Rb-driven transcription limits its tumour-suppressive effects in breast cancer

A new study reveals that the retinoblastoma (Rb) protein, known for its role in suppressing tumor growth, may paradoxically limit its anti-cancer effects through its transcriptional activity in breast cancer. The findings challenge existing assumptions about how Rb functions in response to drug treatments targeting cyclin-dependent kinases (CDKs). Researchers found that while CDK4/6 inhibitors, commonly used in hormone receptor-positive (HR+) breast cancer therapies, activate Rb by reducing its phosphorylation, this activation leads to a significant accumulation of Rb on chromatin, potentially altering transcriptional processes beyond cell cycle control. The Rb protein acts as a tumor suppressor by preventing uncontrolled cell division. It does so primarily by binding to and inhibiting E2F transcription factors at gene promoters, thus blocking the expression of genes necessary for entering the S phase of the cell cycle. This suppression is facilitated by the hypophosphorylated form of Rb, which is considered active. However, during the G1 phase of the cell cycle, CDKs such as CDK2, CDK4, and CDK6 phosphorylate Rb, rendering it inactive and allowing E2F factors to drive the transition into the S phase. In many cancer cells, Rb remains intact, making it a key therapeutic target. Selective CDK inhibitors, including CDK4/6 inhibitors, have been developed to maintain Rb in its hypophosphorylated state, leading to the downregulation of E2F target genes and inducing G1 arrest. Clinically, these inhibitors have shown effectiveness in suppressing tumor proliferation in HR+ breast cancers, becoming central components of first-line treatment strategies for both early and advanced stages of the disease. Similarly, CDK2 inhibitors show promise in tumors characterized by elevated CDK2 activity. Beyond its well-known role in E2F repression, hypophosphorylated Rb appears to have broader functions on chromatin. Earlier studies indicated that active Rb interacts with other transcription factors to modulate their activities. Therefore, pharmacologically activating Rb could influence broader transcriptional programs affecting therapeutic outcomes. However, the downstream transcriptional consequences of sustained Rb activation in cancer remain largely undefined, despite the widespread clinical use of CDK4/6 inhibitors. Mapping the genome-wide chromatin occupancy of Rb has historically been challenging due to technical limitations in chromatin immunoprecipitation followed by sequencing (ChIP-seq), particularly the low signal-to-noise ratio. Previous attempts to address this issue involved using exogenously expressed, tagged Rb, an approach that may not accurately reflect the dynamic changes in endogenous Rb levels throughout the cell cycle and could introduce artifacts related to tagging methods. Most prior Rb ChIP-seq studies have focused on non-transformed cells, leaving the impact of CDK inhibition on Rb's chromatin-binding profile and transcriptional activity in cancer cells unresolved. To overcome these challenges, researchers employed a technique called cleavage under targets and release using nuclease (CUT&RUN), allowing for high-resolution mapping of native, untagged Rb without the need for formaldehyde crosslinking. Using validated monoclonal antibodies, they optimized this method in HR+ breast cancer cell lines (MCF7 and ZR-75-1), identifying specific Rb peaks that were absent in isogenic RB1-knockout cells, confirming the assay’s specificity. When CDK4/6 inhibition was induced with abemaciclib, both cell lines showed Rb hypophosphorylation, reduced total Rb levels, and G1 arrest, as anticipated. Genome-wide analysis revealed a substantial increase in chromatin-bound Rb, 15,218 and 12,868 “up peaks” and only 57 and 8 “down peaks” in MCF7 and ZR-75-1 cells, respectively, indicating a dramatic redistribution of hypophosphorylated, active Rb despite lower overall protein levels. These results were corroborated using palbociclib, another CDK4/6 inhibitor, and extended to CDK2-inhibited cyclin E-driven ovarian cancer lines, suggesting potential broader implications. The findings highlight the complex interplay between Rb activation and transcriptional regulation in cancer cells, indicating that current therapeutic approaches may need to consider more than just cell cycle arrest when evaluating the efficacy of CDK inhibitors. Further research is needed to fully understand the transcriptional consequences of sustained Rb activation and how these might contribute to therapeutic resistance or altered responses to treatment. As the study underscores, clarifying these effects could lead to improved strategies for utilizing CDK inhibitors in cancer therapy.