Heterotrimeric G-Proteins Are Indispensable for FLT3-ITD autophosphorylation and Oncogenic Function

Heterotrimeric G-proteins transmit signals of G-protein coupled receptors and regulate many basic cellular functions. However, their role in normal and malignant hematopoietic stem cells remains obscure. Activating mutations in the heterotrimeric G-protein Gaq were found in various cancers and its expression is enhanced in diffuse large B-cell lymphoma and T-ALL. Our previous data suggested the involvement of heterotrimeric G-proteins in Flt3-ITD-mediated leukemic transformation. FMS-like tyrosine kinase 3 with internal tandem duplication (FLT3-ITD) is a frequent oncoprotein in acute myeloid leukemia causing constitutive active STAT5 signaling. Here, we investigated a novel role of Gaq in Flt3-ITD-induced leukemic transformation.

We could show that Gaq is indispensable for aberrant FLT3-ITD activation and oncogenic function as Gaq activity is necessary to maintain the autophosphorylation of FLT3-ITD. Gaq abrogation resulted in diminished cell proliferation and colony formation as well as delayed leukemogenesis in vivo of Flt3-ITD leukemic cells. Importantly, the growth inhibition could be rescued by addition of IL3 and did not occur in the presence of FLT3 ligand-activated FLT3 wildtype receptor, demonstrating the specificity of Gaq requirement for FLT3-ITD oncogenic signaling. Interestingly, co-immunoprecipitations revealed a direct physical interaction between FLT3-ITD and Gaq which did not require phosphorylation of the receptor tyrosine kinase. Hence, FLT3-ITD hyperphosphorylation seems to be rather a consequence of the interaction than a prerequisite. Flt3-ITD-induced transformation of murine hematopoietic stem/progenitor cells (HSPCs) strictly depended on the presence of Gaq, and the ablation of Gaq/11 in transplanted Flt3-ITD-transduced HSPCs from conditional Gaq/11 double knock-out mice delayed leukemic burden. These findings of an unexpected, yet critical, role of Gaq place the molecule as an important target for antileukemic strategies.