Escape from HER family tyrosine kinase inhibitor therapy by the kinase inactive HER3

Oncogenic tyrosine kinases have proven to be promising targets for the development of highly effective anticancer drugs. However HER family tyrosine kinase inhibitors (TKIs) show only limited activity against HER2-driven cancers despite effective inhibition of EGFR and HER2 in vivo 1–8. The reasons for this are unclear. Signaling in trans is a key feature of this multimember family and the critically important PI3K/Akt pathway is driven predominantly through transphosphorylation of the kinase-inactive HER3 9,10. We report that HER3 and consequently PI3K/Akt signaling evade inhibition by current HER family TKIs in vitro and in tumors in vivo. This is due to a compensatory shift in HER3 phosphorylation-dephosphorylation equilibrium driven by increased membrane HER3 expression driving the phosphorylation reaction and reduced HER3 phosphatase activity impeding the dephosphorylation reaction. These compensatory changes are driven by Akt mediated negative feedback signaling. Although HER3 is not a direct target of TKIs, HER3 substrate resistance undermines their efficacy and has thus far gone undetected. The experimental abbrogation of HER3 resistance by siRNA knockdown restores potent pro-apoptotic effects to otherwise cytostatic HER TKIs, re-affirming the oncogene-addicted nature of HER2-driven tumors and the therapeutic promise of this oncoprotein target. However, since HER3 signaling is buffered against an incomplete inhibition of HER2 kinase, much more potent TKIs or combination strategies are required to effectively silence oncogenic HER2 signaling. The biologic marker to guide HER TKIs should be the transphosphorylation of HER3.