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Objective: Hyperlipidemia is associated with platelet hyperactivity. In the present study, we evaluated the binding of oxidized low-density lipoprotein (oxLDL) on the surface of circulating platelets in patients with stable coronary artery disease and acute coronary syndromes and its possible association with platelet activation. Furthermore, the role of oxLDL binding on platelet adhesion to collagen and endothelial cells in vitro as well as after carotid ligation in mice was investigated., Methods and Results: Using flow cytometry, patients with acute coronary syndromes (n=174) showed significantly enhanced oxLDL binding compared with patients with stable coronary artery disease (n=182; P=0.007). Platelet-bound oxLDL positively correlated with the degree of platelet activation (expression of P-selectin and activated fibrinogen receptor; P<0.001 for both). Plasma oxLDL was increased in patients with acute coronary syndromes compared with stable angina pectoris patients. Preincubation of isolated platelets with oxLDL, but not with native LDL, resulted in enhanced platelet adhesion to collagen and activated endothelial cells under high shear stress in vitro, as well as after carotid ligation in C57BL/6J mice and apolipoprotein E(-/-) mice fed a high cholesterol diet., Conclusions: Increased platelet-bound oxLDL in patients with acute coronary syndromes may play an important role in atherothrombosis, thus providing a potential future therapeutic target.

A mutual relationship exists between metastasizing tumor cells and components of the coagulation cascade. The exact mechanisms as to how platelets influence blood-borne metastasis, however, remain poorly understood. Here, we used murine B16 melanoma cells to observe functional aspects of how platelets contribute to the process of hematogenous metastasis. We found that platelets interfere with a distinct step of the metastasis cascade, as they promote adhesion of melanoma cells to the endothelium in vitro under shear conditions. Constitutively active platelet receptor GPIIb/IIIa (integrin αIIbβ3) expressed on Chinese hamster ovary cells promoted melanoma cell adhesion in the presence of fibrinogen, whereas blocking antibodies to aνβ3 integrin on melanoma cells or to GPIIb/IIIa significantly reduced melanoma cell adhesion to platelets. Furthermore, using intravital microscopy, we observed functional platelet-melanoma cell interactions, as platelet depletion resulted in significantly reduced melanoma cell adhesion to the injured vascular wall in vivo. Using a mouse model of hematogenous metastasis to the lung, we observed decreased metastasis of B16 melanoma cells to the lung by treatment with a mAb blocking the aν subunit of aνβ3 integrin. This effect was significantly reduced when platelets were depleted in vivo. Thus, the engagement of GPIIb/IIIa with aνβ3 integrin interaction mediates tumor cell-platelet interactions and highlights how this interaction is involved in hematogenous tumor metastasis.

Background: CD34(+) progenitor cells play an important role in haematopoiesis and vascular homeostasis. The aim of the present study was to investigate the role of platelet-derived junctional adhesion molecule-C (JAM-C) in adhesion and differentiation of human CD34(+) cells in vitro, as well as its association with platelet-derived P-selectin in patients with coronary artery disease., Methods and Results: Using flow cytometry we observed that JAM-C expression on the surface of washed platelets is increased after activation with thrombin receptor activating peptide-6 in vitro and correlated with platelet-derived P-selectin expression in patients with coronary artery disease (r=0.326, P=0.007). The role of JAM-C and its counter receptor Mac-1 in adhesion of human CD34(+) cells over immobilized platelets was investigated by using a neutralizing soluble protein (sJAM-C-Fc) and a monoclonal antibody against JAM-C or integrin Mac-1 (CD11b/CD18). Treatment with soluble JAM-C-Fc or anti-JAM-C or anti-Mac-1, but not with control-Fc or IgG1, resulted in a significantly decreased adhesion of human CD34(+) cells to platelets under static conditions (P<0.05). In order to validate our findings under high shear stress we performed flow chamber experiments. In a similar manner, inhibiting JAM-C interaction with Mac-1 resulted in a significantly decreased adhesion of CD34(+) cells over immobilized platelets under high shear stress (P<0.05). Colony forming unit assays and coculture assays revealed that inhibition of JAM-C/Mac-1 axis did not influence the platelet-mediated differentiation of CD34(+) cells to endothelial cells or to macrophages/foam cells., Conclusions: Taken together a platelet-derived JAM-C supports CD34(+) cell adhesion, a mechanism potentially involved in homing as well as domiciliation of human CD34(+) cells., (Copyright © 2012 S. Karger AG, Basel.)

Objective: To investigate the role of junctional adhesion molecule A (JAM-A) on adhesion and differentiation of human CD34(+) cells into endothelial progenitor cells., Methods and Results: Tissue healing and vascular regeneration is a multistep process requiring firm adhesion of circulating progenitor cells to the vascular wall and their further differentiation into endothelial cells. The role of JAM-A in platelet-mediated adhesion of progenitor cells was investigated by adhesion assays in vitro and with the help of intravital fluorescence microscopy in mice. Preincubation of human CD34(+) progenitor cells with soluble JAM-A-Fc (sJAM-A-Fc) resulted in significantly decreased adhesion over immobilized platelets or inflammatory endothelium under high shear stress in vitro and after carotid ligation in vivo or ischemia/reperfusion injury in the microcirculation of mice. Human CD34(+) cells express JAM-A, as defined by flow cytometry and Western blot analysis. JAM-A mediates differentiation of CD34(+) cells to endothelial progenitor cells and facilitates CD34(+) cell-induced reendothelialization in vitro. Pretreatment of human CD34(+) cells with sJAM-A-Fc resulted in increased neointima formation 3 weeks after endothelial denudation in the carotid arteries of nonobese diabetic/severe combined immunodeficient mice., Conclusions: These results indicate that the expression of JAM-A on CD34(+) cells mediates adhesion to the vascular wall after injury and differentiation into endothelial progenitor cells, a mechanism potentially involved in vascular regeneration. Human CD34(+) cells express JAM-A, mediating their interaction with platelets and endothelial cells. Specifically, JAM-A expressed on human CD34(+) progenitor cells regulates their adhesion over immobilized platelets or inflammatory endothelium under high shear stress in vitro and after carotid ligation in vivo or ischemia/reperfusion injury in the microcirculation of mice. Moreover, it mediates differentiation of CD34(+) cells to endothelial progenitor cells and facilitates reendothelialization.

Increasing evidence supports the role of stem and progenitor cells in vascular regeneration or injury. Following tissue ischemia, progenitor cells are mobilized from their bone marrow or peripheral niches into circulation, adhere at sites of vascular lesion and differentiate into a variety of mature cell types according to their origin and the local environment. Impairment in this pathophysiological process due to either low numbers of circulating progenitor cells or dysfunctional progenitor cells leads to inadequate vascular repair and upon co-existence with different cardiovascular risk factors to vascular injury and atherosclerosis. Vascular repair is a complex process which includes mobilization, chemotaxis, adhesion, proliferation and differentiation of progenitor cells. The common cardiovascular risk factors can impair this process resulting into inhibition of vascular healing and enhancement of inflammatory pathways which ultimately leads to atherosclerosis. Although homing of progenitor cells into bone marrow has been extensively studied, domiciliation of precursor cells into peripheral tissues and differentiation into mature cells are poorly understood so far. Recently, the role of platelets in domiciliation and subsequent differentiation of progenitor cells has been highlighted. Adherent platelets recruit circulating progenitor cells in vitro and in vivo and induce differentiation of the latter into endothelial cells or macrophages and foam cells. Although further studies are needed to describe the mechanisms that lie underneath these observations, it seems that platelet interaction with progenitor cells is an essential step in both vascular regeneration and injury.