Generation and role of angiostatin in human platelets

Abstract

Platelets regulate new blood vessel growth, because they contain a number of angiogenesis promoters and inhibitors. Additionally, platelets contain matrix metalloproteinases (MMPs), which when released mediate platelet adhesion and aggregation, and plasminogen, a fibrinolytic system enzyme that serves to limit blood clot formation. Enzymatic cleavage of plasminogen by MMPs generates angiostatin, an angiogenesis inhibitor. Therefore, we examined whether platelets generate angiostatin during aggregation in vitro. Platelets were isolated from healthy human donors and then aggregated with collagen, thrombin, or HT-1080 fibrosarcoma cells. Angiostatin was detected by Western blot analysis in the platelet releasates of all blood donors irrespective of the aggregating agent used. Platelet pellet homogenates showed the presence of angiostatin in all donors, which was released upon aggregation. Furthermore, platelet-derived angiostatin was isolated and purified by lysine-Sepharose affinity chromatography from collagen-aggregated platelet releasates. Bioassay of platelet-derived angiostatin showed that it inhibited the formation of capillary structures by human umbilical vein endothelial cells (HUV-EC-Cs) in an in vitro angiogenesis model. Inhibition of angiostatin in platelet releasates promoted the formation of capillary structures by HUV-EC-Cs. We conclude that healthy human platelets contain angiostatin, which is released in active form during platelet aggregation, and platelet-derived angiostatin has the capacity to inhibit angiogenesis. IntroductionPlatelets play a vital role in maintaining hemostasis by forming white aggregates that seal breaches in vascular integrity. In addition to preventing bleeding, platelets are known to play roles in nonhemostatic processes such as immunity 1 and regulation of angiogenesis. 2 Platelets contain a number of proangiogenic and antiangiogenic factors, which have the potential to regulate angiogenesis. Among the angiogenesis promoters found in platelets are vascular endothelial growth factor (VEGF)-A 3 and -C, 4 platelet-derived growth factor (PDGF), 5 basic fibroblastic growth factor (bFGF), 6 and epidermal growth factor (EGF). 7 Furthermore, platelets contain matrix metalloproteinases (MMPs), [8][9][10] which are also known to promote angiogenesis, 11 and often are used as markers of the angiogenic process. To balance off the effects of the angiogenesis promoters, a number of angiogenesis inhibitors are also found in platelets. Included among this group are thrombospondin (TSP-1), 12 platelet factor 4 (PF4), 13 transforming growth factor-␤1 (TGF-␤1), 14 19,20,20, In addition to MMPs, a number of other proteases have been shown to generate angiostatin, including urokinase (uPA) and tissue-type plasminogen activator (tPA). 23 The generation of angiostatin has been found primarily in tumor cells, 24-27 although macrophages have also been shown to form angiostatin as a byproduct of the proteolytic regulation of membrane-bound plasmin. 28 Interestingly, platelets have the ability to...

Activated Platelets Provide a Functional Microenvironment for the Antiangiogenic Fragment of Histidine-Rich Glycoprotein

“…Platelets contain plasminogen and enzymes such as matrix metalloproteinases and urokinase-type plasminogen activator, capable of generating angiostatin. Platelets release functional angiostatin, but the inhibitor is also generated from plasminogen during platelet activation and aggregation (36,37). PF4 and angiostatin exemplify two different mechanisms used by platelets to increase the local concentration of an angiogenesis inhibitor: release from α-granules or generation of the inhibitor during platelet activation.…”

A simple mechanistic model of sprout spacing in tumour-associated angiogenesis

“…Angiostatin is a 38 kDa protein, cleaved from the serum protein plasminogen by the action of such factors as tissue plasminogen activator (tPA) and several of the matrix metalloproteases (MMPs) (Jurasz et al, 2003). It has been shown to inhibit endothelial cell (EC) growth.…”

Platelets, photosensitizers, and PDT

“…Moreover, the balance between these regulator factors can be pharmacologically altered and lead to modulation of angiogenesis [46,112]. Therefore, reducing angiogenesis by inhibition of proangiogenic and stimulation of antiangiogenic factors may be an attractive strategy for a multimodality approach in PDT.…”

“…The biological relevance of this process lies in platelet-sponsored increase in cancer cell survival in response to anticancer treatment, stimulation of blood-borne metastasis, invasion and angiogenesis [45,46,47,48].…”