The Inositol Phosphatase SHIP Inhibits Akt/PKB Activation in B Cells*
- M. Javad Aman‡§,
- Thomas D. Lamkinद,
- Hidetaka Okada‖,
- Tomohiro Kurosaki‖ and
- Kodimangalam S. Ravichandran‡**
- From the ‡Beirne Carter Center for Immunology Research and the Department of Microbiology, University of Virginia, Charlottesville, Virginia 22908 and the ‖Department of Molecular Genetics, Kansai Medical University, 10-15 Fumizono-cho, Moriguchi, Japan
Abstract
The serine-threonine kinase Akt/PKB is activated downstream of phosphatidylinositol 3-kinase in response to several growth factor stimuli and has been implicated in the promotion of cell survival. Although both phosphatidylinositol 3,4,5-trisphosphate (PIP3) and phosphatidylinositol 3,4-bisphosphate (PI 3,4-P2) have been implicated in the regulation of Akt activity in vitro, the relative roles of these two phospholipids in vivo are not well understood. Co-ligation of the B cell receptor (BCR) and the inhibitory FcγRIIB1 on B cells results in the recruitment of the 5′-inositol phosphatase SHIP to the signaling complex. Since SHIP is known to cleave PIP3 to generate PI 3,4-P2 both in vivo and in vitro, and Akt activity has been reported to be regulated by either PIP3 or PI 3,4-P2, we hypothesized that recruitment of SHIP through FcγRIIB1 co-cross-linking to the BCR in B cells might regulate Akt activity. The nature of this regulation, positive or negative, might also reveal the relative contribution of PIP3 and PI 3,4-P2 to Akt activation in vivo. Here we report that Akt is activated by stimulation through the BCR in a phosphatidylinositol 3-kinase-dependent manner and that this activation is inhibited by co-cross-linking of the BCR to FcγRIIB1. Using mutants of FcγRIIB1 and SHIP-deficient B cells, we demonstrate that inhibition of Akt activity is mediated by the immune cell tyrosine-based inhibitory motif within FcγRIIB1 as well as SHIP. The SHIP-dependent inhibition of Akt activation also suggests that PIP3 plays a greater role in Akt activation than PI 3,4-P2 in vivo.
Footnotes
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↵* This work was supported by National Institutes of Health Grants AI43425 and GM55761 (to K. S. R.) and by a grant from the Thomas F. & Kate Miller Jeffress Memorial Trust.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵§ These authors contributed equally to this work.
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↵¶ Recipient of a fellowship from the Cancer Research Institute.
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↵** To whom correspondence should be addressed: Beirne Carter Center for Immunology Research, MR-4, Rm. 4012F, University of Virginia, Charlottesville, VA 22908. Tel.: 804-243-6093; Fax: 804-924-1221; E-mail: kr4h{at}virginia.edu.
- Abbreviations:
- PH
-
pleckstrin homology
- PI3K
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phosphatidylinositol 3-kinase
- PIP3
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phosphatidylinositol 3,4,5-trisphosphate
- PI 3
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4-P2, phosphatidylinositol 3,4-bisphosphate
- BCR
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B cell receptor
- SH2
-
Src homology 2
- ITIM
-
immune cell tyrosine-based inhibitory motif
- GSK-3
-
glycogen sythase kinase-3.
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- Received September 15, 1998.
- The American Society for Biochemistry and Molecular Biology, Inc.
