Overexpression of HER2 in breast cancer is correlated with poor prognosis. HER2-targeted drugs, such as trastuzumab and lapatinib, have been successful to treat HER2-positive breast cancers, however, the acquisition of the drug resistance of the cells is recognized. Here we suggest the novel molecular targets to cure HER2-positive breast cancers.

The oncogenic roles of Rac1, a Rho family small GTPase, in a variety of cancers have been demonstrated. For example, the elevated expression or hyperactivation of Rac1 is frequently observed in human cancers, correlating with their aggressiveness and poor prognosis. In breast cancers, upregulation of Rac1 GEF (GTP exchange factor) and downregulation of Rac1 GAP (GTPase activating protein) have been reported. Moreover, activation of Rac1 contributed to trastuzumab resistance, which poses a serious problem during chemotherapy.

In the present study, we first investigated in detail in which subtypes of breast cancers mRNA expression of Rac1 is correlated with their poor prognosis. Using the METABRIC database, we found that high mRNA expression of Rac1 significantly correlated with the poor prognosis of HER2-positive breast cancer (p=0.0012, High: n=49, Low: n=171). On the other hands, other three types (basal, claudin-low, or luminal-B type) did not show significant correlation between the expression levels of Rac1 mRNA and their prognosis (p=0.15, High: n=97, Low: n=102; p=0.052, High: n=110, Low: n=89; p=0.17, High: n=70, Low: n=391; respectively). In luminal-A type breast cancer, low mRNA expression of Rac1 significantly correlated with poor prognosis (p=0.0046, High: n=492, Low: n=187).

We next investigated the molecular mechanism underlying Rac1 activation in HER2-positive breast cancer cells, SKBR-3 cells. We found that Cullin-3 (CUL3, a subunit of a RING ubiquitin E3 ligase complex) and its adaptor protein KCTD10 are essential for Rac1 activation. Mechanistically, CUL3/KCTD10 ubiquitinate RhoB, a Rho family small GTPase that suppresses the activation of Rac1, leading to the degradation RhoB. We also found that HER2 signaling is essential for the activation of Rac1.

Conclusions: This study reveals that the novel molecular axis CUL3/KCTD10/RhoB regulates the Rac1 activation in HER2-positive breast cancer cells. The interference of CUL3/KCTD10 complex formation may be a new strategy to the treatment of HER2- and Rac1-positive breast cancers.

Citation Format: Murakami A, Maekawa M, Kusakabe E, Yamasawa H, Aoki R, Komatsu S, Taguchi K, Nishiyama K, Yamashita M, Sugimori W, Kawai K, Nakayama J, Araki N, Semba K, Taguchi T, Kamei Y, Takada Y, Higashiyama S. Novel mechanisms of Rac1 activation by the Cullin-3/KCTD10 ubiquitin E3 complex in HER2-positive breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P1-05-02.