The Megakaryocyte/Platelet-Specific Enhancer of the 2β1 Integrin Gene: Two Tandem AP1 Sites and the Mitogen-Activated Protein Kinase Signaling Cascade

Xanthotoxin, a novel inducer of platelet formation, promotes thrombocytopoiesis via IL-1R1 and MEK/ERK signaling

BACKGROUND

Thrombocytopenia is a common hematological disease caused by many factors. It usually complicates critical diseases and increases morbidity and mortality. The treatment of thrombocytopenia remains a great challenge in clinical practice, however, its treatment options are limited. In this study, the active monomer xanthotoxin (XAT) was screened out to explore its medicinal value and provide novel therapeutic strategies for the clinical treatment of thrombocytopenia.

METHODS

The effects of XAT on megakaryocyte differentiation and maturation were detected by flow cytometry, Giemsa and phalloidin staining. RNA-seq identified differentially expressed genes and enriched pathways. The signaling pathway and transcription factors were verified through WB and immunofluorescence staining. Tg (cd41: eGFP) transgenic zebrafish and mice with thrombocytopenia were used to evaluate the biological activity of XAT on platelet formation and the related hematopoietic organ index in vivo.

RESULTS

XAT promoted the differentiation and maturation of Meg-01 cells in vitro. Meanwhile, XAT could stimulate platelet formation in transgenic zebrafish and recover platelet production and function in irradiation-induced thrombocytopenia mice. Further RNA-seq prediction and WB verification revealed that XAT activates the IL-1R1 target and MEK/ERK signaling pathway, and upregulates the expression of transcription factors related to the hematopoietic lineage to promote megakaryocyte differentiation and platelet formation.

CONCLUSION

XAT accelerates megakaryocyte differentiation and maturation to promote platelet production and recovery through triggering IL-1R1 and activating the MEK/ERK signaling pathway, providing a new pharmacotherapy strategy for thrombocytopenia.

Local sequence features that influence AP-1 cis-regulatory activity

In the genome, most occurrences of transcription factor binding sites (TFBS) have no cis-regulatory activity, which suggests that flanking sequences contain information that distinguishes functional from nonfunctional TFBS. We interrogated the role of flanking sequences near Activator Protein 1 (AP-1) binding sites that reside in DNase I Hypersensitive Sites (DHS) and regions annotated as Enhancers. In these regions, we found that sequence features directly adjacent to the core motif distinguish high from low activity AP-1 sites. Some nearby features are motifs for other TFs that genetically interact with the AP-1 site. Other features are extensions of the AP-1 core motif, which cause the extended sites to match motifs of multiple AP-1 binding proteins. Computational models trained on these data distinguish between sequences with high and low activity AP-1 sites and also predict changes in cis-regulatory activity due to mutations in AP-1 core sites and their flanking sequences. Our results suggest that extended AP-1 binding sites, together with adjacent binding sites for additional TFs, encode part of the information that governs TFBS activity in the genome.

A novel signalling screen demonstrates that CALR mutations activate essential MAPK signalling and facilitate megakaryocyte differentiation

Most myeloproliferative neoplasm (MPN) patients lacking JAK2 mutations harbour somatic CALR mutations that are thought to activate cytokine signalling although the mechanism is unclear. To identify kinases important for survival of CALR-mutant cells, we developed a novel strategy (KISMET) that utilizes the full range of kinase selectivity data available from each inhibitor and thus takes advantage of off-target noise that limits conventional small-interfering RNA or inhibitor screens. KISMET successfully identified known essential kinases in haematopoietic and non-haematopoietic cell lines and identified the mitogen activated protein kinase (MAPK) pathway as required for growth of the CALR-mutated MARIMO cells. Expression of mutant CALR in murine or human haematopoietic cell lines was accompanied by myeloproliferative leukemia protein (MPL)-dependent activation of MAPK signalling, and MPN patients with CALR mutations showed increased MAPK activity in CD34 cells, platelets and megakaryocytes. Although CALR mutations resulted in protein instability and proteosomal degradation, mutant CALR was able to enhance megakaryopoiesis and pro-platelet production from human CD34+ progenitors. These data link aberrant MAPK activation to the MPN phenotype and identify it as a potential therapeutic target in CALR-mutant positive MPNs.

Direct regulation of FOXK1 by C-jun promotes proliferation, invasion and metastasis in gastric cancer cells

Forkhead box (FOX) K1 is a member of the FOX transcription factor superfamily. High FOXK1 expression is associated with several cancers. However, whether FOXK1 expression contributes to gastric cancer (GC) development and progression remains unknown. We analyzed the FOXK1 promoter using the Promo software and found several binding sequence transcription factors, including c-jun. However, the molecular mechanism by which FOXK1 affects the c-jun-mediated malignant phenotype is poorly understood. Here, we found that FOXK1 protein expression was higher in 8/10 (80.0%) fresh cancer tissues compared with that in adjacent normal tissues. FOXK1 overexpression enhanced the proliferation, migration and invasion of GC cells. Moreover, FOXK1 expression was stimulated by transforming growth factor-β1 (TGF-β1). FOXK1 acted as a potential epithelial-to-mesenchymal transition (EMT) inducer by stimulating vimentin expression and inducing the loss of E-cadherin in stable FOXK1-transfected cells. The results of promoter reporter and chromatin immunoprecipitation assays demonstrated that c-jun directly binds to and activates the human FOXK1 gene promoter. A positive correlation was observed between the expression patterns of FOXK1 and c-jun in GC cells and tissue. FOXK1 and c-jun expression were correlated with tumor progression and represented significant predictors of overall survival in GC patients. However, the siRNA-mediated repression of c-jun in FOXK1-overexpressing cells reversed EMT, as well as the proliferative and metastatic phenotypes. In vivo, c-jun promoted FOXK1-mediated proliferation and metastasis via orthotopic implantation. The evidence presented here suggests that FOXK1-directed regulation by c-jun promote the development and progression of human GC.

PRL-3 suppresses c-Fos and integrin α2 expression in ovarian cancer cells

Background

Phosphatase of regenerating liver-3 (PRL-3), a protein tyrosine phosphatase, is highly expressed in multiple human cancers and strongly implicated in tumor progression and cancer metastasis. However, the mechanisms by which PRL-3 promotes cancer cell migration, invasion, and metastasis are not very well understood. In this study, we investigated the contribution and molecular mechanisms of PRL-3 in ovarian cancer progression.

Methods

PRL-3 protein expression was detected on ovarian cancer tissue microarrays using immunohistochemistry. Stable PRL-3 depleted cell lines were generated using short hairpin RNA (shRNA) constructs. The migration and invasion potential of these cells were analyzed using Transwell and Matrigel assays, respectively. Immunoblotting and immunofluorescence were used to detect protein levels and distribution in PRL-3-ablated cells and the control cells. Cell morphology was observed with hematoxylin-eosin staining and transmission electron microscopy. Finally, PRL-3-ablated and control cells were injected into nude mice for xenograft tumorigenicity assays.

Results

Elevated PRL-3 expression was detected in 19% (26 out of 135) of human ovarian cancer patient samples, but not in normal ovary tissues (0 out of 14). Stable depletion of PRL-3 in A2780 ovarian cancer cells resulted in decreased migration ability and invasion activity compared with control parental A2780 cells. In addition, PRL-3-ablated cells also exhibited flattened morphology and extended lamellipodia. To address the possible molecular basis for the altered phenotypes associated with PRL-3 down-regulation, we assessed the expression profiles of various proteins involved in cell-matrix adhesion. Depletion of PRL-3 dramatically enhanced both RNA and protein levels of the cell surface receptor integrin α2, but not its heterologous binding partner integrin β1. Inhibition of PRL-3 also correlated with elevated expression and phosphorylation of paxillin. A pronounced increase in the expression and activation of c-fos, a transcriptional activator of integrin α2, was observed in these PRL-3 knock-down cells. Moreover, forced expression of EGFP-PRL-3 resulted in the suppression of both integrin α2 and c-fos expression in A2780 cells. Significantly, using a xenograft tumor model, we observed a greatly reduced tumorigenicity of A2780 PRL-3 knock-down cells in vivo.

Conclusions

These results suggest that PRL-3 plays a critical role in ovarian cancer tumorigenicity and maintaining the malignant phenotype. PRL-3 may inhibit c-fos transcriptional regulation of integrin α2 signaling. Our results strongly support a role for PRL-3 as a promising therapeutic target and potential early biomarker in ovarian cancer progression.

Potentiated activation of VLA-4 and VLA-5 accelerates proplatelet-like formation

Fibronectin (FN) plays important roles in the proliferation, differentiation, and maintenance of megakaryocytic-lineage cells through FN receptors. However, substantial role of FN receptors and their functional assignment in proplatelet-like formation (PPF) of megakaryocytes are not yet fully understood. Herein, we investigated the effects of FN receptors on PPF using the CHRF-288 human megakaryoblastic cell line, which expresses VLA-4 and VLA-5 as FN receptors. FN and phorbol 12-myristate 13-acetate (PMA) were essential for inducing PPF in CHRF-288 cells. Blocking experiments using anti-β1-integrin monoclonal antibodies indicated that the adhesive interaction with FN via VLA-4 and VLA-5 were required for PPF. PPF induced by FN plus PMA was accelerated when CHRF-288 cells were enforced adhering to FN by TNIIIA2, a peptide derived from tenascin-C, which we recently found to induce β1-integrin activation. Adhesion to FN enhanced PMA-stimulated activation of extracellular signal-regulated protein kinase 1 (ERK1)/2 and enforced adhesion to FN via VLA-4 and VLA-5 by TNIIIA2-accelerated activation of ERK1/2 with FN plus PMA. However, c-Jun amino-terminal kinase 1 (JNK1), p38, and phosphoinositide-3 kinase (PI3K)/Akt were not stimulated by FN plus PMA, even with TNIIIA2. Thus, the enhanced activation of ERK1/2 by FN, PMA plus TNIIIA2 was responsible for acceleration of PPF with FN plus PMA.

Transcription of the transforming growth factor beta activating integrin beta8 subunit is regulated by SP3, AP-1, and the p38 pathway

Integrin alphavbeta8 is a critical regulator of transforming growth factor beta activation in vasculogenesis during development, immune regulation, and endothelial/epithelial-mesenchymal homeostasis. Recent studies have suggested roles for integrin beta8 in the pathogenesis of chronic obstructive pulmonary disease, brain arteriovenous malformations, and select cancers (Araya, J., Cambier, S., Markovics, J. A., Wolters, P., Jablons, D., Hill, A., Finkbeiner, W., Jones, K., Broaddus, V. C., Sheppard, D., Barzcak, A., Xiao, Y., Erle, D. J., and Nishimura, S. L. (2007) J. Clin. Invest. 117, 3551-3562; Su, H., Kim, H., Pawlikowska, L., Kitamura, H., Shen, F., Cambier, S., Markovics, J., Lawton, M. T., Sidney, S., Bollen, A. W., Kwok, P. Y., Reichardt, L., Young, W. L., Yang, G. Y., and Nishimura, S. L. (2010) Am. J. Pathol. 176, 1018-1027; Culhane, A. C., and Quackenbush, J. (2009) Cancer Res. 69, 7480-7485; Cambier, S., Mu, D. Z., O'Connell, D., Boylen, K., Travis, W., Liu, W. H., Broaddus, V. C., and Nishimura, S. L. (2000) Cancer Res. 60, 7084-7093). Here we report the first identification and characterization of the promoter for ITGB8. We show that a SP binding site and a cyclic AMP response element (CRE) in the ITGB8 core promoter are required for its expression and that Sp1, Sp3, and several AP-1 transcription factors form a complex that binds to these sites in a p38-dependent manner. Furthermore, we demonstrate the requirement for Sp3, ATF-2, and p38 for the transcription and protein expression of integrin beta8. Additionally, reduction of SP3 or inhibition of p38 blocks alphavbeta8-mediated transforming growth factor beta activation. These results place integrin beta8 expression and activity under the control of ubiquitous transcription factors in a stress-activated and pro-inflammatory pathway.

The mitogen-activated protein kinase signaling pathways: role in megakaryocyte differentiation

Megakaryopoiesis is a process by which bone marrow progenitor cells develop into mature megakaryocytes (MKs), which in turn produce platelets required for normal hemostasis. The mitogen-activated protein kinases (MAPKs) family comprises four main groups of proteins: extracellular signal-related kinases (ERKs) (ERK1/2 or p44/p42), ERK5, p38MAPKs (alpha, beta, gamma, delta) and c-Jun amino-terminal kinases (JNKs) (JNK 1, 2, 3). These intracellular signaling pathways play a pivotal role in many essential cellular processes including proliferation and differentiation. The purpose of this review is to summarize our current knowledge on the role of MAPKs in MKs, specifically regarding differentiation in immortalized cell lines and primary MKs. A critical role of the MEK (MAPK kinase)-ERK1/2 pathway in MK development has been demonstrated although the details remain controversial. There is at present no functional evidence for a role of p38MAPKs whereas the role of JNKs and ERK5 in MK development is not known. Characterization of these molecular event cascades remains crucial for the understanding of the megakaryopoiesis process.

Identification of a minimal cis-element and cognate trans-factor(s) required for induction of Rac2 gene expression during K562 cell differentiation

Rac2 is a Rho family GTPase that is widely expressed in hematopoietic cells and plays a critical role in host defense. This study investigates the mechanisms responsible for increased Rac2 gene expression during myeloid cell differentiation. Treatment of K562 chronic myelogenous leukemia cells with phorbol-12-myristate-13-acetate (PMA) induces megakaryocytic differentiation and Rac2 gene transcription following a lag of 6-12 h. Promoter/luciferase reporter gene assays reveal that a 135 bp cis-element located between -4223 and -4008 bp upstream of the Rac2 transcription start site is necessary and sufficient for PMA-induced gene expression. The AP1 transcription factor binds to three cis-elements within the 135 bp Rac2 gene regulatory region both in vitro and in vivo following PMA treatment, and mutagenesis of the AP1 binding sites ablates the PMA responsiveness of the 135 bp Rac2 gene regulatory region. Over-expression of AP1 is sufficient to induce expression of a transiently transfected Rac2 promoter/luciferase plasmid, but not the endogenous Rac2 gene. Induction of AP1 in vitro DNA-binding activity is apparent within 1 h of PMA stimulation. However, AP1 binding to the endogenous Rac2 promoter exhibits a lag of 5-9 h, which correlates with reduced histone H3-Lys9 methylation, increased histone H3 acetylation, and increased nuclease accessibility within the 135 bp Rac2 gene regulatory region. These results demonstrate that PMA induction of Rac2 expression during terminal myeloid differentiation requires the coordinate induction of transcription factors and remodeling of Rac2 gene chromatin structure.

VLA-5-mediated adhesion to fibronectin accelerates hemin-stimulated erythroid differentiation of K562 cells through induction of VLA-4 expression

Fibronectin plays important roles in erythropoiesis through the fibronectin receptors VLA-4 and VLA-5. However, the substantial role of these fibronectin receptors and their functional assignment in erythroid differentiation are not yet fully understood. Here, we investigated the effects of cell adhesion to fibronectin on erythroid differentiation using K562 human erythroid progenitor cells. Erythroid differentiation could be induced in K562 cells in suspension by stimulating with hemin. This hemin-stimulated erythroid differentiation was highly accelerated when cells were induced to adhere to fibronectin by treatment with TNIIIA2, a peptide derived from tenascin-C, which has recently been found to induce beta1-integrin activation. Another integrin activator, Mn(2+), also accelerated hemin-stimulated erythroid differentiation. Adhesive interaction with fibronectin via VLA-4 as well as VLA-5 was responsible for acceleration of the hemin-stimulated erythroid differentiation in response to TNIIIA2, although K562 cells should have been lacking in VLA-4. Adhesion to fibronectin forced by TNIIIA2 causally induced VLA-4 expression in K562 cells, and this was blocked by the RGD peptide, an antagonist for VLA-5. The resulting adhesive interaction with fibronectin via VLA-4 strongly enhanced the hemin-stimulated activation of p38 mitogen-activated protein kinase, which was shown to serve as a signaling molecule crucial for erythroid differentiation. Suppression of VLA-4 expression by RNA interference abrogated acceleration of hemin-stimulated erythroid differentiation in response to TNIIIA2. Thus, VLA-4 and VLA-5 may contribute to erythropoiesis at different stages of erythroid differentiation.

Rotavirus replication in intestinal cells differentially regulates integrin expression by a phosphatidylinositol 3-kinase-dependent pathway, resulting in increased cell adhesion and virus yield

Changes in the interactions between intestinal cells and their surrounding environment during virus infection have not been well documented. The growth and survival of intestinal epithelial cells, the main targets of rotavirus infection, are largely dependent on the interaction of cell surface integrins with the extracellular matrix. In this study, we detected alterations in cellular integrin expression following rotavirus infection, identified the signaling components required, and analyzed the subsequent effects on cell binding to the matrix component collagen. After rotavirus infection of intestinal cells, expression of alpha2beta1 and beta2 integrins was up-regulated, whereas that of alphaVbeta3, alphaVbeta5, and alpha5beta1 integrins, if present, was down-regulated. This differential regulation of integrins was reflected at the transcriptional level. It was unrelated to the use of integrins as rotavirus receptors, as both integrin-using and integrin-independent viruses induced integrin regulation. Using pharmacological agents that inhibit kinase activity, integrin regulation was shown to be dependent on phosphatidylinositol 3-kinase (PI3K) but independent of the activities of the mitogen-activated protein kinases p38 and ERK1/2, and cyclooxygenase-2. Replication-dependent activation of the PI3K/Akt pathway was observed following infection of intestinal and nonintestinal cell lines. Rotavirus activation of PI3K was important for regulation of alpha2beta1 expression. Blockade of integrin regulation by PI3K inhibition led to decreased adherence of infected intestinal cells to collagen and a concomitant decrease in virus titer. These findings indicate that rotavirus-induced PI3K activation causes regulation of integrin expression in intestinal cells, leading to prolonged adherence of infected cells to collagen and increased virus production.

Transcriptional and epigenetic regulation of the integrin collagen receptor locus ITGA1-PELO-ITGA2

The integrin collagen receptor locus on human chromosome 5q11.2 includes the integrin genes ITGA1 and ITGA2, and the cell cycle regulation gene PELO, embedded within ITGA1 intron 1. ITGA1 contains a CArG box that is bound by serum response factor (SRF), while PELO contains two Sp1 binding elements. A comparison of mRNA levels in megakaryocytic (MK) and non-megakaryocytic (non-MK) cell lines and an analysis of the transcriptional activity of promoter-LUC reporter gene constructs in transfected cells revealed that ITGA1 is selectively suppressed in the MK lineage. Sodium bisulfite genomic sequencing established that a CpG-rich ITGA1 promoter region (-209/+115) is fully methylated at 19 CpG sites in MK cells that do not express alpha1beta1, but completely demethylated in expressing cells. In vitro methylation of ITGA1 suppresses transcription, while treatment of megakaryocytic cells with 5-aza-2'-deoxycytidine, but not Trichostatin A, resulted in de novo expression of ITGA1. During thrombopoietin-induced in vitro differentiation of primary human cord blood mononuclear cells into megakaryocytes, we observed rapid, progressive CpG methylation of ITGA1, but not PELO or ITGA2. Thus, selective CpG methylation of the ITGA1 promoter is a specific feature of alpha1beta1 regulation that coincides with the initiation of megakaryocyte differentiation.

Collagen binding alpha2beta1 and alpha1beta1 integrins play contrasting roles in regulation of Ets-1 expression in human liver myofibroblasts

Activation of hepatic stellate cells from quiescence to myofibroblast-like cells (MFBs) is a pivotal event in hepatic fibrogenesis. Plastic-cultured stellate cells (an established in vitro model of the activated phenotype) recultured on Matrigel revert to quiescence. In the present study we analyzed the molecular mechanism underlying this process, focusing on the effect of collagen receptors alpha(2)beta(1) and alpha(1)beta(1) integrin signaling on the expression of Ets-1 transcription factor and its target gene MMP1 in cultured human MFBs. Cells grown in 3-dimensional (3D) substrates (Matrigel) or collagen type I gel) markedly upregulated Ets-1 and MMP1 messages, in comparison to cells cultured on plastic. A similar effect but less intense was mimicked by stimulation of alpha(2)beta(1) or blocking of alpha(1)beta(1) integrin in cells grown on plastic. We observed increased expression of MMP1 transcripts with parallel changes in MMP1 promoter activity, and in mRNA and protein levels of upstream transcription factors Ets-1 and c-Jun. Interference with alpha(2)beta(1) and alpha(1)beta(1) integrin function in cells cultured in a 3D collagen substrate resulted in an even greater effect. Morphologically, stimulation of alpha(2)beta(1) integrin resulted in formation of multicellular networks, probably by facilitation of cell migration. Thus, we report the novel observation that in cultured human MFBs reverting to quiescence, the expression of transcription factor Ets-1 and its downstream target MMP1 can be modulated by changes in the microenvironment, which are mediated, at least in part, by the balance between collagen receptor integrin alpha(2)beta(1) and alpha(1)beta(1) activities.

Tandem Sp1/Sp3 sites together with an Ets-1 site cooperate to mediate α11 integrin chain expression in mesenchymal cells

Alpha11beta1 integrin is a collagen receptor, which is expressed in a highly regulated manner in a specific subset of ectomesenchymally and mesodermally derived cells. We previously established that a 3 kb region upstream of the transcription start site of the ITGA11 gene efficiently induced alpha11 transcription in a cell-type specific manner. Using the human fibrosarcoma cell line HT1080 and human skin fibroblasts, we now report that the majority of the activity in the proximal promoter resides in a region spanning nt +25 to nt -176. Mutation and deletion analyses using luciferase reporter assays showed that tandem low affinity Sp1/Sp3 binding sites, together with an Ets-1-like binding site, were needed for the proximal promoter activity in mesenchymal cells. EMSAs and supershift assays showed that Sp1 and Sp3 both bind to the Sp1/Sp3 binding sites, whereas occupation of the Ets-1 binding site appears to be Sp3-dependent. Chromatin immunoprecipitation assays verified that Sp1, Sp3 and Ets-1 can bind the promoter in vivo. In heterologous Drosophila SL2 cells, Sp1, Sp3 and Ets-1 all transactivated the alpha11 promoter, with Sp1 being the most efficient activator. The lack of any synergistic effect of Sp1/Sp3 and Ets-1 in SL2 cells indicates that an Ets family member other than Ets-1 might be involved in regulating alpha11 transcription in mesenchymal cells. The central role of Sp1 in regulating alpha11 RNA transcription was further verified by the ability of the Sp1 inhibitor mithramycin A to efficiently attenuate alpha11 RNA and protein levels in primary fibroblasts. The proximal promoter itself was able to confer cell-type specific transcription on HT1080 cells and embryonic fibroblasts but not on U2OS and JAR cells. We speculate that the "mesenchymal signature" of alpha11 integrin gene expression is controlled by the activity of Sp1/Sp3, fibroblast-specific combinations of Ets family members and yet unidentified enhancer-binding transcription factors.

Hypoxia activates MAPK activity in rat nucleus pulposus cells: regulation of integrin expression and cell survival

OBJECTIVE

The aim of the present study was to investigate whether activation of MAPK subtypes ERK and p38 influences integrin expression and promotes nucleus pulposus cell survival in hypoxia.

SUMMARY OF BACKGROUND DATA

We have recently shown that in a low oxygen environment, rat nucleus pulposus cells activate phosphatidylinositol 3-kinase/Akt (PI3K/Akt) and mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) signaling pathways. However, the role of these signaling pathways in regulating cell matrix interactions is not known.

METHODS

Rat nucleus pulposus cells were cultured in hypoxia (2% O2) or normoxia (20% O2). Western blotting and kinase assay were used to analyze expression of MAPK signaling molecules. Cell attachment to collagen was studied using an adhesion assay, whereas flow cytometry and RT-PCR were performed to quantify integrin receptor expression.

RESULTS

In a hypoxic environment (2% O2), rat nucleus pulposus cells showed a persistent phosphorylation of p38 and ERK proteins; pERK catalyzed the phosphorylation of Elk1-Gst fusion protein. When ERK activity was blocked, cell adhesion to Type II collagen, one of the major extracellular matrix proteins in the nucleus pulposus tissue, was impaired. A similar inhibitory effect on collagen adhesion was observed when nucleus pulposus cells were treated with an antibody to alpha2 integrin. Furthermore, when ERK activity was inhibited, there was a decrease in alpha2 integrin mRNA expression. In contrast to ERK, inhibition of p38 activity did not modulate alpha2 integrin subunit mRNA expression. Likewise, inhibition of ERK, but not p38, resulted in downregulation of nucleus pulposus alpha2 integrin protein levels and blocked cell survival in hypoxia.

CONCLUSIONS

Hypoxia activated MAPK signaling pathway activity in nucleus pulposus cells. ERK, but not p38, regulated alpha2 integrin expression and cell survival.

Extracellular signal-regulated kinase induces the megakaryocyte GPIIb/CD41 gene through MafB/Kreisler

Extracellular signal-regulated kinase (ERK) facilitates cell cycle progression in most mammalian cells, but in certain cell types prolonged signaling through this pathway promotes differentiation and lineage-specific gene expression through mechanisms that are poorly understood. Here, we characterize the transcriptional regulation of platelet GPIIb integrin (CD41) by ERK during megakaryocyte differentiation. ERK-dependent transactivation involves the proximal promoter of GPIIb within 114 bp upstream of the transcriptional start site. GATA, Ets, and Sp1 consensus sequences within this region are each necessary and function combinatorially in ERK-activated transcription. MafB/Kreisler is induced in response to ERK and synergizes with GATA and Ets to enhance transcription from the proximal promoter. The requirement for MafB in promoter regulation is demonstrated by inhibition of transactivation following dominant-negative or antisense suppression of MafB function. Thus, ERK promotes megakaryocyte differentiation by coordinate regulation of nuclear factors that synergize in GPIIb promoter regulation. These results establish a novel role for MafB as a regulator of ERK-induced gene expression.

Genetic variation responsible for mouse strain differences in integrin alpha 2 expression is associated with altered platelet responses to collagen

As mouse models have become commonplace for studying hemostasis and thrombosis, we considered whether the mouse system had utility for assessing genetic alterations in platelet receptors. Platelets from 5 mouse strains (C57BL/6 [C57], FVB/N [FVB], BALB/c, C3H/He, and 129Sv) showed only minor differences in the expression of integrin alpha(IIb), integrin beta(3), glycoprotein (GP) Ib alpha, or GPVI across strains. However, FVB platelets expressed approximately 50% the level of integrin alpha(2) as platelets from other strains (P <.0001). We bred FVB mice with C57 and assessed alpha(2) expression in FVB/C57xFVB/C57 (F2) offspring. Linkage analysis demonstrated the gene responsible for alpha(2) levels is tightly linked to the D13mit260 marker (log odds [lod] score 6.7) near the alpha(2) gene. FVB platelets showed reduced aggregation and a longer lag phase to collagen. FVB and C57 platelets aggregated similarly to collagen-related peptide, but FVB platelets showed a reduction in rhodocytin-induced Syk and PLC gamma 2 tyrosine phosphorylation. Thus, FVB platelets express half the level of alpha(2) as other mouse strains, a trait linked to the alpha(2) gene and seemingly responsible for reduced platelet aggregation to collagen. These strain differences serve as a useful model for the 2-fold difference in human platelet alpha(2)beta(1) expression and demonstrate that alpha(2)beta(1) participates in signaling during platelet activation.

Integrin alpha2 and extracellular signal-regulated kinase are functionally linked in highly malignant autocrine transforming growth factor-alpha-driven colon cancer cells

Recently, we have shown that autocrine transforming growth factor-alpha (TGF-alpha) controls the expression of integrin alpha2, cell adhesion to collagen IV and motility in highly progressed HCT116 colon cancer cells (Sawhney, R. S., Zhou, G-H. K., Humphrey, L. E., Ghosh, P., Kreisberg, J. I., and Brattain, M. G. (2002) J. Biol. Chem. 277, 75-86). We now report that expression of basal integrin alpha2 and its biological effects are controlled by constitutive activation of the extracellular signal-regulated/mitogen-activated protein kinase (ERK/MAPK) pathway. Treatment of cells with selective mitogen-activated protein kinase kinase (MEK) inhibitors PD098059 and U0126 showed that integrin alpha2 expression, cell adhesion, and activation of ERK are inhibited in a parallel concentration-dependent fashion. Moreover, autocrine TGF-alpha-mediated epidermal growth factor receptor activation was shown to control the constitutive activation of the ERK/MAPK pathway, since neutralizing antibody to the epidermal growth factor receptor was able to block basal ERK activity. TGF-alpha antisense-transfected cells also showed attenuated activation of ERK. Using a real time electric cell impedance sensing technique, it was shown that ERK-dependent integrin alpha2-mediated cell micromotion signaling is controlled by autocrine TGF-alpha. Thus, this study implicates ERK/MAPK signaling activated by endogenous TGF-alpha as one of the mechanistic features controlling metastatic spread.

Hepatocyte growth factor upregulates alpha2beta1 integrin in Madin-Darby canine kidney cells: implications in tubulogenesis

It has been well established that hepatocyte growth factor (HGF) induces branching tubule formation of Madin-Darby canine kidney (MDCK) cells cultured in collagen gel. Tubulogenesis per se requires the involvement of cell proliferation, migration, focalization proteolysis, cell-cell interaction and differentiation. However, signaling pathways and proteins involved in HGF-induced tubulogenesis by MDCK cells have not been thoroughly studied. Because cell-matrix interactions play important roles in tubulogenesis, we analyzed whether HGF altered the expression of extracellular matrix receptor (alpha2, alpha3, beta1 and alphavbeta3 integrin). We found that among those proteins examined, alpha2beta1 integrin levels were enhanced by HGF. HGF-induced upregulation of alpha2beta1 integrin was mediated via upregulation of alpha2 integrin mRNA abundance. Cycloheximide blocked the HGF-induced increase in alpha2 integrin mRNA expression. To understand the signaling pathways leading to an HGF-induced increase in alpha2beta1 integrin levels, PD98059 (MEK1 inhibitor), LY294002 (PI3-kinase inhibitor), and GF109203X (PKC inhibitor) were used. We found that PD98059 blocked the HGF-induced increase in alpha2beta1 integrin expression. Furthermore, 5E8 (specific anti-alpha2beta1 integrin antibody) was employed to elucidate the potential role of HGF-induced upregulation of alpha2beta1 integrin in branching morphogenesis. 5E8 did not alter HGF-induced scattering effects but disrupted HGF-induced branching tubulogenesis in collagen gel via inhibition of cell-cell interactions and growth. Taken together, HGF upregulates alpha2beta1 integrin expression via an indirect pathway, the results of which contribute to the regulation of cell-cell interactions and cell growth during branching morphogenesis in collagen gel.

Transcription factor activation in response to cutaneous injury: role of AP-1 in reepithelialization

Reepithelialization is the process responsible for restoring an intact epidermis following cutaneous injury. A change in the activity of keratinocytes is required for reepithelialization to occur, and this is likely to be regulated by the altered expression of effector genes, mediated by transcription factors. The injury itself provides a stimulus for transcription factor activation either directly due to mechanical stress, or via paracrine mechanisms such as the release of growth factors from damaged cells. Members of the activator protein-1 family, in particular c-fos and c-jun, have been the most widely studied wound-induced transcription factors. The signal transduction pathways linking cellular injury to activator protein-1 stimulation appear to involve an increase in intracellular Ca2+ and activation of mitogen-activated protein kinases. Given that a number of genes involved in the reepithelialization of wounds are regulated by activator protein-1, a distinct role for this transcription factor in reepithelialization is beginning to emerge. This article reviews the evidence for activator protein-1 involvement in reepithelialization, with particular focus on the activation of this transcription factor in response to wounding, the second messenger/kinase pathways involved, and the modulation of downstream genes that have the capacity to regulate keratinocyte function.

The influence of platelet collagen receptor polymorphisms in hemostasis and thrombotic disease

Extracellular collagens modulate the rate of platelet activation and thereby markedly influence hemostasis and thrombosis. Platelet receptors for collagens, such as the integrin alpha(2)beta(1), platelet glycoprotein (GP) VI or, indirectly, the GPIb complex, are unexploited targets of pharmacological control, and polymorphisms of these receptors have recently become factored into the genetic risk for thrombosis. Seemingly contradictory findings already exist with regard to the contribution of GPIbalpha and integrin alpha(2) polymorphisms, but these discrepancies will be resolved once there is better standardization of clinical studies. There is already substantial evidence that GPIbalpha VNTR A or B alleles, the GPIbalpha-5C allele, and integrin alpha(2) allele 1 (T(807)) each contribute to increased risk for morbidity in thrombotic disease. However, larger, prospective genetic and epidemiological studies are needed to clarify the role of each of these polymorphisms, the contribution of other platelet receptor polymorphisms, and the synergistic effects of combinations of these factors. In addition, in vitro studies that establish the functional relevance of these polymorphisms will provide sound biological explanations for the results of clinical correlation studies.

Genesis of clone size heterogeneity in megakaryocytic and other hemopoietic colonies: the stochastic model revisited

OBJECTIVE

We previously showed that the distributions of the numbers of doublings (NbD) undergone by individual megakaryocyte progenitors before commitment to polyploidization are markedly skewed and can consistently be fitted to straight lines when plotted on semilogarithmic coordinates. The slope of such lines, which yields the probability of polyploidization per doubling, is made less steep by stimulators of megakaryocyte colony formation and is less steep in mixed erythroid-megakaryocyte than in pure megakaryocyte colonies. Therefore, megakaryocytopoiesis provides a unique model for the study of clonal heterogeneity in a hemopoietic lineage, which is the subject of this review.

DATA SOURCES

Articles relevant to the interpretation of these data were selected from the authors' and public databases.

DATA SYNTHESIS

Exponential NbD distributions were first explained by postulating that following the assembly of thrombopoiesis-specific regulators, megakaryocyte progenitors require only a single random event to arrest proliferation and commit to polyploidization. However, this stochastic model was refuted by data indicating that intrinsic properties of individual progenitors affect the NbD they achieve. We suggest that the unequal repartition of critical compounds (including receptors, signaling molecules, and gene regulators) inherent in the stem cell-progenitor transition causes a heritable heterogeneity in megakaryocyte progenitor responsiveness to polyploidization inducers. This model would be compatible with 1) the evidence for intraclonal synchronization in megakaryocyte and other hemopoietic clones generated by committed progenitors; 2) the low probability of polyploidization of the relatively insensitive bipotent megakaryocyte progenitors; and 3) the thesis that stimulators act in part by recruiting megakaryocyte progenitor cells endowed with lesser responsiveness to polyploidization inducers and higher proliferative potential.

CONCLUSION

The responsiveness of individual megakaryocyte progenitors to polyploidization inducers may be a major determinant of the exponential shape of NbD distributions.

The role of platelet collagen receptor (glycoprotein Ia/IIa; integrin alpha2 beta1) polymorphisms in thrombotic disease

Differences in rates of platelet activation induced by extracellular matrix components such as collagens markedly influence normal hemostasis and the pathologic outcome of thrombosis. Thus, platelet collagen receptors, the integrin alpha2beta1, glycoprotein VI, and the glycoprotein Ib complex, represent unexploited targets of pharmacologic control. Polymorphisms of these receptors are now understood as factors that potentially contribute to thrombotic risk. There is substantial evidence that the GPIbalpha variable number of tandem repeats A or B alleles, the -5C allele of GPIbalpha, and the integrin alpha2 allele 1 (T807) each contribute to risk for and morbidity from thrombotic disease. The extent of their individual contributions is disputed. More well-designed, large, prospective, genetic and epidemiologic studies are needed to clarify the role of these and other platelet receptor polymorphisms, and additional in vitro studies are needed to provide a sound biologic explanation for the outcomes of clinical correlations.

Characterization of Inherited Differences in Transcription of the Human Integrin alpha 2 Gene

Inherited, single-base substitutions are found at only two positions, C(-)52T and C(-)92G, within the proximal 5'-regulatory region (within -1096 to +48) of the human integrin alpha(2) gene. We recently reported that the T(-)52 substitution results in decreased binding of transcription factor Sp1 to adjacent binding sites, decreased transcription of the alpha(2) gene, and reduced densities of platelet alpha(2)beta(1). In this study, we identify an additional Sp1-binding site at position -107 to -99 and show that the adjacent dimorphic sequence C(-)92G also influences the rate of gene transcription. In the erythroleukemia cell line Dami, transfected promoter-luciferase constructs bearing the G(-)92 sequence exhibit roughly a 3-fold decrease in activity relative to the C(-)92 constructs. In transfected CHRF-288-11 megakaryocytic cells, the corresponding activity decreases by 5-fold. DNase I footprinting of the promoter region with Dami nuclear extracts showed a protected segment at -107 to -99 that can be deprotected by coincubation with molar excess of a consensus Sp1 oligonucleotide. Gel mobility shift assays and supershift assays with specific antibodies indicate that Sp1 binds to this region of the alpha(2) gene promoter. Mutation of the Sp1 binding element within -107 to -99 in constructs containing either C(-)92 or G(-)92 abolishes basal promoter activity and eliminates the binding of Sp1. The G(-)92 sequence has a gene frequency of 0.15 in a typical Caucasian population, and the presence of this allele correlates with reduced densities of platelet alpha(2)beta(1). The combined substitution G(-)92/T(-)52 has an additive influence on gene transcription, resulting in an 8-fold decrease in transfected Dami cells or a 20-fold decrease in transfected CHRF-288-11 cells. In summary, the natural dimorphism C(-)92G within the proximal 5'-regulatory region of the human integrin alpha(2) gene contributes to the regulation of integrin alpha(2)beta(1) expression on megakaryocytes and blood platelets and must thereby modulate collagen-related platelet function in vivo.

Sustained activation of extracellular signal-regulated kinase stimulated by hepatocyte growth factor leads to integrin alpha 2 expression that is involved in cell scattering

We have previously shown that hepatocyte growth factor (HGF) selectively increases the expression of integrin alpha(2) in Madin-Darby canine kidney (MDCK) cells. In this study, we have further investigated the signal transduction pathways responsible for the event and its role in HGF-induced cell scattering. We found that the level of integrin alpha(2)beta(1) expression induced by HGF correlated with the extent of cell scattering and that a functional blocking antibody against integrin alpha(2) at the concentration of 25 microg/ml partially (40%) inhibited the HGF-induced cell scattering. However, in the presence of the specific phosphatidylinositol 3-kinase inhibitor LY294002 or the selective Src family kinase inhibitor PP1, although cells retained their response to HGF for increasing integrin alpha(2) expression, they failed to scatter, indicating that increased expression of integrin alpha(2) alone is not sufficient for cell scattering. Moreover, epidermal growth factor, which induced a transient (1 h) activation of extracellular signal-regulated kinase (ERK) in MDCK cells, only slightly increased integrin alpha(2) expression and failed to trigger cell scattering. Conversely, HGF induced a sustained (at least 12 h) activation of ERK in the cells. Expression of constitutively active ERK kinase (MEK) in MDCK cells led to increased expression of integrin alpha(2) even in the absence of HGF stimulation. In contrast, expression of ERK phosphatase or dominant negative MEK inhibited HGF-induced integrin alpha(2) expression. Taken together, our results suggest that the increased expression of integrin alpha(2)beta(1) by HGF is at least partially required for cell scattering and that the duration of MEK/ERK activation is likely to be a crucial determinant for cells to activate integrin alpha(2) expression and cell scattering.

Expression and function of the collagen receptor GPVI during megakaryocyte maturation

In this report, the expression and function of the platelet collagen receptor glycoprotein VI (GPVI) were studied in human megakaryocytes during differentiation and maturation of mobilized blood and cord blood derived CD34(+) cells. By flow cytometry, using an anti-GPVI monoclonal antibody or convulxin, a GPVI-specific ligand, GPVI was detected only on CD41(+) cells including some CD41(+)/CD34(+) cells, suggesting expression at a stage of differentiation similar to CD41. These results were confirmed at the mRNA level using reverse transcription-polymerase chain reaction. GPVI expression was low during megakaryocytic differentiation but increased in the more mature megakaryocytes (CD41(high)). As in platelets, megakaryocyte GPVI associates with the Fc receptor gamma chain (FcRgamma). The FcR gamma chain was detected at the RNA and protein level at all stages of megakaryocyte maturation preceding the expression of GPVI. The other collagen receptor, alpha(2)beta(1) integrin (CD49b/CD29), had a pattern of expression similar to GPVI. Megakaryocytic GPVI was recognized as a 55-kDa protein by immunoblotting and ligand blotting, and thus it presented a slightly lower apparent molecular mass than platelet GPVI (58 kDa). Megakaryocytes began to adhere to immobilized convulxin via GPVI after only 8-10 days of culture, at a time when megakaryocytes were maturing. At this stage of maturation, they also adhered to immobilized collagen by alpha(2)beta(1) integrin-dependent and -independent mechanisms. Convulxin induced a very similar pattern of protein tyrosine phosphorylation in megakaryocytes and platelets including Syk, FcRgamma, and PLC(gamma)2. Our results showed that GPVI is expressed early during megakaryocytic differentiation but functionally allows megakaryocyte adherence to collagen only at late stages of differentiation when its expression increases.

Erk is essential for growth, differentiation, integrin expression, and cell function in human osteoblastic cells

Extracellular signal-regulated kinases (Erks), members of the mitogen-activated protein kinase superfamily, play an important role in cell proliferation and differentiation. In this study we employed a dominant negative approach to determine the role of Erks in the regulation of human osteoblastic cell function. Human osteoblastic cells were transduced with a pseudotyped retrovirus encoding either a mutated Erk1 protein with a dominant negative action against both Erk1 and Erk2 (Erk1DN cells) or the LacZ protein (LacZ cells) as a control. Both basal and growth factor-stimulated MAPK activity and cell proliferation were inhibited in Erk1DN cells. Expression of Erk1DN protein suppressed both osteoblast differentiation and matrix mineralization by decreasing alkaline phosphatase activity and the deposition of bone matrix proteins. Cell adhesion to collagen, osteopontin, and vitronectin was decreased in Erk1DN cells as compared with LacZ cells. Cell spreading and migration on these matrices were also inhibited. In Erk1DN cells, expression of alphabeta(1), alpha(v)beta(3), and alpha(v)beta(5) integrins on the surface was decreased. Metabolic labeling indicated that the synthesis of these integrins was inhibited in Erk1DN cells. These data suggest that Erks are not only essential for the growth and differentiation of osteoblasts but also are important for osteoblast adhesion, spreading, migration, and integrin expression.

Thrombopoietin-mediated sustained activation of extracellular signal-regulated kinase in UT7-Mpl cells requires both Ras-Raf-1- and Rap1-B-Raf-dependent pathways

Thrombopoietin (TPO) regulates growth and differentiation of megakaryocytes. We previously showed that extracellular signal-regulated kinases (ERKs) are required for TPO-mediated full megakaryocytic maturation in both normal progenitors and a megakaryoblastic cell line (UT7) expressing the TPO receptor (Mpl). In these cells, intensity and duration of TPO-induced ERK signal are controlled by several regions of the cytoplasmic domain of Mpl. In this study, we explored the signaling pathways involved in this control. We show that the small GTPases Ras and Rap1 contribute together to TPO-induced ERK activation in UT7-Mpl cells and that they do so by activating different Raf kinases as downstream effectors: a Ras-Raf-1 pathway is required to initiate ERK activation while Rap1 sustains this signal through B-Raf. Indeed, (i) in cells expressing wild-type or mutant Mpl, TPO-induced Ras and Rap1 activation correlates with early and sustained phases of ERK signal, respectively; (ii) interfering mutants of Ras and Rap1 both inhibit ERK kinase activity and ERK-dependent Elk1 transcriptional activation in response to TPO; (iii) the kinetics of activation of Raf-1 and B-Raf by TPO follow those of Ras and Rap1, respectively; (iv) RasV12-mediated Elk1 activation was modulated by the wild type or interfering mutants of Raf-1 but not those of B-Raf; (v) Elk1 activation mediated by a constitutively active mutant of Rap1 (Rap1V12) is potentiated by B-Raf and inhibited by an interfering mutant of this kinase. UT7-Mpl cells represent the second cellular model in which Ras and Rap1 act in concert to modulate the duration of ERK signal in response to a growth factor and thereby the differentiation program. This is also, to our knowledge, the first evidence suggesting that Rap1 may play an active role in megakaryocytic maturation.

Allele-dependent transcriptional regulation of the human integrin alpha2 gene

Genetically controlled variation in alpha2beta1 expression by human blood platelets was previously described. Sixty-two haplotype sequences corresponding to the proximal 5' regulatory region (-1096 to +1) of the alpha2 gene were compared, and a dimorphic sequence -52C>T was identified that is located precisely between 2 tandem Sp1/Sp3 binding elements previously shown to be absolutely required for transcriptional activity of this gene in epithelial cell lines and the erythroleukemic cell line K562. The gene frequency of -52T in a random Caucasian population is approximately 0.35, and the expression of -52T correlates directly with reduced densities of platelet alpha2beta1. In mobility shift analyses, the -52T substitution attenuates complex formation with both Sp1 and Sp3. When transfected into the erythroleukemia cell line Dami, promoter-luciferase constructs bearing the -52T sequence exhibit a 5-fold decrease in activity relative to the -52C construct. In transfected CHRF-288-11 megakaryocytic cells, the corresponding activity decreases by 10-fold. The -52T sequence appears to be in linkage disequilibrium with the previously defined allele A3 (807C; HPA-5b), known to be associated with diminished expression of platelet alpha2beta1. In summary, a natural dimorphism has been identified within the proximal 5' regulatory region of the human integrin alpha2 gene that is responsible for decreased expression levels of the integrin alpha2beta1 on blood platelets through a mechanism that is probably mediated by the nuclear regulatory proteins Sp1 and Sp3.

Transcriptional regulation of the human interleukin 1beta gene by fibronectin: role of protein kinase C and activator protein 1 (AP-1)

Interleukin 1beta (IL-1beta) is a multifunctional polypeptide considered a key cytokine during inflammation. Fibronectin (FN), a matrix glycoprotein highly expressed in injured tissues, can induce expression of IL-1beta in human blood monocytic cells. Herein, we explore the intracellular signals and transcriptional mechanisms responsible for IL-1beta induction by FN using human promonocytic U937 cells transfected with the human IL-1beta promoter connected to a reporter gene. Exposure of transfected U937s to FN resulted in increased expression of the full-length IL-1beta promoter. This effect, mediated via the alpha5beta1 integrin, was associated with activation of mitogen-activated protein kinases (MAPKs) and was abolished by pre-treatment of cells with Calphostin C, a specific inhibitor of protein kinase C (PKC) activation. Deletion analysis and co-transfection studies using consensus activator protein 1 (AP-1) oligonucleotides suggested that an AP-1 site present in the 5' end of the IL-1beta promoter was involved in the FN-induced response. Finally, electrophoretic mobility shift assays showed that FN induced binding of AP-1, but not NF-kappaB. Together, these experiments demonstrate that FN binding to the alpha5beta1 integrin activates MAPK-dependent signal pathways, and results in the transcription of the IL-1beta promoter in U937 cells by activating PKC and inducing AP-1.

E-selectin-dependent signaling via the mitogen-activated protein kinase pathway in vascular endothelial cells

E-selectin, a cytokine-inducible adhesion molecule, supports rolling and stable arrest of leukocytes on activated vascular endothelium. Previous studies have suggested that this transmembrane protein can also transduce signals into the endothelial cell. We now demonstrate activation of the mitogen-activated protein kinase (MAPK) signaling cascade in cultured HUVEC in response to E-selectin-dependent leukocyte adhesion and Ab-mediated cross-linking of cell surface E-selectin. Adhesion of increasing numbers of HL60 cells to IL-1beta-activated HUVEC stimulated robust increases in MAPK activity that were abrogated by an E-selectin blocking Ab. Cross-linking of cell surface E-selectin with Abs, as a mimic of multivalent ligand engagement, strongly stimulated MAPK/extracellular signal-related kinase (ERK) kinase (MEK)-dependent MAPK activation and concomitant up-regulation of mRNA for c-fos, an immediate early response gene, whereas Ab cross-linking of HLA class I molecules (present at comparable density) failed to do so. Coimmunoprecipitation documented Ras, Raf-1 and, phospho-MEK complex formation. Unactivated HUVEC transduced with a full-length adenoviral E-selectin construct also exhibited cross-link-induced MAPK activation, macromolecular complex formation, and c-fos up-regulation, whereas HUVEC transduced with a cytoplasmic domain deletion mutant failed to respond. These observations indicate that E-selectin can transduce an activating stimulus via the MAPK cascade into the endothelial cell during leukocyte adhesion.

Megakaryocyte Growth and Development Factor-Induced Proliferation and Differentiation Are Regulated by the Mitogen-Activated Protein Kinase Pathway in Primitive Cord Blood Hematopoietic Progenitors

In several erythroleukemia cell lines, activation of mitogen-activated protein kinases (MAPK) by phorbol esters or megakaryocyte growth and development factor (MGDF) is required for induction of megakaryocytic phenotype and growth arrest. To support this model, we have examined the effect of a specific inhibitor of this pathway (PD98059) on human CD34+ hematopoietic progenitors isolated from cord blood (CB), induced to differentiate along the megakaryocytic lineage in liquid cultures supplemented with rhuMGDF. RhuMGDF induced a sustained activation of MAPK in megakaryocytes and this activation was completely inhibited in the presence of low concentrations of PD98059 (6 to 10 μmol/L). At this concentration, PD98059 induced an increase in cell proliferation, resulting in accumulation of viable cells and a prolongation of the life time of the cultures. This increase correlated with an increase in DNA synthesis rather than with a reduction in apoptosis. This effect was combined with developmental changes indicative of delayed megakaryocytic differentiation: (1) PD98059-treated cells tended to retain markers of immature progenitors as shown by the increased proportion of both CD34+ and CD41+CD34+ cells. (2) PD98059-treated cultures were greatly enriched in immature blasts cells. (3) PD98059 increased megakaryocytic progenitors able to form colonies in semisolid assays. Thus, the MAPK pathway, although not required for megakaryocyte formation, seems to be involved in the transition from proliferation to maturation in megakaryocytes. Inhibition of MAPK activation also led to an increase in the number and size of erythroid colonies without affecting granulocyte/macrophage progenitor numbers suggesting that, in addition to the megakaryocytic lineage, the MAPK pathway could play a role in erythroid lineage differentiation.

Megakaryocyte Growth and Development Factor-Induced Proliferation and Differentiation Are Regulated by the Mitogen-Activated Protein Kinase Pathway in Primitive Cord Blood Hematopoietic Progenitors

In several erythroleukemia cell lines, activation of mitogen-activated protein kinases (MAPK) by phorbol esters or megakaryocyte growth and development factor (MGDF) is required for induction of megakaryocytic phenotype and growth arrest. To support this model, we have examined the effect of a specific inhibitor of this pathway (PD98059) on human CD34+ hematopoietic progenitors isolated from cord blood (CB), induced to differentiate along the megakaryocytic lineage in liquid cultures supplemented with rhuMGDF. RhuMGDF induced a sustained activation of MAPK in megakaryocytes and this activation was completely inhibited in the presence of low concentrations of PD98059 (6 to 10 μmol/L). At this concentration, PD98059 induced an increase in cell proliferation, resulting in accumulation of viable cells and a prolongation of the life time of the cultures. This increase correlated with an increase in DNA synthesis rather than with a reduction in apoptosis. This effect was combined with developmental changes indicative of delayed megakaryocytic differentiation: (1) PD98059-treated cells tended to retain markers of immature progenitors as shown by the increased proportion of both CD34+ and CD41+CD34+ cells. (2) PD98059-treated cultures were greatly enriched in immature blasts cells. (3) PD98059 increased megakaryocytic progenitors able to form colonies in semisolid assays. Thus, the MAPK pathway, although not required for megakaryocyte formation, seems to be involved in the transition from proliferation to maturation in megakaryocytes. Inhibition of MAPK activation also led to an increase in the number and size of erythroid colonies without affecting granulocyte/macrophage progenitor numbers suggesting that, in addition to the megakaryocytic lineage, the MAPK pathway could play a role in erythroid lineage differentiation.