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

Abstract

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. IntroductionMegakaryopoiesis is a complex process that is still not fully understood. Early studies identified thrombopoietin (TPO) as the predominant cytokine responsible for regulating platelet counts. However, many other cytokines have been postulated to participate in regulating megakaryopoiesis by increasing TPO expression in the liver (eg,), enhancing megakaryocyte chemotaxis (eg, stromal-derived factor-1 [SDF-1]), 1 or directly stimulating megakaryocyte development (eg, IL-11). 2 A pathway by which megakaryopoiesis is auto-down-regulated has been suggested based on in vitro studies of platelet factor 4 (PF4) and later by studies of other chemokines that are also stored in ␣-granules, including the related CXC chemokines, neutrophil activating peptide-2 (NAP-2), and 3,4 and the more distantly related CC chemokines, RANTES (regulated upon activation, normal T-cell-expressed and secreted), 5 and MIP-1␣ (macrophage inflammatory peptide-1␣). 4,5 PF4 is a 7.8-kDa protein that is produced primarily in megakaryocytes and expressed in platelets as a tetramer, where it comprises a significant portion of the content of ␣-granules (2.5% on a molar basis). 6 The biological role of PF4 is not fully understood. Unlike other chemokines that have clearly defined chemokine receptors, PF4 appears to function by binding with high affinity to glycosaminoglycans on cell surfaces. 7-9 PF4 has been proposed to participate in many important biological process based primarily on in vitro studies, including roles in angiogenesis, 10 inflammation, 11 atherosclerosis, 12,13 thrombosis, [14][15][16][17] and megakaryopoiesis. 4,5,18 Studies...

Thrombocytopenia: Diagnostic Tool of Neonatal Infection?

“…In neonatological literature the above is explained by the fact that hypoxia in the newborn triggers abundant erythropoesis, which reduces the amount of progenitor cells needed for differentiation of megakaryocytes into thrombocytes [12]. The study implemented by Aman and Hassan showed that 40.7% newborn had some kind of neonatal infection [13], which is similar to our results, according to which 38.2% newborn had an infection. The mechanism that leads from infection to thrombocytopenia development has not been sufficiently clarified yet, but a research conducted by author Lambert et al suggests that the answer hides in increased production of thrombocyte factor 4 which is emitted by activated thrombocytes during a septic condition [14].…”

The importance of calcium in the regulation of megakaryocyte function

“…5,6 With regard to this, we demonstrated that ADP is constitutively released by human Mks in vitro and that it is a crucial molecule in the regulation of platelet production.…”

“…1,3,4 Moreover, early during megakaryopoiesis, Mks develop platelet-specific granules, which release their contents supporting Mk development. 5,6 Among these, we recently demonstrated that Mks constitutively release adenosine diphosphate (ADP) which promotes proplatelet formation by interacting with its receptor P2Y 13 . 7 Most importantly, we described that patients with DeltaStorage Pool Deficiency (δ-SPD), a congenital bleeding diathesis characterized by deficiency of dense granules and their constituents (including ADP) in Mks and platelets, display in vivo a significantly higher prevalence of thrombocytopenia than those observed in other disorders of primary hemostasis.…”

PF-4var/CXCL4L1 predicts outcome in stable coronary artery disease patients with preserved left ventricular function

“…At high concentration (.1 mg/ml) PF-4 inhibits megakaryopoiesis and functions as negative autocrine regulator in vitro [21] and in vivo [22]. Further, PF-4 has pro-thrombotic activity by binding heparin, but it can also function as an anti-coagulant via the generation of activated protein C [23].…”

“…Median levels of PF-4var were significantly lower in men as compared to women (10 [7-15] ng/ml versus 14 [10][11][12][13][14][15][16][17][18][19][20][21][22] ng/ml, p,0.01). No significant correlations were noted between PF-4var and sTNFRI, sTNFRII, number of circulating white blood cells, lymphocytes, granulocytes or monocytes.…”