Abstract P5-10-02: A novel mechanism of phosphatase activation for EGFR by Cullin-3/KCTD10 ubiquitin E3 complex in HER2-positive breast cancer cells

Overexpression of human epidermal growth factor receptor 2 (HER2) in breast cancer is correlated with poor prognosis. HER2-targeting drugs have been successful to treat HER2-positive breast cancer. However, the acquisition of the drug resistance with long-term use is still recognized. Here we report the novel molecular targets to treat HER2-positive breast cancer. HER2 is a key driving forces to proliferate HER2-positive breast cancer cells. Unlike other HER family members (EGFR, HER3, HER4), it recognizes no ligands and forms heterodimers with each other member, leading to the transduction of various cellular signals. Especially, the activated EGFR transactivates HER2, and the phosphorylation of both EGFR and HER2 strongly drives cell proliferation. In this study, we found that the degradation of an endosomal small GTPase, RhoB, by the ubiquitin ligase complex cullin-3 (CUL3)/KCTD10 is essential for both EGFR and HER2 phosphorylation in HER2-positive breast cancer cells. Depletion of CUL3 or KCTD10 drastically reduced the phosphorylation of EGFR and HER2, as well as HER2-positive breast cancer cell proliferation. As mechanisms, we identified a RhoB-interacting scaffold protein and an EGFR phosphatase from human protein arrays prepared by the wheat cell-free protein synthesis system, a unique technology of Ehime University. In HER2-positive breast cancer cells, we showed that the scaffold protein interacts with the EGFR phosphatase at the plasma membrane, which leads to inactivation of the EGFR phosphatase and inhibits EGFR dephosphorylation. Upon CUL3- or KCTD10-knockdwon, accumulated RhoB interacts with the scaffold protein, resulting in the release of active EGFR phosphatase. Using the METABRIC database, we next found that low expression of RhoB mRNA and high expression of scaffold protein correlate with poor prognosis for HER2-positive breast cancer patients. Additionally, high expression of scaffold protein was associated with poor prognosis in immunohistochemistry of HER2-positive breast cancer tissues. These data suggest that high expression of scaffold protein enhances cell proliferative signaling in HER2-positive breast cancers. The inhibitors of the scaffold protein/EGFR phosphatase interaction would be new therapeutic agents for HER2-positive breast cancer cells through EGFR dephosphorylation mediated by activation of the EGFR phosphatase. Citation Format: Kanako Nishiyama, Masashi Maekawa, Akari Murakami, Kaho Utsunomiya, Kana Takemoto, Erina Kusakabe, Haruna Noda, Reina Aoki, Kana Taguchi, Michiko Yamashita, Tomoya Nakagita, Jun Nakayama, Mami Chosei, Takeshi Kiyoi, Yoshiaki Kamei, Hiroyuki Takeda, Yasutsugu Takada, Shigeki Higashiyama. A novel mechanism of phosphatase activation for EGFR by Cullin-3/KCTD10 ubiquitin E3 complex in HER2-positive breast cancer cells [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-10-02.