Abstract P1-03-07: Junctional adhesion molecule-A is a novel upstream regulator of human epidermal growth factor receptor-3 signaling in breast cancer
Authors: Cruz RGB , Hopkins AM 
Published: February 2018

Junctional Adhesion Molecule-A (JAM-A) is a transmembrane protein with important physiological functions in regulating cell-cell adhesion. Pathophysiologically, its high expression in breast tumour tissue has been shown to correlate with that of Human Epidermal Growth Factor Receptor-2 (HER2), whilst JAM-A knockdown in breast cancer cells has been shown to reduce HER2 expression and signaling. Although HER2 has been successfully targeted in the oncology setting for several years, the problem of clinical drug resistance to HER2-targeted therapies (among other factors) has recently put the spotlight on other HER family members and their upstream regulators as potential drug targets. HER3 is the most potent binding partner of HER2 in activating tumor growth signaling, and accumulating evidence suggests that HER3 plays an important role in resistance to anti-HER2 therapies. Furthermore HER3 is frequently overexpressed in HER2-negative breast cancers, and, along with other HER family members, may drive HER2-independent tumorigenic mechanisms. Since JAM-A levels have been reported to regulate HER2 expression (Brennan et al, Oncogene 2013 32(22):2799-804), we hypothesised that JAM-A also regulates expression of HER3 in breast cancer cells. Results from our study showed that stable overexpression of JAM-A in MCF7 breast cancer cells (MCF7-JAM) increased both mRNA and protein expression of HER3. Correspondingly, transient gene silencing of JAM-A reduced the mRNA and protein expression of HER3 in MCF7 and MCF7-JAM cells. JAM-A silencing also reduced HER3 expression in HER2-positive BT474 breast cancer cells. As the cell lines tested were all Estrogen Receptor-α(ERα)-positive, we examined whether ERα was required to permit JAM-dependent regulation of HER3. However concomitant silencing of ERα in MCF7 cells did not alter the capacity of JAM-A silencing to reduce HER3 protein levels. In MCF7 and MCF7-JAM cells, JAM-A gene silencing phenocopied that of HER3 gene silencing by reducing protein expression of the HER downstream effectors phospho-AKT and phospho-ERK, in parallel with significant reductions in cell viability (measured by Alamar Blue assay). To begin exploring the mechanism whereby JAM-A regulates HER3 expression, we focused on the HER3 transcription factor FOXA1. JAM-A knockdown reduced expression of FOXA1 in MCF7 and MCF7-JAM cells, and knockdown of FOXA1 was sufficient to reduce HER3 expression in the same cells. Taken together, our data provide novel evidence of a direct relationship between levels of JAM-A, FOXA1 and HER3 in breast cancer cells. The relationship appears to be uni-directional, since silencing of HER3 did not alter JAM-A expression in any cell line tested. In conclusion, we suggest that JAM-A merits investigation as a novel target to inhibit HER3-dependent tumorigenic signaling in breast cancer. Our ongoing investigations will determine the pharmacological value of inhibiting JAM-A signaling in breast cancer models, and its potential significance in the setting of resistance to HER2-targeted therapies.

Citation Format: Cruz RGB, Hopkins AM. Junctional adhesion molecule-A is a novel upstream regulator of human epidermal growth factor receptor-3 signaling in breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-03-07.