Abstract P4-08-03: The impact of the heregulin-HER receptor signaling axis on response to HER tyrosine kinase inhibitors.

Background: HER receptor tyrosine kinases (EGFR/HER1; HER2; HER3; HER4) and their soluble ligands represent a robust system involved in the regulation of a diverse array of cellular processes. Deregulation of HER receptors, notably HER2, has been linked to the initiation and progression of breast cancer and other solid tumors. Relatively less is known about the role of HER receptor soluble ligands in tumorigenesis and responsiveness to HER targeted therapies. Here we will discuss the impact of the HER3 ligand heregulin (HRG) on the sensitivity of breast cancers to HER tyrosine kinase inhibitors (TKIs), and identify TKI strategies to treat HRG-driven breast cancers. Methods: The effects of exogenous HRG (50ng/ml) on the antitumor activity of a panel of TKIs with different enzymatic properties e.g. a reversible, selective inhibitor (lapatinib) and irreversible, pan-HER inhibitors (neratinib; CI-1033) were assessed in HER2+ breast cancer (BC) cell lines. The concentration of TKIs used was in the 1–2.5 uM range, and cells were treated over a 72 hr course. Vehicle treatment alone served as controls. The impact of the above mentioned treatments on total HER receptor and specific EGFR, HER2, and HER3 phosphotyrosine sites, in addition to the phosphorylation state of components of downstream MAPK and PI3K signaling pathways was analyzed through western blot. In addition to studying the effects of exogenous HRG, the impact of TKIs on a model of triple negative BC (TNBC) known to produce HRG in an autocrine manner was also evaluated. QRT-PCR was used to assess the effects of TKIs on HER receptor mRNA levels. Results: Lapatinib inhibited proliferation and phosphorylation of EGFR, HER2, HER3, and downstream MAPK and PI3K signaling pathways in HER2+ SKBR3 and BT474 cell lines. Pre-treatment with HRG abrogated the antitumor effects of a therapeutic concentration of lapatinib (1 uM), and reversed the inhibitory effects of lapatinib on the phosphorylation of HER receptors and components of their downstream signaling pathways e.g. Erk1/2 and Akt. Neratinib also blocked proliferation and phosphorylation in HER2+ BC cells. In contrast to lapatinib, the antitumor effects of neratinib were not reversed by exogenous HRG. Interestingly, treatment with neratinib and similar pan-HER irreversible TKIs, but not reversible TKIs, resulted in loss of HER2 and EGFR protein expression. QRT-PCR was used to evaluate if this effect was at the level of transcription. Furthermore, neratinib inhibited cell proliferation and blocked EGFR signaling in a TNBC model driven by autocrine produced HRG. Conclusions: Our findings suggest that HRG is a mediator of therapeutic resistance to lapatinib in HER2+ and TNBC. Neratinib demonstrates profound effects on HER signaling by markedly reducing HER2 and EGFR protein expression and blocking the pro-tumorigenic effects of autocrine or paracrine expression of HRG. In contrast to HRG, we previously showed that EGF, an EGFR specific ligand did not reverse the antitumor effects of lapatinib in breast cancer cells. Thus, the selection of HER targeted therapies in a given tumor should take into account not only the HER receptor expression profile, but also the presence of autocrine or paracrine derived ligands activating HER receptors. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P4-08-03.