Functional heterogeneity and metabolic adaptation of fibroblasts in cancer

The tumor microenvironment is comprised of a multitude of cell types producing a milieu of cytokines, growth factors an extracellular matrix. Together, this intricate network of cells plays highly critical roles in determining a tumor’s potential to progress and metastasize. As the predominant cell types in the tumor microenvironment, cancer-associated fibroblasts (CAFs) are a key component in the regulation of tumor development and progression, but their exact functions in tumorigenesis remain poorly understood. This is partly due to the considerable heterogeneity amongst these cells, which confounds the characterization of their roles within the tumor microenvironment. Nevertheless, recent studies have provided fresh insights in CAFs biology and how these cells contribute to tumorigenesis at different stages, suggesting that CAFs can reshape the tumor immune response via interactions with immune cells, as well as induce tumor angiogenesis and support cancer cell proliferation through modifying tumor metabolism. These reports provided excellent groundwork to further unravel the complex and heterotypic functions of this cell type. In this work, we identified phenotypically and functionally distinct CAF subtypes critical in mammary carcinoma progression using multiplex staining techniques and genetically engineered mouse models that enables the specific targeting of CAF subtypes. Specifically, the distinct functional roles played by aSMA+, FAP+ and PDGFRb+ CAFs were characterized. While targeting FAP+ CAFs did not cause any dramatic changes in primary tumor growth or metastasis, depletion of PDGFRb+ CAFs moderately reduced tumor growth and metastasis. On the other and, aSMA+ CAFs-depleted tumors grew significantly slower and showed decreased angiogenesis. Compellingly, despite primary tumor growth stagnation and notably smaller tumors, metastasis to the lungs continued unabated or even slightly increased in mice with tumors lacking aSMA+ CAFs. Intriguingly, we observed a down-regulation of tumor metabolites in tumors depleted of aSMA+ CAFs. We further examined the metabolic relationship between CAFs and cancer cells to arrive at a mechanistic understanding of the observed metabolic phenotypes in CAFs. Our work demonstrates that CAFs’ metabolic reprogramming can be triggered by hypoxia-induced epigenetic modification of HIF-1a and rate-limiting glycolytic enzymes. As an extension to this work, we further explored fibroblasts’ contributions to cancer initiation. Here, we leveraged a previously reported mouse model in which Tgfb signaling in fibroblasts has shown to modulate epithelial growth, resulting in squamous carcinoma of the forestomach. In our studies, we uncovered a complex feed forward loop, in which the loss of Tgfb receptor 2 in FSP1+ fibroblasts leads to a Smad4-dependent up-regulation of HGF production to induce increased proliferation in neighboring epithelial cells. The proliferating epithelial cells release BMP7 which, in turn, promotes fibroblast proliferation. Interestingly, we demonstrated that this signaling cross-talk is specific to FSP1+ fibroblasts, highlighting the importance of fibroblast heterogeneity not only in cancer progression, but also in normal tissue homeostasis. The work presented in this thesis offers novel insights into fibroblast biology at different stages of cancer development and progression. Our studies shed new light on how fibroblast subsets can phenotypically and functionally be defined. We also showed in two settings how fibroblast - epithelial cell interactions are critical for tumor initiation or progression. In one context, we demonstrated the metabolic liaison between CAFs and cancer cells can drive tumorigenesis. In the second context, we showed an intricate signaling cross-talk between fibroblasts and epithelial cells is required to provoke epithelial carcinogenesis