Phosphatidylinositol 3-kinase and Ca2+ influx dependence for ligand-stimulated internalization of the c-Kit receptor

KIT D816V is dimerization-independent and activates downstream pathways frequently perturbed in mastocytosis

KIT, a type III tyrosine kinase receptor, plays a crucial role in haematopoietic development. The KIT receptor forms a dimer after ligand binding; this activates tyrosine kinase activity leading to downstream signal transduction. The D816V KIT mutation is extensively implicated in haematological malignancies, including mastocytosis and leukaemia. KIT D816V is constitutively active, but the molecular nuances that lead to constitutive tyrosine kinase activity are unclear. For the first time, we present experimental evidence that the KIT D816V mutant does not dimerize like KIT wild type. We further show evidence of decreased stabilization of the tyrosine kinase domain in the KIT D816V mutant, a phenomenon that might contribute to its constitutive activity. Since the mechanism of KIT D816V activation varies from that of the wild type, we explored downstream signal transduction events and found that even though KIT D816V targets similar signalling moieties, the signalling is amplified in the mutant compared to stem cell factor-activated wild type receptor. Uniquely, KIT D816V induces infection-related pathways and the spliceosome pathway, providing alternate options for selective as well as combinatorial therapeutic targeting.

Control of Oocyte Reawakening by Kit

In mammals, females are born with finite numbers of oocytes stockpiled as primordial follicles. Oocytes are “reawakened” via an ovarian-intrinsic process that initiates their growth. The forkhead transcription factor Foxo3 controls reawakening downstream of PI3K-AKT signaling. However, the identity of the presumptive upstream cell surface receptor controlling the PI3K-AKT-Foxo3 axis has been questioned. Here we show that the receptor tyrosine kinase Kit controls reawakening. Oocyte-specific expression of a novel constitutively-active Kit^D818V allele resulted in female sterility and ovarian failure due to global oocyte reawakening. To confirm this result, we engineered a novel loss-of-function allele, Kit^L. Kit inactivation within oocytes also led to premature ovarian failure, albeit via a contrasting phenotype. Despite normal initial complements of primordial follicles, oocytes remained dormant with arrested oocyte maturation. Foxo3 protein localization in the nucleus versus cytoplasm explained both mutant phenotypes. These genetic studies provide formal genetic proof that Kit controls oocyte reawakening, focusing future investigations into the causes of primary ovarian insufficiency and ovarian aging.

In mammals, oocyte reawakening controls female fertility, the onset of the menopause, and thus, overall aging. We demonstrate here through complementary genetic experiments that Kit is the upstream receptor regulating oocyte reawakening. Although other cell surface receptors have been proposed as candidates, the data have remained contradictory, and definitive genetic evidence in support of any one receptor has been lacking. We engineered two novel Kit alleles in mice, one an activating (gain-of-function) mutation, the other an inactivating (loss-of-function) mutation. These alleles permitted us to conduct elegant genetic experiments whereby Kit was activated or inactivated in the oocytes of newborn mice. The results were complementary and striking. Oocyte-specific Kit activation resulted in female sterility due to reawakening of all oocytes, leading to premature ovarian failure. In contrast, Kit inactivation also led to female sterility and ovarian failure, but through a contrasting and opposite phenotype: a complete failure of primordial follicle reawakening. Additional studies demonstrated that Foxo3, a known regulator of reawakening, was the mediator of both phenotypes, linking our findings to prior discoveries. These complementary genetic experiments thus definitively incriminate Kit as the upstream receptor regulating reawakening.

Spermatogonial Nature of the Germ Cell Component of Canine Testicular Mixed Germ Cell-Sex Cord Stromal Tumours

The present study has characterized the germ cell component of canine testicular mixed germ cell-sex cord stromal tumours (MGSCTs) by examining the histological nature and histochemical and immunohistochemical features using gonocytic and spermatogonial cellular markers, c-Kit, placental alkaline phosphatase (PLAP), protein gene product 9.5 (PGP9.5), Sal-like protein 4 (SALL4), and the periodic acid-Schiff (PAS) reaction. Histologically, all 45 examples of MGSCTs were classified as spermatocytic seminomas (SSs) and Sertoli cell tumours in combination. The germ cell component of all MGSCTs was negative by PAS staining. Immunohistochemically, PLAP immunoreactivity was lacking in the germ cell component of all MGSCTs, which is not consistent with a gonocytic origin. The germ cell component was positive for PGP9.5 and SALL4 in all MGSCTs and positive for c-Kit in 53% of MGSCTs, which is consistent with the phenotype of spermatogonia. Furthermore, the germ cell component in 71% of MGSCTs had moderate immunoreactivity for SALL4, which is suggestive of a spermatogonial phenotype. Conversely, 29% of cases had a minor population of germ cells showing strong SALL4 immunoreactivity, suggesting a phenotype similar to prespermatogonia. The results suggest that the germ cell component of canine MGSCTs is morphologically classified as SS, with the majority of cases showing the spermatogonial phenotype and some cases containing a small population of prespermatogonia.

Bortezomib enhances the therapeutic efficacy of dasatinib by promoting c-KIT internalization-induced apoptosis in gastrointestinal stromal tumor cells

Dasatinib-based therapy is often used as a second-line therapeutic strategy for imatinib-resistance gastrointestinal stromal tumors (GISTs); however, acquired aberrant activation of dasatinib target proteins, such as c-KIT and PDGFRβ, attenuates the therapeutic efficiency of dasatinib. Combination therapy which inhibits the activation of dasatinib target proteins may enhance the cytotoxicity of dasatinib in GISTs. Bortezomib, a proteasome inhibitor, significantly inhibited cell viability and promoted apoptosis of dasatinib-treated GIST-T1 cells, whereas GIST-T1 cells showed little dasatinib cytotoxicity when treated with dasatinib alone, as the upregulation of c-KIT caused by dasatinib itself interfered with the inhibition of c-KIT and PDGFRβ phosphorylation by dasatinib. Bortezomib induced internalization and degradation of c-KIT by binding c-KIT to Cbl, an E3 ubiquitin-protein ligase, and the subsequent release of Apaf-1, which was originally bound to the c-KIT-Hsp90β-Apaf-1 complex, induced primary apoptosis in GIST-T1 cells. Combined treatment with bortezomib plus dasatinib caused cell cycle arrest in the G1 phase through inactivation of PDGFRβ and promoted bortezomib-induced apoptosis in GIST-T1 cells. Our data suggest that combination therapy exerts better efficiency for eradicating GIST cells and may be a promising strategy for the future treatment of GISTs.

3,6-Bis(1-methyl-4-vinylpyridinium)-carbazole diiodide as a marker for tracking living neural stem/precursor cells

Until now, there has been no appropriate marker to specifically and effectively label living neural stem/precursor cells (NSPCs) to investigate NSPC migration and differentiation. Therefore, the purpose of this study was to develop a specific method for tracking NSPCs by a cell marker, 3,6-bis(1-methyl-4-vinylpyridinium)-carbazole diiodide (BMVC). It was found that the bright fluorescence spots could be rapidly observed in the nuclei of NSPCs, isolated from the embryonic rat cerebral cortex, after incubation with BMVC for just 5 min and could be maintained for exceeding 7 days without obvious decay. Even NSPCs were induced to undergo differentiation; their daughter cells also expressed bright fluorescence spots. In contrast, foreskin fibroblasts, adipose-derived stem cells (ADSCs), bone marrow mesenchymal stem cells (BMMSCs), neurons, and glial cells did not express the bright fluorescence spot in the presence of BMVC. In addition, BMVC-labeled NSPCs could be tracked in wound healing and decellularized rat brain models with minimal cell toxicity. Therefore, BMVC possessed a specific function to distinguish between NSPCs and other stem and neuron-related cells. These results are very encouraging, since a cell marker favorable for long-term labeling NSPCs should be useful in the development of strategies for tracking the development of NSPCs in neuroscience research.

Capsaicin modulates acetylcholine release at the myoneural junction

Transient receptor potential (TRP) proteins are non-selective cation channel proteins that are expressed throughout the body. Previous studies demonstrated the expression of TRP Vanilloid 1 (TRPV1), capsaicin (CAP) receptor, in sensory neurons. Recently, we reported TRPV1 expression in mouse motor nerve terminals [MNTs; (Thyagarajan et al., 2009)], where we observed that CAP protected MNTs from botulinum neurotoxin A (BoNT/A). Phrenic nerve diaphragm nerve muscle preparations (NMP) isolated from isoflurane anesthetized adult mice were analyzed for twitch tension, spontaneous (mEPCs) and nerve stimulus evoked (EPCs) acetylcholine release. When acutely applied to isolated NMP, CAP produced a concentration-dependent decline of twitch tension and produced a significant decline in the amplitude of EPCs and quantal content without any effect on the mEPCs. The suppression of nerve stimulus evoked acetylcholine release by CAP was antagonized by capsazepine (CPZ), a TRPV1 antagonist. CAP did not suppress phrenic nerve stimulus evoked acetylcholine release in TRPV1 knockout mice. Also, CAP treatment, in vitro, interfered with the localization of adapter protein 2 in cholinergic Neuro 2a cells. Wortmannin, (WMN; non-selective phosphoinositol kinase inhibitor), mimicked the effects of CAP by inhibiting the acetylcholine exocytosis. Our data suggest that TRPV1 proteins expressed at the MNT are coupled to the exo-endocytic mechanisms to regulate neuromuscular functions.

Clathrin assembly protein CALM plays a critical role in KIT signaling by regulating its cellular transport from early to late endosomes in hematopoietic cells

CALM is implicated in the formation of clathrin-coated vesicles, which mediate endocytosis and intracellular trafficking of growth factor receptors and nutrients. We previously found that CALM-deficient mice suffer from severe anemia due to the impaired clathrin-mediated endocytosis of transferrin receptor in immature erythroblast. However, CALM has been supposed to regulate the growth and survival of hematopoietic stem/progenitor cells. So, in this study, we focused on the function of CALM in these cells. We here show that the number of Linage⁻Sca-1⁺KIT⁺ (LSK) cells decreased in the fetal liver of CALM^−/− mice. Also, colony forming activity was impaired in CALM^−/− LSK cells. In addition, SCF, FLT3, and TPO-dependent growth was severely impaired in CALM^−/− LSK cells, while they can normally proliferate in response to IL-3 and IL-6. We also examined the intracellular trafficking of KIT using CALM^−/− murine embryonic fibroblasts (MEFs) engineered to express KIT. At first, we confirmed that endocytosis of SCF-bound KIT was not impaired in CALM^−/− MEFs by the internalization assay. However, SCF-induced KIT trafficking from early to late endosome was severely impaired in CALM^−/− MEFs. As a result, although intracellular KIT disappeared 30 min after SCF stimulation in wild-type (WT) MEFs, it was retained in CALM^−/− MEFs. Furthermore, SCF-induced phosphorylation of cytosolic KIT was enhanced and prolonged in CALM^−/− MEFs compared with that in WT MEFs, leading to the excessive activation of Akt. Similar hyperactivation of Akt was observed in CALM^−/− KIT⁺ cells. These results indicate that CALM is essential for the intracellular trafficking of KIT and its normal functions. Also, our data demonstrate that KIT located in the early endosome can activate downstream molecules as a signaling endosome. Because KIT activation is involved in the pathogenesis of some malignancies, the manipulation of CALM function would be an attractive therapeutic strategy.

Expression of protein gene product 9.5 and Sal-like protein 4 in canine seminomas

The aim of the present study was to characterize canine classical seminoma (SE) and spermatocytic seminoma (SS) by immunohistochemical expression of gonocytic and spermatogonial cellular markers (c-Kit, placental alkaline phosphatase [PLAP], protein gene product 9.5 [PGP9.5] and Sal-like protein 4 [Sall4]) and histochemically by the periodic acid-Schiff (PAS) reaction. Twenty-five cases of SE and 23 cases of SS were investigated. Two cases of dysgerminoma were also examined. c-Kit was expressed on the cell membrane of 13 of 25 cases of SE (52%) and four of 23 cases of SS (16%). This marker was not expressed in dysgerminoma. PLAP immunoreactivity was observed in the cytoplasm of neoplastic cells of six of 25 cases of SE (24%). PLAP was not expressed in cases of SS and dysgerminoma. All samples of SE, SS and dysgerminoma showed cytoplasmic expression of PGP9.5 and nuclear immunoreactivity for Sall4. There was fine granular cytoplasmic PAS staining in neoplastic cells in five of 25 cases of SE (20%), while all samples of SS and dysgerminoma cases were PAS negative. These findings suggest that it is not possible to differentiate canine SE and SS using these markers. This may be because canine SS may be derived from spermatogonia that can differentiate to spermatocytes and also because cases of canine SE might consist of neoplastic cells that have lost their gonocytic nature. This study was the first to show positive immunoreactivity for Sall4 in canine seminomas and dysgerminomas and expression of PGP9.5 in canine dysgerminomas.

Listeria monocytogenes alters mast cell phenotype, mediator and osteopontin secretion in a listeriolysin-dependent manner

Whilst mast cells participate in the immune defence against the intracellular bacterium Listeria monocytogenes, there is conflicting evidence regarding the ability of L. monocytogenes to infect mast cells. It is known that the pore-forming toxin listeriolysin (LLO) is important for mast cell activation, degranulation and the release of pro-inflammatory cytokines. Mast cells, however, are a potential source of a wide range of cytokines, chemokines and other mediators including osteopontin, which contributes to the clearing of L. monocytogenes infections in vivo, although its source is unknown. We therefore aimed to resolve the controversy of mast cell infection by L. monocytogenes and investigated the extent of mediator release in response to the bacterium. In this paper we show that the infection of bone marrow-derived mast cells by L. monocytogenes is inefficient and LLO-independent. LLO, however, is required for calcium-independent mast cell degranulation as well as for the transient and selective downregulation of cell surface CD117 (c-kit) on mast cells. We demonstrate that in addition to the key pro-inflammatory cytokines TNF-α and IL-6, mast cells release a wide range of other mediators in response to L. monocytogenes. Osteopontin, IL-2, IL-4, IL-13 and granulocyte macrophage colony-stimulating factor (GM-CSF), and chemokines including CCL2, CCL3, CCL4 and CCL5 are released in a MyD88-dependent manner. The wide range of mediators released by mast cells in response to L. monocytogenes may play an important role in the recruitment and activation of a variety of immune cells in vivo. The cocktail of mediators, however, is unlikely to skew the immune response to a particular effector response. We propose that mast cells provide a hitherto unreported source of osteopontin, and may provide an important role in co-ordinating the immune response during Listeria infection.

Bortezomib interferes with C-KIT processing and transforms the t(8;21)-generated fusion proteins into tumor-suppressing fragments in leukemia cells

The boronic acid dipeptide bortezomib inhibits the chymotrypsin-like activity of the 26S proteasome and shows significant therapeutic efficacy in multiple myeloma. However, recent studies suggest that bortezomib may have more complex mechanisms of action in treating cancer. We report here that the endocytosis and lysosomal degradation of the receptor tyrosine kinase C-KIT are required for bortezomib- but not tyrosine kinase inhibitor imatinib-caused apoptosis of t(8;21) leukemia and gastrointestinal stromal tumor cells, suggesting that C-KIT may recruit an apoptosis initiator. We show that C-KIT binds and phosphorylates heat shock protein 90β (Hsp90β), which sequestrates apoptotic protease activating factor 1 (Apaf-1). Bortezomib dephosphorylates pHsp90β and releases Apaf-1. Although the activated caspase-3 is not sufficient to cause marked apoptosis, it cleaves the t(8;21) generated acute myeloid leukemia 1-eight twenty one (AML1-ETO) and AML1-ETO9a fusion proteins, with production of cleavage fragments that perturb the functions of the parental oncoproteins and further contribute to apoptosis. Notably, bortezomib exerts potent therapeutic efficacy in mice bearing AML1-ETO9a-driven leukemia. These data show that C-KIT-pHsp90β-Apaf-1 cascade is critical for some malignant cells to evade apoptosis, and the clinical therapeutic potentials of bortezomib in C-KIT-driven neoplasms should be further explored.

c-Kit is required for growth and survival of the cells of origin of Brca1-mutation-associated breast cancer

BRCA1 mutation-associated breast cancer originates in oestrogen receptor-alpha-negative (ER(-)) progenitors in the mammary luminal epithelium. These cells also express high levels of the Kit gene and a recent study demonstrated a correlation between Brca1 loss and Kit over-expression in the mammary epithelium. However, the functional significance of c-Kit expression in the mammary gland is unknown. To address this, c-Kit(-) and c-Kit(+) mammary epithelial subsets were isolated by flow cytometry, characterised for expression of lineage-specific cell markers and functionally analysed by in vitro colony forming and in vivo transplantation assays. The results confirm that the majority of luminal ER(-) progenitors are c-Kit(+), but also that most stem cells and the differentiated cell populations are c-Kit(-). A subset of c-Kit(+) cells with high proliferative potential was found in the luminal ER(+) population, however, suggesting the existence of a distinct luminal ER(+) progenitor cell type. Analysis of mouse Brca1 mammary tumours demonstrated that they expressed Kit and its downstream effector Lyn at levels comparable to the most strongly c-Kit(+) luminal ER(-) progenitors. Consistent with c-Kit being a progenitor cell marker, in vitro three-dimensional differentiation of c-Kit(+) cells resulted in a loss of c-Kit expression, whereas c-Kit over-expression prevented normal differentiation in vivo. Furthermore, c-Kit was a functional marker of proliferative potential, as c-Kit inhibition by short hairpin knockdown prevented normal epithelial growth and caused cells to undergo apoptosis. Therefore, c-Kit defines distinct progenitor populations in the mammary epithelium and is critical for mammary progenitor survival and proliferation. Importantly, c-Kit is only the second mammary epithelial stem/progenitor marker to be shown to have a functional role in the mammary epithelium and the first marker to be shown to be required for progenitor cell function. The c-Kit signalling network has potential as a target for therapy and/or prevention in BRCA1-associated breast cancer.

Wiskott-Aldrich syndrome protein is an effector of Kit signaling

The pleiotropic receptor tyrosine kinase Kit can provide cytoskeletal signals that define cell shape, positioning, and migration, but the underlying mechanisms are less well understood. In this study, we provide evidence that Kit signals through Wiskott-Aldrich syndrome protein (WASP), the central hematopoietic actin nucleation-promoting factor and regulator of the cytoskeleton. Kit ligand (KL) stimulation resulted in transient tyrosine phosphorylation of WASP, as well as interacting proteins WASP-interacting protein and Arp2/3. KL-induced filopodia in bone marrow-derived mast cells (BMMCs) were significantly decreased in number and size in the absence of WASP. KL-dependent regulation of intracellular Ca(2+) levels was aberrant in WASP-deficient BMMCs. When BMMCs were derived from WASP-heterozygous female mice using KL as a growth factor, the cultures eventually developed from a mixture of WASP-positive and -negative populations into a homogenous WASP-positive culture derived from the WASP-positive progenitors. Thus, WASP expression conferred a selective advantage to the development of Kit-dependent hematopoiesis consistent with the selective advantage of WASP-positive hematopoietic cells observed in WAS-heterozygous female humans. Finally, KL-mediated gene expression in wild-type and WASP-deficient BMMCs was compared and revealed that approximately 30% of all Kit-induced changes were WASP dependent. The results indicate that Kit signaling through WASP is necessary for normal Kit-mediated filopodia formation, cell survival, and gene expression, and provide new insight into the mechanism in which WASP exerts a strong selective pressure in hematopoiesis.

Ubiquitination and endocytosis of cell adhesion molecule DM-GRASP regulate its cell surface presence and affect its role for axon navigation

DM-GRASP, cell adhesion molecule of the immunoglobulin superfamily, has been shown to promote growth and navigation of axons. We here demonstrate that clustering of DM-GRASP in the plasma membrane induces its rapid internalization via dynamin- and clathrin-dependent endocytosis, which is controlled by phosphatidylinositol 3-kinase and mitogen-activated protein kinase ERK. The clustering of DM-GRASP activates ERK; the intensity and duration of ERK activation by DM-GRASP do not depend on rapid clathrin-mediated internalization of DM-GRASP. Moreover, the preference of retinal ganglion cell axons for DM-GRASP-coated micro-lanes requires clathrin-mediated endocytosis for the appropriate axonal turning reactions at substrate borders. Because the intracellular domain of DM-GRASP does not contain motifs for direct interactions with the endocytosis machinery, we performed a yeast two-hybrid screen to identify intracellular proteins mediating the uptake of DM-GRASP and isolated ubiquitin. Immunoprecipitation of DM-GRASP coexpressed with ubiquitin revealed that one or two ubiquitin(s) are attached to the intracellular domain of cell surface-resident DM-GRASP. Furthermore, elevated ubiquitination levels result in a decrease of cell surface-resident DM-GRASP as well as in the amount of total DM-GRASP. The endocytosis rate is not affected, but the delivery to multivesicular bodies is increased, indicating that DM-GRASP ubiquitination enhances its sorting into the degradation pathway. Together, our data show that ubiquitination and endocytosis of DM-GRASP in concert regulate its cell surface concentration, which is crucial for its function in axon navigation.

Interactions of annexins with the mu subunits of the clathrin assembly proteins

A number of biochemical and genetic studies have suggested that certain annexins play important roles in the endocytic pathway, possibly involving the generation, localization, or fusion of endocytic compartments. In a yeast two-hybrid screen for proteins that interact with the N-terminal domain of annexin A2 we identified the mu2 subunit of the clathrin assembly protein complex AP-2. The interaction depended upon two copies of a Yxx phi amino acid sequence motif (Y = tyrosine, x = variable residue, phi = bulky, hydrophobic residue) in the annexin that is also characteristic of the binding site for mu2 on the cytoplasmic domains of transmembrane receptors. The interaction between mu2 and full-length annexin A2 was demonstrated in vitro to be direct, to require calcium, and to be functional in the sense that annexin A2 was able to recruit the mu2 to immobilized lipids. Examination of other annexins and mu subunits demonstrated that annexin A2 also binds the mu1 subunit of the AP-1 complex, that annexin A6 binds mu1 and mu2, and that annexin A1 binds only mu1. We propose that annexins can "masquerade" as transmembrane receptors when they are attached to membranes in the presence of calcium and that they might therefore function to initiate calcium-regulated coated pit formation at the cell surface or on intracellular organelles.

Stem cell factor and its receptor c-Kit as targets for inflammatory diseases

Stem cell factor (SCF), the ligand of the c-Kit receptor, is expressed by various structural and inflammatory cells in the airways. Binding of SCF to c-Kit leads to activation of multiple pathways, including phosphatidyl-inositol-3 (PI3)-kinase, phospholipase C (PLC)-gamma, Src kinase, Janus kinase (JAK)/Signal Transducers and Activators of Transcription (STAT) and mitogen activated protein (MAP) kinase pathways. SCF is an important growth factor for mast cells, promoting their generation from CD34+ progenitor cells. In vitro, SCF induces mast cells survival, adhesion to extracellular matrix and degranulation, leading to expression and release of histamine, pro-inflammatory cytokines and chemokines. SCF also induces eosinophil adhesion and activation. SCF is upregulated in inflammatory conditions both in vitro and in vivo, in human and mice. Inhibition of the SCF/c-Kit pathway leads to significant decrease of histamine levels, mast cells and eosinophil infiltration, interleukin (IL)-4 production and airway hyperresponsiveness in vivo. Taken together, these data suggest that SCF/c-Kit may be a potential therapeutic target for the control of mast cell and eosinophil number and activation in inflammatory diseases.

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

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

TLR3-induced activation of mast cells modulates CD8+ T-cell recruitment

Mast cells play an important role in host defense against various pathogens, but their role in viral infection has not been clarified in detail. dsRNA, synthesized by various types of viruses and mimicked by polyinosinic-polycytidylic acid (poly(I:C)) is recognized by Toll-like receptor 3 (TLR3). In this study, we demonstrate that poly(I:C) injection in vivo potently stimulates peritoneal mast cells to up-regulate a number of different costimulatory molecules. Therefore, we examined the expression and the functional significance of TLR3 activation in mast cells. Mast cells express TLR3 on the cell surface and intracellularly. After stimulation of mast cells with poly(I:C) and Newcastle disease virus (NDV), TLR3 is phosphorylated and the expression of key antiviral response cytokines (interferon beta, ISG15) and chemokines (IP10, RANTES) is upregulated. Interestingly, mast cells activated via TLR3-poly(I:C) potently stimulate CD8+ T-cell recruitment. Indeed, mast-cell-deficient mice (KitW/KitW-v) given an intraperitoneal injection of poly(I:C) show a decreased CD8+ T-cell recruitment, whereas granulocytes normally migrate to the peritoneal cavity. Mast-cell reconstitution of KitW/KitW-v mice normalizes the CD8+ T-cell influx. Thus, mast cells stimulated through engagement of TLR3 are potent regulators of CD8+ T-cell activities in vitro and in vivo.

Deletion of membrane-bound steel factor results in osteopenia in mice

To examine the functional role of membrane-bound SLF, we evaluated the growing skeletons of WT and SLF mutant (Sl/Sl(d)) mice that do not produce this protein using DXA, bone histomorphometry, cell culture, and flow cytometry. Deletion of membrane-bound SLF delays bone growth, decreases bone mass and BMD, impairs osteoblast function, and increases osteoclast surface in young mice.

INTRODUCTION

Mutations at the murine steel locus lead to a defect in the development of hematopoietic stem cells, mast cells, and germ cells. Two isoforms of steel factor (SLF), soluble and membrane-associated, have been reported. Soluble SLF increases osteoclast formation and activity in cell culture. The effects of deletion of membrane-bound SLF on bone metabolism in mice have yet to be determined and are the subject of this study.

MATERIALS AND METHODS

Five-, 7-, and 12-week-old male and 5-week-old female WCB6F1/J-Kitl(Sl)/Kitl(Sl-d) (Sl/Sl(d)) mice and wildtype (WT) littermates were used. BMD and bone mass, growth, architecture, and turnover were evaluated by DXA (males and females) and histomorphometry (males only). Primary osteoblasts isolated from humeri of 5-week-old male WT and Sl/Sl(d) mice were used to determine osteoblast function, and bone marrow cells from tibias and femurs of these mice were analyzed to determine cell surface expression of osteoclast precursors.

RESULTS AND CONCLUSIONS

Young Sl/Sl(d) mice grew more slowly, had a reduced bone mass, and had shorter bones than WT littermates. Male mutants had significantly decreased whole body BMD in all age groups, largely because of a reduction in BMC. Tibial cross-sectional, cortical, and marrow area of cortical bone and cancellous bone volume was reduced in the mutants at all ages. The osteopenia in Sl/Sl(d) was caused by increased osteoclast surface at all ages and decreased osteoblast surface at 5 weeks of age. [(3)H]thymidine incorporation studies showed that proliferation of osteoblasts derived from mutant mice was significantly suppressed (56%). Moreover, a decrease in mineralization was observed in Sl/Sl(d) osteoblast culture. Fluorescence-activated cell sorting analysis of bone marrow cells from Sl/Sl(d) mice revealed a 65% increase in the percentage of c-Fms(+)CD11b(+)RANK(+) cells compared with WT controls. These findings suggest that membrane-bound SLF/c-Kit signaling plays a role in the regulation of peak bone mass.

Activation of the MAP kinase pathway by c-Kit is PI-3 kinase dependent in hematopoietic progenitor/stem cell lines

The Steel factor (SF) and its receptor c-Kit play a critical role for various cell types at different levels in the hematopoietic hierarchy. Whether similar or distinct signaling pathways are used upon c-Kit activation in different cell types within the hematopoietic hierarchy is not known. To study c-Kit signaling pathways in the hematopoietic system we have compared c-Kit downstream signaling events in SF-dependent hematopoietic stem cell (HSC)-like cell lines to those of mast cells. Both Erk and protein kinase B (PKB)/Akt are activated by ligand-induced activation of the c-Kit receptor in the HSC-like cell lines. Surprisingly, phosphoinositide-3 (PI-3) kinase inhibitors block not only PKB/Akt activation but also activation of Raf and Erk. SF-induced activation of Ras is not affected by inhibition of PI-3 kinase. In mast cells and other more committed hematopoietic precursors, the activation of Erk by SF is not PI-3 kinase dependent. Our results suggest that a molecular signaling switch occurs during differentiation in the hematopoietic system whereby immature hematopoietic progenitor/stem cells use a PI-3 kinase-sensitive pathway in the activation of both Erk and PKB/Akt, which is then switched upon differentiation to the more commonly described PI-3 kinase-independent mitogen-activated protein (MAP) kinase pathway.

Protein Kinase C-ζ and Protein Kinase B Regulate Distinct Steps of Insulin Endocytosis and Intracellular Sorting

We have investigated the molecular mechanisms regulating insulin internalization and intracellular sorting. Insulin internalization was decreased by 50% upon incubation of the cells with the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY294002. PI3K inhibition also reduced insulin degradation and intact insulin release by 50 and 75%, respectively. Insulin internalization was reduced by antisense inhibition of protein kinase C-zeta (PKCzeta) expression and by overexpression of a dominant negative PKCzeta mutant (DN-PKCzeta). Conversely, overexpression of PKCzeta increased insulin internalization as a function of the PKCzeta levels achieved in the cells. Expression of wild-type protein kinase B (PKB)-alpha or of a constitutively active form (myr-PKB) did not significantly alter insulin internalization and degradation but produced a 100% increase of intact insulin release. Inhibition of PKB by a dominant negative mutant (DN-PKB) or by the pharmacological inhibitor ML-9 reduced intact insulin release by 75% with no effect on internalization and degradation. In addition, overexpression of Rab5 completely rescued the effect of PKCzeta inhibition on insulin internalization but not that of PKB inhibition on intact insulin recycling. Indeed, PKCzeta bound to and activated Rab5. Thus, PI3K controls different steps within the insulin endocytic itinerary. PKCzeta appears to mediate the PI3K effect on insulin internalization in a Rab5-dependent manner, whereas PKB directs intracellular sorting toward intact insulin release.

Src family kinases are involved in the differential signaling from two splice forms of c-Kit

In both mice and humans alternate splicing results in isoforms of c-Kit characterized by the presence or the absence of a tetrapeptide sequence, GNNK, in the juxtamembrane region of the extracellular domain. Dramatic differences in the kinetics and magnitude of activation of the intrinsic tyrosine kinase activity of c-Kit between the GNNK- and GNNK+ isoforms has previously been shown. Here we report the analysis of downstream targets of receptor signaling, which revealed that the signaling was differentially regulated in the two splice forms. The kinetics of phosphorylation of Shc, previously demonstrated to be phosphorylated by Src downstream of c-Kit, was stronger and more rapid in the GNNK- form, whereas it showed slower kinetics in the GNNK+ form. Inhibition of Src family kinases with the specific Src family kinase inhibitor SU6656 altered the kinetics of activation of the GNNK- form of c-Kit so that it resembled that of the GNNK+ form. In cells expressing the GNNK- form, SCF was rapidly degraded, whereas in cells expressing the GNNK+ form only showed a very slow rate of degradation of SCF. In the GNNK+ form the Src inhibitor SU6656 only had a weak effect on degradation, whereas in the GNNK- form it dramatically inhibited degradation. In summary, the two splice forms show, despite only a four-amino acid sequence difference, remarkable differences in their signaling capabilities.

Role of p38 and ERK MAP kinase in proliferation of erythroid progenitors in response to stimulation by soluble and membrane isoforms of stem cell factor

Two alternatively spliced stem cell factor (SCF) transcripts encode protein products, which differ in the duration of membrane presentation. One form, soluble SCF (S-SCF) gets rapidly processed to yield predominantly secreted protein. The other form, membrane-associated SCF (MA-SCF) lacks the primary proteolytic cleavage site but is cleaved slowly from an alternate site, and thus represents a more stable membrane form of SCF. Mutants of SCF that lack the expression of MA-SCF (Steel-dickie) or possess a defect in its presentation (Steel17H) manifest deficiencies in erythroid cell development. In this study, we have compared the consequence(s) of activating Kit, the receptor for SCF by MA-SCF with S-SCF, and an obligate membrane-restricted (MR) form of SCF (MR-SCF) on erythroid cell survival, proliferation, cell cycle progression, and the activation of p38 and ERK MAP kinase pathways. Activation of Kit by MR-SCF was associated with a significantly lower incidence of apoptosis and cell death in erythroid cells compared to either other isoform. MR- or MA-SCF–induced stimulation of erythroid cells resulted in similar and significantly greater proliferation and cell cycle progression compared to soluble SCF. The increase in proliferation and cell cycle progression via MA- or MR-SCF stimulation correlated with sustained and enhanced activation of p38 and ERK MAP kinase pathways. In addition, MR- or MA-SCF–induced proliferation was more sensitive to the inhibitory effects of ERK inhibitor compared to S-SCF–induced proliferation. In contrast, soluble SCF-induced proliferation was more sensitive to the inhibitory effects of p38 inhibitor compared with MR- or MA-SCF. These results suggest that different isoforms of SCF may use different biochemical pathways in stimulation of survival and/or proliferation of erythroid cells.

Analysing c-kit internalization using a functional c-kit-EGFP chimera containing the fluorochrome within the extracellular domain

In order to investigate activation and internalization of c-kit we created a functional c-kit-EGFP chimera by inserting EYFP (enhanced yellow fluorescent protein) within the extracellular domain of c-kit immediately downstream of the signal sequence, SS-EYFP-kit. This location was chosen because the C-terminal fusion of EGFP to c-kit unexpectedly caused constitutive activation of the c-kit tyrosine kinase. As analysed in fixed cells and by real time imaging in vivo, SCF induced activation led to internalization of the fusion construct and translocation to punctate structures resembling vesicles. Analysis of the internalization process by time lapse imaging revealed high mobility and discontinuous movement of these vesicles and their predominantly radial tracks. Two subsets of vesicles were observed: Traffic of the majority of vesicles was directed from the periphery to the center of the cell and most likely represents the internalization of activated receptor molecules via the endosomal pathway. However, some vesicular structures were observed to move towards the periphery of the cell and probably contain newly synthesized protein to replace internalized receptor molecules. The calculated velocity of moving vesicles ranged from 0.05 to 0.2 microm per se. Vesicle formation upon SCF induced dimerization of the receptor was strictly dependent on kinase activity of c-kit. Treatment of cells with phenylarsine oxide, an agent blocking receptor internalization, prior to SCF stimulation resulted in abrogation of the translocation of the chimera to vesicles whereas accumulation of vesicles was observed when cells were treated with proteasome inhibitors. Cholesterol depletion of the cell membrane by methyl-beta-cyclodextrin resulted in dose dependent reduction of receptor internalization indicating that c-kit may be present in lipid rafts or that intact lipid rafts are required for efficient internalization of the receptor. Using the induction of vesicular structures as a sign of efficient internalization of the receptor analysis of mutant c-kit constructs deficient either in activation of PI3-Kinase or Src revealed that internalization of c-kit is dependent on recruitment of Src but not PI3-Kinase.

Stem cell factor: laboratory and clinical aspects

Stem cell factor is an essential haemopoietic progenitor cell growth factor with proliferative and anti-apoptotic functions. Molecular biologists have now dissected some of the various pathways through which this cytokine signals to the nucleus. At the same time, new molecules have become available which can inhibit SCF signalling. This provides an exciting prospect for the treatment of Kit+ malignancies such as acute myeloblastic leukaemia. The capacity of SCF to synergize with other cytokines has been exploited in the ex vivo expansion of haemopoietic progenitors and dendritic cells, which may also hold therapeutic promise. In this review the last 5 years' literature on these issues is reviewed and collated.

Phosphatidylinositol 3 kinase contributes to the transformation of hematopoietic cells by the D816V c-Kit mutant

Stem cell factor (SCF) binds the receptor tyrosine kinase c-Kit and is critical for normal hematopoiesis. Substitution of valine for aspartic acid 816 (D816V) constitutively actives human c-Kit, and this mutation is found in patients with mastocytosis, leukemia, and germ cell tumors. Immortalized murine progenitor cells (MIHCs) transduced with wild-type c-Kit proliferate in response to SCF, whereas cells expressing D816V c-Kit (MIHC-D816V) are factor-independent and tumorigenic. However, the mechanisms mediating transformation by D816V c-Kit are unknown. The objective of this study was to identify signaling components that contribute to D816V c-Kit-mediated transformation. SCF stimulates association of p85PI3K with phosphorylated tyrosine 721 of wild-type c-Kit. Phosphatidylinositol 3 kinase (PI3K) subsequently contributes to the activation of Akt and Jnks. In contrast, these studies demonstrated that the D816V c-Kit mutant was constitutively associated with phosphorylated p85PI3K, and, downstream of PI3K, Jnk 1 and Jnk 2 were activated but Akt was not. Interestingly, Erks 1 and 2 were not constitutively activated by D816V c-Kit. Thus, D816V c-Kit maintains the activity of PI3K but not of all signaling pathways activated by wild-type c-Kit. Further, all pathways downstream of PI3K are not constitutively active in MIHC-D816V cells. Studies with a PI3K inhibitor and D816V/Y721F c-Kit, a mutant incapable of recruiting PI3K, indicate that constitutive activation of PI3K through direct recruitment by D816V c-Kit plays a role in factor-independent growth of MIHC and is critical for tumorigenicity.

Differential stimulation of c-Kit mutants by membrane-bound and soluble Steel Factor correlates with leukemic potential

The authors investigated the roles of PI3-kinase and PLC-γ in stimulation by Steel Factor (SLF) through c-Kit. c-Kit mutants YF719, YF728, and a YF719/YF728 double mutant were expressed in 32D myelomonocytic cells. KitYF719 fails to recruit PI3-kinase after stimulation with SLF, whereas KitYF728 fails to stimulate PLC-γ phosphorylation or mobilize Ca++. Both single mutants responded mitogenically to soluble SLF (sSLF) in a manner indistinguishable from wild type (WT), although sSLF failed to stimulate or promote the survival of cells expressing the double mutant. In contrast, although cells expressing WT or YF719 were mitogenically stimulated by membrane-bound SLF (mSLF), stimulation of cells expressing KitYF728 was impaired. Similarly, cells expressing WT or YF719 receptors were stimulated by plate-bound anti-Kit antibodies, whereas cells expressing the YF728 receptor were not stimulated. Neomycin sulfate, a PLC antagonist, inhibited cells expressing YF719 receptors stimulated by sSLF. Neomycin also inhibited cells expressing the WT receptor that were stimulated by mSLF or immobilized anti-Kit antibodies but did not inhibit stimulation of cells expressing WT or YF719 receptors by sSLF. 32D cells expressing KitWT, KitYF719, or KitYF728 were injected into mice and the presence of cells was evaluated by colony assays 6 to 7 weeks later. Although both KitWT and KitYF719 expressing cells could be recovered from the spleen and bone marrow, recovery of KitYF728 cells from these organs was severely reduced. These results indicate that Kit tyrosine 728 is of particular importance for mitogenic stimulation by mSLF or immobilized ligand and is required for full maintenance of cells in vivo, likely through activation of PLC-γ.

Differential stimulation of c-Kit mutants by membrane-bound and soluble Steel Factor correlates with leukemic potential

The authors investigated the roles of PI3-kinase and PLC-γ in stimulation by Steel Factor (SLF) through c-Kit. c-Kit mutants YF719, YF728, and a YF719/YF728 double mutant were expressed in 32D myelomonocytic cells. KitYF719 fails to recruit PI3-kinase after stimulation with SLF, whereas KitYF728 fails to stimulate PLC-γ phosphorylation or mobilize Ca++. Both single mutants responded mitogenically to soluble SLF (sSLF) in a manner indistinguishable from wild type (WT), although sSLF failed to stimulate or promote the survival of cells expressing the double mutant. In contrast, although cells expressing WT or YF719 were mitogenically stimulated by membrane-bound SLF (mSLF), stimulation of cells expressing KitYF728 was impaired. Similarly, cells expressing WT or YF719 receptors were stimulated by plate-bound anti-Kit antibodies, whereas cells expressing the YF728 receptor were not stimulated. Neomycin sulfate, a PLC antagonist, inhibited cells expressing YF719 receptors stimulated by sSLF. Neomycin also inhibited cells expressing the WT receptor that were stimulated by mSLF or immobilized anti-Kit antibodies but did not inhibit stimulation of cells expressing WT or YF719 receptors by sSLF. 32D cells expressing KitWT, KitYF719, or KitYF728 were injected into mice and the presence of cells was evaluated by colony assays 6 to 7 weeks later. Although both KitWT and KitYF719 expressing cells could be recovered from the spleen and bone marrow, recovery of KitYF728 cells from these organs was severely reduced. These results indicate that Kit tyrosine 728 is of particular importance for mitogenic stimulation by mSLF or immobilized ligand and is required for full maintenance of cells in vivo, likely through activation of PLC-γ.

The mechanism of unresponsiveness to circulating tumor antigen MUC1 is a block in intracellular sorting and processing by dendritic cells

Immunity to tumor Ags in patients is typically weak and not therapeutic. We have identified a new mechanism by which potentially immunogenic glycoprotein tumor Ags, such as MUC1, fail to stimulate strong immune responses. MUC1 is a heavily glycosylated membrane protein that is also present in soluble form in sera and ascites of cancer patients. We show that this soluble protein is readily taken up by dendritic cells (DC), but is not transported to late endosomes or MHC class II compartments for processing and binding to class II MHC. MUC1 uptake is mediated by the mannose receptor, and the protein is then retained long term in early endosomes without degradation. Long-term retention of MUC1 does not interfere with the ability of DC to process and present other Ags. We also demonstrate inhibited processing of another important glycoprotein tumor Ag, HER-2/neu. This may, therefore, be a frequent obstacle to presentation of tumor Ags and an important consideration in the design of cancer vaccines. It should be possible to overcome this obstacle by providing DC with a form of tumor Ag that can be better processed. For MUC1 we show that a 140-aa-long synthetic peptide is very efficiently processed by DC.

Association of phosphatidylinositol 3-kinase with the insulin receptor: compartmentation in rat liver

Phosphatidylinositol 3-kinase (PI 3-kinase) plays an important role in a variety of hormone and growth factor-mediated intracellular signaling cascades and has been implicated in the regulation of a number of metabolic effects of insulin, including glucose transport and glycogen synthase activation. In the present study we have examined 1) the association of PI 3-kinase with the insulin receptor kinase (IRK) in rat liver and 2) the subcellular distribution of PI 3-kinase-IRK interaction. Insulin treatment promoted a rapid and pronounced recruitment of PI 3-kinase to IRKs located at the plasma membrane, whereas no increase in association with endosomal IRKs was observed. In contrast to IRS-1-associated PI 3-kinase activity, association of PI 3-kinase with the plasma membrane IRK did not augment the specific activity of the lipid kinase. With use of the selective PI 3-kinase inhibitor wortmannin, our data suggest that the cell surface IRK beta-subunit is not a substrate for the serine kinase activity of PI 3-kinase. The functional significance for the insulin-stimulated selective recruitment of PI 3-kinase to cell surface IRKs remains to be elucidated.

LY-294002-inhibitable PI 3-kinase and regulation of baseline rates of Na(+) transport in A6 epithelia

Blocker-induced noise analysis of epithelial Na(+) channels (ENaCs) was used to investigate how inhibition of an LY-294002-sensitive phosphatidylinositol 3-kinase (PI 3-kinase) alters Na(+) transport in unstimulated and aldosterone-prestimulated A6 epithelia. From baseline Na(+) transport rates (I(Na)) of 4.0 +/- 0.1 (unstimulated) and 9.1 +/- 0.9 microA/cm(2) (aldosterone), 10 microM LY-294002 caused, following a relatively small initial increase of transport, a completely reversible inhibition of transport within 90 min to 33 +/- 6% and 38 +/- 2% of respective baseline values. Initial increases of transport could be attributed to increases of channel open probability (P(o)) within 5 min to 143 +/- 17% (unstimulated) and 142 +/- 10% of control (aldosterone) from baseline P(o) averaging near 0.5. Inhibition of transport was due to much slower decreases of functional channel densities (N(T)) to 28 +/- 4% (unstimulated) and 35 +/- 3% (aldosterone) of control at 90 min. LY-294002 (50 microM) caused larger but completely reversible increases of P(o) (215 +/- 38% of control at 5 min) and more rapid but only slightly larger decreases of N(T). Basolateral exposure to LY-294002 induced no detectable effect on transport, P(o) or N(T). We conclude that an LY-294002-sensitive PI 3-kinase plays an important role in regulation of transport by modulating N(T) and P(o) of ENaCs, but only when presented to apical surfaces of the cells.

Kit signal transduction

The current understanding of kit signaling is that a limited number of signaling proteins interact to build multiple interacting networks that allow diverse cellular responses. Cytoplasmic signaling proteins are increasingly seen to form networks directed through converging and interacting pathways rather than following a simple linear model. There are also numerous cross-connections between signaling proteins more distal to the receptor. Ras thus binds PI3 kinase and potentiates its activation, whereas the Rac-dependent protein kinase PAK phosphorylates MEK and thereby stabilizes its association with Raf. A signaling network with multiple intersecting pathways can obtain a single, coherent response from numerous, potentially conflicting signals. There is still limited information about the effect of activating mutations on various aspects of kit signaling. There is, however, mounting evidence that an activating mutation may enhance kit signaling and also induce factor-independent activation of kit. For instance, this activation could occur through degradation of SHP-1, the protein tyrosine phosphatase that negatively regulates kit signaling. There is also emerging evidence that inherent inhibitory factors may exist in the juxtamembrane of kit and may be suppressed as a result of a mutation in that region. Understanding the impact of these activating mutations on kit signaling is important, not only in contributing to the understanding of the pathogenesis of mastocytosis but ultimately in forming the basis for more effective therapeutic intervention in this disease.

The search for the origin of rhythmicity in intestinal contraction; from tissue to single cells

More than a century ago, rhythmic propulsive contractile activity was observed in the intestine after blockade of nerve conduction, thus demonstrating a form of peristalsis that appeared to be under myogenic control. During this century, light and electron microscopic investigations provided the hypothesis that interstitial cells of Cajal (ICC) could be the cells of origin for this rhythmicity. In recent years, physiological studies demonstrated a link between the presence of electrical slow wave activity and the presence of ICC. The recognition that the ICC cell membrane harbours the Kit protein sparked rapid advancement in ICC research, and has been essential in the identification of ICC in tissue and in culture through Kit immunohistochemistry and kit mRNA reverse transcriptase polymerase chain reaction (RT-PCR). With these techniques, electrophysiology was carried out on positively identified single ICC in culture. These methods revealed that single ICC generate spontaneous rhythmic inward currents and slow waves in membrane potential, thus providing strong evidence that ICC generate the electrical pacemaker activity for the gut musculature.

Early signaling pathways activated by c-Kit in hematopoietic cells

c-Kit is a receptor tyrosine kinase that binds stem cell factor (SCF). Structurally, c-Kit contains five immunoglobulin-like domains extracellularly and a catalytic domain divided into two regions by a 77 amino acid insert intracellularly. Studies in white spotting and steel mice have shown that functional SCF and c-Kit are critical in the survival and development of stem cells involved in hematopoiesis, pigmentation and reproduction. Mutations in c-Kit are associated with a variety of human diseases. Interaction of SCF with c-Kit rapidly induces receptor dimerization and increases in autophosphorylation activity. Downstream of c-Kit, multiple signal transduction components are activated, including phosphatidylinositol-3-kinase, Src family members, the JAK/STAT pathway and the Ras-Raf-MAP kinase cascade. Structure-function studies have begun to address the role of these signaling components in SCF-mediated responses. This review will focus on the biochemical mechanism of action of SCF in hematopoietic cells.

Phosphoinositide-AP-2 interactions required for targeting to plasma membrane clathrin-coated pits

The clathrin-associated AP-2 adaptor protein is a major polyphosphoinositide-binding protein in mammalian cells. A high affinity binding site has previously been localized to the NH(2)-terminal region of the AP-2 alpha subunit (Gaidarov et al. 1996. J. Biol. Chem. 271:20922-20929). Here we used deletion and site- directed mutagenesis to determine that alpha residues 21-80 comprise a discrete folding and inositide-binding domain. Further, positively charged residues located within this region are involved in binding, with a lysine triad at positions 55-57 particularly critical. Mutant peptides and protein in which these residues were changed to glutamine retained wild-type structural and functional characteristics by several criteria including circular dichroism spectra, resistance to limited proteolysis, and clathrin binding activity. When expressed in intact cells, mutated alpha subunit showed defective localization to clathrin-coated pits; at high expression levels, the appearance of endogenous AP-2 in coated pits was also blocked consistent with a dominant-negative phenotype. These results, together with recent work indicating that phosphoinositides are also critical to ligand-dependent recruitment of arrestin-receptor complexes to coated pits (Gaidarov et al. 1999. EMBO (Eur. Mol. Biol. Organ.) J. 18:871-881), suggest that phosphoinositides play a critical and general role in adaptor incorporation into plasma membrane clathrin-coated pits.

The box-1 region of the leukemia inhibitory factor receptor alpha-chain cytoplasmic domain is sufficient for hemopoietic cell proliferation and differentiation

Leukemia inhibitory factor (LIF) is a pleiotropic cytokine that acts on a variety of cell types and regulates cell proliferation and differentiation. The functional receptor for LIF is composed of LIFR alpha-chain (LIFRalpha) and gp130 both of which are shared in the functional receptors for oncostatin M, ciliary neurotrophic factor, and cardiotrophin-1. By using stable transfection of wild-type or cytoplasmic deletion mutants of LIFRalpha together with full-length gp130 into Ba/F3 cells, we found that cells expressing gp130 and an extensively deleted mutant LIFRalpha containing only the box-1 region were capable of proliferating in response to LIF, although LIF-dependent long term growth of these cells was seriously impaired. Using a similar strategy to generate WEHI-3BD+ cells expressing gp130 and wild-type or truncation mutants of LIFRalpha, studies revealed that the box-1 region of the LIFRalpha was also sufficient for LIF-dependent induction of different aspects of differentiation, including up-regulation of macrophage surface marker expression, morphological change, and cell migration in agar culture. However, the C-terminal region of the LIFRalpha, although not essential for intracellular signaling, was important for efficient receptor-mediated ligand internalization. In summary, the membrane-proximal box-1 region plays a dominant role in LIF-induced signal transduction of both proliferation and differentiation.

Transduction pathways involved in rapid hormone receptor regulation in the mammary epithelium

Previous studies have shown that the envelope protein of the mouse mammary tumor virus (MMTV) rapidly upregulates prolactin (PRL) receptors by shifting them from internal pools to the cell surface and downregulates epidermal growth factor (EGF) receptors by inducing their internalization and degradation. This study shows that the effect on PRL receptors is mediated by the nitric oxide (NO)/cGMP pathway, since it can be mimicked by an NO donor or 8-bromo-cGMP and can be blocked by an NO synthase inhibitor. In contrast, the effect on EGF receptors is mediated by tyrosine phosphorylation and phosphatidylinositol 3-kinase (PI3K), since it can be blocked by either a tyrosine kinase inhibitor or by a PI3K inhibitor. Both of these pathways can be activated by a calcium ionophore and inhibited by calcium chelation. Therefore, it appears that the mouse mammary tumor virus envelope protein, like other retroviral envelope proteins, initially elevates cytoplasmic calcium, which can then stimulate both the NO/cGMP and the tyrosine phosphorylation/PI3K pathways, leading to PRL receptor upregulation and EGF receptor downregulation, respectively.