Exploring the functional heterogeneity of the tumor microenvironment

Sammanfattning: Tumors are complex entities, composed of different functional populations of malignant and stromal cells. There is an emerging importance to understand previously unmapped communication routes between different cell and non-cell compartments of the tumor microenvironment. Targeted drugs have been used for years in an attempt to block essential pathways for tumor growth. Specifically, drugs targeting tumor angiogenesis have shown significant success in different types of tumors. However, as effective as some of these compounds are, durable responses are rare and resistance does ultimately occur. Studies exploring therapeutic resistance have largely focused on endothelial cells (ECs). However, more recent reports suggest that development of resistance can be conferred by functional alterations of supporting cells like pericytes. Pericytes are a heterogeneous and highly mystifyingpopulation of mural cells and have been for years suggested to protect ECs from anti-angiogenic insults, promote vessel regrowth and tumor progression. However, most of the knowledge on pericytes in tumor growth is confounding and often conflicting.In the first part of this thesis, we investigated several aspects of pericyte function during tumor progression. Using an experimental model of pancreatic neuroendocrine tumors (PanNETs), we characterized the nature of signals exchanged between pericytes and ECs in tumor vessels. We showed that pericytes, when in the vicinity of ECs, promote the upregulation of genes involved in cell survival like Bcl-w. We further demonstrated that upregulation of survival genes is dependent on autocrine vascular endothelial growth factor A (VEGF-A) signaling in ECs. Moreover, we showed that the amount of pericytes expressing alpha-smooth muscle actin (α-SMA) is increased in the tumor parenchyma upon anti-VEGF-A therapy. This highlights the potential value of using pericyte marker expression to predict clinical response to anti-vascular therapy. Finally, we characterized a novel imaging tool, the PDGFRβ-Affibody, a small molecule showing specific binding to platelet derived growth factor receptor beta (PDGFRβ) in tumors. This Affibody molecule holds the potential of being used to identify pericytes in tumors and to deliver cytotoxic compounds directly to the tumor microvasculature. In the second part of this thesis, we investigated the biological function of the latest identified ligand for PDGFRβ, PDGF-DD, in tumor progression. Making use of a newly generated Pdgfd knockout mouse, we demonstrated that growth of PanNET in the RIP1-TAg2 model is significantly impaired in the absence of PDGF-DD. Deficient PDGF-DD signaling did not affect angiogenesis or pericyte recruitment to blood vessels. Instead, we found that PDGF-DD stimulated the proliferation of the bulk tumor cell population by inducing expression of mitogenic factors by a rare population of malignant cells expressing PDGFRβ.The existence of a heterogeneous population of tumor cells, marked by rare expression of PDGFRβ in malignant clones, was further confirmed in a cohort of human primary and metastatic PanNET.Our studies emphasize the prevalent theory that targeting multiple compartments of the tumor microenvironment may represent a viable alternative to prevent resistance and achieve durable responses in patients. However, elucidating the relationship between the heterogeneous composition of tumors and the therapeutic outcome is still a significant challenge.