Activation of the PI3K-mTOR pathway via HER2: HER3-mediated signaling in HER2+ breast
cancers pose one of the major threats towards the success of
trastuzumab. First,
trastuzumab cannot perturb survival/proliferative signals following HER2: HER3 heterodimerization in HER2+
tumor cells. Second,
trastuzumab treatment has been reported to cause
drug-mediated resistance in over 50% of HER2+ breast
cancers. We have reported that treatment with an
anti-angiogenic drug imparted a significant anti-
tumor advantage when combined with
trastuzumab plus
pertuzumab in the
trastuzumab-resistant model of HER2+ breast
cancers (PMID: 23959459). The very fact as revealed by our study that an inclusion of
anti-angiogenic drug conferred a significant anti-
tumor advantage when combined with dual anti-HER2
therapy clearly indicated a critical and indispensable role of angiogenesis in these
tumors. Hence, we hypothesized that
BEZ235 a dual PI3K/mTOR inhibitor will have an effect on the
tumor as well as the angiogenic stromal compartments. In vitro and in vivo efficacy of
BEZ235 was determined in HER2+
trastuzumab-sensitive,
trastuzumab-resistant and HER2 amplified/PIK3CA mutated cell lines.
BEZ235 alone and in combination with
trastuzumab was tested on the
tumor as well as stromal compartments. AKT-mTOR signal was suppressed following
BEZ235 treatment in a concentration and time-dependent manner. AnnexinV, cl-CASPASE3,
SURVIVIN and p-FOXO1 indicated that BEZ235-induced cell death occurred predominantly via an apoptotic pathway.
Heregulin-induced HIF1α synthesis was also significantly decreased. Oncoprint data (cBioPortal) representing PAM50 Her2 enriched
tumors (TCGA, Nature 2012) and Her2-positive
breast tumors (TCGA, cell 2015) showed 91.4% genetic alterations and 79.2% genetic alterations in a set of four genes comprised of PIK3CA, ERBB2, VEGFA and HIF1alpha. The co-occurrence of HIF1alpha with VEGFA in PAM50 Her2 enriched
tumors (TCGA, Nature 2012) and the co-occurrence of HIF1alpha with VEGFA pair as well as HIF1alpha with PIK3CA pair in Her2-positive
breast tumors (TCGA, cell 2015) were found statistically significant. In xenograft models,
BEZ235 blocked
tumor growth and decreased Ki67, CD31, p-AKT, p-S6RP, p-4EBP1 IHC-expressions. These decreases were more pronounced when
BEZ235 was combined with
trastuzumab in HER2+/
trastuzumab-sensitive,
trastuzumab-resistant and HER2+/PIK3CA mutated models. We demonstrated that combined targeting of HER2 and the PI3K-AKT-mTOR pathway is superior to HER2-directed
therapy alone. Mechanistically the inhibition of
tumor-induced angiogenesis by
BEZ235 caused by the down-regulation of PI3K-mTOR-HIF1alpha signaling irrespective of the
trastuzumab-sensitivity status of HER2+ breast
cancers proving evidence for the first time that the inhibition of angiogenesis is an important component of the anti-
tumor efficacy of
BEZ235 in HER2 defined breast
cancers.