Enhanced expression of FHL2 leads to abnormal myelopoiesis in vivo

FHL2 deficiency aggravates Candida albicans infection through decreased myelopoiesis

Hematopoiesis is a finely tuned process that generates all blood cell types through self-renewal and differentiation, which is crucial for maintaining homeostasis. Acute infections can prompt a hematopoietic response known as emergency myelopoiesis. In this study, using a Candida albicans (C. albicans) infection model, we demonstrated for the first time that disruption of Fhl2 led to increased fungal burden, heightened inflammatory response and reduced survival rates. Impaired myeloid hematopoiesis and immune cell production were evident, as proved by the decreased numbers of hematopoietic stem and progenitor cells (HSPCs) and granulocytes in the bone marrow of Fhl2-deficient mice. In conclusion, FHL2 regulated emergency myelopoiesis in response to C. albicans, affecting the host's defense against pathogens.

Effects of the FHL2 gene on the development of subcutaneous and intramuscular adipocytes in goats

BACKGROUND

Adipose tissue affects not only the meat quality of domestic animals, but also human health. Adipocyte differentiation is regulated by a series of regulatory genes and cyclins. Four and half-LIM protein (FHL2) is positively correlated with the hypertrophy of adipocytes and can cause symptoms such as obesity and diabetes.

RESULT

In the transcriptome sequencing analysis of intramuscular adipocytes after three days of differentiation, the differentially expressed gene FHL2 was found. To further explore the biological significance of the differentially expressed gene FHL2, which was downregulated in the mature adipocytes. We revealed the function of FHL2 in adipogenesis through the acquisition and loss of function of FHL2. The results showed that the overexpression of FHL2 significantly increased the expression of adipogenic genes (PPARγ, C/EBPβ) and the differentiation of intramuscular and subcutaneous adipocytes. However, silencing FHL2 significantly inhibited adipocyte differentiation. The overexpression of FHL2 increased the number of adipocytes stained with crystal violet and increased the mRNA expression of proliferation marker genes such as CCNE, PCNA, CCND and CDK2. In addition, it significantly increased the rate of EdU positive cells. In terms of apoptosis, overexpression of FHL2 significantly inhibited the expression of P53 and BAX in both intramuscular and subcutaneous adipocytes, which are involved in cell apoptosis. However, overexpression of FHL2 promoted the expression of BCL, but was rescued by the silencing of FHL2.

CONCLUSIONS

In summary, FHL2 may be a positive regulator of intramuscular and subcutaneous adipocyte differentiation and proliferation, and acts as a negative regulator of intramuscular and subcutaneous adipocyte apoptosis. These findings provide a theoretical basis for the subsequent elucidation of FHL2 in adipocytes.

Cellular force-sensing through actin filaments

The actin cytoskeleton orchestrates cell mechanics and facilitates the physical integration of cells into tissues, while tissue-scale forces and extracellular rigidity in turn govern cell behaviour. Here, we discuss recent evidence that actin filaments (F-actin), the core building blocks of the actin cytoskeleton, also serve as molecular force sensors. We delineate two classes of proteins, which interpret forces applied to F-actin through enhanced binding interactions: 'mechanically tuned' canonical actin-binding proteins, whose constitutive F-actin affinity is increased by force, and 'mechanically switched' proteins, which bind F-actin only in the presence of force. We speculate mechanically tuned and mechanically switched actin-binding proteins are biophysically suitable for coordinating cytoskeletal force-feedback and mechanical signalling processes, respectively. Finally, we discuss potential mechanisms mediating force-activated actin binding, which likely occurs both through the structural remodelling of F-actin itself and geometric rearrangements of higher-order actin networks. Understanding the interplay of these mechanisms will enable the dissection of force-activated actin binding's specific biological functions.

How (Epi)Genetic Regulation of the LIM-Domain Protein FHL2 Impacts Multifactorial Disease

Susceptibility to complex pathological conditions such as obesity, type 2 diabetes and cardiovascular disease is highly variable among individuals and arises from specific changes in gene expression in combination with external factors. The regulation of gene expression is determined by genetic variation (SNPs) and epigenetic marks that are influenced by environmental factors. Aging is a major risk factor for many multifactorial diseases and is increasingly associated with changes in DNA methylation, leading to differences in gene expression. Four and a half LIM domains 2 (FHL2) is a key regulator of intracellular signal transduction pathways and the FHL2 gene is consistently found as one of the top hyper-methylated genes upon aging. Remarkably, FHL2 expression increases with methylation. This was demonstrated in relevant metabolic tissues: white adipose tissue, pancreatic β-cells, and skeletal muscle. In this review, we provide an overview of the current knowledge on regulation of FHL2 by genetic variation and epigenetic DNA modification, and the potential consequences for age-related complex multifactorial diseases.

Screening and identification of a novel FHL2 mutation by whole exome sequencing in twins with familial Waldenström macroglobulinemia

BACKGROUND

Waldenström macroglobulinemia (WM) is a rare chronic B-cell lymphoma. Familial clustering of WM has been observed over the years. However, little is known about the contribution of inherited genetic variants to familial WM cases.

METHODS

The authors performed whole exome sequencing (WES) of germline DNA samples from twins, one diagnosed with WM and the other diagnosed with immunoglobulin M monoclonal gammopathy of undetermined significance, and their healthy siblings. Bioinformatics analysis of public biological databases was used to identify the most relevant familial WM candidate from WES. Transcript expression and protein levels of the familial WM candidate were evaluated in the WM patient and 2 unaffected members of the kindred.

RESULTS

Among the 10 shared candidate mutations in the twins, the authors identified a novel heterozygous germline mutation in four and a half LIM domains protein 2 (FHL2; c.G226A, p.V76M) as a familial WM-associated mutation. FHL2 appeared to be connected with reported signaling pathways and disease-driving genes such as IL6 and HCK in WM. In addition, the authors found reduced FHL2 messenger RNA and protein expression in peripheral blood samples from the patient with WM in comparison with the healthy siblings.

CONCLUSIONS

Taken together, these findings indicate that an FHL2^g226a mutation may play an important role in familial WM, and they provide new screening possibilities for familial cases.

LAY SUMMARY

Familial clustering in Waldenström macroglobulinemia (WM) has been observed over the years. The authors performed whole exome sequencing of germline DNA samples from twins, one diagnosed with WM and the other diagnosed with immunoglobulin M monoclonal gammopathy of undetermined significance, and their healthy siblings. Among the 10 shared candidate mutations in the twins, a novel heterozygous germline mutation in four and a half LIM domains protein 2 (FHL2; c.G226A, p.V76M) was identified as the most relevant familial WM candidate through bioinformatics analysis of a public database. Also, messenger RNA and protein expression of FHL2 was significantly lower in peripheral blood mononuclear cells of the WM patient in comparison with the healthy siblings, and this suggested that the function of FHL2 was impaired when mutated.

Mechanosensing through Direct Binding of Tensed F-Actin by LIM Domains

Mechanical signals transmitted through the cytoplasmic actin cytoskeleton must be relayed to the nucleus to control gene expression. LIM domains are protein-protein interaction modules found in cytoskeletal proteins and transcriptional regulators. Here, we identify three LIM protein families (zyxin, paxillin, and FHL) whose members preferentially localize to the actin cytoskeleton in mechanically stimulated cells through their tandem LIM domains. A minimal actin-myosin reconstitution system reveals that representatives of all three families directly bind F-actin only in the presence of mechanical force. Point mutations at a site conserved in each LIM domain of these proteins disrupt tensed F-actin binding in vitro and cytoskeletal localization in cells, demonstrating a common, avidity-based mechanism. Finally, we find that binding to tensed F-actin in the cytoplasm excludes the cancer-associated transcriptional co-activator FHL2 from the nucleus in stiff microenvironments. This establishes direct force-activated F-actin binding as a mechanosensing mechanism by which cytoskeletal tension can govern nuclear localization.

Early Dendritic Morphogenesis of Adult-Born Dentate Granule Cells Is Regulated by FHL2

Dentate granule cells (DGCs), the progeny of neural stem cells (NSCs) in the sub-granular zone of the dentate gyrus (DG), must develop and functionally integrate with the mature cohort of neurons in order to maintain critical hippocampal functions throughout adulthood. Dysregulation in the continuum of DGC development can result in aberrant morphology and disrupted functional maturation, impairing neuroplasticity of the network. Yet, the molecular underpinnings of the signaling involved in adult-born DGC maturation including dendritic growth, which correlates with functional integration, remains incompletely understood. Given the high metabolic activity in the dentate gyrus (DG) required to achieve continuous neurogenesis, we investigated the potential regulatory role of a cellular metabolism-linked gene recently implicated in NSC cycling and neuroblast migration, called Four and a half LIM domain 2 (FHL2). The FHL2 protein modulates numerous pathways related to proliferation, migration, survival and cytoskeletal rearrangement in peripheral tissues, interacting with the machinery of the sphingosine-1-phosphate pathway, also known to be highly active especially in the hippocampus. Yet, the potential relevance of FHL2 to adult-born DGC development remains unknown. To elucidate the role of FHL2 in DGC development in the adult brain, we first confirmed the endogenous expression of FHL2 in NSCs and new granule cells within the DG, then engineered viral vectors for genetic manipulation experiments, investigating morphological changes in early stages of DGC development. Overexpression of FHL2 during early DGC development resulted in marked sprouting and branching of dendrites, while silencing of FHL2 increased dendritic length. Together, these findings suggest a novel role of FHL2 in adult-born DGC morphological maturation, which may open up a new line of investigation regarding the relevance of this gene in physiology and pathologies of the hippocampus such as mesial temporal lobe epilepsy (MTLE).

Enhanced expressions of FHL2 and iASPP predict poor prognosis in acute myeloid leukemia

iASPP is a negative regulator of the apoptotic function of p53, and it can enhance the ability of hematopoietic stem cells to self-renew and resist chemo- and radiation therapy. Recent study showed that iASPP could impact the proliferation and apoptosis of leukemia cells by interacting with FHL2. However, whether they have prognostic significance in acute myeloid leukemia (AML) is unknown. Eighty-four AML patients with FHL2 and iASPP expression data from The Cancer Genome Atlas database were enrolled in the study. Patients with high expressions of FHL2 and iASPP had significantly shorter event-free survival (EFS) and overall survival (OS) than patients with low expressions (P = 0.005, P = 0.003, respectively). Univariate analysis indicated that high expressions of FHL2 or iASPP were unfavorable for EFS and OS (all P < 0.05), while multivariate analysis confirmed that high FHL2 expression was an independent risk factor for EFS and OS (all P < 0.05). In patients who underwent allogeneic hematopoietic stem cell transplantation (allo-HSCT), however, EFS and OS were not significantly different between FHL2 or iASPP high- and low-expression groups. Our results suggested that high expressions of FHL2 and iASPP were poor prognostic factors for AML, but the prognostic effect might be overcome by allo-HSCT.

miR-9 upregulation leads to inhibition of erythropoiesis by repressing FoxO3

MicroRNAs (miRNAs) are emerging as critical regulators of normal and malignant hematopoiesis. In previous studies of acute myeloid leukemia miR-9 overexpression was commonly observed. Here, we show that ectopic expression of miR-9 in vitro and in vivo significantly blocks differentiation of erythroid progenitor cells with an increase in reactive oxygen species (ROS) production. Consistent with this observation, ROS scavenging enzymes, including superoxide dismutase (Sod2), Catalase (Cat), and glutathine peroxidase (Gpx1), are down-regulated by miR-9. In addition, miR-9 suppresses expression of the erythroid transcriptional regulator FoxO3, and its down-stream targets Btg1 and Cited 2 in erythroid progenitor cells, while expression of a constitutively active form of FoxO3 (FoxO3-3A) reverses miR-9-induced suppression of erythroid differentiation, and inhibits miR-9-induced ROS production. Thus, our findings indicate that aberrant expression of miR-9 blocks erythropoiesis by deregulating FoxO3-mediated pathways, which may contribute to the ineffective erythropoiesis observed in patients with hematological malignancies.

Genomewide association studies for hematological traits and T lymphocyte subpopulations in a Duroc × Erhualian F resource population

It has been shown that hematological traits can act as important indicators of immune function in both humans and livestock. T lymphocytes are key components of the adaptive immune system, playing a critical role in immune response. To identify genomic regions affecting hematological traits and T lymphocyte subpopulations, we performed both a SNP-based genomewide association study (GWAS) and a haplotype analysis for 20 hematological traits and 8 T cell subpopulations at 3 different time points (d 20, 33, and 35) in a Duroc × Erhualian F intercross population. Bonferroni correction was used to calculate the threshold -values for suggestive and 5% genomewide significance levels. In total, for SNP-based GWAS, we detected 96 significant SNP, including 15 genomewide-significant SNP, associated with 23 hematological traits and 234 significant SNP, including 27 genomewide-significant SNP, associated with 8 T cell subpopulations. Meanwhile, we identified 563 significant SNP, including 7 genomewide-significant SNP, associated with 5 hematological traits and 2,407 significant SNP, including 1,261 genomewide-significant SNP, associated with 8 T cell subpopulations by haplotype analysis. Among the significant regions detected, we propose both the () gene and the () gene on SSC3 as plausible candidate genes associated with CD/CD T lymphocytes at d 20. The findings provide insights into the basis of molecular mechanisms that are involved with immune response in the domestic pig and would aid further identification of causative mutations.

FHL2 Regulates Natural Killer Cell Development and Activation during Streptococcus pneumoniae Infection

Recent in silico studies suggested that the transcription cofactor LIM-only protein FHL2 is a major transcriptional regulator of mouse natural killer (NK) cells. However, the expression and role of FHL2 in NK cell biology are unknown. Here, we confirm that FHL2 is expressed in both mouse and human NK cells. Using FHL2^−/− mice, we found that FHL2 controls NK cell development in the bone marrow and maturation in peripheral organs. To evaluate the importance of FHL2 in NK cell activation, FHL2^−/− mice were infected with Streptococcus pneumoniae. FHL2^−/− mice are highly susceptible to this infection. The activation of lung NK cells is altered in FHL2^−/− mice, leading to decreased IFNγ production and a loss of control of bacterial burden. Collectively, our data reveal that FHL2 is a new transcription cofactor implicated in NK cell development and activation during pulmonary bacterial infection.

A Transcriptomic Signature of Mouse Liver Progenitor Cells

Liver progenitor cells (LPCs) can proliferate extensively, are able to differentiate into hepatocytes and cholangiocytes, and contribute to liver regeneration. The presence of LPCs, however, often accompanies liver disease and hepatocellular carcinoma (HCC), indicating that they may be a cancer stem cell. Understanding LPC biology and establishing a sensitive, rapid, and reliable method to detect their presence in the liver will assist diagnosis and facilitate monitoring of treatment outcomes in patients with liver pathologies. A transcriptomic meta-analysis of over 400 microarrays was undertaken to compare LPC lines against datasets of muscle and embryonic stem cell lines, embryonic and developed liver (DL), and HCC. Three gene clusters distinguishing LPCs from other liver cell types were identified. Pathways overrepresented in these clusters denote the proliferative nature of LPCs and their association with HCC. Our analysis also revealed 26 novel markers, LPC markers, including Mcm2 and Ltbp3, and eight known LPC markers, including M2pk and Ncam. These markers specified the presence of LPCs in pathological liver tissue by qPCR and correlated with LPC abundance determined using immunohistochemistry. These results showcase the value of global transcript profiling to identify pathways and markers that may be used to detect LPCs in injured or diseased liver.

Scaffold protein FHL2 facilitates MDM2-mediated degradation of IER3 to regulate proliferation of cervical cancer cells

The expression of immediate early response 3 (IER3), a protein with a short half-life, is rapidly induced by various cellular stimuli. We recently reported that IER3 induces the apoptosis of cervical cancer cells and that its expression is downregulated in patients with cervical cancer. However, the molecular mechanism involved in the rapid degradation of IER3 remains unknown. Here, we demonstrate that MDM2 is an E3 ligase that interacts with IER3 and promotes its ubiquitination, followed by proteasomal degradation. Polyubiquitination of the conserved lysine 60 of IER3 is essential for its degradation. In addition, four and a half LIM domains protein 2 (FHL2) binds to both IER3 and MDM2, allowing for efficient MDM2-mediated IER3 degradation by facilitating an association between MDM2 and IER3. Moreover, IER3 induces cell cycle arrest in cervical cancer cells and its activity is further enhanced in cells in which FHL2 or MDM2 was silenced, thereby preventing IER3 degradation. The E6 and E7 oncoproteins of human papilloma virus 18 regulated IER3 expression. FHL2 expression was significantly higher in the squamous epithelium of cervical carcinoma tissues than in non-cancerous cervical tissues, whereas cervical carcinoma expression of IER3 was downregulated in this region. Thus, we determined the molecular mechanism responsible for IER3 degradation, involving a ternary complex of IER3, MDM2 and FHL2, which may contribute to cervical tumor growth. Furthermore, we demonstrated that FHL2 serves as a scaffold for E3 ligase and its substrate during the ubiquitination reaction, a function that has not been previously reported for this protein.

Recurrent deletions of IKZF1 in pediatric acute myeloid leukemia

IKAROS family zinc finger 1/IKZF1 is a transcription factor important in lymphoid differentiation, and a known tumor suppressor in acute lymphoid leukemia. Recent studies suggest that IKZF1 is also involved in myeloid differentiation. To investigate whether IKZF1 deletions also play a role in pediatric acute myeloid leukemia, we screened a panel of pediatric acute myeloid leukemia samples for deletions of the IKZF1 locus using multiplex ligation-dependent probe amplification and for mutations using direct sequencing. Three patients were identified with a single amino acid variant without change of IKZF1 length. No frame-shift mutations were found. Out of 11 patients with an IKZF1 deletion, 8 samples revealed a complete loss of chromosome 7, and 3 cases a focal deletion of 0.1-0.9Mb. These deletions included the complete IKZF1 gene (n=2) or exons 1-4 (n=1), all leading to a loss of IKZF1 function. Interestingly, differentially expressed genes in monosomy 7 cases (n=8) when compared to non-deleted samples (n=247) significantly correlated with gene expression changes in focal IKZF1-deleted cases (n=3). Genes with increased expression included genes involved in myeloid cell self-renewal and cell cycle, and a significant portion of GATA target genes and GATA factors. Together, these results suggest that loss of IKZF1 is recurrent in pediatric acute myeloid leukemia and might be a determinant of oncogenesis in acute myeloid leukemia with monosomy 7.

LIM-only protein FHL2 critically determines survival and radioresistance of pancreatic cancer cells

Numerous factors determine the current poor prognosis of pancreatic ductal adenocarcinoma (PDAC). One of the greatest challenges to overcome is treatment resistance. Among a large repertoire of intrinsic resistance mechanisms, integrin-mediated cell adhesion to extracellular matrix (ECM) has been identified to be fundamental. Coalesced in focal adhesion complexes, integrins, receptor tyrosine kinases, protein kinases and adapter proteins mediate prosurvival signaling. Four and a half LIM domains protein 2 (FHL2) is one of these adapter proteins, which operates through protein-protein interactions and shows tumor-specific expression. Based on this, we investigated FHL2 expression in PDAC specimens and three-dimensionally grown cell lines and how FHL2 mechanistically contributes to cell survival, cell cycling and radiation resistance. PDAC exhibited a significantly increased and heterogeneous FHL2 expression. Upon FHL2 depletion, pancreatic cancer cell lines showed significantly decreased cell survival, proliferation and radioresistance as well as enhanced apoptosis and MEK/ERK signaling and cyclin D1, E, A and B1 expression were strongly induced. Targeting of FHL2 and MEK1 was similarly effective than FHL2 depletion alone, suggesting MEK1 as a downstream signaling mediator of FHL2. Taken together, our results provide evidence for the importance of the focal adhesion protein FHL2 in pancreatic cancer cell survival, proliferation and radiosensitivity.

FHL2 regulates hematopoietic stem cell functions under stress conditions

FHL2, a member of the four and one half LIM domain protein family, is a critical transcriptional modulator. Here, we identify FHL2 as a critical regulator of hematopoietic stem cells (HSCs) that is essential for maintaining HSC self-renewal under regenerative stress. We find that Fhl2 loss has limited effects on hematopoiesis under homeostatic conditions. In contrast, Fhl2-null chimeric mice reconstituted with Fhl2-null bone marrow cells developed abnormal hematopoiesis with significantly reduced numbers of HSCs, hematopoietic progenitor cells (HPCs), red blood cells and platelets as well as hemoglobin levels. In addition, HSCs displayed a significantly reduced self-renewal capacity and were skewed toward myeloid lineage differentiation. We find that Fhl2 loss reduces both HSC quiescence and survival in response to regenerative stress, probably as a consequence of Fhl2-loss-mediated down-regulation of cyclin dependent kinase (CDK)-inhibitors, including p21(Cip) and p27(Kip1). Interestingly, FHL2 is regulated under control of a tissue specific promoter in hematopoietic cells and it is down-regulated by DNA hypermethylation in the leukemia cell line and primary leukemia cells. Furthermore, we find that down-regulation of FHL2 frequently occurs in myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) patients, raising a possibility that FHL2 down-regulation plays a role in the pathogenesis of myeloid malignancies.

LIM-only protein FHL2 activates NF-κB signaling in the control of liver regeneration and hepatocarcinogenesis

Four-and-a-half LIM-only protein 2 (FHL2) is an important mediator in many signaling pathways. In this study, we analyzed the functions of FHL2 in nuclear factor κB (NF-κB) signaling in the liver. We show that FHL2 enhanced tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) activity in transcriptional activation of NF-κB targets by stabilizing the protein. TRAF6 is a binding partner of FHL2 and an important component of the Toll-like receptor-NF-κB pathway. Knockdown of FHL2 in 293-hTLR4/MD2-CD14 cells impaired lipopolysaccharide (LPS)-induced NF-κB activity, which regulates expression of inflammatory cytokines. Indeed, FHL2(-/-) macrophages showed significantly reduced production of TNF and interleukin 6 (IL-6) following LPS stimulation. TNF and IL-6 are the key cytokines that prime liver regeneration after hepatic injury. Following partial hepatectomy, FHL2(-/-) mice exhibited diminished induction of TNF and IL-6 and delayed hepatocyte regeneration. In the liver, NF-κB signaling orchestrates inflammatory cross talk between hepatocytes and hepatic immune cells that promote chemical hepatocarcinogenesis. We found that deficiency of FHL2 reduced susceptibility to diethylnitrosamine-induced hepatocarcinogenesis, correlating with the activator function of FHL2 in NF-κB signaling. Our findings demonstrate FHL2 as a positive regulator of NF-κB activity in liver regeneration and carcinogenesis and highlight the importance of FHL2 in both hepatocytes and hepatic immune cells.

Critical role of miR-9 in myelopoiesis and EVI1-induced leukemogenesis

MicroRNA-9 (miR-9) is emerging as a critical regulator of organ development and neurogenesis. It is also deregulated in several types of solid tumors; however, its role in hematopoiesis and leukemogenesis is not yet known. Here we show that miR-9 is detected in hematopoietic stem cells and hematopoietic progenitor cells, and that its expression increases during hematopoietic differentiation. Ectopic expression of miR-9 strongly accelerates terminal myelopoiesis and promotes apoptosis in vitro and in vivo. Conversely, in hematopoietic progenitor cells, the inhibition of miR-9 with a miRNA sponge blocks myelopoiesis. Ecotropic viral integration site 1 (EVI1), required for normal embryogenesis, is considered an oncogene because its inappropriate up-regulation induces malignant transformation in solid and hematopoietic cancers. Here we show that EVI1 binds to the promoter of miR-9-3, leading to DNA hypermethylation of the promoter and repression of miR-9. Moreover, miR-9 expression reverses a myeloid differentiation block that is induced by EVI1. Our findings indicate that EVI1, when inappropriately expressed, delays or blocks myeloid differentiation at least in part by DNA hypermethylation and down-regulation of miR-9. It was reported that Forkhead box class O genes (FoxOs) inhibit myeloid differentiation and prevent differentiation of leukemia-initiating cells. Here we identify both FoxO1 and FoxO3 as direct targets of miR-9 in hematopoietic cells and find that up-regulation of FoxO3 inhibits miR-9-induced myelopoiesis. These results reveal a unique role of miR-9 in myelopoiesis and in the pathogenesis of EVI1-induced myeloid neoplasms and provide insights into the epigenetic regulation of miR9 in tumorigenesis.

The four and a half LIM-only protein 2 regulates liver homeostasis and contributes to carcinogenesis

BACKGROUND & AIMS

The four and a half LIM-only protein 2 (FHL2) is upregulated in diverse pathological conditions. Here, we analyzed the effects of FHL2 overexpression in the liver of FHL2 transgenic mice (Apo-FHL2).

METHODS

We first examined cell proliferation and apoptosis in Apo-FHL2 livers and performed partial hepatectomy to investigate high FHL2 expression in liver regeneration. Expression of FHL2 was then analyzed by real time PCR in human hepatocellular carcinoma and adjacent non-tumorous livers. Finally, the role of FHL2 in hepatocarcinogenesis was assessed using Apo-FHL2;Apc(lox/lox) mice.

RESULTS

Six-fold increase in cell proliferation in transgenic livers was associated with concomitant apoptosis, resulting in normal liver mass. In Apo-FHL2 livers, both cyclin D1 and p53 were markedly increased. Evidence supporting a p53-dependent cell death mechanism was provided by the findings that FHL2 bound to and activated the p53 promoter, and that a dominant negative p53 mutant compromised FHL2-induced apoptosis in hepatic cells. Following partial hepatectomy in Apo-FHL2 mice, hepatocytes displayed advanced G1 phase entry and DNA synthesis leading to accelerated liver weight restoration. Interestingly, FHL2 upregulation in human liver specimens showed significant association with increasing inflammation score and cirrhosis. Finally, while Apo-FHL2 mice developed no tumors, the FHL2 transgene enhanced hepatocarcinogenesis induced by liver-specific deletion of the adenomatous polyposis coli gene and aberrant Wnt/β-catenin signaling in Apc(lox/lox) animals.

CONCLUSIONS

Our results implicate FHL2 in the regulation of signaling pathways that couple proliferation and cell death machineries, and underscore the important role of FHL2 in liver homeostasis and carcinogenesis.

Activation of Wnt/β-catenin protein signaling induces mitochondria-mediated apoptosis in hematopoietic progenitor cells

The canonical Wnt/β-catenin signaling is activated during development, tumorigenesis, and in adult homeostasis, yet its role in maintenance of hematopoietic stem/progenitor cells is not firmly established. Here, we demonstrate that conditional expression of an active form of β-catenin in vivo induces a marked increase in the frequency of apoptosis in hematopoietic stem/progenitor cells (HSCs/HPCs). Activation of Wnt/β-catenin signaling in HPCs in vitro elevates the activity of caspases 3 and 9 and leads to a loss of mitochondrial membrane potential (ΔΨ(m)), indicating that it induces the intrinsic mitochondrial apoptotic pathway. In vivo, expression of activated β-catenin in HPCs is associated with down-regulation of Bcl2 and expression of Casp3. Bone marrow transplantation assays reveal that enhanced cell survival by a Bcl2 transgene re-establishes the reconstitution capacity of HSCs/HPCs that express activated β-catenin. In addition, a Bcl2 transgene prevents exhaustion of these HSCs/HPCs in vivo. Our data suggest that activation of the Wnt/β-catenin pathway contributes to the defective function of HPCs in part by deregulating their survival.

Overexpression of four and a half LIM domains protein 2 promotes epithelial-mesenchymal transition-like phenotype in fish pre-osteoblasts

FHL2 is a multifunctional protein involved in gene transcription regulation and cytoarchitecture modulation that has been recently associated with epithelial-mesenchymal transition (EMT) in colon cancer. Overexpression of FHL2 in a fish pre-osteoblastic cell line promoted cell dedifferentiation and impaired its extracellular matrix mineralization capacity. Cell cultures also acquired a novel three-dimensional structure organization, their proliferation rate was enhanced and gene expression profile was altered in agreement with an EMT-like phenotype upon overexpression of FHL2. Altogether, our results provide additional support to the relevance of FHL2 for cell differentiation and its association with hallmarks of cancer phenotype.

FHL2 interacts with CALM and is highly expressed in acute erythroid leukemia

The t(10;11)(p13;q14) translocation results in the fusion of the CALM (clathrin assembly lymphoid myeloid leukemia protein) and AF10 genes. This translocation is observed in acute myeloblastic leukemia (AML M6), acute lymphoblastic leukemia (ALL) and malignant lymphoma. Using a yeast two-hybrid screen, the four and a half LIM domain protein 2 (FHL2) was identified as a CALM interacting protein. Recently, high expression of FHL2 in breast, gastric, colon, lung as well as in prostate cancer was shown to be associated with an adverse prognosis. The interaction between CALM and FHL2 was confirmed by glutathione S-transferase-pulldown assay and co-immunoprecipitation experiments. The FHL2 interaction domain of CALM was mapped to amino acids 294–335 of CALM. The transcriptional activation capacity of FHL2 was reduced by CALM, but not by CALM/AF10, which suggests that regulation of FHL2 by CALM might be disturbed in CALM/AF10-positive leukemia. Extremely high expression of FHL2 was seen in acute erythroid leukemia (AML M6). FHL2 was also highly expressed in chronic myeloid leukemia and in AML with complex aberrant karyotype. These results suggest that FHL2 may play an important role in leukemogenesis, especially in the case of AML M6.

FHL2 protein is a novel co-repressor of nuclear receptor Nur77

The three members of the NR4A orphan nuclear receptor subfamily Nur77, Nurr1, and NOR-1, regulate a variety of biological functions including vascular disease and metabolism. In this study, we identified Four and a half LIM domains protein-2 (FHL2) as a novel interacting protein of NR4A nuclear receptors by yeast two-hybrid screen and co-immunoprecipitation studies. Each of the four LIM domains of FHL2 can bind Nur77, and both the amino-terminal domain and the DNA binding domain of Nur77 are involved in the interaction between FHL2 and Nur77. FHL2 represses Nur77 transcriptional activity in a dose-dependent manner, and short hairpin RNA-mediated knockdown of FHL2 results in increased Nur77 transcriptional activity. ChIP experiments on the enolase3 promoter revealed that FHL2 inhibits the association of Nur77 with DNA. FHL2 is highly expressed in human endothelial and smooth muscle cells, but not in monocytes or macrophages. To substantiate functional involvement of FHL2 in smooth muscle cell physiology, we demonstrated that FHL2 overexpression increases the growth of these cells, whereas FHL2 knockdown results in reduced DNA synthesis. Collectively, these studies suggest that association of FHL2 with Nur77 plays a pivotal role in vascular disease.

c-myb supports erythropoiesis through the transactivation of KLF1 and LMO2 expression

The c-myb transcription factor is highly expressed in immature hematopoietic cells and down-regulated during differentiation. To define its role during the hematopoietic lineage commitment, we silenced c-myb in human CD34(+) hematopoietic stem/progenitor cells. Noteworthy, c-myb silencing increased the commitment capacity toward the macrophage and megakaryocyte lineages, whereas erythroid differentiation was impaired, as demonstrated by clonogenic assay, morphologic and immunophenotypic data. Gene expression profiling and computational analysis of promoter regions of genes modulated in c-myb-silenced CD34(+) cells identified the transcription factors Kruppel-Like Factor 1 (KLF1) and LIM Domain Only 2 (LMO2) as putative targets, which can account for c-myb knockdown effects. Indeed, chromatin immunoprecipitation and luciferase reporter assay demonstrated that c-myb binds to KLF1 and LMO2 promoters and transactivates their expression. Consistently, the retroviral vector-mediated overexpression of either KLF1 or LMO2 partially rescued the defect in erythropoiesis caused by c-myb silencing, whereas only KLF1 was also able to repress the megakaryocyte differentiation enhanced in Myb-silenced CD34(+) cells. Our data collectively demonstrate that c-myb plays a pivotal role in human primary hematopoietic stem/progenitor cells lineage commitment, by enhancing erythropoiesis at the expense of megakaryocyte diffentiation. Indeed, we identified KLF1 and LMO2 transactivation as the molecular mechanism underlying Myb-driven erythroid versus megakaryocyte cell fate decision.