Promyelocytic leukemia nuclear bodies (PML-NBs) are membraneless phase-separated intranuclear condensates that spatially regulate interactions between multiple proteins, DNA, and transcripts to dynamically organise nuclear architecture. PML-NBs directly control key cellular processes involved in cancer pathology. However, how these phase condensates shape the biochemical landscape of cancer cells and drive oncogenic activity is unclear.
The structure and composition of PML-NBs are closely regulated to maintain PML‑NB function. A recent publication suggested that PML-NBs are associate with the ARP2/3 complex, which nucleates filamentous (F-) actin polymerisation. Our data show that F-actin dramatically altered nuclear architecture to promote the resolution of DNA replication stress, one of the hallmarks of cancer. We, therefore, aimed to understand whether F-actin interacts with PML-NBs and how this may shape nuclear architecture in cancer cells.
Using a combination of super-resolution microscopy, live-cell imaging, bespoke image analysis methods, and mass spectrometry, we revealed a feedback loop between PML-NBs and nuclear F-actin that dramatically promotes cancer cell survival. We showed that PML-NBs function as nucleation hubs from which nuclear F-actin polymers initiate in response to replication stress. The growing actin filaments then impose mechanical pressure on the PML-NBs and change their shape and composition. Critically, we found that this F-actin-dependent structural change releases the AKT oncogene from sequestration within PML-NBs, allowing AKT to be phosphorylated and thereby activated within the nucleoplasm. Phosphorylated AKT exerts a prolific range of anti-apoptotic and pro-anabolic oncogenic effects that drive multiple malignancies.
Our data converge on a novel F-actin/PML-NB/AKT network that emerges during replication stress. PML-NB dysregulation is characteristic of acute promyelocytic leukemia but is also observed across many other cancers, suggesting that our findings may hold translational promise. Our work highlights the significant potential of nuclear F-actin as a novel chemotherapeutic target that could have wide-ranging impacts on oncogene expression and genome stability.