Translocation of Akt/PKB to the nucleus of osteoblast-like MC3T3-E1 cells exposed to proliferative growth factors

An active phosphatidylinositol 3-kinase (PI3K) has been shown in nuclei of different cell types. The products of this enzyme, i.e. inositides phosphor ylated in the D3 position of the inositol ring, may act as second messenger s themselves. Nuclear PI3K translocation has been demonstrated to be relate d to an analogous translocation of a PtdIns(3,4,5)P-3 activated PKC, the ze ta isozyme, We have examined the issue of whether or not in the osteoblast- like clonal cell line MC3T3-E1 there may be observed an insulin-like growth factor-I- (IGF-I) and platelet-derived growth factor- (PDGF) dependent nuc lear translocation of an active Akt/PKB, Western blot analysis showed a max imal nuclear translocation after 20 min of IGF-I stimulation or after 30 mi n of PDGF treatment. Both growth factors increased rapidly and transiently the enzyme activity of immunoprecipitable nuclear Akt/PKB on a similar time scale and after 60 min the values were slightly higher than the basal leve ls. Enzyme translocation was blocked by the specific PI3K inhibitor, LY2940 02, as well as cell entry into S-phase, Confocal microscopy showed an evide nt increase in immunostaining intensity in the nuclear interior after growt h factor treatment but no changes in the subcellular distribution of Akt/PK B when a LY294002 pre-treatment was administered to the cells. These findin gs strongly suggest that the intranuclear translocation of Akt/PKB is an im portant step in signalling pathways that mediate cell proliferation. (C) 20 00 Federation of European Biochemical Societies. Published by Elsevier Scie nce B.V. All rights reserved.