Organization and Physiologically Mediated Differential Release of Angiogenic Regulatory Proteins From Distinct Platelet Alpha Granules.

Abstract 3045

Poster Board II-1021

In addition to their primary roles in hemostasis and thrombosis, platelets have been reported to participate in other physiological and pathological processes, including but not limited to inflammation, wound healing, and tumor metastasis. Although platelets are presumed to contribute to new blood vessel growth by providing numerous pro-and anti-angiogenic factors, the cellular and molecular basis by which platelets regulate angiogenesis is poorly understood. Previously we have shown that platelets differentially package angiogenic regulatory proteins in their alpha granules. More recently, we have further defined the organization of angiogenic proteins within platelet alpha granules using immunofluorescence and immunogold labeling experiments and have uncovered at least three distinct types of alpha granules. These granules were found to contain selective members of pro- and anti-angiogenic regulatory proteins, respectively. In addition, we have analyzed platelets from nine patients with metastatic renal cell carcinoma using immunofluorescence labeling for VEGF and endostatin. In comparison to volunteer control platelets without known malignancy, there is a significant increase in granules containing VEGF in patients with metastatic disease, strongly suggesting a shift of the angiogenic balance towards a more pro-angiogenic environment in the platelets from patients with malignancy. Previously we have shown that differentially packaged alpha granules are susceptible to selective release in response to proteinase-activated receptors. To explore whether selective release also occurred in physiologic conditions, we activated human platelets with known platelet agonists and looked for evidence of selective release. We have demonstrated that differential release of VEGF can be physiologically regulated by activation with ADP. Human platelets exposed to 50uM ADP result in release of VEGF and retention of endostatin, both by immunofluorecence labeling and by ELISA assay. This study provides mechanistic evidence to support an interplay between tumor cells and platelets, by illustrating a key role for platelets in tumor angiogenesis through the delivery of specific angiogenic factors. This work has implications beyond basic platelet biology as it provides a model for the platelet as a delivery system, with important clinical implications.