Abstract
The coexpression of erbB3 and erbB2 is frequently observed in breast cancer; and erbB3 has a critical role in erbB2 promotion of breast cancer progression and anti-estrogen resistance. In this study, we determine the role of erbB3 in erbB2-mediated paclitaxel resistance in breast cancer cells. The overexpression of exogenous erbB3 via either stable or transient transfection in erbB2-overexpressing, but not epidermal growth factor receptor (EGFR)-expressing, breast cancer cells significantly decreases paclitaxel-induced growth inhibition and apoptosis. Consistently, knockdown of erbB3 expression with a specific short hairpin RNA (shRNA) in breast cancer cells with coexpression of both erbB2 and erbB3 enhances paclitaxel-induced apoptosis evidenced by increased DNA fragmentation, poly (ADP-ribose) polymerase (PARP) cleavage and activation of caspase-3 and -8. Furthermore, while forced overexpression of erbB3 increases, specific knockdown of erbB3 decreases the expression levels of Survivin only in the erbB2-overexpressing breast cancer cells. Targeting Survivin with specific shRNA overcomes paclitaxel resistance without effect on the expression levels of either erbB2 or erbB3. Mechanistic studies indicate that the specific phosphoinositide 3-kinase (PI-3K), Akt and mammalian target of rapamycin (mTOR) inhibitors, but not the mitogen-activated protein kinase kinase (MEK) inhibitor, not only abrogate erbB3-mediated upregulation of Survivin, but also reinforce the erbB2/erbB3-coexpressing breast cancer cells to paclitaxel-induced growth inhibition. These data demonstrate that heterodimerization of erbB2/erbB3 is a prerequisite for erbB2 tyrosine kinase activation; and elevated expression of erbB3 is required for erbB2-mediated paclitaxel resistance in breast cancer cells via PI-3K/Akt/mTOR signaling pathway-dependent upregulation of Survivin. Our studies suggest that new strategies targeting erbB3 or Survivin may enhance the efficacy of chemotherapeutic agents against erbB2-overexpressing breast cancer.