Platelets in liver disease, cancer and regeneration

Although viral hepatitis treatments have evolved over the years, the resultant liver cirrhosis still does not completely heal. Platelets contain proteins required for hemostasis, as well as many growth factors required for organ development, tissue regeneration and repair. Thrombocytopenia, which is frequently observed in patients with chronic liver disease (CLD) and cirrhosis, can manifest from decreased thrombopoietin production and accelerated platelet destruction caused by hypersplenism; however, the relationship between thrombocytopenia and hepatic pathogenesis, as well as the role of platelets in CLD, is poorly understood. In this paper, experimental evidence of platelets improving liver fibrosis and accelerating liver regeneration is summarized and addressed based on studies conducted in our laboratory and current progress reports from other investigators. In addition, we describe our current perspective based on the results of these studies. Platelets improve liver fibrosis by inactivating hepatic stellate cells, which decreases collagen production. The regenerative effect of platelets in the liver involves a direct effect on hepatocytes, a cooperative effect with liver sinusoidal endothelial cells, and a collaborative effect with Kupffer cells. Based on these observations, we ascertained the direct effect of platelet transfusion on improving several indicators of liver function in patients with CLD and liver cirrhosis. However, unlike the results of our previous clinical study, the smaller incremental changes in liver function in patients with CLD who received eltrombopag for 6 mo were due to patient selection from a heterogeneous population. We highlight the current knowledge concerning the role of platelets in CLD and cancer and anticipate a novel application of platelet-based clinical therapies to treat liver disease.

Grafting of thrombopoietin-mimetic peptides into cystine knot miniproteins yields high-affinity thrombopoietin antagonists and agonists

Thrombopoietin is the primary regulator of platelet production. We exploited two naturally occurring miniproteins of the inhibitor cystine knot family as stable and rigid scaffolds for the incorporation of peptide sequences that have been shown to act as high-affinity thrombopoietin antagonists. Several miniproteins that antagonistically block thrombopoietin-mediated receptor activation were identified using a microscale reporter assay. Covalent miniprotein dimerization yielded potent bivalent c-Mpl receptor agonists with EC(50) values in the low nanomolar or picomolar range. One selected miniprotein-derived thrombopoietin agonist was almost as active as natural thrombopoietin with regard to stimulation of megakaryocyte colony formation from human bone marrow mononuclear cells, and elicited doubling of platelet counts in mice. Our data suggest that dimeric cystine knot miniproteins have considerable potential for the future development of small and stable receptor agonists. This approach may provide a promising strategy for pharmaceutical interference with other receptors activated by ligand-induced dimerization.

Thrombocytopenia in patients with myelodysplastic syndromes

Thrombocytopenia, common in leukemias and myelodysplastic syndromes (MDS), is responsible for increased risk of bleeding and delay of therapy. Platelet transfusions, although effective in increasing platelet counts, are limited by supply, are associated with risks, and result in limited and transient benefits. Successful development of an alternative treatment approach with thrombopoietin agonists was nearly thwarted when early formulations of recombinant thrombopoietin agonists elicited antibodies that cross-reacted with and neutralized endogenous thrombopoietin. The effectiveness of these recombinant agents led to the development of second-generation thrombopoietin receptor agonists that do not induce cross-reacting neutralizing antibodies against thrombopoietin. Two of the novel thrombopoietin receptor agonists, romiplostim and eltrombopag, have established clinical activity in chronic immune (idiopathic) thrombocytopenic purpura (ITP), and are being explored for the treatment of thrombocytopenia in MDS.

Promegapoietin-1a, an engineered chimeric IL-3 and Mpl-L receptor agonist, stimulates hematopoietic recovery in conventional and abbreviated schedules following radiation-induced myelosuppression in nonhuman primates

Promegapoietin-1a (PMP-1a), a multifunctional agonist for the human interleukin 3 and Mpl receptors, was evaluated for its ability to stimulate hematopoietic reconstitution in nonhuman primates following severe radiation-induced myelosuppression. Animals were total body x-irradiated (250 kVp) to 600 cGy total midline tissue dose. PMP-1a was administered s.c. in several protocols: A) daily (50 microg/kg) for 18 days; B) nine doses (5 microg/kg) every other day for 3 weeks; C) a single high dose (100 microg/kg) at 20 hours, or D) a single high dose (100 microg/kg) at 1 hour following TBI. The irradiation controls received 0.1% autologous serum for 18 consecutive days. Hematopoietic recovery was assessed by bone marrow clonogenic activity, peripheral blood cell nadirs, duration of cytopenias, time to recovery to cellular thresholds, and requirements for clinical support. PMP-1a, irrespective of administration schedule, significantly improved all platelet-related parameters: thrombocytopenia was eliminated, the severity of platelet nadirs was significantly improved, and recovery of platelet counts to > or =20,000/miccrol was significantly reduced in all PMP-1a-treated cohorts. As a consequence, all PMP-1a-treated cohorts were transfusion-independent. Neutrophil regeneration was augmented in all treatment schedules. Additionally, all PMP-1a-treated cohorts showed an improvement in red blood cell nadir and recovery. PMP-1a in conventional or abbreviated schedules induced significant thrombopoietic regeneration relative to the control cohort, whereas significant improvement in neutrophil recovery was schedule-dependent in radiation-myelosuppressed nonhuman primates.

Thrombopoietin potentiates the protein-kinase-C-mediated activation of mitogen-activated protein kinase/ERK kinases and extracellular signal-regulated kinases in human platelets

The thrombopoietin (TPO) receptor is expressed in the megakaryocytic lineage from late progenitors to platelets. We investigated the effect of TPO on the extracellular signal-regulated kinase (ERK) activation pathway in human platelets. TPO by itself did not activate ERK1, ERK2 and protein kinase C (PKC), whereas TPO directly enhanced the PKC-dependent activation of ERKs induced by other agonists including thrombin and phorbol esters, without affecting the PKC activation by those agonists. TPO did not activate the mitogen-activated protein kinase/ERK kinases, MEK1 and MEK2, but activated Raf-1 and directly augmented the PKC-mediated MEK activation, suggesting that TPO primarily potentiates the ERK pathway through regulating MEKs or upstream steps of MEKs including Raf-1. The MEK inhibitor PD098059 failed to affect not only thrombin-induced or phorbol ester-induced aggregation, but also potentiation of aggregation by TPO, denying the primary involvement of ERKs and MEKs in those events. ERKs and MEKs were located mainly in the detergent-soluble/non-cytoskeletal fractions. ERKs but not MEKs were relocated to the cytoskeleton following platelet aggregation and actin polymerization. These data indicate that TPO synergizes with other agonists in the ERK activation pathway of platelets and that this synergy might affect functions of the cytoskeleton possibly regulated by ERKs.

Bleeding and mortality outcomes in ITP clinical trials: a review of thrombopoietin mimetics data

Patients with ITP may have severe thrombocytopenia, putting them at risk for serious bleeding. ITP trials of new treatments must allow use of standard-of-care therapies to prevent serious bleeding. Thrombopoietin mimetic trials used platelet counts and rescue/concomitant medication use as endpoints. These trials were of insufficient size and duration to measure mortality or serious bleeding, which are infrequent with appropriate treatment. A recent Cochrane review criticized the thrombopoietin mimetic registrational trials for inadequately assessing bleeding and survival. We discuss how these endpoints are difficult to measure in clinical trials designed to improve platelet counts and minimize bleeding, in accordance with ethical trial design.

Successful eltrombopag treatment of severe refractory thrombocytopenia in chronic myelomonocytic leukemia: Two cases reports: A CARE-compliant article

RATIONALE

Thrombocytopenia in chronic myelomonocytic leukemia (CMML) is usually attributed to impaired marrow production resulting from cytotoxic drug use or CMML itself ("CMML-induced thrombocytopenia"). In very rare cases, immune thrombocytopenia (ITP) can be a complication of CMML ("CMML-associated ITP"). However, treatment of severe thrombocytopenia in patients with CMML is still a challenge.

PATIENT CONCERNS

Case 1 was a 61-year-old female patient admitted to our hospital because of skin petechiae and purpura for 6 days. She had increased monocyte cell count (1.82 × 10/L), markedly decreased platelet count (2 × 10/L), hypercellularity of the megakaryocyte lineage with many immature megakaryocytes, and ZRSR2(zinc finger CCCH-type, RNA binding motif and serine/arginine rich 2) mutation. She failed to the treatment of corticosteroids, intravenous immunoglobulin (IVIg), TPO (thrombopoietin), and cyclosporin A (CsA). Case 2 was a 72-year-old female patient with thrombocytosis and monocytosis for 4 years, and thrombocytopenia for 6 months. After 10 courses of decitabine therapy, she had a persistent severe thrombocytopenia and decreased number of megakaryocytes, TET2 (tet methylcytosine dioxygenase 2) and SRSF2 (serine and arginine rich splicing factor 2) mutations were detected. She was dependent on platelet transfusion.

DIAGNOSES

Case 1 was diagnosed as CMML-associated ITP, and case 2 as CMML with decitabine therapy-induced thrombocytopenia.

INTERVENTIONS

Both patients were treated with eltrombopag.

OUTCOMES

In both patients, the platelet counts returned to the normal within 1 week after eltrombopag therapy. The platelet count in case 1 patient remained stable at 141-200 × 10/L for 20 months with stopping therapy for 3 months. In case 2 patient, eltrombopag was stopped 1 month later. Her platelet count decreased to 41 × 10/L, but was stable at ∼30 × 10/L for 3 months with platelet transfusion independency for 12 months. Both patients had no adverse effects with eltrombopag.

LESSONS

CMML-associated ITP is very rare and easily misdiagnosed. To the best of our knowledge, case 1 is the first reported case of the successful treatment of CMML-associated ITP with eltrombopag. Both CMML-associated ITP and decitabine therapy-induced thrombocytopenia in these 2 patients were highly sensitive and safe to eltrombopag therapy.

Enhanced effect of vascular endothelial growth factor, thrombopoietin peptide agonist, SCF, and Flt3-L on LTC-IC and reporter gene transduction from umbilical cord blood CD34+ cells

BACKGROUND

Hemangioblastic precursors have been identified that give rise to both endothelial cells and HPCs, suggesting that common growth factor requirements may exist.

STUDY DESIGN AND METHODS

The effect of vascular endothelial growth factor (VEGF) in combination with thrombopoietin peptide agonist (TPOA), Flt-3 L (F), and SCF (S) on long-term culture-initiating cell (LTC-IC), CFU, differentiation, and transduction of cord blood (CB) CD34+ were evaluated up to 4 weeks in culture.

RESULTS

At Week 4, in cultures containing T/F/S and VEGF, the LTC-IC increased 1000-fold (from 37 +/- 8 to 37,012 +/- 14,329) with a frequency of 3.4 in 10,000 cells. In the T/F/S cultures without VEGF, the LTC-IC increased 675-fold (to 25,086 +/- 12,102) with a frequency of one LTC-IC in 10,000 cells. The addition of VEGF significantly increased (p < 0.05) the LTC-IC per 10,000 CB CD34+ cells. Transduction with reporter gene enhanced green fluorescent protein (EGFP), resulted in an increase in EGFP+ CFU at Week 1 and EGFP + LTC-IC at Weeks 1 and 4. The cells maintained their multilineage expression when cultured in conditions for erythroid, myeloid, or megakaryocytic differentiation. Peak percentage EGFP coexpression of GlyA and CD11b was 51 +/- 6 percent and 63 +/- 15 percent, respectively, at Week 2, while CD41a was 34 +/- 17 percent at Week 4.

CONCLUSION

T/F/S and VEGF have an enhanced effect on total LTC-IC output and frequency but do not appear to significantly alter the differentiation or transducibility characteristics of CB HPCs in vitro.

[Thrombopoietin-mediated regulation of hematopoietic stem cells]

Sustaining lifelong hematopoiesis requires maintenance, proliferation, and differentiation of hematopoietic stem cells. Thrombopoietin is a cytokine essential for regulation of hematopoietic stem cells as well as differentiation and maturation of megakaryocytes required for platelet production. Due to these properties, thrombopoietin agonists have been used to treat bone marrow failure syndromes such as aplastic anemia. Through analysis of thrombopoietin gene-deficient mice, my colleagues and I have demonstrated the mechanism of action of thrombopoietin receptor agonists in hematopoietic stem cell maintenance and differentiation. This review focuses on governance of homeostasis in the hematopoietic system by thrombopoietin signaling.

[Effectiveness of perioperative administration of eltrombopag in a patient with idiopathic thrombocytopenic purpura undergoing cardiovascular surgery]

We report a case of chronic aortic dissection and angina pectoris with idiopathic thrombocytopenic purpura treated perioperatively with eltrombopag. A 72-year-old man was admitted to our hospital because of significant enlargement of an ulcer-like projection in the thoracic aorta revealed by chest computed tomography after acute aortic dissection. Laboratory data showed thrombocytopenia with idiopathic thrombocytopenic purpura. Eltrombopag was administered 12.5 mg daily and increased by 12.5 mg every 2 weeks until 37.5 mg/day to control idiopathic thrombocytopenic purpura(ITP). After 7 weeks' eltrombopag therapy, thrombocyte increased, and the patient underwent total arch replacement. Nine months later, coronary angiography revealed progression of coronary artery stenosis at the left main trunk. The patient underwent off-pump coronary artery bypass grafting 10 days after initiation of eltrombopag therapy. His postoperative course was uneventful. Eltrombopag was suggested to be effective in perioperative management in a patient with idiopathic thrombocytopenic purpura undergoing cardiovascular surgery.