Abstract 1424: Her3 up regulation in response to AKT and PI3K inhibition causes differential responses dependent on the presence of neuregulin isoforms in the microenvironmental milieu

Introduction: Triple Negative Breast Cancer (TNBC) is an aggressive form of Breast Cancer (BC) affecting ~20% of BC patients. TNBC is a highly heterogenous disease and numerous tyrosine kinase inhibitors (TKI) targeting different pathways including the PI3K, AKT and MAPK pathway have shown limited benefit in the clinical setting. Intrinsic and extrinsic resistance mechanisms to TKI are the main reason of therapeutic failure. Signalling from the human epidermal growth factor receptor (HER) family of proteins is dependent on a well-orchestrated series of interactions between family members to form either homo- or heterodimers. The heterodimeric complex formed with HER3 is a particularly potent oncogenic signalling unit that can act as a driver of cancer growth. HER3 upregulation in response to small molecule inhibitors has been shown to rescue a large number of cancers including TNBC. In this study we aim to characterize the extent of HER3 reliance in TNBC and to define the effect of neuregulin isoforms in TNBC cancers. Method and Materials: Basal and Claudin type Breast cancer cell lines were treated with a range of small molecule inhibitors, known to exhibit limited efficacy as single agents, including the AKT inhibitor (GDC0068), the PI3K inhibitor (GDC0077) and the EGFR inhibitors (Gefitinib and Lapatinib) or a combination of these drugs in the presence or absence of the HER3 ligand neuregulin. The effect these inhibitors on Akt/ MAPK signalling was characterized using specific biomarkers of the respective pathways. Combination therapy was evaluated in human cancer cell lines through Annexin V staining. Results: Basal (HCC-70 and MDA-MB-468) and Caudin type (MDA-MB-231) TNBC cell lines displayed differential reliance on the HER family of receptors. Dynamic HER3 upregulation was predominant in the TNBC cells that show HER signaling reliance coinciding with the basal subtype. Furthermore, the presence of the natural ligand neuregulin showed potent signaling through the HER3-AKT pathway and significantly diminished the efficacy of the inhibitors tested. We report that neuregulin augments the HER3 feedback mechanism for continued proliferation in TNBC. Interestingly, we show that neuregulin mediated resistance is dependent on the type and dose of the neuregulin isoform. Neuregulin 1β showed potent proliferative effects and overcoming this effect required a 10 fold increase in the drug concentration. In contrast, the presence of neuregulin 1α isoform may paradoxically confer sensitivity to small molecule inhibitors. We show that cellular proliferation is maintained through both the PI3K and MAPK pathway and single targeting is inefficient in overcoming resistance. We demonstrate that combination strategies that block compensatory mechanisms are effective strategies to neuregulin dependent and independent resistance mechanisms. Conclusion: The highly dynamic nature of HER3 expression and signalling combined with redundant HER signalling and downstream mechanisms allow for continued signalling in a subtype of TNBC patients. Targeted therapy to ensure complete abrogation of HER signalling is essential for effective therapeutic response. In conclusion, TNBC cancers display differential mechanism of resistance that should be adequately addressed for therapeutic strategies. Citation Format: Nicoleta Sinevici, Pedram Heidari, Umar Mahmood. Her3 up regulation in response to AKT and PI3K inhibition causes differential responses dependent on the presence of neuregulin isoforms in the microenvironmental milieu [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1424.