PAK2 (p21-activated kinase 2) – A Key Node in FLT3 Dependent Signaling in Acute Lymphoblastic Leukemia

Signaling is a tightly regulated cellular process. Perturbation of critical signaling pathways has been shown to lead to a variety of diseases including cancer. In acute lymphoblastic leukemia (ALL) a substantial number of molecular aberrations affecting signaling pathways have been identified, such as the BCR-ABL translocation leading to constitutive kinase activity. To decipher kinase-dependent signaling networks in childhood ALL, we analysed the expression profile of receptor tyrosine kinases (RTKs) in primary ALL at the protein level. FLT3 and PDGFRb were selected as growth-promoting receptors in ALL to study their downstream signaling using an iTRAQ based mass spectrometry approach. In FLT3 dependent signaling we identified p21-activated kinase (PAK) 2 as a target phosphoprotein. PAKs are described as important regulators in cell signaling and have been implicated in cancer progression. Moreover, PAK2 has been reported to be involved in hematopoietic stem cell development and engraftment. Among PAK family members, PAK1 and PAK2 are widely expressed in ALL cells and we could show that pharmacological inhibition and RNA interference-mediated knockdown of group 1 PAKs lead to apoptosis in vitro. Stimulation of FLT3 leads to an increase in phosphorylation of PAK2, while pharmacological inhibition of FLT3 decreases its phosphoactivity. To determine protein-protein-interaction partners (PPI) of PAK, which might mediate pro-survival effects, we performed mass spectrometry upon stimulation of FLT3 in ALL cells after PAK1 and PAK2 precipitation. We confirmed previously known PPI validating the technological approach and identified unrecognized interaction partners of PAK1 and PAK2, which could potentially explain PAK-dependent survival of ALL cells.