Protein kinase C controls microtubule-based traffic but not proteasomal degradation of c-Met

Spatiotemporal regulation of the hepatocyte growth factor receptor MET activity by sorting nexins 1/2 in HCT116 colorectal cancer cells

Cumulative research findings support the idea that endocytic trafficking is crucial in regulating receptor signaling and associated diseases. Specifically, strong evidence points to the involvement of sorting nexins (SNXs), particularly SNX1 and SNX2, in the signaling and trafficking of the receptor tyrosine kinase (RTK) MET in colorectal cancer (CRC). Activation of hepatocyte growth factor (HGF) receptor MET is a key driver of CRC progression. In the present study, we utilized human HCT116 CRC cells with SNX1 and SNX2 genes knocked out to demonstrate that their absence leads to a delay in MET entering early endosomes. This delay results in increased phosphorylation of both MET and AKT upon HGF stimulation, while ERK1/2 (extracellular signal-regulated kinases 1 and 2) phosphorylation remains unaffected. Despite these changes, HGF-induced cell proliferation, scattering, and migration remain similar between the parental and the SNX1/2 knockout cells. However, in the absence of SNX1 and SNX2, these cells exhibit increased resistance to TRAIL-induced apoptosis. This research underscores the intricate relationship between intracellular trafficking, receptor signaling, and cellular responses and demonstrates for the first time that the modulation of MET trafficking by SNX1 and SNX2 is critical for receptor signaling that may exacerbate the disease.

MET exon 14 skipping mutation is a hepatocyte growth factor (HGF)-dependent oncogenic driver in vitro and in humanised HGF knock-in mice

Exon skipping mutations of the MET receptor tyrosine kinase (METex14), increasingly reported in cancers, occur in 3-4% of non-small-cell lung cancer (NSCLC). Only 50% of patients have a beneficial response to treatment with MET-tyrosine kinase inhibitors (TKIs), underlying the need to understand the mechanism of METex14 oncogenicity and sensitivity to TKIs. Whether METex14 is a driver mutation and whether it requires hepatocyte growth factor (HGF) for its oncogenicity in a range of in vitro functions and in vivo has not been fully elucidated from previous preclinical models. Using CRISPR/Cas9, we developed a METex14/WT isogenic model in nontransformed human lung cells and report that the METex14 single alteration was sufficient to drive MET-dependent in vitro anchorage-independent survival and motility and in vivo tumorigenesis, sensitising tumours to MET-TKIs. However, we also show that human HGF (hHGF) is required, as demonstrated in vivo using a humanised HGF knock-in strain of mice and further detected in tumour cells of METex14 NSCLC patient samples. Our results also suggest that METex14 oncogenicity is not a consequence of an escape from degradation in our cell model. Thus, we developed a valuable model for preclinical studies and present results that have potential clinical implication.

c-Met-integrin cooperation: Mechanisms, tumorigenic effects, and therapeutic relevance

c-Met is a receptor tyrosine kinase which upon activation by its ligand, the hepatocyte growth factor, mediates many important signalling pathways that regulate cellular functions such as survival, proliferation, and migration. Its oncogenic and tumorigenic signalling mechanisms, greatly contributing to cancer development and progression, are well documented. Integrins, heterogeneous adhesion receptors which facilitate cell-extracellular matrix interactions, are important in biomechanically sensitive cell adhesion and motility but also modulate diverse cell behaviour. Here we review the studies which reported cooperation between c-Met and several integrins, particularly β1 and β4, in various cell models including many tumour cell types. From the various experimental models and results analysed, we propose that c-Met-integrin cooperation occurs via inside-out or outside-in signalling. Thus, either c-Met activation triggers integrin activation and cell adhesion or integrin adhesion to its extracellular ligand triggers c-Met activation. These two modes of cooperation require the adhesive function of integrins and mostly lead to cell migration and invasion. In a third, less conventional, mode of cooperation, the integrin plays the role of a signalling adaptor for c-Met, independently from its adhesive property, leading to anchorage independent survival. Recent studies have revealed the influence of endocytic trafficking in c-Met-integrin cooperation including the adaptor function of integrin occurring on endomembranes, triggering an inside-in signalling, believed to promote survival of metastatic cells. We present the evidence of the cooperation in vivo and in human tissues and highlight its therapeutic relevance. A better understanding of the mechanisms regulating c-Met-integrin cooperation in cancer progression could lead to the design of new therapies targeting this cooperation, providing more effective therapeutic approaches than c-Met or integrin inhibitors as monotherapies used in the clinic.

State of the structure address on MET receptor activation by HGF

The MET receptor tyrosine kinase (RTK) and its cognate ligand hepatocyte growth factor (HGF) comprise a signaling axis essential for development, wound healing and tissue homeostasis. Aberrant HGF/MET signaling is a driver of many cancers and contributes to drug resistance to several approved therapeutics targeting other RTKs, making MET itself an important drug target. In RTKs, homeostatic receptor signaling is dependent on autoinhibition in the absence of ligand binding and orchestrated set of conformational changes induced by ligand-mediated receptor dimerization that result in activation of the intracellular kinase domains. A fundamental understanding of these mechanisms in the MET receptor remains incomplete, despite decades of research. This is due in part to the complex structure of the HGF ligand, which remains unknown in its full-length form, and a lack of high-resolution structures of the complete MET extracellular portion in an apo or ligand-bound state. A current view of HGF-dependent MET activation has evolved from biochemical and structural studies of HGF and MET fragments and here we review what these findings have thus far revealed.

CLIP-170 spatially modulates receptor tyrosine kinase recycling to coordinate cell migration

Endocytic sorting of activated receptor tyrosine kinases (RTKs), alternating between recycling and degradative processes, controls signal duration, location and surface complement of RTKs. The microtubule (MT) plus‐end tracking proteins (+TIPs) play essential roles in various cellular activities including translocation of intracellular cargo. However, mechanisms through which RTKs recycle back to the plasma membrane following internalization in response to ligand remain poorly understood. We report that net outward‐directed movement of endocytic vesicles containing the hepatocyte growth factor (HGF) Met RTK, requires recruitment of the +TIP, CLIP‐170, as well as the association of CLIP‐170 to MT plus‐ends. In response to HGF, entry of Met into Rab4‐positive endosomes results in Golgi‐localized γ‐ear‐containing Arf‐binding protein 3 (GGA3) and CLIP‐170 recruitment to an activated Met RTK complex. We conclude that CLIP‐170 co‐ordinates the recycling and the transport of Met‐positive endocytic vesicles to plus‐ends of MTs towards the cell cortex, including the plasma membrane and the lamellipodia, thereby promoting cell migration.

Overexpression-mediated activation of MET in the Golgi promotes HER3/ERBB3 phosphorylation

Ligand-dependent oligomerization of receptor tyrosine kinases (RTKs) results in their activation through highly specific conformational changes in the extracellular and intracellular receptor domains. These conformational changes are unique for each RTK sub-family, limiting cross-activation between unrelated RTKs. The proto-oncogene MET receptor tyrosine kinase overcomes these structural constraints and phosphorylates unrelated RTKs in numerous cancer cell lines. The molecular basis for these interactions is unknown. We investigated the mechanism by which MET phosphorylates the human epidermal growth factor receptor-3 (HER3 or ERBB3), a catalytically impaired RTK whose phosphorylation by MET has been described as an essential component of drug resistance to inhibitors targeting EGFR and HER2. We find that in untransformed cells, HER3 is not phosphorylated by MET in response to ligand stimulation, but rather to increasing levels of MET expression, which results in MET activation in a ligand-independent manner. Phosphorylation of HER3 by its canonical dimerization partners, EGFR and HER2, is achieved by engaging an allosteric site on the HER3 kinase domain, but this site is not required when HER3 is phosphorylated by MET. We also observe that HER3 preferentially interacts with MET during its maturation along the secretory pathway, before MET is post-translationally processed by cleavage within its extracellular domain. This results in accumulation of phosphorylated HER3 in the Golgi apparatus. We further show that in addition to HER3, MET phosphorylates other RTKs in the Golgi, suggesting that this mechanism is not limited to HER3 phosphorylation. These data demonstrate a link between MET overexpression and its aberrant activation in the Golgi endomembranes and suggest that non-canonical interactions between MET and unrelated RTKs occur during maturation of receptors. Our study highlights a novel aspect of MET signaling in cancer that would not be accessible to inhibition by therapeutic antibodies.

The Role of PI3K in Met Driven Cancer: A Recap

The Receptor Tyrosine Kinase (RTK) Met, overexpressed or mutated in cancer, plays a major role in cancer progression and represents an attractive target for cancer therapy. However RTK inhibitors can lead to drug resistance, explaining the necessity to develop therapies that target downstream signaling. Phosphatidylinositide 3-kinase (PI3K) is one of the most deregulated pathways in cancer and implicated in various types of cancer. PI3K signaling is also a major signaling pathway downstream of RTK, including Met. PI3K major effectors include Akt and "mechanistic Target of Rapamycin" (mTOR), which each play key roles in numerous and various cell functions. Advancements made due to the development of molecular and pharmaceutical tools now allow us to delve into the roles of each independently. In this review, we summarize the current understanding we possess of the activation and role of PI3K/Akt/mTOR, downstream of Met, in cancer.

Protein Kinase C-β Dictates B Cell Fate by Regulating Mitochondrial Remodeling, Metabolic Reprogramming, and Heme Biosynthesis

PKCβ-null (Prkcb-/-) mice are severely immunodeficient. Here we show that mice whose B cells lack PKCβ failed to form germinal centers and plasma cells, which undermined affinity maturation and antibody production in response to immunization. Moreover, these mice failed to develop plasma cells in response to viral infection. At the cellular level, we have shown that Prkcb-/- B cells exhibited defective antigen polarization and mTORC1 signaling. While altered antigen polarization impaired antigen presentation and likely restricted the potential of GC development, defective mTORC1 signaling impaired metabolic reprogramming, mitochondrial remodeling, and heme biosynthesis in these cells, which altogether overwhelmingly opposed plasma cell differentiation. Taken together, our study reveals mechanistic insights into the function of PKCβ as a key regulator of B cell polarity and metabolic reprogramming that instructs B cell fate.

The Advantages of Targeted Protein Degradation Over Inhibition: An RTK Case Study

Proteolysis targeting chimera (PROTAC) technology has emerged over the last two decades as a powerful tool for targeted degradation of endogenous proteins. Herein we describe the development of PROTACs for receptor tyrosine kinases, a protein family yet to be targeted for induced protein degradation. The use of VHL-recruiting PROTACs against this protein family reveals several advantages of degradation over inhibition alone: direct comparisons of fully functional, target-degrading PROTACs with target-inhibiting variants that contain an inactivated E3 ligase-recruiting ligand show that degradation leads to more potent inhibition of cell proliferation and a more durable and sustained downstream signaling response, and thus addresses the kinome rewiring challenge seen with many receptor tyrosine kinase inhibitors. Combined, these findings demonstrate the ability to target receptor tyrosine kinases for degradation using the PROTAC technology and outline the advantages of this degradation-based approach.

The Therapeutic Targeting of HGF/c-Met Signaling in Hepatocellular Carcinoma: Alternative Approaches

The poor prognosis of hepatocellular carcinoma (HCC), one of the most devastating cancers worldwide, is due to frequent recurrence and metastasis. Among the metastatic factors in the tumor microenvironment, hepatocyte growth factor (HGF) has been well known to play critical roles in tumor progression, including HCC. Therefore, c-Met is now regarded as the most promising therapeutic target for the treatment of HCC. However, there are still concerns about resistance and the side effects of using conventional inhibitors of c-Met, such as tyrosine kinase inhibitors. Recently, many alternative strategies of c-Met targeting have been emerging. These include targeting the downstream effectors of c-Met, such as hydrogen peroxide-inducible clone 5 (Hic-5), to block the reactive oxygen species (ROS)-mediated signaling for HCC progression. Also, inhibition of endosomal regulators, such as PKCε and GGA3, may perturb the c-Met endosomal signaling for HCC cell migration. On the other hand, many herbal antagonists of c-Met-dependent signaling, such as saponin, resveratrol, and LZ-8, were identified. Taken together, it can be anticipated that more effective and safer c-Met targeting strategies for preventing HCC progression can be established in the future.

Protein Kinase C Signaling in Adenoviral Infection

Activation of protein kinase C (PKC), a serine/threonine protein kinase, ubiquitously influences cellular signal transduction and has been shown to play a role in viral entry. In this study, we explored a role for PKC in human adenovirus type 37 infection of primary human corneal fibroblasts, a major target cell for infection. We sought evidence for an interaction between PKC activation and two potential downstream targets: cSrc kinase, shown previously to play a critical role in adenovirus signaling in these cells, and caveolin-1, reported earlier to be important to entry of adenovirus type 37. Infection of fibroblasts increased PKCα phosphorylation and translocation of PKCα from the cytosol to caveolin-1 containing vesicles. Virus-induced phosphorylation of both cSrc and AKT was abolished in cell lysates pretreated with calphostin C, a chemical inhibitor of PKC. Inhibition of PKC also reduced virus associated phosphorylation of caveolin-1, while inhibition of cSrc by the chemical inhibitor PP2 reduced only caveolin-1 phosphorylation, but not PKCα phosphorylation, in lipid rafts. These results suggest a role for PKCα upstream to both cSrc and caveolin-1. Phosphorylated PKCα was found in the same endosomal fractions as phosphorylated cSrc, and PKCα was present to a greater degree in caveolin-1 pull downs from virus infected than mock infected cell lysates. Calphostin C also reduced early viral gene expression, indicating that PKCα activity may be required for viral entry. PKCα plays a central role in adenovirus infection of corneal fibroblasts and regulation of downstream molecules, including the important lipid raft component caveolin-1.

Beta 1-integrin-c-Met cooperation reveals an inside-in survival signalling on autophagy-related endomembranes

Receptor tyrosine kinases (RTKs) and integrins cooperate to stimulate cell migration and tumour metastasis. Here we report that an integrin influences signalling of an RTK, c-Met, from inside the cell, to promote anchorage-independent cell survival. Thus, c-Met and β1-integrin co-internalize and become progressively recruited on LC3B-positive 'autophagy-related endomembranes' (ARE). In cells growing in suspension, β1-integrin promotes sustained c-Met-dependent ERK1/2 phosphorylation on ARE. This signalling is dependent on ATG5 and Beclin1 but not on ATG13, suggesting ARE belong to a non-canonical autophagy pathway. This β1-integrin-dependent c-Met-sustained signalling on ARE supports anchorage-independent cell survival and growth, tumorigenesis, invasion and lung colonization in vivo. RTK-integrin cooperation has been assumed to occur at the plasma membrane requiring integrin 'inside-out' or 'outside-in' signalling. Our results report a novel mode of integrin-RTK cooperation, which we term 'inside-in signalling'. Targeting integrin signalling in addition to adhesion may have relevance for cancer therapy.

Targeting Protein Kinase C Downstream of Growth Factor and Adhesion Signalling

The signaling outputs of Receptor Tyrosine Kinases, G-protein coupled receptors and integrins converge to mediate key cell process such as cell adhesion, cell migration, cell invasion and cell proliferation. Once activated by their ligands, these cell surface proteins recruit and direct a diverse range of proteins to disseminate the appropriate response downstream of the specific environmental cues. One of the key groups of proteins required to regulate these activities is the family of serine/threonine intracellular kinases called Protein Kinase Cs. The activity and subcellular location of PKCs are mediated by a series of tightly regulated events and is dependent on several posttranslational modifications and the availability of second messengers. Protein Kinase Cs exhibit both pro- and anti-tumorigenic effects making them an interesting target for anti-cancer treatment.

TREK-1 Regulates Cytokine Secretion from Cultured Human Alveolar Epithelial Cells Independently of Cytoskeletal Rearrangements

Background

TREK-1 deficient alveolar epithelial cells (AECs) secrete less IL-6, more MCP-1, and contain less F-actin. Whether these alterations in cytokine secretion and F-actin content are related remains unknown. We now hypothesized that cytokine secretion from TREK-1-deficient AECs was regulated by cytoskeletal rearrangements.

Methods

We determined F-actin and α-tubulin contents of control, TREK-1-deficient and TREK-1-overexpressing human A549 cells by confocal microscopy and western blotting, and measured IL-6 and MCP-1 levels using real-time PCR and ELISA.

Results

Cytochalasin D decreased the F-actin content of control cells. Jasplakinolide increased the F-actin content of TREK-1 deficient cells, similar to the effect of TREK-1 overexpression in control cells. Treatment of control and TREK-1 deficient cells with TNF-α, a strong stimulus for IL-6 and MCP-1 secretion, had no effect on F-actin structures. The combination of TNF-α+cytochalasin D or TNF-α+jasplakinolide had no additional effect on the F-actin content or architecture when compared to cytochalasin D or jasplakinolide alone. Although TREK-1 deficient AECs contained less F-actin at baseline, quantified biochemically, they contained more α-tubulin. Exposure to nocodazole disrupted α-tubulin filaments in control and TREK-1 deficient cells, but left the overall amount of α-tubulin unchanged. Although TNF-α had no effect on the F-actin or α-tubulin contents, it increased IL-6 and MCP-1 production and secretion from control and TREK-1 deficient cells. IL-6 and MCP-1 secretions from control and TREK-1 deficient cells after TNF-α+jasplakinolide or TNF-α+nocodazole treatment was similar to the effect of TNF-α alone. Interestingly, cytochalasin D decreased TNF-α-induced IL-6 but not MCP-1 secretion from control but not TREK-1 deficient cells.

Conclusion

Although cytochalasin D, jasplakinolide and nocodazole altered the F-actin and α-tubulin structures of control and TREK-1 deficient AEC, the changes in cytokine secretion from TREK-1 deficient cells cannot be explained by cytoskeletal rearrangements in these cells.

PKCε-mediated c-Met endosomal processing directs fluctuant c-Met-JNK-paxillin signaling for tumor progression of HepG2

Hepatocyte growth factor (HGF) induced c-Met signaling play critical roles in the progression of hepatocellular carcinoma (HCC). However, c-Met targeting approaches suffered resistance and side effect, thus identification of more suitable downstream targets is needed. Recently, we demonstrated HGF-induced fluctuant ERK/paxillin signaling within 24h. We further examined the underlying mechanisms for fluctuant c-Met/JNK/paxillin signal cascade within 12h. HGF-induced phosphorylation of c-Met, JNK, and paxillin (Ser178) shared a common fluctuation pattern characterized by an initial peak at 0.5h, a middle drop at 4h, and a later peak at 10h. Dynasore, the inhibitor of dynamin, suppressed HGF-induced c-Met internalization and phosphorylation of JNK and paxillin (Ser178) at 0.5h, indicating that endosome formation is required for initial signal enhancement. Further, depletion of PKCε not only enhanced HGF-induced phosphorylation of JNK and paxillin (Ser178) but also prevented c-Met degradation at 0.5h, suggesting that PKCε mediated c-Met degradation for signal declination. On the other hand, HGF induced colocalizations of both phosphorylated JNK and paxillin with the endosomal recycling protein GGA3 at 10h and depletion of GGA3 abolished membrane recycling of c-Met and phosphorylation of JNK/paxillin at the same time point. Interestingly, HGF induced GGA3 phosphorylation in a PKCε-dependent manner during 0.5-4h, which is associated with c-Met degradation in the same period. Finally, HGF-induced cell migration, invasion and intrahepatic metastasis of HepG2 were prevented by the inhibitors of endocytosis. Our results suggest that critical endosomal components are promising therapeutic targets for preventing HGF-induced progression of HCC.

Receptor tyrosine kinase c-Met controls the cytoskeleton from different endosomes via different pathways

Receptor tyrosine kinases (RTKs) are increasingly recognized as having the capacity to signal post-internalization. Signalling outputs and/or duration, and subsequent cellular outcome, are thought to be distinct when emanating from endosomes compared with those from the plasma membrane. Here we show, in invasive, basal-like human breast cell models, that different mechanisms are engaged by the RTK c-Met in two different endosomes to control the actin cytoskeleton via the key migratory signal output Rac1. Despite an acute activation of Rac1 from peripheral endosomes (PEs), c-Met needs to traffic to a perinuclear endosome (PNE) to sustain Rac1 signalling, trigger optimal membrane ruffling, cell migration and invasion. Unexpectedly, in the PNE but not in the PE, PI3K and the Rac-GEF Vav2 are required. Thus we describe a novel endosomal signalling mechanism whereby one signal output, Rac1, is stimulated through distinct pathways by the same RTK depending on which endosome it is localized to in the cell.

The role of ubiquitin-proteasome system in glioma survival and growth

High-grade gliomas represent a group of aggressive brain tumors with poor prognosis due to an inherent capacity of persistent cell growth and survival. The ubiquitin-proteasome system (UPS) is an intracellular machinery responsible for protein turnover. Emerging evidence implicates various proteins targeted for degradation by the UPS in key survival and proliferation signaling pathways of these tumors. In this review, we discuss the involvement of UPS in the regulation of several mediators and effectors of these pathways in malignant gliomas.

Lipopolysaccharide-induced phosphorylation of c-Met tyrosine residue 1003 regulates c-Met intracellular trafficking and lung epithelial barrier function

c-Met, the receptor tyrosine kinase whose natural ligand is hepatocyte growth factor, is known to have a key role in cell motility. We have previously shown that lysophosphatidic acid (LPA) induced a decrease in c-Met activation via serine phosphorylation of c-Met at cell-cell contacts. Here, we demonstrate that lipopolysaccharide (LPS) treatment of human bronchial epithelial cells induced internalization of c-Met via phosphorylation at its tyrosine residue 1003. In addition, it induced epithelial barrier dysfunction as evidenced by a decrease in transepithelial resistance (TER) in a time-dependent manner. Pretreatment with a c-Met inhibitor (PHA-665752) or inhibition of protein kinase C (PKC)-α attenuated the LPS-mediated phosphorylation of c-Met and its internalization. LPS-induced c-Met tyrosine 1003 phosphorylation, activation of PKCα, and c-Met internalization were, however, reversed by pretreatment of cells with LPA, which increased c-Met accumulation at cell-cell contacts. Inhibition of LPS-mediated c-Met tyrosine (Y1003) phosphorylation and internalization by prior treatment with PHA-665752, inhibition of PKCα, or overexpression of c-MetY1003A mutant attenuated LPS-induced reduction of TER. Furthermore, we found that c-Met accumulation at cell-cell contacts contributed to LPA-enhanced epithelial barrier integrity, since downregulation of c-Met by specific small-interfering RNA attenuated LPA-increased TER. The data reveal a novel biological function of c-Met in the regulation of lung epithelial barrier integrity.

Sorting nexin 2-mediated membrane trafficking of c-Met contributes to sensitivity of molecular-targeted drugs

The sorting nexin (SNX) family is a diverse group of cytoplasmic and membrane-associated proteins that are involved in membrane-trafficking steps within the endocytotic network. SNX1 and SNX2 are components of the mammalian retromer complex and they also play critical roles in the membrane trafficking of growth factor receptors including epidermal growth factor receptor (EGFR) and c-Met. The human lung cancer cell lines, which harbor activating mutations in the kinase domain of EGFR gene, are sensitive to EGFR-targeted drugs gefitinib or erlotinib. However, a lung cancer cell line harboring gene amplification of c-Met is sensitive to the c-Met-targeted drug SU11274 but not to EGFR-targeted drugs. C-Met overexpression is identified as one of the bypass mechanisms for acquired resistance to EGFR-targeted drugs. Here we show that the siRNA-mediated knockdown of SNX2 decreases the cell-surface localization of c-Met, but not that of EGFR, resulting in lysosomal degradation of the c-Met protein. SNX2 specifically interacts with c-Met and treatment with lysosomal inhibitors almost completely annihilates downregulation of c-Met protein by SNX2 knockdown. Therefore, silencing of SNX2 markedly alters sensitivity to anticancer drugs targeted to c-Met (SU11274) and EGFR (gefitinib and erlotinib) through promotion of compensatory activation of the EGFR pathway in lung cancer cells. These findings suggest that development of drugs targeting SNX2 could be useful in overcoming drug resistance to EGFR-targeted drugs in lung cancer cells harboring c-Met gene amplification.

The tyrosine kinase c-Met contributes to the pro-tumorigenic function of the p38 kinase in human bile duct cholangiocarcinoma cells

Pro-tumorigenic function of the p38 kinase plays a critical role in human cholangiocarcinogenesis. However, the underlying mechanism remains incompletely understood. Here, we report that c-Met, the tyrosine kinase receptor for hepatocyte growth factor (HGF), contributes to the pro-tumorigenic ability of p38 in human cholangiocarcinoma cells. Both p38 and c-Met promote the proliferation and invasion of human cholangiocarcinoma cells. Importantly, inhibition or knockdown of p38 decreased the basal activation of c-Met. Tyrosine phosphatase inhibitor studies revealed that p38 promotes the activity of c-Met, at least in part, by inhibiting dephosphorylation of the receptor. Moreover, density enhanced phosphatase-1 (DEP-1) is involved in p38-mediated inhibiting dephosphorylation of c-Met. Furthermore, p38 inhibits the degradation of c-Met. Taken together, these data provide a potential mechanism to explain how p38 promotes human cholangiocarcinoma cell proliferation and invasion. We propose that the link between p38 and c-Met is implicated in the progression of human cholangiocarcinoma.

Identification of a pivotal endocytosis motif in c-Met and selective modulation of HGF-dependent aggressiveness of cancer using the 16-mer endocytic peptide

Since c-Met has an important role in the development of cancer, it is considered as an attractive target for cancer therapy. Although molecular mechanisms for oncogenic property of c-Met have been actively investigated, regulatory elements for c-Met endocytosis and its effect on c-Met signaling remain unclear. In this study, we identified a pivotal endocytic motif in c-Met and tested it for selective modulation of HGF-induced c-Met response. Using various chimeric constructs with the cytoplasmic tail of c-Met, we were able to demonstrate that a dileucine motif located in the C-terminus of c-Met acts to regulate its endocytosis. Synthetic peptide Ant-3S, consisting of antennapedia-derived protein transduction domain (designated as Ant) and c-Met-derived 16 amino-acids (designated as 3S, spanning amino-acids 1378 to 1393), rapidly moved into cancer cells and disrupted c-Met trafficking. Importantly, an extension of c-Met retention time on the membrane by Ant-3S peptide significantly decreased phosphorylation-dependent c-Met signal transduction. Additionally, the peptide effectively inhibited HGF-induced cell growth, scattering and migration. The underlying molecular mechanism for these observations has been investigated and revealed that the dileucine motif interacts with endocytic machinery, including adaptin β and caveolin-1, for sustained and enhanced signal transduction. Finally, Ant-3S peptide specifically blocked internalization of interleukin-2 receptor α-subunit/3S chimeric protein, but not the other receptors, including Glut4, Glut8 and transferrin receptor. Such results indicate the presence of a selective endocytic assembly for c-Met. It also suggests a potential for c-Met-specific anti-cancer therapy using the identified endocytic motif in this study.

Disturbance of Ca2+ homeostasis converts pro-Met into non-canonical tyrosine kinase p190MetNC in response to endoplasmic reticulum stress in MHCC97 cells

c-Met, the tyrosine-kinase receptor for hepatocyte growth factor, plays a critical role in the tumorigenesis of hepatocellular carcinoma (HCC). However, the underlying mechanism remains incompletely understood. The mature c-Met protein p190Met(αβ) (consists of a α subunit and a β subunit) is processed from pro-Met. Here we show that pro-Met is processed into p190Met(NC) by sarco/endoplasmic reticulum calcium-ATPase (SERCA) inhibitor thapsigargin. p190Met(NC) compensates for the degradation of p190Met(αβ) and protects human HCC cells from apoptosis mediated by endoplasmic reticulum (ER) stress. In comparison with p190Met(αβ), p190Met(NC) is not cleaved and is expressed as a single-chain polypeptide. Thapsigargin-initiated p190Met(NC) expression depends on the disturbance of ER calcium homeostasis. Once induced, p190Met(NC) is activated independent of hepatocyte growth factor engagement. p190Met(NC) contributes to sustained high basal activation of c-Met downstream pathways during ER calcium disturbance-mediated ER stress. Both p38 MAPK-promoted glucose-regulated protein 78 (GRP78) expression and sustained high basal activation of PI3K/Akt and MEK/ERK are involved in the cytoprotective function of p190Met(NC). Importantly, the expression of p190Met(NC) is detected in some HCC cases. Taken together, these data provide a potential mechanism to explain how c-Met promotes HCC cells survival in response to ER stress. We propose that context-specific processing of c-Met protein is implicated in HCC progression in stressful microenvironments.

Met degradation: more than one stone to shoot a receptor down

The receptor tyrosine kinase Met and its high-affinity ligand, the hepatocyte growth factor/scatter factor (HGF/SF), are essential to embryonic development. Deregulation of their signaling is associated with tumorigenesis and metastasis, notably through receptor overexpression. It is thus important to understand the mechanisms controlling Met expression. The ligand-dependent internalization of Met and its subsequent degradation in the lysosomal compartment are well described. This process is known to attenuate downstream Met signaling pathways. Yet internalized Met takes part directly in intracellular signaling by chaperoning signaling factors in the course of its trafficking. Furthermore, recent studies describe various new degradation mechanisms of membrane-anchored Met, involving proteolytic cleavages or association with novel partners. Although all these degradations are ligand-independent, they share, to different extents, some common features with canonical HGF/SF-dependent degradation. Interestingly, activated Met variants display resistance to degradation, suggesting defective degradation is involved in tumorigenesis. Conversely, forced degradation of Met through reinduction of one or more degradation pathways is a promising therapeutic strategy.

A direct role for Met endocytosis in tumorigenesis

Compartmentalization of signals generated by receptor tyrosine kinase (RTK) endocytosis has emerged as a major determinant of various cell functions. Here, using tumour-associated Met-activating mutations, we demonstrate a direct link between endocytosis and tumorigenicity. Met mutants exhibit increased endocytosis/recycling activity and decreased levels of degradation, leading to accumulation on endosomes, activation of the GTPase Rac1, loss of actin stress fibres and increased levels of cell migration. Blocking endocytosis inhibited mutants' anchorage-independent growth, in vivo tumorigenesis and metastasis while maintaining their activation. One mutant resistant to inhibition by a Met-specific tyrosine kinase inhibitor was sensitive to endocytosis inhibition. Thus, oncogenicity of Met mutants results not only from activation but also from their altered endocytic trafficking, indicating that endosomal signalling may be a crucial mechanism regulating RTK-dependent tumorigenesis.

Anomalous inhibition of c-Met by the kinesin inhibitor aurintricarboxylic acid

c-Met [the hepatocyte growth factor (HGF) receptor] is a receptor tyrosine kinase playing a role in various biological events. Overexpression of the receptor has been observed in a number of cancers, correlating with increased metastatic tendency and poor prognosis. Additionally, activating mutations in c-Met kinase domain have been reported in a subset of familial cancers causing resistance to treatment. Receptor trafficking, relying on the integrity of the microtubule network, plays an important role in activation of downstream targets and initiation of signalling events. Aurintricarboxylic acid (ATA) is a triphenylmethane derivative that has been reported to inhibit microtubule motor proteins kinesins. Additional reported properties of this inhibitor include inhibition of protein tyrosine phosphatases, nucleases and members of the Jak family. Here we demonstrate that ATA prevents HGF-induced c-Met phosphorylation, internalisation, subsequent receptor trafficking and degradation. In addition, ATA prevented HGF-induced downstream signalling which also affected cellular function, as assayed by collective cell migration of A549 cells. Surprisingly, the inhibitory effect of ATA on HGF-induced phosphorylation and signalling in vivo was associated with an increase in basal c-Met kinase activity in vitro. It is concluded that the inhibitory effects of ATA on c-Met in vivo is an allosteric effect mediated through the kinase domain of the receptor. As the currently tested adenosine triphosphate competitive tyrosine kinase inhibitors (TKIs) may lead to tumor resistance (McDermott U, et al., Cancer Res 2010;70:1625-34), our findings suggest that novel anti-c-Met therapies could be developed in the future for cancer treatment.

Cysteine 27 variant of the delta-opioid receptor affects amyloid precursor protein processing through altered endocytic trafficking

Agonist-induced activation of the δ-opioid receptor (δOR) was recently shown to augment β- and γ-secretase activities, which increased the production of β-amyloid peptide (Aβ), known to accumulate in the brain tissues of Alzheimer's disease (AD) patients. Previously, the δOR variant with a phenylalanine at position 27 (δOR-Phe27) exhibited more efficient receptor maturation and higher stability at the cell surface than did the less common cysteine (δOR-Cys27) variant. For this study, we expressed these variants in human SH-SY5Y and HEK293 cells expressing exogenous or endogenous amyloid precursor protein (APP) and assessed the effects on APP processing. Expression of δOR-Cys27, but not δOR-Phe27, resulted in a robust accumulation of the APP C83 C-terminal fragment and the APP intracellular domain, while the total soluble APP and, particularly, the β-amyloid 40 levels were decreased. These changes upon δOR-Cys27 expression coincided with decreased localization of APP C-terminal fragments in late endosomes and lysosomes. Importantly, a long-term treatment with a subset of δOR-specific ligands or a c-Src tyrosine kinase inhibitor suppressed the δOR-Cys27-induced APP phenotype. These data suggest that an increased constitutive internalization and/or concurrent signaling of the δOR-Cys27 variant affects APP processing through altered endocytic trafficking of APP.

MET signalling: principles and functions in development, organ regeneration and cancer

The MET tyrosine kinase receptor (also known as the HGF receptor) promotes tissue remodelling, which underlies developmental morphogenesis, wound repair, organ homeostasis and cancer metastasis, by integrating growth, survival and migration cues in response to environmental stimuli or cell-autonomous perturbations. The versatility of MET-mediated biological responses is sustained by qualitative and quantitative signal modulation. Qualitative mechanisms include the engagement of dedicated signal transducers and the subcellular compartmentalization of MET signalling pathways, whereas quantitative regulation involves MET partnering with adaptor amplifiers or being degraded through the shedding of its extracellular domain or through intracellular ubiquitylation. Controlled activation of MET signalling can be exploited in regenerative medicine, whereas MET inhibition might slow down tumour progression.

PKC and the control of localized signal dynamics

Networks of signal transducers determine the conversion of environmental cues into cellular actions. Among the main players in these networks are protein kinases, which can acutely and reversibly modify protein functions to influence cellular events. One group of kinases, the protein kinase C (PKC) family, have been increasingly implicated in the organization of signal propagation, particularly in the spatial distribution of signals. Examples of where and how various PKC isoforms direct this tier of signal organization are becoming more evident.

PKC{alpha}{beta}{gamma}- and PKC{delta}-dependent endocytosis of NBCe1-A and NBCe1-B in salivary parotid acinar cells

We examined membrane trafficking of NBCe1-A and NBCe1-B variants of the electrogenic Na(+)-HCO(3)(-) cotransporter (NBCe1) encoded by the SLC4A4 gene, using confocal fluorescent microscopy in rat parotid acinar cells (ParC5 and ParC10). We showed that yellow fluorescent protein (YFP)-tagged NBCe1-A and green fluorescent protein (GFP)-tagged NBCe1-B are colocalized with E-cadherin in the basolateral membrane (BLM) but not with the apical membrane marker zona occludens 1 (ZO-1). We inhibited constitutive recycling with monensin and W13 and detected that NBCe1-A and NBCe1-B accumulated in vesicles marked with the early endosomal marker early endosome antigen-1 (EEA1), with a parallel loss from the BLM. We observed that NBCe1-A and NBCe1-B undergo massive carbachol (CCh)-stimulated redistribution from the BLM into early endosomes. We showed that internalization of NBCe1-A and NBCe1-B was prevented by the general PKC inhibitor GF-109203X, the PKCalphabetagamma-specific inhibitor Gö-6976, and the PKCdelta-specific inhibitor rottlerin. We verified the involvement of PKCdelta by blocking CCh-induced internalization of NBCe1-A-cyan fluorescent protein (CFP) in cells transfected with dominant-negative kinase-dead (Lys376Arg) PKCdelta-GFP. Our data suggest that NBCe1-A and NBCe1-B undergo constitutive and CCh-stimulated endocytosis regulated by conventional PKCs (PKCalphabetagamma) and by novel PKCdelta in rat epithelial cells. To help develop a more complete model of the role of NBCe1 in parotid acinar cells we also investigated the initial phase of the secretory response to cholinergic agonist. In an Ussing chamber study we showed that inhibition of basolateral NBCe1 with 5-chloro-2,3-dihydro-3-(hydroxy-2-thienylmethylene)-2-oxo-1H-indole-1-carboxamide (tenidap) significantly decreases an initial phase of luminal anion secretion measured as a transient short-circuit current (I(sc)) across ParC10 cell monolayers. Using trafficking and functional data we propose a model that describes a physiological role of NBC in salivary acinar cell secretion.

Rab GTPase regulation of VEGFR2 trafficking and signaling in endothelial cells

OBJECTIVE

Vascular endothelial growth factor receptor 2 (VEGFR2) is a receptor tyrosine kinase that regulates vascular physiology. However, mechanism(s) by which VEGFR2 signaling and trafficking is coordinated are not clear. Here, we have tested endocytic Rab GTPases for regulation of VEGFR2 trafficking and signaling linked to endothelial cell migration.

METHODS AND RESULTS

Quiescent VEGFR2 displays endosomal localization and colocalization with the Rab5a GTPase, an early endosome fusion regulator. Expression of GTP or GDP-bound Rab5a mutants block activated VEGFR2 trafficking and degradation. Manipulation of Rab7a GTPase activity associated with late endosomes using overexpression of wild-type or mutant proteins blocks activated VEGFR2 trafficking and degradation. Depletion of Rab7a decreased VEGFR2 Y1175 phosphorylation but increased p42/44 (pERK1/2) MAPK phosphorylation. Endothelial cell migration was increased by Rab5a depletion but decreased by Rab7a depletion.

CONCLUSIONS

Rab5a and Rab7a regulate VEGFR2 trafficking toward early and late endosomes. Our data suggest that VEGFR2-mediated regulation of endothelial function is dependent on different but specific Rab-mediated GTP hydrolysis activity required for endosomal trafficking.

Inhibition of fatty acid synthase by luteolin post-transcriptionally down-regulates c-Met expression independent of proteosomal/lysosomal degradation

The hepatocyte growth factor (HGF)/c-Met signaling pathway is involved in the progression of several cancers and associated with increased tumor invasion and metastatic potential. We determined previously that the polyphenol epigallocatechin-3-gallate inhibited HGF-induced c-Met phosphorylation in a variety of tumor cell lines in part by disrupting lipid rafts. Fatty acid synthase (FASN) is implicated in cancer progression and may regulate lipid raft function. We therefore examined the effects of luteolin, a potent FASN inhibitor, on c-Met signaling. Luteolin blocked HGF-induced c-Met phosphorylation and scattering of DU145 prostate cancer cells, but inhibition required at least a 4 h preincubation time. Western blot analysis indicated that inhibition of HGF-induced scattering by luteolin occurred coincident with reduction of total c-Met protein in DU145 cells. In addition, luteolin-induced c-Met down-regulation was mimicked by a pharmacologic inhibitor of FASN, C75, or short hairpin RNA knockdown of FASN. Consistent with a role for FASN, loss of c-Met in cells treated with C75 or luteolin was prevented by exogenous addition of palmitate. Luteolin-induced loss of c-Met primarily occurred at a post-transcriptional level and involved cell surface internalization but did not involve translation inhibition, nor was it dependent on the activity of the 26S proteosome or acidic lysosomes. Taken together, our study shows a novel connection between FASN activity and c-Met protein expression and suggests that luteolin could act as a novel HGF/c-Met inhibitor by reducing expression of this receptor.

Met receptor tyrosine kinase degradation is altered in response to the leucine-rich repeat of the Listeria invasion protein internalin B

Entry of the bacterial pathogen Listeria monocytogenes into host epithelial cells is critical for infection and virulence. One major pathway for Listeria entry involves binding of the bacterial protein Internalin B to the host receptor tyrosine kinase Met (hepatocyte growth factor receptor). Activation of Met and downstream signaling cascades is critical for Listeria entry. Internalin B is composed of several structural domains including an N-terminal leucine-rich repeat that is sufficient for binding Met and stimulating downstream signal transduction. Internalin B is monomeric, whereas the leucine-rich repeat is dimeric when expressed as an isolated fragment. The different quaternary states of Internalin B and the leucine-rich repeat suggest that these two Met ligands might cause distinct biological effects. Here we demonstrate that Internalin B and the leucine-rich repeat fragment exhibit agonist properties that differentially influence Met down-regulation in lysosomes. Specifically, Met stability is increased in response to the leucine-rich repeat fragment compared with Internalin B. Interestingly, Internalin B and the leucine-rich repeat stimulate equivalent rates of clathrin-mediated Met internalization. However, the leucine-rich repeat is defective in promoting lysosomal down-regulation of Met and instead enhances receptor recycling to the cell surface. In addition, the leucine-rich repeat causes prolonged Met activation (phosphorylation) and increased cell motility compared with Internalin B. Taken together, our findings indicate that individual domains of Internalin B differentially regulate Met trafficking. The ability of the leucine-rich repeat fragment to promote Met recycling could account for the increased cell motility induced by this ligand.

Receptor trafficking controls weak signal delivery: a strategy used by c-Met for STAT3 nuclear accumulation

C-Met, the receptor of hepatocyte growth factor (HGF), through overexpression or mutation, is a major protooncogene that provides an attractive molecular target for cancer therapy. HGF/c-Met-induced tumorigenesis is dependent, in part, on the transcription factor and oncogene signal transducer and activator of transcription 3 (STAT3), which is believed to be activated by the receptor at the plasma membrane and then to travel to the nucleus where it acts. We demonstrate that although the robust signal to STAT3 elicited from the cytokine oncostatin-M does indeed support this mechanism of STAT3 action, for the weaker STAT3 signal emanating from c-Met, the activated receptor itself needs to be delivered to a perinuclear endosomal compartment to sustain phosphorylated STAT3 in the nucleus. This is signal specific because c-Met-induced extracellular signal-regulated kinase nuclear accumulation does not require receptor trafficking to the perinuclear compartment. This response is triggered from peripheral endosomes. Thus, control of growth factor receptor traffic determines the nature of the signal output, providing novel opportunities for intervention.

Survival of the weakest: signaling aided by endosomes

The tyrosine kinase receptor c-Met plays a key role in cell proliferation, morphogenesis, and motility in response to hepatocyte growth factor. C-Met is often altered in cancer and is a major target for therapeutic intervention. Despite knowing a great deal of the molecular machinery downstream of this receptor tyrosine kinase, the spatiotemporal regulation of c-Met signaling still remains elusive. In this issue of the Journal of Cell Biology, Kermorgant and Parker (Kermorgant, S. and P.J. Parker. 2008. J. Cell Biol. 182:855–863) provide evidence for a model in which the c-Met–activated STAT3 signal is mediated by endosomal trafficking. This study elegantly highlights how weak signals can be effectively transmitted to the nucleus by exploiting endosomal compartments, raising important mechanistic implications for the signaling research community.

c-Src/Histone Deacetylase 3 Interaction Is Crucial for Hepatocyte Growth Factor–Dependent Decrease of CXCR4 Expression in Highly Invasive Breast Tumor Cells

Hepatocyte growth factor (HGF), a cytokine of tumor microenvironment, exerts opposite effects on CXCR4 expression in MCF-7 (low invasive) and MDA-MB231 (highly invasive) breast carcinoma cells, and here, we show that completely different molecular mechanisms downstream of c-Src activation were involved. As experimental models, we used cells transfected with two CXCR4 promoter constructs and treated with HGF or cotransfected with c-Src wild-type (Srcwt) expression vector; phospho-c-Src formation was enhanced in both cell lines. In MCF-7 cells, consistent with activations of CXCR4Luc constructs after HGF treatment and Srcwt expression, Ets1 and nuclear factor-kappaB (NF-kappaB) transcription factors were activated. In contrast, in MDA-MB231 cells, CXCR4Luc construct, Ets1 and NF-kappaB activities decreased. The divergence point seemed to be downstream of HGF/c-Src and consisted in the interaction between c-Src and the substrate histone deacetylase 3 (HDAC3). Only in MDA-MB231 cells, HDAC3 level was enhanced in membranes and nuclei 30 min after HGF and colocalized/coimmunoprecipitated with phospho-c-Src and phosphotyrosine. Thus, the CXCR4 induction by HGF in MCF-7 cells required NF-kappaB and Ets1 activations, downstream of phosphoinositide-3-kinase/Akt, whereas in HGF-treated MDA-MB231 cells, HDAC3 activation via c-Src probably caused a reduction of transcription factor activities, such as that of NF-kappaB. These results indicate possible roles of HGF in invasive growth of breast carcinomas. By enhancing CXCR4 in low invasive tumor cells, HGF probably favors their homing to secondary sites, whereas by suppressing CXCR4 in highly invasive cells, HGF might participate to retain them in the metastatic sites.

Inhibition of c-Met and prevention of spontaneous metastatic spreading by the 2-indolinone RPI-1

Hepatocyte growth factor (HGF) and its tyrosine kinase receptor Met play a pivotal role in the tumor metastatic phenotype and represent attractive therapeutic targets. We investigated the biochemical and biological effects of the tyrosine kinase inhibitor RPI-1 on the human lung cancer cell lines H460 and N592, which express constitutively active Met. RPI-1-treated cells showed down-regulation of Met activation and expression, inhibition of HGF/Met-dependent downstream signaling involving AKT, signal transducers and activators of transcription 3 and paxillin, as well as a reduced expression of the proangiogenic factors vascular endothelial growth factor and basic fibroblast growth factor. Cell growth in soft agar of H460 cells was strongly reduced in the presence of the drug. Furthermore, RPI-1 inhibited both spontaneous and HGF-induced motility/invasiveness of both H460 and human endothelial cells. Targeting of Met signaling by alternative methods (Met small interfering RNA and anti-phosphorylated Met antibody intracellular transfer) produced comparable biochemical and biological effects. Using the spontaneously metastasizing lung carcinoma xenograft H460, daily oral treatment with well-tolerated doses of RPI-1 produced a significant reduction of spontaneous lung metastases (-75%; P < 0.001, compared with control mice). In addition, a significant inhibition of angiogenesis in primary s.c. tumors of treated mice was observed, possibly contributing to limit the development of metastases. The results provide preclinical evidence in support of Met targeting pharmacologic approach as a new option for the control of tumor metastatic dissemination.

Bacterial entry into cells: a role for the endocytic machinery

Increasing evidence indicates that pathogens have evolved highly efficient strategies to induce their internalization within host cells. Viruses and bacteria express and expose on their surface, molecules that mimic endogenous ligands to cell receptors, thereby inducing specific intracellular signalling cascades. More recently it has become clear that, as most viruses, bacteria can enter cells via the clathrin-mediated pathway, indicating a key role for endocytosis in pathogens entry into cells. Here we review the pathways followed by Listeria monocytogenes to enter into non-phagocytic cells, as a model for the subversion of cellular functions to induce pathogens internalization.

PKC isozymes and diacylglycerol-regulated proteins as effectors of growth factor receptors

Growth factors exert their cellular effects through signal transduction pathways that are initiated by the ligation of growth factors to their cell surface receptors. One of the well-established effectors of growth factor receptors is protein kinase C (PKC), a family of serine-threonine kinases that have been known for years as the main target of the phorbol ester tumor promoters. While there is abundant information regarding downstream PKC effectors and partners, how individual PKC isozymes become activated by growth factors and the regulation of receptor function by PKCs is only partially understood. Moreover, the identification of novel "non-kinase" DAG-binding proteins has added a new level of complexity to the field of DAG signaling.

The Listeria protein internalin B mimics hepatocyte growth factor-induced receptor trafficking

Increased hepatocyte growth factor receptor (HGFR) signaling correlates closely with neoplastic invasion and metastatic potential of many human cancers. Hepatocyte growth factor receptor signaling is initiated by binding the physiological ligand HGF or the internalin B (InlB) protein of Listeria monocytogenes. Subsequent degradation of endocytosed HGFR terminates receptor signaling. Previously reported discrepancies in InlB and HGF-induced HGFR signaling could reflect differences in receptor internalization and degradation in response to these distinct ligands. We report that soluble InlB and HGF are mechanistically equivalent in triggering clathrin-dependent endocytosis and lysosomal degradation of HGFR. After internalization, InlB and HGF colocalize with Rab5, EEA1 and the transferrin receptor in classical early endosomes. Hepatocyte growth factor receptor internalization was prevented by overexpression of dominant negative mutants of dynamin 1 and epidermal growth factor phosphorylation substrate 15, but not caveolin 1, the GTPase Arf6 or the cholesterol-chelating drug Nystatin. Thus, HGFR internalization is principally clathrin-mediated and is not regulated by clathrin- independent pathways. Phosphatidylinositol 3-kinase signaling and HGF-regulated tyrosine kinase substrate were not required for ligand-triggered internalization of HGFR but were essential for subsequent lysosomal degradation. Thus, soluble InlB and HGF induce HGFR endocytosis and degradation by indistinguishable mechanisms, suggesting that InlB may be exploited to regulate pathogenic HGFR signaling.

Listeria hijacks the clathrin-dependent endocytic machinery to invade mammalian cells

The bacterial pathogen Listeria monocytogenes uses the surface protein InlB to invade a variety of cell types. The interaction of InlB with the hepatocyte growth-factor receptor, Met, is crucial for infection to occur. Remarkably, the ubiquitin ligase Cbl is rapidly recruited to InlB-activated Met. Recent studies have shown that ligand-dependent endocytosis of Met and other receptor tyrosine kinases is triggered by monoubiquitination of the receptor, a process that is mediated by Cbl. Here, we show that purified InlB induces the Cbl-dependent monoubiquitination and endocytosis of Met. We then demonstrate that the bacterium exploits the ubiquitin-dependent endocytosis machinery to invade mammalian cells. First, we show that L. monocytogenes colocalizes with Met, EEA1, Cbl, clathrin and dynamin during entry. Then, we assess the role of different proteins of the endocytic machinery during L. monocytogenes infection. Over-expression or down-regulation of Cbl, respectively, increases or decreases bacterial invasion. Furthermore, RNA interference-mediated knock-down of major components of the endocytic machinery (for example, clathrin, dynamin, eps15, Grb2, CIN85, CD2AP, cortactin and Hrs), inhibit bacterial entry, establishing that the endocytic machinery is key to the bacterial internalization process.

Cross-presentation of phage particle antigen in MHC class II and endoplasmic reticulum marker-positive compartments

It has been shown that exogenous antigens can access the MHC class I pathway of professional antigen-processing cells. However, details as to how the MHC class I-peptide complex forms in the presentation pathway are still poorly understood. Here we used MHC class I-peptide-specific antibodies to investigate the formation and intracellular location of class I-peptide complexes in macrophages. We observed that the formation of class I-peptide complexes occurs within a few hours and lasts for another few hours on the cell surface of macrophages following loading with filamentous phage particles. The class I-peptide complexes in the process were co-localized with MHC class II molecules and endocytic system markers. Moreover, endosomal compartments containing class I-peptide complexes were found within intracellular organelles stained by DiOC6 and calnexin. In addition, the cross-presentation of phage particles was transporter associated with antigen processing (TAP)-dependent and sensitive to proteasome inhibitors and NH(4)Cl. These data suggest that endocytosed phage particles may be processed and cross-presented in organelles positive for phagosome and endoplasmic reticulum (ER) markers via a classical ER MHC class I loading mechanism.

Met receptor dynamics and signalling

The receptor for hepatocyte growth factor (HGF), Met, controls a programme of invasive growth that combines proliferation with various moto- and morphogenetic processes. This process is important for development and organ regeneration, but dysregulation in transformed tissues can contribute to cancer progression and metastasis. Acute stimulation of tissue culture cells with HGF leads to Met downregulation via degradation through an endocytic mechanism that also requires proteasome activity. Perturbation of Met trafficking on the endocytic pathway, either at the level of the internalisation step or during sorting at the early endosome, leads to altered signalling outputs. Ubiquitination of Met through the E3-ligase Cbl is required for receptor downregulation, and a mutant receptor defective in Cbl binding is able to transform cells. We discuss the hypothesis that some naturally occurring Met mutants implicated in cancer may transform cells owing to defects in their trafficking along the endosomal degradation pathway.

Hepatocyte growth factor receptor is a coreceptor for adeno-associated virus type 2 infection

After the first attachment of virus to the cell surface through a primary receptor, efficient entry of virus requires the presence of a coreceptor. For adeno-associated virus type 2 (AAV2) infection, heparan sulfate proteoglycan is supposed as the primary receptor, and alphavbeta5 integrin and FGFR1 are reported to act as coreceptors. In this study, we were able to demonstrate that hepatocyte growth factor receptor, c-Met, is also a coreceptor for AAV2 infection. AAV2-mediated transgene analyses revealed that c-Met expression significantly up-regulated transgene expression without increasing AAV2 cell binding. Moreover, a viral overlay assay elucidated the physical interaction between AAV2 and the beta subunit of c-Met. These data suggest that c-Met plays the role of coreceptor for AAV2 infection by facilitating AAV2 internalization into the cytoplasm.

PKC controls HGF-dependent c-Met traffic, signalling and cell migration

The growth factor/receptor pair HGF/c-Met exerts control on proliferation, morphogenesis and motility, and through overexpression and mutation is implicated in cancer. Here we have investigated the relationship between receptor signalling and traffic, and its control by specific PKC isotypes. It is shown that c-Met signalling to the ERK cascade occurs within endosomal compartments and that it is in this compartment that PKCepsilon specifically exerts its control on the pathway with the consequent accumulation of ERK in focal complexes. These events are clearly separated from the subsequent microtubule-dependent sorting of c-Met to its perinuclear destination, which is shown to be under the control of PKCalpha. Thus while it is shown that traffic to endosomes is essential for HGF/c-Met to trigger an ERK response, the subsequent traffic and signalling of c-Met controlled by these two PKC isotypes are unconnected events. The dynamic properties conferred by the PKCepsilon control are shown to be essential for a normal HGF-dependent migratory response. Thus PKCs are shown to control both receptor traffic and signal traffic to relay HGF/c-Met responses.

Protein kinase C-alpha and -delta are required for FcalphaR (CD89) trafficking to MHC class II compartments and FcalphaR-mediated antigen presentation

Studies have demonstrated that receptor-mediated signaling, receptor/antigen complex trafficking, and major histocompatibility complex class II compartments (MIIC) are critically related to antigen presentation to CD4+ T cells. In this study, we investigated the role of protein kinase C (PKC) in FcalphaR/gammagamma (CD89, human IgA receptor)-mediated internalization of immune complexes and subsequent antigen presentation. The classical and novel PKC inhibitor, Calphostin C, inhibits FcalphaR-mediated antigen presentation and interaction of MIIC and cargo vesicle (receptor and antigen). PKC-alpha, PKC-delta, and PKC-epsilon were recruited to lipid rafts following FcalphaR crosslinking, the extent of which was determined by the phenotype of the gamma chain. Mutant gamma chain with an FcgammaRIIA ITAM (immunoreceptor tyrosine-based activation motif) insert was less able to recruit PKC and trigger antigen presentation. Both PKC isoform-specific peptide inhibitors and short interfering RNA (siRNA) showed that PKC-alpha and PKC-delta, but not PKC-epsilon, were required for association of cargo vesicle and MIIC and for FcalphaR-mediated and soluble antigen presentation. Inhibition of PKC (classical and novel) did not alter major histocompatibility class II biosynthesis, assembly, transport, or plasma membrane stability. PKC's role in facilitating interaction of cargo vesicle and MIIC is likely due to regulation of vesicle biology required for fusion of cargo vesicles to MIIC.