Abstract 3023: Cytoplasmic sequestration and autophagic degradation of ErbB receptors in HER2-driven cancer cells by small molecule Sigma1 modulators

Epidermal growth factor receptors (EGFR/ErbB) drive cell growth, survival, metastasis, and resistance in a range of cancers. Heterodimerization of ErbB1/EGFR, ErbB2/HER2, and ErbB3/HER3 drives aggressive tumor growth through hyperactivation of cancer cell survival and growth signaling pathways. The increased protein production required to sustain these activities renders cancer cells acutely dependent on support factors, such as chaperones and scaffolds, that maintain protein homeostasis. ErbB receptors are integral membrane proteins and as such are synthesized in and transported through the secretory pathway, which comprises the endoplasmic reticulum (ER), Golgi, and associated compartments and vesicles. Nascent ErbB receptors are stabilized and chaperoned through this pathway, in part, by heat shock protein 90 family members, HSP90 and GRP94. Sigma1 (also known as sigma1 receptor) is a unique integral membrane protein found primarily in the ER. Emerging lines of evidence suggest that Sigma1 may function as a chaperone or possibly a scaffolding protein. We find that the levels of Sigma1 protein are elevated and aberrantly distributed in HER2-amplified breast tumor biopsies compared to benign breast tissue. These data indicate that the status and potentially the physiological role of Sigma1 are altered in malignancy and that Sigma1 may be a valid drug target in the treatment of HER2-driven breast cancers. Previously, we discovered that certain selective small molecule modulators of Sigma1 could be used to induce the unfolded protein response (UPR) and autophagy in a panel of cancer cell lines. Here, we demonstrate that these responses to Sigma1 modulators can be exploited to alter the trafficking, stability, and thus signaling of ErbB receptors in cancer cells. In vivo, Sigma1 modulators suppress the growth of xenografted HER2-amplified breast tumors. In the tumors, as well as in vitro cell culture, ErbB1-3 all are eliminated in response to treatment with prototypic small molecule Sigma1 modulators. This corresponds with suppression of downstream PI3K/Akt signaling and with induction of UPR and autophagy. Using high resolution microscopy and organelle fractionation techniques, we confirmed that the Sigma1 modulators induce cytoplasmic sequestration and subsequent degradation of ErbB receptors in ubiquitin-enriched autophagosomes. This process is blocked by cotreatment with autolysosome inhibitor, bafilomycin A1, suggesting that autophagy is the primary mechanism of Sigma1 modulator induced ErbB receptor degradation. Altogether, these data suggest that Sigma1 is a unique, ligand-operated scaffolding protein that contributes to the trafficking and stability of ErbB receptors in HER2-driven cancer cells. Furthermore, these data suggest that Sigma1 is a druggable component of the protein homeostasis regulatory apparatus of cancer cells. Citation Format: Christina M. Maher, Jane Y. Tong, Charles G. Longen, Mercedes I. Lioni, Jeffrey D. Thomas, Xing Tan, Logan Tyler, Fernando U. Garcia, Felix J. Kim. Cytoplasmic sequestration and autophagic degradation of ErbB receptors in HER2-driven cancer cells by small molecule Sigma1 modulators. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3023.