Uncovering a Novel Regulation of Human Epidermal Growth Factor Receptor Signalling by Junctional Adhesion Molecule-A in Breast Cancer
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Junctional Adhesion Molecule-A (JAM-A) is a transmembrane protein with important physiological functions in regulating cell-cell adhesion, and an emerging role in the pathophysiology of breast cancer. JAM-A levels have been reported to regulate expression of the important oncogene Human Epidermal Growth Factor Receptor-2 (HER2) in breast cancer cells, thus we aimed to investigate the underlying mechanisms and whether they extended to regulating the expression of other HER family members. This thesis presents novel evidence that JAM-A expression positively regulates that of HER2 and HER3 at a transcriptional level, accordingly influencing signalling through its downstream effectors and functional outcomes including cell viability, migration and colony formation in vitro and proliferation in a semi-in vivo xenograft model. Mechanistically, the transcription factors ZONAB and FOXA1 were identified as potential regulators of HER expression downstream of JAM-A expression. Overexpression and silencing strategies revealed FOXA1 rather than ZONAB as a key regulator of HER2/3 expression in the breast cancer setting. Specifically, JAM-A expression levels influenced those of FOXA1 at a transcriptional level, and FOXA1 gene silencing reduced both HER2 and HER3 expression. Furthermore, FOXA1 bound specifically to oligonucleotides representing its predicted binding sites in the HER2 and HER3 gene promoters, and JAM-A gene silencing reduced FOXA1 binding to these oligonucleotides. The mechanism whereby JAM-A expression regulates that of FOXA1 appears to involve β-catenin activation and its transit to the nucleus of breast cancer cells, and to be dependent on the dimerization and PDZ-binding domains of JAM-A. Accordingly, a 16 proprietary dimerization-blocking anti-JAM-A peptide reduced cell proliferation and AKT/ERK activation in breast cancer cells. Finally, a stepwise connection between the expression of JAM-A, β-catenin, FOXA1 and HER3 was confirmed by immunohistochemical staining in a tissue microarray derived from invasive breast cancer patient tumours. Taken together, our data provide novel evidence of a direct relationship between levels of JAM-A, β-catenin, FOXA1 and HER2/HER3 in breast cancer cells. In conclusion, we suggest that JAM-A merits investigation as a novel target to inhibit HER2/HER3-dependent tumourigenic signalling in breast cancer.