War and peace in the tumor microenvironment : tumor-associated cells as facilitators or adversaries during tumor development

Abstract

The tumor microenvironment, including immune cells, fibroblasts and vasculature, profoundly affects tumor development by initially opposing, but eventually facilitating tumor growth, vascularization and spread. Though corrupted by the growing tumor, such cells remain non-transformed and thus, with proper cues, are possible to direct toward their physiological anti-tumor function. Understanding the mechanisms by which the tumor microenvironment is shaped, before and during tumor growth, has been the principal aim of this thesis.

In paper I, we demonstrate that the composition of tumor-associated macrophages (TAMs) can be modulated by selective proliferation of TAM subsets. We identify SEMA3A as a factor potentiating such selective proliferation of anti-tumor TAMs. In paper II we extend the study of SEMA3A’s effect on tumor immunity by showing that it can functionally alter the phenotype of polymorphonuclear myeloid-derived suppressor cells (PMN-MDSC). As a consequence of its effects on TAMs and PMN-MDSCs, the tumor microenvironment is infiltrated by activated cytotoxic lymphocytes which act to obstruct tumor growth. In paper III we show that regulation of mRNA translation shapes the phenotype of TAMs as they become increasingly pro-tumor during tumor growth. We further show that transcripts translationally activated during tumor growth in TAMs were regulated similarly upon M2-polarization of macrophages in vitro. Selective inhibition of the MNK2/phospho-eIF4E pathway, which impinges on mRNA translation, functionally altered in vitro M2-polarized macrophages toward a pro-inflammatory phenotype. This suggests that modulation of mRNA translation is a potential target in TAM-based anti-tumor therapies.

We further emphasize the importance of mRNA translation in regulating gene expression in the microenvironment in paper IV, where we show changes in its efficiency to drive cancer-associated gene expression alterations in the stroma of patients with chronic obstructive pulmonary disease (COPD). Depending on the lung function, two distinct gene expression programs were discovered. These were enriched for proteins previously identified in fibroblast secretomes that promoted cancer initiation in animal models, highlighting the involvement of non-transformed cells in neoplastic transformation. In paper V, we show that class switch junctions in B cells from patients with BRCA1 mutations display decreased use of non-homologous end joining pathway in favor of the alternative end-joining pathway. This implicates a role for BRCA1 in maintaining genome stability and tumor suppression outside of its recognized role in mediating homologous recombination during cell division.