Differential Effects of Platelet Factor 4 (CXCL4) and Its Non-Allelic Variant (CXCL4L1) on Cultured Human Vascular Smooth Muscle Cells

Platelet factor 4 (CXCL4) is a chemokine abundantly stored in platelets. Upon injury and during atherosclerosis, CXCL4 is transported through the vessel wall where it modulates the function of vascular smooth muscle cells (VSMCs) by affecting proliferation, migration, gene expression and cytokine release. Variant CXCL4L1 is distinct from CXCL4 in function and expression pattern, despite a minor three-amino acid difference. Here, the effects of CXCL4 and CXCL4L1 on the phenotype and function of human VSMCs were compared in vitro. VSMCs were found to constitutively express CXCL4L1 and only exogenously added CXCL4 was internalized by VSMCs. Pre-treatment with heparin completely blocked CXCL4 uptake. A role of the putative CXCL4 receptors CXCR3 and DARC in endocytosis was excluded, but LDL receptor family members appeared to be involved in the uptake of CXCL4. Incubation of VSMCs with both CXCL4 and CXCL4L1 resulted in decreased expression of contractile marker genes and increased mRNA levels of KLF4 and NLRP3 transcription factors, yet only CXCL4 stimulated proliferation and calcification of VSMCs. In conclusion, CXCL4 and CXCL4L1 both modulate gene expression, yet only CXCL4 increases the division rate and formation of calcium-phosphate crystals in VSMCs. CXCL4 and CXCL4L1 may play distinct roles during vascular remodeling in which CXCL4 induces proliferation and calcification while endogenously expressed CXCL4L1 governs cellular homeostasis. The latter notion remains a subject for future investigation.

Platelet-derived chemokines in vascular biology

Undoubtedly, platelets are key elements in the regulation of thrombosis and haemostasis. Along with their primary task to prevent blood loss from injured vessels, platelets have emerged as regulators of a variety of processes in the vasculature. Multiple challenges, from the contact and adhesion to subendothelial matrix after injury of the vessel wall, to interactions with blood cells in inflammatory conditions, result in platelet activation with concomitant shape change and release of numerous substances. Among these, chemokines have been found to modulate several processes in the vasculature, such as atherosclerosis and angiogenesis. In particular, the chemokines connective tissue activating protein III (CTAP-III) and its precursors, or truncation products (CXCL7), platelet factor 4, (PF4, CXCL4) and its variant PF4alt (CXCL4L1) or regulated upon activation and normal T cell expressed and secreted (RANTES, CCL5), have been investigated thoroughly. Defined common properties as their aptitude to bind glycosaminoglycans or their predisposition to associate and form homooligomers are prerequisites for their role in the vasculature and function in vivo. The current review summarizes the development of these single chemokines, and their cooperative effects that may in part be dependent on their physical interactions.

The Role of Cytokine PF4 in the Antiviral Immune Response of Shrimp

During viral infection in vertebrates, cytokines play important roles in the host defense against the virus. However, the function of cytokines in invertebrates has not been well characterized. In this study, shrimp cytokines involved in viral infection were screened using a cytokine antibody microarray. The results showed that three cytokines, the Fas receptor (Fas), platelet factor 4 (PF4) and interleukin-22 (IL-22), were significantly upregulated in the white spot syndrome virus (WSSV)-challenged shrimp, suggesting that these cytokines played positive regulatory roles in the immune response of shrimp against the virus. Further experiments revealed that PF4 had positive effects on the antiviral immunity of shrimp by enhancing the shrimp phagocytic activity and inhibiting the apoptotic activity of virus-infected hemocytes. Therefore, our study presented a novel mechanism of cytokines in the innate immunity of invertebrates.

Specific molecular signatures predict decitabine response in chronic myelomonocytic leukemia

Myelodysplastic syndromes and chronic myelomonocytic leukemia (CMML) are characterized by mutations in genes encoding epigenetic modifiers and aberrant DNA methylation. DNA methyltransferase inhibitors (DMTis) are used to treat these disorders, but response is highly variable, with few means to predict which patients will benefit. Here, we examined baseline differences in mutations, DNA methylation, and gene expression in 40 CMML patients who were responsive or resistant to decitabine (DAC) in order to develop a molecular means of predicting response at diagnosis. While somatic mutations did not differentiate responders from nonresponders, we identified 167 differentially methylated regions (DMRs) of DNA at baseline that distinguished responders from nonresponders using next-generation sequencing. These DMRs were primarily localized to nonpromoter regions and overlapped with distal regulatory enhancers. Using the methylation profiles, we developed an epigenetic classifier that accurately predicted DAC response at the time of diagnosis. Transcriptional analysis revealed differences in gene expression at diagnosis between responders and nonresponders. In responders, the upregulated genes included those that are associated with the cell cycle, potentially contributing to effective DAC incorporation. Treatment with CXCL4 and CXCL7, which were overexpressed in nonresponders, blocked DAC effects in isolated normal CD34+ and primary CMML cells, suggesting that their upregulation contributes to primary DAC resistance.

Proteasome function is required for platelet production

The proteasome inhibiter bortezomib has been successfully used to treat patients with relapsed multiple myeloma; however, many of these patients become thrombocytopenic, and it is not clear how the proteasome influences platelet production. Here we determined that pharmacologic inhibition of proteasome activity blocks proplatelet formation in human and mouse megakaryocytes. We also found that megakaryocytes isolated from mice deficient for PSMC1, an essential subunit of the 26S proteasome, fail to produce proplatelets. Consistent with decreased proplatelet formation, mice lacking PSMC1 in platelets (Psmc1(fl/fl) Pf4-Cre mice) exhibited severe thrombocytopenia and died shortly after birth. The failure to produce proplatelets in proteasome-inhibited megakaryocytes was due to upregulation and hyperactivation of the small GTPase, RhoA, rather than NF-κB, as has been previously suggested. Inhibition of RhoA or its downstream target, Rho-associated protein kinase (ROCK), restored megakaryocyte proplatelet formation in the setting of proteasome inhibition in vitro. Similarly, fasudil, a ROCK inhibitor used clinically to treat cerebral vasospasm, restored platelet counts in adult mice that were made thrombocytopenic by tamoxifen-induced suppression of proteasome activity in megakaryocytes and platelets (Psmc1(fl/fl) Pdgf-Cre-ER mice). These results indicate that proteasome function is critical for thrombopoiesis, and suggest inhibition of RhoA signaling as a potential strategy to treat thrombocytopenia in bortezomib-treated multiple myeloma patients.

Platelets mediate oxidized low-density lipoprotein-induced monocyte extravasation and foam cell formation

OBJECTIVE

A growing body of evidence indicates that platelets contribute to the onset and progression of atherosclerosis by modulating immune responses. We aimed to elucidate the effects of oxidized low-density lipoprotein (OxLDL) on platelet-monocyte interactions and the consequences of these interactions on platelet phagocytosis, chemokine release, monocyte extravasation, and foam cell formation.

APPROACH AND RESULTS

Confocal microscopy and flow cytometric analysis revealed that in vitro and in vivo stimulation with OxLDL resulted in rapid formation of platelet-monocyte aggregates, with a preference for CD16+ monocyte subsets. This platelet-monocyte interaction facilitated OxLDL uptake by monocytes, in a process that involved platelet CD36-OxLDL interaction, release of chemokines, such as CXC motif ligand 4, direct platelet-monocyte interaction, and phagocytosis of platelets. Inhibition of cyclooxygenase with acetylsalicylic acid and antagonists of ADP receptors, P2Y1 and P2Y12, partly abrogated OxLDL-induced platelet-monocyte aggregates and platelet-mediated lipid uptake in monocytes. Platelets also enhanced OxLDL-induced monocyte transmigration across an endothelial monolayer via direct interaction with monocytes in a transwell assay. Importantly, in LDLR(-/-) mice, platelet depletion resulted in a significant decrease of peritoneal macrophage recruitment and foam cell formation in a thioglycollate-elicited peritonitis model. In platelet-depleted wild-type mice, transfusion of ex vivo OxLDL-stimulated platelets induced monocyte extravasation to a higher extent when compared with resting platelets.

CONCLUSIONS

Our results on OxLDL-mediated platelet-monocyte aggregate formation, which promoted phenotypic changes in monocytes, monocyte extravasation and enhanced foam cell formation in vitro and in vivo, provide a novel mechanism for how platelets potentiate key steps of atherosclerotic plaque development and plaque destabilization.

Heparin-induced thrombocytopenia

Heparin is widely used for the prevention and treatment of thrombotic and particularly cardiovascular disorders. Unfortunately, 0.5 to 3.0% of patients given heparin develop an immune reaction, commonly termed Type II heparin-induced thrombocytopenia (HIT). This is characterized by a moderate thrombocytopenia and in some patients, a venous or arterial thrombosis. This frequently leads to disastrous sequelae, such as limb amputation and death. The pathophysiological basis of this serious adverse drug reaction is the production of an immunoglobulin G antibody that reacts with an antigenic complex consisting of heparin and platelet factor 4. A significant risk factor for the development of HIT is recent surgery, and the frequency of developing an antiheparin-platelet factor 4 or HIT antibody is particularly high in cardiac surgery patients, although surprisingly, only a few of these patients actually develop the clinical syndrome of HIT. This review will discuss the frequency, pathophysiology, clinical features, diagnosis and management of HIT.

Lineage tracing of Pf4-Cre marks hematopoietic stem cells and their progeny

The development of a megakaryocyte lineage specific Cre deleter, using the Pf4 (CXCL4) promoter (Pf4-Cre), was a significant step forward in the specific analysis of platelet and megakaryocyte cell biology. However, in the present study we have employed a sensitive reporter-based approach to demonstrate that Pf4-Cre also recombines in a significant proportion of both fetal liver and bone marrow hematopoietic stem cells (HSCs), including the most primitive fraction containing the long-term repopulating HSCs. Consequently, we demonstrate that Pf4-Cre activity is not megakaryocyte lineage-specific but extends to other myeloid and lymphoid lineages at significant levels between 15-60%. Finally, we show for the first time that Pf4 transcripts are present in adult HSCs and primitive hematopoietic progenitor cells. These results have fundamental implications for the use of the Pf4-Cre mouse model and for our understanding of a possible role for Pf4 in the development of the hematopoietic lineage.

The prothrombotic potential of platelet factor 4

Heparin-induced thrombocytopenia (HIT) is a prothrombotic disorder initiated by heparin administration. It is caused by the formation of pathogenic antibodies to complexes of platelet factor-4 (PF4) and heparin on platelet surfaces that cause platelet activation, aggregation and thrombosis. There has been intense research on this intriguing, drug-related thrombocytopenia explaining several characteristic aspects of this condition. However, prothrombotic potential of the key player, PF4 has not been investigated in many studies although it has been shown to be critical in monocyte chemotaxis, monocyte-platelet interaction, and megakaryocyte suppression, all of which can contribute to the pathophysiology of HIT. This article explains the important role of PF4 released during platelet activation with the administration of heparin in the pathogenesis of thrombocytopenia and thrombosis in HIT.

The role of heparin in angiogenesis

A series of experiments is described in which: (i) mast cells were found to accumulate at a tumour site before the ingrowth of new capillaries; (ii) heparin released by mast cells increased the migration of capillary endothelial cells in vitro; and (iii) heparin enhanced tumour angiogenesis in vivo. These experiments led to the discovery that protamine and platelet factor 4 are angiogenesis inhibitors. This finding suggests a central role for heparin or related glycosaminoglycans in the growth regulation of capillary blood vessels.

[Molecular mechanisms inhibiting proliferation of myeloma cells]

Accumulating evidences suggest that many molecules are working as inhibitors of proliferation in myeloma cells e.g., PTEN, mTOR(PI3-kinase signal molecules), p53, RB1, INK4 family and KIP/CIP family (cell cycle check point molecules), PF4 (inhibitor of angiogenesis). In this review, significance of these molecules in myeloma is summarized. Additionally, our finding of growth inhibitory effect by PU.1 is explained.

Elimination of platelet factor 4 (PF4) from platelets reduces atherosclerosis in C57Bl/6 and apoE-/- mice

Activated platelets, which release platelet factor 4 (PF4) are present in patients with atherosclerosis. To date, no direct in-vivo evidence exists for the involvement of PF4 in atherogenesis. In the current study, we tested the hypothesis that PF4 is atherogenic, and that genetic elimination of PF4 would protect mice from atherosclerosis. We have bred PF4(-/-) mice onto two athero-susceptible backgrounds, WT-C57Bl/6(WT) and apoE(-/-) to examine the importance of PF4 in atherogenesis. In order to induce atherosclerosis, WT and PF4(-/-) mice were fed an atherogenic diet for 30 weeks, while apoE(-/-) and apoE(-/-) PF4(-/-) mice were fed a high-fat Western-style diet for 10 weeks. Examination of lesions in the aortic roots of atherogenic diet fed mice demonstrated reduced atherosclerosis in PF4(-/-) (20% compared to WT). Examination of apoE(-/-) mice demonstrated similar changes, with apoE(-/-) PF4(-/-) mice demonstrating 37% of the aortic atherosclerotic burden compared to apoE(-/-) mice. Although we found similar levels of total and non-HDL cholesterol in WT and PF4(-/-) mice, HDL-cholesterol levels were increased in PF4(-/-) on both backgrounds. These data demonstrate, for the first time, that the platelet specific chemokine PF4 promotes atherosclerotic lesion development in vivo.

Endogenous platelet factor 4 stimulates activated protein C generation in vivo and improves survival after thrombin or lipopolysaccharide challenge

Pharmacologic infusion of activated protein C (APC) improves survival in severe sepsis, and platelet factor 4 (PF4) accelerates APC generation in a primate thrombin-infusion model. We now tested whether endogenous platelet PF4 content affects APC generation. Mice completely deficient in PF4 (mPF4(-/-)) had impaired APC generation and survival after thrombin infusion, similar to the impairment seen in heterozygote protein C-deficient (PC(+/-)) mice. Transgenic mice overexpressing human PF4 (hPF4(+)) had increased plasma APC generation. Overexpression of platelet PF4 compensated for the defect seen in PC(+/-) mice. In both a thrombin and a lipopolysaccharide (LPS) survival model, hPF4(+) and PC(+/-)/hPF4(+) mice had improved survival. Further, infusion of hPF4(+) platelets improved survival of wild-type mice after an LPS challenge. These studies suggest that endogenous PF4 release may have biologic consequences for APC generation and survival in clinical sepsis. Infusions of PF4-rich platelets may be an effective strategy to improve outcome in this setting.

Platelet factor 4 is a negative autocrine in vivo regulator of megakaryopoiesis: clinical and therapeutic implications

Platelet factor 4 (PF4) is a negative regulator of megakaryopoiesis in vitro. We have now examined whether PF4 regulates megakaryopoiesis in vivo by studying PF4 knockout mice and transgenic mice that overexpress human (h) PF4. Steady-state platelet count and thrombocrit in these animals was inversely related to platelet PF4 content. Growth of megakaryocyte colonies was also inversely related to platelet PF4 content. Function-blocking anti-PF4 antibody reversed this inhibition of megakaryocyte colony growth, indicating the importance of local PF4 released from developing megakaryocytes. The effect of megakaryocyte damage and release of PF4 on 5-fluorouracil-induced marrow failure was then examined. Severity of thrombocytopenia and time to recovery of platelet counts were inversely related to initial PF4 content. Recovery was faster and more extensive, especially in PF4-overexpressing mice, after treatment with anti-PF4 blocking antibodies, suggesting a means to limit the duration of such a chemotherapy-induced thrombocytopenia, especially in individuals with high endogenous levels of PF4. We found that approximately 8% of 250 healthy adults have elevated (> 2 times average) platelet PF4 content. These individuals with high levels of platelet PF4 may be especially sensitive to developing thrombocytopenia after bone marrow injury and may benefit from approaches that block the effects of released PF4.

Roles of platelet factor 4 in hematopoiesis and angiogenesis

Platelet factor 4 (PF4) has been recognized as a physiological inhibitor of megakaryocytopoiesis and angiogenesis for two decades. Structure-function studies have shown that the DLQ determinant in position 54-56 is necessary for megakaryocytic inhibition whereas mutations of these residues into ELR sequence and more importantly, into DLR sequence, induce a stronger inhibitory activity of peptide p47-70 on angiogenesis. The alpha-helix region of peptides may participate in the fixation of the effector to its cellular receptor and the other important structural domains would activate the receptor. In vivo, PF4 and its related peptides can protect hematopoiesis from chemotherapy by enhancing cell viability and suppress tumor growth through anti-angiogenic pathway. Several PF4 fragments and modified molecules exhibit antiangiogenesis properties and may become an alternative for further therapeutic angiogenesis.

Platelet factor 4 (CXCL4) seals blood clots by altering the structure of fibrin

Platelet factor-4 (PF4/CXCL4) is an orphan chemokine released in large quantities in the vicinity of growing blood clots. Coagulation of plasma supplemented with a matching amount of PF4 results in a translucent jelly-like clot. Saturating amounts of PF4 reduce the porosity of the fibrin network 4.4-fold and decrease the values of the elastic and loss moduli by 31- and 59-fold, respectively. PF4 alters neither the cleavage of fibrinogen by thrombin nor the cross-linking of protofibrils by activated factor XIII but binds to fibrin and dramatically transforms the structure of the ensuing network. Scanning electron microscopy showed that PF4 gives rise to a previously unreported pattern of polymerization where fibrin assembles to form a sealed network. The subunits constituting PF4 form a tetrahedron having at its corners a RPRH motif that mimics (in reverse orientation) the Gly-His-Arg-Pro-amide peptides that co-crystallize with fibrin. Molecular modeling showed that PF4 could be docked to fibrin with remarkable complementarities and absence of steric clashes, allowing the assembly of irregular polymers. Consistent with this hypothesis, as little as 50 microm the QVRPRHIT peptide derived from PF4 affects the polymerization of fibrin.

Oncolytic HSV armed with platelet factor 4, an antiangiogenic agent, shows enhanced efficacy

Oncolytic herpes simplex viruses (HSV) have emerged as a promising platform for cancer therapy. However, efficacy as single agents has thus far been unsatisfactory. Tumor vasculature is critical in supporting tumor growth, but successful antiangiogenic approaches often require maintaining constant levels of antiangiogenic products. We hypothesized that oncolytic HSV has the potential to destroy tumor vasculature and that this effect can be enhanced by combination with antiangiogenic gene transfer. We examined the strategy of arming oncolytic HSV with an antiangiogenic transgene, platelet factor 4 (PF4). The PF4 transgene was inserted into oncolytic HSV G47Delta utilizing a bacterial artificial chromosome construction system. Whereas bG47Delta-empty showed robust cell killing and migration inhibition of proliferating endothelial cells (HUVEC and Py-4-1), the effect was further enhanced by PF4 expression. Importantly, enhanced potency did not impede viral replication. In vivo, bG47Delta-PF4 was more efficacious than its nonexpressing parent bG47Delta-empty at inhibiting tumor growth and angiogenesis in both human U87 glioma and mouse 37-3-18-4 malignant peripheral nerve sheath tumor models. Enhancing the antiangiogenic properties of oncolytic HSV through the expression of antiangiogenic factors such as PF4 is a powerful new strategy that targets both the tumor cells and tumor vasculature.

Mast cells and neutrophils proteolytically activate chemokine precursor CTAP-III and are subject to counterregulation by PF-4 through inhibition of chymase and cathepsin G

The CXC chemokines platelet factor 4 (PF-4/CXCL4) and connective tissue-activating peptide III (CTAP-III) are released by activated human platelets in micromolar concentrations. So far, neutrophils have been recognized to cleave the precursor CTAP-III to form the active chemokine neutrophil-activating peptide 2 (NAP-2/CXCL7) through limited proteolysis by membrane-associated cathepsin G. Here we show for the first time that activated human skin mast cells (MCs) convert CTAP-III into biologically active NAP-2 through proteolytic cleavage by released chymase. A direct comparison on a cell number basis revealed that unstimulated MCs exceed the CTAP-III–processing potency of neutrophils about 30-fold, whereas MCs activated by IgE cross-linking exhibit even 1000-fold higher CTAP-III–processing capacity than fMLP-stimulated neutrophils. Intriguingly, PF-4 counteracted MC- as well as neutrophil-mediated NAP-2 generation at physiologically relevant concentrations. Addressing the underlying mechanism, we obtained evidence that PF-4 acts as an inhibitor of the CTAP-III–processing enzymes cathepsin G and chymase without becoming cleaved itself as a competitive substrate. Because cleavage of the CTAP-III–unrelated substrate substance P was also affected by PF-4, our results suggest a regulatory role for PF-4 not only in NAP-2 generation but also in neutrophil- and MC-mediated processing of other physiologically relevant inflammatory mediators.

Plasma total homocysteine level and its association with carotid intima-media thickness in obesity

BACKGROUND

Elevated plasma concentrations of total homocysteine (tHcy) and obesity are risk factors for cardiovascular disease. The relationship between hyperhomocysteinemia and obesity has not been totally elucidated.

OBJECTIVE

The first aim of the study was to investigate whether anthropometric measurements and insulin resistance contribute to the variation in homocysteine levels in obese adults. Our second aim was to determine if any relationship exists between the carotid intima-media thickness (IMT) and plasma tHcy levels in obese subjects without traditional cardiovascular risk factors.

MATERIAL AND METHODS

Fifty-five obese (15 male, 40 female) and 30 (11 male, 19 female) age- and sex-matched apparently healthy volunteers were included. Exclusion criteria were smoking, hypertension, diabetes, vitamin ingestion, hyperlipidemia, renal failure, liver disease, pregnancy, menopause and secondary obesity such as Cushing's syndrome, hypothyroidism. tHcy, folate, vitamin B12 levels, fasting insulin, glucose, total cholesterol, triglycerides, HDL, LDL particles, uric acid, creatinine and creatinine clearance were measured. Non-invasive ultrasound measurements of carotid IMT were performed.

RESULTS

tHcy levels and carotid IMT were comparable between obese and non-obese subjects. Waist/hip ratio (WHR) was related to tHcy and carotid IMT. Hyperhomocysteinemic subjects (tHcy >19.2 micromol/l) had greater WHR than normo-homocysteinemic subjects. Both tHcy levels and carotid IMT were higher in male subjects both in obese and non-obese subjects. No association was observed between insulin resistance and tHcy and carotid IMT. Renal function and abdominal obesity were significant predictors of plasma tHcy levels.

CONCLUSIONS

We concluded that, in obese subjects who are free from atherosclerosis and impaired renal function, plasma tHcy levels do not differ from healthy subjects. Plasma tHcy concentrations are not related to carotid IMT in obese subjects during the non-atherogenic stage. Although no significant difference was observed between insulin-resistant and insulin-sensitive subjects compared to the plasma tHcy levels, the relationship between tHcy levels and some components of the insulin resistance syndrome may support the opinion that tHcy may be considered a component of the insulin resistance syndrome.

Platelets and chemokines in atherosclerosis: partners in crime

It becomes increasingly evident that blood platelets do not only exert important functions in hemostasis and thrombus formation but are also involved in atherosclerotic vascular disease. A major portion of the underlying mechanisms is related to an intricate functional interaction of platelets with chemokines, which have also been implicated in atherogenesis and neointima formation: (1) Platelets can induce the secretion of chemokines in different cells of the vascular wall; (2) In combination with primary agonists, certain chemokines can potentiate platelet aggregation and adhesion; (3) Activated platelets can release and deposit chemokines and precursors on vascular cell surfaces, which trigger atherogenic recruitment of vascular cells or modulate crucial processes such as angiogenesis and lipoprotein metabolism; (4) Surface-adherent platelets can bind and present vascular cell-derived chemokines to trigger arrest of circulating mononuclear cells. The close linkage between platelets and chemokines as culprits in the pathogenesis of vascular diseases may provide a valuable target for selective interventions.

Platelet factor 4 differentially modulates CD4+CD25+ (regulatory) versus CD4+CD25- (nonregulatory) T cells

Active suppression mediated by CD4(+)CD25(+) T regulatory (Tr) cells plays an important role in the down-regulation of T cell responses to both foreign and self-Ags. Platelet factor 4 (PF4), a platelet-derived CXC chemokine, has been shown to strongly inhibit T cell proliferation as well as IFN-gamma and IL-2 release by isolated T cells. In this report we show that human PF4 stimulates proliferation of the naturally anergic human CD4(+)CD25(+) Tr cells while inhibiting proliferation of CD4(+)CD25(-) T cells. In coculture experiments we found that CD4(+)CD25(+) Tr cells exposed to PF4 lose the ability to inhibit the proliferative response of CD4(+)CD25(-) T cells. Our findings suggest that human PF4, by inducing Tr cell proliferation while impairing Tr cell function, may play a previously unrecognized role in the regulation of human immune responses. Because platelets are the sole source of PF4 in the circulation, these findings may be relevant to the pathogenesis of certain immune-mediated disorders associated with platelet activation, such as heparin-induced thrombocytopenia and autoimmune thrombocytopenic purpura.

Platelet factor 4: an inhibitor of angiogenesis

Platelet factor 4 (PF4) is an antiangiogenic ELR-negative chemokine. PF4 inhibits endothelial cell proliferation and migration and angiogenesis in vitro and in vivo. Three different mechanisms have been proposed to explain PF4's antiangiogenic effects. First, PF4 may bind proteoglycans and interfere with the proteoglycan-bystander effect on growth factor activity. Second, PF4 is able to interact directly with angiogenesis growth factors such as fibroblast growth factors or vascular endothelial growth factors and inhibits their interaction with cell surface receptors. Third, PF4 may activate cell surface receptors on endothelial cells and induce inhibitory signals. Recently, one such receptor, CXCR3-B, was identified. In cardiovascular disease, PF4 may possibly intervene in collateral vessel formation, plaque neovascularization, heparin-induced thrombocytopenia and stent endothelialization. Several PF4 fragments such as PF4-CTF and modified molecules have been made that exhibit antiangiogenesis properties and may serve as leads for further therapeutic development.

Platelet factor 4: a chemokine enigma

Platelet factor 4 (PF4) is a platelet alpha-granule protein sequenced over 25 years ago that is a founding member of the C-X-C chemokine family, yet its physiologic function has yet to be definitively established. Initial investigations focused on possible procoagulant roles for PF4 in platelet function and plasmatic coagulation. Subsequent in vitro studies have, however, described a puzzling array of other apparently unrelated biologic functions, including inhibition of angiogenesis and hematopoiesis, promotion of neutrophil adhesion, and activation, enhancement of oxy-LDL binding to the LDL receptor and stimulation of anti-coagulant activated protein C generation by the thrombomodulin/protein C system. Preliminary studies with a just-described PF4 knockout mouse line support a role for PF4 in platelet-dependent thrombosis in vivo.

[Does colorectal cancer clinical advancement and surgical treatment affect platelet factor 4 concentration]

The aim of the present study was to determine platelet factor 4 (PF 4) concentration, as a marker of platelet activation, in patients with colorectal cancer (CRC), depending on the clinical advancement of carcinoma and surgical treatment.

MATERIAL AND METHODS

We investigated 21 patients with CRC before the surgery (A0) and three days (A1) and twelve days (A2) after the surgery. The patients were divided into three groups, according the clinical advancement--I, II, III (Hutter's classification). The control group (C) consisted 20 healthy subjects. PF 4 concentration was determined using the immunoenzymatic method (ELISA).

RESULTS

Patients with CRC before the surgery had a statistically significantly increased PF 4 concentration versus the control group (p<0.001). The highest PF 4 concentration was observed in patients without metastases (I degree advanced disease). Three days after the surgery PF 4 concentration decreased slightly in patients with III degree advanced disease. Twelve days after surgery (A2) it was again observed an increase of PF 4 concentration in patients with II and III stage of advancement.

CONCLUSION

Results of the study confirm that platelets are involved in development of cancer and indicate significantly that surgical treatment applied in this patients affects platelet activation and morphological parameters. The lower PF 4 concentration in patients with CRC with metastases (II and III group) confirm that more activity platelets play an important role in the metastases.

The control of angiogenesis and tumor invasion by platelet factor-4 and platelet factor-4-derived molecules

Platelet factor-4 (PF-4) is an ELR-negative chemokine that exhibits antiangiogenesis properties. PF-4 inhibits endothelial cell proliferation and migration, angiogenesis in vitro and in vivo, and tumor growth. However, tumor cells are not directly inhibited by PF-4. To date, a cell surface receptor that would explain the biological effects mediated by PF-4 has not been identified. PF-4 is able to interact directly with angiogenesis growth factors such as fibroblast growth factors (FGFs) and vascular endothelial growth factors and inhibits their interaction with cell surface receptors. Furthermore, dimerization of fibroblast growth factors is abrogated by PF-4. Whether PF-4 plays a role as an endogenous angiogenesis regulator is at present not clear. Several PF-4 fragments and modified molecules have been made that exhibit antiangiogenesis properties. Among these is a C-terminal fragment that has a defined structure, retains all the antiangiogenesis properties of the parent molecule, inhibits growth factor receptor binding, associates with FGFs, and destabilizes their three-dimensional structure. The relevance of these observations for the treatment of malignant disease is discussed.

Platelet Factor 4/CXCL4 Induces Phagocytosis and the Generation of Reactive Oxygen Metabolites in Mononuclear Phagocytes Independently of Gi Protein Activation or Intracellular Calcium Transients

Platelet factor 4 (PF-4), a platelet-derived CXC chemokine, is known to prevent human monocytes from apoptosis and to promote differentiation of these cells into HLA-DR(-) macrophages. In this study, we investigated the role of PF-4 in the control of acute monocyte proinflammatory responses involved in the direct combat of microbial invaders. We show that PF-4 increases monocyte phagocytosis and provokes a strong formation of oxygen radicals but lacks a chemotactic activity in these cells. Compared with FMLP, PF-4-induced oxidative burst was later in its onset but was remarkably longer in its duration (lasting for up to 60 min). Furthermore, in PF-4-prestimulated cells, FMLP- as well as RANTES-induced burst responses became synergistically enhanced. As we could show, PF-4-mediated oxidative burst in monocytes does not involve Gi proteins, elevation of intracellular free calcium concentrations, or binding to CXCR3B, a novel PF-4 receptor recently discovered on endothelial cells. Moreover, we found that PF-4 acts on macrophages in a dual manner. On the one hand, very similar to GM-CSF or M-CSF, PF-4 treatment of monocytes generates macrophages with a high capacity for unspecific phagocytosis. On the other hand, short term priming of GM-CSF-induced human macrophages with PF-4 substantially increases their capability for particle ingestion and oxidative burst. A comparable effect was also observed in murine bone marrow-derived macrophages, indicating cross-reactivity of human PF-4 between both species. Taken together, PF-4 may play a crucial role in the induction and maintenance of an unspecific immune response.

Platelet factor 4 modulation of the thrombomodulin–protein C system

OBJECTIVE

To review published studies of the influence of platelet factor 4 (PF4) and other cationic proteins on the generation of activated protein C (APC) by the thrombomodulin-protein C system.

DATA SOURCE

Using the PubMed citation index, literature published from 1973 to 2003 regarding cationic proteins, PF4, and the thrombomodulin-protein C system was reviewed.

DATA SYNTHESIS

All other cationic proteins studied to date either impair or do not affect APC generation via the thrombomodulin-protein C system; however, the platelet alpha-granule protein PF4 causes a 25-fold increase in the ability of thrombomodulin polypeptides to generate APC and a ten-fold increase in the ability of cultured endothelial cell-associated thrombomodulin to generate APC. The mechanism underlying this phenomenon depends on binding of the cationic PF4 to the anionic, vitamin K- dependent gamma-carboxyglutamic acid domain of protein C. The extent of PF4's stimulation of APC generation is further increased by its interaction with the anionic glycosaminoglycan moiety that is variably expressed through posttranslational, O-linked glycosylation of thrombomodulin. In an in vivo thrombin-infusion model of thrombomodulin activation in cynomolgus monkeys, previous intravenous infusion of pharmacologic amounts of PF4 resulted in circulating APC levels and APC-dependent prolongation of activated partial thromboplastin times that were two- to three-fold greater than those observed in saline-infused control animals.

CONCLUSIONS

These findings raise the possibility that PF4 plays a hitherto unsuspected physiologic role in enhancing APC generation in vivo. They also provide a rationale for considering the infusion of PF4 or PF4-related peptides or peptidomimetics as a way of beneficially stimulating "endogenous" APC generation from circulating protein C in pathologic human disease states such as sepsis.

Platelet factor 4 (PF-4)-induced neutrophil adhesion is controlled by src-kinases, whereas PF-4-mediated exocytosis requires the additional activation of p38 MAP kinase and phosphatidylinositol 3-kinase

Among the various chemokines that are functionally active on neutrophils, platelet factor 4 (PF-4; CXCL4) appears to have a specialized role. Lacking typical chemokine activities, PF-4 stimulates neutrophils to undergo firm adhesion to endothelial cells and, in the presence of an appropriate costimulus like tumor necrosis factor (TNF), PF-4 induces exocytosis of secondary granule contents. Analyzing the individual contribution of PF-4 and its costimuli in the control of these functions at the signaling level, we demonstrate that TNF-induced activation of p38 mitogen-activated protein (MAP) kinase (but not extracellular regulated kinase [Erk] kinases) acts as general and essential costimulatory signal in PF-4-dependent neutrophil exocytosis. This was shown by the use of a specific inhibitor (SB203580), by biologic (lipopolysaccharide, N-formyl-methionyl-leucyl-phenylalanine) and pharmacologic (anisomycin) activators of p38 MAP kinase, and by phosphorylation studies. Furthermore, TNF-mediated activation of phosphatidylinositol 3-kinase (PI 3-kinase) represents an additional essential signaling component in this process as demonstrated by studies with its inhibitor wortmannin as well as by analysis of the phosphorylation of AKT kinase. PF-4, however, directly activates src-kinases and PF-4-induced adherence as well as PF-4/TNF-mediated exocytosis was inhibited by an src-kinase inhibitor PP1. Taken together, neutrophil exocytosis and adherence are regulated on p38 MAP kinase, PI 3-kinase, and src-kinase activation.

Molecular targets in the inhibition of angiogenesis

Angiogenesis, the process of blood vessel formation, is crucial for malignant tumour growth and metastases; therefore, it has become an attractive target for anticancer therapy. Theoretically applicable to most solid tumours, this therapy may be advantageous over existing cytotoxic therapy, since it is directed at genetically stable endothelium growing within tumours rather than at malignant cells, which acquire resistance to treatment. Many promising angiogenesis inhibitors have been developed, although their activity has yet to be demonstrated in human clinical trials. To improve therapeutic benefit, this may require further insight into tumour angiogenesis, development of appropriate surrogate markers of activity, treatment of early stage neoplastic disease and probably a combination of different classes of antiangiogenesis agents to overcome redundant mechanisms of angiogenesis control.

Platelet factor 4 enhances generation of activated protein C in vitro and in vivo

Platelet factor 4 (PF4), an abundant platelet alpha-granule protein, accelerates in vitro generation of activated protein C (APC) by soluble thrombin/thrombomodulin (TM) complexes up to 25-fold. To test the hypothesis that PF4 similarly stimulates endothelium-associated TM, we assessed the influence of human PF4 on thrombin-dependent APC generation by cultured endothelial monolayers. APC generated in the presence of 1 to 100 microg PF4 was up to 5-fold higher than baseline for human umbilical vein endothelial cells, 10-fold higher for microvascular endothelial cells, and unaltered for blood outgrowth endothelial cells. In an in vivo model, cynomolgus monkeys (n = 6, each serving as its own control) were infused with either PF4 (7.5 mg/kg) or vehicle buffer, then with human thrombin (1.0 microg/kg/min) for 10 minutes. Circulating APC levels (baseline 3 ng/mL) peaked at 10 minutes, when PF4-treated and vehicle-treated animals had APC levels of 67 +/- 5 ng/mL and 39 +/- 2 ng/mL, respectively (P <.001). The activated partial thromboplastin time (APTT; baseline, 28 seconds) increased maximally by 27 +/- 6 seconds in PF4-treated animals and by 9 +/- 1 seconds in control animals at 30 minutes (P <.001). PF4-dependent increases in circulating APC and APTT persisted more than 2-fold greater than that of controls from 10 through 120 minutes (P < or =.04). All APTT prolongations were essentially reversed by monoclonal antibody C3, which blocks APC activity. Thus, physiologically relevant concentrations of PF4 stimulate thrombin-dependent APC generation both in vitro by cultured endothelial cells and in vivo in a primate thrombin infusion model. These findings suggest that PF4 may play a previously unsuspected physiologic role in enhancing APC generation.

Thrombopoietin-induced CXC chemokines, NAP-2 and PF4, suppress polyploidization and proplatelet formation during megakaryocyte maturation

BACKGROUND

We previously reported that the expressions of two CXC chemokines, neutrophil activating peptide-2 (NAP-2) and platelet factor-4 (PF-4), were induced by megakaryocyte-specific cytokine thrombopoietin (TPO) in mouse bone marrow megakaryocytes. The roles of these chemokines on megakaryocyte maturation/differentiation processes, including polyploidization and proplatelet formation (PPF) remain unresolved.

RESULTS

NAP-2 and PF-4 suppressed the PPF of mature megakaryocytes freshly prepared from mouse bone marrow as well as that of the megakaryocyte progenitors, c-Kit+CD41+ cells, isolated from mouse bone marrow and cultured with TPO. NAP-2 and PF-4 inhibited polyploidization of c-Kit+CD41+ cells in the presence of TPO, and also inhibited the proliferation of c-Kit+CD41+ cells.

CONCLUSIONS

NAP-2 and PF-4 produced by TPO stimulation in megakaryocytes suppress megakaryocyte maturation and proliferation as a feedback control.

Platelet-derived chemokines CXC chemokine ligand (CXCL)7, connective tissue-activating peptide III, and CXCL4 differentially affect and cross-regulate neutrophil adhesion and transendothelial migration

In this study, we have examined the major platelet-derived CXC chemokines connective tissue-activating peptide III (CTAP-III), its truncation product neutrophil-activating peptide 2 (CXC chemokine ligand 7 (CXCL7)), as well as the structurally related platelet factor 4 (CXCL4) for their impact on neutrophil adhesion to and transmigration through unstimulated vascular endothelium. Using monolayers of cultured HUVEC, we found all three chemokines to promote neutrophil adhesion, while only CXCL7 induced transmigration. Induction of cell adhesion following exposure to CTAP-III, a molecule to date described to lack neutrophil-stimulating capacity, depended on proteolytical conversion of the inactive chemokine into CXCL7 by neutrophils. This was evident from experiments in which inhibition of the CTAP-III-processing protease and simultaneous blockade of the CXCL7 high affinity receptor CXCR-2 led to complete abrogation of CTAP-III-mediated neutrophil adhesion. CXCL4 at substimulatory dosages modulated CTAP-III- as well as CXCL7-induced adhesion. Although cell adhesion following exposure to CTAP-III was drastically reduced, CXCL7-mediated adhesion underwent significant enhancement. Transendothelial migration of neutrophils in response to CXCL7 or IL-8 (CXCL8) was subject to modulation by CTAP-III, but not CXCL4, as seen by drastic desensitization of the migratory response of neutrophils pre-exposed to CTAP-III, which was paralleled by selective down-modulation of CXCR-2. Altogether our results demonstrate that there exist multiple interactions between platelet-derived chemokines in the regulation of neutrophil adhesion and transendothelial migration.

[Mechanisms of the effect of circulating neutrophils (granulocytes) in the response of a release reaction of human platelets]

Studied singularities of development platelet release reaction, as of functional activity platelets, at direct effect contents of cytoplasmic granular structures circulating neutrophile leucocytes (granulocytes), possessing a potential capability to selective exocytosis. The model researches were conducted in a system in vitro on isolated viable non active of platelets practically of healthy people. As possible activators the subpopulations them of cytoplasmic granules, distinguished on morphological and enzymetion to indications were applied, previously chosen by methods of an analytical centrifuging from donor neutrophile granulocytes. The development platelets of a response of a release was determined on dynamics quantitative reorganization in platelets an alpha-granules and change of activity in out of platelets to environment of the factor 4 platelets. The capability of a dominance lysosomal initiators neutrophile granulocytes in development of a response of a release of platelets was installed. The scheme of probable trigger participation circulating of neutrophile granulocytes in development syndrome disseminated intravascular coagulation.

Platelet factor 4 inhibits proliferation and cytokine release of activated human T cells

Platelet factor 4 (PF-4), a platelet-derived CXC chemokine, has been shown to induce the differentiation of monocytes into a subset of macrophages that lack the expression of HLA-DR Ag. This suggests a potential role for PF-4 in the modulation of monocyte-dependent T cell activation. Using an Ag-specific stimulation model in which T cells were cocultured with monocytes in the presence of recall Ags, we could show that under these conditions PF-4-treatment caused a strong decrease of T cell proliferation as well as of IFN-gamma release. However, inhibition of T cell functions such as proliferation, IL-2 release, and IL-2 mRNA production did also occur when isolated T cells were activated in the absence of monocytes with immobilized Abs directed against CD3 in combination with cross-linked anti-CD28 Abs. The effect could be reversed when low concentrations of exogenous IL-2 instead of anti-CD28 were used as a costimulus in combination with anti-CD3 Abs. Further evidence for direct modulation of T cell function by PF-4 was obtained by the detection of specific binding sites for the chemokine on the surface of these cells. Taken together, our results show that specific binding of PF-4, resulting in the down-regulation of the IL-2-release correlates with the inhibition of functions in activated T cells.

Heparin-induced thrombocytopenia: pathophysiology and management

Immune-mediated heparin-induced thrombocytopenia (HIT) is a well-defined syndrome. Clinical criteria (thrombocytopenia, resistance to heparin anticoagulation, or thromboses during heparin therapy) are defined, and serologic diagnostic tests are available. Earlier recognition of HIT syndrome has allowed for significant advances in therapy, leading to marked reductions in morbidity and mortality from HIT syndrome. This review addresses the epidemiology, pathobiology, and management of HIT syndrome

Signal transduction of chemokine platelet factor 4 in human erythroleukemia cells

Previous data have demonstrated that CXC-chemokine platelet factor 4 (PF4) inhibits the proliferation of the human erythroleukemia cell line (HEL). However, the mechanism of action is unclear at present. The signaling transduction induced by PF4 in the HEL was compared with that induced by transforming growth factor beta1 (TGF-beta1), which is also a potent inhibitor of HEL growth. It was found that PF4 had no inhibitory effect on intracellular calcium levels in resting HEL cells. When HEL cells were stimulated with interleukin-3 (IL-3), a rapid increase in the intracellular level of free calcium occurred within 15 to 20 seconds, and this increase was followed by a sustained increase that gradually declined until resting levels were reached 30 to 40 minutes later. PF4 dramatically decreased the transient rise of [Ca2+] and protein kinase C (PKC) activity of HEL cells induced by IL-3. However, PF4 had no inhibitory effect on PKC activation in resting HEL cells. Furthermore, PF4 was found to down-regulate significantly protein tyrosine kinase (PTK) activity. In contrast, TGF-beta1 induced an increase in intracellular free calcium concentration and PKC and PTK activity in HEL cells. Furthermore, PF4 significantly increased the messenger RNA (mRNA) level of p21waf1 in HEL cells. These data demonstrate that PF4 acts on HEL cells through a signaling transduction pathway, which is different from that of TGF-beta1 and is related to the up-regulatory mRNA level of p21waf1 in HEL cells.

The role of platelet factor 4 in platelet aggregation induced by the antibodies implicated in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a severe side effect of heparin treatment. Recent studies using immunological methods demonstrated that antibodies contained in plasma, or in purified total immunoglobulin (Ig)G from patients suffering HIT, recognize as target antigen the complex heparin/platelet factor (PF4). In the present study, the role of PF4 in in-vitro platelet aggregation induced by purified total IgG or platelet-poor plasma from patients suffering HIT was investigated. In order to demonstrate the functional role of PF4, an anti-PF4 antibody that specifically blocked PF4 was used. In an experimental system composed of washed platelet suspension, incubation of F(ab')2 fragments (0.125 microg/ml) of the polyclonal anti-PF4 antibody resulted in complete inhibition of platelet aggregation triggered by purified total IgG from patients suffering HIT and heparin. In platelet-rich plasma, a significantly higher concentration (4.25 microg/ml) of the anti-PF4 F(ab')2 was required to inhibit platelet aggregation induced by HIT-PPP and heparin. Intermediate concentrations of the anti-PF4 antibody partially inhibited platelet aggregation. In plasma milieu, the concentration of PF4 was about five-fold higher in comparison with that measured in the purified system. The intensity of platelet aggregation depended on the concentration of HIT-IgG. Platelet aggregation was abolished in the presence of high concentrations of heparin (superior or equal to 10 IU/ml). The present study shows that PF4 is essential for platelet aggregation triggered by the antibodies related to HIT in the presence of heparin. The concentration of PF4 that is available to bind with heparin or with the HIT-related antibodies is critical for platelet aggregation induced by HIT antibodies.

Dual structural requirements for multilineage hematopoietic-suppressive activity of chemokine-derived peptides

Many chemokines have direct suppressive activity in vitro and in vivo on primitive hematopoietic cells. However, few chemokine-derived peptides have shown a significant activity in inhibiting hematopoiesis. Interestingly, a peptide derived from the 34-58 sequence of the CXC chemokine platelet factor 4 (PF4) produced a 30-40% inhibition of proliferation of murine hematopoietic progenitors (CFU-MK, CFU-GM, and BFU-E) in vitro, at concentrations of 30-60-fold lower than PF4. The aim of the present work was to define the structural parameters and motifs involved in conferring biological activity to the peptide PF4(34-58). Both structural predictions and determinations revealed a new helical motif that was further localized between residues 38 and 46. This helix was necessary for binding of the peptide and for permitting the functional DLQ motif at position 54-56 to activate the putative receptor site. Peptides lacking either the helical or the DLQ motif were devoid of inhibitory activity on the hematopoietic progenitors in vitro. However, among inactive peptides, only those having the helical motif counteracted the inhibition induced by the active peptide PF4(34-58). This suggested that the helix might be required for peptide interactions with a putative receptor site, whereas the DLQ motif would be implicated in the activation of this receptor. These results identify for the first time the dual requirements for the design of chemokine-derived peptides with high suppressive activity on hematopoiesis, as well as for the design of molecules with antagonistic action.

The role of cytokines in monocyte apoptosis

Survival or apoptosis, activation and differentiation, phagocytosis and antigen presentation, migration or participation in granuloma formation are features of freshly recruited blood-borne monocytes in the local environment. In this presentation we describe that human monocytes undergo spontaneous apoptosis in vitro which involves Fas/FasL interactions, and that proinflammatory cytokines such as tumor necrosis factor-alpha (TNFalpha), interleukin-1beta and granulocyte-monocyte-colony-stimulating factor prevent spontaneous apoptosis. In vitro infection of purified monocytes with low numbers of Mycobacterium tuberculosis H37Rv prevents spontaneous apoptosis. The apoptosis-preventing effect is correlated to the release of TNFalpha and not due to phagocytosis per se. Furthermore, the minor subset of CD64-negative monocytes is found to be less susceptible to recall antigen-activated CD4-positive T cell-mediated apoptosis than CD64-positive monocytes. Finally, recent findings of our group indicate that the chemokine platelet factor 4 protects monocytes from spontaneous apoptosis and induces the differentiation of monocytes into macrophages. From these findings we conclude that monocyte recruitment, their survival, their differentiation and their functional activity at the site of inflammation are regulated by a cytokine network which needs to be further analyzed in order to design strategies for immune intervention.

The CXC-chemokine platelet factor 4 promotes monocyte survival and induces monocyte differentiation into macrophages

Unstimulated monocytes rapidly undergo physiological changes resulting in programmed cell death (apoptosis) while stimuli promoting differentiation of these cells into macrophages were shown to inhibit apoptotic processes. In the present study, we report that the platelet-derived alpha-chemokine platelet factor 4 (PF4) induces the differentiation of monocytes into macrophages, as is evident from morphological changes as well as from the up-regulation of differentiation markers (carboxypeptidase M/MAX1 and CD71). Significant alterations of the phenotype were observed after 72 hours of culture in the presence of the chemokine and required a minimal concentration of 625 nmol/L PF4. PF4-induced macrophages were characterized by a lack of HLA-DR antigen on their surface but showed a strong increase in the expression of the CD28 ligand B7-2. Furthermore, PF4 stimulation prevented monocytes from undergoing spontaneous apoptosis during 72 hours of culture as determined in an annexin-V-binding assay. Although PF4 induced the secretion of relevant amounts of TNF-alpha, neutralizing antibodies directed against TNF-alpha or granulocyte-macrophage colony-stimulating factor (GM-CSF) did not revert PF4-induced rescue from programmed cell death, suggesting that PF4 exerts its antiapoptotic effects in a TNF-alpha- or GM-CSF-independent fashion. On the basis of these results, we propose a novel role for PF4 in the control of monocyte differentiation during an inflammatory process in vivo. (Blood. 2000;95:1158-1166)

Regulation of megakaryocytopoiesis

After 35 years of research, a physiological regulator of platelet production has been identified and the recombinant protein is available. With the discovery of thrombopoietin (TPO), its potential use in a wide variety of clinical megakaryocytic and platelet disorders has been expected and clinical trials have been undertaken. To date, the reported encouraging pre-clinical studies indicate that, as with erythropoietin or G-CSF, minimal toxicity can be expected. A potential limiting side-effect of TPO could be the induction of thrombosis. Nevertheless, it is too early to know whether this cytokine will be of major therapeutic importance for patients with life-threatening thrombocytopenia, such as patients undergoing bone marrow transplantation or subjected to a high dose of chemotherapy. Several experimental and clinical studies are still needed to determine the efficacy of TPO in the prevention or the amelioration of bleeding, which is the ultimate goal for the appropriate use of cytokines with haemostatic benefit. Basic and clinical studies on regulators of megakaryocytopoiesis have rapidly progressed. Now, there is no doubt that some of these regulators are effective in correcting haematopoietic disorders of various aetiologies. Studies on negative regulators not only are important to understand the regulation of megakaryocytopoiesis in normal and pathological states but also have a potential clinical application. Some of these regulators have been shown to be effective in the treatment of essential thrombocythaemia and other myeloproliferative disorders. Platelet factor 4 (PF4) and some other chemokines are also capable of protecting progenitor cells from the cytotoxicity of chemotherapeutic drugs. However, detailed investigations are still required to determine the precise mechanism(s) of action of these regulators and to establish the optimal clinical protocols of negative regulators alone or in association with positive regulators for the treatment of various haematological diseases and cancer.

Heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia with or without thrombosis has been recognized increasingly as a serious complication of heparin use. This article reviews type II heparin-induced thrombocytopenia, which is mediated by an antibody that in most cases has specificity for a complex between heparin and platelet factor 4, a secreted platelet alpha-granule protein. The antibody-heparin-platelet factor 4 complex can activate platelets and endothelial cells, thereby initiating thrombosis. Clinical thrombosis in this syndrome may be arterial or venous. Treatment of the syndrome requires discontinuation of heparin and institution of an alternative anticoagulant.

Inhibition of human umbilical vein endothelial cell proliferation by the CXC chemokine, platelet factor 4 (PF4), is associated with impaired downregulation of p21(Cip1/WAF1)

Human PF4 is a heparin-binding chemokine known to be capable of inhibiting endothelial cell proliferation and angiogenesis. To explore the biological mechanisms responsible for this action, we investigated the effect of PF4 on epidermal growth factor (EGF)-stimulated human umbilical vein endothelial cells (HUVEC), a model system in which stimulation is essentially independent of interaction with cell-surface glycosaminoglycans. Based on previous findings that PF4 blocks endothelial cell cycle entry and progression into S phase, we studied the molecular mechanism(s) of PF4 interference with cell cycle machinery. PF4 treatment of EGF-stimulated HUVEC caused a decrease in cyclin E-cyclin-dependent kinase 2 (cdk2) activity with resulting attenuation of retinoblastoma protein phosphorylation. PF4-dependent downregulation of cyclin E-cdk2 activity was associated with increased binding of the cyclin-dependent kinase inhibitor, p21(Cip1/WAF1), to the cyclin E-cdk2 complex. Analysis of total cellular p21(Cip1/WAF1) showed that in the presence of PF4, p21(Cip1/WAF1) levels were sustained at time points when p21(Cip1/WAF1) was no longer detectable in cells stimulated by EGF in the absence of PF4. These findings indicate that PF4 inhibition of HUVEC proliferation in response to EGF is associated with impaired downregulation of p21(Cip1/WAF1) and provide the first evidence for interference with cell cycle mechanisms by a chemokine.

Heparin-induced thrombocytopenia: diagnostic tests and biological mechanisms

The discovery of the role of PF4 in the development and pathogenicity of heparin-dependent antibodies which trigger heparin-induced thrombocytopenia (HIT), a rare but severe adverse effect of heparin therapy, has allowed us to revisit the diagnosis of this complication and the pathological mechanisms involved. In this review, diagnostic tests available for confirmation or prediction of HIT are analysed, and new diagnostic strategies are discussed. Factors involved in the development of the heparin-dependent immune response in some heparin-treated patients are then presented. Lastly, it is hypothesized that the presence of antibodies is a risk factor for HIT. The mechanisms which contribute to the development of complications and the role of additional risk factors, including the patient's clinical state and the type of heparin used, are discussed (Magnani, 1993).

Heparin-induced thrombocytopenia: pathophysiology and clinical concerns

Heparin-induced thrombocytopenia (HIT) is a severe immunological adverse effect of heparin treatment. Recently the pathogenesis of HIT has been resolved regarding the mechanisms of platelet activation, the nature of the most important antigens and the involvement of the clotting cascade. HIT seems to be associated with massive generation of thrombin, which contributes to the thromboembolic complications. Based on these findings, treatment of patients with acute HIT should include cessation of all heparins and further treatment with an anticoagulant with antithrombin activity. Currently, the two most important compounds for further anticoagulation of HIT-patients are danaparoid-sodium and recombinant hirudin.