Azacitidine fails to eradicate leukemic stem/progenitor cell populations in patients with acute myeloid leukemia and myelodysplasia

Epigenetic therapies demonstrate significant clinical activity in acute myeloid leukemia (AML) and myelodysplasia (MDS) and constitute an important new class of therapeutic agents. However hematological responses are not durable and disease relapse appears inevitable. Experimentally, leukemic stem/progenitor cells (LSC) propagate disease in animal models of AML and it has been postulated that their relative chemo-resistance contributes to disease relapse. We serially measured LSC numbers in patients with high-risk AML and MDS treated with 5'-azacitidine and sodium valproate (VAL-AZA). Fifteen out of seventy-nine patients achieved a complete remission (CR) or complete remission with incomplete blood count recovery (CRi) with VAL-AZA therapy. There was no significant reduction in the size of the LSC-containing population in non-responders. While the LSC-containing population was substantially reduced in all patients achieving a CR/CRi it was never eradicated and expansion of this population antedated morphological relapse. Similar studies were performed in seven patients with newly diagnosed AML treated with induction chemotherapy. Eradication of the LSC-containing population was observed in three patients all of whom achieved a durable CR in contrast to patients with resistant disease where LSC persistence was observed. LSC quantitation provides a novel biomarker of disease response and relapse in patients with AML treated with epigenetic therapies. New drugs that target this cellular population in vivo are required.

Genetic and pharmacological analyses of involvement of Src-family, Syk and Btk tyrosine kinases in platelet shape change. Src-kinases mediate integrin alphaIIb beta3 inside-out signalling during shape change

Platelet shape change was found to be associated with an increase in protein tyrosine phosphorylation upon stimulation of thrombin-, ADP- and thromboxane A2-G-protein coupled receptors in human platelets and thromboxane A2 receptors in mouse platelets. By using PP1 and PD173956, two structurally unrelated specific inhibitors of Src-family tyrosine kinases, and mouse platelets deficient in the Src-kinase Fyn or Lyn, we show that Src-family kinases cause the increase in protein tyrosine phosphorylation. We further detected that the non-Src tyrosine kinase Syk was activated during shape change in a manner dependent on Src-family kinaseactivation. The pharmacological experiments and the studies on Fyn-, Lyn- and Syk-deficient mouse platelets showed that neither Src-family kinases nor Syk are functionally involved in shape change. Also human platelets deficient of the tyrosine kinase Btk showed a normal shape change. Binding of PAC-1 that recognizes activated integrin alphaIIb beta3 complexes on the platelet surface was enhanced during shape change and blocked by inhibition of Src-kinases. We conclude that the activation of Src-kinases and the subsequent Syk stimulation upon activation of G-protein coupled receptors are not involved in the cytoskeletal changes underlying shape change of human and mouse platelets, but that the stimulation of this evolutionary conserved pathway leads to integrin alphaIIb beta3 exposure during shape change.

Evidence of a role for SHP-1 in platelet activation by the collagen receptor glycoprotein VI

The Src homology (SH)2 domain-containing protein-tyrosine phosphatase SHP-1 is tyrosine phosphorylated in platelets in response to the glycoprotein VI (GPVI)-selective agonist collagen-related peptide (CRP), collagen, and thrombin. Two major unidentified tyrosine-phosphorylated bands of 28 and 32 kDa and a minor band of 130 kDa coprecipitate with SHP-1 in response to all three agonists. Additionally, tyrosine-phosphorylated proteins of 50-55 and 70 kDa specifically associate with SHP-1 following stimulation by CRP and collagen. The tyrosine kinases Lyn, which exists as a 53 and 56-kDa doublet, and Syk were identified as major components of these bands, respectively. Kinase assays on SHP-1 immunoprecipitates performed in the presence of the Src family kinase inhibitor PP1 confirmed the presence of a Src kinase in CRP- but not thrombin-stimulated cells. Lyn, Syk, and SLP-76, along with tyrosine-phosphorylated 28-, 32-, and 130-kDa proteins, bound selectively to a glutathione S-transferase protein encoding the SH2 domains of SHP-1, suggesting that this is the major site of interaction. Platelets isolated from motheaten viable mice (mev/mev) revealed the presence of a heavily tyrosine-phosphorylated 26-kDa protein that was not found in wild-type platelets. CRP-stimulated mev/mev platelets manifested hypophosphorylation of Syk and Lyn and reduced P-selectin expression relative to controls. These observations provide evidence of a functional role for SHP-1 in platelet activation by GPVI.

Phosphatidylinositol 3,4,5-trisphosphate regulates Ca(2+) entry via btk in platelets and megakaryocytes without increasing phospholipase C activity

The role of phosphatidylinositol 3,4,5-trisphosphate (PI3,4,5P(3)) and Btk in signalling by the collagen receptor glycoprotein VI was investigated. PI3,4,5P(3) was increased in platelets from mice deficient in the SH2 domain-containing inositol 5-phosphatase (SHIP), in response to collagen related peptide (CRP). Tyrosine phosphorylation and activation of phospholipase Cgamma2 (PLCgamma2) were unaltered in SHIP(-/-) platelets, whereas Btk was heavily tyrosine phosphorylated under basal conditions and maximally phosphorylated by low concentrations of CRP. There was an increase in basal Ca(2+), maximal expression of P-selectin, and potentiation of Ca(2+) and aminophospholipid exposure to CRP in SHIP(-/-) platelets in the presence of Ca(2+) (1 mM). Microinjection of PI3,4, 5P(3) into megakaryocytes caused a 3-fold increase in Ca(2+) in response to CRP, which was absent in X-linked immunodeficiency (Xid) mice, which have a mutation in the PH domain of Btk. There was a corresponding partial reduction in the sustained level of intracellular Ca(2+) in response to CRP in Xid mice but no change in PLC activity. These results demonstrate a novel pathway of Ca(2+) entry that involves PI3,4,5P(3) and Btk, and which is independent of increased PLC activity.

Thrombopoietin potentiates collagen receptor signaling in platelets through a phosphatidylinositol 3-kinase-dependent pathway

Collagen activates platelets through a tyrosine kinase-dependent pathway, involving phospholipase Cgamma2. Functional responses such as aggregation and secretion induced by collagen are potentiated by preincubation with thrombopoietin (TPO). In this study, we show that collagen and thrombopoietin activate the phosphatidylinositol 3-kinase (PI 3-kinase) pathway and that this contributes to their respective actions. The structurally distinct inhibitors of PI 3-kinase, wortmannin, and LY294002, completely inhibit formation of phosphatidylinositol 3,4,5-trisphosphate by collagen. This leads to a substantial reduction in the formation of inositol phosphates and phosphatidic acid, 2 indices of PLC activity, and the consequent inhibition of intracellular Ca(++) [Ca(++)](i), aggregation and secretion. Potentiation of the collagen response by TPO is prevented in the presence of wortmannin and LY294002. However, when the 2 PI 3-kinase inhibitors are given after the addition of TPO but before the collagen, recovery of potentiation is observed. This suggests that potentiation is mediated through activation of PI 3-kinase. TPO stimulates aggregation of platelets from a low percentage of donors and this is also blocked by wortmannin. These results suggest that the PI 3-kinase pathway plays an important role in signaling by collagen and in the priming action of TPO.

Regulation and function of WASp in platelets by the collagen receptor, glycoprotein VI

Wiskott Aldrich syndrome (WAS) is an X-linked recessive disorder associated with abnormalities in platelets and lymphocytes giving rise to thrombocytopenia and immunodeficiency. WAS is caused by a mutation in the gene encoding the cytoskeletal protein (WASp). Despite its importance, the role of WASp in platelet function is not established. WASp was recently shown to undergo tyrosine phosphorylation in platelets after activation by collagen, suggesting that it may play a selective role in activation by the adhesion molecule. In the present study, we show that WASp is heavily tyrosine phosphorylated by a collagen-related peptide (CRP) that binds to the collagen receptor glycoprotein (GP) VI, but not to the integrin alpha2beta1. Tyrosine phosphorylation of WASp was blocked by Src family kinase inhibitors and reduced by treatment with wortmannin and in patients with X-linked agammaglobulinemia (XLA), a condition caused by a lack of functional expression of Btk. This indicates that Src kinases, phosphatidylinositol 3-kinase (PI 3-kinase), and Btk all contribute to the regulation of tyrosine phosphorylation of WASp. The functional importance of WASp was investigated in 2 WAS brothers who show no detectable expression of WASp. Platelet aggregation and secretion from dense granules induced by CRP and thrombin was slightly enhanced in the WAS platelets relative to controls. Furthermore, there was no apparent difference in morphology in WAS platelets after stimulation by these agonists. These observations suggest that WASp does not play a critical role in intracellular signaling downstream of tyrosine kinase-linked and G protein-coupled receptors in platelets.

LAT is required for tyrosine phosphorylation of phospholipase cgamma2 and platelet activation by the collagen receptor GPVI

In the present study, we have addressed the role of the linker for activation of T cells (LAT) in the regulation of phospholipase Cgamma2 (PLCgamma2) by the platelet collagen receptor glycoprotein VI (GPVI). LAT is tyrosine phosphorylated in human platelets heavily in response to collagen, collagen-related peptide (CRP), and FcgammaRIIA cross-linking but only weakly in response to the G-protein-receptor-coupled agonist thrombin. LAT tyrosine phosphorylation is abolished in CRP-stimulated Syk-deficient mouse platelets, whereas it is not altered in SLP-76-deficient mice or Btk-deficient X-linked agammaglobulinemia (XLA) human platelets. Using mice engineered to lack the adapter LAT, we showed that tyrosine phosphorylation of Syk and Btk in response to CRP was maintained in LAT-deficient platelets whereas phosphorylation of SLP-76 was slightly impaired. In contrast, tyrosine phosphorylation of PLCgamma2 was substantially reduced in LAT-deficient platelets but was not completely inhibited. The reduction in phosphorylation of PLCgamma2 was associated with marked inhibition of formation of phosphatidic acid, a metabolite of 1,2-diacylglycerol, phosphorylation of pleckstrin, a substrate of protein kinase C, and expression of P-selectin in response to CRP, whereas these parameters were not altered in response to thrombin. Activation of the fibrinogen receptor integrin alpha(IIb)beta(3) in response to CRP was also reduced in LAT-deficient platelets but was not completely inhibited. These results demonstrate that LAT tyrosine phosphorylation occurs downstream of Syk and is independent of the adapter SLP-76, and they establish a major role for LAT in the phosphorylation and activation of PLCgamma2, leading to downstream responses such as alpha-granule secretion and activation of integrin alpha(IIb)beta(3). The results further demonstrate that the major pathway of tyrosine phosphorylation of SLP-76 is independent of LAT and that there is a minor, LAT-independent pathway of tyrosine phosphorylation of PLCgamma2. We propose a model in which LAT and SLP-76 are required for PLCgamma2 phosphorylation but are regulated through independent pathways downstream of Syk.

A novel inhibitory action of wheat germ agglutinin on phospholipase C in HEL and MEG-01 cell lines

Stimulation of HEL megakaryocytic cells by Fc gammaRIIA crosslinking is associated with tyrosine phosphorylation of syk and phospholipase C gamma2 (PLCgamma2) and is accompanied by formation of inositol phosphates and release of intracellular Ca2+. These responses are inhibited by the kinase inhibitors, staurosporine and ST271. In contrast, the G-protein receptor agonist, thrombin induces formation of inositol phosphates and release of intracellular calcium without an increase in tyrosine phosphorylation. The plant lectin wheat germ agglutinin (WGA) stimulates tyrosine phosphorylation of syk and PLCgamma2 but surprisingly does not stimulate formation of inositol phosphates and induce release of intracellular Ca2+. WGA also inhibited formation of inositol phosphates and release of intracellular Ca2+ by Fc gammaRIIA crosslinking and thrombin-stimulation. A similar inhibitory effect of WGA was observed against elevation of Ca2+ by the same two stimuli in MEG-01 megakaryotic cells. The results demonstrate a novel pathway of inhibition of PLC on crosslinking of cell surface proteins that is not present in platelets.