Furin gene (fur) regulation in differentiating human megakaryoblastic Dami cells: involvement of the proximal GATA recognition motif in the P1 promoter and impact on the maturation of furin substrates

Allele-Specific Epigenetic Regulation of FURIN Expression at a Coronary Artery Disease Susceptibility Locus

Genome-wide association studies have revealed an association between the genetic variant rs17514846 in the FURIN gene and coronary artery disease. We investigated the mechanism through which rs17514846 modulates FURIN expression. An analysis of isogenic monocytic cell lines showed that the cells of the rs17514846 A/A genotype expressed higher levels of FURIN than cells of the C/C genotype. Pyrosequencing showed that the cytosine (in a CpG motif) at the rs17514846 position on the C allele was methylated. Treatment with the DNA methylation inhibitor 5-aza-2'-deoxycytidine increased FURIN expression. An electrophoretic mobility super-shift assay with a probe corresponding to the DNA sequence at and around the rs17514846 position of the C allele detected DNA-protein complex bands that were altered by an anti-MeCP2 antibody. A chromatin immunoprecipitation assay with the anti-MeCP2 antibody showed an enrichment of the DNA sequence containing the rs17514846 site. siRNA-mediated knockdown of MeCP2 caused an increase in FURIN expression. Furthermore, MeCP2 knockdown increased monocyte migration and proliferation, and this effect was diminished by a FURIN inhibitor. The results of our study suggest that DNA methylation inhibits FURIN expression and that the coronary artery disease-predisposing variant rs17514846 modulates FURIN expression and monocyte migration via an allele-specific effect on DNA methylation.

The emerging role of furin in neurodegenerative and neuropsychiatric diseases

Furin is an important mammalian proprotein convertase that catalyzes the proteolytic maturation of a variety of prohormones and proproteins in the secretory pathway. In the brain, the substrates of furin include the proproteins of growth factors, receptors and enzymes. Emerging evidence, such as reduced FURIN mRNA expression in the brains of Alzheimer's disease patients or schizophrenia patients, has implicated a crucial role of furin in the pathophysiology of neurodegenerative and neuropsychiatric diseases. Currently, compared to cancer and infectious diseases, the aberrant expression of furin and its pharmaceutical potentials in neurological diseases remain poorly understood. In this article, we provide an overview on the physiological roles of furin and its substrates in the brain, summarize the deregulation of furin expression and its effects in neurodegenerative and neuropsychiatric disorders, and discuss the implications and current approaches that target furin for therapeutic interventions. This review may expedite future studies to clarify the molecular mechanisms of furin deregulation and involvement in the pathogenesis of neurodegenerative and neuropsychiatric diseases, and to develop new diagnosis and treatment strategies for these diseases.

Inhibitors of Activin Receptor-like Kinase 5 Interfere with SARS-CoV-2 S-Protein Processing and Spike-Mediated Cell Fusion via Attenuation of Furin Expression

Screening of a protein kinase inhibitor library identified SB431542, targeting activin receptor-like kinase 5 (ALK5), as a compound interfering with SARS-CoV-2 replication. Since ALK5 is implicated in transforming growth factor β (TGF-β) signaling and regulation of the cellular endoprotease furin, we pursued this research to clarify the role of this protein kinase for SARS-CoV-2 infection. We show that TGF-β1 induces the expression of furin in a broad spectrum of cells including Huh-7 and Calu-3 that are permissive for SARS-CoV-2. The inhibition of ALK5 by incubation with SB431542 revealed a dose-dependent downregulation of both basal and TGF-β1 induced furin expression. Furthermore, we demonstrate that the ALK5 inhibitors SB431542 and Vactosertib negatively affect the proteolytic processing of the SARS-CoV-2 Spike protein and significantly reduce spike-mediated cell-cell fusion. This correlated with an inhibitory effect of ALK5 inhibition on the production of infectious SARS-CoV-2. Altogether, our study shows that interference with ALK5 signaling attenuates SARS-CoV-2 infectivity and cell-cell spread via downregulation of furin which is most pronounced upon TGF-β stimulation. Since a TGF-β dominated cytokine storm is a hallmark of severe COVID-19, ALK5 inhibitors undergoing clinical trials might represent a potential therapy option for COVID-19.

Identification of Novel Genetic Regulatory Region for Proprotein Convertase FURIN and Interferon Gamma in T Cells

The proprotein convertase enzyme FURIN promotes the proteolytic maturation of pro-proteins and thereby it serves as an important factor for maintaining cellular homeostasis. In T cells, FURIN is critical for maintaining the T regulatory cell dependent peripheral immune tolerance and intact T helper cell polarization. The enzymatic activity of FURIN is directly associated with its expression levels, but genetic determinants for cell-type specific Furin gene regulation have remained elusive. By exploring the histone acetyltransferase p300 binding patterns in T helper cells, a putative regulatory region at ca. 20kB upstream of Furin gene was identified. When this region was deleted with CRISPR/Cas9 the production of Furin mRNA was significantly reduced in activated mouse T cells. Genome-wide RNA profiling by sequencing revealed that the novel Furin regulator region also impacted the expression of several genes that have previously been associated with the Th1 type hall mark cytokine IFNγ regulation or function. Finally, Furin genetic regulatory region was found to specifically promote the secretion of IFNγ by activated T cells. In sum, our data unravels the presence of Furin expression regulatory region in T cells that has characteristics of a super-enhancer for Th1 cell fate.

TGF-β-induced intracellular PAI-1 is responsible for retaining hematopoietic stem cells in the niche

Hematopoietic stem and progenitor cells (HSPCs) reside in the supportive stromal niche in bone marrow (BM); when needed, however, they are rapidly mobilized into the circulation, suggesting that HSPCs are intrinsically highly motile but usually stay in the niche. We questioned what determines the motility of HSPCs. Here, we show that transforming growth factor (TGF)-β-induced intracellular plasminogen activator inhibitor (PAI)-1 activation is responsible for keeping HSPCs in the BM niche. We found that the expression of PAI-1, a downstream target of TGF-β signaling, was selectively augmented in niche-residing HSPCs. Functional inhibition of the TGF-β-PAI-1 signal increased MT1-MMP-dependent cellular motility, causing a detachment of HSPCs from the TGF-β-expressing niche cells, such as megakaryocytes. Furthermore, consistently high motility in PAI-1-deficient HSPCs was demonstrated by both a transwell migration assay and reciprocal transplantation experiments, indicating that intracellular, not extracellular, PAI-1 suppresses the motility of HSPCs, thereby causing them to stay in the niche. Mechanistically, intracellular PAI-1 inhibited the proteolytic activity of proprotein convertase Furin, diminishing MT1-MMP activity. This reduced expression of MT1-MMP in turn affected the expression levels of several adhesion/deadhesion molecules for determination of HSPC localization, such as CD44, VLA-4, and CXCR4, which then promoted the retention of HSPCs in the niche. Our findings open up a new field for the study of intracellular proteolysis as a regulatory mechanism of stem cell fate, which has the potential to improve clinical HSPC mobilization and transplantation protocols.

Phorbol esters dPPA/dPA promote furin expression involving transcription factor CEBPβ in neuronal cells

Using high-throughput small molecule screening targeting furin gene, we identified that phorbol esters dPPA (12-Deoxyphorbol 13-phenylacetate 20-acetate) and dPA (12-Deoxyphorbol 13-acetate) significantly increased furin protein and mRNA expression in SH-SY5Y cells. This effect was prevented by PKC (protein kinase C) inhibitor calphostin C but not Ro318220, suggesting that the C1 domain, rather than the catalytic domain of PKC plays an important role. Luciferase assay revealed that nucleotides -7925 to -7426 were sufficient to mediate dPPA/dPA enhancement of furin P1 promoter activity. RNA interference of transcriptional factors CEBPβ (CCAAT/enhancer-binding protein β) and GATA1 revealed that knockdown of CEBPβ significantly attenuated the effect of dPPA on furin expression. Pharmacological inhibition of ERK and PI3K but not TGFβ receptor diminished the up-regulation of furin by dPPA. These results suggested that in neuronal cells, transcriptional activation of furin by dPPA/dPA may be initiated by C1 domain containing proteins including PKC; the intracellular signaling involves ERK and PI3K and transcription factor CEBPβ.

Notch1 Autoactivation via Transcriptional Regulation of Furin, Which Sustains Notch1 Signaling by Processing Notch1-Activating Proteases ADAM10 and Membrane Type 1 Matrix Metalloproteinase

Notch1 is an evolutionarily conserved transmembrane receptor involved in melanoma growth. Notch1 is first cleaved by furin in the Golgi apparatus to produce the biologically active heterodimer. Following ligand binding, Notch1 is cleaved at the cell membrane by proteases such as ADAM10 and -17 and membrane type 1 matrix metalloproteinase (MT1-MMP), the latter of which we recently identified as a novel protease involved in Notch1 processing. The final cleavage is γ-secretase dependent and releases the active Notch intracellular domain (NIC). We now demonstrate that Notch1 directly regulates furin expression. Aside from activating Notch1, furin cleaves and activates several proteases, including MT1-MMP, ADAM10, and ADAM17. By chromatin immunoprecipitation and a reporter assay, we demonstrate that Notch1 binds at position -1236 of the furin promoter and drives furin expression. The Notch1-dependent enhancement of furin expression increases the activities of MT1-MMP and ADAM10 but not that of ADAM17, as demonstrated by short hairpin RNA (shRNA) knockdown of furin, and promotes the cleavage of Notch1 itself. These data highlight a novel positive-feedback loop whereby Notch1-dependent furin expression can induce Notch1 signaling by increasing Notch1 processing and by potentiating the activity of the proteases responsible for Notch1 activation. This leads to Notch1 signal amplification, which can promote melanoma tumor growth and progression, as demonstrated by the inhibition of cell migration and invasion upon furin inhibition downstream of Notch1. Disruption of such feedback signaling might represent an avenue for the treatment of melanoma.

Single Nucleotide Polymorphism (rs4932178) in the P1 Promoter of FURIN Is Not Prognostic to Colon Cancer

High expression of the proprotein processing enzyme FURIN has been associated with tumor progression and metastasis. A SNP (rs4932178) in the promoter of FURIN has been reported to affect expression in liver, with the T allele resulting in higher expression than the C allele. In this study we have investigated the association of this SNP with prognostic and biological subgroups of colorectal cancer (CRC). In a panel of 1382 patients with CRC, this SNP had no impact on overall survival or on postoperative risk of relapse. This SNP also could not be linked with FURIN expression levels in CRC samples from the patients. Furthermore, we demonstrate in luciferase reporter experiments in the colon cancer cell lines Caco-2 and SW480 and in the hepatocellular carcinoma cell line Huh 7 that expression is not affected by the SNP. Since, FURIN inhibition in human colon cancer cell lines has previously been shown to repress tumor metastases, association between FURIN gene expression levels and postoperative relapse-free survival was also investigated. However, no association could be found. Altogether, we could not confirm an effect of the SNP on FURIN expression in vitro and no correlations could be found in vivo with FURIN expression or outcome.

The cAMP-responsive element binding protein (CREB) transcription factor regulates furin expression during human trophoblast syncytialization

INTRODUCTION

The multinucleated syncytiotrophoblast is formed and maintained by cytotrophoblast cell fusion and serves multiple functions to ensure a successful pregnancy. We have previously reported that the proprotein convertase furin is required for trophoblast syncytialization by processing type 1 insulin-like growth factor receptor (IGF1R).

METHODS

Utilizing trophoblast cell fusion models including induced fusion of choriocarcinoma BeWo cells and spontaneous fusion of primary cultured term cytotrophoblast cells, the expression of furin was evaluated by quantitative real-time PCR, Western blotting and immunofluorescence. The key transcription factor regulating the FUR gene promoter and critical responsive elements were identified by luciferase reporter assays, truncated mutants analysis, site-directed mutagenesis and ChIP.

RESULTS

We demonstrated that the levels of FUR mRNA were significantly stimulated by cAMP/PKA signaling pathway during spontaneous fusion of cytotrophoblast cells and forskolin-induced fusion of BeWo cells. cAMP-responsive element binding protein (CREB) was proven to be the key transcription factor which regulated the FUR P1 promoter during forskolin-induced BeWo cell fusion, and two critical cAMP-responsive elements (CREs) in the P1 promoter were further identified. Finally, we showed that CREB mediated endogenous furin activation and that CREB siRNA attenuated forskolin-induced furin expression and cell fusion in BeWo cells.

DISCUSSION

This provides the first evidence of the upstream regulator of furin during trophoblast cell fusion.

CONCLUSIONS

The above results suggest that the FUR transcription is activated by CREB-dependent stimulation of the FUR P1 promoter during human trophoblast syncytialization.

Plasminogen activator inhibitor 1 is an intracellular inhibitor of furin proprotein convertase

Proprotein convertases (PCs) are a family of serine proteases that are involved in the post-translational processing and activation of a wide range of regulatory proteins. The upstream role of PCs in the control of many physiological and pathological processes generates a growing interest in understanding their regulation. Here, we demonstrate that the serine protease inhibitor plasminogen activator inhibitor 1 (PAI-1) forms an SDS-stable complex with the PC furin, which leads to the inhibition of the intra-Golgi activity of furin. It is known that elevated PAI-1 plasma levels are correlated with the occurrence of the metabolic syndrome and type 2 diabetes, and we show that PAI-1 reduces the furin-dependent maturation and activity of the insulin receptor and ADAM17: two proteins involved in the onset of these metabolic disorders. In addition to demonstrating that PAI-1 is an intracellular inhibitor of furin, this study also provides arguments in favor of an active role for PAI-1 in the development of metabolic disorders.

Furin targeted drug delivery for treatment of rhabdomyosarcoma in a mouse model

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Improvement of treatment efficacy and decreased side effects through tumor-targeted drug delivery would be desirable. By panning with a phage-displayed cyclic random peptide library we selected a peptide with strong affinity for RMS in vitro and in vivo. The peptide minimal binding motif Arg-X-(Arg/Lys)(Arg/Lys) identified by alanine-scan, suggested the target receptor to be a proprotein convertase (PC). Expression profiling of all PCs in RMS biopsies and cell lines revealed consistent high expression levels for the membrane-bound furin and PC7. Direct binding of RMS-P3 peptide to furin was demonstrated by affinity chromatography and supported by activity and colocalization studies. Treatment of RMS in mice with doxorubicin coupled to the targeting peptide resulted in a two-fold increase in therapeutic efficacy compared to doxorubicin treatment alone. Our findings indicate surface-furin binding as novel mechanism for therapeutic cell penetration which needs to be further investigated. Furthermore, this work demonstrates that specific targeting of membrane-bound furin in tumors is possible for and suggests that RMS and other tumors might benefit from proprotein convertases targeted drug delivery.

Influence of a single nucleotide polymorphism in the P1 promoter of the furin gene on transcription activity and hepatitis B virus infection

Hepatitis B e antigen (HBeAg) is a viral strategy of immune response evasion associated with hepatitis B virus (HBV) persistence. Spontaneous HBeAg seroconversion is usually accompanied by liver disease remission. Unfortunately, this goal is difficult to achieve and requires expensive and time-consuming treatment. Furin, a proprotein convertase, is involved in HBeAg maturation and is therefore a potential therapeutic target or indicator for predicting disease progression and antiviral response. Here we demonstrate that healthy Han Chinese from southern China (an endemic area of HBV infection) harbor a common single nucleotide polymorphism (SNP; -229 C/T) in a 1268-bp region of the P1 promoter of the furin gene [FES upstream region (Fur)]. A luciferase reporter gene assay showed that transcription activity is about 3 times higher in allele T carriers than in allele C carriers of this SNP. Allele T includes a suboptimal transcription factor NF-E2 [i.e., nuclear factor (erythroid-derived 2)]-binding motif according to bioinformatics and studies using site-directed mutagenesis. We also observed that individuals carrying allele T were more likely to become persistently infected. When persistently infected patients were divided into subgroups according to recent guidelines and HBeAg-defective virus infection was taken into account, patients with allele T or genotype TT had a decreased likelihood of HBeAg seroconversion or an increased likelihood of progressing to HBeAg-negative chronic hepatitis B or liver cirrhosis if accompanied by HBeAg-defective virus infection.

CONCLUSION

The common SNP in the P1 promoter of the Fur gene affects furin transcription activity and HBV infection outcome, possibly by increasing furin messenger RNA expression, and this suggests that furin is a potential therapeutic target and that this SNP is a potential predictor of disease progression or therapeutic response.

Thyroid hormone promotes cell invasion through activation of furin expression in human hepatoma cell lines

The objective of this study was to identify genes regulated by thyroid hormone (T(3)) and associated with tumor invasion. The gene encoding furin, as previously identified by cDNA microarray, is known to be up-regulated by T(3) treatment, and stimulated furin production occurs in thyroidectomized rats after administration of T(3). Presently, by using serial deletion of the promoter and EMSAs, the T(3) response element on the furin promoter was localized to the -6317/-6302 region. T(3)-mediated furin up-regulation was cooperative with TGF-beta because T(3) induction increased after Smad3/4 addition. Furthermore, the invasiveness of HepG2-thyroid hormone receptor (TR) cells was significantly increased by T(3) treatment, perhaps due to furin processing of matrix metalloproteinase-2 and -9. In addition, furin up-regulation either by stable overexpression or T(3) and/or TGF-beta induction was evident in severe-combined immune-deficient mice inoculated with HepG2-TRalpha1 cells. The HepG2-furin mice displayed a higher metastasis index and tumor size than HepG2-neo mice. Notably, the increased liver and lung tumor number or size in the hyperthyroid severe-combined immune-deficient mice as well as TGF-beta mice was attributed specifically to furin overexpression in the HepG2-TRalpha1 cells. Furthermore, this study demonstrated that furin overexpression in some types of hepatocellular carcinomas is TR dependent and might play a crucial role in the development of hepatocellular carcinoma. Thus, T(3) regulates furin gene expression via a novel mechanism or in cooperation with TGF-beta to enhance tumor metastasis in vitro and in vivo.

Sox9-dependent transcriptional regulation of the proprotein convertase furin

The proprotein convertase furin participates in the maturation/bioactivation of a variety of proproteins involved in chondrogenesis events. These include parathyroid hormone-related peptide (PTHrP), an autocrine/paracrine factor that is crucial to both normal cartilage development and cartilage-related pathological processes. Despite the known importance of furin activity in the bioactivation of the polypeptides, the mechanisms that control furin regulation in chondrogenesis remain unknown. To gain insight into the molecular regulation of furin, we used the mouse prechondrogenic ATDC5 cell line, an established in vitro model of cartilage differentiation. Peak expression of both furin mRNA and furin PTHrP maturation was observed during chondrocyte nodule formation stage, an event that correlated with increased mRNA levels of Sox9, a potent high-mobility-group (HMG) box-containing transcription factor required for cartilage formation. Inhibition of furin activity led to a diminution in maturation of PTHrP, suggesting a relationship between Sox9-induced regulation of furin and chondrogenesis events. Transient transfection of Sox9 in nonchondrogenic cells resulted in a marked increase in furin mRNA and in the transactivation of the furin P1A promoter. Direct Sox9 action on the P1A promoter was narrowed down to a critical paired site with Sox9 binding capability in vitro and in vivo. Sox9 transactivation effect was inhibited by L-Sox5 and Sox-6, two Sox9 homologs also expressed in ATDC5 cells. Sox6 inhibitory effect was reduced when using Sox6-HMG-box mutants, indicating a repressive effect through direct HMG-box/DNA binding. Our work suggests a mechanism by which furin is regulated during chondrogenesis. It also adds to the complexity of Sox molecule interaction during gene regulation.

Transforming growth factor beta1 induces hypoxia-inducible factor-1 stabilization through selective inhibition of PHD2 expression

The hypoxia-inducible transcription factor-1 (HIF-1) is central to a number of pathological processes through the induction of specific genes such as vascular endothelial growth factor (VEGF). Even though HIF-1 is highly regulated by cellular oxygen levels, other elements of the inflammatory and tumor microenvironment were shown to influence its activity under normal oxygen concentration. Among others, recent studies indicated that transforming growth factor (TGF) beta increases the expression of the regulatory HIF-1alpha subunit, and induces HIF-1 DNA binding activity. Here, we demonstrate that TGFbeta acts on HIF-1alpha accumulation and activity by increasing HIF-1alpha protein stability. In particular, we demonstrate that TGFbeta markedly and specifically decreases both mRNA and protein levels of a HIF-1alpha-associated prolyl hydroxylase (PHD), PHD2, through the Smad signaling pathway. As a consequence, the degradation of HIF-1alpha was inhibited as determined by impaired degradation of a reporter protein containing the HIF-1alpha oxygen-dependent degradation domain encompassing the PHD-targeted prolines. Moreover, inhibition of the TGFbeta1 converting enzyme, furin, resulted in increased PHD2 expression, and decreased basal HIF-1alpha and VEGF levels, suggesting that endogenous production of bioactive TGFbeta1 efficiently regulates HIF-1-targeted genes. This was reinforced by results from HIF-1alpha knock-out or HIF-1alpha-inhibited cells that show impairment in VEGF production in response to TGFbeta. This study reveals a novel mechanism by which a growth factor controls HIF-1 stability, and thereby drives the expression of specific genes, through the regulation of PHD2 levels.

The CDX2 transcription factor regulates furin expression during intestinal epithelial cell differentiation

CDX2, a member of the caudal family of transcription factors, is involved in enterocyte lineage specification. CDX2 activates many intestine-specific genes, such as sucrase-isomaltase and lactase-phlorizin hydrolase (LPH), and adhesion proteins, namely, LI-cadherin and claudin-2. In this study, we show that the proprotein convertase furin, involved in proteolytic maturation of proprotein substrates including LPH and cell surface proteins, is a CDX2 target. Indeed, expression of the rat furin homolog was induced 1.5-fold, as determined by microarray experiments that compared control with CDX2-expressing intestinal epithelial cells (IEC-6). As determined by transient transfection assays in Caco-2/15 cells, the furin P1 promoter 1.3-kb fragment between SacI and NheI was essential for CDX2 transcriptional activation. Electrophoretic mobility shift/supershift assays followed by site-specific mutagenesis and chromatin immunoprecipitation identified the CDX DNA-binding site (CBS)2 sequence from nt -1827 to -1821 as the major CBS involved in furin P1 promoter activation. Increased furin mRNA and protein expression correlated with both CDX2 expression and intestinal epithelial cell differentiation. In addition, furin mRNAs were detected predominantly in differentiated epithelial cells of the villus, as determined by in situ hybridization. Treatment of Caco-2/15 cells with a furin inhibitor led to inhibition of LPH activity. Morphological differentiation of enterocyte-like features in Caco-2/15 such as epithelial cell polarity and brush-border formation were strongly attenuated by furin inhibition. These results suggest that CDX2 regulates furin expression in intestinal epithelial cells. Furin may be important in modulating the maturation and/or activation of key factors involved in enterocyte differentiation.

Discovery of the Proprotein Convertases and their Inhibitors

The members of the convertase family play a central role in the processing of various protein precursors ranging from hormones and growth factors to viral envelope proteins and bacterial toxins. The proteolysis of these precursors that occurs at basic residues is mediated by the proprotein convertases (PCs), namely: PC1, PC2, Furin, PACE4, PC4, PC5 and PC7. The proteolysis at non-basic residues is performed by subtilisin/kexin-like isozyme-1 (S1P/SKI-1) and the newly identified neural apoptosis-regulated convertase-1 (NARC-1/PCSK9). These proteases have key roles in many physiological processes and various pathologies including cancer, obesity, diabetes, neurodegenerative diseases and autosomal dominant hypercholesterolermia. Here we summarize the discovery of the proprotein convertases and their inhibitors, discuss their properties, roles, resemblance and differences

Hypoxia-enhanced expression of the proprotein convertase furin is mediated by hypoxia-inducible factor-1: impact on the bioactivation of proproteins

Hypoxia is a common tumorigenesis enhancer, mostly owing to its impact on gene expression of many angiogenic and invasion-related mediators, some of which are natural substrates for the proprotein convertase furin. Analysis of furin promoters revealed the presence of putative binding sites for hypoxia-inducible factor-1 (HIF-1), a transcription complex that plays a pivotal role in cellular adaptation to hypoxia. In fact, we demonstrate herein that the levels of fur mRNA, encoding furin, are remarkably increased upon hypoxic challenge. Cotransfection of a HIF-1alpha dominant negative form in wild-type (WT) cells or transfection of a furin promoter-reporter gene in HIF-1-deficient cells indicated the requirement of HIF-1 for furin promoter activation by hypoxia. Direct HIF-1 action on the furin promoter was identified as a canonical hypoxia-responsive element site with enhancer capability. The hypoxic/HIF-1 regulation of furin correlated with an increased proteolytic activation of the substrates membrane-type 1 matrix metalloproteinase and transforming growth factor-beta1. Our findings unveil a new facet of the physiological consequences of hypoxia/HIF-1, through enhanced furin-induced proteolytic processing/activation of proproteins known to be involved in tumorigenesis.

Potential role of APRIL as autocrine growth factor for megakaryocytopoiesis

A proliferation-inducing ligand (APRIL) is a new tumor necrosis factor family member implicated in tumor cell proliferation. We investigated the role of APRIL in megakaryocytopoiesis, a developmental hematopoietic process responsible for progenitor cell differentiation to megakaryoblasts and megakaryocytes, leading to platelet formation. APRIL is not expressed in CD34+ progenitor cells from healthy donors, but it is massively up-regulated during the proliferative phase of megakaryocytic cell differentiation. Exogenous APRIL expression in primary cells increases megakaryocytic cell growth, suggesting that APRIL acts as a proliferative factor in megakaryocytopoiesis. More importantly, neutralization of endogenous APRIL was able to dramatically reduce megakaryocyte expansion and platelet production. Thus, our data provide evidence that APRIL acts as a growth factor for terminal megakaryocytopoiesis and may promote physiologic platelet production.