PI3K-independent mTOR activation promotes lapatinib resistance and IAP expression that can be effectively reversed by mTOR and Hsp90 inhibition

Abstract

Although HER2 targeted therapies have substantially improved outcomes in HER2 overexpressing (HER2C) breast cancer, resistance to these therapies remains a clinical challenge. To better understand the mechanisms of resistance to lapatinib, a HER2 and EGFR dual kinase inhibitor, we treated HER2 C breast cancer cells with lapatinib for an extended period to generate a lapatinib-resistant (LapR) cell line model and examined cancer-promoting signaling activation in LapR cells. We found that LapR cells possess enhanced mTOR activation, which was independent of PI3K and other known mTOR activators. Lapatinib resistance could be reversed by mTOR kinase inhibition. Intriguingly, LapR cells had constitutive cytosolic cytochrome C, indicating that LapR cells suppress lapatinib-induced apoptosis downstream of cytochrome C release from mitochondria into the cytosol rather than by preventing its release into the cytosol. Consistent with this notion, LapR cells possessed increased levels of 2 of the inhibitors of apoptosis (IAPs), survivin and c-IAP-2, which are reported to block caspase activation downstream of cytosolic cytochrome C release. Further, treatment with the mTOR kinase inhibitor AZD8055 or the Hsp90 inhibitor 17-AAG reversed expression of IAPs and overcame lapatinib resistance in LapR cells. Together, these data suggest that suppression of apoptosis downstream of cytosolic cytochrome C release, possibly through increased expression of IAPs or other caspase-suppressing proteins, may promote lapatinib resistance. Further, PI3K is thought to be the main driver of lapatinib resistance, but our findings indicate that PI3K inhibitors may be ineffective in some lapatinib-resistant HER2C breast cancers with PI3Kindependent activation of mTOR kinase, which may instead benefit from mTOR or Hsp90 inhibitors.