BIOMECHANICAL REGULATION OF CELL PROLIFERATION, EPITHELIAL-MESENCHYMAL TRANSITION AND INVASION

Abstract

Cells in epithelial tissues adhere to an underlying extracellular matrix (ECM) and respond to changes in its physical properties by altering gene expression and behavior. The mechanical properties of the ECM, including the matrix stiffness, are therefore central to developmental, physiological, and pathological processes. In the context of cancer, increased tissue stiffness and interstitial fluid pressure are inherent features of tumorigenesis, which promote malignant transformation of surrounding cells and correlate with poor survival. These processes depend on the synergistic effects of mechanical signals such as ECM stiffness and chemical signals such as Wnt, TGFβ and proteases in the cellular microenvironment. This dissertation explores how mechanical and biochemical signals are integrated to control cell behaviors including proliferation, apoptosis, epithelial-mesenchymal transition (EMT), and invasion. We used engineered substrata to recapitulate the mechanical stiffnesses of the normal mammary gland as well as that of breast tumors.

We found that exposure to Wnt3a increased proliferation of mammary epithelial cells cultured on stiff substrata, with compliances characteristic of breast tumors, but not of cells on soft substrata, with compliances comparable to that of normal mammary tissue. Depleting integrin-linked kinase (ILK), which functions as a signaling hub for cellular mechanotransduction, rendered cells unresponsive to Wnt3a on both substrata. Ectopic expression of ILK permitted Wnt3a to induce proliferation of cells on both microenvironments. We further showed that ILK regulates expression of the Wnt receptor frizzled-1 (Fzd1), suggesting the existence of a positive feedback loop between Wnt3a, ILK and Fzd1.

Following these findings, we examined the role of ILK and tissue stiffness in regulating cell fate in response to TGFβ1. We found that ILK controls the switch between apoptosis and EMT downstream of TGFβ1 through modulating the fine balance between cell-cell and cell-matrix adhesions. Specifically, depletion of ILK leads to increased E-cadherin-based cell-cell adhesion and cortical actin and decreased focal adhesions and stress fibers in mammary epithelial cells. These changes favor apoptosis and suppress EMT downstream of TGFβ1, regardless of matrix stiffness. Similarly, soft matrix also favors apoptosis downstream of TGFβ1 while stiff matrix promotes EMT, suggesting that depletion of ILK disrupts the ability of a cell to sense a mechanically stiff microenvironment.

Taken together, these findings suggest that tissue mechanics regulates the cellular response to Wnt and TGFβ under physiological and pathological microenvironmental conditions. We also highlighted the role of ILK in mediating the responses to matrix stiffness and tuning the proliferative and potential tumorigenic behaviors of mammary epithelial cells to Wnt3a and TGFβ1. These results provide insight into the molecular mechanisms underlying the effects of a pathological microenvironment on neoplastic progression.