Phosphorylation of RSK2 at Tyr529 by FGFR2-p38 enhances human mammary epithelial cells migration.

The members of p90 ribosomal S6 kinase (RSK) family of Ser/Thr kinases are downstream effectors of MAPK/ERK pathway that regulate diverse cellular processes including cell growth, proliferation and survival. In carcinogenesis, RSKs are thought to modulate cell motility, invasion and metastasis. Herein, we have studied an involvement of RSKs in FGF2/FGFR2-driven behaviours of mammary epithelial and breast cancer cells. We found that both silencing and inhibiting of FGFR2 attenuated phosphorylation of RSKs, whereas FGFR2 overexpression and/or its stimulation with FGF2 enhanced RSKs activity. Moreover, treatment with ERK, Src and p38 inhibitors revealed that p38 kinase acts as an upstream RSK2 regulator. We demonstrate for the first time that in FGF2/FGFR2 signalling, p38 but not MEK/ERK, indirectly activated RSK2 at Tyr529, which facilitated phosphorylation of its other residues (Thr359/Ser363, Thr573 and Ser380). In contrast to FGF2-triggered signalling, inhibition of p38 in the EGF pathway affected only RSK2-Tyr529, without any impact on the remaining RSK phosphorylation sites. p38-mediated phosphorylation of RSK2-Tyr529 was crucial for the transactivation of residues located at kinase C-terminal domain and linker-region, specifically, in the FGF2/FGFR2 signalling pathway. Furthermore, we show that FGF2 promoted anchorage-independent cell proliferation, formation of focal adhesions and cell migration, which was effectively abolished by treatment with RSKs inhibitor (FMK). These indicate that RSK2 activity is indispensable for FGF2/FGFR2-mediated cellular effects. Our findings identified a new FGF2/FGFR2-p38-RSK2 pathway, which may play a significant role in the pathogenesis and progression of breast cancer and, hence, may present a novel therapeutic target in the treatment of FGFR2-expressing tumours.