Exploiting a negative feedback loop linking s6 kinases and axl to reduce pyrimidine biosynthesis in pten-deficient glioblastoma

Abstract PTEN inactivation triggers oncodependent signal transduction through the PI3K/Akt signal transduction pathway in glioblastoma (GBM). Targeting the PI3K/Akt pathway to counteract PTEN loss in GBM has been impeded by negative feedback signal transduction networks mediated by the downstream protein kinases S6K1 and S6K2. Here, we show that the receptor tyrosine kinase AXL is a major target of that S6K feedback signaling and that combined inactivation of just S6K1 and AXL is an effective therapeutic strategy for treatment of PTEN-deficient GBM. Chemical-genetic interaction studies in gliomasphere and GBM cell lines revealed critical and independent roles for S6K1 and S6K2 in mediating GBM growth in PTEN-deficient cells. Interestingly, S6K2 exerted a dual role in signal transduction: it sustained GBM growth while also exerting negative feedback control on the upstream receptor tyrosine kinase AXL. Genetic inactivation of S6K2 sensitized PTEN-deficient GBMs to AXL inhibition, indicating that derepressed AXL is required for compensatory signaling upon targeting of the PI3K/Akt pathway. Combining the AXL inhibitor BMS-777607 with S6K1 inhibitor LY-2584702 also prevented compensatory AXL signaling and triggered cytotoxic responses selectively in the PTEN-deficient condition. Importantly, combination inhibition of AXL and S6K1 reduced pyrimidine biosynthesis, a known vulnerability of PTEN-deficient GBMs. The oral inhibitors of S6K1 and AXL were found to be brain-penetrant and effective in reducing GBM growth in several mouse models. These results establish that targeting of AXL can circumvent S6 Kinase dependent feedback to reduce pyrimidine biosynthesis and trigger cytotoxic responses in PTEN-deficient GBMs.