Investigating the role of RAC1 signalling in small cell lung cancer plasticity

Small cell lung cancer (SCLC) is an aggressive tumour, characterised by rapidly acquired chemoresistance and disease progression. Approximately 80% of patients have metastatic disease at diagnosis resulting in a median survival of less than one year. Development of new treatments is hampered by insufficient understanding of SCLC biology and a lack of actionable driver mutations. Most SCLC tumours contain two different tumour cell types, the majority being neuroendocrine (NE) with a small subset of non-neuroendocrine (non-NE) tumour cells. A phenotype switch from NE to non-NE, likened to epithelial-to-mesenchymal transition in epithelial tumours, generates the non-NE phenotype. Importantly, co-operation between NE and non-NE SCLC cells is required for metastasis in mouse models. We propose that this phenotype transition presents a vulnerability that could be inhibited to impede metastasis. Inhibiting the transition would also maximise the benefit of chemotherapy as non-NE cells are more chemoresistant than NE cells. The NE to non-NE transition is known to be driven by the transcriptional regulators NOTCH1 and MYC; however, the downstream molecular events that drive it are poorly understood. This application will build on our recent publication demonstrating a role for the small GTPase RAC1 in SCLC NE cell survival as well as our pilot data revealing elevated RAC1 activity in non-NE cells and a role for RAC1 signalling in promoting NE to non-NE transition. RAC1 is a molecular switch, cycling between inactive GDP and active GTP-bound forms, which regulates many processes within cells. Amongst these, and relevant for this proposal, active RAC1 has an established role in stimulating signalling cascades that rewire gene expression and induce membrane protrusions and substrate adhesion to drive migration and invasion of cells during metastasis. Given the established roles of RAC1 signalling in migration/invasion of other cancer cells, this proposal aims to decipher not only the molecular mechanisms by which RAC1 promotes the NE to non-NE transition but also the mechanisms by which RAC1 signalling stimulates invasion and metastasis of SCLC. Our aims are: The identification of proteins that activate RAC1 to drive NE to non-NE transition. The direct activation of RAC1 can be performed by one of more than 40 proteins in addition to multiple other regulators of RAC1 GTP-hydrolysis, stability and localisation. Therefore, we will identify which of these regulators is responsible for RAC1 activation during the NE to non-NE transition. Understanding the mechanism by which RAC1 drives the NE to non-NE transition. This will include the identification of downstream effectors of RAC1 and exploring how these drive the transition. Studying NE and non-NE cell interactions to understand how these two cell types cooperate to promote invasion, the first step of the metastatic process. Specifically, we will investigate how targeting RAC1 signalling in non-NE cells influences SCLC invasion and metastasis. This work addresses the poor understanding of the NE to non-NE transition and the cooperation between NE and non-NE cells during invasion and metastasis in order to identify potential therapeutic targets. This application benefits from the synergy between internationally leading expertise in RAC1 signalling biology in the Malliri group, in SCLC translational research in the Simpson/Dive group, and in thoracic oncology in the adjacent Christie Hospital, allowing for continued close collaboration between investigators with deep and highly specialised expertise in basic, translational and clinical SCLC research.