Evolutionary Mechanisms for Host Resistance to Tumor Growth and Subsequent Cancer Cell Counter-Adaptations

Cancer is well-recognized as an evolutionary system, as first proposed by Cairns and Nowell more than 60 years ago. In an evolutionary context, cancers growing in vivo typically consist of heterogeneous subpopulations of cells that interact with each other and with host cells through selection forces operating at many temporal and spatial scales. Moreover, the tumor environment comprises more than just cancer cells; it includes a rich cancer stroma and cancer-driving molecules such as cytokines and metabolites. The tumor’s environment comprises intratumoral heterogeneity that often leads to therapy resistance attributed to the essential roles of many genetic and nongenetic mechanisms. My dissertation investigated possible outcomes from complex eco-evolutionary interactions between cancer cells and their host organism. By exploring phenotypic, genetic, and epigenetic mechanisms and responses, I discovered that both immune and non-immune resistance strategies are evolutionarily possible. Thus, my findings from three related studies provide novel insights into the evolutionary “arms race” of tumor progression in immune-competent and immune-deficient mice. In the first study involving interactions between the tumor and the host immune system, I identified the consequence of disturbing the equilibrium phase of the dynamic process that consists of immunosurveillance and tumor progression (i.e., cancer immunoediting). This phase is a characteristic of tumor dormancy that is achieved when a complex equilibrium occurs between the tumor cell and the immune system, and the tumor remains in stasis. My studies have shown that perturbation of this equilibrium by a stress stimulus, such as administration of volatile and intravenous anesthetics, enhances tumor growth in immune-competent mice but not in immune-deficient mice. This suggests that the immune system can be a key component in the oncological stress response for those pharmacologic agents for hosts that are not immune compromised. In the second study, I identified different strategies that allow tumors to be resistant to one type of cancer by applying selective breeding over 10 generations to laboratory immune-competent and immune-deficient mice inoculated with subcutaneous tumors. My studies showed that both mice strains evolved greater cancer resistance and suppression mechanisms after 10 generations of selection, but the tumors of these mice responded differently. In the absence of an intact adaptive immune system, the immune-deficient mice evolved with changes in mesenchymal cells that limited resources and cancer cell growth. In contrast, the immune-competent mice evolved with improved immune-mediated killing of cancer cells through changes in immune cell frequency, phenotype, and function. Cancer cells deployed observable counter- responses to the hosts’ cancer suppression mechanisms. These counter-responses included increased proliferation in immune-competent mice and both less cell proliferation and higher necrosis in immune-deficient mice. My studies suggest that host species can rapidly evolve both immunologic and non-immunologic tumor defenses depending on the lineage. However, cancer cells maintain sufficient plasticity to deploy effective phenotypic and population-based counterstrategies quickly. For example, variation in tumor gene expression was largely explained by the differences between the hosts and the fact that the hosts responded differently to selection for resistance to the tumor. In the third study, I examined how transcriptomic responses evolved in the hosts in response to selection for resistance as well as the transcriptomic response of the original cancer cell line. In immune-competent mice compared to immune-deficient mice, I found increased expression in genes enriched for developmental processes, cell migration and movement, and cell membrane composition. The gene with the highest fold expression increase was Semaphorin 3D (Sema3D). This gene is implicated in the development and formation of blood vessels during angiogenesis for the regulation of the epithelial to mesenchymal transition. In addition, genes related to integrin binding, cell adhesion molecular binding, and extracellular matrix (ECM) binding were differentially expressed in immune-deficient mice. Expression levels of extracellular matrix markers, such as collagen type VI (Col18a1), alpha, prolyl 3-hydroxylase 2 (p3h2), and collagen, type XII alpha (Col12a1), were decreased. My future plans include associating the genome-wide differentially expressed genes with methylation changes, as well as how examining how patterns of gene expression and methylation may change across the tumor and the host in response to selection for cancer resistance. The data presented here demonstrates the importance of molecular-level mechanisms that can be effectively targeted for therapeutic benefits. Furthermore, the mice strains developed in these studies can be used to discover more mechanisms of tumor growth resistance and metastasis that may lead to significant advancements in clinical treatments for patients with cancer