The role of histology in the study of tumor cell metabolism: the TRAP1 paradigm

Background. Metabolic reprogramming is a key feature of neoplastic transformation and mitochondria are the most important organelles in such oncogenic process. Recent evidence suggests that TRAP1 is a key player in tumor-related metabolic rewiring. Most studies have addressed TRAP1- related oncogenesis by in vitro and in vivo analyses. Little is however known on the possible contribution of histology to such studies. Study aims. This study assessed the role of histology in the study of TRAP1-related metabolic reprogramming. Specifically, it aimed: (i) to integrate the results of in vitro and in vivo studies with the histological analysis of tumor samples; (ii) to verify the correspondence between primary human neoplasms and animal tumor models; (iv) to identify novel fields for the study of TRAP1-related oncogenic cascades. Materials and methods. This project considered the following neoplastic settings: (i) neurofibromatosis type 1 (NF1)-related benign and malignant nerve sheath tumors; and (ii) germinal center (GC)-derived lymphoproliferative disorders. For the NF1-related tumors, morphological analysis and phenotypic characterization (TRAP1, HIF1a and related metabolic markers) were performed on: (i) human samples of plexiform neurofibroma (PN) and malignant nerve sheath tumors (MPNST); (ii) engineered mouse models of NF1-related neoplasms; and (iii) xenografts of MPNST. For GC-derived lymphomas, TRAP1 expression was assessed in non-neoplastic lymphoid tissues and in Burkitt lymphoma (BL), diffuse large B-cell lymphoma (DLBCL) and Hodgkin lymphoma (HL) samples. The immunohistochemical results were integrated with the results of in silico gene expression studies (Oncomine database). Results. Histological analysis of human PNs and MPNSTs documented the expression of TRAP1, HIF1a and downstream metabolic markers in both benign and malignant samples. A progressive increase in the positivity for such proteins was noted along the oncogenic cascade from nonneoplastic nerves to benign (PN) and malignant (MPNST) tumors. Similar expression patterns were observed in the animal tumor models. In this context, histological evaluation also proved instrumental: (i) to confirm the correspondence between human and animal tumors; (ii) to investigate the metastatic potential of MPNST xenografts; (iii) to detect the effects of TRAP1 knock-down on tumor cell growth and metabolic reprograming; and (iv) to highlight strongly versus minimally activated metabolic pathways in NF1-related oncogenic cascades. The histological characterization of reactive lymphoid tissues highlighted TRAP1 expression in subsets of GC blasts (i.e. differentiating immunoblasts and re-cycling centroblasts). The joint expression of TRAP1 and HIF1a in GC blasts confirmed the presence and activation of the TRAP1/HIF1a axis in GC physiology. In silico studies of GC-derived lymphomas showed very high TRAP1 mRNA levels in BL and (to a lesser extent) DLBCL and HL. Immunohistochemical analysis of primary tumor samples confirmed the in silico results. Conclusions. Histological analysis contributes to the understanding of tumor metabolism and integrates the results of in vitro and in vivo biochemical studies. In particular, it confirms the relevance of TRAP1 activation in NF1-related peripheral nerve sheath tumors and discloses a tight correspondence between primary human samples and animal tumor models. Immunohistochemical characterization of reactive lymphoid tissues and primary lymphoma samples also identifies specific TRAP1 expression profiles, possibly subtending tumor-related metabolic networks.