1. ¹Center of Experimental Medicine, Institute of Biochemistry and Molecular Biology I: Cellular Signal Transduction, University Hospital Hamburg-Eppendorf, Hamburg, Germany
2. ²Department of Immunology and Cell Biology, Research Center Borstel, Borstel, Germany
3. ³Bone Marrow Transplantation Center, University Hospital Hamburg-Eppendorf, Hamburg, Germany

Correspondence: Dr M Jücker, Center of Experimental Medicine, Institute of Biochemistry and Molecular Biology I: Cellular Signal Transduction, University Hospital Hamburg-Eppendorf, Martinistr. 52, D-20246 Hamburg, Germany. Fax: +49 40 42803 6818; E-mail: juecker@uke.uni-hamburg.de

Received 13 July 2004; Accepted 24 August 2004; Published online 30 September 2004.

Abstract

The inositol 5-phosphatase SHIP (SHIP-1) is a negative regulator of signal transduction in hematopoietic cells and targeted disruption of SHIP in mice leads to a myeloproliferative disorder. We analyzed the effects of SHIP on the human leukemia cell line Jurkat in which expression of endogenous SHIP protein is not detectable. Restoration of SHIP expression in Jurkat cells with an inducible expression system caused a 69% reduction of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P₃) and a 65% reduction of Akt kinase activity, which was associated with reduced phosphorylation of glycogen synthase kinase 3 (GSK-3) (Ser-9) without changing the phosphorylation of Bad (Ser-136), FKHR (Ser-256) or MAPK (Thr-202/Tyr-204). SHIP-expressing Jurkat cells showed an increased transit time through the G1 phase of the cell cycle, but SHIP did not cause a complete cell cycle arrest or apoptosis. Extension of the G1 phase was associated with an increased stability of the cell cycle inhibitor p27^Kip1 and reduced phosphorylation of the retinoblastoma protein Rb at serine residue 780. Our data indicate that restoration of SHIP activity in a human leukemia cell line, which has lost expression of endogenous SHIP, downregulates constitutively activated phosphatidylinositol 3-kinase/Akt/GSK-3 signaling and leads to an increased transit time through the G1 phase of the cell cycle.