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J. Cell Biol.,
Volume 144, Number 1, January 11, 1999 175-184
IIb
3
Regulates Synthesis of Proteins in Activated Human Platelets


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§

* Nora Eccles Harrison Cardiovascular Research and Training Institute, Integrins are widely expressed plasma membrane adhesion molecules that tether cells to matrix
proteins and to one another in cell-cell interactions. Integrins also transmit outside-in signals that regulate
functional responses of cells, and are known to influence gene expression by regulating transcription. In
previous studies we found that platelets, which are naturally occurring anucleate cytoplasts, translate preformed mRNA transcripts when they are activated by
outside-in signals. Using strategies that interrupt engagement of integrin
Program in Human Molecular Biology and Genetics,
Eccles Institute of Human Genetics, § Department of Biochemistry,
Department of Internal Medicine, and ¶ Department of
Pathology, University of Utah, Salt Lake City, Utah 84112; and the ** Department of Medicine, Johns Hopkins University
School of Medicine, Baltimore, Maryland 21205
IIb
3 by fibrinogen and platelets deficient in this integrin, we found that
IIb
3 regulates
the synthesis of B cell lymphoma 3 (Bcl-3) when platelet aggregation is induced by thrombin. We also found
that synthesis of Bcl-3, which occurs via a specialized
translation control pathway regulated by mammalian
target of rapamycin (mTOR), is induced when platelets adhere to immobilized fibrinogen in the absence of
thrombin and when integrin
IIb
3 is engaged by a conformation-altering antibody against integrin
IIb
3.
Thus, outside-in signals delivered by integrin
IIb
3 are
required for translation of Bcl-3 in thrombin-stimulated aggregated platelets and are sufficient to induce translation of this marker protein in the absence of thrombin. Engagement of integrin
2
1 by collagen also triggered synthesis of Bcl-3. Thus, control of translation
may be a general mechanism by which surface adhesion
molecules regulate gene expression.
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