A potential role for protein kinase C-epsilon in regulating megakaryocytic lineage commitment

Protein kinase C signaling "in" and "to" the nucleus: Master kinases in transcriptional regulation

PKC is a multifunctional family of Ser-Thr kinases widely implicated in the regulation of fundamental cellular functions, including proliferation, polarity, motility, and differentiation. Notwithstanding their primary cytoplasmic localization and stringent activation by cell surface receptors, PKC isozymes impel prominent nuclear signaling ultimately impacting gene expression. While transcriptional regulation may be wielded by nuclear PKCs, it most often relies on cytoplasmic phosphorylation events that result in nuclear shuttling of PKC downstream effectors, including transcription factors. As expected from the unique coupling of PKC isozymes to signaling effector pathways, glaring disparities in gene activation/repression are observed upon targeting individual PKC family members. Notably, specific PKCs control the expression and activation of transcription factors implicated in cell cycle/mitogenesis, epithelial-to-mesenchymal transition and immune function. Additionally, PKCs isozymes tightly regulate transcription factors involved in stepwise differentiation of pluripotent stem cells toward specific epithelial, mesenchymal, and hematopoietic cell lineages. Aberrant PKC expression and/or activation in pathological conditions, such as in cancer, leads to profound alterations in gene expression, leading to an extensive rewiring of transcriptional networks associated with mitogenesis, invasiveness, stemness, and tumor microenvironment dysregulation. In this review, we outline the current understanding of PKC signaling "in" and "to" the nucleus, with significant focus on established paradigms of PKC-mediated transcriptional control. Dissecting these complexities would allow the identification of relevant molecular targets implicated in a wide spectrum of diseases.

Factor-specific generative pattern from large-scale drug-induced gene expression profile

Drug discovery is a complex and interdisciplinary field that requires the identification of potential drug targets for specific diseases. In this study, we present FacPat, a novel approach that identifies the optimal factor-specific pattern explaining the drug-induced gene expression profile. FacPat uses a genetic algorithm based on pattern distance to mine the optimal factor-specific pattern for each gene in the LINCS L1000 dataset. We applied Benjamini-Hochberg correction to control the false discovery rate and identified significant and interpretable factor-specific patterns consisting of 480 genes, 7 chemical compounds, and 38 human cell lines. Using our approach, we identified genes that show context-specific effects related to chemical compounds and/or human cell lines. Furthermore, we performed functional enrichment analysis to characterize biological features. We demonstrate that FacPat can be used to reveal novel relationships among drugs, diseases, and genes.

Mechanosensitive Pathways Involved in Cardiovascular Development and Homeostasis in Zebrafish

Cardiovascular diseases such as coronary heart disease, myocardial infarction, and cardiac arrhythmia are the leading causes of morbidity and mortality in developed countries and are steadily increasing in developing countries. Fundamental mechanistic studies at the molecular, cellular, and animal model levels are critical for the diagnosis and treatment of these diseases. Despite being phylogenetically distant from humans, zebrafish share remarkable similarity in the genetics and electrophysiology of the cardiovascular system. In the last 2 decades, the development and deployment of innovative genetic manipulation techniques greatly facilitated the application of zebrafish as an animal model for studying basic biology and diseases. Hemodynamic shear stress is intimately involved in vascular development and homeostasis. The critical mechanosensitive signaling pathways in cardiovascular development and pathophysiology previously studied in mammals have been recapitulated in zebrafish. In this short article, we reviewed recent knowledge about the role of mechanosensitive pathways such as Notch, PKCε/PFKFB3, and Wnt/Ang2 in cardiovas-cular development and homeostasis from studies in the -zebrafish model.

Down-regulation of superoxide dismutase 1 by PMA is involved in cell fate determination and mediated via protein kinase D2 in myeloid leukemia cells

Myeloid leukemia cells maintain a high intracellular ROS level and use redox signals for survival. The metabolism of ROS also affects cell fate, including cell death and differentiation. Superoxide dismutases (SODs) are major antioxidant enzymes that have high levels of expression in myeloid leukemia cells. However, the role of SODs in the regulation of myeloid leukemia cells' biological function is still unclear. To investigate the function of SODs in myeloid leukemia cell death and differentiation, we used myeloid leukemia cell lines K562, MEG-01, TF-1, and HEL cells for this study. We found that PMA-induced megakaryocytic differentiation in myeloid leukemia cells is accompanied by cell death and SOD1 down-regulation, while SOD2 expression is not affected. The role of SOD1 is verified when ATN-224, a SOD1 specific inhibitor, inhibits cell proliferation and promotes cell death in myeloid leukemia cells without PMA treatment. Moreover, inhibition or silencing of SODs further increases cell death and decreases polyploidization induced by PMA while they were partially reversed by SOD1 overexpression. Thus, SOD1 expression is required for myeloid leukemia cell fate determination. In addition, the knockdown of PKD2 reduces cell death and promotes polyploidization induced by PMA. PMA/PKD2-mediated necrosis via PARP cleavage involves both SOD1-dependent and -independent pathways. Finally, ATN-224 enhanced the inhibition of cell proliferation by Ara-C. Taken together, the results demonstrate that SOD1 regulates cell death and differentiation in myeloid leukemia cells. ATN-224 may be beneficial for myeloid leukemia therapy.

Biphasic Effects of Ingenol 3,20-Dibenzoate on the Erythropoietin Receptor: Synergism at Low Doses and Antagonism at High Doses

Although ingenol 3,20-dibenzoate (IDB) is known as a selective novel protein kinase C (PKC) agonist, its biologic actions and underlying mechanisms remain incompletely understood. In this study, we identified IDB as a proliferative agent for an erythropoietin (EPO)-dependent cell line, UT-7/EPO, through the screening of a natural compound library. To clarify the underlying mechanism of IDB's EPO-like activities, we thoroughly analyzed the mutual relation between EPO and IDB in terms of in vitro and in vivo activities, signaling molecules, and a cellular receptor. IDB substantially induced the proliferation of UT-7/EPO cells, but not as much as EPO. IDB also lessened the anemia induced by 5-fluorouracil in an in vivo mouse model. Interestingly, IDB showed a synergistic effect on EPO at low concentration, but an antagonistic effect at higher concentration. Physical interaction and activation of PKCs by IDB- and EPO-competitive binding of IDB to EPO receptor (EPOR) explain these synergistic and antagonistic activities, respectively. Importantly, we addressed IDB's mechanism of action by demonstrating the direct binding of IDB to PKCs, and by identifying EPOR as a novel molecular target of IDB. Based on these dual targeting properties, IDB holds promise as a new small molecule modulator of EPO-related pathologic conditions.

GATA transcription factors in vertebrates: evolutionary, structural and functional interplay

GATA transcription factors perform conserved and essential roles during animal development, including germ-layer specification, hematopoiesis, and cardiogenesis. The evolutionary history and the changes in selection pressures following duplication of the six GATA family members in vertebrates have not been completely understood. Recently, we explored multiple databases to find GATAs in different vertebrate species. Using these sequences, we have performed molecular phylogenetic analyses using Maximum Likelihood and Bayesian methods, and statistical tests of tree topologies, to ascertain the phylogenetic relationship and selection pressures among GATA proteins. Seventy-one full-length cDNA sequences from 24 vertebrate species were extracted from multiple databases. By phylogenetic analyses, we investigated the origin, conservation, and evolution of the GATAs. Six GATA genes in vertebrates might be formed by gene duplication. The inferred evolutionary transitions that separate members which belong to different gene clusters correlated with changes in functional properties. Selection analysis and protein structure analysis were combined to explain Darwinian selection in GATA sequences and these changes brought putative biological significance. 26 positive selection sites were detected in this process. This study reveals the evolutionary history of vertebrate GATA paralogous and positively selected sites likely relevant for the distinct functional properties of the paralogs. It provides a new perspective for understanding the origin and evolution and biological functions of GATAs, which will help to uncover the GATAs' biological roles, evolution and their relationship with associated diseases; in addition, other complex multidomain families and also larger superfamilies can be investigated in a similar way.

Unique in vitro and in vivo thrombopoietic activities of ingenol 3,20 dibenzoate, a Ca(++)-independent protein kinase C isoform agonist

Thrombopoiesis following severe bone marrow injury frequently is delayed, thereby resulting in life-threatening thrombocytopenia for which there are limited treatment options. The reasons for these delays in recovery are not well understood. Protein kinase C (PKC) agonists promote megakaryocyte differentiation in leukemia cell lines and primary cells. However, little is known about the megakaryopoietic effects of PKC agonists on primary CD34+ cells grown in culture or in vivo. Here we present evidence that the novel PKC isoform-selective agonist 3,20 ingenol dibenzoate (IDB) potently stimulates early megakaryopoiesis of human CD34+ cells. In contrast, broad spectrum PKC agonists failed to do so. In vivo, a single intraperitoneal injection of IDB selectively increased platelets in mice without affecting hemoglobin or white counts. Finally, IDB strongly mitigated radiation-induced thrombocytopenia, even when administered 24 hours after irradiation. Our data demonstrate that novel PKC isoform agonists such as IDB may represent a unique therapeutic strategy for accelerating the recovery of platelet counts following severe marrow injury.

Protein kinase D-HDAC5 signaling regulates erythropoiesis and contributes to erythropoietin cross-talk with GATA1

In red cell development, the differentiation program directed by the transcriptional regulator GATA1 requires signaling by the cytokine erythropoietin, but the mechanistic basis for this signaling requirement has remained unknown. Here we show that erythropoietin regulates GATA1 through protein kinase D activation, promoting histone deacetylase 5 (HDAC5) dissociation from GATA1, and subsequent GATA1 acetylation. Mice deficient for HDAC5 show resistance to anemic challenge and altered marrow responsiveness to erythropoietin injections. In ex vivo studies, HDAC5(-/-) progenitors display enhanced entry into and passage through the erythroid lineage, as well as evidence of erythropoietin-independent differentiation. These results reveal a molecular pathway that contributes to cytokine regulation of hematopoietic differentiation and offer a potential mechanism for fine tuning of lineage-restricted transcription factors by lineage-specific cytokines.

Mild hyperthermia promotes and accelerates development and maturation of erythroid cells

Hyperthermia treatment has at times been associated with increased platelet levels in humans. The heat shock protein HSP70, which can be induced by hyperthermia in megakaryocytes and erythrocytes, was recently shown to protect GATA-1 from degradation and to be required for erythroid differentiation. Based on these findings, we hypothesize that mild hyperthermia (MH), such as fever (39°C), could impact the differentiation of hematopoietic progenitors into erythrocytes and their subsequent maturation. Cell growth and erythroid differentiation increased dramatically in cord blood CD34(+) cell cultures incubated under MH. Erythroid maturation was also strongly promoted, which resulted in an increased proportion of hemoglobinized and enucleated erythroids. The rise in erythroid development was traced to a strong synergistic activity between MH and erythropoietin (EPO). The molecular basis for this potent synergy appears to originate from the capacity of MH to increase the basal activation of several signaling molecules downstream of the EPO receptor and the transcriptional activity of GATA-1. Moreover, the potent impact of MH on erythroid development was found be dependent on increased intracellular levels of reactive oxygen species. Thus, fever-like temperatures can promote the differentiation of progenitors along the erythroid lineage and accelerate their maturation through normal regulatory circuitry.

Essential role for ALCAM gene silencing in megakaryocytic differentiation of K562 cells

Background

Activated leukocyte cell adhesion molecule (ALCAM/CD166) is expressed by hematopoietic stem cells. However, its role in hematopoietic differentiation has not previously been defined.

Results

In this study, we show that ALCAM expression is silenced in erythromegakaryocytic progenitor cell lines. In agreement with this finding, the ALCAM promoter is occupied by GATA-1 in vivo, and a cognate motif at -850 inhibited promoter activity in K562 and MEG-01 cells. Gain-of-function studies showed that ALCAM clusters K562 cells in a process that requires PKC. Induction of megakaryocytic differentiation in K562 clones expressing ALCAM activated PKC-δ and triggered apoptosis.

Conclusions

There is a lineage-specific silencing of ALCAM in bi-potential erythromegakaryocytic progenitor cell lines. Marked apoptosis of ALCAM-expressing K562 clones treated with PMA suggests that aberrant ALCAM expression in erythromegakaryocytic progenitors may contribute to megakaryocytopenia.

Induction of erythroid differentiation in human erythroleukemia cells by depletion of malic enzyme 2

Malic enzyme 2 (ME2) is a mitochondrial enzyme that catalyzes the conversion of malate to pyruvate and CO2 and uses NAD as a cofactor. Higher expression of this enzyme correlates with the degree of cell de-differentiation. We found that ME2 is expressed in K562 erythroleukemia cells, in which a number of agents have been found to induce differentiation either along the erythroid or the myeloid lineage. We found that knockdown of ME2 led to diminished proliferation of tumor cells and increased apoptosis in vitro. These findings were accompanied by differentiation of K562 cells along the erythroid lineage, as confirmed by staining for glycophorin A and hemoglobin production. ME2 knockdown also totally abolished growth of K562 cells in nude mice. Increased ROS levels, likely reflecting increased mitochondrial production, and a decreased NADPH/NADP+ ratio were noted but use of a free radical scavenger to decrease inhibition of ROS levels did not reverse the differentiation or apoptotic phenotype, suggesting that ROS production is not causally involved in the resultant phenotype. As might be expected, depletion of ME2 induced an increase in the NAD+/NADH ratio and ATP levels fell significantly. Inhibition of the malate-aspartate shuttle was insufficient to induce K562 differentiation. We also examined several intracellular signaling pathways and expression of transcription factors and intermediate filament proteins whose expression is known to be modulated during erythroid differentiation in K562 cells. We found that silencing of ME2 leads to phospho-ERK1/2 inhibition, phospho-AKT activation, increased GATA-1 expression and diminished vimentin expression. Metabolomic analysis, conducted to gain insight into intermediary metabolic pathways that ME2 knockdown might affect, showed that ME2 depletion resulted in high orotate levels, suggesting potential impairment of pyrimidine metabolism. Collectively our data point to ME2 as a potentially novel metabolic target for leukemia therapy.

MicroRNA-34a inhibits cell proliferation by repressing mitogen-activated protein kinase kinase 1 during megakaryocytic differentiation of K562 cells

Phorbol 12-myristate 13-acetate (PMA) induces megakaryocytic differentiation of the human chronic myelocytic leukemia cell line K562. We examined the potential regulatory role of microRNAs (miRNAs) in this process. Genome-wide expression profiling identified 21 miRNAs (miRs) that were induced by the treatment of K562 cells with PMA. Among them, the expression of miR-34a, miR-221, and miR-222 was induced in the early stages and maintained throughout the late stages of differentiation. Cell signaling analysis showed that the activation of extracellular signal-regulated protein kinase (ERK) in response to PMA strongly induced miR-34a expression by transactivation via the activator protein-1 binding site in the upstream region of the miR-34a gene. Reporter gene assays identified mitogen-activated protein kinase kinase 1 (MEK1) as a direct target of miR-34a and c-fos as a direct target of miR-221/222. Although overexpression of the three miRNAs had little effect on cell differentiation, overexpression of miR-34a significantly repressed the proliferation of K562 cells with a concomitant reduction in MEK1 protein expression. Conversely, a locked nucleic acid probe against miR-34a significantly enhanced the proliferation of PMA-treated K562 cells. Taken together, the results show that PMA activates the MEK-ERK pathway and strongly induces miRNA-34a expression, which in turn inhibits cell proliferation by repressing the expression of MEK1. Thus, the results highlight an important regulatory role for miR-34a in the process of megakaryocytic differentiation, especially in the arrest of cell growth, which is a prerequisite for cells to enter differentiation.

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.

Activation of PKC-ε counteracts maturation and apoptosis of HL-60 myeloid leukemic cells in response to TNF family members

Protein kinase C (PKC)-ε, a component of the serine/threo-nine PKC family, has been shown to influence the survival and differentiation pathways of normal hematopoietic cells. Here, we have modulated the activity of PKC-ε with specific small molecule activator or inhibitor peptides. PKC-ε inhibitor and activator peptides showed modest effects on HL-60 maturation when added alone, but PKC-ε activator peptide significantly counteracted the pro-maturative activity of tumor necrosis factor (TNF)-α towards the monocytic/macrophagic lineage, as evaluated in terms of CD14 surface expression and morphological analyses. Moreover, while PKC-ε inhibitor peptide showed a reproducible increase of TNF-related apoptosis inducing ligand (TRAIL)-induced apoptosis, PKC-ε activator peptide potently counteracted the pro-apoptotic activity of TRAIL. Taken together, the anti-maturative and anti-apoptotic activities of PKC-ε envision a potentially important proleukemic role of this PKC family member.

TPO-independent megakaryocytopoiesis

Megakaryocytopoiesis is a continuous developmental process of platelet production. In this process, a complex network of hemopoietic growth factors are involved, among which TPO (thrombopoietin) is the most thoroughly investigated regulator of MKs (megakaryocytes). In addition to TPO, other regulators also have non-negligible effects on megakaryocytopoiesis. The majority of their effects are independent of TPO signaling. To date, TPO-independent megakaryocytopoiesis forms a regulatory system that includes four signals and (an) unknown signaling pathway(s). These four pathways are the gp 130 (glycoprotein 130)-dependent signaling pathway, the Notch pathway, NMDA (N-methyl-d-aspartate) receptor-mediated signaling, and the SDF-1 (stromal cell-derived factor-1)/FGF-4 (fibroblast growth factor-4) paradigm. Understanding of the TPO-independent regulatory system is important because the system may offer additional opportunities to understand the developmental process and the mechanisms of disorders characterized by abnormal MK and platelet production, such as thrombocytopenia and thrombocythemia, and to advance the development of therapeutics.

Protein kinase Calpha is differentially activated during neonatal and adult erythropoiesis and favors expression of a reporter gene under the control of the (A)gamma globin-promoter in cellular models of hemoglobin switching

PKCalpha was found to be expressed (mRNA and protein) throughout the in vitro maturation of primary human erythroblasts but its activity (phosphorylation levels and nuclear localization) was consistently higher in cells derived from human neonatal rather than adult blood. Since the gamma/gamma + beta globin expression ratio represented the major difference between neonatal and adult erythroblasts (58 +/- 12 vs. 7 +/- 3, respectively), we tested the hypothesis that PKCalpha might affect gamma-globin expression by measuring the levels of (A)gamma- or beta-promoter-driven reporter activity in erythroid cells stably (GM979) or transiently (K562, primary adult and neonatal erythroblasts) transfected with a dual microLCRbetaprRluc(A)gammaprFluc reporter in the presence of transient expression of either the constitutively active (sPKCalpha) or catalytically inactive (iPKCalpha) PKCalpha. As further control, GM979 cells were incubated with the PKC inhibitor rottlerin (30 microM). In all the cells analyzed, sPKCalpha significantly increased (by two- to sixfold) the levels of luciferase activity driven by the (A)gamma-promoter and the (A)gamma-F/((A)gamma-F + 2beta-R) expression ratio. In GM979 cells, rottlerin inhibited (by 50%) the (A)gamma-driven luciferase activity and the (A)gamma-F/((A)gamma-F + 2beta-R) expression ratio. These results suggest that different PKC isoforms may exert ontogenetic-specific functions in erythropoiesis and that modulation of PKCalpha might affect the activity of (A)gamma-promoter-driven reporters.

Involvement of nPKC-MAPK pathway in the decrease of nucleophosmin/B23 during megakaryocytic differentiation of human myelogenous leukemia K562 cells

Human myelogenous leukemia K562 cells were induced to undergo megakaryocytic differentiation by long-term treatment with phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA). The protein level of nucleophosmin/B23 (NPM/B23), a nucleolar protein, was substantially decreased upon TPA treatment. In this study, we found that the proteasome inhibitors blocked the decrease of NPM/B23 protein in response to TPA, suggesting the proteasomes were involved in the downregulation of NPM/B23 upon megakaryocytic differentiation. To investigate the signaling pathway in the downregulation of NPM/B23 during early TPA-induced megakaryocytic differentiation of K562 cells, K562 cells were treated with TPA in the presence of the PKC isozyme-selective inhibitors, GF109203X and Gö 6976, or MEK1 inhibitor, PD98059. The decrease of NPM/B23 protein in the TPA-treated K562 cells was blocked by GF109203X but not by Gö 6976, suggesting the involvement of novel PKCs in the downregulation of NPM/B23 during TPA-induced megakaryocytic differentiation of K562 cells. The application of MEK1 inhibitor PD98059 upon TPA treatment blocked the TPA-induced decrease of NPM/B23 protein and aborted the megakaryocytic differentiation but not to break through the cell growth arrest. Unlike NPM/B23, the degradation of nucleolin in the TPA-treated K562 cells could not be blocked by PD98059 while the TPA-induced megakaryocytic differentiation was abrogated. The decrease of NPM/B23 protein seems to be more correlated with the novel PKC-MAPK-induced megakaryocytic differentiation than another nucleolar protein, nucleolin. Taken together, our results indicated that novel PKC-MAPK pathway was required for the decrease of NPM/B23 during TPA-induced megakaryocytic differentiation.

CD34+ hematopoietic stem-progenitor cell microRNA expression and function: a circuit diagram of differentiation control

MicroRNAs (miRNAs) are a recently identified class of epigenetic elements consisting of small noncoding RNAs that bind to the 3' untranslated region of mRNAs and down-regulate their translation to protein. miRNAs play critical roles in many different cellular processes including metabolism, apoptosis, differentiation, and development. We found 33 miRNAs expressed in CD34+ hematopoietic stem-progenitor cells (HSPCs) from normal human bone marrow and mobilized human peripheral blood stem cell harvests. We then combined these data with human HSPC mRNA expression data and with miRNA-mRNA target predictions, into a previously undescribed miRNA:mRNA interaction database called the Transcriptome Interaction Database. The in silico predictions from the Transcriptome Interaction Database pointed to miRNA control of hematopoietic differentiation through translational control of mRNAs critical to hematopoiesis. From these predictions, we formulated a model for miRNA control of stages of hematopoiesis in which many of the genes specifying hematopoietic differentiation are expressed by HSPCs, but are held in check by miRNAs until differentiation occurs. We validated miRNA control of several of these target mRNAs by demonstrating that their translation in fact is decreased by miRNAs. Finally, we chose miRNA-155 for functional characterization in hematopoiesis, because we predicted that it would control both myelopoiesis and erythropoiesis. As predicted, miRNA-155 transduction greatly reduced both myeloid and erythroid colony formation of normal human HSPCs.

The role of PKC and PDK1 in monocyte lineage specification by Ras

Although hyperactivation of Ras is a common feature of myeloid malignancy, its role in subverting hematopoiesis is unclear. We have examined the influence of Ras on normal human uncommitted myeloid subsets and show that expression of this oncogene strongly favors monocyte lineage selection in bipotential granulocyte/macrophage progenitors while inhibiting colony formation in other uncommitted subsets. Ras also promoted monocytic differentiation but not the proliferation of these cells. The mechanism through which Ras drives monocyte lineage selection was dependent on PKC activity and Ras was found to promote the expression, membrane translocation, and phosphorylation of conventional and novel PKC isoforms. We further show that Ras promoted the expression of the AGC kinase master regulator, PDK1, which maintains the stability and activity of PKC isoforms. Consistent with this, overexpression of PDK1 itself promoted monocyte colony formation and translocation of PKC. Overexpression of PDK1 was found to be a common feature of acute myeloid leukemia (45% of patients) and was closely associated with hyperphosphorylation of PKC. These data demonstrate that Ras is able to promote monocyte lineage selection via PKC and show for the first time the involvement of the kinase master regulator, PDK1, in both lineage specification and in human leukemia.

Apoptosis and erythroid differentiation triggered by Bcr-Abl inhibitors in CML cell lines are fully distinguishable processes that exhibit different sensitivity to caspase inhibition

Imatinib targets the Bcr-Abl oncogene that causes chronic myelogenous leukemia (CML) in humans. Recently, we demonstrated that besides triggering apoptosis in K562 cells, imatinib also mediated their erythroid differentiation. Although both events appear to proceed concomitantly, it is not known at present whether or not imatinib-induced apoptosis and differentiation are interdependent processes. Hence, we investigated the requirements for Bcr-Abl inhibitor-mediated apoptosis and erythroid differentiation in several established and engineered CML cell lines. Imatinib triggered apoptosis and erythroid differentiation of different CML cell lines, but only apoptosis exhibited sensitivity to ZVAD-fmk inhibition. Conversely, the p38 mitogen-activated protein (MAP) kinase inhibitor, SB202190, significantly slowed down erythroid differentiation without affecting caspase activation. Furthermore, imatinib and PD166326, another Bcr-Abl inhibitory molecule, triggered erythroid differentiation of K562 cell clones, nevertheless resistant to Bcr-Abl inhibitor-induced apoptosis. Finally, short hairpin RNA inhibitor (shRNAi) silencing of caspase 3 efficiently inhibited caspase activity but had no effect on erythroid differentiation, whereas silencing of Bcr-Abl mimicked imatinib or PD166326 treatment, leading to increased apoptosis and erythroid differentiation of K562 cells. Taken together, our findings not only demonstrate that Bcr-Abl inhibitor-mediated apoptosis and differentiation are fully distinguishable events, but also that caspases are dispensable for erythroid differentiation of established CML cell lines.

Stress-induced cholinergic signaling promotes inflammation-associated thrombopoiesis

To study the role of the stress-induced “readthrough” acetylcholinesterase splice variant, AChE-R, in thrombopoiesis, we used transgenic mice overexpressing human AChE-R (TgR). Increased AChE hydrolytic activity in the peripheral blood of TgR mice was associated with increased thrombopoietin levels and platelet counts. Bone marrow (BM) progenitor cells from TgR mice presented an elevated capacity to produce mixed (GEMM) and megakaryocyte (Mk) colonies, which showed intensified labeling of AChE-R and its interacting proteins RACK1 and PKC. When injected with bacterial lipopolysaccharide (LPS), parent strain FVB/N mice, but not TgR mice, showed reduced platelet counts. Therefore, we primed human CD34+ cells with the synthetic ARP26 peptide, derived from the cleavable C-terminus of AChE-R prior to transplantation, into sublethally irradiated NOD/SCID mice. Engraftment of human cells (both CD45+ and CD41+ Mk) was significantly increased in mice that received ARP26-primed CD34+ human cells versus mice that received fresh nonprimed CD34+ human cells. Moreover, ARP26 induced polyploidization and proplatelet shedding in human MEG-01 promegakaryotic cells, and human platelet engraftment increased following ex vivo expansion of ARP26-treated CD34+ cells as compared to cells expanded with thrombopoietin and stem cell factor. Our findings implicate AChE-R in thrombopoietic recovery, suggesting new therapeutic modalities for supporting platelet production.

A survey of the signaling pathways involved in megakaryocytic differentiation of the human K562 leukemia cell line by molecular and c-DNA array analysis

The K562 cell line serves as a model to study the molecular mechanisms associated with leukemia differentiation. We show here that cotreatment of K562 cells with PMA and low doses of SB202190 (SB), an inhibitor of the p38 MAPK pathway, induced a majority of cells to differentiate towards the megakaryocytic lineage. Electronic microscopy analysis showed that K562 cells treated with PMA+SB exhibited characteristic features of physiological megakaryocytic differentiation including the presence of vacuoles and demarcation membranes. Differentiation was also accompanied by a net increase in megakaryocytic markers and a reduction of erythroid markers, especially when both effectors were present. PMA effect was selectively mediated by new PKC isoforms. Differentiation of K562 cells by the combination of PMA and SB required Erk1/2 activation, a threshold of JNK activation and p38 MAPK inhibition. Interestingly, higher concentrations of SB, which drastically activated JNK, blocked megakaryocytic differentiation, and considerably increased cell death in the presence of PMA. c-DNA microarray membranes and PCR analysis allow us to identify a set of genes modulated during PMA-induced K562 cell differentiation. Several gene families identified in our screening, including ephrins receptors and some angiogenic factors, had never been reported so far to be regulated during megakaryocytic differentiation.

MicroRNA modulation of megakaryoblast fate involves cholinergic signaling

MicroRNAs (miRNAs) are abundant small regulatory RNAs with multiple roles in cell fate determination. The processes regulating cellular miRNA levels are still unclear and experimental oligonucleotide tools to readily mimic their effects are not yet available. Here, we report that thapsigargin-induced intracellular Ca(++) release suppressed pre-miR-181a levels in human promegakaryotic Meg-01 cells, induced differentiation-associated nuclear endoreduplication and caspase-3 activation and replaced the acetylcholinesterase 3' splice variant AChE-S with AChE-R. AChE, PKC and PKA inhibitors all attenuated the pre-miR-181a decline and the induced differentiation. AChmiON, a synthetic 23-mer 2'-oxymethylated oligonucleotide mimicking the miR-181a sequence, blocked the calcium-induced differentiation while elevating cellular pre-miR-181a levels and inducing DNA fragmentation and cell death. Moreover, when added to RW 264.7 macrophages, AChmiON at 100 nM induced nitric oxide production with efficiency close to that of bacterial endotoxin, demonstrating physiologically relevant activities also in blood-born monocytes/macrophages. The stress-induced modulation of hematopoietic miR-181a levels through AChE, PKC and PKA cascade(s) suggests using miRNA mimics for diverting the fate of hematopoietic tumor cells towards differentiation and/or apoptosis.

Jun blockade of erythropoiesis: role for repression of GATA-1 by HERP2

Although Jun upregulation and activation have been established as critical to oncogenesis, the relevant downstream pathways remain incompletely characterized. In this study, we found that c-Jun blocks erythroid differentiation in primary human hematopoietic progenitors and, correspondingly, that Jun factors block transcriptional activation by GATA-1, the central regulator of erythroid differentiation. Mutagenesis of c-Jun suggested that its repression of GATA-1 occurs through a transcriptional mechanism involving activation of downstream genes. We identified the hairy-enhancer-of-split-related factor HERP2 as a novel gene upregulated by c-Jun. HERP2 showed physical interaction with GATA-1 and repressed GATA-1 transcriptional activation. Furthermore, transduction of HERP2 into primary human hematopoietic progenitors inhibited erythroid differentiation. These results thus define a novel regulatory pathway linking the transcription factors c-Jun, HERP2, and GATA-1. Furthermore, these results establish a connection between the Notch signaling pathway, of which the HERP factors are a critical component, and the GATA family, which participates in programming of cellular differentiation.

Disabled-2 is a negative regulator of integrin alpha(IIb)beta(3)-mediated fibrinogen adhesion and cell signaling

Disabled-2 (DAB2) is an adapter protein that is up-reg-ulated during megakaryocytic differentiation of hematopoietic cells and is abundantly expressed in platelets. In this study, the role of DAB2 in integrin alpha(IIb)beta(3)-mediated matrix protein fibrinogen adhesion and cell signaling was investigated. In K562 cells differentiating to the megakaryocytic lineage, down-regulation of DAB2 by DAB2 small interfering RNA augmented integrin alpha(IIb)beta(3) activation and resulted in an increase in cell adhesion to fibrinogen. Ectopic expression of DAB2 reversed the DAB2 small interfering RNA effect or, by itself, decreased fibrinogen adhesion of K562 cells. Mutational analysis revealed that a DAB2 Ser(24) phosphorylation mutant (S24A) abrogated the inhibitory function of DAB2. The spatial and temporal association/interaction of DAB2 and platelet integrin alpha(IIb)beta(3) (CD61) in both megakaryocytic cells and platelets led us to examine the effect of Ser(24) phosphorylation on the interaction between DAB2 and integrin beta(3). Through cellular localization and co-immunoprecipitation analysis, we demonstrate for the first time that Ser(24) phosphorylation promotes membrane translocation of DAB2 and its subsequent interaction with integrin beta(3), thereby defining a mechanism for DAB2 in regulating integrin alpha(IIb)beta(3) activation and inside-out signaling. Consistent with the effect on fibrinogen adhesion, Ser(24) phosphorylation of DAB2 was also involved in the negative regulation of alpha(IIb)beta(3)-induced T cell factor transcriptional activity. In contrast, the S24A mutant acted like wild-type DAB2 and inhibited both beta-catenin- and plakoglobin-mediated T cell factor transactivation. Hence, DAB2 elicits distinct regulatory mechanisms in alpha(IIb)beta(3) and beta-catenin/plakoglobin signaling in a Ser(24) phosphorylation-dependent and -independent manner, respectively. These findings indicate Ser(24) phosphorylation as a molecular basis for DAB2 acting as a negative regulator in alpha(IIb)beta(3) inside-out signaling and contribute to our understanding of DAB2 in megakaryocytic differentiation and platelet function.

Association of CBFA2 mutation with decreased platelet PKC-θ and impaired receptor-mediated activation of GPIIb-IIIa and pleckstrin phosphorylation: proteins regulated by CBFA2 play a role in GPIIb-IIIa activation

The mechanisms by which agonists activate glycoprotein (GP) IIb-IIIa function remain unclear. We have reported data on a patient with thrombocytopenia and impaired receptor-mediated aggregation, phosphorylation of pleckstrin (a protein kinase C [PKC] substrate), and activation of the GPIIb-IIIa complex. Abnormalities in hematopoietic transcription factors have been associated with thrombocytopenia and platelet dysfunction. To define the molecular mechanisms, we amplified from patient platelet RNA exons 3 to 6 of core-binding factor A2 (CBFA2) cDNA, which encompasses the DNA-binding Runt domain; a 13-nucleotide (nt) deletion was found (796-808 nt). The gDNA revealed a heterozygous mutation (G>T) in intron 3 at the splice acceptor site for exon 4, leading to a frameshift with premature termination in the Runt domain. On immunoblotting, platelet CBFA2, PKC-θ, albumin, and IgG were decreased, but pleckstrin, PKC-α, -βI, -βII, -η, -ϵ, -δ, and -ζ, and fibrinogen were normal. Our conclusions are that (1) CBFA2 mutation is associated with not only thrombocytopenia, but also impaired platelet protein phosphorylation and GPIIb-IIIa activation; (2) proteins regulated by CBFA2 are required for inside-out signal transduction-dependent activation of GPIIb-IIIa; and (3) we have documented the first deficiency of a human PKC isozyme (PKC-θ), suggesting a major role of this isozyme in platelet production and function. (Blood. 2004;103:948-954)

RUNX1 and GATA-1 coexpression and cooperation in megakaryocytic differentiation

Megakaryocytic and erythroid lineages derive from a common bipotential progenitor and share many transcription factors, most prominently factors of the GATA zinc-finger family. Little is known about transcription factors unique to the megakaryocytic lineage that might program divergence from the erythroid pathway. To identify such factors, we used the K562 system in which megakaryocyte lineage commitment is dependent on sustained extracellular regulatory kinase (ERK) activation and is inhibited by stromal cell contact. During megakaryocytic induction in this system, the myeloid transcription factor RUNX1 underwent up-regulation, dependent on ERK signaling and inhibitable by stromal cell contact. Immunostaining of healthy human bone marrow confirmed a strong expression of RUNX1 and its cofactor, core-binding factor beta (CBFbeta), in megakaryocytes and a minimal expression in erythroblasts. In primary human hematopoietic progenitor cultures, RUNX1 and CBFbeta up-regulation preceded megakaryocytic differentiation, and down-regulation of these factors preceded erythroid differentiation. Functional studies showed cooperation among RUNX1, CBFbeta, and GATA-1 in the activation of a megakaryocytic promoter. By contrast, the RUNX1-ETO leukemic fusion protein potently repressed GATA-1-mediated transactivation. These functional interactions correlated with physical interactions observed between GATA-1 and RUNX1 factors. Enforced RUNX1 expression in K562 cells enhanced the induction of the megakaryocytic integrin proteins alphaIIb and alpha2. These results suggest that RUNX1 may participate in the programming of megakaryocytic lineage commitment through functional and physical interactions with GATA transcription factors. By contrast, RUNX1-ETO inhibition of GATA function may constitute a potential mechanism for the blockade of erythroid and megakaryocytic differentiation seen in leukemias with t(8;21).

Stromal inhibition of megakaryocytic differentiation is associated with blockade of sustained Rap1 activation

Coculture with stromal cells tends to maintain normal hematopoietic progenitors and their leukemic counterparts in an undifferentiated, proliferative state. An example of this effect is seen with megakaryocytic differentiation, wherein stromal contact renders many cell types refractory to potent induction stimuli. This inhibitory effect of stroma on megakaryocytic differentiation correlates with a blockade within hematopoietic cells of protein kinase C-epsilon (PKC-epsilon) up-regulation and of extracellular signal-regulated kinase/mitogen-activated protein (ERK/MAP) kinase activation, both of which have been implicated in promoting megakaryocytic differentiation. In this study K562DeltaRafER.5 cells, expressing an estradiol-responsive mutant of the protein kinase Raf-1, were used to determine the relevance and stage of ERK/MAPK pathway blockade by stromal contact. Activation of DeltaRafER by estradiol overrode stromal blockade of megakaryocytic differentiation, implicating the proximal stage of the ERK/MAPK pathway as a relevant control point. Because stromal contact blocked delayed but not early ERK activation, the small guanosine triphosphatase (GTPase) Rap1 was considered as a candidate inhibitory target. Activation assays confirmed that Rap1 underwent sustained activation as a result of megakaryocytic induction, as previously described. As with ERK activation, stromal contact selectively blocked delayed but not early Rap1 activation, having no effect on Ras activation. Enforced expression of either wild-type Rap1 or the GTPase (GAP) resistant mutant Rap1 V12 failed to override stromal inhibition, suggesting that the inhibitory mechanism does not involve GAP up-regulation but rather may target upstream guanine nucleotide exchange factor (GEF) complexes. Accordingly, coimmunoprecipitation demonstrated stromally induced alterations in a protein complex associated with c-Cbl, a scaffolding factor for Rap1-GEF complexes.

Molecular cloning and identification of a putative PKC epsilon cDNA from Limulus polyphemus brain

The protein kinase C (PKC) family of enzymes is broadly distributed and has been implicated in a diverse array of cellular functions. Recent evidence supporting PKC involvement in the regulation of the Limulus choline cotransporter prompted us to clone PKC from a Limulus central nervous system (CNS) cDNA library. An Aplysia californica calcium independent PKC (Apl II) cDNA probe was used to screen the library and 5' RACE SMART PCR was used to obtain the full-length sequence. The resulting cDNA, which included 5' and 3' nontranslation regions, was 4675 bp. Analysis of the encoded peptide sequence using the Swiss-prot database revealed at least 58% identity to PKC epsilon. A commercial polyclonal antibody against PKC epsilon was used in Western blots to positively label a 30 kDa protein from Limulus CNS and the expressed fusion protein of the encoded sequence. These data support the presence of a newly identified PKC-like homolog in Limulus which likely represents a PKC epsilon equivalent.

Characterization of a pan-species reactive monoclonal antibody specific for a cell surface epitope which could serve as a marker for human monocytic and megakaryocytic differentiation

A mouse IgG(1)-producing hybridoma, CSC-31, was isolated and characterized. The monoclonal antibody (MAb) was originally raised against monkey kidney cell-surface molecules. FACS analysis further showed that CSC-31 exhibited broad tissue and species reactivity. Although most human T- and B-cell lines failed to react with CSC-31, myeloid, and erythroleukemia cells lines such as THP-1 and K562 expressed the CSC-31 cell surface marker. Furthermore, in vitro differentiation of HL-60, U-937, and K562 showed that expression of the CSC-31 marker is associated with monocytic or megakaryocytic differentiation. However, up-regulation of the CSC-31 marker expression was not detected during granulocytic or erythroid differentiation. Through the in vitro differentiation of K562, it was demonstrated that up-regulation of the CSC-31 marker required novel PKCs and might be regulated by the MAPK signaling pathway. Last, limited biochemical analysis demonstrated that the CSC-31-specific epitope is sensitive to digestion by papain yet highly resistant to other proteases.

Protein kinase C mediates mutant N-Ras-induced developmental abnormalities in normal human erythroid cells

RAS mutations are one of the most frequent molecular abnormalities associated with myeloid leukemia and preleukemia, yet there is a poor understanding of how they contribute to the pathogenesis of these conditions. Here, we describe the consequences of ectopic mutant N-Ras (N-Ras*) expression on normal human erythropoiesis. We show that during early (erythropoietin [EPO]-independent) erythropoiesis, N-Ras* promoted the amplification of a phenotypically primitive but functionally defective subpopulation of CD34(+) erythroblasts. N-Ras* also up-regulated the expression of megakaryocyte antigens on human erythroblasts. Although early erythroblasts expressing N-Ras* were able to respond to erythropoietin and generate mature progeny, this occurred with greatly reduced efficiency, probably explaining the poor colony growth characteristics of these cells. We further report that this oncogene promoted the expression and activation of protein kinase C (PKC) and that the effects of N-Ras* on erythropoiesis could be abrogated or attenuated by inhibition of PKC. Similarly, the effects of this oncogene could be partially mimicked by treatment with PKC agonist. Together, these data suggest that expression of N-Ras* is able to subvert the normal developmental cues that regulate erythropoiesis by activating PKC. This gives rise to phenotypic and functional abnormalities commonly observed in preleukemia, suggesting a direct link between RAS mutations and the pathogenesis of preleukemia.

Role of protein kinase C alpha in primary human osteoblast proliferation

Protein kinase C (PKC) isoforms have been shown to have specific expression profiles and individual isoforms are believed to play distinct roles in the cells in which they are found. The goal here was to determine which specific isoform(s) is involved in proliferation of primary human osteoblasts. In primary human osteoblasts, 10 microM of acute sphingosine-1-phosphate (S1P) treatment induced an increase in proliferation that correlated with an increase in PKCalpha and PKCiota expression. To further delineate which isoforms are involved in osteoblastic cell proliferation, the effect of low versus high serum culture conditions on PKC isoform expression was determined. Likewise, the effect of antisense oligodeoxynucleotides (ODNs) to specific PKC isoforms on proliferation and MAPK activation was studied. The effect of S1P on intracellular translocation of activated PKC isoforms was also evaluated. The results indicated that in primary human osteoblasts, PKCalpha was not expressed under conditions of low proliferative rate while PKCdelta and PKCiota expression was not affected. The specific inhibition of PKCalpha by antisense ODNs resulted in inhibition of MAPK activity leading to a significant decrease in proliferation. S1P up-regulated antisense ODN inhibited PKCalpha expression and MAPK activity and led to an increase in proliferation. Subsequent experiments using platelet-derived growth factor (PDGF) as an additional mitogen generated similar data. PDGF stimulation resulted in a significant increase in proliferation that correlated with an up-regulation of inhibited PKCalpha expression in antisense ODN-treated cells. Immunofluorescence methods showed that mitogenic stimulation of PKCa resulted in nuclear translocation. Our findings present original data that PKCalpha is the isoform specifically involved in the proliferation of primary human osteoblasts.

Hematopoietic cytokines, transcription factors and lineage commitment

The past two decades have witnessed significant advances in our understanding of the cellular physiology and molecular regulation of hematopoiesis. At the heart of stem cell self-renewal and lineage commitment decisions lies the relative expression levels of lineage-specific transcription factors. The expression of these transcription factors in early stem cells may be promiscuous and fluctuate, but ultimately comes under the influence of extracellular regulatory signals in the form of hematopoietic cytokines. In this review, we first summarize our current understanding of the phenotypic characterization of hematopoietic stem cells. Next, we describe key known transcription factors which govern stem cell self-renewal and lineage commitment decisions. Finally, we review data concerning the role of specific cytokines in influencing these decisions. From this review, a picture emerges in which stem cell fate decisions are governed by the integrated effects of intrinsic transcription factors and external signaling pathways initiated by regulatory cytokines.

Protein kinase C epsilon mediates the induction of P-glycoprotein in LNCaP prostate carcinoma cells

P-glycoprotein (P-gp) mediates drug resistance. Protein kinase C (PKC) expression correlates with drug resistance in several types of cancer. We determined whether PKC signals the induction of P-gp in LNCaP human prostate cancer cells, and identified a specific isozyme involved, in a model of aspirin-induced P-glycoprotein expression. An inhibitor of PKC activity, and a specific peptide inhibitor of PKC epsilon translocation, suppressed the induction of P-gp. The PKC activator ingenol, but not OAG, induced P-gp expression in a dose-dependent manner. Based on our results, we conclude that PKC epsilon mediates the induction of P-gp. Accordingly, PKC epsilon is activated and translocates from the membrane fraction to the cytoskeleton fraction in aspirin-treated cells. The findings of this study point to PKC epsilon as a signalling molecule for the induction of P-gp in LNCaP prostate cancer cells.

Stromal inhibition of megakaryocytic differentiation correlates with blockade of signaling by protein kinase C-epsilon and ERK/MAPK

Contact with bone marrow stromal cells maintains normal and leukemic hematopoietic progenitors in an undifferentiated state. Recently, stromal contact has been shown to diminish the yield of megakaryocytes in cultures of primary human hematopoietic stem cells. This inhibition may explain the poor megakaryocytic engraftment frequently observed after bone marrow transplantation. In the current study, stromal co-culture is shown to render K562 cells refractory to megakaryocytic induction. This stromal inhibition correlated with the selective down-regulation in K562 cells of protein kinase C-epsilon (PKC-epsilon), which has recently been implicated in regulation of megakaryocytic lineage commitment. In addition, the stromal inhibition correlated with inactivation of the ERK/MAPK pathway, which has also been implicated in promoting megakaryocytic development. Forced expression of PKC-epsilon by retroviral transduction was insufficient to reverse the stromal blockade of ERK/MAPK signaling or of megakaryocytic induction. Thus stromal interruption of ERK/MAPK signaling occurred independently of PKC-epsilon levels and correlated more closely with megakaryocytic blockade. These findings provide potential mechanisms for stromal inhibition of hematopoietic differentiation and possibly for the poor megakaryocytic engraftment seen after bone marrow transplantation.

Induction of megakaryocytic differentiation in primary human erythroblasts: a physiological basis for leukemic lineage plasticity

In myelodysplasias and acute myeloid leukemias, abnormalities in erythroid development often parallel abnormalities in megakaryocytic development. Erythroleukemic cells in particular have been shown to possess the potential to undergo megakaryocytic differentiation in response to a variety of stimuli. Whether or not such lineage plasticity occurs as a consequence of the leukemic phenotype has not previously been addressed. In this study, highly purified primary human erythroid progenitors were subjected to stimuli known to induce megakaryocytic differentiation in erythroleukemic cells. Remarkably, the primary erythroid progenitors rapidly responded with morphological and immunophenotypic evidence of megakaryocytic differentiation, equivalent to that seen in erythroleukemic cells. Even erythroblasts expressing high levels of hemoglobin manifested partial megakaryocytic differentiation. These results indicate that the lineage plasticity observed in erythroleukemic cells reflects an intrinsic property of cells in the erythroid lineage rather than an epiphenomenon of leukemic transformation.