IntroductionLeukocyte-platelet cross-talk plays multiple and important roles in inflammatory and thrombotic responses. 1 Activated platelets and platelet-released mediators, such as PDGF and PAF, activate leukocytes enhancing their responses such as adhesion, chemotaxis, phagocytosis, and superoxide generation. [2][3][4][5] Conversely, activated leukocytes induce platelet activation as evidenced by increased platelet P-selectin expression. 6 In vivo studies show that leukocytes and platelets colocalize within atherosclerotic and restenotic lesions, at sites of hemorrhage and ischemia-reperfusion injury. [7][8][9][10] Indeed, increased levels of leukocyte-platelet aggregates are found in patients with diabetes, acute coronary syndromes, trauma, and sepsis. [11][12][13] Numerous studies have analyzed the receptors involved in leukocyte-platelet interactions. P-selectin on activated platelets binds PSGL-1 on leukocytes 14,15 to support their rolling on adherent platelets and to activate leukocyte integrin ␣ M ␤ 2 , which mediates firm leukocyte adhesion. [16][17][18] Several ␣ M ␤ 2 counterreceptors on platelets have been proposed, including Fg-bound ␣ IIb ␤ 3 , 19 GPIb␣,20,21 and JAM-3. 22 Recognition of these counterreceptors requires that ␣ M ␤ 2 be in an activated conformation. ␣ M ␤ 2 can be activated by stimulation of leukocytes by agonists and can be monitored with monoclonal antibodies (mAbs), CBRM1/5 and mAb24, which react selectively with the activated conformation.Microparticles (MPs) are released by membrane blebbing from cells undergoing activation or apoptosis. MPs are usually defined by 2 criteria: size (Ͻ 1 m) and surface expression of negatively charged phosphatidylserine (reviewed in Boulanger et al, 23 Morel et al, 24 Martinez et al, 25 and Distler et al 26 ). MPs are not simply the markers of cell activation or damage but may exert pleiotropic effects on blood and vascular cells. MPs of platelet and endothelial origin have procoagulant and proinflammatory activities (reviewed in Boulanger et al, 23 Morel et al, 24 Martinez et al, 25 and Distler et al 26 ) and are markers of acute coronary syndromes. 27,28 The functions of leukocyte-derived MPs are less clear. MPs shed by PMNs behave as inflammatory mediators and activate endothelial cells 29,30 but also exert antiinflammatory effects on macrophages. 31 One of the most important features of leukocyte-derived MPs is the expression of tissue factor 32 and negatively charged phospholipids, suggesting that they may contribute to blood coagulation. Indeed, in vivo studies have shown that leukocyte MPs are captured by activated platelets within thrombi by a P-selectin/PSGL-1-dependent mechanism that leads to accumulation of tissue factor and ultimately enhanced fibrin deposition. 33,34 Although the interactions of leukocyte-derived MPs with activated platelets have been well documented, additional mechanisms in which leukocyte MPs may activate resting platelets, when platelet P-selectin expression is low, have not been examined. In this study, we show...

The FERM domain containing protein Kindlin-3 has been recognized as a major regulator of integrin function in hematopoietic cells, but its role in neoplasia is totally unknown. We have examined the relationship between Kindlin-3 and breast cancer in mouse models and human tissues. Human breast tumors showed a ∼7-fold elevation in Kindlin-3 mRNA compared with nonneoplastic tissue by quantitative polymerase chain reaction. Kindlin-3 overexpression in a breast cancer cell line increased primary tumor growth and lung metastasis by 2.5- and 3-fold, respectively, when implanted into mice compared with cells expressing vector alone. Mechanistically, the Kindlin-3-overexpressing cells displayed a 2.2-fold increase in vascular endothelial growth factor (VEGF) secretion and enhanced β1 integrin activation. Increased VEGF secretion resulted from enhanced production of Twist, a transcription factor that promotes tumor angiogenesis. Knockdown of Twist diminished VEGF production, and knockdown of β1 integrins diminished Twist and VEGF production by Kindlin-3-overexpressing cells, while nontargeting small interfering RNA had no effect on expression of these gene products. Thus, Kindlin-3 influences breast cancer progression by influencing the crosstalk between β1 integrins and Twist to increase VEGF production. This signaling cascade enhances breast cancer cell invasion and tumor angiogenesis and metastasis.

Interplay between tumor cells and host cells in the tumor microenvironment dictates the development of all cancers. In breast cancer, malignant cells educate host macrophages to adopt a pro-tumorigenic phenotype. In this study, we show how the integrin regulatory protein kindlin-2 (FERMT2) promotes metastatic progression of breast cancer through the recruitment and subversion of host macrophages. Kindlin-2 expression was elevated in BC biopsy tissues where its levels correlated with reduced patient survival. Based on these observations, we used CRISPR/Cas9 technology to ablate Kindlin-2 expression in human MDA-MB-231 and murine 4T1 breast cancer cells. Kindlin-2 deficiency inhibited invasive and migratory properties in vitro without affecting proliferation rates. However, in vivo tumor outgrowth was inhibited by >80% in a manner associated with reduced macrophage infiltration and secretion of the macrophage attractant and growth factor CSF-1. The observed loss of CSF-1 appeared to be caused by a more proximal deficiency in TGF-β-dependent signaling in Kindlin-2 deficient cells. Collectively, our results illuminate a Kindlin-2/TGF-β/CSF-1 signaling axis employed by breast cancer cells to capture host macrophage functions that drive tumor progression.

Kindlin-2, a widely distributed cytoskeletal protein, has been implicated in integrin activation, and its absence is embryonically lethal in mice and causes severe developmental defects in zebrafish. Knockdown of kindlin-2 levels in endothelial cells resulted in defective adhesive and migratory responses, suggesting that angiogenesis might be aberrant even with partial reduction of kindlin-2. This hypothesis has now been tested in the kindlin-2 ؉/؊ mice. RM1 prostate tumors grown in kindlin-2 ؉/؊ mice had fewer blood vessels, which were thinner and shorter and supported less tumor growth compared with wild-type littermates. The vessels that did form in the kindlin-2 ؉/؊ mice lacked smooth muscle cells and pericytes and had thinner basement membranes, indicative of immature vessels. VEGF-induced angiogenesis in matrigel implants was also abnormal in the kindlin-2 ؉/؊ mice. Vessels in the kindlin-2 ؉/؊ mice were leaky, and BM transplantation from kindlin-2 ؉/؊ to WT mice did not correct this defect. Endothelial cells derived from kindlin-2 ؉/؊ mice had integrin expression levels similar to WT mice but reduced ␣V␤3-dependent signaling, migration, adhesion, spreading, and tube formation. Developmental angiogenesis was markedly impaired by kindlin-2 morpholinos in zebrafish. Taken together, kindlin-2 plays an important role in pathologic and developmental angiogenesis, which arises from defective activation of integrin ␣V␤3. IntroductionAlthough endothelial cells (ECs) are the primary cell type that forms blood vessel tubes, other cell types, including BM-derived cells, smooth muscle cells, and pericytes, are all engaged in forming blood-bearing conduits. Growth factor-growth factor receptor interactions are involved in initial recruitment of these cells while other ligand-receptor systems govern migration of the responding cells into angiogenic tissues (reviewed in Folkman 1 ). Among the cellular receptors that specialize in regulating the adhesive and migratory responses of cells during angiogenesis are members of the integrin family. 2 Integrins of the ␤1 and ␤3 subfamilies on ECs have been implicated in angiogenesis in vivo through knockout 3 and inhibitor approaches 4 and integrin ␣V␤3 and ␣5␤1 are targets of antiangiogenic drugs for cancer treatment (reviewed in Avraamides et al 5 ).The regulation of integrin function in angiogenesis as well as many other responses depends on their ability to undergo activation, a rapid transition from a low-to a high-affinity state for recognition of their ligands. Particularly relevant to angiogenesis is the ability of VEGF to activate integrin ␣V␤3 and ␣5␤1 by engaging one of its EC receptors, VEGFR2. 6 Indeed, VEGFR2 and integrin ␣V␤3 can form a physical complex in EC and influence the functions of each other. 6,7 Two sets of cytoskeletal proteins, the talins and the kindlins, bind to the ␤ cytoplasmic tails and are now known to be involved in integrin activation. [8][9][10] Talin has long been known to be critical for integrin activation 11 and more recent studies have sh...