Cellular and Metabolic Processes in Tumor Growth and Metastasis - Recruitment of Myeloid-Derived Suppressor Cells to the Primary and Metastatic Tumor Microenvironment and Multiparametric Metabolic Fingerprinting of Murine Breast Cancer Cells

Abstract

Cancer and metastatic disease are a leading cause of morbidity and mortality worldwide. Molecular, biochemical and cellular traits of the multi-step malignant transformation of normal tissue cells are similar independent of the cancer type and referred to as the hallmarks of cancer. This work focused on aspects in cancer immune evasion and tumor metabolism, two of the hallmarks of cancer. Myeloid-derived suppressor cells (MDSCs), pathologically activated immunosuppressive immature cells of the myeloid compartment, are of particular interest in cancer immune evasion due to their potent immunosuppressive function supporting primary tumor growth, pre-metastatic niche and macrometastases formation. While overall MDSC levels in peripheral blood of cancer patients have been correlated with disease stage, progression and overall survival in multiple cancer types, the migration kinetics and homing dynamics of the polymorphonuclear (PMN-) and monocytic (M-)MDSC subpopulations remain largely elusive. Therefore, it was hypothesized that the migration kinetics and homing dynamics of PMN- and M-MDSCs could be visualized and quantified with non-invasive imaging techniques in vivo. To this end, a direct antibody-based radiolabeling approach for positron emission tomography (PET) was established, evaluated and applied to murine PMN- and M-MDSCs. This radiolabeling approach could be successfully utilized for in vivo PMN- and M-MDSC tracking by PET in syngenic preclinical murine primary cancer and cancer metastasis models for breast cancer and melanoma elucidating both differences in PMN- and M-MDSC recruitment kinetics to the primary and metastatic tumor microenvironment and a certain tumor tropism of the murine MDSC subpopulations in these models. This work presents the first quantitative visualization of MDSC trafficking in vivo with non-invasive PET imaging. The metabolic switch towards aerobic glycolysis in cancer cells was already observed in the early 20th century and is considered a hallmark of cancer. Recent literature describes a certain metabolic flexibility in tumorigenesis that further promotes metastatic invasion. Metabolic flexibility and the link between cancer metabolism and metastatic potential were therefore examined in two murine breast cancer cell lines, in a parental cell line and in it’s more aggressive, metastatic progeny cell line, both derived from the endogenous MMTV-PyVmT tumor model. To this end, multiparametric and metabolic fingerprinting of the murine cancer cell lines and allograft tumors was performed employing non-invasive and molecular imaging modalities including PET, magnetic resonance imaging (MRI) and spectroscopy (MRS), dynamic nuclear polarization (DNP-) MRS with [1-13C]pyruvate, proteomics, transcriptomics and 1H nuclear magnetic resonance metabolomics. In accordance to current literature, fingerprints of glucose, pyruvate and choline metabolism could be correlated to the molecular phenotype and aggressive growth of the metastatic progeny cell line stressing the significance of cancer metabolism in cancer aggressiveness and metastasis.