The Characterization of ABL Tyrosine Kinase-Regulated Transcriptional Networks

Abstract

The ABL family of tyrosine kinases are multifaceted signaling molecules that link diverse extracellular stimuli to intracellular signaling pathways that control cell growth, survival, migration, and invasion during development and normal cellular homeostasis. In recent years, it has been established that multiple solid tumor types hijack ABL kinase signaling to support tumor progression and metastasis. The ABL kinases often potentiate these processes by inducing the stability and/or transactivation of transcriptional regulators. Using in vivo mouse models of solid tumor metastasis combined with mechanistic cell signaling and biochemistry, the studies presented herein uncovered multiple transcriptional networks modulated by the ABL kinases through the regulation of their key transcriptional regulators. First, we report on the discovery of an actionable signaling pathway utilized by brain 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 find that HSF1 is required for survival and outgrowth by metastatic lung cancer cells in the brain parenchyma. Further, we identify the ABL2 tyrosine kinase as an upstream regulator of HSF1 protein expression and show that the SRC-Homology 3 (SH3) domain of ABL2 directly interacts with HSF1 protein at a noncanonical, proline- independent SH3 interaction motif. Importantly, knockdown as well as pharmacologic inhibition of ABL2 using allosteric inhibitors impairs expression of HSF1 protein and HSF1-E2F transcriptional gene targets. We have recently identified that expression of the master transcriptional regulator of the cellular response of hypoxia, HIF-1α, as well as expression of the oncogenic transcription factor MYC are dependent on ABL kinase activity. The hypoxia inducible factor 1-α (HIF-1α) enables cells to adapt and respond to hypoxia, and the activity of this transcription factor is regulated by several oncogenic signals and cellular stressors. While the pathways controlling normoxic degradation of HIF-1α are well understood, the mechanisms supporting the sustained stabilization and activity of HIF-1α under hypoxia are less clear. We report that ABL kinase activity protects HIF-1α from proteasomal degradation during hypoxia. Using a FACS-based CRISPR/Cas9 screen we identified HIF-1α as a substrate of the Cleavage and Polyadenylation Specificity Factor-1 (CPSF1), an E3-ligase which targets HIF-1α for degradation whose activity is regulated by ABL kinases. Interestingly, MYC was also shown to be a substrate of CPSF1, and its expression was decreased upon ABL inhibition. These studies highlight the importance of the ABL kinases in the activation of oncogenic transcriptional networks driven by HSF1, HIF-1α, and MYC.