Breast cancer therapy resistance and disease recurrence are driven by an infrequent population of stem-like tumor cells, termed breast cancer stem cells or tumor-initiating cells (BTIC). Whereas drugs that target BTIC could be combined with conventional therapies to provide durable remissions, identifying such agents has been difficult. To achieve the latter, our lab screened more than 35,000 compounds for their capacity to reduce the activity of BTIC-enriched mouse mammary tumorspheres, wherein we identified numerous antagonists of multiple serotonin receptors (HTRs). The serotonergic antagonist that prevented sphere formation with the highest potency is a highly selective antagonist of HTR5A, SB-699551. We subsequently demonstrated that this agent affects BTIC activity in breast tumor cell lines representative of all clinical and molecular subtypes of breast cancer. Whereas the primary target of SB-699551 is known, the downstream signaling pathways responsible for its anti-BTIC effect remains enigmatic. The goal of this thesis work was to elucidate the signaling pathways downstream of HTR5A in human breast tumor cell lines. We used a phospho-proteomic approach to establish that treatment of human SB-699551 affects the phosphorylation of proteins involved in the Gi-coupled and the PI3K/AKT/mTOR signaling axes. Moreover, we demonstrated that selective antagonists of PI3K, AKT, and mTOR phenocopied the effect of SB-699551 in tumorsphere forming assays. Taken together, our data suggests that SB-699551 elicits its effect through the PI3K/AKT/mTOR signaling pathways downstream of HTR5A. / Thesis / Master of Health Sciences (MSc) / Accumulating data suggests that the progression of breast cancer is driven by a rare population of breast tumor-initiating cells (BTIC). BTIC lie dormant during conventional therapy and initiate recurrence after such therapies are withdrawn. Hence, there is an urgent need to develop drugs that target BTIC that can be combined with the current standard of care to improve the durability of remission. With the latter objective in mind, our lab previously determined that antagonists of serotonin signaling target BTIC. One of the agents that we identified in our screen inhibits the activity of serotonin receptor 5A (HTR5A). The exact signaling mechanism whereby inhibition of HTR5A leads to a loss in BTIC activity was enigmatic. Hence, this thesis aims to elucidate the signaling pathways downstream of HTR5A in breast cancer. Knowledge of the latter will help identify a plausible mechanism in addition to identifying biomarkers of therapy efficacy.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/24743 |
| Date | January 2019 |
| Creators | Shakeel, Mirza Shahbaz |
| Contributors | Hassell, John A., Biochemistry |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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