p210BCR/ABL-induced alteration of pre-mRNA splicing in primary human CD34+ hematopoietic progenitor cells

Biological function and research progress of N6-methyladenosine binding protein heterogeneous nuclear ribonucleoprotein A2B1 in human cancers

N6-methyladenosine (m6A) is the most prevalent internal modification found in both mRNA and lncRNA. It exerts reversible regulation over RNA function and affects RNA processing and metabolism in various diseases, especially tumors. The m6A binding protein, hnRNPA2B1, is extensively studied as a member of the heterogeneous nuclear ribonucleoprotein (hnRNP) protein family. It is frequently dysregulated and holds significant importance in multiple types of tumors. By recognizing m6A sites for variable splicing, maintaining RNA stability, and regulating translation and transport, hnRNPA2B1 plays a vital role in various aspects of tumor development, metabolism, and regulation of the immune microenvironment. In this review, we summarized the latest research on the functional roles and underlying molecular mechanisms of hnRNPA2B1. Moreover, we discussed its potential as a target for tumor therapy.

Inhibition of SRPK1, a key splicing regulator, exhibits antitumor and chemotherapeutic-sensitizing effects on extranodal NK/T-cell lymphoma cells

Background

Increasing evidence has convincingly shown that abnormal pre-mRNA splicing is implicated in the development of most human malignancies. Serine/arginine-rich protein kinase 1 (SRPK1), a key splicing regulator, is reported to be overexpressed in leukemia and other cancer types, which suggests the therapeutic potential of targeting SRPK1.

Methods

SRPK1 expression was measured in 41 ENKTL patients by immunohistochemistry and mRNA expression was analyzed by qRT‒PCR. We knocked down SRPK1 expression in the ENKTL cell line YT by siRNA transfection and inhibited SRPK1 using inhibitors (SPHINX31 and SRPIN340) in YT cells and peripheral blood lymphocytes (PBLs) isolated from ENKTL patients to investigate its role in cell proliferation and apoptosis. Then, RNA-seq analysis was performed to predict the potential signaling pathway by which SRPK1 inhibition induces cell death and further verified this prediction by Western blotting.

Results

In the present study, we initially evaluated the clinical significance of SRPK1 in extranodal natural killer/T-cell lymphoma (ENKTL), a very aggressive subtype of non-Hodgkin lymphoma. The expression of SRPK1 in ENKLT patients was examined by immunohistochemistry and qRT‒PCR, which revealed SRPK1 overexpression in more than 60% of ENKTL specimens and its association with worse survival. Cellular experiments using the human ENKTL cell line YT and PBLs from ENKTL patients, demonstrated that inhibition of SRPK1 suppressed cell proliferation and induced apoptosis. Subsequently, we investigated the downstream targets of SRPK1 by RNA-seq analysis and found that SRPK1 inhibition induced ATF4/CHOP pathway activation and AKT1 inhibition. Furthermore, ENKTL patients presenting high SRPK1 expression showed resistance to cisplatin-based chemotherapy. The association of SRPK1 expression with cisplatin resistance was also confirmed in YT cells. SRPK1 overexpression via pLVX-SRPK1 plasmid transfection dramatically decreased the sensitivity of YT cells to cisplatin, while siRNA-mediated SRPK1 knockdown or SRPK1 inhibitor treatment significantly increased cisplatin cytotoxicity.

Conclusion

In summary, these results support that SRPK1 might be a useful clinical prognostic indicator and therapeutic target for ENKTL, especially for patients who relapse after cisplatin-based chemotherapies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-10158-6.

The Spliceosome: A New Therapeutic Target in Chronic Myeloid Leukaemia

Simple Summary

RNA splicing factors are frequently altered in cancer and have been found mutated or deregulated in myeloid malignancies, justifying the growing interest in new therapeutic strategies. We recently showed that the DNA methylation alterations of CD34⁺CD15⁻ chronic myeloid leukaemia (CML) cells affect alternative splicing genes, suggesting that spliceosome actors might be altered in chronic-phase (CP)-CML. We investigated the expression of 12 splicing genes in primary CP-CML CD34⁺ cells at diagnosis (n = 15). We found that CP-CML CD34⁺ cells had a distinct splicing signature profile, suggesting: (i) a spliceosome deregulation from the diagnosis time and (ii) an intraclonal heterogeneity. In vitro incubation of a spliceosome-targeted drug (TG003) showed that CP-CML CD34⁺ cells are spliceosome dependent; moreover, with the combination of TKI, the two drugs showing an additive effect while sparing healthy donors cells. Our results suggest that the spliceosome may be a new potential target for the treatment of CML.

Abstract

RNA splicing factors are frequently altered in cancer and can act as both oncoproteins and tumour suppressors. They have been found mutated or deregulated, justifying the growing interest in the targeting of splicing catalysis, splicing regulatory proteins, and/or specific, key altered splicing events. We recently showed that the DNA methylation alterations of CD34⁺CD15⁻ chronic myeloid leukaemia (CML) cells affect, among others, alternative splicing genes, suggesting that spliceosome actors might be altered in chronic-phase (CP)-CML. We investigated the expression of 12 spliceosome genes known to be oncogenes or tumour suppressor genes in primary CP-CML CD34⁺ cells at diagnosis (n = 15). We found that CP-CML CD34⁺ cells had a distinct splicing signature profile as compared with healthy donor CD34⁺ cells or whole CP-CML cells, suggesting: (i) a spliceosome deregulation from the diagnosis time and (ii) an intraclonal heterogeneity. We could identify three profile types, but there was no relationship with a patient’s characteristics. By incubating cells with TKI and/or a spliceosome-targeted drug (TG003), we showed that CP-CML CD34⁺ cells are both BCR::ABL and spliceosome dependent, with the combination of the two drugs showing an additive effect while sparing healthy donors cells. Our results suggest that the spliceosome may be a new potential target for the treatment of CML.

Splicing Anomalies in Myeloproliferative Neoplasms: Paving the Way for New Therapeutic Venues

Since the discovery of spliceosome mutations in myeloid malignancies, abnormal pre-mRNA splicing, which has been well studied in various cancers, has attracted novel interest in hematology. However, despite the common occurrence of spliceosome mutations in myelo-proliferative neoplasms (MPN), not much is known regarding the characterization and mechanisms of splicing anomalies in MPN. In this article, we review the current scientific literature regarding “splicing and myeloproliferative neoplasms”. We first analyse the clinical series reporting spliceosome mutations in MPN and their clinical correlates. We then present the current knowledge about molecular mechanisms by which these mutations participate in the pathogenesis of MPN or other myeloid malignancies. Beside spliceosome mutations, splicing anomalies have been described in myeloproliferative neoplasms, as well as in acute myeloid leukemias, a dreadful complication of these chronic diseases. Based on splicing anomalies reported in chronic myelogenous leukemia as well as in acute leukemia, and the mechanisms presiding splicing deregulation, we propose that abnormal splicing plays a major role in the evolution of myeloproliferative neoplasms and may be the target of specific therapeutic strategies.

SRSF1 mediates cytokine-induced impaired imatinib sensitivity in chronic myeloid leukemia

Patients with chronic myeloid leukemia (CML) who are treated with tyrosine kinase inhibitors (TKIs) experience significant heterogeneity regarding depth and speed of responses. Factors intrinsic and extrinsic to CML cells contribute to response heterogeneity and TKI resistance. Among extrinsic factors, cytokine-mediated TKI resistance has been demonstrated in CML progenitors, but the underlying mechanisms remain obscure. Using RNA-sequencing, we identified differentially expressed splicing factors in primary CD34⁺ chronic phase (CP) CML progenitors and controls. We found SRSF1 expression to be increased as a result of both BCR-ABL1- and cytokine-mediated signaling. SRSF1 overexpression conferred cytokine independence to untransformed hematopoietic cells and impaired imatinib sensitivity in CML cells, while SRSF1 depletion in CD34⁺ CP CML cells prevented the ability of extrinsic cytokines to decrease imatinib sensitivity. Mechanistically, PRKCH and PLCH1 were upregulated by elevated SRSF1 levels, and contributed to impaired imatinib sensitivity. Importantly, very high SRSF1 levels in the bone marrow of CML patients at presentation correlated with poorer clinical TKI responses. In summary, we find SRSF1 levels to be maintained in CD34⁺ CP CML progenitors by cytokines despite effective BCR-ABL1 inhibition, and that elevated levels promote impaired imatinib responses. Together, our data support an SRSF1/PRKCH/PLCH1 axis in contributing to cytokine-induced impaired imatinib sensitivity in CML.

Serine arginine protein kinase 1 (SRPK1): a moonlighting protein with theranostic ability in cancer prevention

Serine/arginine protein kinase 1 (SRPK1); a versatile functional moonlighting protein involved in varied cellular activities comprised of cell cycle progression, innate immune response, chromatin reorganization, negative and positive regulation of viral genome replication, protein amino acid phosphorylation, regulation of numerous mRNA-processing pathways, germ cell development as well as inflammation due to acquaintances with many transcription factors and signaling pathways. Several diseases including cancer have been associated with dysregulation of SRPK1. The function of SRPK1 in cancer is contradictory and inexplicable because it acts as both tumor suppressor and promoter based on the type of cell and locale. Over expression of SRPK1 including its role has been recently narrated and associated with several cancers, which includes, lung, glioma, prostate and breast via dysregulated signals from the Akt/eIF4E/HIF-1/VEGF, Erk or MAPK, PI3K/AKT/mTOR, TGF-β, and Wnt/β-catenin signaling pathways. Therefore, SRPK1 has occurred as a promising and possible curative target in cancer. In recent years, few natural and synthetic SRPK1 inhibitors have been discovered. This review emphasizes and highlights the complicated connections between SRPK1 and oncogenic signaling circuits together with the possibility of aiming SRPK1 in the treatment of cancer.

Role of Pyk2 in Human Cancers

Proline-rich tyrosine kinase 2 (Pyk2) plays essential roles in tumorigenesis and tumor progression. Pyk2 serves as a non-receptor tyrosine kinase regulating tumor cell survival, proliferation, migration, invasion, metastasis, and chemo-resistance, and is associated with poor prognosis and shortened survival in various cancer types. Thus, Pyk2 has been traditionally regarded as an oncogene and potential therapeutic target for cancers. However, a few studies have also demonstrated that Pyk2 exerts tumor-suppressive effects in some cancers, and anti-cancer treatment of Pyk2 inhibitors may only achieve marginal benefits in these cancers. Therefore, more detailed knowledge of the contradictory functions of Pyk2 is needed. In this review, we summarized the tissue distribution, expression, interactive molecules of Pyk2 in the signaling pathway, and roles of Pyk2 in cancers, and focused on regulation of the interconnectivity between Pyk2 and its downstream targets. The potential use of inhibitors of Pyk2 and its related pathways in cancer therapy is also discussed.

Potential Antileukemia Effect and Structural Analyses of SRPK Inhibition by N-(2-(Piperidin-1-yl)-5-(Trifluoromethyl)Phenyl)Isonicotinamide (SRPIN340)

Dysregulation of pre-mRNA splicing machinery activity has been related to the biogenesis of several diseases. The serine/arginine-rich protein kinase family (SRPKs) plays a critical role in regulating pre-mRNA splicing events through the extensive phosphorylation of splicing factors from the family of serine/arginine-rich proteins (SR proteins). Previous investigations have described the overexpression of SRPK1 and SRPK2 in leukemia and other cancer types, suggesting that they would be useful targets for developing novel antitumor strategies. Herein, we evaluated the effect of selective pharmacological SRPK inhibition by N-(2-(piperidin-1-yl)-5-(trifluoromethyl)phenyl)isonicotinamide (SRPIN340) on the viability of lymphoid and myeloid leukemia cell lines. Along with significant cytotoxic activity, the effect of treatments in regulating the phosphorylation of the SR protein family and in altering the expression of MAP2K1, MAP2K2, VEGF and FAS genes were also assessed. Furthermore, we found that pharmacological inhibition of SRPKs can trigger early and late events of apoptosis. Finally, intrinsic tryptophan fluorescence emission, molecular docking and molecular dynamics were analyzed to gain structural information on the SRPK/SRPIN340 complex. These data suggest that SRPK pharmacological inhibition should be considered as an alternative therapeutic strategy for fighting leukemias. Moreover, the obtained SRPK-ligand interaction data provide useful structural information to guide further medicinal chemistry efforts towards the development of novel drug candidates.

Splicing Regulators and Their Roles in Cancer Biology and Therapy

Alternative splicing allows cells to expand the encoding potential of their genomes. In this elegant mechanism, a single gene can yield protein isoforms with even antagonistic functions depending on the cellular physiological context. Alterations in splicing regulatory factors activity in cancer cells, however, can generate an abnormal protein expression pattern that promotes growth, survival, and other processes, which are relevant to tumor biology. In this review, we discuss dysregulated alternative splicing events and regulatory factors that impact pathways related to cancer. The SR proteins and their regulatory kinases SRPKs and CLKs have been frequently found altered in tumors and are examined in more detail. Finally, perspectives that support splicing machinery as target for the development of novel anticancer therapies are discussed.

Toward the development of a global induced pluripotent stem cell library

The ability to preselect the donor genotype of iPSC lines provides important opportunities for immune matching in cell therapy. Here we propose that an international assessment should be made of how immune incompatibility can best be managed and how a network of GMP HLA homozygous haplobanks could be operated.

Focal adhesion kinase splice variants maintain primitive acute myeloid leukemia cells through altered Wnt signaling

Focal adhesion kinase (FAK) activity contributes to many advanced cancer phenotypes, but little is known about its role in human acute myeloid leukemia (AML). Here, we show that FAK splice variants are abnormally expressed in the primitive leukemic cells of poor prognosis AML patients. In the CD34(+) 38(-) 123(+) long-term culture-initiating cell-enriched leukemic cells of these patients, FAK upregulates expression of Frizzled-4 and phosphorylates Pyk2 to enable the required association of Pyk2 with the Wnt5a/Frizzled-4/LRP5 endocytosis complex and downstream activation of β-catenin, thereby replacing the Wnt3a-controlled canonical pathway used by normal hematopoietic stem cells. Transduction of primitive normal human hematopoietic cells with FAK splice variants induces a marked increase in their clonogenic activity and signaling via the Wnt5a-controlled canonical pathway. Targeting FAK or β-catenin efficiently eradicates primitive leukemic cells in vitro suggesting that FAK could be a useful therapeutic target for improved treatment of poor prognosis AML cases.

Cancer-Associated Perturbations in Alternative Pre-messenger RNA Splicing

For most of our 25,000 genes, the removal of introns by pre-messenger RNA (pre-mRNA) splicing represents an essential step toward the production of functional messenger RNAs (mRNAs). Alternative splicing of a single pre-mRNA results in the production of different mRNAs. Although complex organisms use alternative splicing to expand protein function and phenotypic diversity, patterns of alternative splicing are often altered in cancer cells. Alternative splicing contributes to tumorigenesis by producing splice isoforms that can stimulate cell proliferation and cell migration or induce resistance to apoptosis and anticancer agents. Cancer-specific changes in splicing profiles can occur through mutations that are affecting splice sites and splicing control elements, and also by alterations in the expression of proteins that control splicing decisions. Recent progress in global approaches that interrogate splicing diversity should help to obtain specific splicing signatures for cancer types. The development of innovative approaches for annotating and reprogramming splicing events will more fully establish the essential contribution of alternative splicing to the biology of cancer and will hopefully provide novel targets and anticancer strategies. Metazoan genes are usually made up of several exons interrupted by introns. The introns are removed from the pre-mRNA by RNA splicing. In conjunction with other maturation steps, such as capping and polyadenylation, the spliced mRNA is then transported to the cytoplasm to be translated into a functional protein. The basic mechanism of splicing requires accurate recognition of each extremity of each intron by the spliceosome. Introns are identified by the binding of U1 snRNP to the 5' splice site and the U2AF65/U2AF35 complex to the 3' splice site. Following these interactions, other proteins and snRNPs are recruited to generate the complete spliceosomal complex needed to excise the intron. While many introns are constitutively removed by the spliceosome, other splice junctions are not used systematically, generating the phenomenon of alternative splicing. Alternative splicing is therefore the process by which a single species of pre-mRNA can be matured to produce different mRNA molecules (Fig. 1). Depending on the number and types of alternative splicing events, a pre-mRNA can generate from two to several thousands different mRNAs leading to the production of a corresponding number of proteins. It is now believed that the expression of at least 70 % of human genes is subjected to alternative splicing, implying an enormous contribution to proteomic diversity, and by extension, to the development and the evolution of complex animals. Defects in splicing have been associated with human diseases (Caceres and Kornblihtt, Trends Genet 18(4):186-93, 2002, Cartegni et al., Nat Rev Genet 3(4):285-98, 2002, Pagani and Baralle, Nat Rev Genet 5(5):389-96, 2004), including cancer (Brinkman, Clin Biochem 37(7):584-94, 2004, Venables, Bioessays 28(4):378-86, 2006, Srebrow and Kornblihtt, J Cell Sci 119(Pt 13):2635-2641, 2006, Revil et al., Bull Cancer 93(9):909-919, 2006, Venables, Transworld Res Network, 2006, Pajares et al., Lancet Oncol 8(4):349-57, 2007, Skotheim and Nees, Int J Biochem Cell Biol 39:1432-1449, 2007). Numerous studies have now confirmed the existence of specific differences in the alternative splicing profiles between normal and cancer tissues. Although there are a few cases where specific mutations are the primary cause for these changes, global alterations in alternative splicing in cancer cells may be primarily derived from changes in the expression of RNA-binding proteins that control splice site selection. Overall, these cancer-specific differences in alternative splicing offer an immense potential to improve the diagnosis and the prognosis of cancer. This review will focus on the functional impact of cancer-associated alternative splicing variants, the molecular determinants that alter the splicing decisions in cancer cells, and future therapeutic strategies.

Elevated expression of the serine-arginine protein kinase 1 gene in ovarian cancer and its role in Cisplatin cytotoxicity in vitro

Alternatively spliced variants of several oncogenes and tumor suppressors have been shown to be important for their tumorigenicity. In the present study we have tested whether serine-arginine protein kinase 1 (SRPK1), a major regulator of splicing factors, is involved in ovarian cancer progression and plays a role in chemo-sensitivity. By Western blot analyses, SRPK1 protein was found to be overexpressed in 4 out of 6 ovarian cancer cell lines as compared with an immortalized ovarian surface epithelial cell line; and in 55% of ovarian tumor samples as compared with non-neoplastic ovarian tissue samples. Reduction of SRPK1 expression using small interfering RNA (siRNA) encoding small hairpin RNA in ovarian cancer cells led to (i) reduced cell proliferation rate, slower cell cycle progression and compromised anchorage-independent growth and migration ability in vitro, (ii) decreased level of phosphorylation of multiple serine-arginine proteins, and P44/42MAPK and AKT proteins, and (iii) enhanced sensitivity to cisplatin. Together, these results suggest that elevated SRPK1 expression may play a role in ovarian tumorigenesis and SRPK1 may be a potential target for ovarian cancer therapy.

Chronic myeloid leukemia stem cell biology

Leukemia progression and relapse is fueled by leukemia stem cells (LSC) that are resistant to current treatments. In the progression of chronic myeloid leukemia (CML), blast crisis progenitors are capable of adopting more primitive but deregulated stem cell features with acquired resistance to targeted therapies. This in turn promotes LSC behavior characterized by aberrant self-renewal, differentiation, and survival capacity. Multiple reports suggest that cell cycle alterations, activation of critical signaling pathways, aberrant microenvironmental cues from the hematopoietic niche, and aberrant epigenetic events and deregulation of RNA processing may facilitate the enhanced survival and malignant transformation of CML progenitors. Here we review the molecular evolution of CML LSC that promotes CML progression and relapse. Recent advances in these areas have identified novel targets that represent important avenues for future therapeutic approaches aimed at selectively eradicating the LSC population while sparing normal hematopoietic progenitors in patients suffering from chronic myeloid malignancies.

Phosphoprotein profiling predicts response to tyrosine kinase inhibitor therapy in chronic myeloid leukemia patients

Tyrosine kinase inhibitors (TKIs) have dramatically improved treatment outcomes in chronic myeloid leukemia (CML), but a proportion of patients fail to achieve optimal molecular response. By using a phosphoproteomic approach, we aimed to discover aberrant signaling pathways and putative biomarkers in bone marrow samples of suboptimally responding patients, which could be used to guide treatment selection at the diagnosis. The study consisted of 20 chronic-phase CML patients (10 optimal and 10 suboptimal response patients based on 18 months European-Leukemia-Net criteria) and healthy bone marrow cells, and CML cell lines were used as controls. The phosphorylation profile of normal bone marrow cells diverged from CML patients expectedly but, interestingly, CML cell lines (such as K562) also showed marked difference with primary CML cells. Several phosphoproteins were elevated in suboptimal patients compared to optimal response group. Most prominent differences were seen in signal transducers and activators of transcription 5b, phospholipase C γ-1, proline-rich tyrosine kinase 2, Hck, and Paxillin. These phosphoproteins were also increased in three additional nonresponder patients studied, but each of them also had unique phosphorylation patterns, such as highly active HSP27 protein in one patient. In conclusion, suboptimal imatinib response is related to increased phosphorylation of several proteins at diagnosis, which might guide the selection of TKI therapy. Furthermore, the activation of additional BCR-ABL-independent pathways in nonresponder patients (such as the anti-apoptotic HSP27 pathway) may reveal novel therapy targets.

Three novel alternative splicing mutations in BCR-ABL1 detected in CML patients with resistance to kinase inhibitors

INTRODUCTION

Multiple types of mutations in the BCR-ABL1 kinase domain have been reported. We previously reported a common alternatively spliced BCR-ABL mRNA with a 35-nucleotide insertion (35INS). We report three novel alternative splicing mutants expressed as the dominant transcripts in patient with chronic myelogenous leukemia and resistance to kinase inhibitors.

METHODS

We screened RNA from more than 200 patients with resistance to more than one of the three kinase inhibitors for ABL1 kinase domain mutations by direct sequencing.

RESULTS

We found three not previously described splice mutants. All three showed >90% mutant transcript. The first resulted from the insertion of 79 nucleotides into the ABL1 exon 8-9 junction. The inserted sequence contained a sequence from regions of intron 8, located 120 bp apart: the 35-nucleotide sequence previously described, and an additional 44-nucleotide segment downstream from 35INS. The combined 79-nucleotide insertion splice mutant showed the same protein change as 35INS (p C475YfsX11). The second splice mutation comprised an 84-nucleotide sequence from intron 7 inserted into the ABL1 exon7-8 junction, also causing a frameshift and protein truncation (p A424EfsX18). The third splice derived from a 231-nucleotide sequence from intron 4 retained in the ABL1 exon 4-5 junction adding 40 intron-encoded amino acids and leading to a frameshift and early termination (p E275LfsX41).

CONCLUSION

These findings, when combined with the data on 35INS, support the concept that loss of the C-terminus of BCR-ABL1 is associated with significant resistance to kinase inhibitors; this mechanism appears to be a major source of resistance to kinase inhibitors.

BCR-ABL1 kinase-dependent alteration of mRNA metabolism: potential alternatives for therapeutic intervention

The use of first- and second-generation tyrosine kinase inhibitors (TKIs) significantly improves prognosis for patients with early chronic phase chronic myeloid leukemia (CML) and efficiently counteracts leukemia in most patients with CML bearing a disease characterized by the expression of BCR-ABL1 mutants. However, the so-called 'tinib' TKIs (e.g. imatinib, nilotinib, dasatinib, and bosutinib) are both ineffective in patients who undergo blastic transformation and unable to eradicate CML at the stem cell level. This raises a few important questions. Is BCR-ABL1 expression and/or activity essential for blastic transformation? Is blastic transformation the result of genetic or epigenetic events that occur at the stem cell level which only become apparent in the granulocyte-macrophage progenitor (GMP) cell pool, or does it arise directly at the GMP level? As altered mRNA metabolism contributes to the phenotype of blast crisis CML progenitors (decreased translation of tumor suppressor genes and transcription factors essential for terminal differentiation and increased translation of anti-apoptotic genes), one attractive concept is to restore levels of these essential molecules to their normal levels. In this review, we discuss the mechanisms by which mRNA processing, translation, and degradation are deregulated in BCR-ABL1 myeloid blast crisis CML progenitors, and present encouraging results from studies with pharmacologic inhibitors which support their inclusion in the clinic.

The ratio of SRPK1/SRPK1a regulates erythroid differentiation in K562 leukaemic cells

SRPK1, the prototype of the serine/arginine family of kinases, has been implicated in the regulation of multiple cellular processes such as pre-mRNA splicing, chromatin structure, nuclear import and germ cell development. SRPK1a is a much less studied isoform of SRPK1 that contains an extended N-terminal domain and so far has only been detected in human testis. In the present study we show that SRPK1 is the predominant isoform in K562 cells, with the ratio of the two isoforms being critical in determining cell fate. Stable overexpression of SRPK1a induces erythroid differentiation of K562 cells. The induction of globin synthesis was accompanied by a marked decrease in proliferation and a significantly reduced clonogenic potential. Small interfering RNA-mediated down-regulation of SRPK1 in K562 cells results similarly in a decrease in proliferative capacity and induction of globin synthesis. A decreased SRPK1/SRPK1a ratio is also observed upon hemin/DMSO-induced differentiation of K562 cells as well as in normal human erythroid progenitor cells. Mass spectrometric analysis of SRPK1a-associated proteins identified multiple classes of RNA-binding proteins including RNA helicases, heterogeneous nuclear ribonucleoproteins, ribosomal proteins, and mRNA-associated proteins. Several of the SRPK1a-copurifying proteins have been previously identified in ribosomal and pre-ribosomal complexes, thereby suggesting that SRPK1a may play an important role in linking ribosomal assembly and/or function to erythroid differentiation in human leukaemic cells.

Chronic myeloid leukemia: mechanisms of blastic transformation

The BCR-ABL1 oncoprotein transforms pluripotent HSCs and initiates chronic myeloid leukemia (CML). Patients with early phase (also known as chronic phase [CP]) disease usually respond to treatment with ABL tyrosine kinase inhibitors (TKIs), although some patients who respond initially later become resistant. In most patients, TKIs reduce the leukemia cell load substantially, but the cells from which the leukemia cells are derived during CP (so-called leukemia stem cells [LSCs]) are intrinsically insensitive to TKIs and survive long term. LSCs or their progeny can acquire additional genetic and/or epigenetic changes that cause the leukemia to transform from CP to a more advanced phase, which has been subclassified as either accelerated phase or blastic phase disease. The latter responds poorly to treatment and is usually fatal. Here, we discuss what is known about the molecular mechanisms leading to blastic transformation of CML and propose some novel therapeutic approaches.

Hematopoietic Pyk2 regulates migration of differentiated HL-60 cells

Background

Pyk2 is a non-receptor cytoplasmic tyrosine kinase that belongs to the focal adhesion kinase family and has been implicated in neutrophil spreading and respiratory burst activity caused by TNF-α. However, the role of Pyk2 in neutrophil migration is incompletely defined. In this study, we tested the hypothesis that Pyk2 regulates the migration of neutrophil-like differentiated HL-60 cells subsequent to β2-integrin mediated cell adhesion.

Methods

HL-60 cells were induced to differentiate into neutrophil-like cells (dHL60) by incubation in medium containing 1.25% DMSO for up to 4 days. Pyk2 expression and tyrosine phosphorylation was measured by Western blot analysis. Adhesion of dHL60 cells to plated fibrinogen was measured by residual myeloperoxidase activity. dHL60 cell migration was evaluated using a 96-well chemoTx chamber.

Results

Western blot analysis demonstrated that hematopoietic Pyk2 was predominantly expressed after HL60 cell differentiation. Pyk2 was tyrosine phosphorylated upon adhesion of dHL60 cells to plated fibrinogen in the presence of fMLP. By contrast, tyrosine phosphorylation of Pyk2 was insignificant in dHL60 cells treated in suspension with fMLP. Antibodies against CD18 blocked both phosphorylation of Pyk2 and adhesion of dHL60 cells to fibrinogen, demonstrating that phosphorylation of Pyk2 was β₂-integrin dependent. TAT-Pyk2-CT, a dominant negative fusion protein in which the TAT protein transduction domain was fused to the c-terminal Pyk2, attenuated fMLP-stimulated spreading, migration and phosphorylation of endogenous Pyk2 without blocking adhesion of dHL-60 cells to fibrinogen. Similarly, silencing of Pyk2 expression by siRNA in dHL60 cells also attenuated dHL60 cell migration caused by fMLP. Phospho-Pyk2 was evenly distributed around cell membrane circumferentially in unstimulated dHL-60 cells adherent to plated fibrinogen. In dHL60 cells treated with fMLP to cause cell spreading and polarization, Pyk2 was concentrated at the leading edge of pseudopods or at the trailing edge of uropods during migration of neutrophilic dHL-60 cells.

Conclusions

We conclude that Pyk2 is activated by β2-integrin adhesion. The activated concentration of Pyk2 and colocalization with F-actin in pseudopodia suggests that Pyk2 may regulate cell spreading and migration in dHL60 cells.

Alternative Splicing Modulates Stem Cell Differentiation

Stem cells have the surprising potential to develop into many different cell types. Therefore, major research efforts have focused on transplantation of stem cells and/or derived progenitors for restoring depleted diseased cells in degenerative disorders. Understanding the molecular controls, including alternative splicing, that arise during lineage differentiation of stem cells is crucial for developing stem cell therapeutic approaches in regeneration medicine. Alternative splicing to allow a single gene to encode multiple transcripts with different protein coding sequences and RNA regulatory elements increases genomic complexities. Utilizing differences in alternative splicing as a molecular marker may be more sensitive than simply gene expression in various degrees of stem cell differentiation. Moreover, alternative splicing maybe provide a new concept to acquire induced pluripotent stem cells or promote cell–cell transdifferentiation for restorative therapies and basic medicine researches. In this review, we highlight the recent advances of alternative splicing regulation in stem cells and their progenitors. It will hopefully provide much needed knowledge into realizing stem cell biology and related applications.

Regulation of PERK signaling and leukemic cell survival by a novel cytosolic isoform of the UPR regulator GRP78/BiP

The unfolded protein response (UPR) is an evolutionarily conserved mechanism to allow cells to adapt to stress targeting the endoplasmic reticulum (ER). Induction of ER chaperone GRP78/BiP increases protein folding capacity; as such it represents a major survival arm of UPR. Considering the central importance of the UPR in regulating cell survival and death, evidence is emerging that cells evolve feedback regulatory pathways to modulate the key UPR executors, however, the precise mechanisms remain to be elucidated. Here, we report the fortuitous discovery of GRP78va, a novel isoform of GRP78 generated by alternative splicing (retention of intron 1) and alternative translation initiation. Bioinformatic and biochemical analyses revealed that expression of GRP78va is enhanced by ER stress and is notably elevated in human leukemic cells and leukemia patients. In contrast to the canonical GRP78 which is primarily an ER lumenal protein, GRP78va is devoid of the ER signaling peptide and is cytosolic. Through specific knockdown of endogenous GRP78va by siRNA without affecting canonical GRP78, we showed that GRP78va promotes cell survival under ER stress. We further demonstrated that GRP78va has the ability to regulate PERK signaling and that GRP78va is able to interact with and antagonize PERK inhibitor P58(IPK). Our study describes the discovery of GRP78va, a novel cytosolic isoform of GRP78/BiP, and the first characterization of the modulation of UPR signaling via alternative splicing of nuclear pre-mRNA. Our study further reveals a novel survival mechanism in leukemic cells and other cell types where GRP78va is expressed.

Glycogen synthase kinase 3beta missplicing contributes to leukemia stem cell generation

Recent evidence suggests that a rare population of self-renewing cancer stem cells (CSC) is responsible for cancer progression and therapeutic resistance. Chronic myeloid leukemia (CML) represents an important paradigm for understanding the genetic and epigenetic events involved in CSC production. CML progresses from a chronic phase (CP) in hematopoietic stem cells (HSC) that harbor the BCR-ABL translocation, to blast crisis (BC), characterized by aberrant activation of beta-catenin within granulocyte-macrophage progenitors (GMP). A major barrier to predicting and inhibiting blast crisis transformation has been the identification of mechanisms driving beta-catenin activation. Here we show that BC CML myeloid progenitors, in particular GMP, serially transplant leukemia in immunocompromised mice and thus are enriched for leukemia stem cells (LSC). Notably, cDNA sequencing of Wnt/beta-catenin pathway regulatory genes, including adenomatous polyposis coli, GSK3beta, axin 1, beta-catenin, lymphoid enhancer factor-1, cyclin D1, and c-myc, revealed a novel in-frame splice deletion of the GSK3beta kinase domain in the GMP of BC samples that was not detectable by sequencing in blasts or normal progenitors. Moreover, BC CML progenitors with misspliced GSK3beta have enhanced beta-catenin expression as well as serial engraftment potential while reintroduction of full-length GSK3beta reduces both in vitro replating and leukemic engraftment. We propose that CP CML is initiated by BCR-ABL expression in an HSC clone but that progression to BC may include missplicing of GSK3beta in GMP LSC, enabling unphosphorylated beta-catenin to participate in LSC self-renewal. Missplicing of GSK3beta represents a unique mechanism for the emergence of BC CML LSC and might provide a novel diagnostic and therapeutic target.

Alternative splicing in cancer: Noise, functional, or systematic?

Pre-messenger RNA splicing is a fine-tuned process that generates multiple functional variants from individual genes. Various cell types and developmental stages regulate alternative splicing patterns differently in their generation of specific gene functions. In cancers, splicing is significantly altered, and understanding the underlying mechanisms and patterns in cancer will shed new light onto cancer biology. Cancer-specific transcript variants are promising biomarkers and targets for diagnostic, prognostic, and treatment purposes. In this review, we explore how alternative splicing cannot simply be considered as noise or an innocent bystander, but is actively regulated or deregulated in cancers. A special focus will be on aspects of cell biology and biochemistry of alternative splicing in cancer cells, addressing differences in splicing mechanisms between normal and malignant cells. The systems biology of splicing is only now applied to the field of cancer research. We explore functional annotations for some of the most intensely spliced gene classes, and provide a literature mining and clustering that reflects the most intensely investigated genes. A few well-established cancer-specific splice events, such as the CD44 antigen, are used to illustrate the potential behind the exploration of the mechanisms of their regulation. Accordingly, we describe the functional connection between the regulatory machinery (i.e., the spliceosome and its accessory proteins) and their global impact on qualitative transcript variation that are only now emerging from the use of genomic technologies such as microarrays. These studies are expected to open an entirely new level of genetic information that is currently still poorly understood.

Acute myeloid leukemia with the 8q22;21q22 translocation: secondary mutational events and alternative t(8;21) transcripts

Nonrandom and somatically acquired chromosomal translocations can be identified in nearly 50% of human acute myeloid leukemias. One common chromosomal translocation in this disease is the 8q22;21q22 translocation. It involves the AML1 (RUNX1) gene on chromosome 21 and the ETO (MTG8, RUNX1T1) gene on chromosome 8 generating the AML1-ETO fusion proteins. In this review, we survey recent advances made involving secondary mutational events and alternative t(8;21) transcripts in relation to understanding AML1-ETO leukemogenesis.

Serine-Arginine Protein Kinase 1 Overexpression Is Associated with Tumorigenic Imbalance in Mitogen-Activated Protein Kinase Pathways in Breast, Colonic, and Pancreatic Carcinomas

Aberrant patterns of pre-mRNA processing are typical of human malignancies, yet the mechanisms responsible for these changes remain undefined. We have recently shown overexpression of a core splice regulatory protein, serine-arginine protein kinase 1 (SRPK1), in dysplastic and neoplastic pancreatic ductular cells. In the present study, we have established that SRPK1 levels are similarly up-regulated in breast and colonic tumors where its expression increases coordinately with tumor grade. Targeting SRPK1 for inhibition using small interfering RNA in breast and colonic tumor cell lines in vitro resulted in both increased apoptotic potential and enhanced cell killing after treatment with gemcitabine and cisplatin. Recent reports have described multifaceted interactions between the mitogen-activated protein kinase (MAPK) and AKT signaling networks and the splice regulatory machinery. Consequently, we have shown that targeted inhibition of SRPK1 in tumor cells results in reduced phosphorylation of MAPK3, MAPK1, and AKT. Alterations in the splice pattern and resulting expression of MAPK kinase are implicated in mediating the antitumoral effects resulting from SRPK1 down-regulation. The up-regulation of SRPK1 in multiple cancers and its ability to regulate multiple relevant signaling pathways provide support for developing agents to inhibit this kinase for possible broad application to treat epithelial cancers.

Unbalanced alternative splicing and its significance in cancer

Alternative pre-mRNA splicing leads to distinct products of gene expression in development and disease. Antagonistic splice variants of genes involved in differentiation, apoptosis, invasion and metastasis often exist in a delicate equilibrium that is found to be perturbed in tumours. In several recent examples, splice variants that are overexpressed in cancer are expressed as hyper-oncogenic proteins, which often correlate with poor prognosis, thus suggesting improved diagnosis and follow up treatment. Global gene expression technologies are just beginning to decipher the interplay between alternatively spliced isoforms and protein-splicing factors that will lead to identification of the mutations in these trans-acting factors responsible for pathogenic alternative splicing in cancer.

BCR-ABL1 induces aberrant splicing of IKAROS and lineage infidelity in pre-B lymphoblastic leukemia cells

Pre-B lymphoblastic leukemia cells carrying a BCR-ABL1 gene rearrangement exhibit an undifferentiated phenotype. Comparing the genome-wide gene expression profiles of normal B-cell subsets and BCR-ABL1+ pre-B lymphoblastic leukemia cells by SAGE, the leukemia cells show loss of B lymphoid identity and aberrant expression of myeloid lineage-specific molecules. Consistent with this, BCR-ABL1+ pre-B lymphoblastic leukemia cells exhibit defective expression of IKAROS, a transcription factor needed for early lymphoid lineage commitment. As shown by inducible expression of BCR-ABL1 in human and murine B-cell precursor cell lines, BCR-ABL1 induces the expression of a dominant-negative IKAROS splice variant, termed IK6. Comparing matched leukemia sample pairs from patients before and during therapy with the BCR-ABL1 kinase inhibitor STI571 (Imatinib), inhibition of BCR-ABL1 partially corrected aberrant expression of IK6 and lineage infidelity of the leukemia cells. To elucidate the contribution of IK6 to lineage infidelity in BCR-ABL1+ cell lines, IK6 expression was silenced by RNA interference. Upon inhibition of IK6, BCR-ABL1+ leukemia cells partially restored B lymphoid lineage commitment. Therefore, we propose that BCR-ABL1 induces aberrant splicing of IKAROS, which interferes with lineage identity and differentiation of pre-B lymphoblastic leukemia cells.

Diversification of stem cell molecular repertoire by alternative splicing

Complete information regarding transcriptional and posttranscriptional gene regulation in stem cells is necessary to understand the regulation of self-renewal and differentiation. Alternative splicing is a prevalent mode of posttranscriptional regulation, and occurs in approximately one half of all mammalian genes. The frequency and functional impact of alternative splicing in stem cells are yet to be determined. In this study we combine computational and experimental methods to identify splice variants in embryonic and hematopoietic stem cells on a genome-wide scale. Using EST collections derived from stem cells, we detect alternative splicing in >1,000 genes. Systematic RT-PCR and sequencing studies show confirmation of computational predictions at a level of 80%. We find that alternative splicing can modify multiple components of signaling pathways important for stem cell function. We also analyze the distribution of splice variants across different classes of genes. We find that tissue-specific genes have a higher tendency to undergo alternative splicing than ubiquitously expressed genes. Furthermore, the patterns of alternative splicing are only weakly conserved between orthologous genes in human and mouse. Our studies reveal extensive modification of the stem cell molecular repertoire by alternative splicing and provide insights into its overall role as a mechanism of generating genomic diversity.

Evaluation of CML model cell lines and imatinib mesylate response: Determinants of signaling profiles

Our understanding of the mechanisms by which BCR-ABL drives CML is based, in part, on the use of model cell lines such as the K562 cell line. However, the BCR-ABL translocation may occur via a number of different junction points. In addition, CML is a disease of hematopoietic stem cells and, as a result, can give rise to multiple lineages of tumor cells. In this study, we examined the cellular signaling profiles following imatinib mesylate treatment of eight model CML and ALL cell lines that encompass three BCR-ABL junction points and multiple lineages. We used phosphorylation-specific antibodies and flow cytometry to determine the kinase and pathway activation states with each of the cell lines before and after imatinib mesylate exposure. The comparisons of signaling response profiles, junction points and lineages indicate that cell line lineage rather than BCR-ABL junction point may determine cellular response to imatinib mesylate. The large amount of variation observed among the cell lines suggests that further analysis is required to understand the complex signaling profiles present in CML patients.

Serological identification of adult T-cell leukaemia-associated antigens

Adult T-cell leukaemia (ATL) is a peripheral T-cell neoplasm caused by human T-cell leukaemia virus type I (HTLV-I). Several clinical observations suggest that some tumour-associated antigens in ATL may be recognised by the immune system. In this study, we performed the serological screening of an expression library to identify ATL-associated antigens by using materials from a unique ATL patient with long-term stable disease. Among five distinct genes isolated, serine/arginine protein kinase 1 (SRPK1), which has been reported to have a restricted normal tissue distribution, was found to be overexpressed in most acute type ATL samples, but not in chronic type ATL or in normal peripheral blood mononuclear cells by real-time reverse transcription polymerase chain reaction. Interestingly, the overexpression of SRPK1 in aggressive types of ATL was more exclusively observed at the protein level than at the mRNA level. Autologous antibody to SRPK1 was confirmed in the ATL patient using Western blot analysis with plasma, but not detected in asymptomatic HTLV-I carriers or in healthy volunteers. These results indicate that SRPK1 may be useful for the development of therapeutic and diagnostic methods for patients with ATL.

Mimicry of a constitutively active pre-B cell receptor in acute lymphoblastic leukemia cells

Pre-B cells undergo apoptosis unless they are rescued by pre-B cell receptor-dependent survival signals. We previously showed that the BCR-ABL1 kinase that is expressed in pre-B lymphoblastic leukemia bypasses selection for pre-B cell receptor-dependent survival signals. Investigating possible interference of BCR-ABL1 with pre-B cell receptor signaling, we found that neither SYK nor SLP65 can be phosphorylated in response to pre-B cell receptor engagement. Instead, Bruton's tyrosine kinase (BTK) is constitutively phosphorylated by BCR-ABL1. Activated BTK is essential for survival signals that otherwise would arise from the pre-B cell receptor, including activation of PLCgamma1, autonomous Ca2+ signaling, STAT5-phosphorylation, and up-regulation of BCLX(L). Inhibition of BTK activity specifically induces apoptosis in BCR-ABL1+ leukemia cells to a similar extent as inhibition of BCR-ABL1 kinase activity itself. However, BCR-ABL1 cannot directly bind to full-length BTK. Instead, BCR-ABL1 induces the expression of a truncated splice variant of BTK that acts as a linker between the two kinases. As opposed to full-length BTK, truncated BTK lacks kinase activity yet can bind to BCR-ABL1 through its SRC-homology domain 3. Acting as a linker, truncated BTK enables BCR-ABL1-dependent activation of full-length BTK, which initiates downstream survival signals and mimics a constitutively active pre-B cell receptor.

BCR/ABL-mediated downregulation of genes implicated in cell adhesion and motility leads to impaired migration toward CCR7 ligands CCL19 and CCL21 in primary BCR/ABL-positive cells

The mechanism underlying p210(BCR/ABL) oncoprotein-mediated transformation in chronic myelogenous leukemia (CML) is not fully understood. We hypothesized that p210(BCR/ABL) suppresses expression of genes which may explain at least some of the pathogenetic features of CML. A subtractive cDNA library was created between BCR/ABL-enhanced-green-fluorescent-protein (GFP)-transduced umbilical cord blood (UCB) CD34+ cells and GFP-transduced UCB CD34+ cells to identify genes whose expression is downregulated by p210(BCR/ABL). At least 100 genes were identified. We have confirmed for eight of these genes that expression was suppressed by quantitative real-time-RT-PCR (Q-RT-PCR) of additional p210(BCR/ABL)-transduced CD34+ UCB cells as well as primary early chronic phase (CP) bone marrow (BM) CML CD34+ cells. Imatinib mesylate reversed downregulation of some genes, to approximately normal levels. Several of the genes are implicated in cell adhesion and motility, including L-selectin, intercellular adhesion molecule-1 (ICAM-1), and the chemokine receptor, CCR7, consistent with the known defect in adhesion and migration of CML cells. Compared with GFP UCB or normal (NL) BM CD34+ cells, p210 UCB and CML CD34+ cells migrated poorly towards the CCR7 ligands, CCL19 and CCL21, suggesting a possible role for CCR7 in the abnormal migratory behavior of CML CD34+ cells.

Alterations of pre-mRNA splicing in cancer

Recent genomewide analyses of alternative splicing (AS) indicate that up to 70% of human genes may have alternative splice forms, suggesting that AS together with various posttranslational modifications plays a major role in the production of proteome complexity. Splice-site selection under normal physiological conditions is regulated in the developmental stage in a tissue type-specific manner by changing the concentrations and the activity of splicing regulatory proteins. Whereas spliceosomal errors resulting in the production of aberrant transcripts rarely occur in normal cells, they seem to be an intrinsic property of cancer cells. Changes in splice-site selection have been observed in various types of cancer and may affect genes implicated in tumor progression (for example, CD44, MDM2, and FHIT) and in susceptibility to cancer (for example, BRCA1 and APC). Splicing defects can arise from inherited or somatic mutations in cis-acting regulatory elements (splice donor, acceptor and branch sites, and exonic and intronic splicing enhancers and silencers) or variations in the composition, concentration, localization, and activity of regulatory proteins. This may lead to altered efficiency of splice-site recognition, resulting in overexpression or down-regulation of certain splice variants, a switch in splice-site usage, or failure to recognize splice sites correctly, resulting in cancer-specific splice forms. At least in some cases, changes in splicing have been shown to play a functionally significant role in tumorigenesis, either by inactivating tumor suppressors or by gain of function of proteins promoting tumor development. Moreover, cancer-specific splicing events may generate novel epitopes that can be recognized by the host's immune system as cancer specific and may serve as targets for immunotherapy. Thus, the identification of cancer-specific splice forms provides a novel source for the discovery of diagnostic or prognostic biomarkers and tumor antigens suitable as targets for therapeutic intervention.

Phosphatidylinositol-3-kinase activation mediates proline-rich tyrosine kinase 2 phosphorylation and recruitment to β1-integrins in human CD34+ cells

OBJECTIVE

beta1-integrins mediate hematopoietic stem and progenitor cell homing and retention in the bone marrow (BM) and inhibit hematopoietic proliferation and differentiation. Having no intrinsic kinase activity, integrins recruit intracellular kinases, such as the focal adhesion kinase (FAK) or the related proline-rich tyrosine kinase 2 (PYK2), to initiate signal transduction. Phosphatidylinositol-3-kinase (PI3K), which is involved in beta1-integrin signaling in many cell types, is physically and functionally associated with FAK in anchorage-dependent cells. Because PYK2 is the principal focal adhesion kinase expressed in primary human CD34+ cells, we assessed its functional relationship with PI3K in CD34+ cells in response to integrin engagement.

METHODS

beta1-integrins on primary mobilized peripheral blood CD34+ cells and CD34+ KG1A cells were engaged by adhesion to fibronectin (FN) or by cross-linking with an anti-beta1 integrin antibody, respectively. PI3K activity and PYK2 phosphorylation were then assessed in the presence or absence of the PI3K inhibitor, wortmannin. Association between PI3K, PYK2, and the beta1-integrin subunit were also evaluated in co-immunoprecipitation experiments.

RESULTS

beta1-integrin engagement induced PI3K activation, which was required for, and temporally preceded, PYK2 phosphorylation, indicating that PI3K lies upstream of PYK2 in CD34+ cells. Furthermore, although PYK2 and PI3K were constitutively associated, interaction of the PYK2/PI3K complex with beta1-integrins required prior integrin engagement and PI3K activation.

CONCLUSION

Activation of PI3K following beta1-integrin engagement on human CD34+ cells results in subsequent phosphorylation of PYK2, and is required for the recruitment of the PI3K/PYK2 complex to beta1-integrins at the cell surface.

Integrin engagement-induced inhibition of human myelopoiesis is mediated by proline-rich tyrosine kinase 2 gene products

OBJECTIVE

Hematopoietic progenitor proliferation and differentiation are inhibited by integrin engagement of fibronectin (FN). Focal adhesion kinases have been shown to mediate intracellular signaling from integrins, and we recently demonstrated that gene expression and pre-mRNA splicing of the focal adhesion kinase, PYK2, is abnormal in CD34(+) cells from chronic myelogenous leukemia (CML) patients. Here we investigated whether PYK2 gene products mediate integrin signaling in hematopoietic stem and progenitor cells.

METHODS

Cord blood CD34(+) cells were retrovirally transduced with vectors encoding Pyk2H, Pyk2, or the dominant negative-acting, kinase-deficient, C-terminal PYK2 fragment, PRNK, and myeloid proliferation and differentiation was assessed using colony-forming cell (CFC), long-term culture-initiating cell (LTC-IC), and liquid culture assays.

RESULTS

CD34(+) cells overexpressing Pyk2H or Pyk2 generated 50% less colony-forming unit granulocyte/macrophage (CFU-GM) than eGFP-transduced controls. Although the number of CFC generated by PRNK-expressing cells was unchanged, LTC-IC were significantly reduced. Culture of CD34(+) cells on FN significantly reduced the generation of mature myeloid cells vs those cultured on BSA-coated wells, and could be overcome by addition of SCF. As is observed when integrins are engaged, overexpression of either Pyk2H or Pyk2 decreased committed myeloid progenitor proliferation and differentiation; however, SCF could not override this inhibition. Finally, as is observed when integrins are not engaged, PRNK-mediated inhibition of endogenous Pyk2H resulted in integrin-nonresponsive proliferation and differentiation of myeloid precursors and accelerated differentiation of primitive hematopoietic progenitors.

CONCLUSION

These studies indicate that PYK2 gene products mediate integrin-induced signals that regulate myelopoiesis.