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A more recent version of this article appeared on February 1, 2008
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Submitted on July 10, 2007
Revised on October 10, 2007
Accepted on November 9, 2007
Departments of *Biochemistry and
Medicine, University of Washington, Seattle, WA 98195;
Millipore, Inc., Temecula, CA 92590;
The Center for Vascular and Inflammatory Diseases, and the Departments of Surgery and Physiology, University of Maryland, Rockville, MD 20855
Monitoring Editor: Josephine Adams
TSPs 1 and 2 function as endogenous inhibitors of angiogenesis. Although TSPs have been shown to induce apoptosis in HMVEC, we reasoned that a homeostatic mechanism would also be needed to inhibit EC growth without causing cell death, e.g., in the maintenance of a normal vascular endothelium. HMVEC, cultured in low serum, responded to VEGF with an increase in [3H]thymidine incorporation that was inhibited by TSPs and was accompanied by decreases in the phosphorylation of Akt and MAPK, without an increase in apoptosis. RAP, an inhibitor of the LDL family of endocytic receptors, and blocking antibodies to VLDLR, were as effective as TSPs in the inhibition of thymidine uptake in response to VEGF, and the effects of these agents were not additive. Supportive evidence for the role of the VLDLR in mediating this inhibition was provided by the demonstration of a high-affinity interaction between TSPs and the VLDLR. We propose that TSP1 and TSP2, together with the VLDLR, initiate a nonapoptotic pathway for maintenance of the normal adult vascular endothelium in a quiescent state, similar to that invoked for the regulation of mitogenesis by PDGF, but involving signaling via the VLDLR rather than LRP1.
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