Tumor-Intrinsic and -Extrinsic Mechanisms in Breast Cancer Metastasis

Abstract

In breast cancer, mortality is predominantly associated with metastasis, cancerous spread to distant organs. Understanding tumor dissemination is vitally important to continued therapeutic advancement, but research is hindered by the inherent complexity within the metastatic cascade, including tumor-intrinsic and tumor-extrinsic mechanisms. This dissertation examines the interplay between these processes, defining methods by which tumor cells disseminate and uncovering novel metastasis therapies.

The first study examines the mechanism by which tumor cells influence osteoclast differentiation, a crucial component of osteolytic metastasis. We report broad microRNA (miRNA) expression changes in osteoclasts after exposure to tumor-conditioned media, due in part to NFκB signaling by soluble intracellular adhesion molecule (sICAM1) from bone-metastatic cells. Ectopic expression of multiple miRNAs suppressed osteoclast differentiation by targeting important osteoclast genes. In vivo delivery of these miRNAs inhibited osteoclast activity and reduced osteolytic bone metastasis. Functional studies revealed impaired osteoclast development, and reduced bone metastasis burden, after ectopic expression of miR-141 and miR-219. These findings establish miRNAs as potential therapeutic targets and clinical biomarkers of bone metastasis via a tumor-extrinsic mechanism.

The second and third studies examine tumor-intrinsic mechanisms involved in breast cancer lung metastasis. The miR-23b/27b/24 miRNAs were associated with lung metastasis samples from human patients, and correlated with increased metastatic potential in breast cancer cell lines. Ectopic expression of the miRNAs in weakly metastatic cells inhibited in vitro migration, but enhanced metastasis. MiR-24 and miR-27b were found to target Prosaposin, a protein with potential metastasis-suppressing functions. The final section examines the dynamics of TGFβ-induced epithelial-to-mesenchymal transition (EMT). Utilizing a mathematical model, we examined cellular commitment to EMT during the first steps of metastasis. Our findings show that EMT is predicated by a positive feedback loop between transcription factors and regulatory miRNAs. This feedback loop produces a bistable, hysteretic system, in which picomolar concentrations of TGFβ are capable of inducing a stable EMT. Furthermore, TGFβ functions through an autocrine pathway, producing additional TGFβ that influences surrounding cells. In vivo analysis found that hysteresis within TGFβ-induced EMT is capable of enhancing metastasis. These results further the understanding of miRNAs and EMT during metastasis, and suggest additional therapeutic strategies.