Project Summary / Abstract
HER2-positive breast cancers are highly aggressive and associated with poor prognosis. HER2-targeted
therapy is the preferred treatment for these cancers, but drug resistance is a major problem. Our previous studies
demonstrated that dysregulation of G protein coupled receptors, in particular, a subgroup of GPCRs that couple
to G protein ai/o subunits (Gi/o-GPCRs), contributes to HER2-induced breast cancer initiation and progression.
Targeting Gi/o-GPCR signaling blocks tumor progression and enhances the efficacy of HER2-targeted therapy.
This proposal aims to delineate how dysregulated Gi/o-GPCR signaling may control HER2+ breast cancer
progression through a poorly studied CTLH E3 ubiquitin ligase complex. The CTLH (carboxy-terminal to LisH
domain) complex is a mammalian ortholog of the yeast GID (glucose-induced-degradation-deficient) complex
that contains multiple subunits, including an adaptor/scaffold protein, WDR26, which may be required for CTLH
complex assembly and recruitment of specific substrates for ubiquitination and degradation. The CTLH complex
was implicated in tumorigenesis but its exact functions in tumor development remain largely unknown.
We previously found WDR26 is a scaffolding protein that regulates GPCR signaling and is highly upregulated
in all molecular subtypes of invasive breast carcinoma and associated with worse prognosis. In preliminary
studies, we tested WDR26 gene deletion in a Neu transgenic mouse model of HER2+ breast cancer and showed
mammary-specific WDR26 gene deletion recapitulated a Gi/o-GPCR-signaling blockade: both inhibited tumor
initiation, growth, and lung metastasis. WDR26 likely promotes both G protein signaling and controls CTLH-
ubiquitin-ligase-driven degradation of SNF5 (an epigenetic tumor suppressor) in tumor cells. Based on these
exciting preliminary data, we hypothesize that, in HER2-driven breast tumors, WDR26 upregulation promotes
G-protein signaling and facilitates nuclear CTLH-complex assembly and E3 ubiquitin ligase activity, leading to
ubiquitination and proteasomal degradation of SNF5; and this SNF5 depletion activates oncogenic transcriptional
programs, in part, by upregulating EZH2 (enhancer of zeste homology 2), ultimately promoting tumor growth and
metastasis and resistance to HER2-targeted therapy. In this study, using a combination of cell lines, several
newly developed genetic and patient-derived xenograft mouse models, we will determine 1) how WDR26
manifests dysregulated Gi/o-GPCR signaling to drive breast cancer progression via the CTLH complex; 2) how
the CTLH complex regulates breast cancer development by targeting SNF5 for proteasomal degradation; and
3) whether targeting the CTLH and SNF5 function improves the efficacy of HER2-targeted therapy.
The results of our studies should fundamentally advance understanding of how the poorly studied CTLH E3
ubiquitin ligase complex targets the epigenetic tumor suppressor SNF5 to drive tumor progression and drug
resistance. This knowledge should help us identify new strategies for augmenting HER2-targeted therapy in
breast cancer.