Mpl ligand increases P2Y1 receptor gene expression in megakaryocytes with no concomitant change in platelet response to ADP

Blood cells: an historical account of the roles of purinergic signalling

The involvement of purinergic signalling in the physiology of erythrocytes, platelets and leukocytes was recognised early. The release of ATP and the expression of purinoceptors and ectonucleotidases on erythrocytes in health and disease are reviewed. The release of ATP and ADP from platelets and the expression and roles of P1, P2Y(1), P2Y(12) and P2X1 receptors on platelets are described. P2Y(1) and P2X(1) receptors mediate changes in platelet shape, while P2Y(12) receptors mediate platelet aggregation. The changes in the role of purinergic signalling in a variety of disease conditions are considered. The successful use of P2Y(12) receptor antagonists, such as clopidogrel and ticagrelor, for the treatment of thrombosis, myocardial infarction and stroke is discussed.

A new role for the A2b adenosine receptor in regulating platelet function

BACKGROUND

Activation of platelets is a critical component of atherothrombosis and plays a central role in the progression of unstable cardiovascular syndromes. Adenosine, acting through adenosine receptors, increases intracellular cAMP levels and inhibits platelet aggregation. The A2a adenosine receptor has already been recognized as a mediator of adenosine-dependent effects on platelet aggregation, and here we present a new role for the A2b adenosine receptor (A2bAR) in this process.

METHODS AND RESULTS

As compared with platelets from wild-type controls, platelets derived from A2bAR knockout mice have significantly greater ADP receptor activation-induced aggregation. Although mouse megakaryocytes and platelets express low levels of the A2bAR transcript, this gene is highly upregulated following injury and systemic inflammation in vivo. Under these conditions, A2bAR-mediated inhibition of platelet aggregation significantly increases. Our studies also identify a novel mechanism by which the A2bAR could regulate platelet aggregation; namely, ablation of the A2bAR leads to upregulated expression of the P2Y1 ADP receptor, whereas A2bAR-mediated or direct elevation of cAMP has the opposite effect. Thus, the A2bAR regulates platelet function beyond mediating the immediate effect of adenosine on aggregation.

CONCLUSIONS

Taken together, these investigations show for the first time that the platelet A2bAR is upregulated under stress in vivo, plays a significant role in regulating ADP receptor expression, and inhibits agonist-induced platelet aggregation.

Pegylated recombinant human megakaryocyte growth and development factor suppresses the development of megakaryoblastic leukemia in mice

We examined the effects of pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) on the development of L-8057, a murine megakaryoblastic leukemia that expresses the thrombopoietin receptor c-Mpl, in mice. PEG-rHuMGDF administration prolonged survival of L-8057 leukemic mice, in which L-8057 cell growth in the spleen was decreased. L-8057 cells harvested from PEG-rHuMGDF-treated leukemic mice had decreased ability to generate leukemic colonies in vitro as well as to induce leukemia in vivo. PEG-rHuMGDF administration also resulted in prolonged survival of mice transplanted with a c-Mpl-expressing erythroleukemia, but had no effect on survival of mice transplanted with a myeloblastic leukemia that does not possess c-Mpl. Thus, PEG-rHuMGDF suppresses the development of c-Mpl-expressing leukemia in vivo in mice.

Transcriptional down-regulation of the platelet ADP receptor P2Y(12) and clusterin in patients with systemic lupus erythematosus

Cardiovascular complications are common in systemic lupus erythematosus (SLE) and myocardial infarctions are the leading cause of increased mortality. The ADP receptor P2Y(12) plays a central role in platelet activation and the P2Y(12) blocker clopidogrel reduces the incidence of cardiovascular events. Clusterin, a complement inhibitory protein suggested to be involved in the pathogenesis of SLE, has been found recently in a microarray study to be expressed at very high levels in platelets. Using a new protocol for mRNA quantification in platelets we set out to study if gene expression is altered in SLE patients compared with a healthy control group. Quantitative assay based on real-time PCR was used to measure mRNA expression, Western blot for P2 receptor protein expression and PFA-100 for platelet aggregation. The P2Y(12) receptor expression was decreased in SLE compared to the controls (P < 0.05), while expression of P2Y(1) and P2X(1) were unaltered. These findings were consistent at the protein level. The clusterin mRNA expression was very high. However, SLE patients had significantly lower levels than controls (P < 0.05). Platelet aggregation was similar in both groups. It may be suggested that a decreased level of P2Y(12) receptors could represent a protective response in SLE against thrombotic complications. Lowered clusterin levels could be involved in the pathogenesis of SLE due to decreased protective effects. These findings could help to achieve a better understanding of the platelet function in SLE and serve as a guide for further research and drug use.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

A selective effect of Mpl ligand on mRNA stabilization during megakaryocyte differentiation

Megakaryocytes, the platelet precursors, are induced to differentiate in response to Mpl ligand. Here we report that stability of the megakaryocyte-specific platelet factor 4 (PF4) mRNA is substantially augmented in the presence of Mpl ligand. This stabilization requires protein synthesis, but the 3'-untranslated region of PF4 mRNA is not sufficient for granting the effect. This cytokine also significantly or mildly stabilizes Mpl receptor or GAPDH mRNAs, respectively, in contrast to a previously reported lack of effect on P2Y(1) receptor mRNA. Our study is the first to suggest that Mpl ligand-induced lineage specification is also determined by message stabilization.