Hypoxia-induced and calpain-dependent cleavage of filamin A regulates the hypoxic response

Pathogenic FLNA variants affecting the hinge region of filamin A are associated with male survival

Pathogenic variants in FLNA cause a diversity of X-linked developmental disorders associated with either preserved or diminished levels of filamin A protein and are conceptualized dichotomously as relating to underlying gain- or loss-of-function pathogenic mechanisms. Hemizygosity for germline deletions or truncating variants in FLNA is generally considered to result in embryonic lethality. Structurally, filamin A is composed of an N-terminal actin-binding region, followed by 24 immunoglobulin-like repeat units. The repeat domains are separated into distinct segments by two regions of low-complexity known as hinge-1 and hinge-2. Hinge-1 is proposed to confer flexibility to the otherwise rigid protein and is a target for cleavage by calpain with the resultant filamin fragments mediating crucial cellular signaling processes. Here, three families with pathogenic variants in FLNA that impair the function of hinge-1 in males are described, leading to distinct clinical phenotypes. One large in-frame deletion that includes the hinge leads to frontometaphyseal dysplasia in affected males and females, while two germline truncating variants located within the exon encoding hinge 1 result in phenotypes in males that are explained by exon skipping and under-expression of a transcript that deletes hinge-1 from the resultant protein. These three variants affecting hinge-1 indicate that this domain does not mediate cellular functions that, when deficientresult in embryonic lethality in males and that germline truncating variants in this region of FLNA can result in viable phenotypes in males.

Role of Filamin A in Growth and Migration of Breast Cancer-Review

Despite ongoing research in the field of breast cancer, the morbidity rates indicate that the disease remains a significant challenge. While patients with primary tumors have relatively high survival rates, these chances significantly decrease once metastasis begins. Thus, exploring alternative approaches, such as targeting proteins overexpressed in malignancies, remains significant. Filamin A (FLNa), an actin-binding protein (ABP), is involved in various cellular processes, including cell migration, adhesion, proliferation, and DNA repair. Overexpression of the protein was confirmed in samples from patients with numerous oncological diseases such as prostate, lung, gastric, colorectal, and pancreatic cancer, as well as breast cancer. Although most researchers concur on its role in promoting breast cancer progression and aggressiveness, discrepancies exist among studies. Moreover, the precise mechanisms through which FLNa affects cell migration, invasion, and even cancer progression remain unclear, highlighting the need for further research. To evaluate FLNa's potential as a therapeutic target, we have summarized its roles in breast cancer.

Upregulated Cytoskeletal Proteins Promote Pathological Angiogenesis in Moyamoya Disease

BACKGROUND

Moyamoya disease (MMD) is a rare progressive vascular disease that leads to intracranial internal carotid artery stenosis and eventual occlusion. However, its pathogenesis remains unclear. The purpose of this study is to explore the role of abnormally expressed proteins in the pathogenesis of MMD.

METHODS

Data-independent acquisition mass spectrometry identifies the differentially expressed proteins in MMD serum by detecting the serum from 60 patients with MMD and 20 health controls. The differentially expressed proteins were validated using enzyme linked immunosorbent assays. Immunofluorescence for superficial temporal artery and middle cerebral artery specimens was used to explore the morphological changes of vascular wall in MMD. In vitro experiments were used to explore the changes and mechanisms of differentially expressed proteins on endothelial cells.

RESULTS

Proteomic analysis showed that a total of 14 726 peptides and 1555 proteins were quantified by mass spectrometry data. FLNA (filamin A) and ZYX (zyxin) proteins were significantly higher in MMD serum compared with those in health controls (Log2FC >2.9 and >2.8, respectively). Immunofluorescence revealed an intimal hyperplasia in superficial temporal artery and middle cerebral artery specimens of MMD. FLNA and ZYX proteins increased the proportion of endothelial cells in S phase and promoted their proliferation, angiogenesis, and cytoskeleton enlargement. Mechanistic studies revealed that AKT (serine/threonine kinase)/GSK-3β (glycogen synthase kinase 3β)/β-catenin signaling pathway plays a major role in these FLNA- and ZYX-induced changes in endothelial cells.

CONCLUSIONS

This study provides proteomic data on a large sample size of MMD. The differential expression of FLNA and ZYX in patient with MMD and following in vitro experiments suggest that these upregulated proteins are related to the pathology of cerebrovascular intimal hyperplasia in MMD and are involved in MMD pathogenesis, with diagnostic and therapeutic ramifications.

Filamin A facilitates NLRP3 inflammasome activation during arsenic-induced nonalcoholic steatohepatitis

Prolonged exposure to arsenic can cause nonalcoholic steatohepatitis (NASH). The NOD-like receptor protein 3 (NLRP3) inflammasome plays an essential role in the process of NASH. However, the mechanism by which arsenic promotes NLRP3 expression remains unclear. Three-month NaAsO2 gavage led to the nuclear factor-κB (NF-κB) signaling pathway activation and NASH. Additionally, NaAsO2 upregulated the level of Filamin A (FLNA) and pyroptosis, thereby activating the NLRP3 inflammasome in SD rat liver. Using FLNA siRNA, NASH-associated inflammation and pyroptosis were clearly mitigated by reducing activation of the NLRP3 inflammasome. Furthermore, arsenic treatment facilitated activation of the NF-κB signaling pathway and promoted p-p65 translocation into the nucleus. Chromatin immunoprecipitation (Ch-IP) assay indicated that FLNA promoted p65 binding to the NLRP3 gene and upregulated the transcription of NLRP3, ultimately leading to pyroptosis and NASH. Our findings indicate that FLNA and pyroptosis are strongly associated with NASH induced by NaAsO2. Collectively, the findings of this study indicated that FLNA mediates NF-κB signaling pathway-induced activation of the NLRP3 inflammasome and ultimately activates pyroptosis and NASH upon NaAsO2 exposure. This information may be useful for improving therapeutic strategies against arsenic-induced NASH.

Transcriptional Response to Hypoxia: The Role of HIF-1-Associated Co-Regulators

The Hypoxia Inducible Factor 1 (HIF-1) plays a major role in the cellular response to hypoxia by regulating the expression of many genes involved in adaptive processes that allow cell survival under low oxygen conditions. Adaptation to the hypoxic tumor micro-environment is also critical for cancer cell proliferation and therefore HIF-1 is also considered a valid therapeutical target. Despite the huge progress in understanding regulation of HIF-1 expression and activity by oxygen levels or oncogenic pathways, the way HIF-1 interacts with chromatin and the transcriptional machinery in order to activate its target genes is still a matter of intense investigation. Recent studies have identified several different HIF-1- and chromatin-associated co-regulators that play important roles in the general transcriptional activity of HIF-1, independent of its expression levels, as well as in the selection of binding sites, promoters and target genes, which, however, often depends on cellular context. We review here these co-regulators and examine their effect on the expression of a compilation of well-characterized HIF-1 direct target genes in order to assess the range of their involvement in the transcriptional response to hypoxia. Delineating the mode and the significance of the interaction between HIF-1 and its associated co-regulators may offer new attractive and specific targets for anticancer therapy.

Deficiency of filamin A in smooth muscle cells protects against hypoxia‑mediated pulmonary hypertension in mice

Filamin A (FLNA) is a high molecular weight cytoskeleton protein important for cell locomotion. A relationship between FLNA mutations and pulmonary arterial hypertension (PAH) has previously been reported; however, the detailed mechanism remains unclear. The present study aimed to explore the role of FLNA in vascular smooth muscle cells during the development of PAH. Smooth muscle cell (SMC)‑specific FLNA‑deficient mice were generated and the mice were then exposed to hypoxia for 28 days to build the mouse model of PAH. Human pulmonary arterial smooth muscle cells (PASMCs) were also cultured and transfected with FLNA small interfering RNA or overexpression plasmids to investigate the effects of FLNA on PASMC proliferation and migration. Notably, compared with control individuals, the expression levels of FLNA were increased in lung tissues from patients with PAH, and it was obviously expressed in the PASMCs of pulmonary arterioles. FLNA deficiency in SMCs attenuated hypoxia‑induced pulmonary hypertension and pulmonary vascular remodeling. In vitro studies suggested that absence of FLNA impaired PASMC proliferation and migration, and produced lower levels of phosphorylated (p)‑PAK‑1 and RAC1 activity. However, FLNA overexpression promoted PASMC proliferation and migration, and increased the expression levels of p‑PAK‑1 and RAC1 activity. The present study highlights the role of FLNA in pulmonary vascular remodeling; therefore, it could serve as a potential target for the treatment of PAH.

LPA receptor 1 (LPAR1) is a novel interaction partner of Filamin A that promotes Filamin A phosphorylation, MRTF-A transcriptional activity and oncogene-induced senescence

Myocardin-related transcription factors A and B (MRTFs) are coactivators of Serum Response Factor (SRF), which controls fundamental biological processes such as cell growth, migration, and differentiation. MRTF and SRF transcriptional activity play an important role in hepatocellular carcinoma (HCC) growth, which represents the second leading cause of cancer-related mortality in humans worldwide. We, therefore, searched for druggable targets in HCC that regulate MRTF/SRF transcriptional activity and can be exploited therapeutically for HCC therapy. We identified the G protein-coupled lysophosphatidic acid receptor 1 (LPAR1) as a novel interaction partner of MRTF-A and Filamin A (FLNA) using fluorescence resonance energy transfer-(FRET) and proximity ligation assay (PLA) in vitro in HCC cells and in vivo in organoids. We found that LPAR1 promotes FLNA phosphorylation at S2152 which enhances the complex formation of FLNA and MRTF-A, actin polymerization, and MRTF transcriptional activity. Pharmacological blockade or depletion of LPAR1 prevents FLNA phosphorylation and complex formation with MRTF-A, resulting in reduced MRTF/SRF target gene expression and oncogene-induced senescence. Thus, inhibition of the LPAR1-FLNA-MRTF-A interaction represents a promising strategy for HCC therapy.

Silencing of YAP attenuates pericyte-myofibroblast transition and subretinal fibrosis in experimental model of choroidal neovascularization

Age-related macular degeneration (AMD) is the main reason of irreversible vision loss in the elderly. The subretinal fibrosis subsequent to choroidal neovascularization (CNV) is an important feature in the late stage of wet AMD and is considered to be one reason for incomplete response to anti-VEGF drugs. Recent studies have shown that pericyte-myofibroblast transition (PMT) is an important pathological process involving fibrotic diseases of various organs. However, the specific role and mechanism of PMT in the subretinal fibrosis of CNV have not been clarified. It has been clear that the Hippo pathway along with its downstream effector Yes-associated protein (YAP) plays an important role in both epithelial and endothelial myofibroblast development. Therefore, we speculate whether YAP participates in PMT of pericytes and promotes fibrosis of CNV. In this study, experimental CNV was induced by laser photocoagulation in C57BL/6J (B6) mice, and aberrant YAP overexpression was detected in the retinal pigment epithelial/choroid/sclera tissues of the laser-injured eyes. YAP knockdown reduced the proliferation, migration, and differentiation of human retinal microvascular pericytes in vitro. It also reduced subretinal fibrosis of laser-induced CNV in vivo. Moreover, by proteomics-based analysis of pericyte conditioned medium (PC-CM) and bioinformatic analyses, we identified that the crosstalk between Hippo/YAP and MAPK/Erk was involved in expression of filamin A in hypoxic pericytes. These findings suggest that Hippo/YAP and MAPK/Erk are linked together to mediate pericyte proliferation, migration as well as differentiation, which may embody potential implications for treatment in diseases related to CNV.

Calpain-1 mediates vascular remodelling and fibrosis via HIF-1α in hypoxia-induced pulmonary hypertension

Calpain‐1, a calcium‐activated neutral cysteine proteases, has been reported to be involved in the formation of pulmonary hypertension. HIF‐1α, an oxygen‐sensitive transcription factor, has been reported to activate genes involved in cell proliferation and extracellular matrix recombination. This study was designed to investigate the effect of calpain‐1 in hypoxic pulmonary hypertension (HPH) and to explore whether there is a relationship between calpain‐1 and HIF‐1α in this disease. In the hypoxia‐induced model of HPH, we found that hypoxia resulted in increased right ventricular systolic pressure, right ventricular hypertrophy, pulmonary vascular remodelling and collagen deposition in lung tissues of mice. The levels of calpain‐1 and HIF‐1α were up‐regulated in the lung tissues of hypoxia‐treated mice and pulmonary arterial smooth muscle cells (PASMCs). Knock‐out of calpain‐1 restrained haemodynamic and histological changes induced by chronic hypoxia in mice, and inhibition of calpain‐1 also repressed the abnormal proliferation and migration of PASMCs. Besides, knock‐out or inhibition of calpain‐1 suppressed hypoxia‐induced expression of HIF‐1α, VEGF, PCNA, TGF‐β1, MMP2 and collagen I in vivo and in vitro. While inhibition of HIF‐1α abolished the above effects of calpain‐1. Furthermore, we found that calpain‐1 mediates the expression of HIF‐1α through NF‐κB (P65) under hypoxia conditions. In conclusion, our results suggest that calpain‐1 plays a pivotal role in hypoxia‐induced pulmonary vascular remodelling and fibrosis through HIF‐1α, providing a better understanding of the pathogenesis of HPH.

Calpain as a therapeutic target in cancer

INTRODUCTION

Calpain-1 and calpain-2 are prototypical classical isoforms of the calpain family of calcium-activated cysteine proteases. Their substrate proteins participate in a wide range of cellular processes, including transcription, survival, proliferation, apoptosis, migration, and invasion. Dysregulated calpain activity has been implicated in tumorigenesis, suggesting that calpains may be promising therapeutic targets.

AREAS COVERED

This review covers clinical and basic research studies implicating calpain-1 and calpain-2 expression and activity in tumorigenesis and metastasis. We highlight isoform specific functions and provide an overview of substrates and cancer-related signalling pathways affected by calpain-mediated proteolytic cleavage. We also discuss efforts to develop clinically relevant calpain specific inhibitors and spotlight the challenges facing inhibitor development.

EXPERT OPINION

Rationale for targeting calpain-1 and calpain-2 in cancer is supported by pre-clinical and clinical studies demonstrating that calpain inhibition has the potential to attenuate carcinogenesis and block metastasis of aggressive tumors. The wide range of substrates and cleavage products, paired with inconsistencies in model systems, underscores the need for more complete understanding of physiological substrates and how calpain cleavage alters their function in cellular processes. The development of isoform specific calpain inhibitors remains an important goal with therapeutic potential in cancer and other diseases.

Filamin A increases aggressiveness of human neuroblastomas

Background

The actin-binding protein filamin A (FLNA) regulates oncogenic signal transduction important for tumor growth, but the role of FLNA in the progression of neuroblastoma (NB) has not been explored.

Methods

We analyzed FLNA mRNA expression in the R2 NB-database and FLNA protein expression in human NB tumors. We then silenced FLNA expression in human SKNBE2 and IMR32 NB cells by lentiviral vector encoding shRNA FLNA and assayed the cells for proliferation, migration, colony, spheroid formation, and apoptosis. SKNBE2 xenografts expressing or lacking FLNA in BALB/c nude mice were analyzed by both routine histopathology and immunohistochemistry.

Results

We observed shorter patient survival with higher expression of FLNA mRNA than patients with lower FLNA mRNA expression, and high-risk NB tumors expressed higher FLNA levels. SKNBE2 cells expressing higher FLNA levels proliferated more than IMR32 cells expressing lower FLNA levels. NB cell lines transfected with siRNA FLNA proliferated and migrated less, expressed lower levels of phosphorylated AKT and ERK1/2, formed smaller colonies and spheroids, as well as increased apoptosis. After inoculation of SKNBE2 cells infected with lentivirus expressing shRNA FLNA, size of NB tumors and number of proliferating cells were decreased. Furthermore, we identified STAT3 as an interacting partner of FLNA. Silencing FLNA mRNA reduced levels of phosphorylated STAT3 and MYCN, and induced expression NF-κB, cleaved caspase 3, and p53.

Conclusion

Inhibition of FLNA impaired NB cell signaling and function and reduced NB tumor size in vivo, suggesting that drugs targeting either FLNA or its interaction with STAT3 may be useful in the treatment of NB.

Filamin-A expression in laryngeal squamous cell carcinoma and its clinical significance

BACKGROUND

Laryngeal squamous cell carcinomas (LSCCs) are tumours with a high incidence of treatment failure and recurrence. Recent strategies to improve the five-year survival rate and to decrease the rates of recurrence and metastases did not improve outcomes significantly. Research efforts in recent years have started focusing on discovering biomarkers of prognosis and management in LSCCs. Filamin-A reportedly has been associated with metastatic disease in a recent study. Analysis of this protein's expression in LSCCs is lacking in the literature.

MATERIALS AND METHODS

This study analysed the expression of filamin-A, using immunohistochemistry, in a tissue microarray of 80 cases of laryngeal squamous cell cancers. Clinical-pathological parameters were analysed according to filamin-A expression in the tissue microarray. Furthermore, a review of possible mechanisms of this protein in cancer, in general, was presented, along with a review of the protein's expression in other head and neck tumours.

RESULTS

A significant majority of laryngeal squamous cell cancers exhibited positive expression of filamin-A protein. All the filamin-A positive tumours expressed it in their cytoplasm. Significant correlation between filamin-A expression and grade, stage, lymph node status and metastases were found.

CONCLUSION

The above may suggest an important role for filamin-A in LSCCs. Overall, filamin-A expression in laryngeal cancer is in line with evidence seen in other head and neck cancers. Further studies are in order to pinpoint the exact role of this protein in LSCCs, and its possible utilization in the management of these difficult-to-treat tumours.

Calpain-2 regulates hypoxia/HIF-induced plasticity toward amoeboid cancer cell migration and metastasis

Hypoxia, through hypoxia inducible factor (HIF), drives cancer cell invasion and metastatic progression in various cancer types. In epithelial cancer, hypoxia induces the transition to amoeboid cancer cell dissemination, yet the molecular mechanisms, relevance for metastasis, and effective intervention to combat hypoxia-induced amoeboid reprogramming remain unclear. Here, we identify calpain-2 as a key regulator and anti-metastasis target of hypoxia-induced transition from collective to amoeboid dissemination of breast and head and neck (HN) carcinoma cells. Hypoxia-induced amoeboid dissemination occurred through low extracellular matrix (ECM)-adhesive, predominantly bleb-based amoeboid movement, which was maintained by a low-oxidative and -glycolytic energy metabolism ("eco-mode"). Hypoxia induced calpain-2-mediated amoeboid conversion by deactivating β1 integrins through enzymatic cleavage of the focal adhesion adaptor protein talin-1. Consequently, targeted downregulation or pharmacological inhibition of calpain-2 restored talin-1 integrity and β1 integrin engagement and reverted amoeboid to elongated phenotypes under hypoxia. Calpain-2 activity was required for hypoxia-induced amoeboid conversion in the orthotopic mouse dermis and upregulated in invasive HN tumor xenografts in vivo, and attenuation of calpain activity prevented hypoxia-induced metastasis to the lungs. This identifies the calpain-2/talin-1/β1 integrin axis as a druggable mechanosignaling program that conserves energy yet enables metastatic dissemination that can be reverted by interfering with calpain activity.

Cancer biology: Hypoxia-induced talin tail-docking sparks cancer metastasis

Hypoxia drives cancer metastasis and induces cancer cells to switch from collective to amoeboid migration. A new study identifies a molecular pathway in which hypoxia stimulates calpain-2-mediated cleavage of talin-1, resulting in a reduction of integrin β1 activity and the promotion of blebbing amoeboid cancer cell migration and metastasis.

HIF-1 mediated activation of antimicrobial peptide LL-37 in type 2 diabetic patients

Abstract

Infections are common in patients with diabetes, but increasing antibiotic resistance hampers successful bacterial clearance and calls for alternative treatment strategies. Hypoxia-inducible factor 1 (HIF-1) is known to influence the innate immune defense and could therefore serve as a possible target. However, the impact of high glucose on HIF-1 has received little attention and merits closer investigation. Here, we show that higher levels of proinflammatory cytokines and CAMP, encoding for the antimicrobial peptide cathelicidin, LL-37, correlate with HIF-1 in type 2 diabetic patients. Chemical activation of HIF-1 further enhanced LL-37, IL-1β, and IL-8 in human uroepithelial cells exposed to high glucose. Moreover, HIF-1 activation of transurethrally infected diabetic mice resulted in lower bacterial load. Drugs activating HIF-1 could therefore in the future potentially have a therapeutic role in clearing bacteria in diabetic patients with infections where antibiotic treatment failed.

Key messages

• Mohanty et al. “HIF-1 mediated activation of antimicrobial peptide LL-37 in type 2 diabetic patients.”

• Our study highlights induction of the antimicrobial peptide, LL-37, and strengthening of the innate immunity through hypoxia-inducible factor 1 (HIF-1) in diabetes.

• Our key observations are:

1. HIF-1 activation increased LL-37 expression in human urothelial cells treated with high glucose. In line with that, we demonstrated that patients with type 2 diabetes living at high altitude had increased levels of the LL-37.

2. HIF-1 activation increased IL-1β and IL-8 in human uroepithelial cells treated with high glucose concentration.

3. Pharmacological activation of HIF-1 decreased bacterial load in the urinary bladder of mice with hereditary diabetes.

• We conclude that enhancing HIF-1 may along with antibiotics in the future contribute to the treatment in selected patient groups where traditional therapy is not possible.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00109-021-02134-7.

Calpain-Mediated Mitochondrial Damage: An Emerging Mechanism Contributing to Cardiac Disease

Calpains belong to the family of calcium-dependent cysteine proteases expressed ubiquitously in mammals and many other organisms. Activation of calpain is observed in diseased hearts and is implicated in cardiac cell death, hypertrophy, fibrosis, and inflammation. However, the underlying mechanisms remain incompletely understood. Recent studies have revealed that calpains target and impair mitochondria in cardiac disease. The objective of this review is to discuss the role of calpains in mediating mitochondrial damage and the underlying mechanisms, and to evaluate whether targeted inhibition of mitochondrial calpain is a potential strategy in treating cardiac disease. We expect to describe the wealth of new evidence surrounding calpain-mediated mitochondrial damage to facilitate future mechanistic studies and therapy development for cardiac disease.

Filamin A Regulates Cardiovascular Remodeling

Filamin A (FLNA) is a large actin-binding cytoskeletal protein that is important for cell motility by stabilizing actin networks and integrating them with cell membranes. Interestingly, a C-terminal fragment of FLNA can be cleaved off by calpain to stimulate adaptive angiogenesis by transporting multiple transcription factors into the nucleus. Recently, increasing evidence suggests that FLNA participates in the pathogenesis of cardiovascular and respiratory diseases, in which the interaction of FLNA with transcription factors and/or cell signaling molecules dictate the function of vascular cells. Localized FLNA mutations associate with cardiovascular malformations in humans. A lack of FLNA in experimental animal models disrupts cell migration during embryogenesis and causes anomalies, including heart and vessels, similar to human malformations. More recently, it was shown that FLNA mediates the progression of myocardial infarction and atherosclerosis. Thus, these latest findings identify FLNA as an important novel mediator of cardiovascular development and remodeling, and thus a potential target for therapy. In this update, we summarized the literature on filamin biology with regard to cardiovascular cell function.

LXN deficiency regulates cytoskeleton remodelling by promoting proteolytic cleavage of Filamin A in vascular endothelial cells

Endothelial cells (ECs) respond to blood shear stress by changing their morphology is important for maintaining vascular homeostasis. Studies have documented a relationship between endothelial cell shape and the stress flow, and however, the mechanism underlying this cytoskeletal rearrangement due to shear stress remains uncertain. In this paper, we demonstrate that laminar shear stress (LSS) significantly reduces latexin (LXN) expression in ECs. By using siRNA and cell imaging, we demonstrated that LXN knockdown results in the morphologic change and F‐actin remodelling just like what LSS does in ECs. We further demonstrate that LXN interacts with Filamin A (FLNA) and regulates FLNA proteolytic cleavage and nuclei translocation. By constructing LXN^‐/‐ mice and ApoE^‐/‐LXN^‐/‐ double knockout mice, we evaluated the effect of LXN knockout on aortic endothelium damage in mice. We found that LXN deficiency significantly improves vascular permeability, vasodilation and atherosclerosis in mice. Our findings provide confident evidence, for the first time, that LXN is a novel regulator for morphological maintenance of ECs, and LXN deficiency has a protective effect on vascular homeostasis. This provides new strategies and drug targets for the treatment of vascular diseases.

The function and pathogenic mechanism of filamin A

Filamin A(FLNa) is an actin-binding protein, which participates in the formation of the cytoskeleton, anchors a variety of proteins in the cytoskeleton and regulates cell adhesion and migration. It is involved in signal transduction, cell proliferation and differentiation, pseudopodia formation, vesicle transport, tumor resistance and genetic diseases by binding with interacting proteins. In order to fully elucidate the structure, function and pathogenesis of FLNa, we summarized all substances which directly or indirectly act on FLNa so far, upstream and downstream targets which having effect on it, signaling pathways and their functions. It also recorded the expression and effect of FLNa in different diseases, including hereditary disease and tumors.

Recent advances in proteome-wide label-free target deconvolution for bioactive small molecules

Small-molecule drugs modulate biological processes and disease states through engagement of target proteins in cells. Assessing drug-target engagement on a proteome-wide scale is of utmost importance in better understanding the molecular mechanisms of action of observed beneficial and adverse effects, as well as in developing next generation tool compounds and drugs with better efficacies and specificities. However, systematic assessment of drug-target engagement has been an arduous task. With the continuous development of mass spectrometry-based proteomics instruments and techniques, various chemical proteomics approaches for drug target deconvolution (i.e., the identification of molecular target for drugs) have emerged. Among these, the label-free target deconvolution approaches that do not involve the chemical modification of compounds of interest, have gained increased attention in the community. Here we provide an overview of the basic principles and recent biological applications of the most important label-free methods including the cellular thermal shift assay, pulse proteolysis, chemical denaturant and protein precipitation, stability of proteins from rates of oxidation, drug affinity responsive target stability, limited proteolysis, and solvent-induced protein precipitation. The state-of-the-art technical implications and future outlook for the label-free approaches are also discussed.

Target identification and validation of natural products with label-free methodology: A critical review from 2005 to 2020

Natural products (NPs) have been an important source of therapeutic drugs in clinic use and contributed many chemical probes for research. The usefulness of NPs is however often marred by the incomplete understanding of their direct cellular targets. A number of experimental methods for drug target identification have been developed over the years. One class of methods, termed "label-free" methodology, exploits the energetic and biophysical features accompanying the association of macromolecules with drugs and other compounds in their native forms. Herein we review the working principles, assay implementations, and key applications of the most important approaches, and also give examples where they have been applied to NPs. We also assess the key advantages and limitations of each method. Furthermore, we address when and how the label-free methodology can be particularly useful considering some of the unique features of NP chemistry and bioactivation.

Exploration of Oxygen-Induced Retinopathy Model to Discover New Therapeutic Drug Targets in Retinopathies

Oxygen-induced retinopathy (OIR) is a pure hypoxia-driven angiogenesis model and the most widely used model for ischemic retinopathies, such as retinopathy of prematurity (ROP), proliferative diabetic retinopathy (PDR), and retinal vein occlusion (RVO). OIR model has been used to test new potential anti-angiogenic factors for human diseases. We have recently performed the most comprehensive characterization of OIR by a relatively novel mass spectrometry (MS) technique, sequential window acquisition of all theoretical fragment ion mass spectra (SWATH-MS) proteomics and used genetically modified mice strains to identify novel molecular drug targets in angiogenic retinal diseases. We have confirmed the relevance of the identified molecular targets to human diseases by determining their expression pattern in neovascular membranes obtained from PDR and RVO patients. Based on our results, crystallins were the most prominent proteins induced by early hypoxic environment during the OIR, while actomyosin complex and Filamin A-R-Ras axis, that regulates vascular permeability of the angiogenic blood vessels, stood out at the peak of angiogenesis. Our results have revealed potential new therapeutic targets to address hypoxia-induced pathological angiogenesis and the associated vascular permeability in number of retinal diseases.

Calpain proteolytic systems counteract endothelial cell adaptation to inflammatory environments

Vascular endothelial cells (ECs) make up the innermost surface of arteries, veins, and capillaries, separating the remaining layers of the vessel wall from circulating blood. Under non-inflammatory conditions, ECs are quiescent and form a robust barrier structure; however, exposure to inflammatory stimuli induces changes in the expression of EC proteins that control transcellular permeability and facilitate angiogenic tube formation. Increasing evidence suggests that dysfunction in intracellular proteolytic systems disturbs EC adaptation to the inflammatory environment, leading to vascular disorders such as atherosclerosis and pathological angiogenesis. Recent work has highlighted the contribution of the calpain–calpastatin stress-responsive intracellular proteolytic system to adaptation failure in ECs. In this review, we summarize our current knowledge of calpain–calpastatin-mediated physiologic and pathogenic regulation in ECs and discuss the molecular basis by which disruption of this system perturbs EC adaptation to the inflammatory environment.

Targeting Filamin A Reduces Macrophage Activity and Atherosclerosis

BACKGROUND

The actin-binding protein FLNA (filamin A) regulates signal transduction important for cell locomotion, but the role of macrophage-specific FLNA during atherogenesis has not been explored.

METHODS

We analyzed FLNA expression in human carotid atherosclerotic plaques by immunofluorescence. We also produced mice with Flna-deficient macrophages by breeding conditional Flna-knockout mice ( Flna ^o/fl) with mice expressing Cre from the macrophage-specific lysosome M promoter ( LC). Atherosclerosis in vivo was studied by transplanting bone marrow from male Flna ^o/fl/ LC mice to atherogenic low-density lipoprotein receptor-deficient ( Ldlr^-/-) mice; and by infecting Flna ^o/fl and Flna ^o/fl/ LC mice with AdPCSK9 (adenoviral vector overexpressing proprotein convertase subtilisin/kexin type 9). Furthermore, C57BL/6 mice were infected with AdPCSK9 and then treated with the calpain inhibitor calpeptin to inhibit FLNA cleavage.

RESULTS

We found that macrophage FLNA expression was higher in advanced than in intermediate human atherosclerotic plaques. Flna ^o/fl/ LC macrophages proliferated and migrated less than controls; expressed lower levels of phosphorylated AKT and ERK1/2; exhibited reduced foam cell formation and lipid uptake; and excreted more lipids. The deficiency of Flna in macrophages markedly reduced the size of aortic atherosclerotic plaques in both Ldlr^{-/-BMT: Flnao/fl/LC} and AdPCSK9-infected Flna ^o/fl/ LC mice. Intima/media ratios and numbers of CD68-positive macrophages in atherosclerotic plaques were lower in Flna-deficient mice than in control mice. Moreover, we found that STAT3 interacts with a calpain-cleaved carboxyl-terminal fragment of FLNA. Inhibiting calpain-mediated FLNA cleavage with calpeptin in macrophages reduced nuclear levels of phosphorylated STAT3, interleukin 6 secretion, foam cell formation, and lipid uptake. Finally, calpeptin treatment reduced the size of atherosclerotic plaques in C57BL/6 mice infected with AdPCSK9.

CONCLUSIONS

Genetic inactivation of Flna and chemical inhibition of calpain-dependent cleavage of FLNA impaired macrophage signaling and function, and reduced atherosclerosis in mice, suggesting that drugs targeting FLNA may be useful in the treatment of atherosclerosis.

SWATH-MS Proteomic Analysis of Oxygen-Induced Retinopathy Reveals Novel Potential Therapeutic Targets

Purpose

Oxygen-induced retinopathy (OIR) is the most widely used model for ischemic retinopathies such as retinopathy of prematurity (ROP), proliferative diabetic retinopathy (PDR), and retinal vein occlusion (RVO). The purpose of this study was to perform the most comprehensive characterization of OIR by a recently developed technique, sequential window acquisition of all theoretical mass spectra (SWATH-MS) proteomics.

Methods

Control and OIR retina samples collected from various time points were subjected to SWATH-MS and detailed data analysis. Immunohistochemistry from mouse retinas as well as neovascular membranes from human PDR and RVO patients were used for the detection of the localization of the proteins showing altered expression in the retina and to address their relevance to human ischemic retinopathies.

Results

We report the most extensive proteomic profiling of OIR to date by quantifying almost 3000 unique proteins and their expression differences between control and OIR retinas. Crystallins were the most prominent proteins induced by hypoxia in the retina, while angiogenesis related proteins such as Filamin A and nonmuscle myosin IIA stand out at the peak of angiogenesis. Majority of the changes in protein expression return to normal at P42, but there is evidence to suggest that proteins involved in neurotransmission remain at reduced level.

Conclusions

The results reveal new potential therapeutic targets to address hypoxia-induced pathological angiogenesis taking place in number of retinal diseases. The extensive proteomic profiling combined with pathway analysis also identifies novel molecular networks that could contribute to the pathogenesis of retinal diseases.

RNA editing of Filamin A pre-mRNA regulates vascular contraction and diastolic blood pressure

Epitranscriptomic events such as adenosine-to-inosine (A-to-I) RNA editing by ADAR can recode mRNAs to translate novel proteins. Editing of the mRNA that encodes actin crosslinking protein Filamin A (FLNA) mediates a Q-to-R transition in the interactive C-terminal region. While FLNA editing is conserved among vertebrates, its physiological function remains unclear. Here, we show that cardiovascular tissues in humans and mice show massive editing and that FLNA RNA is the most prominent substrate. Patient-derived RNA-Seq data demonstrate a significant drop in FLNA editing associated with cardiovascular diseases. Using mice with only impaired FLNA editing, we observed increased vascular contraction and diastolic hypertension accompanied by increased myosin light chain phosphorylation, arterial remodeling, and left ventricular wall thickening, which eventually causes cardiac remodeling and reduced systolic output. These results demonstrate a causal relationship between RNA editing and the development of cardiovascular disease indicating that a single epitranscriptomic RNA modification can maintain cardiovascular health.

Genetic disruption of calpain-1 and calpain-2 attenuates tumorigenesis in mouse models of HER2+ breast cancer and sensitizes cancer cells to doxorubicin and lapatinib

Calpains are a family of calcium activated cysteine proteases which participate in a wide range of cellular functions including migration, invasion, autophagy, programmed cell death, and gene expression. Calpain-1 and calpain-2 isoforms are ubiquitously expressed heterodimers composed of isoform specific catalytic subunits coupled with an obligate common regulatory subunit encoded by capns1. Here, we report that conditional deletion of capns1 disrupted calpain-1 and calpain-2 expression and activity, and this was associated with delayed tumorigenesis and altered signaling in a transgenic mouse model of spontaneous HER2⁺ breast cancer and effectively blocked tumorigenesis in an orthotopic engraftment model. Furthermore, capns1 knockout in a tumor derived cell line correlated with enhanced sensitivity to the chemotherapeutic doxorubicin and the HER2/EGFR tyrosine kinase inhibitor lapatinib. Collectively, these results indicate pro-tumorigenic roles for calpains-1/2 in HER2⁺ breast cancer and provide evidence that calpain-1/2 inhibitors could have anti-tumor effects if used either alone or in combination with chemotherapeutics and targeted agents.

Potential biomarkers and targets in reversibility of pulmonary arterial hypertension secondary to congenital heart disease: an explorative study

Whether pulmonary arterial hypertension (PAH) is reversible in congenital heart disease (CHD) is important for the operability of CHD. However, little is known about that. Our research was aimed at exploring novel biomarkers and targets in the reversibility of CHD-PAH. CHD-PAH patients diagnosed with right heart catheterization (RHC) were enrolled (n = 14). Lung biopsy was performed during the repair surgery. After one year follow-up, mean pulmonary arterial pressures (mPAP) were evaluated by RHC to determine the diagnosis of reversible (mPAP < 25 mmHg, n = 10) and irreversible (mPAP ≥ 25 mmHg, n = 4) PAH. Harvested normal lung tissues (n = 6) were included as the control group. Pulmonary arteriole lesions were identified by pathological grading in tissue staining. iTRAQ-labelled mass-spectrometry analysis followed by immunohistochemistry and western blot was used to explore the most meaningful differential proteins. For enrolled patients, the histopathological grading of pulmonary vascular lesions in reversible CHD-PAH patients was all at grades 0-II while grades III-IV were shown only in irreversible CHD-PAH patients. Proteomic analysis identified 85 upregulated and 75 downregulated proteins, including cytoskeletal proteins and collagen chains, mainly involved in cell adhesion, extracellular matrix, cytoskeleton, immune response, and complement pathways. Among them, caveolin-1, filamin A expression, and cathepsin D combined with macrophagocytes counts were significantly increased; glutathione S-transferase mu1 (GSTM1) expression was significantly decreased in the irreversible CHD-PAH group (all P < 0.05). Caveolin-1, filamin A, and cathepsin D expression showed a positive relation and GSTM1 showed a negative relation with pathological grading. Upregulated caveolin-1, filamin A, and cathepsin D combined with increased macrophagocytes and downregulated GSTM1 may be potential biomarkers and targets in the irreversibility CHD-PAH, and which may be useful in evaluating the operability and understanding the irreversibility of CHD-PAH. Expression of these pathological biomarkers combined with pathological changes in lung biopsy may have great value in predicting the irreversibility of PAH.

Signaling regulation and role of filamin A cleavage in Ca2+-stimulated migration of androgen receptor-deficient prostate cancer cells

Ca²⁺, a ubiquitous cellular signal, and filamin A, an actin-binding protein, play an important role in the regulation of cell adhesion, shape and motility. Using transwell filters to analyze cell migration, we found that extracellular Ca²⁺ (Ca_o²⁺) promotes the migration of androgen receptor (AR)-deficient and highly metastatic prostate cancer cell lines (DU145 and PC-3) compared to AR-positive and relatively less metastatic prostate cancer cells (LNCaP). Furthermore, we found that expression of filamin A is up-regulated in DU145 and PC-3 cells, and that Ca_o²⁺ significantly induces the cleavage of filamin A. Silencing expression of Ca²⁺-sensing receptor (CaR) and p115RhoGEF, and treating with leupeptin, a protease inhibitor, and ALLM, a calpain specific inhibitor, we further demonstrate that Ca_o²⁺-induced filamin A cleavage occurs via a CaR- p115RhoGEF-calpain dependent pathway. Our data show that Ca_o²⁺ via CaR- mediated signaling induces filamin A cleavage and promotes the migration in AR-deficient and highly metastatic prostate cancer cells.

FLNA is implicated in pulmonary neuroendocrine tumors aggressiveness and progression

Pulmonary neuroendocrine tumors (PNTs) comprise different neoplasms, ranging from low grade carcinoids to the highly malignant small cell lung cancers. Several studies identified the cytoskeleton protein Filamin A (FLNA) as determinant in cancer progression and metastasis, but the role of FLNA in PNT aggressiveness and progression is still unknown.

We evaluated FLNA expression in PNTs with different grade of differentiation, the role of FLNA in cell proliferation, colony formation, angiogenesis, cell adhesion and migration in PNT cell line (H727 cells) and primary cultures and the possible interaction between FLNA and Rap1-GTPase. FLNA is highly expressed in PNTs with high malignant grade. FLNA silencing reduces cyclin D1 levels (-51±5, p<0.001) and cell proliferation in PNT cells (-37±4, p<0.05), colony formation and VEGF expression (-39±9%, p<0.01) in H727 cells.

FLNA and Rap1 co-localize in cellular protrusions and FLNA silencing up-regulates Rap1 expression (+73±18%, p<0.01).

Rap1 silencing prevents cell adhesion increase (+43%±18%, p<0.01) and cell migration decrease (-56±7%, p<0.01) induced by FLNA silencing, without affecting cell proliferation reduction. In conclusion, FLNA is implicated in PNT progression, in part through Rap1, thus providing a potential diagnostic and therapeutic target.

Dysregulation of Calpain Proteolytic Systems Underlies Degenerative Vascular Disorders

Chronic vascular diseases such as atherosclerosis, aneurysms, diabetic angiopathy/retinopathy as well as fibrotic and proliferative vascular diseases are generally complicated by the progression of degenerative insults, which are characterized by endothelial dysfunction, apoptotic/necrotic cell death in vascular/immune cells, remodeling of extracellular matrix or breakdown of elastic lamella. Increasing evidence suggests that dysfunctional calpain proteolytic systems and defective calpain protein metabolism in blood vessels contribute to degenerative disorders. In vascular endothelial cells, the overactivation of conventional calpains consisting of calpain-1 and -2 isozymes can lead to the disorganization of cell-cell junctions, dysfunction of nitric oxide synthase, sensitization of Janus kinase/signal transducer and activator of transcription cascades and depletion of prostaglandin I₂, which contributes to degenerative disorders. In addition to endothelial cell dysfunctions, calpain overactivation results in inflammatory insults in macrophages and excessive fibrogenic/proliferative signaling in vascular smooth muscle cells. Moreover, calpain-6, a non-proteolytic unconventional calpain, is involved in the conversion of macrophages to a pro-atherogenic phenotype, leading to the pinocytotic deposition of low-density lipoprotein cholesterol in the cells. Here, we discuss the recent progress that has been made in our understanding of how calpain contributes to degenerative vascular disorders.

Endothelial cell calpain as a critical modulator of angiogenesis

Calpains are a family of calcium-dependent non-lysosomal cysteine proteases. In particular, calpains residing in the endothelial cells play important roles in angiogenesis. It has been shown that calpain activity can be increased in endothelial cells by growth factors, primarily vascular endothelial growth factor (VEGF). VEGF/VEGFR2 induces calpain 2 dependent activation of PI3K/AMPK/Akt/eNOS pathway, and consequent nitric oxide production and physiological angiogenesis. Under pathological conditions such as tumor angiogenesis, endothelial calpains can be activated by hypoxia. This review focuses on the molecular regulatory mechanisms of calpain activation, and the newly identified mechanistic roles and downstream signaling events of calpains in physiological angiogenesis, and in the conditions of pathological tumor angiogenesis and diabetic wound healing, as well as retinopathy and atherosclerosis that are also associated with an increase in calpain activity. Further discussed include the differential strategies of modulating angiogenesis through manipulating calpain expression/activity in different pathological settings. Targeted limitation of angiogenesis in cancer and targeted promotion of angiogenesis in diabetic wound healing via modulations of calpains and calpain-dependent signaling mechanisms are of significant translational potential. Emerging strategies of tissue-specific targeting, environment-dependent targeting, and genome-targeted editing may turn out to be effective regimens for targeted manipulation of angiogenesis through calpain pathways, for differential treatments including both attenuation of tumor angiogenesis and potentiation of diabetic angiogenesis.

A compendium of proteins that interact with HIF-1α

Hypoxia-inducible factor 1 (HIF-1) is the founding member of a family of transcription factors that function as master regulators of oxygen homeostasis. HIF-1 is composed of an O₂-regulated HIF-1α subunit and a constitutively expressed HIF-1β subunit. This review provides a compendium of proteins that interact with the HIF-1α subunit, many of which regulate HIF-1 activity in either an O₂-dependent or O₂-independent manner.

The putative involvement of actin-binding proteins and cytoskeleton proteins in pathological mechanisms of ketamine cystitis-Revealed by a prospective pilot study using proteomic approaches

PURPOSE

Ketamine-induced cystitis (KC) among chronic ketamine young abusers has increased dramatically and it has brought attention for Urologists. The underlying pathophysiological mechanism(s) of KC is still unclear. Therefore, the purpose of this study is to elucidate the possible pathophysiological mechanism(s) of KC through proteomic techniques.

EXPERIMENTAL DESIGN

Bladder tissues are obtained from seven patients with KC, seven patients with interstitial cystitis/bladder pain syndrome, and five control subjects who underwent video-urodynamic study followed by augmentation enterocystoplasty to increase bladder capacity. 2DE/MS/MS-based approach, functional classifications, and network analyses are used for proteomic and bioinformatics analyses and protein validation is carried out by Western blot analysis.

RESULTS

Among the proteins identified, bioinformatics analyses revealed that several actin binding related proteins such as cofilin-1, myosin light polypeptide 9, filamin A, gelsolin, lamin A are involved in the apoptosis. Besides, the contractile proteins and cytoskeleton proteins such as myosin light polypeptide 9, filamin A, and calponin are found downregulated in KC bladders.

CONCLUSIONS AND CLINICAL RELEVANCE

Increased apoptosis in KC might be mediated by actin-binding proteins and a Ca²⁺ -activated protease. Rapid detrusor contraction in KC might be induced by contractile proteins and cytoskeleton proteins.

Cytoskeletal Filamin A Differentially Modulates RNA Polymerase III Gene Transcription in Transformed Cell Lines

Cytoskeletal filamin A (FLNA) is an important protein involved in multiple cellular processes. Previous studies have shown that FLNA can promote or inhibit cancer growth and development; however, the mechanisms underlying these events are not fully understood. Here we show that, in both 293T and SaOS2 cells, knockdown of FLNA significantly enhanced transcription of RNA polymerase (pol) III-transcribed genes except for a subset of tRNA genes. In contrast, re-expression of FLNA in an FLNA-deficient melanoma cell line (A7) repressed transcription of all pol III-transcribed genes, suggesting that FLNA inhibits pol III transcription in a cell type-specific manner. Chromatin immunoprecipitation assays revealed that the repression of pol III gene transcription by FLNA correlates with the decreased occupancy of the RNA pol III transcription machinery at promoters. Immunofluorescence microscopy and coimmunoprecipitation assays revealed that FLNA can associate with the RNA pol III transcription machinery through its actin-binding domain within nuclei. Mechanistic analysis revealed that FLNA suppresses pol III gene transcription by confining the recruitment of the RNA pol III transcription machinery at the promoters of the genes that are sensitive to the alteration of FLNA expression. These findings not only extend the understanding of FLNA function in cells but also provide novel insights into the mechanism by which FLNA represses cell proliferation.

The peptidomimetic Vasotide targets two retinal VEGF receptors and reduces pathological angiogenesis in murine and nonhuman primate models of retinal disease

Blood vessel growth from preexisting vessels (angiogenesis) underlies many severe diseases including major blinding retinal diseases such as retinopathy of prematurity (ROP) and aged macular degeneration (AMD). This observation has driven development of antibody inhibitors that block a central factor in AMD, vascular endothelial growth factor (VEGF), from binding to its receptors VEGFR-1 and mainly VEGFR-2. However, some patients are insensitive to current anti-VEGF drugs or develop resistance, and the required repeated intravitreal injection of these large molecules is costly and clinically problematic. We have evaluated a small cyclic retro-inverted peptidomimetic, D(Cys-Leu-Pro-Arg-Cys) [D(CLPRC)], and hereafter named Vasotide, that inhibits retinal angiogenesis by binding selectively to the VEGF receptors VEGFR-1 and neuropilin-1 (NRP-1). Delivery of Vasotide via either eye drops or intraperitoneal injection in a laser-induced monkey model of human wet AMD, a mouse genetic knockout model of the AMD subtype called retinal angiomatous proliferation (RAP), and a mouse oxygen-induced model of ROP decreased retinal angiogenesis in all three animal models. This prototype drug candidate is a promising new dual receptor inhibitor of the VEGF ligand with potential for translation into safer, less-invasive applications to combat pathological angiogenesis in retinal disorders.

Discovery of Manassantin A Protein Targets Using Large-Scale Protein Folding and Stability Measurements

Manassantin A is a natural product that has been shown to have anticancer activity in cell-based assays, but has a largely unknown mode-of-action. Described here is the use of two different energetics-based approaches to identify protein targets of manassantin A. Using the stability of proteins from rates of oxidation technique with an isobaric mass tagging strategy (iTRAQ-SPROX) and the pulse proteolysis technique with a stable isotope labeling with amino acids in cell culture strategy (SILAC-PP), over 1000 proteins in a MDA-MB-231 cell lysate grown under hypoxic conditions were assayed for manassantin A interactions (both direct and indirect). A total of 28 protein hits were identified with manassantin A-induced thermodynamic stability changes. Two of the protein hits (filamin A and elongation factor 1α) were identified using both experimental approaches. The remaining 26 hit proteins were only assayed in either the iTRAQ-SPROX or the SILAC-PP experiment. The 28 potential protein targets of manassantin A identified here provide new experimental avenues along which to explore the molecular basis of manassantin A's mode of action. The current work also represents the first application iTRAQ-SPROX and SILAC-PP to the large-scale analysis of protein-ligand binding interactions involving a potential anticancer drug with an unknown mode-of-action.

Unravelling a new mechanism linking actin polymerization and gene transcription

In the recent years, the role of actin and actin-binding proteins in gene transcription has received considerable attention. Nuclear monomeric and polymerized actin and several actin binding proteins have been detected in the mammalian cell nucleus, although their roles in transcription are just beginning to emerge. Our group recently reported that the actin-binding protein Filamin A interacts with the transcriptional coactivator MKL1 to link actin polymerization with transcriptional activity of Serum Response Factor. Here we summarize the regulation and function of MKL1, and highlight this novel mechanism of MKL1 regulation through binding to Filamin A and its implications for cell migration.

The Oxygen Sensor PHD2 Controls Dendritic Spines and Synapses via Modification of Filamin A

Neuronal function is highly sensitive to changes in oxygen levels, but how hypoxia affects dendritic spine formation and synaptogenesis is unknown. Here we report that hypoxia, chemical inhibition of the oxygen-sensing prolyl hydroxylase domain proteins (PHDs), and silencing of Phd2 induce immature filopodium-like dendritic protrusions, promote spine regression, reduce synaptic density, and decrease the frequency of spontaneous action potentials independently of HIF signaling. We identified the actin cross-linker filamin A (FLNA) as a target of PHD2 mediating these effects. In normoxia, PHD2 hydroxylates the proline residues P2309 and P2316 in FLNA, leading to von Hippel-Lindau (VHL)-mediated ubiquitination and proteasomal degradation. In hypoxia, PHD2 inactivation rapidly upregulates FLNA protein levels because of blockage of its proteasomal degradation. FLNA upregulation induces more immature spines, whereas Flna silencing rescues the immature spine phenotype induced by PHD2 inhibition.

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The oxygen sensor PHD2 is present in dendritic spines

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PHD2 inhibition by hypoxia reduces spine maturation, synaptic density, and activity

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Through hydroxylation, PHD2 targets filamin A for proteasomal degradation

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Filamin A stabilization promotes dendritic spine remodeling

Filamin-A is required to mediate SST2 effects in pancreatic neuroendocrine tumours

Somatostatin receptor type 2 (SST2) is the main pharmacological target of somatostatin (SS) analogues widely used in patients with pancreatic neuroendocrine tumours (P-NETs), this treatment being ineffective in a subset of patients. Since it has been demonstrated that Filamin A (FLNA) is involved in mediating GPCR expression, membrane anchoring and signalling, we investigated the role of this cytoskeleton protein in SST2 expression and signalling, angiogenesis, cell adhesion and cell migration in human P-NETs and in QGP1 cell line. We demonstrated that FLNA silencing was not able to affect SST2 expression in P-NET cells in basal conditions. Conversely, a significant reduction in SST2 expression (-43 ± 21%, P < 0.05 vs untreated cells) was observed in FLNA silenced QGP1 cells after long term SST2 activation with BIM23120. Moreover, the inhibitory effect of BIM23120 on cyclin D1 expression (-46 ± 18%, P < 0.05 vs untreated cells), P-ERK1/2 levels (-42 ± 14%; P < 0.05 vs untreated cells), cAMP accumulation (-24 ± 3%, P < 0.05 vs untreated cells), VEGF expression (-31 ± 5%, P < 0.01 vs untreated cells) and in vitro release (-40 ± 24%, P < 0.05 vs untreated cells) was completely lost after FLNA silencing. Interestingly, BIM23120 promoted cell adhesion (+86 ± 45%, P < 0.05 vs untreated cells) and inhibited cell migration (-24 ± 2%, P < 0.00001 vs untreated cells) in P-NETs cells and these effects were abolished in FLNA silenced cells. In conclusion, we demonstrated that FLNA plays a crucial role in SST2 expression and signalling, angiogenesis, cell adhesion and cell migration in P-NETs and in QGP1 cell line, suggesting a possible role of FLNA in determining the different responsiveness to SS analogues observed in P-NET patients.

Filamin A interacts with the coactivator MKL1 to promote the activity of the transcription factor SRF and cell migration

Megakaryoblastic leukemia 1 (MKL1) is a coactivator of serum response factor (SRF) that promotes the expression of genes associated with cell proliferation, motility, adhesion, and differentiation-processes that also involve dynamic cytoskeletal changes in the cell. MKL1 is inactive when bound to monomeric globular actin (G-actin), but signals that activate the small guanosine triphosphatase RhoA cause actin polymerization and MKL1 dissociation from G-actin. We found a new mechanism of MKL1 activation that is mediated through its binding to filamin A (FLNA), a protein that binds filamentous actin (F-actin). The interaction of FLNA and MKL1 was required for the expression of MKL1 target genes in primary fibroblasts, melanoma, mammary and hepatocellular carcinoma cells. We identified the regions of interaction between MKL1 and FLNA, and cells expressing an MKL1 mutant that was unable to bind FLNA exhibited impaired cell migration and reduced expression of MKL1-SRF target genes. Induction and repression of MKL1-SRF target genes correlated with increased or decreased MKL1-FLNA interaction, respectively. Lysophosphatidic acid-induced RhoA activation in primary human fibroblasts promoted the association of endogenous MKL1 with FLNA, whereas exposure to an actin polymerization inhibitor dissociated MKL1 from FLNA and decreased MKL1-SRF target gene expression in melanoma cells. Thus, FLNA functions as a positive cellular transducer linking actin polymerization to MKL1-SRF activity, counteracting the known repressive complex of MKL1 and monomeric G-actin.

High-level secretion of tissue factor-rich extracellular vesicles from ovarian cancer cells mediated by filamin-A and protease-activated receptors

Thromboembolic events occur frequently in ovarian cancer patients. Tissue factor (TF) is often overexpressed in tumours, including ovarian clear-cell carcinoma (CCC), a subtype with a generally poor prognosis. TF-coagulation factor VII (fVII) complexes on the cell surface activate downstream coagulation mechanisms. Moreover, cancer cells secrete extracellular vesicles (EVs), which act as vehicles for TF. We therefore examined the characteristics of EVs produced by ovarian cancer cells of various histological subtypes. CCC cells secreted high levels of TF within EVs, while the high-TF expressing breast cancer cell line MDA-MB-231 shed fewer TF-positive EVs. We also found that CCC tumours with hypoxic tissue areas synthesised TF and fVII in vivo, rendering the blood of xenograft mice bearing these tumours hypercoagulable compared with mice bearing MDA-MB-231 tumours. Incorporation of TF into EVs and secretion of EVs from CCC cells exposed to hypoxia were both dependent on the actin-binding protein, filamin-A (filA). Furthermore, production of these EVs was dependent on different protease-activated receptors (PARs) on the cell surface. These results show that CCC cells could produce large numbers of TF-positive EVs dependent upon filA and PARs. This phenomenon may be the mechanism underlying the increased incidence of venous thromboembolism in ovarian cancer patients.

Calpain in Breast Cancer: Role in Disease Progression and Treatment Response

The calpains are a family of intracellular cysteine proteases that function in a wide array of cellular activities, including cytoskeletal remodelling, survival and apoptosis. The ubiquitously expressed micro (µ)-calpain and milli (m)-calpain are archetypal family members that require calcium for function and can be inhibited by their endogenous inhibitor calpastatin. This review describes the role of the calpain system in the prognosis of breast cancer and disease progression, in addition to the role of the calpain system in the response to breast cancer treatments, including chemotherapeutic, endocrine and targeted therapies.

Hypoxia induces calpain activity and degrades SMAD2 to attenuate TGFβ signaling in macrophages

Background

Under inflammatory conditions or during tumor progression macrophages acquire distinct phenotypes, with factors of the microenvironment such as hypoxia and transforming growth factor β (TGFβ) shaping their functional plasticity. TGFβ is among the factors causing alternative macrophage activation, which contributes to tissue regeneration and thus, resolution of inflammation but may also provoke tumor progression. However, the signal crosstalk between TGFβ and hypoxia is ill defined.

Results

Exposing human primary macrophages to TGFβ elicited a rapid SMAD2/SMAD3 phosphorylation. This early TGFβ-signaling remained unaffected by hypoxia. However, with prolonged exposure periods to TGFβ/hypoxia the expression of SMAD2 declined because of decreased protein stability. In parallel, hypoxia increased mRNA and protein amount of the calpain regulatory subunit, with the further notion that TGFβ/hypoxia elicited calpain activation. The dual specific proteasome/calpain inhibitor MG132 and the specific calpain inhibitor 1 rescued SMAD2 degradation, substantiating the ability of calpain to degrade SMAD2. Decreased SMAD2 expression reduced TGFβ transcriptional activity of its target genes thrombospondin 1, dystonin, and matrix metalloproteinase 2.

Conclusions

Hypoxia interferes with TGFβ signaling in macrophages by calpain-mediated proteolysis of the central signaling component SMAD2.

Electronic supplementary material

The online version of this article (doi:10.1186/s13578-015-0026-x) contains supplementary material, which is available to authorized users.