The Role of the PI3K-AKT-mTOR Pathway in NK Cell Anti-Tumor Reactivity.

Abstract 3690

Poster Board III-626

The phosphatidylinositol 3-kinase – protein kinase B – mammalian target of rapamycin (PI3K – AKT – mTOR) pathway was found to be abnormally activated in many malignancies. Thus, protein kinase (PK) inhibitors (PKI) targeting different signaling molecules of this pathway are presently under clinical evaluation e.g. in sarcoma, multiple myeloma, or renal cell cancer. However, PK are also responsible for most of the signal transduction in immune effector cells and control various effector mechanisms including proliferation, cellular cytotoxicity, and cytokine release. Among those immunoregulatory signaling pathways, the PI3K – AKT – mTOR pathway was found to play a central role in TLR-mediated release of cytokines in macrophages and DC as well as in the regulation of T cell functions. Little is known about the role of this pathway in NK cell-mediated anti-tumor reactivity. Here we analyzed the tumor cell-induced activation of PI3K, AKT, and mTOR in NK cells and the consequences of an inhibition of these molecules by therapeutic PKI for NK cell anti-tumor reactivity. We found that, in response to tumor target cells, PI3K, AKT, and mTOR are consecutively activated in NK cells as revealed by western blot analyses using phospho-specific antibodies. Presence of the specific PI3K-inhbitor BKM-120 concentration-dependently inhibited cytotoxicity and IFN-g production of NK cells, which is in line with available data defining PI3K as a central regulator of NK cell target recognition. The mTOR inhibitors Sirolimus, Temsirolimus, and Everolimus did not alter cytotoxicity but significantly impaired NK cell IFN-γ production. In contrast, Triciribine, a compound which inhibits the phosphorylation and thus activation of AKT, did not influence cytotoxicity and, tantalizingly, even enhanced NK cell IFN-γ production. Thus, after target cell recognition and the activation of proximal PK like PI3K, different and at least partially independent signaling events govern NK cell cytokine production and cellular cytotoxicity. While the activity of PI3K followed by the activation of mitogen-activated PK is known to be crucial for NK cell cytotoxicity, we here identified the AKT – mTOR pathway as a yet unknown central component in the regulation of NK cell IFN-γ production. Moreover, in light of the important role of NK cells in tumor immune surveillance our data indicate that the choise and dosing of the most suitable PKI for a given cancer patient requires careful consideration. In the future it will be critical to define potential differences in immunosuppressive and immunostimulatory side effects of different compounds among the rapidly growing assortment of multi-targeted PKI to enable therapeutic approaches combining targeting of crucial signaling pathways in tumor cells with immunotherapy.