FGFR3 Associates with and Tyrosine-Phosphorylates p90RSK2, Leading to RSK2 Activation That Mediates Hematopoietic Transformation

Dysregulation of receptor tyrosine kinase FGFR3 has been implicated to play a pathogenic role in a number of human hematopoietic malignancies and solid tumors. These include t(4;14) multiple myeloma associated with ectopic expression of FGFR3 and t(4;12)(p16;p13) acute myeloid leukemia associated with expression of a constitutively activated fusion tyrosine kinase TEL-FGFR3. We recently reported that FGFR3 directly tyrosine phosphorylates p90 Ribosomal S6 Kinase2(RSK2) at Y529, which consequently regulates RSK2 activation [Kang et al, Cancer Cell 2007 Sep;12(3):201–14]. Here we identified Y707 as an additional tyrosine site of RSK2 that is phosphorylated by FGFR3. Phosphorylation at Y707 contributes to RSK2 activation, through a putative disruption of the autoinhibitory αL-helix on the C-terminus of RSK2, unlike Y529 phosphorylation that facilitates ERK binding. To elucidate the role of tyrosine phosphorylation at Y707 induced by FGFR3 in RSK2 activation, we characterized the RSK2 mutants with single Y→A and Y→F substitutions at Y707. RSK2 Y707F demonstrated decreased kinase activity, suggesting substitution of Y707 attenuates activation of RSK2 induced by FGFR3. Tyrosine phosphorylation at Y529 by FGFR3 regulates RSK2 activation by facilitating inactive ERK binding, whereas substitution of Y707 in RSK2 does not similarly attenuate inactive ERK binding to RSK2. Phosphorylation at Y707 may regulate RSK2 activation by affecting the structure of the autoinhibitory C-terminal domain of RSK2 since the Y707 is localized at the C-terminal tail region which represents a conserved putative auto-inhibitory alpha helix. Since other tyrosine kinases including FGFR1 and Src also phosphorylate RSK2 at Y529 and Y707, tyrosine phosphorylation may be a general requirement for RSK2 activation through the ERK/MAPK pathway. Together, our current and previous findings represent a paradigm for tyrosine phosphorylation-dependent regulation of serine-threonine kinases. Moreover, we found that FGFR3 interacts with RSK2 through residue W332 in the linker region of RSK2, and that this association is required for FGFR3-dependent phosphorylation of RSK2 at Y529 and Y707, and subsequent RSK2 activation. Furthermore, in a murine bone marrow transplant assay, genetic deficiency in RSK2 resulted in a significantly delayed and attenuated myeloproliferative syndrome induced by TEL-FGFR3 as compared with wild type cells, suggesting a critical role of RSK2 in FGFR3-induced hematopoietic transformation.