Mechanical Regulation of Autophagy, Chemoresistance and Genomic Instability

Abstract

The mechanical properties of the tumor microenvironment influence breast cancer

progression by regulating cell proliferation, invasion and dissemination. Stiffening of the

extracellular matrix (ECM), a defining feature of breast tumors, promotes cancer cell

division, genomic instability and metastasis. During metastasis, the dissemination of

breast cancer cells from a primary tumor to secondary sites with different mechanical

properties could lead to prolonged growth arrest, or dormancy, and resistance to

conventional hormone- and chemotherapy. In line with this, breast cancer relapse is often

detected in tissues that are softer than the normal mammary gland or the primary breast

tumor, such as the bone marrow, the brain, and the lung. Although metastasis is the main

cause of death from cancer, the microenvironmental factors that influence resistance to

therapy and the duration of dormancy are largely unknown. This dissertation explores

how the stiffness of the microenvironment at secondary sites regulates tumor dormancy

and the response of breast cancer cells to hormone- and chemotherapy. We show that in

soft microenvironments reminiscent of common metastatic sites, breast cancer cells resist

the effects of the estrogen receptor modulator tamoxifen by increasing autophagy and

decreasing expression of estrogen receptor-α. Consistently, we demonstrate that

pharmacologic inhibition or genetic downregulation of autophagy sensitizes breast cancer

cells to tamoxifen on soft substrata. Further, we find that stimulating cell-ECM adhesion

by ectopically expressing integrin-linked kinase is sufficient to decrease autophagy on

soft substrata. We then investigate the mechanisms by which stiff microenvironments

regulate genomic instability in healthy mammary epithelial and breast cancer cells. We

demonstrate that TGFβ-induced epithelial to mesenchymal transition and ECM stiffening

promote multinucleation, a marker of aneuploidy, and that mitotic failures on stiff

microenvironments correlate highly with DNA damage. Altogether, our data show that

the crosstalk between cancer cells and their mechanical microenvironment regulates

therapeutic outcome, long-term survival, and genomic integrity.