Metabolic Profiling of Thymic Epithelial Tumors Hints to a Strong Warburg Effect, Glutaminolysis and Precarious Redox Homeostasis as Potential Therapeutic Targets

Abstract

Thymomas and thymic carcinomas (TC) are malignant thymic epithelial tumors (TETs) with poor outcome, if non-resectable. Metabolic signatures of TETs have not yet been studied and may offer new therapeutic options. Metabolic profiles of snap-frozen thymomas (WHO types A, AB, B1, B2, B3, n = 12) and TCs (n = 3) were determined by high resolution magic angle spinning 1H nuclear magnetic resonance (HRMAS 1H-NMR) spectroscopy. Metabolite-based prediction of active KEGG metabolic pathways was achieved with MetPA. In relation to metabolite-based metabolic pathways, gene expression signatures of TETs (n = 115) were investigated in the public “The Cancer Genome Atlas” (TCGA) dataset using gene set enrichment analysis. Overall, thirty-seven metabolites were quantified in TETs, including acetylcholine that was not previously detected in other non-endocrine cancers. Metabolite-based cluster analysis distinguished clinically indolent (A, AB, B1) and aggressive TETs (B2, B3, TCs). Using MetPA, six KEGG metabolic pathways were predicted to be activated, including proline/arginine, glycolysis and glutathione pathways. The activated pathways as predicted by metabolite-profiling were generally enriched transcriptionally in the independent TCGA dataset. Shared high lactic acid and glutamine levels, together with associated gene expression signatures suggested a strong “Warburg effect”, glutaminolysis and redox homeostasis as potential vulnerabilities that need validation in a large, independent cohort of aggressive TETs. If confirmed, targeting metabolic pathways may eventually prove as adjunct therapeutic options in TETs, since the metabolic features identified here are known to confer resistance to cisplatin-based chemotherapy, kinase inhibitors and immune checkpoint blockers, i.e., currently used therapies for non-resectable TETs.

Ultrahigh‐performance liquid chromatography–high‐resolution mass spectrometry‐based plasma metabolomics study of thymoma and thymic hyperplasia

“…Lactic acid is a byproduct of glycolysis and it can promote tumor progression 41 . Tissue lactic acid was reported to have higher levels in aggressive (B2, B3 and TC) compared with indolent TETs and normal thymic, suggesting a strong “Warburg effect” as potential vulnerabilities in thymoma development and progression 5 . Another sequela of the metabolic program Warburg effect is the acidification of the tumor microenvironment, which is a key factor in cancer somatic evolution and progression to malignancy 42 .…”

“…As a consequence, it is hard to achieve accurate pre‐operative diagnosis of thymoma by only imaging and new objective and reliable biomarkers are needed 9 . For those patients with unresectable disease or those who progress after surgery and radiotherapy, the current treatment strategy is platinum‐based chemotherapy; however, no standard treatments are clinically implemented if patients are resistant to platinum‐based therapies 3,5 . Immune checkpoint inhibitors such as PD‐1 and PD‐L1 inhibitors represent one of the new targeted therapeutic strategies; however, these may induce severe autoimmune adverse reactions in thymoma patients 5 …”

“…Due to the low incidence of thymoma, there have been very few metabolomics studies of thymoma samples. Alwahsh et al performed NMR‐based metabolic profiling study of 12 thymoma and 3 thymic carcinoma (TC) tissue samples and quantified 37 metabolites, including lactic acid, glutamine, histidine, choline and isoleucine 5 . The results indicated a strong “Warburg effect,” glutaminolysis and redox homeostasis as potential vulnerabilities of aggressive TETs, and they might be used as targets for TET treatment, once validated with a larger sample cohort 5 .…”

“…Type AB tumors are like type A tumors but have neoplastic lymphocytic foci. Types A, AB and B1 thymomas are at low tumor stages and are considered as indolent thymomas, while types B2 and B3 are more aggressive 5 . The prognosis becomes gradually worse from type A to type B3 thymomas 4 .…”

Metabolomic and Transcriptomic Profiling Identified Significant Genes in Thymic Epithelial Tumor

“…There was only a single study of metabolomic analysis in TETs by nuclear magnetic resonance spectroscopy, which included tissue samples from 15 TETs and 4 normal controls. It was suggested that pathways associated with cysteine, glutathione, lactate, and glutamine appear to be promising therapeutic targets based on metabolite differences between thymic epithelial tumor tissue and normal thymic tissue proline [25]. Inhibitors of glutaminolysis and the downstream tricarboxylic acid cycle (TCA) are also expected to be reasonable therapeutic strategies.…”