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

PDGFRα/β heterodimer activation negatively affects downstream ERK1/2 signaling and cellular proliferation

The platelet-derived growth factor receptor (PDGFR) family of receptor tyrosine kinases allows cells to communicate with one another by binding to growth factors at the plasma membrane and activating intracellular signaling pathways to elicit responses such as migration, proliferation, survival and differentiation. The PDGFR family consists of two receptors, PDGFRα and PDGFRβ, that dimerize to form PDGFRα homodimers, PDGFRα/β heterodimers and PDGFRβ homodimers. Here, we overcame prior technical limitations in visualizing and purifying PDGFRα/β heterodimers by generating a cell line stably expressing C-terminal fusions of PDGFRα and PDGFRβ with bimolecular fluorescence complementation fragments corresponding to the N-terminal and C-terminal regions of the Venus fluorescent protein, respectively. We found that these receptors heterodimerize relatively quickly in response to PDGF-BB ligand treatment, with a peak of receptor autophosphorylation following 5 minutes of ligand stimulation. Moreover, we demonstrated that PDGFRα/β heterodimers are rapidly internalized into early endosomes, particularly signaling endosomes, where they dwell for extended lengths of time. We showed that PDGFRα/β heterodimer activation does not induce downstream phosphorylation of ERK1/2 and significantly inhibits cell proliferation. Further, we characterized the PDGFR dimer-specific interactome and identified MYO1D as a novel protein that preferentially binds PDGFRα/β heterodimers. We demonstrated that knockdown of MYO1D leads to retention of PDGFRα/β heterodimers at the plasma membrane, resulting in increased phosphorylation of ERK1/2 and increased cell proliferation. Collectively, our findings impart valuable insight into the molecular mechanisms by which specificity is introduced downstream of PDGFR activation to differentially propagate signaling and generate distinct cellular responses.

RAB4A GTPase regulates epithelial-to-mesenchymal transition by modulating RAC1 activation

Epithelial-to-mesenchymal transition (EMT) is a critical underpinning process for cancer progression, recurrence and resistance to drug treatment. Identification of new regulators of EMT could lead to the development of effective therapies to improve the outcome of advanced cancers. In the current study we discovered, using a variety of in vitro and in vivo approaches, that RAB4A function is essential for EMT and related manifestation of stemness and invasive properties. Consistently, RAB4A suppression abolished the cancer cells' self-renewal and tumor forming ability. In terms of downstream signaling, we found that RAB4A regulation of EMT is achieved through its control of activation of the RAC1 GTPase. Introducing activated RAC1 efficiently rescued EMT gene expression, invasion and tumor formation suppressed by RAB4A knockdown in both the in vitro and in vivo cancer models. In summary, this study identifies a RAB4A-RAC1 signaling axis as a key regulatory mechanism for the process of EMT and cancer progression and suggests a potential therapeutic approach to controlling these processes.

PDGFR dimer-specific activation, trafficking and downstream signaling dynamics

Signaling through the platelet-derived growth factor receptors (PDGFRs) plays a critical role in multiple cellular processes during development. The two PDGFRs, PDGFRα and PDGFRβ, dimerize to form homodimers and/or heterodimers. Here, we overcome previous limitations in studying PDGFR dimer-specific dynamics by generating cell lines stably expressing C-terminal fusions of each PDGFR with bimolecular fluorescence complementation (BiFC) fragments corresponding to the N-terminal or C-terminal regions of the Venus fluorescent protein. We find that PDGFRβ receptors homodimerize more quickly than PDGFRα receptors in response to PDGF ligand, with increased levels of autophosphorylation. Furthermore, we demonstrate that PDGFRα homodimers are trafficked and degraded more quickly, whereas PDGFRβ homodimers are more likely to be recycled back to the cell membrane. We show that PDGFRβ homodimer activation results in a greater amplitude of phospho-ERK1/2 and phospho-AKT signaling, as well as increased proliferation and migration. Finally, we demonstrate that inhibition of clathrin-mediated endocytosis leads to changes in cellular trafficking and downstream signaling, particularly for PDGFRα homodimers. Collectively, our findings provide significant insight into how biological specificity is introduced to generate unique responses downstream of PDGFR engagement. This article has an associated First Person interview with the first author of the paper.

Increased abundance of Cbl E3 ligases alters PDGFR signaling in recessive dystrophic epidermolysis bullosa

In recessive dystrophic epidermolysis bullosa (RDEB), loss of collagen VII, the main component of anchoring fibrils critical for epidermal-dermal cohesion, affects several intracellular signaling pathways and leads to impaired wound healing and fibrosis. In skin fibroblasts, wound healing is also affected by platelet-derived growth factor receptor (PDGFR) signaling. To study a potential effect of loss of collagen VII on PDGFR signaling we performed unbiased disease phosphoproteomics. Whereas RDEB fibroblasts exhibited an overall weaker response to PDGF, Cbl E3 ubiquitin ligases, negative regulators of growth factor signaling, were stronger phosphorylated. This increase in phosphorylation was linked to higher Cbl mRNA and protein levels due to increased TGFβ signaling in RDEB. In turn, increased Cbl levels led to increased PDGFR ubiquitination, internalization, and degradation negatively affecting MAPK and AKT downstream signaling pathways. Thus, our results indicate that elevated TGFβ signaling leads to an attenuated response to growth factors, which contributes to impaired dermal wound healing in RDEB.

The complexities of PKCα signaling in cancer

Protein kinase C α (PKCα) is a ubiquitously expressed member of the PKC family of serine/threonine kinases with diverse functions in normal and neoplastic cells. Early studies identified anti-proliferative and differentiation-inducing functions for PKCα in some normal tissues (e.g., regenerating epithelia) and pro-proliferative effects in others (e.g., cells of the hematopoietic system, smooth muscle cells). Additional well documented roles of PKCα signaling in normal cells include regulation of the cytoskeleton, cell adhesion, and cell migration, and PKCα can function as a survival factor in many contexts. While a majority of tumors lose expression of PKCα, others display aberrant overexpression of the enzyme. Cancer-related mutations in PKCα are uncommon, but rare examples of driver mutations have been detected in certain cancer types (e. g., choroid gliomas). Here we review the role of PKCα in various cancers, describe mechanisms by which PKCα affects cancer-related cell functions, and discuss how the diverse functions of PKCα contribute to tumor suppressive and tumor promoting activities of the enzyme. We end the discussion by addressing mutations and expression of PKCα in tumors and the clinical relevance of these findings.

The emerging complexity of PDGFRs: activation, internalization and signal attenuation

The platelet-derived growth factor receptor (PDGFR) family of receptor tyrosine kinases allows cells to communicate with the environment to regulate diverse cellular activities. Here, we highlight recent data investigating the structural makeup of individual PDGFRs upon activation, revealing the importance of the whole receptor in the propagation of extracellular ligand binding and dimerization. Furthermore, we review ongoing research demonstrating the significance of receptor internalization and signal attenuation in the regulation of PDGFR activity. Interactions with internalization machinery, signaling from endosomes, receptor degradation and receptor recycling are physiological means by which cells fine-tune PDGFR responses to growth factor stimulation. In this review, we discuss the biophysical, structural, in silico and biochemical data that have provided evidence for these mechanisms. We further highlight the commonalities and differences between PDGFRα and PDGFRβ signaling, revealing critical gaps in knowledge. In total, this review provides a conclusive summary on the state of the PDGFR field and underscores the need for novel techniques to fully elucidate the mechanisms of PDGFR activation, internalization and signal attenuation.

Protein Kinase Cα Promotes Proliferation and Migration of Schwann Cells by Activating ERK Signaling Pathway

Wallerian degeneration (WD) and axon regeneration generally take place following peripheral nerve injury (PNI). Schwann cells (SCs) and macrophages play major role in WD. SCs, acting as repair cells and primary signal mediators, dedifferentiate and proliferate to remove the debris, form Büngner's bands and secrete trophic factors during these processes. However, the underlying mechanisms remain poorly understood. Here, we found that protein kinase Cα (PKCα), a serine/threonine kinase, expressed in SCs was significantly up-regulated after PNI. Activating PKCα with phorbol 12-myristate 13-acetate (PMA), a phorbol ester binds and activates PKCα) promoted SCs proliferation and migration. While, silence of PKCα by siRNAs inhibited these processes. PD184352, an inhibitor of MEK1, reversed the effect induced by PMA on SCs. Mechanism studies revealed that PKCα functioned through activating the ERK signaling pathway. Furthermore, PKCα also exhibited a neuroprotective role by upregulating the expression of neurotrophic factors in SCs. To sum up, this study offers novel insights for clarifying our understanding of the involvement of PKCα in the mechanism of peripheral nerve degeneration as well as regeneration.

Targeting Lysophosphatidic Acid in Cancer: The Issues in Moving from Bench to Bedside

Since the clear demonstration of lysophosphatidic acid (LPA)'s pathological roles in cancer in the mid-1990s, more than 1000 papers relating LPA to various types of cancer were published. Through these studies, LPA was established as a target for cancer. Although LPA-related inhibitors entered clinical trials for fibrosis, the concept of targeting LPA is yet to be moved to clinical cancer treatment. The major challenges that we are facing in moving LPA application from bench to bedside include the intrinsic and complicated metabolic, functional, and signaling properties of LPA, as well as technical issues, which are discussed in this review. Potential strategies and perspectives to improve the translational progress are suggested. Despite these challenges, we are optimistic that LPA blockage, particularly in combination with other agents, is on the horizon to be incorporated into clinical applications.

Receptor Tyrosine Kinase Ubiquitination and De-Ubiquitination in Signal Transduction and Receptor Trafficking

Receptor tyrosine kinases (RTKs) are membrane-based sensors that enable rapid communication between cells and their environment. Evidence is now emerging that interdependent regulatory mechanisms, such as membrane trafficking, ubiquitination, proteolysis and gene expression, have substantial effects on RTK signal transduction and cellular responses. Different RTKs exhibit both basal and ligand-stimulated ubiquitination, linked to trafficking through different intracellular compartments including the secretory pathway, plasma membrane, endosomes and lysosomes. The ubiquitin ligase superfamily comprising the E1, E2 and E3 enzymes are increasingly implicated in this post-translational modification by adding mono- and polyubiquitin tags to RTKs. Conversely, removal of these ubiquitin tags by proteases called de-ubiquitinases (DUBs) enables RTK recycling for another round of ligand sensing and signal transduction. The endocytosis of basal and activated RTKs from the plasma membrane is closely linked to controlled proteolysis after trafficking and delivery to late endosomes and lysosomes. Proteolytic RTK fragments can also have the capacity to move to compartments such as the nucleus and regulate gene expression. Such mechanistic diversity now provides new opportunities for modulating RTK-regulated cellular responses in health and disease states.

Involvement of platelet-derived growth factor ligands and receptors in tumorigenesis

Platelet-derived growth factor (PDGF) isoforms and their receptors have important roles during embryogenesis, particularly in the development of various mesenchymal cell types in different organs. In the adult, PDGF stimulates wound healing and regulates tissue homeostasis. However, overactivity of PDGF signalling is associated with malignancies and other diseases characterized by excessive cell proliferation, such as fibrotic conditions and atherosclerosis. In certain tumours, genetic or epigenetic alterations of the genes for PDGF ligands and receptors drive tumour cell proliferation and survival. Examples include the rare skin tumour dermatofibrosarcoma protuberance, which is driven by autocrine PDGF stimulation due to translocation of a PDGF gene, and certain gastrointestinal stromal tumours and leukaemias, which are driven by constitute activation of PDGF receptors due to point mutations and formation of fusion proteins of the receptors, respectively. Moreover, PDGF stimulates cells in tumour stroma and promotes angiogenesis as well as the development of cancer-associated fibroblasts, both of which promote tumour progression. Inhibitors of PDGF signalling may thus be of clinical usefulness in the treatment of certain tumours.

Different protein kinase C isoenzymes mediate inhibition of cardiac rapidly activating delayed rectifier K+ current by different G-protein coupled receptors

BACKGROUND AND PURPOSE

Elevated angiotensin II (Ang II) and sympathetic activity contributes to a high risk of ventricular arrhythmias in heart disease. The rapidly activating delayed rectifier K⁺ current (I_Kr ) carried by the hERG channels plays a critical role in cardiac repolarization, and decreased I_Kr is involved in increased cardiac arrhythmogenicity. Stimulation of α_1A -adrenoreceptors or angiotensin II AT₁ receptors is known to inhibit I_Kr via PKC. Here, we have identified the PKC isoenzymes mediating the inhibition of I_Kr by activation of these two different GPCRs.

EXPERIMENTAL APPROACH

The whole-cell patch-clamp technique was used to record I_Kr in guinea pig cardiomyocytes and HEK293 cells co-transfected with hERG and α_1A -adrenoreceptor or AT₁ receptor genes.

KEY RESULTS

A broad spectrum PKC inhibitor Gö6983 (not inhibiting PKCε), a selective cPKC inhibitor Gö6976 and a PKCα-specific inhibitor peptide, blocked the inhibition of I_Kr by the α_1A -adrenoreceptor agonist A61603. However, these inhibitors did not affect the reduction of I_Kr by activation of AT₁ receptors, whereas the PKCε-selective inhibitor peptide did block the effect. The effects of angiotensin II and the PKCε activator peptide were inhibited in mutant hERG channels in which 17 of the 18 PKC phosphorylation sites were deleted, whereas a deletion of the N-terminus of the hERG channels selectively prevented the inhibition elicited by A61603 and the cPKC activator peptide.

CONCLUSIONS AND IMPLICATIONS

Our results indicated that inhibition of I_Kr by activation of α_1A -adrenoreceptors or AT₁ receptors were mediated by PKCα and PKCε isoforms respectively, through different molecular mechanisms.

Neuropilin 1 binds platelet-derived growth factor (PDGF)-D and is a co-receptor in PDGF-D/PDGF receptor β signaling

Platelet-derived growth factor (PDGF)-D is a PDGF receptor β (PDGFRβ) specific ligand implicated in a number of pathological conditions, such as cardiovascular disease and cancer, but its biological function remains incompletely understood.

In this study, we demonstrate that PDGF-D binds directly to NRP1, with the requirement of the C-terminal Arg residue of PDGF-D. Stimulation with PDGF-D, but not PDGF-B, induced PDGFRβ/NRP1 complex formation in fibroblasts. Additionally, PDGF-D induced translocation of NRP1 to cell-cell junctions in endothelial cells, independent of PDGFRβ, altering the availability of NRP1 for VEGF-A/VEGF receptor 2 signaling. PDGF-D showed differential effects on pericyte behavior in ex vivo sprouting assays, compared to PDGF-B. Furthermore, PDGF-D induced PDGFRβ/NRP1 interaction in the trans-configuration between endothelial cells and pericytes.

In summary, we show that NRP1 can act as a co-receptor for PDGF-D in PDGFRβ signaling, possibly implicated in intercellular communication in the vascular wall.

Protein Kinase C-α is a Critical Protein for Antisense Oligonucleotide-mediated Silencing in Mammalian Cells

We have identified the existence of a productive, PKC-α-dependent endocytotic silencing pathway that leads gymnotically-delivered locked nucleic acid (LNA)-gapmer phosphorothioate antisense oligonucleotides (ASOs) into late endosomes. By blocking the maturation of early endosomes to late endosomes, silencing the expression of PKC-α results in the potent reduction of ASO silencing ability in the cell. We have also demonstrated that silencing of gene expression in the cytoplasm is vitiated when PKC-α expression is reduced. Restoring PKC-α expression via a reconstitution experiment reinstates the ability of ASOs to silence. These results advance our understanding of intracellular ASO trafficking and activity following gymnotic delivery, and further demonstrate the existence of two distinct silencing pathways in mammalian cells, one in the cytoplasmic and the other in the nuclear compartment.

On the move: endocytic trafficking in cell migration

Directed cell migration is a fundamental process underlying diverse physiological and pathophysiological phenomena ranging from wound healing and induction of immune responses to cancer metastasis. Recent advances reveal that endocytic trafficking contributes to cell migration in multiple ways. (1) At the level of chemokines and chemokine receptors: internalization of chemokines by scavenger receptors is essential for shaping chemotactic gradients in tissue, whereas endocytosis of chemokine receptors and their subsequent recycling is key for maintaining a high responsiveness of migrating cells. (2) At the level of integrin trafficking and focal adhesion dynamics: endosomal pathways do not only modulate adhesion by delivering integrins to their site of action, but also by supplying factors for focal adhesion disassembly. (3) At the level of extracellular matrix reorganization: endosomal transport contributes to tumor cell migration not only by targeting integrins to invadosomes but also by delivering membrane type 1 matrix metalloprotease to the leading edge facilitating proteolysis-dependent chemotaxis. Consequently, numerous endocytic and endosomal factors have been shown to modulate cell migration. In fact key modulators of endocytic trafficking turn out to be also key regulators of cell migration. This review will highlight the recent progress in unraveling the contribution of cellular trafficking pathways to cell migration.

Rab proteins and the compartmentalization of the endosomal system

Of the approximately 70 human Rab GTPases, nearly three-quarters are involved in endocytic trafficking. Significant plasticity in endosomal membrane transport pathways is closely coupled to receptor signaling and Rab GTPase-regulated scaffolds. Here we review current literature pertaining to endocytic Rab GTPase localizations, functions, and coordination with regulatory proteins and effectors. The roles of Rab GTPases in (1) compartmentalization of the endocytic pathway into early, recycling, late, and lysosomal routes; (2) coordination of individual transport steps from vesicle budding to fusion; (3) effector interactomes; and (4) integration of GTPase and signaling cascades are discussed.

Rab proteins and the compartmentalization of the endosomal system

Of the approximately 70 human Rab GTPases, nearly three-quarters are involved in endocytic trafficking. Significant plasticity in endosomal membrane transport pathways is closely coupled to receptor signaling and Rab GTPase-regulated scaffolds. Here we review current literature pertaining to endocytic Rab GTPase localizations, functions, and coordination with regulatory proteins and effectors. The roles of Rab GTPases in (1) compartmentalization of the endocytic pathway into early, recycling, late, and lysosomal routes; (2) coordination of individual transport steps from vesicle budding to fusion; (3) effector interactomes; and (4) integration of GTPase and signaling cascades are discussed.

Platelet-derived growth factor β-receptor, transforming growth factor β type I receptor, and CD44 protein modulate each other's signaling and stability

Growth factors, such as platelet-derived growth factor BB (PDGF-BB) and transforming growth factor β (TGFβ), are key regulators of cellular functions, including proliferation, migration, and differentiation. Growth factor signaling is modulated by context-dependent cross-talk between different signaling pathways. We demonstrate in this study that PDGF-BB induces phosphorylation of Smad2, a downstream mediator of the canonical TGFβ pathway, in primary dermal fibroblasts. The PDGF-BB-mediated Smad2 phosphorylation was dependent on the kinase activities of both TGFβ type I receptor (TβRI) and PDGF β-receptor (PDGFRβ), and it was prevented by inhibitory antibodies against TGFβ. Inhibition of the activity of the TβRI kinase greatly reduced the PDGF-BB-dependent migration in dermal fibroblasts. Moreover, we demonstrate that the receptors for PDGF-BB and TGFβ interact physically in primary dermal fibroblasts and that stimulation with PDGF-BB induces internalization not only of PDGFRβ but also of TβRI. In addition, silencing of PDGFRβ by siRNA decreased the stability of TβRI and delayed TGFβ-induced signaling. We further show that the hyaluronan receptor CD44 interacts with both PDGFRβ and TβRI. Depletion of CD44 by siRNA increased signaling via PDGFRβ and TβRI by stabilizing the receptor proteins. Our data suggest that cross-talk between PDGFRβ and TβRI occurs in dermal fibroblasts and that CD44 negatively modulates signaling via these receptors.

Labeling of platelet-derived growth factor by reversible biotinylation to visualize its endocytosis by microscopy

Microscopical analyses of endocytic trafficking require tools for efficient detection of internalized cargo. Due to the lack of suitable reagents and limitations related to its biological properties, visualization of platelet-derived growth factor (PDGF) by microscopy remained a challenge. To overcome these restrictions, we generated a biologically active PDGF labeled with up to five biotins on cleavable linkers. Subsequently, we stimulated cells with such ligand followed by removal of extracellular biotins. PDGF captured in endocytic vesicles was successfully detected with antibiotin antibodies with parallel detection of PDGF receptor, as well as other markers of endocytic compartments. Labeled PDGF was successfully validated and can be utilized in various microscopical techniques.

Targeting the PDGF signaling pathway in tumor treatment

Platelet-derived growth factor (PDGF) isoforms and PDGF receptors have important functions in the regulation of growth and survival of certain cell types during embryonal development and e.g. tissue repair in the adult. Overactivity of PDGF receptor signaling, by overexpression or mutational events, may drive tumor cell growth. In addition, pericytes of the vasculature and fibroblasts and myofibroblasts of the stroma of solid tumors express PDGF receptors, and PDGF stimulation of such cells promotes tumorigenesis. Inhibition of PDGF receptor signaling has proven to useful for the treatment of patients with certain rare tumors. Whether treatment with PDGF/PDGF receptor antagonists will be beneficial for more common malignancies is the subject for ongoing studies.

Effects of membrane trafficking on signaling by receptor tyrosine kinases

The intracellular trafficking machinery contributes to the spatial and temporal control of signaling by receptor tyrosine kinases (RTKs). The primary role in this process is played by endocytic trafficking, which regulates the localization of RTKs and their downstream effectors, as well as the duration and the extent of their activity. The key regulatory points along the endocytic pathway are internalization of RTKs from the plasma membrane, their sorting to degradation or recycling, and their residence in various endosomal compartments. Here I will review factors and mechanisms that modulate RTK signaling by (1) affecting receptor internalization, (2) regulating the balance between degradation and recycling of RTK, and (3) compartmentalization of signals in endosomes and other organelles. Cumulatively, these mechanisms illustrate a multilayered control of RTK signaling exerted by the trafficking machinery.

Transcriptomic and phospho-proteomic analyzes of erythroblasts expanded in vitro from normal donors and from patients with polycythemia vera

Erythropoiesis is a tightly regulated process which becomes decoupled from its normal differentiation program in patients with polycythemia vera (PV). Somatic mutations in JAK2 are commonly associated with this myeloid proliferative disorder. To gain insight into the molecular events that are required for abnormally developing erythroid cells to escape dependence on normal growth signals, we performed in vitro expansion of mature erythroblasts (ERY) from seven normal healthy donors and from seven polycythemic patients in the presence of IL3, EPO, SCF for 10, 11, or 13 days. Normal ERYs required exposure to the glucocorticoid dexamethasone (Dex) for expansion, while PV-derived ERYs expanded in the absence of dexamethasone. RNA expression profiling revealed enrichment of two known oncogenes, GPR56 and RAB4a, in PV-derived ERYs along with reduced expression levels of transcription factor TAL1 (ANOVA FDR < 0.05). While both normal and polycythemic-derived ERYs integrated signaling cascades for growth, they did so via different signaling pathways which are represented by their differential phospho-profiles. Our results show that normal ERYs displayed greater levels of phosphorylation of EGFR, PDGFRβ, TGFβ, and cKit, while PV-derived ERYs were characterized by increased phosphorylation of cytoplasmic kinases in the JAK/STAT, PI3K, and GATA1 pathways. Together these data suggest that PV erythroblast expansion and maturation may be maintained and enriched in the absence of dexamethasone through reduced TAL1 expression and by accessing additional signaling cascades. Members of this acquired repertoire may provide important insight into the pathogenesis of aberrant erythropoiesis in myeloproliferative neoplasms such as polycythemia vera.

Structural and functional properties of platelet-derived growth factor and stem cell factor receptors

The receptors for platelet-derived growth factor (PDGF) and stem cell factor (SCF) are members of the type III class of PTK receptors, which are characterized by five Ig-like domains extracellularly and a split kinase domain intracellularly. The receptors are activated by ligand-induced dimerization, leading to autophosphorylation on specific tyrosine residues. Thereby the kinase activities of the receptors are activated and docking sites for downstream SH2 domain signal transduction molecules are created; activation of these pathways promotes cell growth, survival, and migration. These receptors mediate important signals during the embryonal development, and control tissue homeostasis in the adult. Their overactivity is seen in malignancies and other diseases involving excessive cell proliferation, such as atherosclerosis and fibrotic diseases. In cancer, mutations of PDGF and SCF receptors-including gene fusions, point mutations, and amplifications-drive subpopulations of certain malignancies, such as gastrointestinal stromal tumors, chronic myelomonocytic leukemia, hypereosinophilic syndrome, glioblastoma, acute myeloid leukemia, mastocytosis, and melanoma.

Protein kinase C-δ-mediated recycling of active KIT in colon cancer

PURPOSE

Abnormal signaling through receptor tyrosine kinase (RTK) moieties is important in tumorigenesis and drug targeting of colorectal cancers. Wild-type KIT (WT-KIT), a RTK that is activated upon binding with stem cell factor (SCF), is highly expressed in some colon cancers; however, little is known about the functional role of SCF-dependent KIT activation in colon cancer pathogenesis. We aimed to elucidate the conditions and roles of WT-KIT activation in colon cancer tumorigenesis.

EXPERIMENTAL DESIGN

Colorectal cancers with KIT expression were characterized by immunoblotting and immunohistochemistry. The biologic alterations after KIT-SCF binding were analyzed with or without protein kinase C (PKC) activation.

RESULTS

We found that WT-KIT was expressed in a subset of colon cancer cell lines and was activated by SCF, leading to activation of downstream AKT and extracellular signal-regulated kinase (ERK) signaling pathways. We also showed that KIT expression gradually decreased, after prolonged SCF stimulation, due to lysosomal degradation. Degradation of WT-KIT after SCF binding was significantly rescued when PKC was activated. We also showed the involvement of activated PKC-δ in the recycling of WT-KIT. We further showed that a subset of colorectal cancers exhibit expressions of both WT-KIT and activated PKC-δ and that expression of KIT is correlated with poor patient survival (P = 0.004).

CONCLUSIONS

Continuous downstream signal activation after KIT-SCF binding is accomplished through PKC-δ-mediated recycling of KIT. This sustained KIT activation may contribute to tumor progression in a subset of colon cancers with KIT expression and might provide the rationale for a therapeutic approach targeting KIT.

Fibroblast Migration Is Regulated by Myristoylated Alanine-Rich C-Kinase Substrate (MARCKS) Protein

Myristoylated alanine-rich C-kinase substrate (MARCKS) is a ubiquitously expressed substrate of protein kinase C (PKC) that is involved in reorganization of the actin cytoskeleton. We hypothesized that MARCKS is involved in regulation of fibroblast migration and addressed this hypothesis by utilizing a unique reagent developed in this laboratory, the MANS peptide. The MANS peptide is a myristoylated cell permeable peptide corresponding to the first 24-amino acids of MARCKS that inhibits MARCKS function. Treatment of NIH-3T3 fibroblasts with the MANS peptide attenuated cell migration in scratch wounding assays, while a myristoylated, missense control peptide (RNS) had no effect. Neither MANS nor RNS peptide treatment altered NIH-3T3 cell proliferation within the parameters of the scratch assay. MANS peptide treatment also resulted in inhibited NIH-3T3 chemotaxis towards the chemoattractant platelet-derived growth factor-BB (PDGF-BB), with no effect observed with RNS treatment. Live cell imaging of PDGF-BB induced chemotaxis demonstrated that MANS peptide treatment resulted in weak chemotactic fidelity compared to RNS treated cells. MANS and RNS peptides did not affect PDGF-BB induced phosphorylation of MARCKS or phosphoinositide 3-kinase (PI3K) signaling, as measured by Akt phosphorylation. Further, no difference in cell migration was observed in NIH-3T3 fibroblasts that were transfected with MARCKS siRNAs with or without MANS peptide treatment. Genetic structure-function analysis revealed that MANS peptide-mediated attenuation of NIH-3T3 cell migration does not require the presence of the myristic acid moiety on the amino-terminus. Expression of either MANS or unmyristoylated MANS (UMANS) C-terminal EGFP fusion proteins resulted in similar levels of attenuated cell migration as observed with MANS peptide treatment. These data demonstrate that MARCKS regulates cell migration and suggests that MARCKS-mediated regulation of fibroblast migration involves the MARCKS amino-terminus. Further, this data demonstrates that MANS peptide treatment inhibits MARCKS function during fibroblast migration and that MANS mediated inhibition occurs independent of myristoylation.

Endocytosis of receptor tyrosine kinases

Endocytosis is the major regulator of signaling from receptor tyrosine kinases (RTKs). The canonical model of RTK endocytosis involves rapid internalization of an RTK activated by ligand binding at the cell surface and subsequent sorting of internalized ligand-RTK complexes to lysosomes for degradation. Activation of the intrinsic tyrosine kinase activity of RTKs results in autophosphorylation, which is mechanistically coupled to the recruitment of adaptor proteins and conjugation of ubiquitin to RTKs. Ubiquitination serves to mediate interactions of RTKs with sorting machineries both at the cell surface and on endosomes. The pathways and kinetics of RTK endocytic trafficking, molecular mechanisms underlying sorting processes, and examples of deviations from the standard trafficking itinerary in the RTK family are discussed in this work.

The role of endosomal signaling triggered by metastatic growth factors in tumor progression

Within tumor microenvironment, a lot of growth factors such as hepatocyte growth factor and epidermal growth factor may induce similar signal cascade downstream of receptor tyrosine kinase (RTK) and trigger tumor metastasis synergistically. In the past decades, the intimate relationship of RTK-mediated receptor endocytosis with signal transduction was well established. In general, most RTK undergoes clathrin-dependent endocytosis and/or clathrin-independent endocytosis. The internalized receptors may sustain the signaling within early endosome, recycling to plasma membrane for subsequent ligand engagement or sorting to late endosomes/lysosome for receptor degradation. Moreover, receptor endocytosis influences signal transduction in a temporal and spatial manner for periodical and polarized cellular processes such as cell migration. The endosomal signalings triggered by various metastatic factors are quite similar in some critical points, which are essential for triggering cell migration and tumor progression. There are common regulators for receptor endocytosis including dynamin, Rab4, Rab5, Rab11 and Cbl. Moreover, many critical regulators within the RTK signal pathway such as Grb2, p38, PKC and Src were also modulators of endocytosis. In the future, these may constitute a new category of targets for prevention of tumor metastasis.

Dynamin inhibitors impair endocytosis and mitogenic signaling of PDGF

Platelet-derived growth factor (PDGF) isoforms regulate cell proliferation, migration and differentiation both in embryonic development and adult tissue remodeling. At the cellular level, growth-factor signaling is often modulated by endocytosis. Despite important functions of PDGF, its endocytosis remains poorly studied, mainly for lack of tools to track internalized ligand by microscopy. Here, we developed such a tool and quantitatively analyzed internalization and endosomal trafficking of PDGF-BB in human fibroblasts. We further show that PDGF can be internalized in the presence of dynamin inhibitors, arguing that both dynamin-dependent and dynamin-independent pathways can mediate PDGF uptake. Although these routes operate with somewhat different kinetics, they both ultimately lead to lysosomal degradation of PDGF. Although acute inhibition of dynamin activity only moderately affects PDGF endocytosis, it specifically decreases downstream signaling of PDGF via signal transducer and activator of transcription 3 (STAT3). This correlates with reduced expression of MYC and impaired cell entry into S-phase, indicating that dynamin activity is required for PDGF-induced mitogenesis. Our data support a general view that the components governing endocytic trafficking may selectively regulate certain signaling effectors activated by a growth factor.

TBC1D16 is a Rab4A GTPase activating protein that regulates receptor recycling and EGF receptor signaling

Rab4A is a master regulator of receptor recycling from endocytic compartments to the plasma membrane. The protein TBC1D16 is up-regulated in melanoma, and TBC1D16-overexpressing melanoma cells are dependent on TBC1D16. We show here that TBC1D16 enhances the intrinsic rate of GTP hydrolysis by Rab4A. TBC1D16 is both cytosolic and membrane associated; the membrane-associated pool colocalizes with transferrin and EGF receptors (EGFRs) and early endosome antigen 1, but not with LAMP1 protein. Expression of two TBC1D16 isoforms, but not the inactive R494A mutant, reduces transferrin receptor recycling but has no effect on transferrin receptor internalization. Expression of TBC1D16 alters GFP-Rab4A membrane localization. In HeLa cells, overexpression of TBC1D16 enhances EGF-stimulated EGFR degradation, concomitant with decreased EGFR levels and signaling. Thus, TBC1D16 is a GTPase activating protein for Rab4A that regulates transferrin receptor recycling and EGFR trafficking and signaling.

Macropinocytosis of the PDGF β-receptor promotes fibroblast transformation by H-RasG12V

Fibroblast transformation by H-RasG12V induces internalization of PDGFRβ by macropinocytosis, enhancing its signaling activity and increasing anchorage-independent proliferation. It is proposed that H-Ras transformation promotes tumor progression by enhancing growth factor receptor signaling through increased receptor macropinocytosis.

Receptor tyrosine kinase (RTK) signaling is frequently increased in tumor cells, sometimes as a result of decreased receptor down-regulation. The extent to which the endocytic trafficking routes can contribute to such RTK hyperactivation is unclear. Here, we show for the first time that fibroblast transformation by H-RasG12V induces the internalization of platelet-derived growth factor β-receptor (PDGFRβ) by macropinocytosis, enhancing its signaling activity and increasing anchorage-independent proliferation. H-RasG12V transformation and PDGFRβ activation were synergistic in stimulating phosphatidylinositol (PI) 3-kinase activity, leading to receptor macropinocytosis. PDGFRβ macropinocytosis was both necessary and sufficient for enhanced receptor activation. Blocking macropinocytosis by inhibition of PI 3-kinase prevented the increase in receptor activity in transformed cells. Conversely, increasing macropinocytosis by Rabankyrin-5 overexpression was sufficient to enhance PDGFRβ activation in nontransformed cells. Simultaneous stimulation with PDGF-BB and epidermal growth factor promoted macropinocytosis of both receptors and increased their activation in nontransformed cells. We propose that H-Ras transformation promotes tumor progression by enhancing growth factor receptor signaling as a result of increased receptor macropinocytosis.

PDGF: the nuts and bolts of signalling toolbox

PDGF is a growth factor and is extensively involved in multi-dimensional cellular dynamics. It switches on a plethora of molecules other than its classical pathway. It is engaged in various transitions of development; however, if the unleashed potentials lead astray, it brings forth tumourigenesis. Conventionally, it has been assumed that the components of this signalling pathway show fidelity and act with a high degree of autonomy. However, as illustrated by the PDGF signal transduction, reinterpretation of recent data suggests that machinery is often shared between multiple pathways, and other components crosstalk to each other through multiple mechanisms. It is important to note that metastatic cascade is an intricate process that we have only begun to understand in recent years. Many of the early steps of this PDGF cascade are not readily targetable in the clinic. In this review, we will unravel the paradoxes with reference to mitrons and cellular plasticity and discuss how disruption of signalling cascade triggers cellular proliferation phase transition and metastasis. We will also focus on the therapeutic interventions to counteract resultant molecular disorders.

Cooperation between integrins and growth factor receptors in signaling and endocytosis

All multicellular animals express receptors for growth factors (GFs) and extracellular matrix (ECM) molecules. Integrin-type ECM receptors anchor cells to their surroundings and concomitantly activate intracellular signal transduction pathways. The same signaling mechanisms are regulated by GF receptors (GFRs). Recently, intensive research efforts have revealed novel mechanisms describing how the two receptor systems collaborate at many different levels. Integrins can directly bind to GFs and promote their activation. Adhesion receptors also organize signaling platforms and assist GFRs or even activate them via ligand-independent mechanisms. Furthermore, integrins can orchestrate endocytosis and recycling of GFRs. Here, we review the present knowledge about the interplay between integrins and GFRs and discuss recent ideas of how this collaboration may explain some previous controversies in integrin research.

Role of platelet-derived growth factors in the testis

Normal development and function of the testis are controlled by endocrine and paracrine signaling pathways. Platelet-derived growth factors (PDGFs) are growth factors that mediate epithelial-mesenchymal interactions in various tissues during normal and abnormal processes such as embryo development, wound healing, tissue fibrosis, vascular disorders, and cancer. PDGFs and their receptors (PDGFRs) have emerged as key players in the regulation of embryonic and postnatal development of the male gonad. Cells that express PDGFs and PDGFRs are found in the testis of mammals, birds, and reptiles, and their distribution, regulation, and function vary across species. Testicular PDGFs and PDGFRs appear after the process of sex determination in animals that use either genetic sex determination or environmental sex determination. Sertoli cells are the main PDGF-producing cells during the entire period of prenatal and postnatal testis development. Fetal Leydig cells and their precursors, adult Leydig cells and their stem cell precursors, peritubular myoid cells, cells of the blood vessels, and gonocytes are the testicular cell types expressing PDGFRs. Genetically targeted deletions of PDGFs, PDGFRs, PDGFR target genes or pharmacological silencing of PDGF signaling produce profound damage on the target cells that, depending on the developmental period, are under direct or indirect control of PDGF. PDGF signaling may also serve diverse functions outside of the realm of testis development, including testicular tumors. In this review, we provide a framework of the current knowledge to clarify the useful information regarding how PDGFs function in individual cells of the testis.

Effects of Lewis Y antigen on the gene expression of multiple drug resistance-associated proteins in human ovarian cancer RMG-I-H cells

The effects of Lewis Y antigen on the gene expression of multiple drug resistance-associated proteins in human ovarian cancer RMG-I-H cells were unclear by now. In this study, we detected the gene expression of multiple drug resistance-associated proteins (MRP) in RMG-I-H cells and RMG-I-H cells treated with anti-Lewis Y monoclonal antibody to investigate the association between Lewis Y antigen and the gene expression of drug resistance-associated proteins. Compared with RMG-I cells, the expression of MRP1, MRP2, protein kinase C-alpha (PKC-alpha), and topoisomerase I (Topo I) mRNAs in RMG-I-H cells were significantly upregulated, while the MDR-1 mRNA was downregulated. Immunochemistry analyses indicated that the in vitro and in vivo expression levels of MDR-1 protein (P-gp) in RMG-I-H cells were significantly higher than those in RMG-I cells. After RMG-I-H cells were treated with anti-Lewis Y monoclonal antibody, the expression levels of MDR-1, MRP1, MRP2, PKC-alpha, and Topo I mRNAs gradually decreased with the prolongation of treatment duration. In contrast, no obvious changes were noted in the expression levels of these mRNAs in the non-treatment group. At 6 h after treatment, the relative levels of MDR-1, MRP1, MRP2, PKC-alpha, and Topo I mRNAs in the antibody treatment group were significantly lower than those in the non-treatment group. In conclusion, Lewis Y antigen is closely associated with regulating the gene expression of multiple drug resistance-associated proteins.

Critical role of the platelet-derived growth factor receptor (PDGFR) beta transmembrane domain in the TEL-PDGFRbeta cytosolic oncoprotein

The fusion of TEL with platelet-derived growth factor receptor (PDGFR) beta (TPbeta) is found in a subset of patients with atypical myeloid neoplasms associated with eosinophilia and is the archetype of a larger group of hybrid receptors that are produced by rearrangements of PDGFR genes. TPbeta is activated by oligomerization mediated by the pointed domain of TEL/ETV6, leading to constitutive activation of the PDGFRbeta kinase domain. The receptor transmembrane (TM) domain is retained in TPbeta and in most of the described PDGFRbeta hybrids. Deletion of the TM domain (DeltaTM-TPbeta) strongly impaired the ability of TPbeta to sustain growth factor-independent cell proliferation. We confirmed that TPbeta resides in the cytosol, indicating that the PDGFRbeta TM domain does not act as a transmembrane domain in the context of the hybrid receptor but has a completely different function. The DeltaTM-TPbeta protein was expressed at a lower level because of increased degradation. It could form oligomers, was phosphorylated at a slightly higher level, co-immunoprecipitated with the p85 adaptor protein, but showed a much reduced capacity to activate STAT5 and ERK1/2 in Ba/F3 cells, compared with TPbeta. In an in vitro kinase assay, DeltaTM-TPbeta was more active than TPbeta and less sensitive to imatinib, a PDGFR inhibitor. In conclusion, we show that the TM domain is required for TPbeta-mediated signaling and proliferation, suggesting that the activation of the PDGFRbeta kinase domain is not enough for cell transformation.

Interplay between cell adhesion and growth factor receptors: from the plasma membrane to the endosomes

The emergence of multicellular animals could only take place once evolution had produced molecular mechanisms for cell adhesion and communication. Today, all metazoans express integrin-type adhesion receptors and receptors for growth factors. Integrins recognize extracellular matrix proteins and respective receptors on other cells and, following ligand binding, can activate the same cellular signaling pathways that are regulated by growth factor receptors. Recent reports have indicated that the two receptor systems also collaborate in many other ways. Here, we review the present information concerning the role of integrins as assisting growth factor receptors and the interplay between the receptors in cell signaling and in the orchestration of receptor recycling.