# Dual Targeting of IDH2 and the Ubiquitin-Proteasome System Reveals a Functional Vulnerability in Breast Cancer Models

## TL;DR

Combining IDH2 and proteasome inhibitors causes stronger cell death in breast cancer models, especially in aggressive triple-negative types.

## Contribution

The study shows that dual targeting of IDH2 and the ubiquitin-proteasome system enhances therapeutic sensitivity in breast cancer.

## Key findings

- Dual IDH2-proteasome inhibition induces stronger apoptosis than either treatment alone in breast cancer models.
- Combination treatments significantly reduce clonogenic potential and alter survival pathways like NF-κB and PI3K-p85.
- The synergy is most pronounced in triple-negative breast cancer cell lines like MDA-MB-231 and 4T1.

## Abstract

In recent years, proteasome inhibitors have shown strong activity in hematologic malignancies, but their application in solid tumors such as breast cancer remains limited and insufficiently characterized. To identify strategies that could improve their therapeutic impact, we examined whether inhibition of the mitochondrial enzyme IDH2 creates a vulnerability to proteasome blockade. Across several breast cancer models, including aggressive triple-negative subtypes, dual IDH2-proteasome inhibition induced substantially stronger apoptotic responses than either agent alone. These findings reveal a metabolic dependency that may be exploited to enhance therapeutic sensitivity to proteasome inhibitors in triple-negative breast cancer.

Background/Objectives: Breast cancer cells rely on both mitochondrial metabolism and proteostatic mechanisms for cell fitness. The mitochondrial enzyme IDH2 supports redox balance and biosynthesis, while the ubiquitin-proteasome system (UPS) preserves protein quality. This study aimed to determine whether inhibiting IDH2 enhances sensitivity to proteasome-targeting agents across breast cancer subtypes. Methods: A panel of human and murine breast cancer cell lines was treated with the IDH2 inhibitor AGI-6780, alone or in combination with the proteasome inhibitor carfilzomib (CFZ) or the E1 ubiquitin-activating enzyme inhibitor TAK-243. Synergy was evaluated using Bliss scoring. Apoptosis, clonogenicity, and pathway modulation were assessed through Western blotting, colony-formation assays, and reverse-phase protein array (RPPA) profiling. Results: We observed that co-targeting IDH2 and the UPS produced strong synergistic cytotoxicity in multiple breast cancer models, including in triple-negative MDA-MB-231 and 4T1 cells (Bliss > 25). Combination treatments led to pronounced apoptosis, evidenced by cleaved PARP-1 and Caspase-3 cleavage, and a marked loss of clonogenic potential. RPPA analysis revealed significant alterations in key survival and stress-response pathways, including NF-κB, PI3K-p85, Src, and p38-MAPK. Conclusions: Inhibition of IDH2 markedly enhances the cytotoxic effects of proteasome-targeting by disrupting metabolic–proteostatic balance and promoting apoptotic cell death. These findings identify a growth-inhibitory effect that may be leveraged to improve functional dependency in breast cancer, particularly in triple-negative breast cancer, which currently lacks efficient drug treatments.