Serotonin transporter antagonists target tumor-initiating cells in a transgenic mouse model of breast cancer

Abstract

Accumulating data suggests that the initiation and progression of human breast tumors is fueled by a rare subpopulation of tumor cells, termed breast tumor-initiating cells (BTIC), which resist radiotherapy and chemotherapy. Consequently, therapies that abrogate BTIC activity are needed to achieve durable cures for breast cancer patients. To identify such therapies we used a sensitive assay to complete a high-throughput screen of small molecules, including approved drugs, with BTIC-rich mouse mammary tumor cell populations. We found that inhibitors of the serotonin reuptake transporter (SERT) and serotonin receptors, which include approved drugs used to treat mood disorders, were potent inhibitors of mouse BTIC activity as determined by functional sphere-forming assays and the initiation of tumor formation by transplant of drug-exposed tumor cells into syngeneic mice. Moreover, sertraline (Zoloft), a selective serotonin reuptake inhibitor (SSRI), synergized with docetaxel (Taxotere) to shrink mouse breast tumors in vivo. Hence drugs targeting the serotonergic system might be repurposed to treat breast cancer patients to afford more durable breast cancer remissions.

Serotonergic system antagonists target breast tumor initiating cells and synergize with chemotherapy to shrink human breast tumor xenografts

“…Interestingly, overexpressing cDNAs encoding the 5-HT 2C or 5-HT 2A receptors results in the transformation of mouse 3T3 fibroblasts demonstrating that they function as oncogenes in the focus-forming assay [ 55 , 56 ]. Consistent with our findings serotonergic system antagonists, including several that we independently discovered [ 17 ], were recently found to inhibit sphere formation by glioblastoma cell lines [ 57 ] and to affect the growth of tumor allografts and xenografts of neuroendocrine origin [ 58 ].…”

“…We have shown that spheres arise from dispersed tumorsphere-derived cells in direct proportion to the number of cells plated into the medium, and that the frequency of sphere-forming cells, which averages 5% of the total tumor cell population in human breast tumor cell line-derived tumorspheres, can be accurately quantified over a range of cell densities [ 27 ]. Moreover plating single tumorsphere-derived cells into the wells of 96-well plates yields spheres at the same frequency as those forming when the cells are plated at higher cell densities [ 17 ].…”

“…Our high-throughput screen, which identified serotonergic system antagonists as candidate anti-BTIC agents, was carried out in mammary tumor cells from an HER2 overexpressing mouse model of breast cancer [ 17 ]. Moreover, the HCC1954 breast tumor cell line we initially used for sphere-forming assays also overexpresses HER2 ( Supplementary Table 1 ).…”

“…To identify human SERT in tumor cell lysates we used a polyclonal antibody to SERT (Alomone Labs; Jerusalem, Israel), elicited by immunization of rabbits with a peptide corresponding to amino acids 388-400. The polyclonal antibody was also used to detect SERT in breast tumor xenograft sections as described [ 17 ]. A polyclonal antibody to TPH1 (LifeSpan BioSciences, Inc.; Burlington, ON, Canada) generated by immunizing rabbits with a peptide (amino acids 231-280) was used to detect the protein in breast tumor cell lines [ 17 ].…”

“…We previously reported that tumors from multiple transgenic mouse models of breast cancer comprise a high BTIC frequency [ 14 ], which is maintained when the cells are propagated in chemically-defined, serum-free medium [ 15 ] as non-adherent spheres, which we termed tumorspheres [ 16 ]. The capacity to propagate BTIC-enriched tumor cells in vitro enabled a high-throughput phenotypic screen using a sensitive cell viability assay with approximately 35,000 compounds [ 17 ]. We found that neurotransmitter antagonists comprised a high frequency of the small molecules of known mechanism of action that affected the viability of sphere-derived mouse tumor cells.…”