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.

We reported that the class I HDAC inhibitor entinostat induced apoptosis in erbB2-overexpressing breast cancer cells via downregulation of erbB2 and erbB3. Here, we study the molecular mechanism by which entinostat dual-targets erbB2/erbB3. Treatment with entinostat had no effect on erbB2/erbB3 mRNA, suggesting a transcription-independent mechanism. Entinostat decreased endogenous but not exogenous erbB2/erbB3, indicating it did not alter their protein stability. We hypothesized that entinostat might inhibit erbB2/erbB3 protein translation via specific miRNAs. Indeed, entinostat significantly upregulated miR-125a, miR-125b, and miR-205, that have been reported to target erbB2 and/or erbB3. Specific inhibitors were then used to determine whether these miRNAs had a causal role in entinostat-induced downregulation of erbB2/erbB3 and apoptosis. Transfection with a single inhibitor dramatically abrogated entinostat induction of miR-125a, miR-125b, or miR-205; however, none of the inhibitors blocked entinostat action on erbB2/erbB3. In contrast, co-transfection with two inhibitors not only reduced their corresponding miRNAs, but also significantly abrogated entinostat-mediated reduction of erbB2/erbB3. Moreover, simultaneous inhibition of two, but not one miRNA significantly attenuated entinostat-induced apoptosis. Interestingly, although the other HDAC inhibitors, such as SAHA and panobinostat, exhibited activity as potent as entinostat to induce growth inhibition and apoptosis in erbB2-overexpressing breast cancer cells, they had no significant effects on the three miRNAs. Instead, both SAHA- and panobinostat-decreased erbB2/erbB3 expression correlated with the reduction of their mRNA levels. Collectively, we demonstrate that entinostat specifically induces expression of miR-125a, miR-125b, and miR-205, which act in concert to downregulate erbB2/erbB3 in breast cancer cells. Our data suggest that epigenetic regulation via miRNA-dependent or -independent mechanisms may represent a novel approach to treat breast cancer patients with erbB2-overexpressing tumors.