Abstract B007: Identification of Asporin as a HER3 ligand exposes a therapeutic vulnerability in metastatic prostate cancer
Authors: Hesterberg Amanda B. , Wong Hong Yuen , Jackson Jorgen , Antunovic Monika , Rios Brenda L. , Watkins Evan , Bergman Riley E. , Davidson Brad A. , Ginther Sarah E. , Graves Diana , Nahmias Elliott F. , Googel Jared A. , Martin Lillian B. , Sanchez Violeta , Ericsson Paula Gonzalez , Sheng Quanhu , Brown Benjamin P. , Meiler Jens , Park Ben H. , Schaffer Kerry R. , Gordetsky Jennifer B. , Hurley Paula J. 
Published: November 2024

Abstract
Background: Rates of prostate cancer incidence and advanced-stage diagnoses are increasing. Despite curative-intent therapies, nearly all patients with advanced disease progress to metastatic castration-resistant prostate cancer (mCRPC), a lethal form of the disease. There is an urgent unmet need to understand the mechanisms driving prostate cancer progression and identify alternative therapies for patients with mCRPC. It is well-established that cancer-associated fibroblasts (CAF) in the tumor microenvironment secrete factors that enable cancer cells to establish metastases. Asporin (ASPN) is an extracellular protein secreted by a subset of CAF and is associated with worse oncologic outcomes, yet the mechanisms remain poorly understood. In this study, we sought to identify the molecular interactions between the CAF-secreted ASPN and metastatic prostate cancer cells. Methods: ASPN-induced HER2/HER3 activation and downstream signaling was identified by RNA-sequencing and then confirmed by immunoblotting in multiple human metastatic prostate cancer cell lines. Binding interactions were assessed computationally by AlphaFOld2 with Rosetta refinement and substantiated by co-immunoprecipitation assays. CRISPR-cas9 targeting of HER2 and HER3 and therapeutic inhibition of HER2 with Tucatinib were used to determine the role of HER2/HER3 in ASPN-induced signaling and migration. The efficacy of Tucatinib and antibody-drug conjugates (ADC) targeting HER2 (Trastuzumab-deruxtecan) and HER3 (Patritumab-deruxtecan) were assessed in multiple metastatic prostate cancer cell lines in vitro. Trastuzumab-deruxtecan was assessed in PC3 xenografts in vivo. Prostate cancer metastases from 33 patients were assessed by IHC for HER2 and HER3 and by RNAscope for ASPN. Results: We report that ASPN is a novel ligand of HER3 and induces HER3 heterodimerization with its preferred dimerization partner, HER2. ASPN activates established ErbB-associated pathways including Phosphoinositide 3-kinase (PI3K), Mitogen-activated protein kinase (MAPK), and calcium signaling, in multiple metastatic prostate cancer cell lines to promote cell migration. Genetic and molecular inhibition of HER2/HER3 mitigates ASPN-induced signaling and cell migration, suggesting these receptors are required for paracrine activation by ASPN. Importantly, small molecule and ADC therapies targeting HER2/HER3 significantly diminished prostate cancer cell growth in vitro, with enzalutamide-resistant cells showing increased sensitivity. Trastuzumab-deruxtecan nearly resolved tumors in an in vivo xenograft model. Lastly, ASPN+ CAF in the TME of HER2/HER3-expressing metastatic prostate cancer is frequently observed in patient samples, supporting the clinical relevance of these findings. Conclusions: Collectively, these findings indicate ASPN functions as a HER3 ligand to induce cellular migration, and inhibition with anti-HER2/HER3 therapies highlights potential clinical utility for patients with HER2-expressing metastatic prostate cancer.
Citation Format: Amanda B. Hesterberg, Hong Yuen Wong, Jorgen Jackson, Monika Antunovic, Brenda L. Rios, Evan Watkins, Riley E. Bergman, Brad A. Davidson, Sarah E. Ginther, Diana Graves, Elliott F. Nahmias, Jared A. Googel, Lillian B. Martin, Violeta Sanchez, Paula Gonzalez Ericsson, Quanhu Sheng, Benjamin P. Brown, Jens Meiler, Ben H. Park, Kerry R. Schaffer, Jennifer B. Gordetsky, Paula J. Hurley. Identification of Asporin as a HER3 ligand exposes a therapeutic vulnerability in metastatic prostate cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Tumor-body Interactions: The Roles of Micro- and Macroenvironment in Cancer; 2024 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2024;84(22_Suppl):Abstract nr B007.