Lysosome-dependent degradation of Notch3

E3 ubiquitin ligase Smurf1 promotes cardiomyocyte pyroptosis by mediating ubiquitin-dependent degradation of TRIB2 in a rat model of heart failure

OBJECTIVE

Heart failure (HF) causes structural and functional changes in the heart, with the pyroptosis-mediated inflammatory response as the core link in HF pathogenesis. E3 ubiquitin ligases participate in cardiovascular disease progression. Here, we explored the underlying molecular mechanisms of E3 ubiquitin ligase Smurf1 in governing HF.

METHODS

HF rat/H9C2 cell models were established by doxorubicin intraperitoneal injections/hypoxia-reoxygenation (H/R), and treated with Smurf1 siRNA and oe-TRIB2 lentivirus plasmids or the NF-κB pathway inhibitor PDTC/si-smurf1, si-TRIB2, protease inhibitor MG132, or lysosomal inhibitor NH4Cl. The cardiac function/cardiac tissue pathological changes/fibrosis in HF rats were evaluated by echocardiography/H&E and Masson staining. GSDMD-N expression was determined by immunohistochemistry. Cell viability/lactate dehydrogenase (LDH) activity/IL-1β and IL-18 levels were measured by CCK-8/LDH kit/ELISA. The interaction between TRIB2 and Smurf1/TRIB2 ubiquitination levels was assessed by co-immunoprecipitation assay. The expression levels of Smurf1 and TRIB2 messenger RNA (mRNA) were determined by RT-qPCR. Levels of Smurf1/TRIB2/the NF-κB pathway-related factors/pyroptosis-related factors and TRIB2 mRNA were determined by Western blot/RT-qPCR.

RESULTS

Smurf1 was highly expressed in H/R-induced H9C2 cells/HF rats, while its knockdown up-regulated TRIB2 and repressed the NF-κB pathway, reduced cardiomyocyte pyroptosis, and attenuated HF. Mechanistically, Smurf1 promoted TRIB2 degradation through an ubiquitin-dependent manner and activated the NF-κB pathway under H/R conditions. TRIB2 silencing annulled Smurf1 knockdown-regulated NF-κB pathway and cardiomyocyte pyroptosis. TRIB2 overexpression inactivated the NF-κB pathway and reduced cardiomyocyte pyroptosis, thus retarding HF.

CONCLUSION

Smurf1 was highly expressed in HF rats, which promoted TRIB2 ubiquitination degradation and activated the NF-κB pathway, thereby promoting cardiomyocyte pyroptosis in HF rats.

Myofibrillogenesis Regulator-1 Regulates the Ubiquitin Lysosomal Pathway of Notch3 Intracellular Domain Through E3 Ubiquitin-Protein Ligase Itchy Homolog in the Metastasis of Non-Small Cell Lung Cancer

Myofibrillogenesis regulator-1 (MR-1) is a multifunctional protein involved in the development of various human tumors. The study is the first to report the promoting effect of MR-1 on the development and metastasis of non-small cell lung cancer (NSCLC). MR-1 is upregulated in NSCLC and positively associated with poor prognosis. The overexpression of MR-1 promotes the metastasis of NSCLC cells by stabilizing the expression of Notch3-ICD (NICD3) in the cytoplasm through enrichment analysis, in vitro and in vivo experimental researches. And Notch3 signaling can upregulate many genes related to metastasis. The stabilizing effect of MR-1 on NICD3 is achieved through the mono-ubiquitin lysosomal pathway and the specific E3 ubiquitin ligase is Itchy homolog (ITCH). There is a certain interaction between MR-1 and NICD3. Elevated MR-1 can affect the level of ITCH phosphorylation, reduce its E3 enzyme activity, and thus lead to reduce the ubiquitination and degradation of NICD3. Interference with the interaction between MR-1 and NICD3 can increase the degradation of NICD3 and impair the metastatic ability of NSCLC cells, which is a previously overlooked treatment option in NSCLC. In summary, interference with the interaction between MR-1 and NICD3 in the progression of lung cancer may be a promising therapeutic target.

NOTCH3 inhibits transcription factor ZEB1 expression and metastasis of breast cancer cells via transcriptionally upregulating miR-223

Background: NOTCH receptor 3 (NOTCH3) and zinc finger E-box binding protein 1 (ZEB1) play important roles in breast cancer respectively. NOTCH3 maintains the luminal phenotype and inhibits epithelial-mesenchymal transition (EMT) in breast cancer, while ZEB1 and NOTCH3 have the opposite effects. Methods: Public databases were used to predict the expression of NOTCH3 and ZEB1 in breast cancer cell lines. The regulatory effect of NOTCH3 on ZEB1 expression was verified by western blot and RT-PCR. MiRNAs regulating ZEB1 expression were identified by using multiple databases and confirmed by reporter gene experiments. Cellular function experiments were conducted to evaluate the role of NOTCH3/miR-223/ZEB1 in the proliferation and invasion of triple-negative breast cancer (TNBC). Results: NOTCH3 and ZEB1 have opposite expression pattern in MCF-7 cells that over-express LncATB or were incubated in TGF-β to induce EMT. Western blotting and RT-PCR showed that NOTCH3 could regulate expression of ZEB1. MiR-223 inhibited the proliferation and invasion of breast cancer cells via down-regulating the expression of ZEB1. NOTCH3 inhibited the proliferation and invasion of breast cancer cells via up-regulating the expression of miR-223. Clinically, high expression of NOTCH3, miR-223 or low expression of ZEB1 were related to good prognosis of breast cancer patients. Conclusion: The current study reports a novel NOTCH3/miR-223/ZEB1 axis, which can inhibit the proliferation and invasion of breast cancer cells, and may serve as a potential biomarker for the prognosis of breast cancer.

Mutant NOTCH3ECD Triggers Defects in Mitochondrial Function and Mitophagy in CADASIL Cell Models

Background

Cerebral autosomal-dominant arteriopathy with subcortical infarction and leukoencephalopathy (CADASIL) is an inherited small-vessel disease that affects the white matter of the brain. Recent studies have confirmed that the deposition of NOTCH3ECD is the main pathological basis of CADASIL; however, whether different mutations present the same pathological characteristics remains to be further studied. Some studies have found that mitochondrial dysfunction is related to CADASIL; however, the specific effects of NOTCH3ECD on mitochondrial remain to be determined.

Objective

We aimed to explore the role of mitochondrial dysfunction in CADASIL.

Methods

We established transgenic human embryonic kidney-293T cell models (involving alterations in cysteine and non-cysteine residues) via lentiviral transfection. Mitochondrial function and structure were assessed using flow cytometry and transmission electron microscopy, respectively. Mitophagy was assessed using western blotting and immunofluorescence.

Results

We demonstrated that NOTCH3ECD deposition affects mitochondrial morphology and function, and that its protein levels are significantly correlated with mitochondrial quality and can directly bind to mitochondria. Moreover, NOTCH3ECD deposition promoted the induction of autophagy and mitophagy. However, these processes were impaired, leading to abnormal mitochondrial accumulation.

Conclusions

This study revealed a common pathological feature of NOTCH3ECD deposition caused by different NOTCH3 mutations and provided new insights into the role of NOTCH3ECD in mitochondrial dysfunction and mitophagy.

Investigation of Mitochondrial Related Variants in a Cerebral Small Vessel Disease Cohort

Monogenic forms of cerebral small vessel disease (CSVD) can be caused by both variants in nuclear DNA and mitochondrial DNA (mtDNA). Mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) is known to have a phenotype similar to Cerebral Autosomal Dominant Arteriopathy with Sub-cortical Infarcts and Leukoencephalopathy (CADASIL), and can be caused by variants in the mitochondrial genome and in several nuclear-encoded mitochondrial protein (NEMP) genes. The aim of this study was to screen for variants in the mitochondrial genome and NEMP genes in a NOTCH3-negative CADASIL cohort, to identify a potential link between mitochondrial dysfunction and CSVD pathology. Whole exome sequencing was performed for 50 patients with CADASIL-like symptomology on the Ion Torrent system. Mitochondrial sequencing was performed using an in-house designed protocol with sequencing run on the Ion GeneStudio S5 Plus (S5 +). NEMP genes and mitochondrial sequencing data were examined for rare (MAF < 0.001), non-synonymous variants that were predicted to have a deleterious effect on the protein. We identified 29 candidate NEMP variants that had links to either MELAS-, encephalopathy-, or Alzheimer's disease-related phenotypes. Based on these changes, variants affecting POLG, MTO1, LONP1, NDUFAF6, NDUFB3, and TCIRG1 were thought to play a potential role in CSVD pathology in this cohort. Overall, the exploration of the mitochondrial genome identified a potential role for mitochondrial related proteins and mtDNA variants contributing to CSVD pathologies.

Cigarette Smoke Activates NOTCH3 to Promote Goblet Cell Differentiation in Human Airway Epithelial Cells

Chronic obstructive pulmonary disease (COPD) is the third leading cause of death in the United States and is primarily caused by cigarette smoking. Increased numbers of mucus-producing secretory ("goblet") cells, defined as goblet cell metaplasia or hyperplasia (GCMH), contributes significantly to COPD pathophysiology. The objective of this study was to determine whether NOTCH signaling regulates goblet cell differentiation in response to cigarette smoke. Primary human bronchial epithelial cells (HBECs) from nonsmokers and smokers with COPD were differentiated in vitro on air-liquid interface and exposed to cigarette smoke extract (CSE) for 7 days. NOTCH signaling activity was modulated using 1) the NOTCH/γ-secretase inhibitor dibenzazepine (DBZ), 2) lentiviral overexpression of the NICD3 (NOTCH3-intracellular domain), or 3) NOTCH3-specific siRNA. Cell differentiation and response to CSE were evaluated by quantitative PCR, Western blotting, immunostaining, and RNA sequencing. We found that CSE exposure of nonsmoker airway epithelium induced goblet cell differentiation characteristic of GCMH. Treatment with DBZ suppressed CSE-dependent induction of goblet cell differentiation. Furthermore, CSE induced NOTCH3 activation, as revealed by increased NOTCH3 nuclear localization and elevated NICD3 protein levels. Overexpression of NICD3 increased the expression of goblet cell-associated genes SPDEF and MUC5AC, whereas NOTCH3 knockdown suppressed CSE-mediated induction of SPDEF and MUC5AC. Finally, CSE exposure of COPD airway epithelium induced goblet cell differentiation in a NOTCH3-dependent manner. These results identify NOTCH3 activation as one of the important mechanisms by which cigarette smoke induces goblet cell differentiation, thus providing a novel potential strategy to control GCMH-related pathologies in smokers and patients with COPD.

Costunolide represses hepatic fibrosis through WW domain-containing protein 2-mediated Notch3 degradation

BACKGROUND AND PURPOSE

This study investigates the antifibrotic activities and potential mechanisms of costunolide (COS), a natural sesquiterpene compound.

EXPERIMENTAL APPROACH

Rats subjected to bile duct ligation and mice challenged with CCl₄ were used to study the antifibrotic effects of COS in vivo. Mouse primary hepatic stellate cells (pHSCs) and human HSC line LX-2 also served as an in vitro liver fibrosis models. The expression of fibrogenic genes and signaling proteins in the neurogenic locus notch homologue protein 3 (Notch3)-hairy/enhancer of split-1 (HES1) pathway was examined using western blot and/or real-time PCR. Notch3 degradation was analysed using immunofluorescence and coimmunoprecipitation.

KEY RESULTS

In animals, COS administration attenuated hepatic histopathological injury and collagen accumulation and reduced the expression of fibrogenic genes. COS time- and dose-dependently suppressed the levels of fibrotic markers in LX-2 cells and mouse pHSCs. Mechanistic studies showed COS destabilized Notch3 and subsequently inhibited the Notch3-HES1 pathway, thus inhibiting HSC activation. Furthermore, COS blocked the WW domain-containing protein 2 (WWP2)/protein phosphatase 1G (PPM1G) interaction and enhanced the effect of WWP2 on Notch3 degradation.

CONCLUSIONS AND IMPLICATIONS

COS exerted potent antifibrotic effects in vitro and in vivo by disrupting the WWP2/PPM1G complex, promoting Notch3 degradation and inhibiting the Notch3/HES1 pathway. This indicates that COS may be a potential therapeutic candidate for the treatment of liver fibrosis.

A phase I, dose-escalation study of PF-06650808, an anti-Notch3 antibody-drug conjugate, in patients with breast cancer and other advanced solid tumors

Background PF-06650808 is a novel anti-Notch3 antibody-drug conjugate (ADC) able to deliver an auristatin-based cytotoxic payload to target cells. In this first-in-human, dose-finding, phase I study (NCT02129205), we investigated safety, pharmacokinetics, immunogenicity, and preliminary antitumor activity of single-agent PF-06650808 in 40 patients with advanced breast cancer (BC) and other solid tumors unselected for Notch3 expression. Primary endpoint was dose-limiting toxicity (DLT). PF-06650808 was administered intravenously every 3 weeks at a starting dose of 0.2 mg/kg, escalated up to 6.4 mg/kg following the modified continual reassessment method. An additional dose level, 2.0 mg/kg, was evaluated in patients with advanced, estrogen receptor-positive (ER⁺) BC. Results The majority of patients had advanced BC (60%) and almost all (90%) had received ≥3 prior lines of anticancer therapy. Treatment with PF-06650808 was generally well tolerated at dose levels ≤2.0 mg/kg with no DLTs. The maximum tolerated dose (MTD) was estimated to be 2.4 mg/kg. The most common treatment-related AEs in all patients were fatigue (40.0%), decreased appetite (37.5%), nausea (35.0%), alopecia (32.5%), abdominal pain (25.0%), pruritus (25.0%), and vomiting (25.0%). Five patients achieved a partial response (PR), including 2 unconfirmed PRs; 4 of the responders had ER⁺/PR⁺/HER2^- BC. Sixteen (51.6%) patients achieved stable disease, including 8 (57.1%) of 14 patients with ER⁺ BC. Tumor samples from all responders tested positive for NOTCH3 expression in a retrospective, exploratory analysis. Conclusions The anti-Notch3 ADC PF-06650808 has demonstrated a manageable safety profile and early signs of antitumor activity in patients with advanced BC.

Autophagy-lysosomal defect in human CADASIL vascular smooth muscle cells

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is a familial progressive degenerative disorder and is caused by mutations in NOTCH3 gene. Previous study reported that mutant NOTCH3 is more prone to form aggregates than wild-type NOTCH3 and the mutant aggregates are resistant to degradation. We hypothesized that aggregation or accumulation of NOTCH3 could be due to impaired lysosomal-autophagy machinery in VSMC. Here, we investigated the possible cause of accumulation/aggregation of NOTCH3 in CADASIL using cerebral VSMCs derived from control and CADASIL patients carrying NOTCH3^R133C mutation. Thioflavin-S-staining confirmed the increased accumulation of aggregated NOTCH3 in VSMC^R133C compared to VSMC^WT. Increased levels of the lysosomal marker, Lamp2, were detected in VSMC^R133C, which also showed co-localization with NOTCH3 using double-immunohistochemistry. Increased level of LC3-II/LC3-I ratio was observed in VSMC^R133C suggesting an accumulation of autophagosomes. This was coupled with the decreased co-localization of NOTCH3 with LC3, and Lamp2 and, further, increase of p62/SQSTM1 levels in VSMC^R133C compared to the VSMC^WT. In addition, Western blot analysis indicated phosphorylation of p-ERK, p-S6RP, and p-P70 S6K. Altogether, these results suggested a dysfunction in the autophagy-lysosomal pathway in VSMC^R133C. The present study provides an interesting avenue of the research investigating the molecular mechanism of CADASIL.

Histone deacetylase 6 controls Notch3 trafficking and degradation in T-cell acute lymphoblastic leukemia cells

Several studies have revealed that endosomal sorting controls the steady-state levels of Notch at the cell surface in normal cells and prevents its inappropriate activation in the absence of ligands. However, whether this highly dynamic physiologic process can be exploited to counteract dysregulated Notch signaling in cancer cells remains unknown. T-ALL is a malignancy characterized by aberrant Notch signaling, sustained by activating mutations in Notch1 as well as overexpression of Notch3, a Notch paralog physiologically subjected to lysosome-dependent degradation in human cancer cells. Here we show that treatment with the pan-HDAC inhibitor Trichostatin A (TSA) strongly decreases Notch3 full-length protein levels in T-ALL cell lines and primary human T-ALL cells xenografted in mice without substantially reducing NOTCH3 mRNA levels. Moreover, TSA markedly reduced the levels of Notch target genes, including pTα, CR2, and DTX-1, and induced apoptosis of T-ALL cells. We further observed that Notch3 was post-translationally regulated following TSA treatment, with reduced Notch3 surface levels and increased accumulation of Notch3 protein in the lysosomal compartment. Surface Notch3 levels were rescued by inhibition of dynein with ciliobrevin D. Pharmacologic studies with HDAC1, 6, and 8-specific inhibitors disclosed that these effects were largely due to inhibition of HDAC6 in T-ALL cells. HDAC6 silencing by specific shRNA was followed by reduced Notch3 expression and increased apoptosis of T-ALL cells. Finally, HDAC6 silencing impaired leukemia outgrowth in mice, associated with reduction of Notch3 full-length protein in vivo. These results connect HDAC6 activity to regulation of total and surface Notch3 levels and suggest HDAC6 as a potential novel therapeutic target to lower Notch signaling in T-ALL and other Notch3-addicted tumors.

Redox-sensitive MAPK and Notch3 regulate fibroblast differentiation and activation: a dual role of ERK1/2

Myofibroblastic transformation, characterized by upregulation of α-smooth muscle actin in response to profibrotic agents such as TGF-β1, is considered as a major event leading to fibrosis. The mechanistic basis linking myofibroblast differentiation to idiopathic pulmonary fibrosis and the disease treatment remain elusive. In this study, we studied roles of MAPK, Notch, and reactive oxygen species (ROS) during the differentiation of IMR-90 lung fibroblasts at basal level and induced by TGF-β1. Our results demonstrated that ROS-dependent activation of p38, JNK1/2 and Notch3 promoted basal and TGF-β1-induced differentiation and expression of extracellular matrix proteins. In stark contrast, ERK1/2 was suppressed by ROS and exhibited an inhibitory effect on the differentiation but showed a weak promotion on the expression of extracellular matrix proteins. TGF-β1-induced Notch3 expression depended on p38 and JNK1/2. Interestingly, Notch3 was also downstream of ERK1/2, suggesting a complex role of ERK1/2 in lung function. Our results suggest a novel ROS-mediated shift of dominance from the inhibitory ERK1/2 to the stimulatory p38, JNK1/2 and Notch3 during the pathological progression of IPF. Thus, targeting ERK1/2 signaling for activation and p38, JNK1/2 and Notch3 for inhibition may be of clinical potential against lung fibrosis.

N-acetylcysteine negatively regulates Notch3 and its malignant signaling

Notch3 receptor is expressed in a variety of cancers and the excised active intracellular domain (N3ICD) initiates its signaling cascade. N-acetylcysteine (NAC) as an antioxidant has been implicated in cancer prevention and therapy. In this study, we demonstrated a negative regulation of Notch3 by NAC in cancer cells. HeLa cells treated with NAC exhibited a time- and concentration-dependent decrease in Notch3 levels and its downstream effectors Hes1 and HRT1 in a manner independent of f-secretase or glutathione. In contrast, NAC did not affect protein levels of Notch1, the full length Notch3 precursor, or ectopically expressed N3ICD. Although SOD, catalase and NAC suppressed reactive oxygen species in HeLa cells, the first two antioxidants did not impact on Notch3 levels. While the mRNA expression of Notch3 was not altered by NAC, functional inhibition of lysosome, but not proteasome, blocked the NAC-dependent reduction of Notch3 levels. Furthermore, results from Notch3 silencing and N3ICD overexpression demonstrated that NAC prevented malignant phenotypes through down-regulation of Notch3 protein in multiple cancer cells. In summary, NAC reduces Notch3 levels through lysosome-dependent protein degradation, thereby negatively regulates Notch3 malignant signaling in cancer cells. These results implicate a novel NAC treatment in sensitizing Notch3-expressing tumors.

A lignan induces lysosomal dependent degradation of FoxM1 protein to suppress β-catenin nuclear translocation

Colon cancer is one of the most common cancers. In this study, we isolated a lignan [(−)-(2R,3R)-1,4-O-diferuloylsecoisolariciresinol, DFS] from Alnus japonica (Betulaceae) and investigated its biological activity and mechanism of action on colon cancer. DFS reduced the viability of colon cancer cells and induced cell cycle arrest. DFS also suppressed β-catenin nuclear translocation and β-catenin target gene expression through a reduction in FoxM1 protein. To assess the mechanism of the action of DFS, we investigated the effect of DFS on endogenous and exogenous FoxM1 protein degradation in colon cancer cells. DFS-induced FoxM1 protein degradation was suppressed by lysosomal inhibitors, chloroquine and bafilomycin A1, but not by knock-down of proteasomal proteins. The mechanism of DFS for FoxM1 degradation is lysosomal dependent, which was not reported before. Furthermore, we found that FoxM1 degradation was partially lysosomal-dependent under normal conditions. These observations indicate that DFS from A. japonica suppresses colon cancer cell proliferation by reducing β-catenin nuclear translocation. DFS induces lysosomal-dependent FoxM1 protein degradation. This is the first report on the lysosomal degradation of FoxM1 by a small molecule. DFS may be useful in treating cancers that feature the elevated expression of FoxM1.

Targeting Notch degradation system provides promise for breast cancer therapeutics

Notch receptor signaling pathways play an important role, not only in normal breast development but also in breast cancer development and progression. As a group of ligand-induced proteins, different subtypes of mammalian Notch (Notch1-4) are sensitive to subtle changes in protein levels. Thus, a clear understanding of mechanisms of Notch protein turnover is essential for understanding normal and pathological mechanisms of Notch functions. It has been suggested that there is a close relationship between the carcinogenesis and the dysregulation of Notch degradation. However, this relationship remains mostly undefined in the context of breast cancer, as protein degradation is mediated by numerous signaling pathways as well as certain molecule modulators (activators/inhibitors). In this review, we summarize the published data regarding the regulation of Notch family member degradation in breast cancer, while emphasizing areas that are likely to provide new therapeutic modalities for mechanism-based anti-cancer drugs.

Notch3 interactome analysis identified WWP2 as a negative regulator of Notch3 signaling in ovarian cancer

The Notch3 signaling pathway is thought to play a critical role in cancer development, as evidenced by the Notch3 amplification and rearrangement observed in human cancers. However, the molecular mechanism by which Notch3 signaling contributes to tumorigenesis is largely unknown. In an effort to identify the molecular modulators of the Notch3 signaling pathway, we screened for Notch3-intracellular domain (N3-ICD) interacting proteins using a human proteome microarray. Pathway analysis of the Notch3 interactome demonstrated that ubiquitin C was the molecular hub of the top functional network, suggesting the involvement of ubiquitination in modulating Notch3 signaling. Thereby, we focused on functional characterization of an E3 ubiquitin-protein ligase, WWP2, a top candidate in the Notch3 interactome list. Co-immunoprecipitation experiments showed that WWP2 interacted with N3-ICD but not with intracellular domains from other Notch receptors. Wild-type WWP2 but not ligase-deficient mutant WWP2 increases mono-ubiquitination of the membrane-tethered Notch3 fragment, therefore attenuating Notch3 pathway activity in cancer cells and leading to cell cycle arrest. The mono-ubiquitination by WWP2 may target an endosomal/lysosomal degradation fate for Notch3 as suggested by the fact that the process could be suppressed by the endosomal/lysosomal inhibitor. Analysis of The Cancer Genome Atlas dataset showed that the majority of ovarian carcinomas harbored homozygous or heterozygous deletions in WWP2 locus, and there was an inverse correlation in the expression levels between WWP2 and Notch3 in ovarian carcinomas. Furthermore, ectopic expression of WWP2 decreased tumor development in a mouse xenograft model and suppressed the Notch3-induced phenotypes including increase in cancer stem cell-like cell population and platinum resistance. Taken together, our results provide evidence that WWP2 serves as a tumor suppressor by negatively regulating Notch3 signaling in ovarian cancer.

Notch pathway is important for many cellular activities, and its dysregulation leads to several diseases in humans, including cancer. Although Notch hyperactivity has been observed in many types of cancers, the interactome of Notch receptor remains largely unknown, especially for Notch3, which is involved in ovarian cancer pathogenesis. This article is the first study, to our knowledge, that delineates the Notch3 interacting network, and demonstrates that one of the Notch3 interacting proteins, WWP2, an E3 ubiquitin-protein ligase, plays a major role in negative regulation of Notch3 signaling in cancer cells. WWP2 locus was found to be deleted, and its mRNA down-regulated in a significant fraction of ovarian carcinomas. Ectopic expression of WWP2 reduced tumorigenicity of ovarian cancer cells, and counteracted Notch3-mediated phenotypes, including promotion of cancer stem-like cell phenotype and platinum resistance, further supporting its tumor suppressor role. The results from this study provide new insights into how Notch3 signaling contributes to cancer development, and should have implications for the design of Notch3-based cancer therapy.

Ligand stimulation induces clathrin- and Rab5-dependent downregulation of the kinase-dead EphB6 receptor preceded by the disruption of EphB6-Hsp90 interaction

Ligand-induced internalisation and subsequent downregulation of receptor tyrosine kinases (RTKs) serve to determine biological outputs of their signalling. Intrinsically kinase-deficient RTKs control a variety of biological responses, however, the mechanism of their downregulation is not well understood and its analysis is focused exclusively on the ErbB3 receptor. The Eph group of RTKs is represented by the EphA and EphB subclasses. Each bears one kinase-inactive member, EphA10 and EphB6, respectively, suggesting an important role for these molecules in the Eph signalling network. While EphB6 effects on cell behaviour have been assessed, the mechanism of its downregulation remains elusive. Our work reveals that EphB6 and its kinase-active relative, and signalling partner, EphB4, are downregulated in a similar manner in response to their common ligand, ephrin-B2. Following stimulation, both receptors are internalised through clathrin-coated pits and are degraded in lysosomes. Their targeting for lysosomal degradation relies on the activity of an early endosome regulator, the Rab5 GTPase, as this process is inhibited in the presence of a Rab5 dominant-negative mutant. EphB6 also interacts with the Hsp90 chaperone and EphB6 downregulation is preceded by their rapid dissociation. Moreover, the inhibition of Hsp90 results in EphB6 degradation, mimicking its ligand-induced downregulation. These processes appear to rely on overlapping mechanisms, since Hsp90 inhibition does not significantly enhance ligand-induced EphB6 elimination. Taken together, our observations define a novel mechanism for intrinsically kinase-deficient RTK downregulation and support an intriguing model, where Hsp90 dissociation acts as a trigger for ligand-induced receptor removal.

Streptococcus pneumoniae invades endothelial host cells via multiple pathways and is killed in a lysosome dependent manner

Streptococcus pneumoniae is one of the major causative agents of pneumonia, sepsis, meningitis and other morbidities. In spite of its heavy disease burden, surprisingly little is known about the mechanisms involved in the switch of life style, from commensal colonizer of the nasopharynx to invasive pathogen. In vitro experiments, and mouse models have shown that S. pneumoniae can be internalized by host cells, which coupled with intracellular vesicle transport through the cells, i.e. transcytosis, is suggested to be the first step of invasive disease. To further dissect the process of S. pneumoniae internalization, we chemically inhibited discrete parts of the cellular uptake system. We show that this invasion of the host cells was facilitated via both clathrin- and caveolae-mediated endocytosis. After internalization we demonstrated that the bulk of the internalized S. pneumoniae was killed in the lysosome. Interestingly, inhibition of the lysosome altered transcytosis dynamics as it resulted in an increase in the transport of the internalized bacteria out of the cells via the basal side. These results show that uptake of S. pneumoniae into host cells occurs via multiple pathways, as opposed to the often proposed view of invasion being dependent on specific, and singular receptor-mediated endocytosis. This indicates that the endothelium not only has a critical role as a physical barrier against S. pneumoniae in the blood stream, but also in degrading S. pneumonia cells that have adhered to, and invaded the endothelial cells.

Active form Notch4 promotes the proliferation and differentiation of 3T3-L1 preadipocytes

Adipose tissue is composed of adipocytes, which differentiate from precursor cells in a process called adipogenesis. Many signal molecules are involved in the transcriptional control of adipogenesis, including the Notch pathway. Previous adipogenic studies of Notch have focused on Notch1 and HES1; however, the role of other Notch receptors in adipogenesis remains unclear. Q-RT-PCR analyses showed that the augmentation of Notch4 expression during the differentiation of 3T3-L1 preadipocytes was comparable to that of Notch1. To elucidate the role of Notch4 in adipogenesis, the human active form Notch4 (N4IC) was transiently transfected into 3T3-L1 cells. The expression of HES1, Hey1, C/EBPδ and PPARγ was up-regulated, and the expression of Pref-1, an adipogenic inhibitor, was down-regulated. To further characterize the effect of N4IC in adipogenesis, stable cells expressing human N4IC were established. The expression of N4IC promoted proliferation and enhanced differentiation of 3T3-L1 cells compared with those of control cells. These data suggest that N4IC promoted proliferation through modulating the ERK pathway and the cell cycle during the early stage of 3T3-L1 adipogenesis and facilitated differentiation through up-regulating adipogenic genes such as C/EBPα, PPARγ, aP2, LPL and HSL during the middle and late stages of 3T3-L1 adipogenesis.

Biochemical characterization and cellular effects of CADASIL mutants of NOTCH3

Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy (CADASIL) is the best understood cause of dominantly inherited stroke and results from NOTCH3 mutations that lead to NOTCH3 protein accumulation and selective arterial smooth muscle degeneration. Previous studies show that NOTCH3 protein forms multimers. Here, we investigate protein interactions between NOTCH3 and other vascular Notch isoforms and characterize the effects of elevated NOTCH3 on smooth muscle gene regulation. We demonstrate that NOTCH3 forms heterodimers with NOTCH1, NOTCH3, and NOTCH4. R90C and C49Y mutant NOTCH3 form complexes which are more resistant to detergents than wild type NOTCH3 complexes. Using quantitative NOTCH3-luciferase clearance assays, we found significant inhibition of mutant NOTCH3 clearance. In coculture assays of NOTCH function, overexpressed wild type and mutant NOTCH3 significantly repressed NOTCH-regulated smooth muscle transcripts and potently impaired the activity of three independent smooth muscle promoters. Wildtype and R90C recombinant NOTCH3 proteins applied to cell cultures also blocked canonical Notch fuction. We conclude that CADASIL mutants of NOTCH3 complex with NOTCH1, 3, and 4, slow NOTCH3 clearance, and that overexpressed wild type and mutant NOTCH3 protein interfere with key NOTCH-mediated functions in smooth muscle cells.

Notch signaling modulates MUC16 biosynthesis in an in vitro model of human corneal and conjunctival epithelial cell differentiation

PURPOSE

Notch proteins are a family of transmembrane receptors that coordinate binary cell fate decisions and differentiation in wet-surfaced epithelia. We sought to determine whether Notch signaling contributes to maintaining mucosal homeostasis by modulating the biosynthesis of cell surface-associated mucins in an in vitro model of human corneal (HCLE) and conjunctival (HCjE) epithelial cell differentiation.

METHODS

HCLE and HCjE cells were grown at different stages of differentiation, representing nondifferentiated (preconfluent and confluent) and differentiated (stratified) epithelial cultures. Notch signaling was blocked with the γ-secretase inhibitor dibenzazepine (DBZ). The presence of Notch intracellular domains (Notch1 to Notch3) and mucin protein (MUC1, -4, -16) was evaluated by electrophoresis and Western blot analysis. Mucin gene expression was determined by TaqMan real-time polymerase chain reaction.

RESULTS

Here we demonstrate that Notch3 is highly expressed in undifferentiated and differentiated HCLE and HCjE cells, and that Notch1 and Notch2 biosynthesis is enhanced by induction of differentiation with serum-containing media. Inhibition of Notch signaling with DBZ impaired MUC16 biosynthesis in a concentration-dependent manner in undifferentiated cells at both preconfluent and confluent stages, but not in postmitotic stratified cells. In contrast to protein levels, the amount of MUC16 transcripts were not significantly reduced after DBZ treatment, suggesting that Notch regulates MUC16 posttranscriptionally. Immunoblots of DBZ-treated epithelial cells grown at different stages of differentiation revealed no differences in the levels of MUC1 and MUC4.

CONCLUSIONS

These results indicate that MUC16 biosynthesis is posttranscriptionally regulated by Notch signaling at early stages of epithelial cell differentiation, and suggest that Notch activation contributes to maintaining a mucosal phenotype at the ocular surface.

Regulation of Notch1 signaling by the APP intracellular domain facilitates degradation of the Notch1 intracellular domain and RBP-Jk

The Notch1 receptor is a crucial controller of cell fate decisions, and is also a key regulator of cell growth and differentiation in a variety of contexts. In this study, we have demonstrated that the APP intracellular domain (AICD) attenuates Notch1 signaling by accelerated degradation of the Notch1 intracellular domain (Notch1-IC) and RBP-Jk, through different degradation pathways. AICD suppresses Notch1 transcriptional activity by the dissociation of the Notch1-IC-RBP-Jk complex after processing by γ-secretase. Notch1-IC is capable of forming a trimeric complex with Fbw7 and AICD, and AICD enhances the protein degradation of Notch1-IC through an Fbw7-dependent proteasomal pathway. AICD downregulates the levels of RBP-Jk protein through the lysosomal pathway. AICD-mediated degradation is involved in the preferential degradation of non-phosphorylated RBP-Jk. Collectively, our results demonstrate that AICD functions as a negative regulator in Notch1 signaling through the promotion of Notch1-IC and RBP-Jk protein degradation.

Notch signaling in the vasculature

Notch signaling is an evolutionarily conserved, intercellular signaling mechanism that plays myriad roles during vascular development and physiology in vertebrates. These roles include the regulation of arteriovenous specification and differentiation in both endothelial cells and vascular smooth muscle cells, regulation of blood vessel sprouting and branching during normal and pathological angiogenesis, and the physiological responses of vascular smooth muscle cells. Defects in Notch signaling also cause inherited vascular diseases, such as the degenerative vascular disorder cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy. This review summarizes recent studies that highlight the multiple roles the Notch signaling pathway plays during vascular development and physiology.

SOX9 determines RUNX2 transactivity by directing intracellular degradation

Mesenchymal stem cell differentiation is controlled by the cooperative activity of a network of signaling mechanisms. Among these, RUNX2 and SOX9 are the major transcription factors for osteogenesis and chondrogenesis, respectively. Their expression is overlapped both temporally and spatially during embryogenesis. Here we have demonstrated that RUNX2 and SOX9 physically interact in intact cells and have confirmed that SOX9 can inhibit the transactivation of RUNX2. In addition, RUNX2 exerts reciprocal inhibition on SOX9 transactivity. In analyses of the mechanism by which SOX9 regulated RUNX2 function, we demonstrated that SOX9 induced a dose-dependent degradation of RUNX2. Although RUNX2 is normally degraded by the ubiquitin-proteasome pathway, we found that SOX9-mediated degradation was proteasome-independent but phosphorylation-dependent and required the presence of the RUNX2 C-terminal domain, which contains a nuclear matrix targeting sequence (NMTS). Furthermore, SOX9 was able to decrease the level of ubiquitinated RUNX2 and direct RUNX2 to the lysosome for degradation. SOX9 also preferentially directed β-catenin, an intracellular mediator of canonical Wnt signaling, for lysosomal breakdown. Consequently, the mechanisms by which SOX9 regulates RUNX2 function may underlie broader signaling pathways that can influence osteochondrogenesis and mesenchymal fate.