THE ROLE AND REGULATION OF STORE OPERATED CALCIUM ENTRY (SOCE) IN BREAST CANCER

Abstract

Breast cancer (BC) is the most common cancer in women and represents the leading cause of cancer deaths among this gender. An estimated 1.7 million cases of breast cancer were documented in 2012 that were associated with over 0.5 million deaths. Despite many advances in the treatment of breast cancer, it remains the cancer with the highest mortality. Importantly, over 90% of all cancer deaths are primarily due to metastasis. Ca2+ signalling is critical for cell migration, and as such metastasis of cancer cells to secondary sites. In particular store-operated Ca2+ entry (SOCE) plays an important role in cell migration and has been shown to be needed for cancer metastasis. SOCE is a pathway that mediates Ca2+ entry into cells in response to depletion of intracellular Ca2+ stores. SOCE is mediated by two primary protein families, STIM and Orai. STIM1 is an endoplasmic reticulum (ER) Ca2+ sensor, and Orai1 is Ca2+ channel at the plasma membrane. Inhibition of Ca2+ influx through SOCE represents a potential therapeutic target to attenuate cancer metastasis. The overarching objective of this dissertation is to begin to understand the role of SOCE in breast cancer. During my PhD studies I approached this problem on three fronts: assessing the role of the miRNA machinery in regulating STIM1 expression in breast cancer cells, elucidate the mechanisms underlying SOCE inhibition in new hypomorph mouse model and its effect on cell migration, and finally assessing the role of STIM1 phosphorylation in cell migration. First, I built on the observation that STIM1 is differentially expressed in two breast cancer cell lines with distinct metastatic potential, the highly aggressive MDA-MB-231 as compared to the less aggressive MCF7 cells. I investigated the mechanisms controlling STIM1 expression and showed an important role for the miRNA machinery, specifically Ago2. My second line of investigation was focused on defining the molecular mechanisms responsible for downregulation of SOCE in a novel mouse model with a duplication of the STIM1 C-terminus that results in ~70% decreased in SOCE activity. This mouse line provides a good model to assess the ability of breast cancer cells to metastasize when SOCE activity is reduced. Finally, I investigated the role of STIM1 phosphorylation in cell migration using a knock-in mouse line that expresses a non-phosphorylatable STIM1 (STIM1-10A). Homozygous STIM1-10A mice developed and reproduced normally following Mendelian ratios with no obvious growth defects or other abnormalities. This suggests that STIM1 phosphorylation is not essential for cell motility. To directly test this hypothesis, we established mouse embryonic fibroblasts (MEFs) cell line from STIM1-10A mice and compared them to the MEFs derived from wild-type mice (WT). Our results show that STIM1 phosphorylation is dispensable for cell migration. In conclusion, our findings support an important role for STIM1 in cancer cell migration. Therefore STIM1 is an attractive therapeutic target to inhibit cancer migration and metastasis.

Date of Award	2020
Original language	American English
Awarding Institution	HBKU College of Health & Life Sciences