Stabilization of RUNX1 Induced by O-GlcNAcylation Promotes PDGF-BB-Mediated Resistance to CDK4/6 Inhibitors in Breast Cancer.

Cyclin-dependent kinases 4 and 6 (CDK4/6) are crucial in regulating cell-cycle progression and cancer development. Targeting CDK4/6 has shown considerable promise in treating various cancers, including breast cancer. Despite significant therapeutic efficacy, resistance to CDK4/6 inhibitors (CDK4/6i), such as palbociclib, remains a substantial hurdle in clinical practice. Using a coculture system, cytokine array, and quantitative high-throughput combinatorial screening, we discovered a mechanism by which the Runt-related transcription factor (RUNX) 1-platelet-derived growth factor (PDGF)-BB axis regulates palbociclib resistance in breast cancer cells. Specifically, RUNX1 functioned as a transcription factor to drive expression of PDGFB, leading to resistance to palbociclib by enhancing the Akt pathway and suppressing senescence. Furthermore, in resistant cells, RUNX1 was O-GlcNAcylated at serine 252 by O-GlcNAc transferase, resulting in the stabilization of RUNX1 by preventing ubiquitin-mediated degradation. Inhibition of the RUNX1-PDGF-BB axis by specific inhibitors overcame palbociclib resistance both in vitro and in vivo. Notably, the RUNX1-PDGF-BB axis was upregulated in resistant patient-derived xenograft lines and in patients with breast cancer following treatment with CDK4/6i. These findings not only unveil O-GlcNAcylation-mediated activation of a RUNX1-PDGF-BB pathway as a driver of palbociclib resistance but also provide clinical evidence supporting the repurposing of FDA-approved PDGFR inhibitors as a therapeutic strategy to treat patients with CDK4/6i-resistant breast cancer. Significance: RUNX1-PDGF-BB signaling drives resistance to CDK4/6 inhibition in breast cancer, providing the foundation to develop approaches to target the RUNX1-PDGF-BB axis to overcome CDK4/6 inhibitor resistance in breast cancer patients.