Membrane-anchored growth factor, HB-EGF, on the cell surface targeted to the inner nuclear membrane

The influence of environmental factors on premature ovarian insufficiency and ovarian aging

Premature ovarian insufficiency and ovarian aging are complex conditions that affect women's reproductive health and overall well-being. They are both characterized by hypergonadotropic hypogonadism and infertility, and together affect about 1 in 100 women by the age of 40. This review explores the influence of environmental factors on the development and progression of premature ovarian insufficiency and ovarian aging. When referring to environmental factors, we include a wide range of external agents and conditions, including chemicals, socioeconomic factors and lifestyle choices. Through a review of the literature, we attempt to highlight the link between environmental factors and ovarian health. We examine the impact of endocrine-disrupting chemicals, such as bisphenol A and phthalates, on ovarian function and investigate the mechanisms by which these chemicals can disrupt hormone signaling pathways, leading to alterations in ovarian reserve, oocyte quality, and folliculogenesis. Moreover, we explore lifestyle factors like obesity, stress, smoking and alcohol in relation to their effects on ovarian aging. Epigenetic changes may play a crucial role in the prevalence of premature ovarian insufficiency. Understanding the impact of environmental factors on premature ovarian insufficiency and ovarian aging is very important in public and clinical health contexts. By identifying risk factors, healthcare providers can develop targeted and strategic prevention and intervention plans. Furthermore, this knowledge can promote reproductive health and minimize exposure to harmful environmental agents.

Histone methyltransferase SUV39H1 regulates the Golgi complex via the nuclear envelope-spanning LINC complex

Cell motility is related to the higher-order structure of chromatin. Stimuli that induce cell migration change chromatin organization; such stimuli include elevated histone H3 lysine 9 trimethylation (H3K9me3). We previously showed that depletion of histone H3 lysine 9 methyltransferase, SUV39H1, suppresses directional cell migration. However, the molecular mechanism underlying this association between chromatin and cell migration remains elusive. The Golgi apparatus is a cell organelle essential for cell motility. In this study, we show that loss of H3K9 methyltransferase SUV39H1 but not SETDB1 or SETDB2 causes dispersion of the Golgi apparatus throughout the cytoplasm. The Golgi dispersion triggered by SUV39H1 depletion is independent of transcription, centrosomes, and microtubule organization, but is suppressed by depletion of any of the following three proteins: LINC complex components SUN2, nesprin-2, or microtubule plus-end-directed kinesin-like protein KIF20A. In addition, SUN2 is closely localized to H3K9me3, and SUV39H1 affects the mobility of SUN2 in the nuclear envelope. Further, inhibition of cell motility caused by SUV39H1 depletion is restored by suppression of SUN2, nesprin-2, or KIF20A. In summary, these results show the functional association between chromatin organization and cell motility via the Golgi organization regulated by the LINC complex.

Identification of HBEGF+ fibroblasts in the remission of rheumatoid arthritis by integrating single-cell RNA sequencing datasets and bulk RNA sequencing datasets

Background

Fibroblasts are important structural cells in synovium and play key roles in maintaining the synovial homeostasis. By single-cell RNA sequencing (scRNA-seq), subpopulation of synovium-resident cells has been reported to protect intra-articular structures from chronic inflammation and promote tissue repair. However, a significant number of researchers have concentrated on the role of fibroblasts in the progress of rheumatoid arthritis (RA) while few reports had described the contribution of distinct fibroblast subsets in the RA remission. It is helpful to understand the role of fibroblast subpopulations in the RA process to provide predictive biomarkers and address RA remission mechanisms. Here, we found HBEGF+ fibroblasts that contributed to RA remission by integrating scRNA-seq datasets and bulk RNA sequencing (bulk RNA-seq) datasets.

Method

Three single-cell RNA datasets of cells harvested from RA patients were processed and integrated by Seurat and Harmony R packages. After identifying cell types by classic marker genes, the integrated dataset was used to run CellChat for analysis of cell-cell communication. Specially, EGF signaling pathway was found and HBEGF+ fibroblasts were identified based on HBEGF expression. Differential expressed genes of HBEGF+ were shown in heatmap and volcano plot and used to run gene ontology (GO) enrichment analysis. Next, bulk RNA-seq datasets of synovium under different conditions (health, osteoarthritis (OA), rheumatoid arthritis, before and after classical treatment) were compared to show expression change of HBEGF and gene markers that are mainly expressed by HBEGF+ fibroblasts such as CLIC5, PDGFD, BDH2, and ENPP1. Finally, two single-cell RNA sequencing datasets of synovial cells from mice were integrated to identify Hbegf+ fibroblasts and calculate the population of Hbegf+ fibroblasts under different joint conditions (health, K/BxN serum transfer arthritis (STA), and remission of STA).

Result

After integrating three single-cell RNA sequencing datasets, we identified 11 clusters of synovial cells, such as fibroblasts, mural cells, endothelial cells, CD4+ T cells, CD8+ T cells, natural killer cells, synovium macrophage, peripheral blood macrophages, plasma cells, B cells, and STMN1+ cells. We found fibroblasts had an extensive communication network with other clusters and interacted with synovial macrophages through EGF signaling pathway via analysis of cell-cell communication between synovial cells. HBEGF, ligand to EGF signaling pathway, was highly expressed by a subset of fibroblasts and macrophages, and EGFR, receptor to EGF signaling pathway, was highly expressed by fibroblasts and meniscus cells. Moreover, HBEGF was downregulated under RA state and it had an increase after classical treatment. We then defined fibroblasts with high expression of HBEGF as HBEGF+ fibroblasts. In addition, we also found that HBEGF+ fibroblasts highly expressed CRTAC1, ITGB8, SCARA5, THBS4, and ITGBL1, genes relative to encoding extracellular matrix proteins and engaged in positive regulation of cell migration and motility, cellular component movement, and cell growth by GO enrichment analysis. We eventually identified HBEGF+ fibroblasts specially expressed CLIC5, PDGFD, BDH2, and ENPP1, which positively correlated with the expression of HBEGF. Moreover, the expression of CLIC5, PDGFD, BDH2, and ENPP1 was downregulated under RA state and elevated by classical therapy. On the contrary, the HBEGF+ macrophages specially expressed SLAMF8, GK, L1RN, and JAK2, which negatively correlated with the expression of HBEGF. The expression was upregulated in SLAMF8, GK, L1RN, and JAK2 under the RA state, whereas it was decreased after classical treatment. In mice, the number of Hbegf+ fibroblasts was reduced in the RA synovium but increased in the RA remitting synovium.

Conclusions

HBEGF+ fibroblasts play a role in the remission of rheumatoid arthritis, and HBEGF has potential to become a novel biomarker for prediction of RA progress.

Inner Nuclear Membrane Protein, SUN1, is Required for Cytoskeletal Force Generation and Focal Adhesion Maturation

The linker of nucleoskeleton and cytoskeleton (LINC) complex is composed of the inner nuclear membrane-spanning SUN proteins and the outer nuclear membrane-spanning nesprin proteins. The LINC complex physically connects the nucleus and plasma membrane via the actin cytoskeleton to perform diverse functions including mechanotransduction from the extracellular environment to the nucleus. Mammalian somatic cells express two principal SUN proteins, namely SUN1 and SUN2. We have previously reported that SUN1, but not SUN2, is essential for directional cell migration; however, the underlying mechanism remains elusive. Because the balance between adhesive force and traction force is critical for cell migration, in the present study, we focused on focal adhesions (FAs) and the actin cytoskeleton. We observed that siRNA-mediated SUN1 depletion did not affect the recruitment of integrin β1, one of the ubiquitously expressed focal adhesion molecules, to the plasma membrane. Consistently, SUN1-depleted cells normally adhered to extracellular matrix proteins, including collagen, fibronectin, laminin, and vitronectin. In contrast, SUN1 depletion reduced the activation of integrin β1. Strikingly, the depletion of SUN1 interfered with the incorporation of vinculin into the focal adhesions, whereas no significant differences in the expression of vinculin were observed between wild-type and SUN1-depleted cells. In addition, SUN1 depletion suppressed the recruitment of zyxin to nascent focal adhesions. These data indicate that SUN1 is involved in the maturation of focal adhesions. Moreover, disruption of the SUN1-containing LINC complex abrogates the actin cytoskeleton and generation of intracellular traction force, despite the presence of SUN2. Thus, a physical link between the nucleus and cytoskeleton through SUN1 is required for the proper organization of actin, thereby suppressing the incorporation of vinculin and zyxin into focal adhesions and the activation of integrin β1, both of which are dependent on traction force. This study provides insights into a previously unappreciated signaling pathway from the nucleus to the cytoskeleton, which is in the opposite direction to the well-known mechanotransduction pathways from the extracellular matrix to the nucleus.

Contribution of ADAM17 and related ADAMs in cardiovascular diseases

A disintegrin and metalloproteases (ADAMs) are key mediators of cell signaling by ectodomain shedding of various growth factors, cytokines, receptors and adhesion molecules at the cellular membrane. ADAMs regulate cell proliferation, cell growth, inflammation, and other regular cellular processes. ADAM17, the most extensively studied ADAM family member, is also known as tumor necrosis factor (TNF)-α converting enzyme (TACE). ADAMs-mediated shedding of cytokines such as TNF-α orchestrates immune system or inflammatory cascades and ADAMs-mediated shedding of growth factors causes cell growth or proliferation by transactivation of the growth factor receptors including epidermal growth factor receptor. Therefore, increased ADAMs-mediated shedding can induce inflammation, tissue remodeling and dysfunction associated with various cardiovascular diseases such as hypertension and atherosclerosis, and ADAMs can be a potential therapeutic target in these diseases. In this review, we focus on the role of ADAMs in cardiovascular pathophysiology and cardiovascular diseases. The main aim of this review is to stimulate new interest in this area by highlighting remarkable evidence.

The SUN2-nesprin-2 LINC complex and KIF20A function in the Golgi dispersal

The morphology of the Golgi complex is influenced by the cellular context, which strictly correlates with nuclear functions; however, the mechanism underlying this association remains elusive. The inner nuclear membrane SUN proteins, SUN1 and SUN2, have diverse functions together with the outer nuclear membrane nesprin proteins, which comprise the LINC complex. We found that depletion of SUN1 leads to Golgi complex dispersion with maintenance of ministacks and retained function for vesicle transport through the Golgi complex. In addition, SUN2 associates with microtubule plus-end-directed motor KIF20A, possibly via nesprin-2. KIF20A plays a role in the Golgi dispersion in conjunction with the SUN2-nesprin-2 LINC complex in SUN1-depleted cells, suggesting that SUN1 suppresses the function of the SUN2-nesprin-2 LINC complex under a steady-state condition. Further, SUN1-knockout mice, which show impaired cerebellar development and cerebellar ataxia, presented altered Golgi morphology in Purkinje cells. These findings revealed a regulation of the Golgi organization by the LINC complex.

Role of hypoxia-related proteins in adenoid cystic carcinoma invasion

BACKGROUND

Among cancers affecting the oral cavity, adenoid cystic carcinoma (ACC) is a relatively common malignant neoplasm. It has high rates of metastasis and recurrence and is associated with significant morbidity. During the progression of ACC, the oxygen concentration is reduced in specific areas of the tumour microenvironment, leading to intratumoural hypoxia. The expression of NOTCH1, a disintegrin and metalloproteinase 12 (ADAM-12), hypoxia-inducible factor 1 alpha (HIF-1α), and heparin-binding epidermal growth factor (HB-EGF) under hypoxic conditions has been implicated in invadopodia formation, tumour invasiveness, and metastasis. The aim of this study was to analyse the expression of these proteins to elucidate the mechanisms underlying ACC invasiveness.

METHODS

Fifteen ACC samples and 10 normal-looking salivary gland (SG) samples were used to investigate the expression of these proteins by immunohistochemistry. Primary antibodies against NOTCH1, ADAM-12, HIF-1α, and HB-EGF were used.

RESULTS

The immunoexpression of all proteins was higher in ACC samples than in SG samples (p < 0.05).

CONCLUSIONS

There was increased expression of proteins associated with hypoxia and tumour invasiveness in ACC samples, which indicates a possible role of these proteins in the biological behaviour of this tumour.

Reversible interconversion and maintenance of mammary epithelial cell characteristics by the ligand-regulated EGFR system

Epithelial cell plasticity is controlled by extracellular cues, but the underlying mechanisms remain to be fully understood. Epidermal growth factor (EGF) and amphiregulin (AREG) are high- and low-affinity ligands for EGF receptor (EGFR), respectively. EGFR signaling is known to promote epithelial-mesenchymal transition (EMT) by the activation of ERK and the induction of an EMT transcription factor, ZEB1. Here, we demonstrate that ligand-switching between EGF and AREG at equivalent molarity reversibly interconverts epithelial and mesenchymal-like states of EGFR signal-dependent mammary epithelial cells. The EGF- and AREG-cultured cells also differ in their epithelial characteristics, including the expression of cell surface markers, the mode of migration and the ability for acinus-formation. The ligand-switching between EGF and AREG temporally alters strength of the shared EGFR-ERK signaling. This alteration inverts relative expression levels of ZEB1 and its antagonizing microRNAs, miR-205 and miR-200c, those are critical determinants of the epithelial phenotype. Further, AREG-induced EGFR accumulation on the plasma membrane compensates for the weak association between AREG and EGFR. The EGFR dynamics enables AREG to support proliferation as efficiently as EGF at equivalent molarity and to maintain epithelial characteristics. Our findings reveal a role of EGFR ligands-generated signal strength in the regulation of mammary epithelial cell plasticity.

Targeting autocrine HB-EGF signaling with specific ADAM12 inhibition using recombinant ADAM12 prodomain

Dysregulation of ErbB-family signaling underlies numerous pathologies and has been therapeutically targeted through inhibiting ErbB-receptors themselves or their cognate ligands. For the latter, “decoy” antibodies have been developed to sequester ligands including heparin-binding epidermal growth factor (HB-EGF); however, demonstrating sufficient efficacy has been difficult. Here, we hypothesized that this strategy depends on properties such as ligand-receptor binding affinity, which varies widely across the known ErbB-family ligands. Guided by computational modeling, we found that high-affinity ligands such as HB-EGF are more difficult to target with decoy antibodies compared to low-affinity ligands such as amphiregulin (AREG). To address this issue, we developed an alternative method for inhibiting HB-EGF activity by targeting its cleavage from the cell surface. In a model of the invasive disease endometriosis, we identified A Disintegrin and Metalloproteinase 12 (ADAM12) as a protease implicated in HB-EGF shedding. We designed a specific inhibitor of ADAM12 based on its recombinant prodomain (PA12), which selectively inhibits ADAM12 but not ADAM10 or ADAM17. In endometriotic cells, PA12 significantly reduced HB-EGF shedding and resultant cellular migration. Overall, specific inhibition of ligand shedding represents a possible alternative to decoy antibodies, especially for ligands such as HB-EGF that exhibit high binding affinity and localized signaling.

Establishment and characterization of cetuximab resistant head and neck squamous cell carcinoma cell lines: focus on the contribution of the AP-1 transcription factor

BACKGROUND

After an initial response to EGFR targeted therapy, secondary resistance almost invariably ensues, thereby limiting the clinical benefit of the drug. Hence, it has been recognized that the successful implementation of targeted therapy in the treatment of HNSCC cancer is very much dependent on predictive biomarkers for patient selection.

METHODS

We generated an in vitro model of acquired cetuximab resistance by chronically exposing three HNSCC cell lines to increasing cetuximab doses. Gene expression profiles of sensitive parental cells and resistant daughter cells were compared using microarray analysis. Growth inhibitory experiments were performed with an HB-EGF antibody and the MMP inhibitor, both in combination with cetuximab. Characteristics of EMT were analyzed using migration and invasion assays, immunofluorescent vimentin staining and qRT-PCR for several genes involved in this process. The function of the transcription factor AP-1 was investigated using qRT-PCR for several genes upregulated or downregulated in cetuximab resistant cells. Furthermore, anchorage-independent growth was investigated using the soft agar assay.

RESULTS

Gene expression profiling shows that cetuximab resistant cells upregulate several genes, including interleukin 8, the EGFR ligand HB-EGF and the metalloproteinase ADAM19. Cytotoxicity experiments with neutralizing HB-EGF antibody could not induce any growth inhibition, whereas an MMP inhibitor inhibited cell growth in cetuximab resistant cells. However, no synergetic effects combined with cetuximab could be observed. Cetuximab resistant cells showed traits of EMT, as witnessed by increased migratory potential, increased invasive potential, increased vimentine expression and increased expression of several genes involved in EMT. Furthermore, expression of upregulated genes could be repressed by the treatment with apigenin. The cetuximab resistant LICR-HN2 R10.3 cells tend to behave differently in cell culture, forming spheres. Therefore, soft agar assay was performed and showed more and larger colonies when challenged with cetuximab compared to PBS challenged cells.

CONCLUSIONS

In summary, our results indicate that increased expression of the ligand HB-EGF could contribute to resistance towards cetuximab in our cetuximab resistant HNSCC cells. Furthermore, several genes upregulated or downregulated in cetuximab resistant cells are under control of the AP-1 transcription factor. However, more studies are warranted to further unravel the role of AP-1 in cetuximab resistance.

Nuclear magnetic resonance structure of the cytoplasmic tail of heparin binding EGF-like growth factor (proHB-EGF-CT) complexed with the ubiquitin homology domain of Bcl-2-associated athanogene 1 from Mus musculus (mBAG-1-UBH)

The membrane form of heparin binding EGF-like growth factor (proHB-EGF) yields secreted HB-EGF and a membrane-anchored cytoplasmic tail (proHB-EGF-CT), which may be targeted to the nuclear membrane after a shedding stimulus. Bcl-2-associated athanogene 1 (BAG-1) accumulates in the nuclei and inhibits apoptosis in adenoma-derived cell lines. The maintenance of high levels of nuclear BAG-1 enhances cell survival. However, the ubiquitin homology domain of BAG-1 from Mus musculus (mBAG-1-UBH) is proposed to interact with proHB-EGF-CT, and this interaction may enhance the cytoprotection against the apoptosis inducer. The mechanism of the synergistic anti-apoptosis function of proHB-EGF-CT and mBAG-1-UBH is still unknown. We offer a hypothesis that proHB-EGF-CT can maintain high levels of nuclear BAG-1. In this study, we first report the three-dimensional nuclear magnetic resonance structure of proHB-EGF-CT complexed with mBAG-1-UBH. In the structure of the complex, the residues in the C-terminus and one turn between β-strands β1 and β2 of mBAG-1-UBH bind to two terminals of proHB-EGF-CT, which folds into a loop with end-to-end contact. This end-to-end folding of proHB-EGF-CT causes the basic amino acids to colocalize and form a positively charged groove. The dominant forces in the binding interface between proHB-EGF-CT and mBAG-1-UBH are charge-charge interactions. On the basis of our mutagenesis results, the basic amino acid cluster in the N-terminus of proHB-EGF-CT is the crucial binding site for mBAG-1-UBH, whereas another basic amino acid in the C-terminus facilitates this interaction. Interestingly, the mBAG-1-UBH binding region on the proHB-EGF-CT peptide is also involved in the region found to be important for nuclear envelope targeting, supporting the hypothesis that proHB-EGF-CT is most likely able to trigger the nuclear translocation of BAG-1 in keeping its level high.

Heparin-binding epidermal growth factor-like growth factor/diphtheria toxin receptor in normal and neoplastic hematopoiesis

Heparin-binding EGF-like growth factor (HB-EGF) belongs to the EGF family of growth factors. It is biologically active either as a molecule anchored to the membrane or as a soluble form released by proteolytic cleavage of the extracellular domain. HB-EGF is involved in relevant physiological and pathological processes spanning from proliferation and apoptosis to morphogenesis. We outline here the main activities of HB-EGF in connection with normal or neoplastic differentiative or proliferative events taking place primitively in the hematopoietic microenvironment.

Increased ectodomain shedding of lung epithelial cell adhesion molecule 1 as a cause of increased alveolar cell apoptosis in emphysema

Rationale

Alveolar epithelial cell apoptosis and protease/antiprotease imbalance based proteolysis play central roles in the pathogenesis of pulmonary emphysema but molecular mechanisms underlying these two events are not yet clearly understood. Cell adhesion molecule 1 (CADM1) is a lung epithelial cell adhesion molecule in the immunoglobulin superfamily. It generates two membrane associated C terminal fragments (CTFs), αCTF and βCTF, through protease mediated ectodomain shedding.

Objective

To explore the hypothesis that more CADM1-CTFs are generated in emphysematous lungs through enhanced ectodomain shedding, and cause increased apoptosis of alveolar epithelial cells.

Methods and results

Western blot analyses revealed that CADM1-CTFs increased in human emphysematous lungs in association with increased ectodomain shedding. Increased apoptosis of alveolar epithelial cells in emphysematous lungs was confirmed by terminal nucleotide nick end labelling (TUNEL) assays. NCI-H441 lung epithelial cells expressing mature CADM1 but not CTFs were induced to express αCTF both endogenously (by shedding inducers phorbol ester and trypsin) and exogenously (by transfection). Cell fractionation, immunofluorescence, mitochondrial membrane potentiometric JC-1 dye labelling and TUNEL assays revealed that CADM1-αCTF was localised to mitochondria where it decreased mitochondrial membrane potential and increased cell apoptosis. A mutation in the intracytoplasmic domain abrogated all three abilities of αCTF.

Conclusions

CADM1 ectodomain shedding appeared to cause alveolar cell apoptosis in emphysematous lungs by producing αCTF that accumulated in mitochondria. These data link proteolysis to apoptosis, which are two landmark events in emphysema.

Omeprazole and PGC-formulated heparin binding epidermal growth factor normalizes fasting blood glucose and suppresses insulitis in multiple low dose streptozotocin diabetes model

PURPOSE

Our objective was to develop novel nanocarriers (protected graft copolymer, PGC) that improve the stability of heparin binding EGF (HBEGF) and gastrin and then to use PGC-formulated HBEGF (PGC-HBEGF) and Omeprazole (+/- PGC-gastrin) for normalizing fasting blood glucose (FBG) and improving islet function in diabetic mice.

METHODS

HBEGF, PGC-HBEGF, Omeprazole, Omeprazole + PGC-HBEGF, Omeprazole + PGC-gastrin + PGC-HBEGF and epidermal growth factor (EGF) + gastrin were tested in multiple low dose streptozotocin diabetic mice.

RESULTS

Omeprazole + PGC-HBEGF normalized FBG and is better than EGF + gastrin at improving islet function and decreasing insulitis. Groups treated with Omeprazole, Omeprazole + PGC-HBEGF, or EGF + gastrin have significantly improved islet function versus saline control. All animals that received PGC-HBEGF had significantly reduced islet insulitis versus saline control. Non-FBG was lower for Omeprazole + PGC-gastrin + PGC-HBEGF but Omeprazole + PGC-HBEGF alone showed better FBG and glucose tolerance.

CONCLUSIONS

Omeprazole + PGC-HBEGF provides a sustained exposure to both EGFRA and gastrin, improves islet function, and decreases insulitis in multiple low dose streptozotocin diabetic mice. Although HBEGF or EGF elevates non-FBG, it facilitates a reduction of insulitis and, in the presence of Omeprazole, provides normalization of FBG at the end of treatment. The study demonstrates Omeprazole and PGC-HBEGF is a viable treatment for diabetes.

Recent advances in the epidermal growth factor receptor/ligand system biology on skin homeostasis and keratinocyte stem cell regulation

The epidermal growth factor (EGF) receptor/ligand system stimulates multiple pathways of signal transduction, and is activated by various extracellular stimuli and inter-receptor crosstalk signaling. Aberrant activation of EGF receptor (EGFR) signaling is found in many tumor cells, and humanized neutralizing antibodies and synthetic small compounds against EGFR are in clinical use today. However, these drugs are known to cause a variety of skin toxicities such as inflammatory rash, skin dryness, and hair abnormalities. These side effects demonstrate the multiple EGFR-dependent homeostatic functions in human skin. The epidermis and hair follicles are self-renewing tissues, and keratinocyte stem cells are crucial for maintaining these homeostasis. A variety of molecules associated with the EGF receptor/ligand system are involved in epidermal homeostasis and hair follicle development, and the modulation of EGFR signaling impacts the behavior of keratinocyte stem cells. Understanding the roles of the EGF receptor/ligand system in skin homeostasis is an emerging issue in dermatology to improve the current therapy for skin disorders, and the EGFR inhibitor-associated skin toxicities. Besides, controlling of keratinocyte stem cells by modulating the EGF receptor/ligand system assures advances in regenerative medicine of the skin. We present an overview of the recent progress in the field of the EGF receptor/ligand system on skin homeostasis and regulation of keratinocyte stem cells.

Cancer-associated upregulation of histone H3 lysine 9 trimethylation promotes cell motility in vitro and drives tumor formation in vivo

Global histone modification patterns correlate with tumor phenotypes and prognostic factors in multiple tumor types. Recent studies suggest that aberrant histone modifications play an important role in cancer. However, the effects of global epigenetic rearrangements on cell functions remain poorly understood. In this study, we show that the histone H3 lysine 9 (H3K9) methyltransferase SUV39H1 is clearly involved in regulating cell migration in vitro. Overexpression of wild-type SUV39H1, but not enzymatically inactive SUV39H1, activated migration in breast and colorectal cancer cells. Inversely, migration was reduced by knockdown of SUV39H1 or chemical inhibition by chaetocin. In addition, H3K9 trimethylation (H3K9me3) was specifically increased in invasive regions of colorectal cancer tissues. Moreover, the presence of H3K9me3 positively correlated with lymph node metastasis in colorectal cancer patients. Furthermore, overexpression of SUV39H1 drove tumorigenesis in mouse, resulting in a considerable decrease in survival rate. These data indicate that H3K9 trimethylation plays an important role in human colorectal cancer progression, possibly by promoting collective cell invasion.

Emerging roles of recycling endosomes

Cells internalize extracellular solutes, ligands and proteins and lipids in the plasma membrane (PM) by endocytosis. The removal of membrane from the PM is counteracted by endosomal recycling pathways that return the endocytosed proteins and lipids back to the PM. Recycling to the PM can occur from early endosomes. However, many cells have a distinct subpopulation of endosomes that have a mildly acidic pH of 6.5 and are involved in the endosomal recycling. These endosomes are dubbed recycling endosomes (REs). In recent years, studies have begun to reveal that function of REs is not limited to the endosomal recycling. In this review, I summarize the nature of membrane trafficking pathways that pass through REs and the cell biological roles of these pathways.

Telmisartan inhibits cell proliferation by blocking nuclear translocation of ProHB-EGF C-terminal fragment in colon cancer cells

Background & Aims

Current treatment target toward advanced colorectal cancers is mainly focused on the epidermal growth factor receptor (EGFR) signaling, but its additive effects with chemotherapy are still limited. A disintegrin and metalloproteinase (ADAM) cleaves the proheparin-binding epidermal growth factor like growth factor (proHB-EGF). And soluble HB-EGF activates EGFR. In parallel, the carboxy-terminal fragment of proHB-EGF (HB-EGF-CTF) translocates into the inner nuclear membrane, and subsequently exerts on the regulation of cell proliferation by binding nuclear promyelocytic leukemia zinc finger (PLZF) protein, a transcriptional repressor, thereby causing its nuclear export. We hypothesized that the inhibition of HB-EGF-CTF nuclear translocation may be a new strategy in preventing cell proliferation.

Methods

12-O-tetradecanoylphorbor-13-acetate (TPA) was treated to activate ADAM. Nine-thousand chemical compounds were screened for their efficacies in blocking the binding of HB-EGF-CTF to promyelocytic leukemia zinc finger (PLZF) with Alphascreen system. The obtained candidates were then used to block the binding of HB-EGF-CTF to PLZF in colon cancer cells, HT29 and HCT116. Cell proliferation was investigated with a growth curve assay. The intracellular localization, and association between HB-EGF-CTF and PLZF, was assessed with immunofluorescent staining, and immunoprecipitation and Western blotting, respectively. The effects of obtained candidates on EGFR phosphorylation and on nuclear translocation of HB-EGF-CTF and export of PLZF during the angiotensin II type1 receptor (AT1R) knockdown were also investigated.

Results

Telmisartan and candesartan were found to be potential candidates. Telmisartan inhibited TPA-induced cell proliferation stronger than candesartan. Telmisartan, but not candesartan blocked the nuclear translocation of HB-EGF-CTF, and binding of HB-EGF-CTF to PLZF, during TPA stimulation. Both telmisartan and candesartan did not inhibit TPA-induced EGFR phosphorylation, and telmisartan, but not candesartan, inhibited TPA-induced nuclear translocation of HB-EGF-CTF after knockdown of AT1R.

Conclusions

The inhibition of HB-EGF-CTF nuclear translocation with telmisartan may be a novel strategy in preventing cell proliferation.

Identification of the cancer cell proliferation and survival functions of proHB-EGF by using an anti-HB-EGF antibody

PURPOSE

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) is a member of the epidermal growth factor family. The membrane-bound proHB-EGF is known to be a precursor of the soluble form of HB-EGF (sHB-EGF), which promotes cell proliferation and survival. While the functions of sHB-EGF have been extensively studied, it is not yet fully understood if proHB-EGF is also involved in cellular signaling events. In this study, we utilized the anti-HB-EGF monoclonal antibodies Y-142 and Y-073, which have differential specificities toward proHB-EGF, in order to elucidate proHB-EGF functions in cancer cells.

EXPERIMENTAL DESIGN

The biological activities of proHB-EGF were assessed in cell proliferation, caspase activation, and juxtacrine activity assays by using a 3D spheroid culture of NUGC-3 cells.

RESULTS

Y-142 and Y-073 exhibited similar binding and neutralizing activities for sHB-EGF. However, only Y-142 bound to proHB-EGF. We could detect the function of endogenously expressed proHB-EGF in a 3D spheroid culture. Blocking proHB-EGF with Y-142 reduced spheroid formation, suppressed cell proliferation, and increased caspase activation in the 3D spheroid culture of NUGC-3 cells.

CONCLUSIONS

Our results show that proHB-EGF acts as a cell proliferation and cell survival factor in cancer cells. The results suggest that proHB-EGF may play an important role in tumor progression.

Epidermal growth factor cytoplasmic domain affects ErbB protein degradation by the lysosomal and ubiquitin-proteasome pathway in human cancer cells

The cytoplasmic domains of EGF-like ligands, including EGF cytoplasmic domain (EGFcyt), have important biological functions. Using specific constructs and peptides of human EGF cytoplasmic domain, we demonstrate that EGFcyt facilitates lysosomal and proteasomal protein degradation, and this coincided with growth inhibition of human thyroid and glioma carcinoma cells. EGFcyt and exon 22-23-encoded peptide (EGF22.23) enhanced procathepsin B (procathB) expression and procathB-mediated lysosomal degradation of EGFR/ErbB1 as determined by inhibitors for procathB and the lysosomal ATPase inhibitor BafA1. Presence of mbEGFctF, EGFcyt, EGF22.23, and exon 23-encoded peptides suppressed the expression of the deubiqitinating enzyme ubiquitin C-terminal hydrolase-L1 (UCH-L1). This coincided with hyperubiquitination of total cellular proteins and ErbB1/2 and reduced proteasome activity. Upon small interfering RNA-mediated silencing of endogenously expressed UCH-L1, a similar hyperubiquitinylation phenotype, reduced ErbB1/2 content, and attenuated growth was observed. The exon 23-encoded peptide region of EGFcyt was important for these biologic actions. Structural homology modeling of human EGFcyt showed that this molecular region formed an exposed surface loop. Peptides derived from this EGFcyt loop structure may aid in the design of novel peptide therapeutics aimed at inhibiting growth of cancer cells.

The cytoplasmic tail of heparin-binding EGF-like growth factor regulates bidirectional intracellular trafficking between the plasma membrane and ER

Heparin-binding epidermal growth factor (EGF)- like growth factor (HB-EGF) is synthesized in the ER, transported along the exocytic pathway, and expressed on the plasma membrane as a type I transmembrane protein. Upon extracellular stimulation, HB-EGF, either proHB-EGF or the shed form HB-EGF-CTF, undergoes endocytosis and is then transported retrogradely to the ER. In this study, we showed the essential contribution of the short cytoplasmic tail of HB-EGF (HB-EGF-cyto) to the bidirectional intracellular trafficking between the ER and plasma membrane and revealed several critical amino acids residues that are responsible for internalization from the plasma membrane and ER targeting. We suggest that these anterograde and retrograde sorting signals within HB-EGF-cyto are strictly regulated by protein modification and conformation.

Rab6-mediated retrograde transport regulates inner nuclear membrane targeting of caveolin-2 in response to insulin

Here, we have identified a retrograde transport pathway of caveolin-2 (cav-2) for its regulatory function in the nucleus. Confocal microscopy analysis, photoactivation experiments and subcellular fractionation revealed that cav-2 localized in the Golgi was transported to the inner nuclear membrane (INM) in response to insulin. Exogenous caveolin-1 (cav-1) and P132L-cav-1 expression did not affect the Golgi localization and insulin-induced INM targeting of cav-2. Cav-2(DKV) mutant in the endoplasmic reticulum (ER) was unable to translocate to the INM in response to insulin. The GTP-bound form of Rab6 promoted, but Rab6 siRNA and the GDP-bound form of Rab6 abrogated, retrograde trafficking of cav-2 from the Golgi to ER. Colchicine or nocodazole treatment abolished insulin-induced INM targeting of cav-2. Knock down of gp210 inhibited insulin-induced import of cav-2 from ER/outer nuclear membrane (ONM) to the INM. The INM-targeted cav-2 prevented heterochromatinization and promoted transcriptional activation of Elk-1 and signal transducer and activator of transcription 3 (STAT3). The results provide molecular mechanisms for insulin-induced INM translocation of cav-2 initiated (i) by Golgi-to-ER retrograde trafficking of cav-2 via microtubule-based Rab6-GTP-dependent transport and subsequently processed (ii) by gp210-mediated import of cav-2 from ER/ONM to INM.

Nuclear translocation of the cytoplasmic domain of HB-EGF induces gastric cancer invasion

Background

Membrane-anchored heparin-binding epidermal growth factor-like growth factor (proHB-EGF) yields soluble HB-EGF, which is an epidermal growth factor receptor (EGFR) ligand, and a carboxy-terminal fragment of HB-EGF (HB-EGF-CTF) after ectodomain shedding. We previously reported that HB-EGF-CTF and unshed proHB-EGF which has the cytoplasmic domain of proHB-EGF (HB-EGF-C), translocate from the plasma membrane to the nucleus and regulate cell cycle after shedding stimuli. However, the significance of nuclear exported HB-EGF-C in human gastric cancer is unclear.

Methods

We investigated the relationship between intracellular localization of HB-EGF-C and clinical outcome in 96 gastric cancer patients treated with gastrectomy. Moreover, we established stable gastric cancer cell lines overexpressing wild-type HB-EGF (wt-HB-EGF) and mutated HB-EGF (HB-EGF-mC), which prevented HB-EGF-C nuclear translocation after shedding. Cell motility between these 2 gastric cancer cell lines was investigated using a transwell invasion assay and a wound healing assay.

Results

Of the 96 gastric cancer cases, HB-EGF-C immunoreactivity was detected in both the nucleus and cytoplasm in 19 cases (19.8 %) and in the cytoplasm only in 25 cases (26.0 %). The nuclear immunoreactivity of HB-EGF-C was significantly increased in stage pT3/4 tumors compared with pT1/2 tumors (T1/2 vs. T3/4: 11.1 % vs. 36.4 %, P < 0.01). The growth of wt-HB-EGF- and HB-EGF-mC-expressing cells significantly increased compared with control cells, but the growth of HB-EGF-mC-expressing cells was significantly decreased compared with wt-HB-EGF-expressing cells. Gastric cancer cell invasion obviously increased in wt-HB-EGF-expressing cells, but invasion in HB-EGF-mC-expressing cells showed a slight increase compared with control cells. Moreover, wt-HB-EGF overexpression increased the effectiveness of wound healing, but had no significant effect in HB-EGF-mC-expressing cells.

Conclusions

Both the function of HB-EGF as an EGFR ligand and a novel signal for HB-EGF-C nuclear translocation induce gastric cancer growth, whereas HB-EGF-C nuclear translocation independently plays a critical role in gastric cancer invasion. The present study demonstrated that HB-EGF-C nuclear translocation might be crucial in gastric cancer invasion. HB-EGF-C nuclear translocation may offer a prognostic marker and a new molecular target for gastric cancer therapy.

Regulated intramembrane cleavage of the EGF receptor

Following the addition of EGF or ionomycin to A431 cells, protease activity mediates cleavage of the EGF receptor producing a 60 kDa fragment that includes the intracellular domain (ICD). This fragment is located in both membrane and nuclear fractions. On the basis of sensitivity to chemical inhibitors and overexpression of cDNAs, the rhomboid intramembrane proteases, not γ-secretase proteases, are identified as responsible for the cleavage event. Agonist-initiated cleavage occurs slowly over 3-24 h. Inhibition of calpain protease activity significantly increased the detectable level of ICD fragment.

MT1-MMP regulates the PI3Kδ·Mi-2/NuRD-dependent control of macrophage immune function

Macrophages play critical roles in events ranging from host defense to obesity and cancer, where they infiltrate affected tissues and orchestrate immune responses in tandem with the remodeling of the extracellular matrix (ECM). Despite the dual roles played by macrophages in inflammation, the functions of macrophage-derived proteinases are typically relegated to tissue-invasive or -degradative events. Here we report that the membrane-tethered matrix metalloenzyme MT1-MMP not only serves as an ECM-directed proteinase, but unexpectedly controls inflammatory gene responses wherein MT1-MMP(-/-) macrophages mount exaggerated chemokine and cytokine responses to immune stimuli both in vitro and in vivo. MT1-MMP modulates inflammatory responses in a protease-independent fashion in tandem with its trafficking to the nuclear compartment, where it triggers the expression and activation of a phosphoinositide 3-kinase δ (PI3Kδ)/Akt/GSK3β signaling cascade. In turn, MT1-MMP-dependent PI3Kδ activation regulates the immunoregulatory Mi-2/NuRD nucleosome remodeling complex that is responsible for controlling macrophage immune response. These findings identify a novel role for nuclear MT1-MMP as a previously unsuspected transactivator of signaling networks central to macrophage immune responses.

Membrane-bound trafficking regulates nuclear transport of integral epidermal growth factor receptor (EGFR) and ErbB-2

Nuclear localization of multiple receptor-tyrosine kinases (RTKs), such as EGF receptor (EGFR), ErbB-2, FGF receptor (FGFR), and many others, has been reported by several groups. We previously showed that cell surface EGFR is trafficked to the nucleus through a retrograde pathway from the Golgi to the endoplasmic reticulum (ER) and that EGFR is then translocated to the inner nuclear membrane (INM) through the INTERNET (integral trafficking from the ER to the nuclear envelope transport) pathway. However, the nuclear trafficking mechanisms of other membrane RTKs, apart from EGFR, remain unclear. The purpose of this study was to compare the nuclear transport of EGFR family proteins with that of FGFR-1. Interestingly, we found that digitonin permeabilization, which selectively releases soluble nuclear transporters from the cytoplasm and has been shown to inhibit nuclear transport of FGFR-1, had no effects on EGFR nuclear transport, raising the possibility that EGFR and FGFR-1 use different pathways to be translocated into the nucleus. Using the subnuclear fractionation assay, we further demonstrated that biotinylated cell surface ErbB-2, but not FGFR-1, is targeted to the INM, associating with Sec61β in the INM, similar to the nuclear trafficking of EGFR. Thus, ErbB-2, but not FGFR-1, shows a similar trafficking pathway to EGFR for translocation to the nucleus, indicating that at least two different pathways of nuclear transport exist for cell surface receptors. This finding provides a new direction for investigating the trafficking mechanisms of various nuclear RTKs.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

Cell surface annexins regulate ADAM-mediated ectodomain shedding of proamphiregulin

Ectodomain shedding of EGFR ligands by ADAM17 is a key step of transactivation of epidermal growth factor receptor (EGFR) and the downstream signaling network. In this study, we identified cell surface annexins as regulators of ectodomain shedding of amphiregulin precursor. We propose that cell surface annexins act as a shedding platform to determine the substrate selectivity of ADAM17.

A disintegrin and metalloproteinase (ADAM) is a family of enzymes involved in ectodomain shedding of various membrane proteins. However, the molecular mechanism underlying substrate recognition by ADAMs remains unknown. In this study, we successfully captured and analyzed cell surface transient assemblies between the transmembrane amphiregulin precursor (proAREG) and ADAM17 during an early shedding phase, which enabled the identification of cell surface annexins as components of their shedding complex. Annexin family members annexin A2 (ANXA2), A8, and A9 interacted with proAREG and ADAM17 on the cell surface. Shedding of proAREG was increased when ANXA2 was knocked down but decreased with ANXA8 and A9 knockdown, because of enhanced and impaired association with ADAM17, respectively. Knockdown of ANXA2 and A8 in primary keratinocytes altered wound-induced cell migration and ultraviolet B–induced phosphorylation of epidermal growth factor receptor (EGFR), suggesting that annexins play an essential role in the ADAM-mediated ectodomain shedding of EGFR ligands. On the basis of these data, we propose that annexins on the cell surface function as “shedding platform” proteins to determine the substrate selectivity of ADAM17, with possible therapeutic potential in ADAM-related diseases.

Nuclear envelope-localized EGF family protein amphiregulin activates breast cancer cell migration in an EGF-like domain independent manner

Amphiregulin (AREG), an EGF family protein, is synthesized as a type I transmembrane precursor (proAREG) and expressed on the cell surface with an extracellular EGF-like domain and an intracellular short cytoplasmic tail. The ectodomain shedding yields a soluble EGF receptor ligand (soluble AREG) which binds to EGF receptor (EGFR) and concomitantly induces migration of unshed proAREG from the plasma membrane to the nuclear envelope (NE). AREG is known to play a potential role in breast cancer and has been intensively investigated as an EGF receptor ligand, while the function of the NE-localized proAREG remains unknown. In this study we used a truncated mutant that mimics NE-localized proAREG without shedding stimuli to discriminate between the functions of NE-localized and plasma membrane-localized proAREG and demonstrate that NE-localized proAREG activates breast cancer cell migration, but suppresses cell growth. Moreover, the present study shows that induction of cell migration by NE-localized proAREG does not require the extracellular growth factor domain or EGF receptor function. Collectively these data demonstrate a novel function mediated by the intracellular domain of proAREG and suggest a significant role for NE-localized proAREG in driving human breast cancer progression.

Monoubiquitination of pro-amphiregulin regulates its endocytosis and ectodomain shedding

All members of epidermal growth factor (EGF) family are expressed as transmembrane precursors on cell surfaces and then proteolytically converted to soluble ligands for EGF receptor (EGFR) by a disintegrin and metalloproteases (ADAMs). As enzyme-substrate complex formation is essential for this "ectodomain shedding", alteration of cell surface retention could affect their physical interaction with ADAMs and eventually contribute to shedding efficiency. Here, we showed that monoubiquitination of pro-amphiregulin (pro-AREG, an EGFR ligand) accelerated its half-life on cell surface. Monoubiquitination occurred at lysine 240 of pro-AREG as the primary acceptor site. Using a chimeric protein of pro-AREG and a monomeric ubiquitin mutant (pro-AREGmUb), immunocytochemical analysis and a cell surface biotinylation assay revealed that a significant portion of pro-AREGmUb was expressed on the cell surface, immediately endocytosed, and predominantly localized to early endosomes. Importantly, ectodomain shedding of pro-AREGmUb induced by tetradecanoyl phorbol acetate was significantly reduced in comparison to wild-type pro-AREG. These results suggested that pro-AREG monoubiquitination and the subsequent trafficking to intracellular organelles is a novel shedding regulatory mechanism that contributes to the secretion of EGFR ligands in growth factor signaling.

Nuclear translocation of pro-amphiregulin induces chemoresistance in gastric cancer

Amphiregulin (AR) is derived from a membrane-anchored form (proAR) by ectodomain shedding, and is a ligand that activates epidermal growth factor receptor (EGFR). We have recently shown that proAR translocates from the plasma membrane to the nucleus after truncation of 11 amino acids at the C-terminus, which is independent of the conventional EGFR signaling pathway. Although proAR immunoreactivity has reportedly been detected in the nucleus of cancer cells, its biological meaning has never been investigated. This study was performed to investigate the roles of proAR nuclear translocation in human gastric cancer. We constructed proAR truncated 11 amino acids at the C-terminus (proARΔC11) that spontaneously translocates to the nucleus, and established proARΔC11-expression regulatable gastric cancer cells (MKN45, MKN28) using the tet-off system. Using these cells, we found that proAR nuclear translocation significantly induced chemoresistance in vitro and in vivo. Analyzing the relationship between immunoreactive localization of proAR and the clinical outcome for 46 advanced gastric cancer cases treated with chemotherapy, median survival time was 311 days in 16 patients with AR-positive staining in the nucleus and 387 days in 30 patients with AR-negative staining (P < 0.05). The present study demonstrates that proAR nuclear translocation increases resistance to anti-cancer drugs, which might be associated with poor prognosis in human gastric cancer.

VIP36 protein is a target of ectodomain shedding and regulates phagocytosis in macrophage Raw 264.7 cells

Ectodomain shedding is a posttranslational modification mechanism, which liberates extracellular domains of membrane proteins through juxtamembrane processing executed mainly by the ADAM (a disintegrin and metalloprotease) family of metalloproteases. Shedding is a unique and effective mechanism for inducing multifaceted effects through the soluble extracellular domains released and/or the remaining membrane-bound portions; however, the physiological functions of shedding are not yet fully understood. In this study, we performed unbiased proteomic screening for shedding targets in a lipopolysaccharide (LPS)-stimulated macrophage cell line to elucidate a new immunological function of shedding. We identified VIP36 (36-kDa vesicular integral membrane protein), a lectin domain-containing transmembrane protein postulated as a cargo receptor for Golgi-to-endoplasmic reticulum transport, as a new target for shedding and found that the shedding of VIP36 occurs mainly on the cell surface. In addition, we demonstrate that the amount of VIP36 precisely regulates phagocytosis in macrophages and that the shedding of VIP36 is required for this regulation. These results substantially expand our knowledge of the immunological and cell biological functions of both the shedding process and VIP36 itself.

Ectodomain shedding and remnant peptide signalling of EGFRs and their ligands

Both receptor tyrosine kinases epidermal growth factor receptors (EGFRs) and their ligands are transmembrane proteins. It has been known that ligand binding activates cytoplasmic tyrosine kinase domains of EGFRs, resulting in the transduction of signals for cell proliferation, migration, differentiation or survival. In an EGFRs-ligands system, however, signal transduction occurs not only unidirectionally but also bidirectionally, which is regulated by cell-cell contact and proteolytic cleavage. Recent studies of proteolytic cleavage 'ectodomain shedding' of EGFRs and their ligands mediated by membrane-type metalloproteinases, a disintegrin and metalloproteinases have been unveiling novel functions and molecular mechanism of their remnant peptides. In addition, the study of the remnant peptide signalling would be essential for understanding the physiological and pathological relevance of anti-shedding therapeutic strategies for diseases such as cancer.

ZNF689 suppresses apoptosis of hepatocellular carcinoma cells through the down-regulation of Bcl-2 family members

ZNF689, a C2H2-type of zinc finger transcription factor, was suggested to play a key role in hepatocarcinogenesis. However, none of the target genes or potential roles of ZNF689 in hepatocellular carcinoma (HCC) have been elucidated. Here, we investigated the role of ZNF689 in HCC cell lines focusing on cell viability and apoptosis.We found that the knockdown of ZNF689 by its specific siRNA decreased cell viability of Huh7. Cell cycle analysis revealed that the ZNF689 knockdown increased the proportion of the sub-G1 population, accompanied by an increase of annexin V- and TUNEL-positive cells.Western blot analysis revealed that ZNF689 knockdown induced the expression of pro-apoptotic factors of Bcl-2 family, Bax, Bak and jBid. There was a correlation between the expression of ZNF689 and an anticancer drug 5-fluorouracil (5-FU) resistance of HCC cells. In vivo, ZNF689 siRNA reduced tumor viability in HepG2-bearing mice with statistical significance. Furthermore, immunohistochemical analysis demonstrated that nuclei of a significant portion of human HCC surgical specimens were positive for ZNF689. Taken together, our results indicate that ZNF689 blocks pro-apoptotic signaling by suppressing the Bak/Bax/Bid pathway, resulting in the progression of liver cancer and resistance to 5-FU. ZNF689 may be a promising chemotherapeutic target against liver cancer.

Functional K(v)10.1 channels localize to the inner nuclear membrane

Ectopically expressed human K_V10.1 channels are relevant players in tumor biology. However, their function as ion channels at the plasma membrane does not totally explain their crucial role in tumors. Both in native and heterologous systems, it has been observed that a majority of K_V10.1 channels remain at intracellular locations. In this study we investigated the localization and possible roles of perinuclear K_V10.1. We show that K_V10.1 is expressed at the inner nuclear membrane in both human and rat models; it co-purifies with established inner nuclear membrane markers, shows resistance to detergent extraction and restricted mobility, all of them typical features of proteins at the inner nuclear membrane. K_V10.1 channels at the inner nuclear membrane are not all transported directly from the ER but rather have been exposed to the extracellular milieu. Patch clamp experiments on nuclei devoid of external nuclear membrane reveal the existence of channel activity compatible with K_V10.1. We hypothesize that K_V10.1 channels at the nuclear envelope might participate in the homeostasis of nuclear K⁺, or indirectly interact with heterochromatin, both factors known to affect gene expression.

Linking L1CAM-mediated signaling to NF-κB activation

The cell adhesion molecule L1 (L1CAM) was originally identified as a neural adhesion molecule essential for neurite outgrowth and axon guidance. Many studies have now shown that L1CAM is overexpressed in human carcinomas and associated with poor prognosis. So far, L1CAM-mediated cellular signaling has been largely attributed to an association with growth factor receptors, referred to as L1CAM-'assisted' signaling. New data demonstrate that L1CAM can signal via two additional mechanisms: 'forward' signaling via regulated intramembrane proteolysis and 'reverse' signaling via the activation of the transcription factor nuclear factor (NF)-κB. Taken together, these findings lead to a new understanding of L1CAM downstream signaling that is fundamental for the development of anti-L1CAM antibody-mediated therapeutics in human tumor cells.

RNF13: an emerging RING finger ubiquitin ligase important in cell proliferation

Protein ubiquitination mediated by ubiquitin ligases plays a very important role in a wide spectrum of biological processes including development and disease pathogenesis. RING finger protein 13 (RNF13) is a recently identified ubiquitin ligase which contains an N-terminal protease-associated domain and a C-terminal RING finger domain separated by a transmembrane region. RNF13 is an evolutionarily conserved protein. Most interestingly, RNF13 expression is developmentally regulated during myogenesis and is upregulated in various human tumors. These data suggest that RNF13, acting as an ubiquitin ligase, might have profound biological functions during development and disease. This minireview summarizes recent work on RNF13 functions related to cell proliferation, differentiation and cancer development.

Trafficking and proteolytic processing of RNF13, a model PA-TM-RING family endosomal membrane ubiquitin ligase

RING finger protein 13 (RNF13) is a ubiquitously expressed, highly regulated ubiquitin ligase anchored in endosome membranes. A RING domain located in the cytoplasmic half of this type 1 membrane protein mediates ubiquitination in vitro but physiological substrates have not yet been identified. The protein localized in endosomal membranes undergoes extensive proteolysis in a proteasome-dependent manner, but the mRNA level can be increased and the encoded protein stabilized under specific physiological conditions. The cytoplasmic half of RNF13 is released from the membrane by regulatory proteases and therefore has the potential to mediate ubiquitination at distant sites independent of the full-length protein. In response to protein kinase C activation, the full-length protein is stabilized and moves to recycling endosomes and to the inner nuclear membrane, which exposes the RING domain to the nucleoplasm. Thus RNF13 is a ubiquitin ligase that can potentially mediate ubiquitination in endosomes, on the plasma membrane, in the cytoplasm, in the nucleoplasm or on the inner nuclear membrane, with the site(s) regulated by signaling events that modulate protein targeting and proteolysis.

Sequential and gamma-secretase-dependent processing of the betacellulin precursor generates a palmitoylated intracellular-domain fragment that inhibits cell growth

Betacellulin (BTC) belongs to the family of epidermal growth factor (EGF)-like growth factors that are expressed as transmembrane precursors and undergo proteolytic ectodomain shedding to release soluble mature ligands. BTC is a dual-specificity ligand for ErbB1 and ErbB4 receptors, and can activate unique signal-transduction pathways that are beneficial for the function, survival and regeneration of pancreatic beta-cells. We have previously shown that BTC precursor (proBTC) is cleaved by ADAM10 to generate soluble ligand and a stable, transmembrane remnant (BTC-CTF). In this study, we analyzed the fate of the BTC-CTF in greater detail. We demonstrated that proBTC is cleaved by ADAM10 to produce BTC-CTF, which then undergoes intramembrane processing by presenilin-1- and/or presenilin-2-dependent gamma-secretase to generate an intracellular-domain fragment (BTC-ICD). We found that the proBTC cytoplasmic domain is palmitoylated and that palmitoylation is not required for ADAM10-dependent cleavage but is necessary for the stability and gamma-secretase-dependent processing of BTC-CTF to generate BTC-ICD. Additionally, palmitoylation is required for nuclear-membrane localization of BTC-ICD, as demonstrated by the redistribution of non-palmitoylated BTC-ICD mutant to the nucleoplasm. Importantly, a novel receptor-independent role for BTC-ICD signaling is suggested by the ability of BTC-ICD to inhibit cell growth in vitro.

Membrane type 1-matrix metalloproteinase cleaves off the NH2-terminal portion of heparin-binding epidermal growth factor and converts it into a heparin-independent growth factor

Epidermal growth factor (EGF) receptors (ErbB) and EGF family members represent promising targets for cancer therapy. Heparin-binding EGF (HB-EGF) is a member of the EGF family and is an important target for therapy in some types of human cancers. Processing of HB-EGF by proprotein convertases, and successively, by ADAM family proteases, generates a soluble growth factor that requires heparin as a cofactor. Although heparin potentiates HB-EGF activity in vitro, it is not clear how the heparin-binding activity of HB-EGF is regulated. Here, we show that membrane type 1-matrix metalloproteinase (MT1-MMP; MMP14), a potent invasion-promoting protease, markedly enhances HB-EGF-dependent tumor formation in mice. MT1-MMP additionally cleaves HB-EGF and removes the NH(2)-terminal 20 amino acids that are important for binding heparin. Consequently, the processing of HB-EGF by MT1-MMP converts HB-EGF into a heparin-independent growth factor with enhanced mitogenic activity, and thereby, expression of both proteins costimulates tumor cell growth in vitro and in vivo. The ErbB family of receptors expressed in human gastric carcinoma cells play a role in mediating enhanced HB-EGF activity by MT1-MMP during invasive cell growth in collagen. Thus, we shed light on a new mechanism whereby HB-EGF activity is regulated that should be considered when designing HB-EGF-targeted cancer therapy.

Nuclear trafficking of the epidermal growth factor receptor family membrane proteins

Multiple membrane-bound receptor tyrosine kinases (RTKs), such as the epidermal growth factor receptor (EGFR) and ErbB-2, have been reported to be localized in the nucleus, where emerging evidence suggests that they are involved in transcriptional regulation, cell proliferation, DNA repair and chemo- and radio-resistance. Recent studies have shown that endocytosis and endosomal sorting are involved in the nuclear transport of cell surface RTKs. However, the detailed mechanism by which the full-length receptors embedded in the endosomal membrane travel all the way from the cell surface to the early endosomes and pass through the nuclear pore complexes is unknown. This important area has been overlooked for decades, which has hindered progress in our understanding of nuclear RTKs' functions. Here, we discuss the putative mechanisms by which EGFR family RTKs are shuttled into the nucleus. Understanding the trafficking mechanisms as to how RTKs are transported from the cell surface to the nucleus will significantly contribute to understanding the functions of the nuclear RTKs.

Nuclear targeting of an endosomal E3 ubiquitin ligase

Ring finger protein 13 (RNF13) is an E3 ubiquitin ligase embedded in endosome membranes. The protein undergoes constitutive post-translational proteolysis, making its detection difficult unless cells are incubated with a proteasome inhibitor to allow biosynthetic forms to accumulate. When cells were treated with phorbol 12-myristate 13-acetate (PMA), RNF13 avoided proteolysis. A similar stabilization was seen on ionomycin treatment of cells. Drug treatment stabilized both the full-length protein and a membrane-embedded C-terminal fragment generated following ectodomain shedding. Immunofluorescence staining revealed that PMA treatment caused the protein to accumulate in recycling endosomes, where it colocalized with transferrin receptor, and on the inner nuclear membrane, where it colocalized with lamin B. Expression of dominant-negative Rab11 inhibited nuclear localization, suggesting RNF13 was targeted to the inner nuclear membrane through recycling endosomes. New protein synthesis was necessary for this targeting. Nuclear localization was confirmed by immunoelectron microscopy and by purification of the inner nuclear membrane. Stress-induced transport of an endosomal protein to the inner nuclear membrane is a novel mechanism for introduction of regulatory proteins to the DNA environment. RNF13, with its ubiquitin ligase-active RING domain, has the potential to turn over key nuclear proteins in response to signals received at the plasma membrane.

The endocytic matrix

Endocytosis has long been thought of as simply a way for cells to internalize nutrients and membrane-associated molecules. But an explosive growth in knowledge has given a new dimension to our understanding of this process. It now seems that endocytosis is a master organizer of signalling circuits, with one of its main roles being the resolution of signals in space and time. Many of the functions of endocytosis that are emerging from recent research cannot yet be reconciled with the canonical view of intracellular trafficking but, instead, point to endocytosis being integrated at a deeper level in the cellular 'master plan' (the cellular network of signalling circuits that lie at the base of the cell's make-up). Deconvolution of this level, which we call the 'endocytic matrix', might uncover a fundamental aspect of how a cell is built.