Ligand-induced ubiquitination of the platelet-derived growth factor beta-receptor plays a negative regulatory role in its mitogenic signaling

The E3 Ubiquitin Ligase TRIM21 Regulates Basal Levels of PDGFRβ

Activation of platelet-derived growth factor (PDGF) receptors α and β (PDGFRα and PDGFRβ) at the cell surface by binding of PDGF isoforms leads to internalization of receptors, which affects the amplitude and kinetics of signaling. Ubiquitination of PDGF receptors in response to ligand stimulation is mediated by the Casitas b-lineage lymphoma (Cbl) family of ubiquitin ligases, promoting internalization and serving as a sorting signal for vesicular trafficking of receptors. We report here that another E3 ligase, i.e., tripartite motif-containing protein 21 (TRIM21), contributes to the ubiquitination of PDGFRβ in human primary fibroblasts AG1523 and the osteosarcoma cell line U2OS and regulates basal levels of PDGFRβ. We found that siRNA-mediated depletion of TRIM21 led to decreased ubiquitination of PDGFRβ in response to PDGF-BB stimulation, while internalization from the cell surface and the rate of ligand-induced degradation of the receptor were not affected. Moreover, induction of TRIM21 decreased the levels of PDGFRβ in serum-starved cells, and even more in growing cells, in the absence of PDGF stimulation. Consistently, siRNA knockdown of TRIM21 caused accumulation of the total amount of PDGFRβ, both in the cytoplasm and on the cell surface, without affecting mRNA levels of the receptor. We conclude that TRIM21 acts post-translationally and maintains basal levels of PDGFRβ, thus suggesting that ubiquitination of PDGFRβ by TRIM21 may direct a portion of receptor for degradation in growing cells in a ligand-independent manner.

The Active Role of Pericytes During Neuroinflammation in the Adult Brain

Microvessels in the central nervous system (CNS) have one of the highest populations of pericytes, indicating their crucial role in maintaining homeostasis. Pericytes are heterogeneous cells located around brain microvessels; they present three different morphologies along the CNS vascular tree: ensheathing, mesh, and thin-strand pericytes. At the arteriole-capillary transition ensheathing pericytes are found, while mesh and thin-strand pericytes are located at capillary beds. Brain pericytes are essential for the establishment and maintenance of the blood-brain barrier, which restricts the passage of soluble and potentially toxic molecules from the circulatory system to the brain parenchyma. Pericytes play a key role in regulating local inflammation at the CNS. Pericytes can respond differentially, depending on the degree of inflammation, by secreting a set of neurotrophic factors to promote cell survival and regeneration, or by potentiating inflammation through the release of inflammatory mediators (e.g., cytokines and chemokines), and the overexpression of cell adhesion molecules. Under inflammatory conditions, pericytes may regulate immune cell trafficking to the CNS and play a role in perpetuating local inflammation. In this review, we describe pericyte responses during acute and chronic neuroinflammation.

Platelet-derived growth factor signalling in neurovascular function and disease

Platelet-derived growth factors are critical for cerebrovascular development and homeostasis. Abnormalities in this signalling pathway are implicated in neurological diseases, especially those where neurovascular dysfunction and neuroinflammation plays a prominent role in disease pathologies, such as stroke and Alzheimer's disease; the angiogenic nature of this pathway also draws its significance in brain malignancies such as glioblastoma where tumour angiogenesis is profuse. In this review, we provide an updated overview of the actions of the platelet-derived growth factors on neurovascular function, their role in the regulation of perivascular cell types expressing the cognate receptors, neurological diseases associated with aberrance in signalling, and highlight the clinical relevance and therapeutic potentials of this pathway for central nervous system diseases.

Mechanisms for Regulating and Organizing Receptor Signaling by Endocytosis

Intricate relationships between endocytosis and cellular signaling, first recognized nearly 40 years ago through the study of tyrosine kinase growth factor receptors, are now known to exist for multiple receptor classes and to affect myriad physiological and developmental processes. This review summarizes our present understanding of how endocytosis orchestrates cellular signaling networks, with an emphasis on mechanistic underpinnings and focusing on two receptor classes-tyrosine kinase and G protein-coupled receptors-that have been investigated in particular detail. Together, these examples provide a useful survey of the current consensus, uncertainties, and controversies in this rapidly advancing area of cell biology.

Interferon-γ blocks signalling through PDGFRβ in human brain pericytes

BACKGROUND

Neuroinflammation and blood-brain barrier (BBB) disruption are common features of many brain disorders, including Alzheimer's disease, epilepsy, and motor neuron disease. Inflammation is thought to be a driver of BBB breakdown, but the underlying mechanisms for this are unclear. Brain pericytes are critical cells for maintaining the BBB and are immunologically active. We sought to test the hypothesis that inflammation regulates the BBB by altering pericyte biology.

METHODS

We exposed primary adult human brain pericytes to chronic interferon-gamma (IFNγ) for 4 days and measured associated functional aspects of pericyte biology. Specifically, we examined the influence of inflammation on platelet-derived growth factor receptor-beta (PDGFRβ) expression and signalling, as well as pericyte proliferation and migration by qRT-PCR, immunocytochemistry, flow cytometry, and western blotting.

RESULTS

Chronic IFNγ treatment had marked effects on pericyte biology most notably through the PDGFRβ, by enhancing agonist (PDGF-BB)-induced receptor phosphorylation, internalization, and subsequent degradation. Functionally, chronic IFNγ prevented PDGF-BB-mediated pericyte proliferation and migration.

CONCLUSIONS

Because PDGFRβ is critical for pericyte function and its removal leads to BBB leakage, our results pinpoint a mechanism linking chronic brain inflammation to BBB dysfunction.

Membrane-to-Nucleus Signals and Epigenetic Mechanisms for Myofibroblastic Activation and Desmoplastic Stroma: Potential Therapeutic Targets for Liver Metastasis?

Cancer-associated fibroblasts (CAFs), the most abundant cells in the tumor microenvironment (TME), are a key source of the extracellular matrix (ECM) that constitutes the desmoplastic stroma. Through remodeling of the reactive tumor stroma and paracrine actions, CAFs regulate cancer initiation, progression, and metastasis, as well as tumor resistance to therapies. The CAFs found in stroma-rich primary hepatocellular carcinomas (HCC) and liver metastases of primary cancers of other organs predominantly originate from hepatic stellate cells (HSTC), which are pericytes associated with hepatic sinusoids. During tumor invasion, HSTCs transdifferentiate into myofibroblasts in response to paracrine signals emanating from either tumor cells or a heterogeneous cell population within the hepatic tumor microenvironment. Mechanistically, HSTC-to-myofibroblast transdifferentiation, also known as, HSTC activation, requires cell surface receptor activation, intracellular signal transduction, gene transcription, and epigenetic signals, which combined ultimately modulate distinct gene expression profiles that give rise to and maintain a new phenotype. The current review defines a paradigm that explains how HSTCs are activated into CAFs to promote liver metastasis. Furthermore, a focus on the most relevant intracellular signaling networks and epigenetic mechanisms that control HSTC activation is provided. Finally, we discuss the feasibility of targeting CAF/activated HSTCs, in isolation or in conjunction with targeting cancer cells, which constitutes a promising and viable therapeutic approach for the treatment of primary stroma-rich liver cancers and liver metastasis.

The role of ubiquitylation in receptor endocytosis and endosomal sorting

Ligand-induced activation of transmembrane receptors activates intracellular signaling cascades that control vital cellular processes, such as cell proliferation, differentiation, migration and survival. Receptor signaling is modulated by several mechanisms to ensure that the correct biological outcome is achieved. One such mechanism, which negatively regulates receptor signaling, involves the modification of receptors with ubiquitin. This post-translational modification can promote receptor endocytosis and targets receptors for lysosomal degradation, thereby ensuring termination of receptor signaling. In this Commentary, we review the roles of ubiquitylation in receptor endocytosis and degradative endosomal sorting by drawing on the epidermal growth factor receptor (EGFR) as a well-studied example. Furthermore, we elaborate on the molecular basis of ubiquitin recognition along the endocytic pathway through compartment-specific ubiquitin-binding proteins and highlight how endocytic sorting machineries control these processes. In addition, we discuss the importance of ubiquitin-dependent receptor endocytosis for the maintenance of cellular homeostasis and in the prevention of diseases such as cancer.

Potentiation of platelet-derived growth factor receptor-β signaling mediated by integrin-associated MFG-E8

OBJECTIVE

Pericytes/pericyte precursors produce milk fat globule-associated protein with epidermal growth factor and factor VIII-like domains (MFG-E8) in vivo, and this α(v) integrin ligand enhances angiogenesis in tumors and in oxygen-induced retinopathy in mice. Inhibition of MFG-E8 production or function attenuates platelet-derived growth factor-BB (PDGF-BB)-induced migration of pericyte/pericyte precursor-like 10T1/2 cells in vitro. Herein, we describe mechanisms by which MFG-E8 modulates PDGF-BB:PDGF receptor β (PDGFRβ) signaling in 10T1/2 cells.

METHODS AND RESULTS

Small interfering RNA depletion of MFG-E8 from 10T1/2 cells or antibody inhibition of MFG-E8 action enhanced PDGF-BB-dependent degradation of PDGFRβ and attenuated signaling. Coimmunoprecipitation revealed transient association of MFG-E8 with PDGFRβ in PDGF-BB-treated 10T1/2 cells and reduced PDGFRβ-focal adhesion kinase association in MFG-E8-depleted cells. Confocal microscopy demonstrated that MFG-E8 binding to 10T1/2 cells was RGD motif and α(v) dependent but PDGF-BB treatment independent, whereas colocalization of MFG-E8 with PDGFRβ was enhanced by PDGF-BB. Ubiquitination of PDGFRβ was also increased in MFG-E8 small interfering RNA-transfected cells.

CONCLUSION

Integrin α(v)-bound MFG-E8 associates with PDGFRβ and focal adhesion kinase after PDGF-BB treatment, results in cell surface retention of PDGFRβ, delays receptor degradation, potentiates downstream signaling, and enhances migration of 10T1/2 cells. MFG-E8 may promote angiogenesis, in part, via cell autonomous actions on pericytes or pericyte precursors that result in enhanced PDGF-BB:PDGFRβ signaling mediated via integrin-growth factor receptor cross-talk.

Ubiquitination and endocytosis of the high affinity receptor for IgE

The high affinity receptor for IgE (FcvarepsilonRI) is constitutivelly expressed on the surface of mast cells and basophils as a multimeric complex. Upon antigen ligation to FcvarepsilonRI-bound IgE molecules, the receptor complex transduces intracellular signals leading to the release of preformed and newly synthesised pro-inflammatory mediators. FcvarepsilonRI engagement also generates negative intracellular signals involving the coordinated action of adapters, phosphatases and ubiquitin ligases that limits the intensity and duration of positive signals. Relevant to this, antigen-induced FcvarepsilonRI ubiquitination has become recognized as an important signal for the internalization and delivery of engaged receptor complexes to lysosomes for degradation. In this article, we review recent advances in our understanding of molecular mechanisms that guarantee the clearance of antigen-stimulated FcvarepsilonRI complexes from the cell surface. A particular emphasis will be given on how lipid rafts and the ubiquitin pathway cooperate to ensure receptor internalization and sorting along the endocytic compartments. A brief discussion regarding how ubiquitination regulates the endocytosis of Fc receptors other than FcvarepsilonRI will be included.

Endothelial and smooth muscle-derived neuropilin-like protein regulates platelet-derived growth factor signaling in human vascular smooth muscle cells by modulating receptor ubiquitination

Endothelial and smooth muscle cell-derived neuropilin-like protein (ESDN) is up-regulated in the neointima of remodeling arteries and modulates vascular smooth muscle cell (VSMC) growth. Platelet-derived growth factor (PDGF) is the prototypic growth factor for VSMCs and plays a key role in vascular remodeling. Here, we sought to further define ESDN function in primary human VSMCs. ESDN down-regulation by RNA interference significantly enhanced PDGF-induced VSMC DNA synthesis and migration. This was associated with increased ERK1/2, Src, and PDGF receptor (PDGFR)beta phosphorylation, without altering total PDGFRbeta expression levels. In binding assays, ESDN down-regulation significantly increased (125)I-PDGF maximum binding (B(max)) to PDGF receptors on VSMCs without altering the binding constant (K(d)), raising the possibility that ESDN regulates PDGFR processing. ESDN down-regulation significantly reduced ligand-induced PDGFRbeta ubiquitination. This was associated with a significant reduction in the expression level of c-Cbl, an E3 ubiquitin ligase that ubiquitinylates PDGFRbeta. Thus, ESDN modulates PDGF signaling in VSMCs via regulation of PDGFR surface levels. The ESDN effect is mediated, at least in part, through effects on PDGFRbeta ubiquitination. ESDN may serve as a target for regulating PDGFRbeta signaling in VSMCs.

Degradation of activated protein kinases by ubiquitination

Protein kinases are important regulators of intracellular signal transduction pathways and play critical roles in diverse cellular functions. Once a protein kinase is activated, its activity is subsequently downregulated through a variety of mechanisms. Accumulating evidence indicates that the activation of protein kinases commonly initiates their downregulation via the ubiquitin/proteasome pathway. Failure to regulate protein kinase activity or expression levels can cause human diseases.

Stable isotope labeling by amino acids in cell culture and differential plasma membrane proteome quantitation identify new substrates for the MARCH9 transmembrane E3 ligase

The regulation of cell surface receptor expression is essential for immune cell differentiation and function. At the plasma membrane ubiquitination is an important post-translational mechanism for regulating expression of a wide range of surface proteins. MARCH9, a member of the RING-CH family of transmembrane E3 ubiquitin ligases, down-regulates CD4, major histocompatibility complex-I (MHC), and ICAM-1 in lymphoid cells. To identify novel MARCH9 substrates, we used high throughput flow cytometry and quantitative mass spectrometry by stable isotope labeling by amino acids in cell culture (SILAC) to determine the differential expression of plasma membrane proteins in a MARCH9-expressing B cell line. This combined approach identified 13 potential new MARCH9 targets. All of the SILAC-identified targets for which antibodies were available were subsequently confirmed by flow cytometry, validating the proteomics results. A close correlation (r(2) = 0.93) between -fold down-regulation as determined by SILAC and flow cytometry was found, with no false positive hits detected. The potential new MARCH9 substrates cover a wide range of functions and include receptor-type protein-tyrosine phosphatases (e.g. PTPRJ/CD148) as well as Fc gamma receptor IIB (CD32B), HLA-DQ, signaling lymphocytic activation molecule (CD150), and polio virus receptor (CD155). The identification of plasma membrane targets by SILAC with confirmation by flow cytometry represents a novel and powerful approach to analyze changes in the plasma membrane proteome.

Activation of protein kinase C alpha is necessary for sorting the PDGF beta-receptor to Rab4a-dependent recycling

Previous studies showed that loss of the T-cell protein tyrosine phosphatase (TC-PTP) induces Rab4a-dependent recycling of the platelet-derived growth factor (PDGF) beta-receptor in mouse embryonic fibroblasts (MEFs). Here we identify protein kinase C (PKC) alpha as the critical signaling component that regulates the sorting of the PDGF beta-receptor at the early endosomes. Down-regulation of PKC abrogated receptor recycling by preventing the sorting of the activated receptor into EGFP-Rab4a positive domains on the early endosomes. This effect was mimicked by inhibition of PKCalpha, using myristoylated inhibitory peptides or by knockdown of PKCalpha with shRNAi. In wt MEFs, short-term preactivation of PKC by PMA caused a ligand-induced PDGF beta-receptor recycling that was dependent on Rab4a function. Together, these observations demonstrate that PKC activity is necessary for recycling of ligand-stimulated PDGF beta-receptor to occur. The sorting also required Rab4a function as it was prevented by expression of EGFP-Rab4aS22N. Preventing receptor sorting into recycling endosomes increased the rate of receptor degradation, indicating that the sorting of activated receptors at early endosomes directly regulates the duration of receptor signaling. Activation of PKC through the LPA receptor also induced PDGF beta-receptor recycling and potentiated the chemotactic response to PDGF-BB. Taken together, our present findings indicate that sorting of PDGF beta-receptors on early endosomes is regulated by sequential activation of PKCalpha and Rab4a and that this sorting step could constitute a point of cross-talk with other receptors.

Binding of Cbl to a phospholipase Cgamma1-docking site on platelet-derived growth factor receptor beta provides a dual mechanism of negative regulation

Ubiquitin conjugation to receptor tyrosine kinases is a critical biochemical step in attenuating their signaling through lysosomal degradation. Our previous studies have established Cbl as an E3 ubiquitin ligase for ubiquitinylation and degradation of platelet-derived growth factor receptor (PDGFR) alpha and PDGFRbeta. However, the role of endogenous Cbl in PDGFR regulation and the molecular mechanisms of this regulation remain unclear. Here, we demonstrate that endogenous Cbl is essential for ligand-induced ubiquitinylation and degradation of PDGFRbeta; this involves the Cbl TKB domain binding to PDGFRbeta phosphotyrosine 1021, a known phospholipase C (PLC) gamma1 SH2 domain-binding site. Lack of Cbl or ablation of the Cbl-binding site on PDGFRbeta impedes receptor sorting to the lysosome. Cbl-deficient cells also show more PDGF-induced PLCgamma1 association with PDGFRbeta and enhanced PLC-mediated cell migration. Thus, Cbl-dependent negative regulation of PDGFRbeta involves a dual mechanism that concurrently promotes ubiquitin-dependent lysosomal sorting of the receptor and competitively reduces the recruitment of a positive mediator of receptor signaling.

Role of Ubiquitylation in Cellular Membrane Transport

Ubiquitylation of membrane proteins has gained considerable interest in recent years. It has been recognized as a signal that negatively regulates the cell surface expression of many plasma membrane proteins both in yeast and in mammalian cells. Moreover, it is also involved in endoplasmic reticulum-associated degradation of membrane proteins, and it acts as a sorting signal both in the secretory pathway and in endosomes, where it targets proteins into multivesicular bodies in the lumen of vacuoles/lysosomes. In this review we discuss the progress in understanding these processes, achieved during the past several years.

Presenilins mediate phosphatidylinositol 3-kinase/AKT and ERK activation via select signaling receptors. Selectivity of PS2 in platelet-derived growth factor signaling

The Alzheimer's disease-linked genes, PS1 and PS2, are required for intramembrane proteolysis of multiple type I proteins, including Notch and amyloid precursor protein. In addition, it has been documented that PS1 positively regulates, whereas PS1 familial Alzheimer disease mutations suppress, phosphatidylinositol 3-kinase (PI3K)/Akt activation, a pathway known to inactivate glycogen synthase kinase-3 and reduce tau phosphorylation. In this study, we show that the loss of presenilins not only inhibits PI3K/Akt signaling and increases tau phosphorylation but also suppresses the MEK/ERK pathway. The deficits in Akt and ERK activation in cells deficient in both PS1 and PS2 (PS-/-) are evident after serum withdrawal and stimulation with fetal bovine serum or ligands of select receptor tyrosine kinases, platelet-derived growth factor receptor beta (PDGFR beta) and PDGFR alpha, but not insulin-like growth factor-1R and epidermal growth factor receptor. The defects in PDGF signaling in PS-/- cells are due to reduced expression of PDGF receptors. Whereas fetal bovine serum-induced Akt activation is reconstituted by both PS1 and PS2 in PS-/- cells, PDGF signaling is selectively restored by PS2 but not PS1 and is dependent on the N-terminal fragment of PS2 but not gamma-secretase activity or the hydrophilic loop of PS2. The rescue of PDGF receptor expression and activation by PS2 is facilitated by FHL2, a PS2-interacting transcriptional co-activator. Finally, we present evidence that PS1 mutations interfere with this PS2-mediated activity by reducing PS2 fragments. These findings highlight important roles of both presenilins in Akt and ERK signaling via select signaling receptors.

Low Density Lipoprotein Receptor-related Protein 1 (LRP1) Controls Endocytosis and c-CBL-mediated Ubiquitination of the Platelet-derived Growth Factor Receptor β (PDGFRβ)

The low density lipoprotein receptor-related protein 1 (LRP1) has been implicated in intracellular signaling functions as well as in lipid metabolism. Recent in vivo and in vitro studies suggest that LRP1 is a physiological modulator of the platelet-derived growth factor (PDGF) signaling pathway. Here we show that in mouse fibroblasts LRP1 modulates PDGF-BB signaling by controlling endocytosis and ligand-induced down-regulation of the PDGF receptor beta (PDGFRbeta). In LRP1-deficient fibroblasts, basal PDGFRbeta tyrosine kinase activity was derepressed, and PDGF-BB-induced endocytosis and degradation of PDGFRbeta were accelerated as compared with control cells. This was accompanied by rapid uptake of receptor-bound PDGF-BB into the cells and by attenuated ERK activation in response to PDGF-BB stimulation. Pulse-chase analysis indicated that the steady-state turnover rate of PDGFRbeta was also accelerated in LRP-deficient fibroblasts. The rapid degradation of PDGFRbeta in the LRP1-deficient fibroblasts was prevented by MG132 and chloroquine. Furthermore, the association of PDGFRbeta with c-Cbl, a ubiquitin E3-ligase, as well as the ligand-induced ubiquitination of PDGFRbeta were increased in LRP1-deficient fibroblasts. We show that LRP1 can directly interact with c-Cbl, suggesting a Sprouty-like role for LRP1 in regulating the access of the PDGFRbeta to the ubiquitination machinery. Thus, LRP1 modulates PDGF signaling by controlling ubiquitination and endocytosis of the PDGFRbeta.

Endogenous glucocorticoids modulate neutrophil function in a murine model of haemolytic uraemic syndrome

Haemolytic uraemic syndrome (HUS) is caused by Shiga-toxin-producing Escherichia coli (STEC). Although, Shiga toxin type 2 (Stx2) is responsible for the renal pathogenesis observed in patients, the inflammatory response, including cytokines and polymorphonuclear neutrophils (PMN), plays a key role in the development of HUS. Previously, we demonstrated that Stx2 injection generates an anti-inflammatory reaction characterized by endogenous glucocorticoid (GC) secretion, which attenuates HUS severity in mice. Here, we analysed the effects of Stx2 on the pathogenic function of PMN and the potential role of endogenous GC to limit PMN activation during HUS development in a murine model. For this purpose we assessed the functional activity of isolated PMN after in vivo treatment with Stx2 alone or in simultaneous treatment with Ru486 (GC receptor antagonist). We found that Stx2 increased the generation of reactive oxygen intermediates (ROI) under phobol-myristate-acetate (PMA) stimulation and that the simultaneous treatment with Ru486 strengthened this effect. Conversely, both treatments significantly inhibited in vitro phagocytosis. Furthermore, Stx2 augmented in vitro PMN adhesion to fibrinogen (FGN) and bovine serum albumin (BSA) but not to collagen type I (CTI). Stx2 + Ru486 caused enhanced adhesion to BSA and CTI compared to Stx2. Whereas Stx2 significantly increased migration towards N-formyl-methionyl-leucyl-phenylalanine (fMLP), Stx2 + Ru486 treatment enhanced and accelerated this process. The percentage of apoptotic PMN from Stx2-treated mice was higher compared with controls, but equal to Stx2 + Ru486 treated mice. We conclude that Stx2 activates PMN and that the absence of endogenous GC enhances this activation suggesting that endogenous GC can, at least partially, counteract PMN inflammatory functions.

Insulin inhibits platelet-derived growth factor-induced cell proliferation

Cellular behavior can be considered to be the result of a very complex spatial and temporal integration of intracellular and extracellular signals. These signals arise from serum-soluble factors as well as from cell-substrate or cell-cell interactions. The current approach in mitogenesis studies is generally to analyze the effect of a single growth factor on serum-starved cells. In this context, a metabolic hormone such as insulin is found to be a mitogenic agent in many cellular types. In the present study, we have considered the effect of insulin stimulation in platelet-derived growth factor (PDGF)-activated NIH-3T3 and C2C12 cells. Our results show that insulin is able to inhibit strongly both NIH-3T3 and C2C12 cell growth induced by PDGF, one of the most powerful mitotic agents for these cell types. This inhibitory effect of insulin is due primarily to a premature down-regulation of the PDGF receptor. Thus, when NIH-3T3 or C2C12 cells are stimulated with both PDGF and insulin, we observe a decrease in PDGF receptor phosphorylation with respect to cells treated with PDGF alone. In particular, we find that costimulation with insulin leads to a reduced production of H2O2 with respect to cell stimulation with PDGF alone. The relative low concentration of H2O2 in PDGF/insulin-costimulated cell leads to a limited down-regulation of protein tyrosine phosphatases, and, consequently, to a reduced PDGF receptor phosphorylation efficiency. The latter is very likely to be responsible for the insulin-dependent inhibition of PDGF-receptor mitogenic signaling.

Ubiquitination and proteasomal degradation of nucleophosmin-anaplastic lymphoma kinase induced by 17-allylamino-demethoxygeldanamycin: role of the co-chaperone carboxyl heat shock protein 70-interacting protein

Nucleophosmin-anaplastic lymphoma kinase (NPM-ALK) is a constitutively active fusion tyrosine kinase involved in lymphomagenesis of human anaplastic large cell lymphomas (ALCL), the maturation and activity of which depend on the association with the heat shock protein (hsp) 90 protein chaperone. Targeting hsp90 by the ansamycins geldanamycin and 17-allyl-amino-demethoxygeldanamycin (17-AAG) promotes degradation of several proteins through the ubiquitin-proteasome pathway, including oncogenic Raf, v-Src, erbB2, and BCR-ABL. We have previously shown that 17-AAG prevents hsp90/NPM-ALK complex formation and fosters NPM-ALK turnover, perhaps through its association with the hsp70 chaperone. Here, we show that inhibition of the proteasome activity by the potent and specific compound pyrazylcarbonyl-Phe-Leu-boronate (PS-341) blocks 17-AAG-induced down-regulation of NPM-ALK, which becomes detergent-insoluble and relocates into ubiquitin-rich perinuclear vesicles that represent aggregated polyubiquitinated forms of the protein. Kinase activity was not mandatory for proteasomal degradation of NPM-ALK, because kinase-defective NPM-ALK was even more rapidly degraded upon 17-AAG treatment. Prolonged exposure to the proteasome inhibitor was shown to trigger caspase-3-mediated apoptosis in proliferating ALCL cells at nanomolar concentrations. However, we verified that the accumulation of detergent-insoluble NPM-ALK in ALCL cells was not a spurious consequence of PS341-committed apoptosis, because caspase inhibitors prevented poly(ADP-ribose) polymerase cleavage whereas they did not affect partitioning of aggregated NPM-ALK. In line with these observations, the carboxyl hsp70-interacting ubiquitin ligase (CHIP), was shown to increase basal ubiquitination and turnover of NPM-ALK kinase, supporting a mechanism whereby NPM-ALK proceeds rapidly toward hsp70-assisted ubiquitin-dependent proteasomal degradation, when chaperoning activity of hsp90 is prohibited by 17-AAG.

Dioxygen activation in methane monooxygenase: a theoretical study

Using broken-symmetry unrestricted Density Functional Theory, the mechanism of enzymatic dioxygen activation by the hydroxylase component of soluble methane monooxygenase (MMOH) is determined to atomic detail. After a thorough examination of mechanistic alternatives, an optimal pathway was identified. The diiron(II) state H(red) reacts with dioxygen to give a ferromagnetically coupled diiron(II,III) H(superoxo) structure, which undergoes intersystem crossing to the antiferromagnetic surface and affords H(peroxo), a symmetric diiron(III) unit with a nonplanar mu-eta(2):eta(2)-O(2)(2)(-) binding mode. Homolytic cleavage of the O-O bond yields the catalytically competent intermediate Q, which has a di (mu-oxo)diiron(IV) core. A carboxylate shift involving Glu243 is essential to the formation of the symmetric H(peroxo) and Q structures. Both thermodynamic and kinetic features agree well with experimental data, and computed spin-exchange coupling constants are in accord with spectroscopic values. Evidence is presented for pH-independent decay of H(red) and H(peroxo). Key electron-transfer steps that occur in the course of generating Q from H(red) are also detailed and interpreted. In contrast to prior theoretical studies, a requisite large model has been employed, electron spins and couplings have been treated in a quantitative manner, potential energy surfaces have been extensively explored, and quantitative total energies have been determined along the reaction pathway.

Variable coordination geometries at the diiron(II) active site of ribonucleotide reductase R2

The R2 subunit of Escherichia coli ribonucleotide reductase contains a dinuclear iron center that generates a catalytically essential stable tyrosyl radical by one electron oxidation of a nearby tyrosine residue. After acquisition of Fe(II) ions by the apo protein, the resulting diiron(II) center reacts with O(2) to initiate formation of the radical. Knowledge of the structure of the reactant diiron(II) form of R2 is a prerequisite for a detailed understanding of the O(2) activation mechanism. Whereas kinetic and spectroscopic studies of the reaction have generally been conducted at pH 7.6 with reactant produced by the addition of Fe(II) ions to the apo protein, the available crystal structures of diferrous R2 have been obtained by chemical or photoreduction of the oxidized diiron(III) protein at pH 5-6. To address this discrepancy, we have generated the diiron(II) states of wildtype R2 (R2-wt), R2-D84E, and R2-D84E/W48F by infusion of Fe(II) ions into crystals of the apo proteins at neutral pH. The structures of diferrous R2-wt and R2-D48E determined from these crystals reveal diiron(II) centers with active site geometries that differ significantly from those observed in either chemically or photoreduced crystals. Structures of R2-wt and R2-D48E/W48F determined at both neutral and low pH are very similar, suggesting that the differences are not due solely to pH effects. The structures of these "ferrous soaked" forms are more consistent with circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopic data and provide alternate starting points for consideration of possible O(2) activation mechanisms.

Signaling through monoubiquitination

Ubiquitination is a post-translational modification in which a small conserved peptide, ubiquitin, is appended to target proteins in the cell, through a series of complex enzymatic reactions. Recently, a particular form of ubiquitination, monoubiquitination, has emerged as a nonproteolytic reversible modification that controls protein function. In this review, we highlight recent findings on monoubiquitination as a signaling-induced modification, controlled, among others, by pathways originating from active receptor tyrosine kinases. Furthermore, we review the major cellular processes controlled by ubiquitin modification, including membrane trafficking, histone function, transcription regulation, DNA repair, and DNA replication.

The PHD domain of MEKK1 acts as an E3 ubiquitin ligase and mediates ubiquitination and degradation of ERK1/2

ERK1/2 MAP kinases are important regulators in cellular signaling, whose activity is normally reversibly regulated by threonine-tyrosine phosphorylation. In contrast, we have found that stress-induced ERK1/2 activity is downregulated by ubiquitin/proteasome-mediated degradation of ERK1/2. The PHD domain of MEKK1, a RING finger-like structure, exhibited E3 ubiquitin ligase activity toward ERK2 in vitro and in vivo. Moreover, both MEKK1 kinase activity and the docking motif on ERK1/2 were involved in ERK1/2 ubiquitination. Significantly, cells expressing ERK2 with the docking motif mutation were resistant to sorbitol-induced apoptosis. Therefore, MEKK1 functions not only as an upstream activator of the ERK and JNK through its kinase domain, but also as an E3 ligase through its PHD domain, providing a negative regulatory mechanism for decreasing ERK1/2 activity.

c-Cbl associates directly with the C-terminal tail of the receptor for the macrophage colony-stimulating factor, c-Fms, and down-modulates this receptor but not the viral oncogene v-Fms

The receptor for the macrophage colony-stimulating factor (CSF-1, also termed M-CSF), the tyrosine kinase c-Fms, was originally determined to be the oncogene product of the McDonough strain of feline sarcoma virus, v-Fms. The structural difference between c-Fms and v-Fms amounts to only five point mutations in the extracellular domain, two mutations in the cytoplasmic domain, and the replacement of 50 amino acids by 14 unrelated amino acids at the C-terminal tail. Here, we have identified c-Cbl as the direct binding partner for c-Fms. c-Cbl binds to phosphotyrosine residue 977 at the C-terminal end of feline c-Fms, which is absent in v-Fms. The replacement of the C-terminal end of v-Fms by the corresponding part of c-Fms (vc-Fms) restored the binding potential. As a result, vc-Fms reduced the transforming potency of v-Fms. The overexpression of Cbl did not influence the v-Fms-transformed phenotype, although c-Cbl forms a complex with v-Fms indirectly. In contrast, the expression of Cbl drastically reduced the vc-Fms-transformed phenotype and the activation of Erk and enhanced Fms ubiquitination via phosphotyrosine residue 977. Furthermore, the replacement of tyrosine 977 into phenylalanine in feline c-Fms and vc-Fms reduced the Cbl-dependent ubiquitination. These data suggest that an indirect association of c-Cbl via multimeric complex induced a different signaling pathway from the pathway induced by c-Cbl direct interaction.

Inhibition of proteasome activity blocks the ability of TNF alpha to down-regulate G(i) proteins and stimulate lipolysis

Prolonged treatment of rat adipocytes with TNF alpha increases lipolysis through a mechanism mediated, in part, by down-regulation of inhibitory G proteins (G(i)). Separately, down-regulation of G(i) by prolonged treatment with an A(1)-adenosine receptor agonist, N(6)-phenylisopropyl adenosine (PIA) increases lipolysis. To investigate the role of proteolysis in TNF alpha and PIA-mediated G(i) down-regulation and stimulation of lipolysis, we used the protease inhibitors lactacystin (proteasome inhibitor) and calpeptin (calpain inhibitor). Rat adipocytes were preincubated for 1 h with lactacystin (10 microM) or calpeptin (50 microM), before 30-h treatment with either TNF alpha (50 ng/ml) or PIA (300 nM). We then measured lipolysis (glycerol release), abundance of alpha-subunits of G(i)1 and G(i)2 in plasma membranes (Western blotting) and protease activities (in specific fluorogenic assays). TNF alpha and PIA stimulated lipolysis approximately 2-fold and caused G(i) down-regulation. Although neither lactacystin nor calpeptin affected basal lipolysis, lactacystin completely inhibited both TNF alpha and PIA-stimulated lipolysis (the 50% inhibitory concentration was approximately 2 microM), whereas calpeptin had no effect. Similarly, lactacystin but not calpeptin blocked both PIA and TNF alpha-induced G(i) down-regulation. These findings provide further evidence that the chronic lipolytic effect of TNF alpha and PIA is secondary to G(i) down-regulation and suggest that the mechanism involves proteolytic degradation mediated through the proteasome pathway.

Regulation of cell growth by redox-mediated extracellular proteolysis of platelet-derived growth factor receptor beta

Redox-regulated processes are important elements in various cellular functions. Reducing agents, such as N-acetyl-l-cysteine (NAC), are known to regulate signal transduction and cell growth through their radical scavenging action. However, recent studies have shown that reactive oxygen species are not always involved in ligand-stimulated intracellular signaling. Here, we report a novel mechanism by which NAC blocks platelet-derived growth factor (PDGF)-induced signaling pathways in hepatic stellate cells, a fibrogenic player in the liver. Unlike in vascular smooth muscle cells, we found that reducing agents, including NAC, triggered extracellular proteolysis of PDGF receptor-beta, leading to desensitization of hepatic stellate cells toward PDGF-BB. This effect was mediated by secreted mature cathepsin B. In addition, type II transforming growth factor-beta receptor was also down-regulated. Furthermore, these events seemed to cause a dramatic improvement of rat liver fibrosis. These results indicated that redox processes impact the cell's response to growth factors by regulating the turnover of growth factor receptors and that "redox therapy" is promising for fibrosis-related disease.

Connecting signaling and cell cycle progression in growth factor-stimulated cells

A widely used model system to investigate cell proliferation is stimulation of serum-arrested cells with growth factors. Recent data suggest that there are two waves of growth factor-dependent signaling events required for a proliferative response. One is an acute burst of signaling, which occurs immediately after growth factor stimulation and lasts for 30 - 60 min. The other occurs in a different time frame (8 - 12 h post stimulation), and involves activation of cyclin dependent kinases (Cdks). In addition to a general overview of growth factor-dependent signaling, we present our 'two wave' hypothesis for how signaling and cell cycle progression are linked.

PDGF-beta receptor expression and ventilatory acclimatization to hypoxia in the rat

Activation of platelet-derived growth factor-beta (PDGF-beta) receptors in the nucleus of the solitary tract (nTS) modulates the late phase of the acute hypoxic ventilatory response (HVR) in the rat. We hypothesized that temporal changes in PDGF-beta receptor expression could underlie the ventilatory acclimatization to hypoxia (VAH). Normoxic ventilation was examined in adult Sprague-Dawley rats chronically exposed to 10% O(2), and at 0, 1, 2, 7, and 14 days, Northern and Western blots of the dorsocaudal brain stem were performed for assessment of PDGF-beta receptor expression. Although no significant changes in PDGF-beta receptor mRNA occurred over time, marked attenuation of PDGF-beta receptor protein became apparent after day 7 of hypoxic exposure. Such changes were significantly correlated with concomitant increases in normoxic ventilation, i.e., with VAH (r: -0.56, P < 0.005). In addition, long-term administration of PDGF-BB in the nTS via osmotic pumps loaded with either PDGF-BB (n = 8) or vehicle (Veh; n = 8) showed that although no significant changes in the magnitude of acute HVR occurred in Veh over time, the typical attenuation of HVR by PDGF-BB decreased over time. Furthermore, PDGF-BB microinjections did not attenuate HVR in acclimatized rats at 7 and 14 days of hypoxia (n = 10). We conclude that decreased expression of PDGF-beta receptors in the dorsocaudal brain stem correlates with the magnitude of VAH. We speculate that the decreased expression of PDGF-beta receptors is mediated via internalization and degradation of the receptor rather than by transcriptional regulation.

Platelet-derived growth factor receptor association with Na(+)/H(+) exchanger regulatory factor potentiates receptor activity

Platelet-derived growth factor (PDGF) is a potent mitogen for many cell types. The PDGF receptor (PDGFR) is a receptor tyrosine kinase that mediates the mitogenic effects of PDGF by binding to and/or phosphorylating a variety of intracellular signaling proteins upon PDGF-induced receptor dimerization. We show here that the Na(+)/H(+) exchanger regulatory factor (NHERF; also known as EBP50), a protein not previously known to interact with the PDGFR, binds to the PDGFR carboxyl terminus (PDGFR-CT) with high affinity via a PDZ (PSD-95/Dlg/Z0-1 homology) domain-mediated interaction and potentiates PDGFR autophosphorylation and extracellular signal-regulated kinase (ERK) activation in cells. A point-mutated version of the PDGFR, with the terminal leucine changed to alanine (L1106A), cannot bind NHERF in vitro and is markedly impaired relative to the wild-type receptor with regard to PDGF-induced autophosphorylation and activation of ERK in cells. NHERF potentiation of PDGFR signaling depends on the capacity of NHERF to oligomerize. NHERF oligomerizes in vitro when bound with PDGFR-CT, and a truncated version of the first NHERF PDZ domain that can bind PDGFR-CT but which does not oligomerize reduces PDGFR tyrosine kinase activity when transiently overexpressed in cells. PDGFR activity in cells can also be regulated in a NHERF-dependent fashion by stimulation of the beta(2)-adrenergic receptor, a known cellular binding partner for NHERF. These findings reveal that NHERF can directly bind to the PDGFR and potentiate PDGFR activity, thus elucidating both a novel mechanism by which PDGFR activity can be regulated and a new cellular role for the PDZ domain-containing adapter protein NHERF.

Src family kinases negatively regulate platelet-derived growth factor alpha receptor-dependent signaling and disease progression

We tested the hypothesis that Src family kinases (SFK) contribute to c-Cbl-mediated degradation of the platelet-derived growth factor (PDGF) alpha receptor (alphaPDGFR). Using either a receptor mutant that does not engage SFKs (F72/74), or cells that that lack SFKs, we found that SFKs contributed to degradation of the alphaPDGFR. Overexpression of c-Cbl also reduced the receptor half-life, but only if the receptor was able to engage SFKs. In cultured cells, prolonging the half-life of the receptor correlated with enhanced signaling and more efficient S phase entry, whereas accelerating receptor degradation had the opposite effect. Consistent with these tissue culture findings, there was a statistically significant increase in the onset of a proliferative retinal disease when animals were injected with cells expressing the F72/74 receptor, as compared with cells expressing the WT receptor. Our findings suggest that SFKs cooperate with c-Cbl to negatively regulate the alphaPDGFR, and that the SFK/c-Cbl suppression of alphaPDGFR output is relevant to the onset and progression of a proliferative disease.

The progesterone receptor and ubiquitin are differentially regulated within the endometrial glands of the natural and stimulated cycle

The initiation of human pregnancy requires precisely timed development of the endometrium to receive the implanting blastocyst. The ovarian steroid hormones are essential for development and maintenance of a hospitable uterine environment. The hormonal regimes employed in assisted reproduction procedures are known to alter the abundance of specific endometrial receptors for these steroids. Since, in the presence of ligand, the progesterone receptor (PR) is known to be modified by the small intracellular protein ubiquitin, we have investigated the localization of ubiquitin and PR within the endometrial glands of 28 fertile women during a monitored menstrual cycle and also during a stimulated cycle prior to oocyte donation. We have also observed the number of gland cells undergoing cell division as demonstrated by the presence of Ki67 immunostaining. We demonstrate that the percentage of ubiquitin-positive nuclei increases from day four post-ovulation to day 10 post-ovulation in the natural cycle, but that this increase is not seen during a stimulated cycle. The presence of PR within glandular epithelium and the proliferation of gland cells were only observed during the early secretory phase and did not appear to vary significantly between the two cycles. We conclude that ubiquitin may play an important role in endometrial development and that perturbation of ubiquitin may be related to the lower implantation rate seen in the stimulated cycle.

Mechanism of action and in vivo role of platelet-derived growth factor

Platelet-derived growth factor (PDGF) is a major mitogen for connective tissue cells and certain other cell types. It is a dimeric molecule consisting of disulfide-bonded, structurally similar A- and B-polypeptide chains, which combine to homo- and heterodimers. The PDGF isoforms exert their cellular effects by binding to and activating two structurally related protein tyrosine kinase receptors, denoted the alpha-receptor and the beta-receptor. Activation of PDGF receptors leads to stimulation of cell growth, but also to changes in cell shape and motility; PDGF induces reorganization of the actin filament system and stimulates chemotaxis, i.e., a directed cell movement toward a gradient of PDGF. In vivo, PDGF has important roles during the embryonic development as well as during wound healing. Moreover, overactivity of PDGF has been implicated in several pathological conditions. The sis oncogene of simian sarcoma virus (SSV) is related to the B-chain of PDGF, and SSV transformation involves autocrine stimulation by a PDGF-like molecule. Similarly, overproduction of PDGF may be involved in autocrine and paracrine growth stimulation of human tumors. Overactivity of PDGF has, in addition, been implicated in nonmalignant conditions characterized by an increased cell proliferation, such as atherosclerosis and fibrotic conditions. This review discusses structural and functional properties of PDGF and PDGF receptors, the mechanism whereby PDGF exerts its cellular effects, and the role of PDGF in normal and diseased tissues.

Tyrosine kinase-dependent ubiquitination of CD16 zeta subunit in human NK cells following receptor engagement

We investigated whether aggregation of the low-affinity immunoglobulin G receptor (CD16) on human NK cells results in receptor ubiquitination. We found that the CD16 zeta subunit becomes ubiquitinated in response to receptor engagement. We then investigated whether protein tyrosine kinase (PTK) activation is required for CD16-mediated receptor ubiquitination. Pretreatment with the PTK inhibitor genistein substantially decreased ligand-induced zeta ubiquitination, suggesting a requirement for PTK activation in receptor ubiquitination. We further analyzed PTK involvement in controlling receptor ubiquitination by using the vaccinia virus expression system. Overexpression of wild-type active lck, but not a kinase-deficient mutant, enhanced both ligand-induced tyrosine phosphorylation and ubiquitination of the CD16 zeta subunit. Taken together, our data demonstrate that CD16 engagement induces zeta chain ubiquitination and strongly suggest a role for lck in regulating this modification.

Evidence that downregulation of the M-CSF receptor is not dependent upon receptor kinase activity

The downregulation of tyrosine kinase receptors attenuates signalling and is thought to be dependent upon intrinsic receptor kinase activity, largely because down-regulation is inhibited by a kinase-inactivating mutation of an invariant lysine residue of the receptors for EGF, insulin, M-CSF and PDGF. We confirmed that this mutation inhibited the degradation of the M-CSF receptor. However, two different kinase inactivating mutations of the invariant amino acids Gly 591 and Glu 633 did not prevent M-CSF-induced receptor degradation, so demonstrating that receptor kinase activity is not essential for this process. Three other kinase-inactivating mutations were found to cause constitutive receptor degradation in the absence of M-CSF, most probably by disrupting the structure of the activating loop of the kinase domain. It is known that extensive movement of the A-loop is necessary for kinase activation and is normally induced by ligand-binding. It is therefore suggested that some aspect or consequence of the change in structure of the A-loop caused by ligand binding also activates receptor downregulation, so ensuring that downregulation is coupled to but is not necessarily dependent upon receptor kinase activity.

Cbl-mediated negative regulation of platelet-derived growth factor receptor-dependent cell proliferation. A critical role for Cbl tyrosine kinase-binding domain

The Cbl proto-oncogene product has emerged as a novel negative regulator of receptor and non-receptor tyrosine kinases. Our previous observations that Cbl overexpression in NIH3T3 cells enhanced the ubiquitination and degradation of the platelet-derived growth factor receptor-alpha (PDGFRalpha) and that the expression of oncogenic Cbl mutants up-regulated the PDGFRalpha signaling machinery strongly suggested that Cbl negatively regulates PDGFRalpha signaling. Here, we show that, similar to PDGFRalpha, selective stimulation of PDGFRbeta induces Cbl phosphorylation, and its physical association with the receptor. Overexpression of wild type Cbl in NIH3T3 cells led to an enhancement of the ligand-dependent ubiquitination and subsequent degradation of the PDGFRbeta, as observed with PDGFRalpha. We show that Cbl-dependent negative regulation of PDGFRalpha and beta results in a reduction of PDGF-induced cell proliferation and protection against apoptosis. A point mutation (G306E) that inactivates the tyrosine kinase binding domain in the N-terminal transforming region of Cbl compromised the PDGF-inducible tyrosine phosphorylation of Cbl although this mutant could still associate with the PDGFR. More importantly, the G306E mutation abrogated the ability of Cbl to enhance the ligand-induced ubiquitination and degradation of the PDGFR and to inhibit the PDGF-dependent cell proliferation and protection from apoptosis. These results demonstrate that Cbl can negatively regulate PDGFR-dependent biological responses and that this function requires the conserved tyrosine kinase binding domain of Cbl.

Signal transduction via platelet-derived growth factor receptors

Platelet-derived growth factor (PDGF) exerts its stimulatory effects on cell growth and motility by binding to two related protein tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation, allowing binding and activation of cytoplasmic SH2-domain containing signal transduction molecules. Thereby, a number of different signaling pathways are initiated leading to cell growth, actin reorganization migration and differentiation. Recent observations suggest that extensive cross-talk occurs between different signaling pathways, and that stimulatory signals are modulated by inhibitory signals arising in parallel.

Intracellular proteinases of invertebrates: calcium-dependent and proteasome/ubiquitin-dependent systems

Cytosolic proteinases carry out a variety of regulatory functions by controlling protein levels and/or activities within cells. Calcium-dependent and ubiquitin/proteasome-dependent pathways are common to all eukaryotes. The former pathway consists of a diverse group of Ca(2+)-dependent cysteine proteinases (CDPs; calpains in vertebrate tissues). The latter pathway is highly conserved and consists of ubiquitin, ubiquitin-conjugating enzymes, deubiquitinases, and the proteasome. This review summarizes the biochemical properties and genetics of invertebrate CDPs and proteasomes and their roles in programmed cell death, stress responses (heat shock and anoxia), skeletal muscle atrophy, gametogenesis and fertilization, development and pattern formation, cell-cell recognition, signal transduction and learning, and photoreceptor light adaptation. These pathways carry out bulk protein degradation in the programmed death of the intersegmental and flight muscles of insects and of individuals in a colonial ascidian; molt-induced atrophy of crustacean claw muscle; and responses of brine shrimp, mussels, and insects to environmental stress. Selective proteolysis occurs in response to specific signals, such as in modulating protein kinase A activity in sea hare and fruit fly associated with learning; gametogenesis, differentiation, and development in sponge, echinoderms, nematode, ascidian, and insects; and in light adaptation of photoreceptors in the eyes of squid, insects, and crustaceans. Proteolytic activities and specificities are regulated through proteinase gene expression (CDP isozymes and proteasomal subunits), allosteric regulators, and posttranslational modifications, as well as through specific targeting of protein substrates by a diverse assemblage of ubiquitin-conjugases and deubiquitinases. Thus, the regulation of intracellular proteolysis approaches the complexity and versatility of transcriptional and translational mechanisms.

The tyrosine kinase regulator Cbl enhances the ubiquitination and degradation of the platelet-derived growth factor receptor alpha

The Cbl protooncogene product has emerged as a negative regulator of receptor and nonreceptor tyrosine kinases. We recently demonstrated that oncogenic Cbl mutants upregulate the endogenous tyrosine kinase signaling machinery when expressed in the NIH 3T3 cells, and identified the platelet-derived growth factor receptor-alpha (PDGFRalpha) as one of the tyrosine kinases targeted by these oncogenes. These findings suggested a role for the normal Cbl protein in negative regulation of the PDGFRalpha. However, the mechanism of such negative regulation remained to be determined. Here we show that overexpression of the wild-type Cbl enhances the ligand-induced ubiquitination of the PDGFRalpha. Concomitantly, the PDGFRalpha in Cbl-overexpressing cells undergoes a faster ligand-induced degradation compared with that in the control cells. These results identify a role for Cbl in the regulation of ligand-induced ubiquitination and degradation of receptor tyrosine kinases and suggest one potential mechanism for evolutionarily conserved negative regulatory influence of Cbl on tyrosine kinases.

The itchy locus encodes a novel ubiquitin protein ligase that is disrupted in a18H mice

Non-agouti-lethal 18H (a18H) mice are dark agouti with black pinna hairs. What makes these mice unique is that they develop a spectrum of immunological diseases not seen in other agouti mutant mice. On the JU/Ct background, a18H mice develop an inflammatory disease of the large intestine. On the C57BL/6J background, they develop a fatal disease characterized by pulmonary chronic interstitial inflammation and alveolar proteinosis, inflammation of the glandular stomach and skin resulting in scarring due to constant itching, and hyperplasia of lymphoid cells, haematopoietic cells and the forestomach epithelium. Previous studies suggested that the a18H mutation results from a paracentric inversion that affects two loci: agouti and another, as yet unidentified locus designated itchy (the provisional gene symbol is Itch), that is responsible for the immunological phenotype of a18H mice. Here we confirm that a18H results from an inversion and show that Itch encodes a novel E3 ubiquitin protein ligase, a protein involved in ubiquitin-mediated protein degradation. Our results indicate that ubiquitin-dependent proteolysis is an important mediator of the immune response in vivo and provide evidence for Itch's role in inflammation and the regulation of epithelial and haematopoietic cell growth.

Heregulin beta1 induces the down regulation and the ubiquitin-proteasome degradation pathway of p185HER2 oncoprotein

Analysis of the fate of the p185HER2 oncoprotein following activation by heregulin beta1 revealed the induction of the tyrosine-phosphorylation, down-modulation, and polyubiquitination of p185HER2. Receptor ubiquitination was suppressed in cells treated with heregulin beta1 in the presence of sodium azide, an inhibitor of ATP-dependent reactions, or genistein, a tyrosine kinase protein inhibitor, indicating the requirement for kinase activity and ATP in p185HER2 polyubiquitination. Ubiquitinated p185HER2 was degradated by the 26S proteasome proteolytic pathway. Kinetics and inhibition experiments indicated that endocytosis of the receptor occurs downstream of the initiation of the degradation process.

Ubiquitin and the control of protein fate in the secretory and endocytic pathways

The modification of proteins by chains of ubiquitin has long been known to mediate targeting of cytosolic and nuclear proteins for degradation by proteasomes. In this article, we discuss recent developments that reveal the involvement of ubiquitin in the degradation of proteins retained within the endoplasmic reticulum (ER) and in the internalization of plasma membrane proteins. Both luminal and transmembrane proteins retained in the ER are now known to be retrotranslocated into the cytosol in a process that involves ER chaperones and components of the protein import machinery. Once exposed to the cytosolic milieu, retro-translocated proteins are degraded by the proteasome, in most cases following polyubiquitination. There is growing evidence that both the ubiquitin-conjugating machinery and proteasomes may be associated with the cytosolic face of the ER membrane and that they could be functionally coupled to the process of retrotranslocation. The ubiquitination of plasma membrane proteins, on the other hand, mediates internalization of the proteins, which in most cases is followed by lysosomal/vacuolar degradation. There is, however, a well-documented case of a plasma membrane protein (the c-Met receptor) for which ubiquitination results in proteasomal degradation. These recent findings imply that ubiquitin plays more diverse roles in the regulation of the fate of cellular proteins than originally anticipated.

Identification of p185neu sequences required for monoclonal antibody- or ligand-mediated receptor signal attenuation

Anti-p185neu antibodies downmodulate constitutively active p185neu receptors from the cell surface, which is associated with a reduction in the transformed phenotype. We have analyzed a group of mutant p185neu forms with carboxyl (C)-terminal truncations and/or an internal deletion of amino acids 1008-1057. Receptor endocytosis and degradation were examined by flow cytometric analysis and pulse-chase assays following anti-p185neu monoclonal antibody (MAb) treatment. Deletion of a sequence within the distal carboxyl terminus, including three known autophosphorylation sites, did not affect MAb-mediated receptor surface downmodulation and degradation of surface receptor. However, kinase-active deletion mutants with elimination of the putative internalization sequence (Tint delta), or Tint delta mutants also containing a large C-terminal truncation, displayed markedly impaired receptor endocytosis in response to MAb treatment. Cells expressing endocytosis-defective mutant proteins became insensitive to anti-p185neu MAb-mediated inhibition of anchorage-independent growth and were more oncogenic in vivo. Cells expressing endocytosis-defective mutant EGFR/neu chimeric proteins were more transforming upon EGF addition when compared to cells expressing wild-type EGFR/neu receptors. Taken together, these data suggest that, in addition to kinase activity, p185neu receptor endocytosis requires a functional modular structure, i.e., an internalization sequence, possibly to serve as target for endocytotic adapter proteins. Unattenuated signaling from oncogenic p185neu forms resulting from prolonged surface localization may result in enhanced cellular transformation and desensitization to MAb-mediated downregulation and phenotypic reversion.

Identification of an ubiquitin-ligation system for the epidermal-growth-factor receptor--herbimycin A induces in vitro ubiquitination in rabbit-reticulocyte lysate

Some receptor tyrosine kinases such as the receptors for epidermal-growth factor (EGF) and platelet-derived growth factor undergo polyubiquitination as a consequence of ligand binding. The EGF receptor is also ubiquitinated by treatment with herbimycin A, an ansamycin antibiotic widely used as a tyrosine kinase inhibitor. To investigate the mechanism of the receptor ubiquitination, we have established an assay system in which herbimycin-A-induced ubiquitination processes can be analyzed in vitro. We now show that herbimycin A treatment of the purified EGF receptor induces polyubiquitination of the receptor in rabbit-reticulocyte lysate. Both DEAE unadsorbed material (fraction I) and high salt eluate (fraction II) of the reticulocyte lysate are involved cooperatively in the ubiquitination process, where the ubiquitin-conjugating enzyme UBC4 can functionally substitute for fraction I. A ubiquitin-protein ligase-like activity, partially purified from fraction II by DEAE anion-exchange chromatography, also functions in concert with UBC4. The precise mechanism of herbimycin A-induced ubiquitination of the EGF receptor is not fully understood, however, our present findings suggest that direct interaction with herbimycin A results in some modification of the receptor which is recognized by the ubiquitin-conjugating system in rabbit-reticulocyte lysate.

Increased ubiquitin immunoreactivity in hepatocellular carcinomas and precancerous lesions of the liver

BACKGROUND/AIMS

Ubiquitin covalently attaches to abnormal and short-lived proteins, thus marking them for ATP-dependent proteolysis in eukaryotic cells. Increased ubiquitin immunoreactivity was recently observed immunohistochemically in human malignant tumors. To clarify the change in protein metabolism during hepatocarcinogenesis, we studied ubiquitin immunoreactivity in hepatocellular carcinomas (HCCs) and precancerous lesions using immunohistochemistry and immunoblot analysis.

METHODS

A total of 72 HCCs (37 advanced, 19 early, 16 early-advanced (advanced HCC component in early HCC nodule) type HCCs) and 18 precancerous lesions (8 atypical adenomatous hyperplasias (AAHs), 10 adenomatous hyperplasias (AHs)) were studied immunohistochemically. Immunoblot analysis was also performed in advanced HCC and early HCC cases.

RESULTS

Non-tumorous hepatocytes were either immunonegative or weakly stained in their nuclei. Advanced HCCs showed strong immunoreactivity in most cases, while early HCCs showed relatively weaker immunoreactivity. In 14 of 16 early-advanced type tumors, the inner portion of the nodules, which corresponds to advanced HCC, showed stronger immunoreactivity than the outer low-grade portion. In 8 of 8 AAHs and 7 of 10 AHs, positive but weak staining was found. Immunoblot analysis showed an increase in 42 kDa ubiquitinated protein(s) in 8 of 16 advanced HCC cases (50%) and in 1 of 6 early HCC cases (16.7%), as well as an increase in several other bands in tumor tissues.

CONCLUSIONS

The intensity of ubiquitin staining appeared to increase in a stepwise manner from AH to advanced HCC, and the results suggest a possible correlation between changes in the ubiquitinated proteins and multistep hepatocarcinogenesis.