Tissue polarity meets nuclear function: Polarity as a modulator of cell proliferation and DNA repair

Abstract

Establishment of baso-apical polarity, a fundamental property of tissue architecture in epithelial cells is involved in the maintenance of tissue homeostasis. Recent data suggests that nuclear organization also participates in tissue homeostasis. I have used three-dimensional (3D) culture system that can reproduce different levels of polarity to explore the relationship between nuclear organization and tissue polarity in the control of cell fate. The HMT-3522 non-neoplastic mammary epithelial cells (S1) differentiate to form baso-apically polarized structures (acini) in 3D culture in the presence of laminin rich basement membrane (BM). Under similar 3D culture conditions, tumor (T4-2) cells form tumor-like nodules and upon induction of phenotypic reversion, T4-2 cells form tissue structures (spheroids) that have basal, but no apical polarity (RT4-2). I show that RT4-2 cells display nuclear structural characteristics of acinar differentiation. Alteration of nuclear organization in S1 acini and RT4-2 spheroids by treatment with anti-NuMA (nuclear mitotic apparatus protein) antibodies leads to loss of differentiation. However, while non-neoplastic cells subsequently undergo apoptosis, reverted tumor cells enter the cell cycle. My results demonstrate that the impact of nuclear organization on cell fate depends on the status of tissue polarity. Cell contact with the BM is required for the establishment of basal polarity. Loss of cell-BM interaction in cancer is characterized by accumulation of genomic instability in the form of mutations. I asked whether cell-BM interaction will affect DNA repair in mammary epithelial cells. My results demonstrate that basally polarized S1 and RT4-2 cells undergo DNA repair and the DNA damage response in S1 and RT4-2 cells involves activation of ATM and p53. Furthermore, inhibition of PI3 kinase decreased DNA repair in S1 and RT4-2 cells. When RT4-2 cells were cultured in the absence of BM, the DNA repair activity was decreased. Blocking alpha 6 integrin signaling in RT4-2 cells also resulted in decrease in DNA repair activity. The above results demonstrate the importance of cell-BM interaction in the DNA repair of RT4-2 cells. Overall, the results from this thesis reinforce the importance of tissue polarity in nuclear function