Combined PDK1 and CHK1 inhibition is required to kill glioblastoma stem-like cells in vitro and in vivo

Glioblastoma (GBM) is the most common and deadly adult brain tumor. Despite aggressive surgery, radiation and chemotherapies, the life expectancy of patients diagnosed with GBM is approximately 14 months. The extremely aggressive nature of GBM results from glioblastoma stem-like cells (GSCs) that sustain GBM growth and survival, survive intensive chemotherapy and give rise to tumor recurrence. There is accumulating evidence revealing that GSC resilience is due to concomitant activation of multiple survival pathways. In order to decode the signal transduction networks responsible for the malignant properties of GSCs, we analyzed a collection of GSC lines using a dual, but complementary experimental approach, i.e. Reverse-Phase Protein Microarray (RPMA) and kinase inhibitor library screening. GSCs are relatively insensitive to temozolomide (TMZ) treatment in terms of pathway activation and, although displaying heterogeneous individual phospho-proteomic profiles, most GSCs are resistant to specific inhibition of the major signaling pathways involved in cell survival and proliferation. However, simultaneous multi-pathway inhibition by the staurosporin derivative UCN-01, demonstrates efficacy in vitro and in vivo in GSCs. The sensitivity to UCN-01 is dependent on activation of survival signals mediated by PDK1 and the DNA damage response initiated by CHK1. Since several novel small molecules targeting multiple kinases are approved or in clinical development, our results provide a rationale for the development of new therapeutic approaches based on simultaneous inhibition of PDK1 and CHK1 in GBM