2018-09-172018-09-172018-042018-09-17April 2018http://hdl.handle.net/10388/10647The human Fyn-related kinase (FRK) is a non-receptor tyrosine kinase known to have tumor suppressor activity in breast cancer cells. FRK expression is mostly absent in mesenchymal breast cancer cells but present in epithelial cells. Overexpression of FRK in breast cancer cells was shown to suppress cell growth by interacting, phosphorylating and stabilizing the tumor suppressor PTEN, inhibiting breast cancer cell proliferation by arresting the G1 phase of the cell cycle, by interacting with pRb (Retinoblastoma) and decreasing STAT3 phosphorylation. However, STAT3 has not been validated as a target of FRK and the mechanisms by which FRK suppresses cell proliferation, and migration has not been fully characterized. We used the FRK-negative MDA-MB 231 breast cancer cell line in which we stably overexpressed FRK, and analyzed the effect on FRK on STAT3 signaling, cell proliferation, migration, and invasiveness. We employed both immunoblotting and RT-PCR to identify/validate FRK-regulated targets (proteins and genes) in these cells. Finally, we interrogated the TCGA and GENT gene expression databases to determine the correlation between the expression of FRK and epithelial/mesenchymal markers in breast cancer cell lines and tissues. We observed that FRK overexpression inhibited JAK/STAT signaling pathway by suppressing the expression of some STAT3 target genes. Overexpression of FRK also increased transcription of the epithelial marker gene E-cadherin, and down-regulated the transcript levels of Vimentin, Fibronectin, and Slug. We also observed an inverse correlation between FRK expression and mesenchymal markers in a large cohort of breast cancer cells. FRK suppresses breast cancer cell proliferation by inducing cellular senescence through upregulation of p21 thus, resulting in pRb dephosphorylation and the consequent inhibition of E2F1. Finally, we found that FRK suppressed mammary tumorigenesis in xenograft mice. Our data indicate that the suppression of EMT and upregulation of p21 is one of the mechanisms by which FRK suppresses breast cancer tumorigenesis. We conclude therefore that FRK acts as a tumor suppressor in breast cancer by repressing cell proliferation, migration and invasiveness by suppressing epithelial-to-mesenchymal transition and inducing cellular senescence.application/pdfFyn-related kinase (FRK)Basal A, basal B and Luminal breast cancer subtypesSTAT3Epithelial to Mesenchymal Transition (EMT)cellular senescence and Cell cycleThesis2018-09-17