Phosphorylation of FGFR1 at Ser777 by p38 MAPK regulates translocation of exogenous FGF1 to cytosol and nucleus

Centre for Cancer Biomedicine, Faculty Division Norwegian Radium Hospital, University of Oslo; and Department of Biochemistry, and Department of Immunology, Institute for Cancer Research, Norwegian Radium Hospital, Rikshospitalet University Hospital, Oslo, Norway; and Faculty of Biotechnology, University of Wroclaw, Wroclaw, Poland

^* To whom correspondence should be addressed. Email: antoni.wiedlocha{at}rr-research.no.

	Abstract

Exogenous FGF1 signals through activation of transmembrane FGFRs, but may also regulate cellular processes after translocation to the cytosol and nucleus of target cells. Translocation of FGF1 occurs across the limiting membrane of intracellular vesicles and is a regulated process that depends on the C-terminal tail of the FGFR. Here, we report that translocation of FGF1 requires activity of the isoform of p38 MAPK. FGF1 translocation was inhibited after chemical inhibition of p38 MAPK or after siRNA knock down of p38. Translocation was increased after stimulation of p38 MAPK with anisomycin, mannitol or H₂O₂. The activity level of p38 MAPK was not found to affect endocytosis or intracellular sorting of FGF1/FGFR1. Instead, we found that p38 MAPK regulates FGF1 translocation by phosphorylation of FGFR1 at Ser777. The FGFR1 mutation, S777A, abolished FGF1 translocation, while phosphomimetic mutations of Ser777 to Asp or Glu allowed translocation to take place and bypassed the requirement of active p38 MAPK. Ser777 in FGFR1 was directly phosphorylated by p38 in a cell-free system. These data demonstrate a crucial role for p38 MAPK in the regulated translocation of exogenous FGF1 into cytosol/nucleus and they reveal a specific role for p38 MAPK mediated serine phosphorylation of FGFR1.