Role of RAF kinases in Dendritic Cells
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Abstract
Mitogen-activated protein kinases (MAPKs) are a family of serine/threonine protein kinases that regulate fundamental cellular processes like cell division, migration, differentiation and cell survival. There are 14 mammalian MAPKs described which define 7 distinct MAPK pathways. The classical MAPK signaling pathway consisting of RAF-MEK1/2-ERK1/2 gained enormous interest as the pathway is deregulated in many human cancers. Since RAS, which activates the RAF-MEK1/2-ERK1/2 cascade, as well as BRAF are among the most frequently activated oncogenes, lot of efforts were made to develop therapeutics to target the pathway. Drugs like vemurafenib or dabrafenib targeting the activated form of BRAF have shown enormous clinical success, but they unexpectedly induced MAPK activation in a BRAF-mutation free context by triggering RAF dimerization leading to paradoxical MAPK activation. Apart from targeted therapy, an efficient anti-cancer immune response is required for effective killing of cancer. But tumors can exploit several strategies, like secreting immune suppressive cytokines or inhibiting the presentation of tumor antigens to dampen immune responses. Thus, synergistic treatment regimes combining both targeted and immune therapy have been shown to benefit tumor patients. Since the role of the classical RAF-MAPK pathway in tumorigenesis has already been well described, we wanted to go further and investigate how MAPK signaling regulates innate and adaptive immune responses. Therefore, we initially evaluated the role of the classical RAF-MEK1/2-ERK1/2 pathway and especially of RAF kinases in dendritic cells (DCs). By employing a well-established in vitro culture, in which human monocytes are differentiated to moDCs, we investigated how RAF kinases contribute to the differentiation and maturation of moDCs. We observed that RAF kinases were stabilized at the protein level during moDC differentiation and detected RAF heterodimers between all three RAFs in moDCs. To validate the functional role of RAF kinases in moDCs we employed pan-RAF as well as MEK inhibitors to block MAPK signaling. Inhibition of RAF kinases but not MEK1/2 impaired the activation and migration of DCs. Furthermore, DCs treated with RAF inhibitors showed a reduced ability to activate allogeneic CD4+ T cells. Additionally, we showed that RAF and MEK1/2 inhibition directly inhibited proliferation of CD4+ T cells. Thus, our results point to a role of RAF kinases in moDCs in a MEK1/2-independent manner and that RAF kinases are required for DC activation and function.