SIK2 is potential mediator of cross-talk between FGF and PKA pathways

Abstract

FGF signaling pathway is initiated upon FGF binding to RTKs and the signal transduction is achieved by a series of phosphorylation/dephosphorylation events mediated by several kinase and phosphatases. The pathway is tightly controlled through negative feedback mechanisms adopted by the activated effector proteins in the pathway as well as via interacting pathways. Recent studies suggested interplay between Ras/ERK pathway and PKA pathways. Results obtained in our laboratory raise the possibility that serine/threonine kinase SIK2 may be part of this cross-talk as PKA modulates FGF9-dependent ERK activation and nuclear localization of SIK2. In order to gain further insight into the involvement of SIK2 in FGF signal transduction and in the cross-talk between FGF and PKA pathways, we focused on the prototypic member of the family, FGF2. The results show that SIK2 is both serine and threonine phosphorylated in response to FGF2 stimulation and PKA activation. Upon FGF2 induction, Ras/ERK pathway is transiently activated, resulting in the proliferation of MIO-M1 cells. The inhibition of PKA activity prior to FGF2 treatment leads to an increase in pERK levels and a retardation in signal attenuation. The proliferation data indicates prior PKA activation leads to no significant changes, while inhibition of PKA activity leads to an increased FGF2-dependent proliferation of MIO-M1 cells. Immunocytochemical studies reveal that 10 minutes or longer durations of FGF2 stimulus exported SIK2 out of nuclei; and that inhibition of PKA activity results in retardation of this translocation. We also demonstrated that the expression profile of SIK2 changes with PKA activity and upon FGF2 induction indicating a feedback mechanism. These results firmly places SIK2 in FGF signal transduction cascade, and provides evidence that this protein might be an important component of cross-talk machinery between growth factor and metabolic pathways.