC. Type B thymomas primarily exhibited a cortical epithelial immunophenotype. Type AB thymomas showed cortical, medullary, or mixed corticomedullary epithelial immunophenotype. Our results demonstrated that thymic cortical and medullary epithelial markers including β5t, PRSS16, cathepsin V, and AIRE could be used as ancillary markers in the diagnosis and prognosis of thymic epithelial tumors.

Structural and Functional Thymic Biomarkers Are Involved in the Pathogenesis of Thymic Epithelial Tumors: An Overview

The normal human thymus originates from the third branchial cleft as two paired anlages that descend into the thorax and fuse on the midline of the anterior–superior mediastinum. Alongside the epithelial and lymphoid components, different types of lymphoid accessory cells, stromal mesenchymal and endothelial cells migrate to, or develop in, the thymus. After reaching maximum development during early postnatal life, the human thymus decreases in size and lymphocyte output drops with age. However, thymic immunological functions persist, although they deteriorate progressively. Several major techniques were fundamental to increasing the knowledge of thymic development and function during embryogenesis, postnatal and adult life; these include immunohistochemistry, immunofluorescence, flow cytometry, in vitro colony assays, transplantation in mice models, fetal organ cultures (FTOC), re-aggregated thymic organ cultures (RTOC), and whole-organ thymic scaffolds. The thymic morphological and functional characterization, first performed in the mouse, was then extended to humans. The purpose of this overview is to provide a report on selected structural and functional biomarkers of thymic epithelial cells (TEC) involved in thymus development and lymphoid cell maturation, and on the historical aspects of their characterization, with particular attention being paid to biomarkers also involved in Thymic Epithelial Tumor (TET) pathogenesis. Moreover, a short overview of targeted therapies in TET, based on currently available experimental and clinical data and on potential future advances will be proposed.

At the Cutting Edge against Cancer: A Perspective on Immunoproteasome and Immune Checkpoints Modulation as a Potential Therapeutic Intervention

Immunoproteasome is a noncanonical form of proteasome with enzymological properties optimized for the generation of antigenic peptides presented in complex with class I MHC molecules. This enzymatic property makes the modulation of its activity a promising area of research. Nevertheless, immunotherapy has emerged as a front-line treatment of advanced/metastatic tumors providing outstanding improvement of life expectancy, even though not all patients achieve a long-lasting clinical benefit. To enhance the efficacy of the currently available immunotherapies and enable the development of new strategies, a broader knowledge of the dynamics of antigen repertoire processing by cancer cells is needed. Therefore, a better understanding of the role of immunoproteasome in antigen processing and of the therapeutic implication of its modulation is mandatory. Studies on the potential crosstalk between proteasome modulators and immune checkpoint inhibitors could provide novel perspectives and an unexplored treatment option for a variety of cancers.

Role of immunoproteasomes and thymoproteasomes in health and disease

The proteasome is a multisubunit protease that degrades intracellular proteins into small peptides. Besides playing a pivotal role in many cellular processes indispensable for survival, it is involved in the production of peptides presented by major histocompatibility complex class I molecules. In addition to the standard proteasome shared in all eukaryotes, jawed vertebrates have two specialized forms of proteasome known as immunoproteasomes and thymoproteasomes. The immunoproteasome, which contains cytokine-inducible catalytic subunits with distinct cleavage specificities, produces peptides presented by class I molecules more efficiently than the standard proteasome. The thymoproteasome, which contains a unique catalytic subunit β5t, is a tissue-specific proteasome expressed exclusively in cortical thymic epithelial cells. It plays a critical role in CD8⁺ cytotoxic T cell development via positive selection. This review provides a brief overview on the structure and function of these specialized forms of proteasome and their involvement in human disease.

Expression of cathepsins B, D and K in thymic epithelial tumours

AIM

Cathepsins are proteases that regulate a wide range of physiological processes, including protein turnover, cell signalling and antigen presentation. Recent studies have shown that cathepsins are highly upregulated in many types of tumours. Of the 15 cathepsins in humans, cathepsins V and S are abundantly expressed in the thymus, and we previously showed that the immunostaining of these cathepsins could serve as diagnostic markers for thymic epithelial tumours. However, little is known about the expression of other cathepsins in thymic epithelial tumours. To determine the diagnostic implications of cathepsins, we performed immunohistochemical analysis of cathepsin B (CTB), cathepsin D (CTD) and cathepsin K (CTK), all of which have been reported to correlate with the progression of squamous cell carcinoma.

METHODS

The association between cathepsin expression and clinicopathological features was evaluated in 122 cases of thymoma and thymic carcinoma.

RESULTS

CTB and CTD were frequently expressed in type A and type AB thymomas. In contrast, CTB and CTD were significantly less common in type B thymomas than in type A or AB thymomas. In type AB thymomas, the expression of CTB correlated with histological features, and was found predominantly in the type A component. Notably, CTK was expressed most commonly in thymic carcinomas, and patients who died of the disease showed increased expression of CTK.

CONCLUSIONS

The expression of CTB and CTD correlated with the histological subtype of thymoma. In addition, the expression of CTK appears to be useful for the diagnosis of thymic carcinomas and as a prognostic marker.

Proteasomes and Several Aspects of Their Heterogeneity Relevant to Cancer

The life of every organism is dependent on the fine-tuned mechanisms of protein synthesis and breakdown. The degradation of most intracellular proteins is performed by the ubiquitin proteasome system (UPS). Proteasomes are central elements of the UPS and represent large multisubunit protein complexes directly responsible for the protein degradation. Accumulating data indicate that there is an intriguing diversity of cellular proteasomes. Different proteasome forms, containing different subunits and attached regulators have been described. In addition, proteasomes specific for a particular tissue were identified. Cancer cells are highly dependent on the proper functioning of the UPS in general, and proteasomes in particular. At the same time, the information regarding the role of different proteasome forms in cancer is limited. This review describes the functional and structural heterogeneity of proteasomes, their association with cancer as well as several established and novel proteasome-directed therapeutic strategies.

Proteolytic dynamics of human 20S thymoproteasome

An efficient immunosurveillance of CD8⁺ T cells in the periphery depends on positive/negative selection of thymocytes and thus on the dynamics of antigen degradation and epitope production by thymoproteasome and immunoproteasome in the thymus. Although studies in mouse systems have shown how thymoproteasome activity differs from that of immunoproteasome and strongly impacts the T cell repertoire, the proteolytic dynamics and the regulation of human thymoproteasome are unknown. By combining biochemical and computational modeling approaches, we show here that human 20S thymoproteasome and immunoproteasome differ not only in the proteolytic activity of the catalytic sites but also in the peptide transport. These differences impinge upon the quantity of peptide products rather than where the substrates are cleaved. The comparison of the two human 20S proteasome isoforms depicts different processing of antigens that are associated to tumors and autoimmune diseases.

The immunoproteasome and thymoproteasome: functions, evolution and human disease

The basic principle of adaptive immunity is to strictly discriminate between self and non-self, and a central challenge to overcome is the enormous variety of pathogens that might be encountered. In cell-mediated immunity, immunological discernment takes place at a molecular or cellular level. Central to both mechanisms of discernment is the generation of antigenic peptides associated with MHC class I molecules, which is achieved by a proteolytic complex called the proteasome. To adequately accomplish the discrimination between self and non-self that is essential for adaptive immunity and self-tolerance, two proteasome subtypes have evolved via gene duplication: the immunoproteasome and the thymoproteasome. In this Review, we describe various aspects of these immunity-dedicated proteasomes, from their discovery to recent findings.

Expression of cathepsins V and S in thymic epithelial tumors

Cathepsins are a group of proteolytic enzymes of the endosomal/lysosomal pathway involved in the thymic development of T cells restricted by major histocompatibility complex class II molecules. In the normal thymus, cathepsin V (CTV) and cathepsin S (CTS) are expressed in cortical and medullary epithelial cells, respectively. To investigate whether cathepsins could serve as a diagnostic marker, we performed immunohistochemical analysis for CTV and CTS in 77 cases of thymic epithelial tumors. Almost all cases (59/60) of thymoma expressed CTV, whereas 28 of 60 cases of thymoma expressed CTS. Notably, CTS was expressed in most cases of type A and type AB thymomas, but not in type B thymoma. The expression of cathepsins in type AB thymoma showed a clear correlation with histologic features; CTV was found predominantly in the type B component, and CTS was frequently expressed in the type A component. In thymic carcinoma, CTV was expressed in less than half cases (7/17), and the ratio of CTS-positive cases was equivalent to that of thymoma (8/17). Cases of CTV-negative thymic carcinoma tended to have a higher incidence of recurrence than did CTV-positive cases. Although further studies with a larger number of cases are required to confirm the utility of cathepsin immunostaining, CTV and CTS appear to serve as auxiliary diagnostic and/or prognostic markers in thymic epithelial tumors.