BSCI-05. ABL2-HSF1-E2F signaling axis promotes lung adenocarcinoma brain metastasis

Brain metastases are the most common intracranial tumors in adults and are associated with increased patient morbidity and mortality. Limited therapeutic options are currently available for the treatment of brain metastasis. We have identified an actionable signaling pathway utilized by metastatic tumor cells whereby the transcriptional regulator Heat Shock Factor 1 (HSF1) drives a transcriptional program, divergent from its canonical role as the master regulator of the heat shock response, leading to enhanced expression of a subset of E2F transcription factor family gene targets. We showed that HSF1 is required for survival and outgrowth by metastatic lung cancer cells in the brain parenchyma. Unexpectedly, we identified the ABL2 tyrosine kinase as an upstream regulator of HSF1 protein expression, and showed that the Src-homology 3 (SH3) domain of ABL2 directly interacts with HSF1 protein at a non-canonical, proline-independent SH3 interaction motif. Importantly, knockdown of ABL2 impairs expression of HSF1 protein and HSF1-E2F transcriptional gene targets. Notably, we found that pharmacologic inhibition of the ABL kinases using selective ABL allosteric inhibitors, but not ATP-competitive inhibitors, ablates the physical interaction between ABL2 and HSF1, leading to markedly decreased expression of HSF1, E2F1 and E2F8 proteins in brain-metastatic lung cancer cells, and depletion of HSF1-E2F transcriptional targets. These findings highlight potential differences affecting intra- and inter-molecular protein-protein interactions induced by allosteric versus ATP-competitive kinase inhibitors that have important therapeutic implications. Importantly, the targetable nature of the ABL2-HSF1-E2F signaling network identifies ABL allosteric inhibitors as a potentially effective therapy for the treatment of metastatic lung cancers characterized by high expression of HSF1.