Tumor microenvironment : the paradoxical action of fibroblasts

Abstract

The term tumor refers to an abnormal and pathological tissue characterized by a massive cell growth; it comprises various populations of transformed and malignant cells. These cells cross-communicate with each other and with different types of cells in the surrounding microenvironment. The nature of communication and interactions within the tumor microenvironment (TME) directs the fate of transformed cells via inducing pro- or antitumorigenic signals. Consequently, these cells will either succeed or fail to progress into a malignant growth phenotype. In the TME, fibroblasts are considered as one of the major cellular compartments and the primary source of non-cellular elements, including the extracellular matrix (ECM) and soluble factors.

It has been shown that tumor cells can recruit fibroblasts to induce growth-stimulatory signals. On the other hand, normal fibroblasts may also act as tumor growth repressors. However, these actions have not been thoroughly addressed. The results of this thesis demonstrate the dual functionality of fibroblasts in the TME. First, we examined the phenomenon, whereby the normal fibroblasts inhibit tumor growth and development. We found that fibroblasts reduced tumor cell proliferation and motility through two sets of signals, the first set involved transmembrane proteins and the ECM.

The second set was only effective after induction of the first signal, and included soluble factors secreted upon direct contact of the fibroblasts and tumor cells. Next, we uncovered the signaling pathways that were involved in the process of tumor growth inhibition and fibroblasts activation. We revealed a switch in fibroblasts from tumor suppressive cells to ones characterized by tumor stimulatory functions. Genetic ablation of the RhoA gene in fibroblasts significantly reduced tumor cell proliferation and motility in vitro, and induced tumor cell clustering in 3D-collagen matrix.

Loosing of the suppressive function was accompanied by gaining a tumor inducing ability, since RhoA deficient fibroblasts enhanced tumor initiation and development by a small number of PC3 prostate cancer cells injected subcutaneously into immunodeficient mice. In addition, knocking out the RhoA gene altered the cytoskeletal organization and reduced αSMA expression in fibroblasts. These changes conferred the cells stiffer but less contractile compared to control cells. Furthermore, upon the crosstalk with tumor cells, the RhoA deficient fibroblasts overexpressed several pro-inflammatory genes encoding for IL6, IL8, CXCL1, CXCL5, and CCL5. Such a biochemical and mechanical shift in the fibroblasts reflected their protumorigenic phenotype. Using patient-derived cancer-associated fibroblasts (CAFs).

We demonstrated that CAFs rescued tumor cells from apoptosis and could even enhance their growth under cis-platinum treatment. Beside the molecular mechanistic results, this thesis introduces a comprehensive quantitative live-cell imaging tool to investigate tumor cellfibroblast interactions dynamically, providing the opportunity to measure and observe cellular proliferation, motility, and phenotypic plasticity simultaneously. Taken together, the current thesis uncovers two opposite effects of fibroblasts on tumor growth. These results emphasize the demand for targeting both CAFs and tumor cells to treat and cure cancer patients and may open novel avenues for cancer prevention approaches.