Cross-talk between GLI transcription factors and FOXC1 promotes T-cell acute lymphoblastic leukemia dissemination.

T-cell acute lymphoblastic leukemia (T-ALL) is a highly malignant pediatric leukemia, where few therapeutic options are available for patients which relapse. We find that therapeutic targeting of GLI transcription factors by GANT-61 is particularly effective against NOTCH1 unmutated T-ALL cells. Investigation of the functional role of GLI1 disclosed that it contributes to T-ALL cell proliferation, survival, and dissemination through the modulation of AKT and CXCR4 signaling pathways. Decreased CXCR4 signaling following GLI1 inactivation was found to be prevalently due to post-transcriptional mechanisms including altered serine 339 CXCR4 phosphorylation and cortactin levels. We also identify a novel cross-talk between GLI transcription factors and FOXC1. Indeed, GLI factors can activate the expression of FOXC1 which is able to stabilize GLI1/2 protein levels through attenuation of their ubiquitination. Further, we find that prolonged GLI1 deficiency has a double-edged role in T-ALL progression favoring disease dissemination through the activation of a putative AKT/FOXC1/GLI2 axis. These findings have clinical significance as T-ALL patients with extensive central nervous system dissemination show low GLI1 transcript levels. Further, T-ALL patients having a GLI2-based Hedgehog activation signature are associated with poor survival. Together, these findings support a rationale for targeting the FOXC1/AKT axis to prevent GLI-dependent oncogenic Hedgehog signaling.