Induction of epithelial-mesenchymal transition (EMT) in women's cancer : protective role of differentiation factors

Abstract

Metastatic spread of cancer cells to vital organs is the predominant cause of death among women suffering from breast and ovarian cancer, and invasive cancer cells are in many cases resilient to standard drugs used in the clinic. Consequently, further understanding of the metastatic process and development of new strategies to target invasive cancer cells are needed. One process that has been closely linked to cancer cell invasion and migration is epithelial-mesenchymal transition (EMT), a developmental process, which can be reactivated during cancer progression. EMT allows carcinoma cells, with an epithelial origin, to acquire mesenchymal and migratory properties that are employed to invade the surrounding tumor tissue. The overall aim of this thesis was to investigate how EMT is induced in breast and ovarian cancer cells and to study the role of EMT in drug resistance.

Relapse of resilient cancer cells after surgery and first line of drug treatments is a major cause of death in ovarian and breast cancer. Currently, little is known about the functional properties of cancer cells that develop resistance to existing drug treatments and how they can be targeted. The aim of study I was to characterize the phenotypic properties of ovarian cancer cells that developed resistance to cisplatin, a chemotherapeutic drug commonly used in the clinic. We found that human SKOV-3 ovarian cancer cells that acquired resistance to cisplatin gained properties of EMT and cancer stem cells, suggesting that they were more invasive than drug-sensitive cells. Indeed, functional experiments showed that cisplatin-resistant SKOV-3 cells were more migratory in invasion assays and displayed an increased tumor initiating capacity compared to cisplatin-sensitive cells. The results from these studies link EMT to drug resistance in ovarian cancer cells, and emphasize that further understanding of EMT is needed and to be able to target EMT for therapy.

In study II-IV we investigated how cellular sensitivity to EMT is regulated. In particular, we focused on identifiying epithelial differentiation factors that regulate EMT in breast cancer cells. We identified two transcription factors – C/EBPβ and Foxp4 that were lost during breast cancer progression, which conferred cells an enhanced capacity to undergo EMT as well as to gain invasive and metastatic properties in experimental in vitro and in vivo models of breast cancer. In addition, we identified the coxsackie- and adenovirus receptor (CAR), a tight junction-based cell adhesion molecule, as a novel regulator of Akt signaling and TGF-β-induced EMT in breast cancer cells. The mechanism was traced to a role of CAR in regulating localization, stability and function of the phosphatase Pten, a potent Akt inhibitor, at tight junctions. The results from these studies indicate that the EMT process is not solely regulated by factors that drive a mesenchymal differentiation program, but also, is under tight control by epithelial differentiation factors. Loss of C/EBPβ, Foxp4 and CAR may lead to increased cellular sensitivity to EMT and thereby open up the possibility that cancer cells acquire invasive and migratory properties. Based on this, we propose that novel therapies aiming to strengthen, or preserve, epithelial differentiation mechanisms in breast or ovarian cancer cells, might be useful as a type of differentiation therapy to inhibit cancer cell invasion and metastasis.