The combination of pharmacological cyclin dependent kinases 4 and 6 (CDK4/6) inhibitors and endocrine therapy has become an established standard of care for treating advanced hormone receptor-positive breast cancer. Clinical studies have demonstrated that the combination is more effective than either therapy alone, and pre-clinical studies suggest the combination is synergistic. However, the mechanisms underpinning this synergism are still unclear. Our project aims to understand why the concomitant inhibition of CDK4/6 and estrogen receptor (ER) signalling works best. We hypothesize that CDK4/6 inhibition leads to the redistribution of estrogen receptor on the chromatin, activating an ER-driven transcriptional program, rendering the cancer cell more sensitive to endocrine manipulation.
Abemaciclib and fulvestrant, a selective estrogen receptor downregulator, were used to inhibit CDK4/6 and ER, respectively. The synergy of CDK4/6 inhibition and endocrine therapy was characterised in human ER-positive breast cancer cell lines using cell counting, cell cycle analysis, and measurement of cell death. Gene expression after treatment was measured using RNA-sequencing. To investigate the redistribution of ER on chromatin and enhancer landscape, ChIP-sequencing was performed.
Our results support the notion that inhibiting CDK4/6 made cells more ER-dependent. Treatment using physiologically relevant concentrations of abemaciclib and fulvestrant in breast cancer cell lines synergistically inhibited cell proliferation. CDK4/6 inhibition induced enrichment of estrogen response gene expression signatures and ER redistribution to CDK4/6i-induced enhancer regions. Blocking ER reversed this enrichment in estrogen response. These results suggest that inhibition of CDK4/6 can lead to activation of estrogen response signalling, explaining why combining CDK4/6 inhibitors with endocrine therapy is synergistic. Future directions include investigating whether these findings also occur in endocrine therapy-resistant ER-positive breast cancer.