MicroRNAs in erythroid and megakaryocytic differentiation and megakaryocyte-erythroid progenitor lineage commitment

MicroRNAs (miRNAs) are a class of small regulatory noncoding RNAs that modulate the expression of their target genes through either mRNA degradation or inhibition of protein translation. In recent years, miRNAs have been shown to be critical regulators of hematopoiesis and have important roles in the differentiation of specific lineages. Here, we summarize our current understanding of miRNAs involved in hematopoiesis with a focus on the role of miRNAs in regulating erythroid and megakaryocytic differentiation and megakaryocyte-erythroid progenitor lineage commitment.

Specific cell-cell contacts are essential for induction of gene expression during differentiation of Dictyostelium discoideum

Postaggregation Dictyostelium discoideum cells contain 2000-3000 mRNA species that are absent from pre-aggregation cells. These aggregation-dependent sequences compose 30% of the mass of the late mRNA and represent the transcription products of an additional 11% of the single-copy genome. By analysis of mutants that are blocked at different stages of differentiation, we show that induction of expression of these genes is correlated with the formation of tight cell-cell contacts that resist EDTA. In particular, mutants that exhibit chemotaxis and aggregate to form loose mounds but do not form cell-cell contacts that resist EDTA fail to induce these late mRNA and protein species. By contrast, mutants that form normal contacts but progress no further through development do express the late mRNA species. Thus, interactions at the cell surface are involved in developmental induction of a large group of coregulated mRNAs. We have employed two independent assays for these developmentally regulated mRNAs: hybridization of gel-separated RNAs to cloned nuclear DNAs and hybridization of mRNA to a cDNA probe specific for the population of 2000-3000 regulated sequences.

A model of secretory protein trafficking in recombinant AtT-20 cells

After their synthesis, secretory proteins in animal cells undergo a series of transport and processing steps before they are secreted. The amount and quality of protein obtained in culture medium depends on the rates of these intracellular steps. We present a model of recombinant protein trafficking in mouse pituitary AtT-20 cells based on currently available biological knowledge, plausible hypotheses, and quantitative secretion results, and we use it to simulate the dynamics of basal and induced secretion and to predict the dynamics of intracellular trafficking events. Besides the endoplasmic reticulum and Golgi, the model recognizes a conversion compartment (CC) where final processing of protein occurs, a storage compartment from which protein is secreted only in the presence of secretion stimulus, and constitutive and pseudoregulated (PR) pathways of secretion. The model further assumes that the protein flux is split between CC and PR and that the storage compartment exerts a negative feedback on protein flux through CC. The model predictions are compared with experimental results on secretion of human growth hormone (hGH) and insulin related peptides and on accumulation of hGH upon removal of secretion stimulus. The model is in agreement with data when either of two hypotheses is implemented: (a) cells always exhibit a high sorting efficiency at the trans-Golgi, but CC has the capacity to process only a fraction of the protein flux leaving the Golgi compartment; (b) the processing capacity of CC never becomes saturated, but significant missorting at the trans-Golgi occurs; in the case, the protein flux toward the plasma membrane becomes split both at the trans-Golgi cisternae and between CC and PR. The usefulness of the type of model considered in providing a quantitative description of intracellular events and in designing new experiments is discussed.

In vitro erythropoiesis from bone marrow-derived progenitors provides a physiological assay for toxic and mutagenic compounds

The goal of this study was to create an in vitro cell culture system that captures essential features of the in vivo erythroid micronucleus (MN) genotoxicity assay, thus enabling increased throughput and controlled studies of the hematopoietic DNA damage response. We show that adult bone marrow (BM) cultures respond to erythropoietin, the principal hormone that stimulates erythropoiesis, with physiological erythropoietic proliferation, differentiation, and enucleation. We then show that this in vitro erythropoietic system clearly signals exposure to genotoxicants through erythroid MN formation. Furthermore, we determined that DNA repair-deficient (MGMT(-/-)) BM displayed sensitivity to genotoxic exposure in vivo compared with WT BM and that this phenotypic response was reflected in erythropoietic cultures. These findings suggest that this in vitro erythroid MN assay is capable of screening for genotoxicity on BM in a physiologically reflective manner. Finally, responses to genotoxicants during erythroid differentiation varied with exposure time, demonstrating that this system can be used to study the effect of DNA damage at specific developmental stages.

Microenvironment-driven changes in the expression profile of hematopoietic cobblestone area-forming cells

Studies with ex vivo cultures of bone marrow have indicated the importance of the adherent layer as a primary reservoir of the most primitive hematopoietic stem cells, from which derivative stem cells and more differentiated progenitors are continuously generated. We used the Affymetrix GeneChip to analyze the mRNA expressions between bone marrow-derived hematopoietic progenitor cells in the cobblestone areas (CA) and the free-floating cells released from the CA formations. Mouse bone marrow hematopoietic progenitor cell line FDCP-Mix and S17 stromal cells were used in this study. Of the 12,000 genes on the chip, only 29 showed more than fivefold higher in CAFC; and for cells in the supernatant, only 55 showed fivefold higher expressions than in the cobblestone area-forming cells (CAFC). The hematopoietic cells in CAFC expressed genes associated with homing, adhesion, and suppression of differentiation, while the free-floating hematopoietic cells showed mature lineage markers and differentiation-specific genes. This confirmed the more primitive nature of the hematopoietic cells in the adherent layer. Of interest in the findings were the discoveries of many secreted and surface protein expressions in CA hematopoietic cells. This may imply interactions among the hematopoietic cells, stromal cells, and the extracellular matrix in CA, which drive the growth, maturation, and differentiation of the hematopoietic cells.

Ineffective erythropoiesis in Stat5a(-/-)5b(-/-) mice due to decreased survival of early erythroblasts

Erythropoietin (Epo) controls red cell production in the basal state and during stress. Epo binding to its receptor, EpoR, on erythroid progenitors leads to rapid activation of the transcription factor Stat5. Previously, fetal anemia and increased apoptosis of fetal liver erythroid progenitors were found in Stat5a(-/-)5b(-/-) mice. However, the role of Stat5 in adult erythropoiesis was not clear. The present study shows that some adult Stat5a(-/-)5b(-/-) mice have a near-normal hematocrit but are deficient in generating high erythropoietic rates in response to stress. Further, many adult Stat5a(-/-)5b(-/-) mice have persistent anemia despite a marked compensatory expansion in their erythropoietic tissue. Analysis of erythroblast maturation in Stat5a(-/-)5b(-/-) hematopoietic tissue shows a dramatic increase in early erythroblast numbers, but these fail to progress in differentiation. Decreased expression of bcl-x(L) and increased apoptosis in Stat5a(-/-)5b(-/-) early erythroblasts correlate with the degree of anemia. Hence, Stat5 controls a rate-determining step regulating early erythroblast survival.

The N-terminal domain of Janus kinase 2 is required for Golgi processing and cell surface expression of erythropoietin receptor

We show that Janus kinase 2 (JAK2), and more specifically just its intact N-terminal domain, binds to the erythropoietin receptor (EpoR) in the endoplasmic reticulum and promotes its cell surface expression. This interaction is specific as JAK1 has no effect. Residues 32 to 58 of the JAK2 JH7 domain are required for EpoR surface expression. Alanine scanning mutagenesis of the EpoR membrane proximal region reveals two modes of EpoR-JAK2 interaction. A continuous block of EpoR residues is required for functional, ligand-independent binding to JAK2 and cell surface receptor expression, whereas four specific residues are essential in switching on prebound JAK2 after ligand binding. Thus, in addition to its kinase activity required for cytokine receptor signaling, JAK is also an essential subunit required for surface expression of cytokine receptors.

Erythropoiesis in the absence of janus-kinase 2: BCR-ABL induces red cell formation in JAK2(-/-) hematopoietic progenitors

The receptor-associated protein tyrosine kinase janus-kinase 2 (JAK2) is essential for normal red cell development and for erythropoietin receptor (EpoR) signaling. JAK2(-/-) embryos are severely deficient in erythropoiesis and die at an early stage of development from fetal anemia. The binding of erythropoietin (Epo) to the EpoR triggers the activation of JAK2, the phosphorylation of the EpoR, and the initiation of the EpoR signaling cascade. In addition to Epo binding to its receptor, signaling pathways downstream of the EpoR can also be stimulated by the BCR-ABL oncoprotein. This study explored whether JAK2 is required for BCR-ABL-mediated stimulation of erythropoiesis. Here, it is shown that JAK2 is constitutively tyrosine phosphorylated in cultured and primary erythroid cells expressing BCR-ABL. However, BCR-ABL effectively supports normal erythroid proliferation, differentiation, and maturation in JAK2-deficient fetal liver cells. Using mutants of BCR-ABL, this study shows that certain signaling pathways activated by BCR-ABL segments distinct from its tyrosine kinase domain are essential for rescue of erythropoiesis in JAK2(-/-) progenitors. The consequences of these multiple signaling pathways for normal erythroid development are discussed.

A novel mechanism for regulating transforming growth factor beta (TGF-beta) signaling. Functional modulation of type III TGF-beta receptor expression through interaction with the PDZ domain protein, GIPC

Transforming growth factor beta (TGF-beta) mediates its biological effects through three high-affinity cell surface receptors, the TGF-beta type I, type II, and type III receptors, and the Smad family of transcription factors. Although the functions of the type II and type I receptors are well established, the precise role of the type III receptor in TGF-beta signaling remains to be established. While expression cloning signaling molecules downstream of TGF-beta, we cloned GIPC (GAIP-interacting protein, C terminus), a PDZ domain-containing protein. GIPC binds a Class I PDZ binding motif in the cytoplasmic domain of the type III receptor resulting in regulation of expression of the type III receptor at the cell surface. Increased expression of the type III receptor mediated by GIPC enhanced cellular responsiveness to TGF-beta both in terms of inhibition of proliferation and in plasminogen-activating inhibitor (PAI)-based promoter gene induction assays. In all cases, deletion of the Class I PDZ binding motif of the type III receptor prevented the type III receptor from binding to GIPC and abrogated the effects of GIPC on type III receptor expressing cells. These results establish, for the first time, a protein that interacts with the cytoplasmic domain of the type III receptor, determine that expression of the type III receptor is regulated at the protein level and that increased expression of the type III receptor is sufficient to enhance TGF-beta signaling. These results further support an essential, non-redundant role for the type III receptor in TGF-beta signaling.

Nucleocytoplasmic shuttling of Smad1 conferred by its nuclear localization and nuclear export signals

Smad1 mediates signaling by bone morphogenetic proteins (BMPs). In the resting state, Smad1 is found in both the nucleus and cytosol. BMP addition triggers Smad1 serine phosphorylation, binding of Smad4, and its accumulation in the nucleus. Mutations in the Smad1 N-terminal basic nuclear localization signal (NLS)-like motif, conserved among all Smad proteins, eliminated its ligand-induced nuclear translocation without affecting its other functions, including DNA binding and complex formation with Smad4. Addition of leptomycin B, an inhibitor of nuclear export, induced rapid nuclear accumulation of Smad1, whereas overexpression of CRM1, the receptor for nuclear export, resulted in Smad1 re-localization to the cytoplasm and inhibition of BMP-induced nuclear accumulation. Thus, in addition to the NLS, Smad1 also contains a functional nuclear export signal (NES). We identified a leucine-rich NES motif in the C terminus of Smad1; its disruption led to constitutive Smad1 nuclear distribution. Reporter gene activation assays demonstrated that both the NLS and NES are required for optimal transcriptional activation by Smad1. Despite its constitutive nuclear accumulation, a Smad1 NES mutant did not display higher basal reporter gene activity. We conclude that Smad1 is under constant nucleocytoplasmic shuttling conferred by its NLS and NES; nuclear accumulation after ligand-induced phosphorylation represents a change in the balance of the activities of these opposing signals and is essential for transcriptional activation.

Functional cloning of the proto-oncogene brain factor-1 (BF-1) as a Smad-binding antagonist of transforming growth factor-beta signaling

Using the plasminogen activator inhibitor (PAI) promoter to drive the expression of a reporter gene (mouse CD2), we devised a system to clone negative regulators of the transforming growth factor-beta (TGF-beta) signaling pathway. We infected a TGF-beta-responsive cell line (MvLu1) with a retroviral cDNA library, selecting by fluorescence-activated cell sorter single cells displaying low PAI promoter activity in response to TGF-beta. Using this strategy we cloned the proto-oncogene brain factor-1 (BF-1). BF-1 represses the PAI promoter in part by associating with both unphosphorylated Smad3 (in the cytoplasm) and phosphorylated Smad3 (in the nucleus), thus preventing its binding to DNA. BF-1 also associates with Smad1, -2, and -4; the Smad MH2 domain binds to BF-1, and the C-terminal segment of BF-1 is uniquely and solely required for binding to Smads. Further, BF-1 represses another TGF-beta-induced promoter (p15), it up-regulates a TGF-beta-repressed promoter (Cyclin A), and it reverses the growth arrest caused by TGF-beta. Our results suggest that BF-1 is a general inhibitor of TGF-beta signaling and as such may play a key role during brain development.

TGF-beta-induced apoptosis is mediated by the adapter protein Daxx that facilitates JNK activation

Transforming growth factor-beta (TGF-beta) is a multifunctional growth factor that has a principal role in growth control through both its cytostatic effect on many different epithelial cell types and its ability to induce programmed cell death in a variety of other cell types. Here we have used a screen for proteins that interact physically with the cytoplasmic domain of the type II TGF-beta receptor to isolate the gene encoding Daxx - a protein associated with the Fas receptor that mediates activation of Jun amino-terminal kinase (JNK) and programmed cell death induced by Fas. The carboxy-terminal portion of Daxx functions as a dominant-negative inhibitor of TGF-beta-induced apoptosis in B-cell lymphomas, and antisense oligonucleotides to Daxx inhibit TGF-beta-induced apoptosis in mouse hepatocytes. Furthermore, Daxx is involved in mediating JNK activation by TGF-beta. Our findings associate Daxx directly with the TGF-beta apoptotic-signalling pathway, and make a biochemical connection between the receptors for TGF-beta and the apoptotic machinery.

TGF-beta-induced apoptosis is mediated by the adapter protein Daxx that facilitates JNK activation

Transforming growth factor-beta (TGF-beta) is a multifunctional growth factor that has a principal role in growth control through both its cytostatic effect on many different epithelial cell types and its ability to induce programmed cell death in a variety of other cell types. Here we have used a screen for proteins that interact physically with the cytoplasmic domain of the type II TGF-beta receptor to isolate the gene encoding Daxx - a protein associated with the Fas receptor that mediates activation of Jun amino-terminal kinase (JNK) and programmed cell death induced by Fas. The carboxy-terminal portion of Daxx functions as a dominant-negative inhibitor of TGF-beta-induced apoptosis in B-cell lymphomas, and antisense oligonucleotides to Daxx inhibit TGF-beta-induced apoptosis in mouse hepatocytes. Furthermore, Daxx is involved in mediating JNK activation by TGF-beta. Our findings associate Daxx directly with the TGF-beta apoptotic-signalling pathway, and make a biochemical connection between the receptors for TGF-beta and the apoptotic machinery.

Fatty acid transport proteins: a current view of a growing family

Long-chain fatty acids (LCFAs) are a major caloric component of our diet and are key metabolites for energy generation and storage. Physiological uptake of LCFAs across cell membranes is a saturable and competable process occurring at low concentrations, indicative of protein-mediated transport. Fatty acid transport proteins are a family of transmembrane proteins that enhance LCFA uptake and are produced in all fatty acid-utilizing tissues. Here, we review our current understanding of the function, expression patterns and regulation and subcellular localization of this interesting family of proteins.

Functional roles for the cytoplasmic domain of the type III transforming growth factor beta receptor in regulating transforming growth factor beta signaling

Transforming growth factor beta (TGF-beta) signals through three high affinity cell surface receptors, TGF-beta type I, type II, and type III receptors. The type III receptor, also known as betaglycan, binds to the type II receptor and is thought to act solely by "presenting" the TGF-beta ligand to the type II receptor. The short cytoplasmic domain of the type III receptor is thought to have no role in TGF-beta signaling because deletion of this domain has no effect on association with the type II receptor, or with the presentation role of the type III receptor. Here we demonstrate that the cytoplasmic domains of the type III and type II receptors interact specifically in a manner dependent on the kinase activity of the type II receptor and the ability of the type II receptor to autophosphorylate. This interaction results in the phosphorylation of the cytoplasmic domain of the type III receptor by the type II receptor. The type III receptor with the cytoplasmic domain deleted is able to bind TGF-beta, to bind the type II receptor, and to enhance TGF-beta binding to the type II receptor but is unable to enhance TGF-beta2 signaling, determining that the cytoplasmic domain is essential for some functions of the type III receptor. The type III receptor functions by selectively binding the autophosphorylated type II receptor via its cytoplasmic domain, thus promoting the preferential formation of a complex between the autophosphorylated type II receptor and the type I receptor and then dissociating from this active signaling complex. These studies, for the first time, elucidate important functional roles of the cytoplasmic domain of the type III receptor and demonstrate that these roles are essential for regulating TGF-beta signaling.

Insulin-responsive compartments containing GLUT4 in 3T3-L1 and CHO cells: regulation by amino acid concentrations

In fat and muscle, insulin stimulates glucose uptake by rapidly mobilizing the GLUT4 glucose transporter from a specialized intracellular compartment to the plasma membrane. We describe a method to quantify the relative proportion of GLUT4 at the plasma membrane, using flow cytometry to measure a ratio of fluorescence intensities corresponding to the cell surface and total amounts of a tagged GLUT4 reporter in individual living cells. Using this assay, we demonstrate that both 3T3-L1 and CHO cells contain intracellular compartments from which GLUT4 is rapidly mobilized by insulin and that the initial magnitude and kinetics of redistribution to the plasma membrane are similar in these two cell types when they are cultured identically. Targeting of GLUT4 to a highly insulin-responsive compartment in CHO cells is modulated by culture conditions. In particular, we find that amino acids regulate distribution of GLUT4 to this kinetically defined compartment through a rapamycin-sensitive pathway. Amino acids also modulate the magnitude of insulin-stimulated translocation in 3T3-L1 adipocytes. Our results indicate a novel link between glucose and amino acid metabolism.

Ligand-independent oligomerization of cell-surface erythropoietin receptor is mediated by the transmembrane domain

Binding of erythropoietin (Epo) to the Epo receptor (EpoR) is crucial for production of mature red cells. Although it is well established that the Epo-bound EpoR is a dimer, it is not clear whether, in the absence of ligand, the intact EpoR is a monomer or oligomer. Using antibody-mediated immunofluorescence copatching (oligomerizing) of epitope-tagged receptors at the surface of live cells, we show herein that a major fraction of the full-length murine EpoR exists as preformed dimers/oligomers in BOSC cells, which are human embryo kidney 293T-derived cells. This observed oligomerization is specific because, under the same conditions, epitope-tagged EpoR did not oligomerize with several other tagged receptors (thrombopoietin receptor, transforming growth factor beta receptor type II, or prolactin receptor). Strikingly, the EpoR transmembrane (TM) domain but not the extracellular or intracellular domains enabled the prolactin receptor to copatch with EpoR. Preformed EpoR oligomers are not constitutively active and Epo binding was required to induce signaling. In contrast to tyrosine kinase receptors (e.g., insulin receptor), which cannot signal when their TM domain is replaced by the strongly dimerizing TM domain of glycophorin A, the EpoR could tolerate the replacement of its TM domain with that of glycophorin A and retained signaling. We propose a model in which TM domain-induced dimerization maintains unliganded EpoR in an inactive state that can readily be switched to an active state by physiologic levels of Epo.

Ski/Sno and TGF-beta signaling

Transforming growth factor-beta is a potent inhibitor of epithelial cell proliferation. Proteins involved in TGF-beta signaling are bona fide tumor suppressors and many tumor cells acquire the ability to escape TGF-beta growth inhibition through the loss of key signaling transducers in the pathway or through the activation of oncogenes. Recent studies indicate that there is a specific connection between the TGF-beta signaling pathway and the Ski/SnoN family of oncoproteins. We summarize evidence that Ski and SnoN directly associate with Smad proteins and block the ability of the Smads to activate expression of many if not all TGF-beta-responsive genes. This appears to cause abrogation of TGF-beta growth inhibition in epithelial cells.

Proteolytic cleavage product of 30-kDa adipocyte complement-related protein increases fatty acid oxidation in muscle and causes weight loss in mice

Adipocyte complement-related protein (30 kDa) (Acrp30), a secreted protein of unknown function, is exclusively expressed in differentiated adipocytes; its mRNA is decreased in obese humans and mice. Here we describe novel pharmacological properties of the protease-generated globular head domain of Acrp30 (gAcrp30). Acute treatment of mice with gAcrp30 significantly decreased the elevated levels of plasma free fatty acids caused either by administration of a high fat test meal or by i.v. injection of Intralipid. This effect of gAcrp30 was caused, at least in part, by an acute increase in fatty acid oxidation by muscle. As a result, daily administration of a very low dose of gAcrp30 to mice consuming a high-fat/sucrose diet caused profound and sustainable weight reduction without affecting food intake. Thus, gAcrp30 is a novel pharmacological compound that controls energy homeostasis and exerts its effect primarily at the peripheral level.

The erythropoietin receptor cytosolic juxtamembrane domain contains an essential, precisely oriented, hydrophobic motif

We report that the erythropoietin receptor cytosolic juxtamembrane region is conformationally rigid and contains a hydrophobic motif, composed of residues L253, I257, and W258, that is crucial for Janus kinase 2 (JAK2) activation and receptor signaling. Alanine insertion mutagenesis shows that the orientation of this motif and not its distance from the membrane bilayer is critical. Intragenic complementation studies suggest that L253 is contained within an alpha helix functionally continuous to the transmembrane alpha helix. The alpha-helical orientation of L53 is required not for JAK2 activation but for activated JAK2 to induce phosphorylation of the erythropoietin receptor. This motif is highly conserved among cytokine receptors and couples ligand-induced conformational changes in the receptor to intracellular activation of JAK2.

Identification of a novel immunoreceptor tyrosine-based activation motif-containing molecule, STAM2, by mass spectrometry and its involvement in growth factor and cytokine receptor signaling pathways

In an effort to clone novel tyrosine-phosphorylated substrates of the epidermal growth factor receptor, we have initiated an approach coupling affinity purification using anti-phosphotyrosine antibodies to mass spectrometry-based identification. Here, we report the identification of a signaling molecule containing a Src homology 3 domain as well as an immunoreceptor tyrosine-based activation motif (ITAM). This molecule is 55% identical to a previously isolated molecule designated signal transducing adaptor molecule (STAM) that was identified as an interleukin (IL)-2-induced phosphoprotein and is therefore designated STAM2. Tyrosine phosphorylation of STAM2 is induced by growth factors such as epidermal growth factor and platelet-derived growth factor as well as by cytokines like IL-3. Several of the deletion mutants tested except the one containing only the amino-terminal region underwent tyrosine phosphorylation upon growth factor stimulation, implying that STAM2 is phosphorylated on several tyrosine residues. STAM2 is downstream of the Jak family of kinases since coexpression of STAM2 with Jak1 or Jak2 but not an unrelated Tec family kinase, Etk, resulted in its tyrosine phosphorylation. In contrast to epidermal growth factor receptor-induced phosphorylation, this required the ITAM domain since mutants lacking this region did not undergo tyrosine phosphorylation. Finally, overexpression of wild type STAM2 led to an increase in IL-2-mediated induction of c-Myc promoter activation indicating that it potentiates cytokine receptor signaling.

Tankyrase is a golgi-associated mitogen-activated protein kinase substrate that interacts with IRAP in GLUT4 vesicles

The poly(ADP-ribose) polymerase tankyrase was originally described as a telomeric protein whose catalytic activity was proposed to regulate telomere function. Subsequent studies revealed that most tankyrase is actually extranuclear, but a discordant pattern of cytoplasmic targeting was reported. Here we used fractionation and immunofluorescence to show in 3T3-L1 fibroblasts that tankyrase is a peripheral membrane protein associated with the Golgi. We further colocalized tankyrase with GLUT4 storage vesicles in the juxtanuclear region of adipocytes. Consistent with this colocalization, we found that tankyrase binds specifically to a resident protein of GLUT4 vesicles, IRAP (insulin-responsive amino peptidase). The binding of tankyrase to IRAP involves the ankyrin repeats of tankyrase and a defined sequence ((96)RQSPDG(101)) in the IRAP cytosolic domain (IRAP(1-109)). Tankyrase is a novel signaling target of mitogen-activated protein kinase (MAPK); it is stoichiometrically phosphorylated upon insulin stimulation. Phosphorylation enhances the poly(ADP-ribose) polymerase activity of tankyrase but apparently does not mediate the acute effect of insulin on GLUT4 targeting. Taken together, tankyrase is a novel target of MAPK signaling in the Golgi, where it is tethered to GLUT4 vesicles by binding to IRAP. We speculate that tankyrase may be involved in the long term effect of the MAPK cascade on the metabolism of GLUT4 vesicles.

Disruption of TGF-beta growth inhibition by oncogenic ras is linked to p27Kip1 mislocalization

Expression of oncogenic Ras in epithelial tumor cells is linked to the loss of transforming growth factor-beta (TGF-beta) anti-proliferative activity, and was proposed to involve inhibition of Smad2/3 nuclear translocation. Here we studied several epithelial cell lines expressing oncogenic N-RasK61 and show that TGF-beta-induced nuclear translocation of and transcriptional activation by Smad2/3 were unaffected. In contrast, oncogenic Ras mediated nuclearto-cytoplasmic mislocalization of p27KiP1 (p27) and of the cyclin-dependent kinase (CDK) CDK6, but not CDK2. Concomitantly, oncogenic Ras abrogated the ability of TGF-beta to release p27 from CDK6, to enhance its binding to CDK2 and to inhibit CDK2 activity. Inactivation of Ras by a specific antagonist restored the growth inhibitory response to TGF-beta with concurrent normalization of p27 and CDK6 localization. Therefore, the disruption of TGF-beta-mediated growth inhibition by oncogenic Ras appears to be due to lack of inhibition of CDK2, caused by the sequestration of p27 and CDK2 in different subcellular compartments and by the loss of TGF-beta-induced partner switching of p27 from CDK6 to CDK2.

Critical role of Smads and AP-1 complex in transforming growth factor-beta -dependent apoptosis

Transforming growth factor-beta1 (TGF-beta1) induces not only cell growth inhibition but also apoptosis in hepatocytes, myeloid cells, and epithelial cells. Although Smad proteins are identified as key signal transducers in TGF-beta1-dependent growth inhibition, their roles in the induction of apoptosis are unclear. We show here that both Smad proteins and AP-1 complex are involved in TGF-beta1 signaling for apoptosis. Overexpression of a dominant-negative Smad3 mutant or Smad7, both of which impair Smad-mediated signal transduction, inhibits TGF-beta1-dependent apoptosis. Only the JunD. FosB form of the AP-1 complex is markedly activated during TGF-beta1-dependent apoptosis. FosB substantially enhances Smad3. Smad4-dependent transcription, and dominant-negative FosB blocks TGF-beta1-dependent apoptosis but not growth inhibition. Expression of JunD.FosB enhances induction of apoptosis by TGF-beta1. Moreover, JunD.FosB binds to the 12-O-tetradecanoyl-13-acetate-responsive gene promoter element and recruits Smad3.Smad4 to form a multicomponent complex. These results suggest that Smad proteins and AP-1 complex synergize to mediate TGF-beta1-dependent apoptosis.

Synergism between transcription factors TFE3 and Smad3 in transforming growth factor-beta-induced transcription of the Smad7 gene

Activation of transforming growth factor-beta (TGF-beta) receptors triggers phosphorylation of Smad2 and Smad3. After binding to Smad4, the complex enters the nucleus and interacts with other transcription factors to activate gene transcription. Unlike other Smads, Smad7 inhibits phosphorylation of Smad2 and Smad3, and its transcription is induced by TGF-beta, suggesting a negative feedback loop. Here, we show that TFE3 and Smad3 synergistically mediate TGF-beta-induced transcription from the Smad7 promoter by binding to an E-box and two adjacent Smad binding elements (SBEs), respectively. A precise 3-base pair spacer between one SBE and the E-box is essential. Previously, we showed that a similar arrangement between a SBE and an E-box of an element is essential for TGF-beta-dependent transcription of the plasminogen activator inhibitor-1 gene (PAI-1) and that TGF-beta-induced phosphorylation of Smad3 triggers its association with TFE3. Thus, TFE3-Smad3 response elements may represent a common target for TGF-beta-induced gene expression.

Importin beta mediates nuclear translocation of Smad 3

Smad proteins are intracellular mediators of transforming growth factor-beta (TGF-beta) and related cytokines. Although ligand-induced nuclear translocation of Smad proteins is clearly established, the pathway mediating this import is yet to be determined. We previously identified a nuclear localization signal (NLS) in the N-terminal region of Smad 3, the major Smad protein involved in TGF-beta signal transduction. This basic motif (Lys(40-)Lys-Leu-Lys-Lys(44)), conserved among all the pathway-specific Smad proteins, is required for Smad 3 nuclear import in response to ligand. Here we studied the nuclear import pathway of Smad 3 mediated by this NLS. We demonstrate that the isolated Smad 3 MH1 domain displays significant specific binding to importin beta, which is diminished or eliminated by mutations in the NLS. Full-size Smad 3 exhibits weak but specific binding to importin beta, which is enhanced after phosphorylation by the type I TGF-beta receptor. In contrast, no interaction was observed between importin alpha and Smad 3 or its MH1 domain, indicating that nuclear translocation of Smad proteins may occur through direct binding to importin beta. We propose that activation of all of the pathway-specific Smad proteins (Smads 1, 2, 3, 5, 8, and 9) exposes the conserved NLS motif, which then binds directly to importin beta and triggers nuclear translocation.

A distinct nuclear localization signal in the N terminus of Smad 3 determines its ligand-induced nuclear translocation

Smad proteins are intracellular mediators of transforming growth factor beta (TGF-beta) and related cytokines and undergo ligand-induced nuclear translocation. Here we describe the identification of a nuclear localization signal (NLS) in the N-terminal region of Smad 3, the major Smad protein involved in TGF-beta signaling. An NLS-like basic motif (Lys(40)-Lys-Leu-Lys-Lys(44)), conserved among all pathway-specific Smad proteins, not only is responsible for constitutive nuclear localization of the isolated Smad 3 MH1 domain but also is crucial for Smad 3 nuclear import in response to ligand. Mutations in this motif completely abolished TGF-beta-induced nuclear translocation but had no impact on ligand-induced phosphorylation of Smad 3, complex formation with Smad 4, or specific binding to DNA. Hence Smad 3 proteins with NLS mutations are dominant-negative inhibitors of TGF-beta-induced transcriptional activation. Smad 4, which cannot translocate into the nucleus in the absence of Smad 3 or another pathway-specific Smad, contains a Glu in place of the last Lys in this motif. Smad 3 harboring the same mutation (K44E) does not undergo ligand-induced nuclear import. Conversely, the isolated Smad 4 MH1 domain does not accumulate in the nucleus but becomes nuclear enriched when Glu(49) is replaced with Lys. We propose that this highly conserved five-residue NLS motif determines ligand-induced nuclear translocation of all pathway-specific Smads.

Cloning of a receptor subunit required for signaling by thymic stromal lymphopoietin

Signaling by type I cytokines involves the formation of receptor homodimers, heterodimers or higher order receptor oligomers. Here we report the cloning of a type I cytokine receptor subunit that is most closely related to the common cytokine receptor gamma chain (gamma c). Binding and crosslinking experiments demonstrate that this protein is the receptor for a recently described interleukin 7 (IL-7)-like factor, thymic stromal lymphopoietin (TSLP). Binding of TSLP to the thymic stromal lymphopoietin receptor (TSLPR) is increased markedly in the presence of the IL-7 receptor alpha chain (IL-7R alpha). IL-7R alpha-expressing but not parental 32D cells proliferate in the presence of exogenous TSLP. Moreover, a combination of IL-7R alpha and TSLPR is required for TSLP-dependent activation of a STAT5-dependent reporter construct. Thus it is shown that IL-7R alpha is a component of both the IL-7 and TSLP receptors, which helps to explain why deletion of the gene that encodes IL-7R alpha affects the lymphoid system more severely than deletion of the gene encoding IL-7 does. Cloning of TSLPR should facilitate an understanding of TSLP function and its signaling mechanism.

Generation of mammalian cells stably expressing multiple genes at predetermined levels

Expression of cloned genes at desired levels in cultured mammalian cells is essential for studying protein function. Controlled levels of expression have been difficult to achieve, especially for cell lines with low transfection efficiency or when expression of multiple genes is required. An internal ribosomal entry site (IRES) has been incorporated into many types of expression vectors to allow simultaneous expression of two genes. However, there has been no systematic quantitative analysis of expression levels in individual cells of genes linked by an IRES, and thus the broad use of these vectors in functional analysis has been limited. We constructed a set of retroviral expression vectors containing an IRES followed by a quantitative selectable marker such as green fluorescent protein (GFP) or truncated cell surface proteins CD2 or CD4. The gene of interest is placed in a multiple cloning site 5' of the IRES sequence under the control of the retroviral long terminal repeat (LTR) promoter. These vectors exploit the approximately 100-fold differences in levels of expression of a retrovirus vector depending on its site of insertion in the host chromosome. We show that the level of expression of the gene downstream of the IRES and the expression level and functional activity of the gene cloned upstream of the IRES are highly correlated in stably infected target cells. This feature makes our vectors extremely useful for the rapid generation of stably transfected cell populations or clonal cell lines expressing specific amounts of a desired protein simply by fluorescent activated cell sorting (FACS) based on the level of expression of the gene downstream of the IRES. We show how these vectors can be used to generate cells expressing high levels of the erythropoietin receptor (EpoR) or a dominant negative Smad3 protein and to generate cells expressing two different cloned proteins, Ski and Smad4. Correlation of a biologic effect with the level of expression of the protein downstream of the IRES provides strong evidence for the function of the protein placed upstream of the IRES.

Pantophysin is a phosphoprotein component of adipocyte transport vesicles and associates with GLUT4-containing vesicles

Pantophysin, a protein related to the neuroendocrine-specific synaptophysin, recently has been identified in non-neuronal tissues. In the present study, Northern blots showed that pantophysin mRNA was abundant in adipose tissue and increased during adipogenesis of 3T3-L1 cells. Immunoblot analysis of subcellular fractions showed pantophysin present exclusively in membrane fractions and relatively evenly distributed in the plasma membrane and internal membrane fractions. Sucrose gradient ultracentrifugation demonstrated that pantophysin and GLUT4 exhibited overlapping distribution profiles. Furthermore, immunopurified GLUT4 vesicles contained pantophysin, and both GLUT4 and pantophysin were depleted from this vesicle population following treatment with insulin. Additionally, a subpopulation of immunopurified pantophysin vesicles contained insulin-responsive GLUT4. Consistent with the interaction of synaptophysin with vesicle-associated membrane protein 2 in neuroendocrine tissues, pantophysin associated with vesicle-associated membrane protein 2 in adipocytes. Furthermore, in [(32)P]orthophosphate-labeled cells, pantophysin was phosphorylated in the basal state. This phosphorylation was unchanged in response to insulin; however, insulin stimulated the phosphorylation of a 77-kDa protein associated with alpha-pantophysin immunoprecipitates. Although the functional role of pantophysin in vesicle trafficking is unclear, its presence on GLUT4 vesicles is consistent with the emerging role of soluble N-ethylmaleimide-sensitive protein receptor (SNARE) factor complex and related proteins in regulated vesicle transport in adipocytes. In addition, pantophysin may provide a marker for the analysis of other vesicles in adipocytes.

Analysis of receptor signaling pathways by mass spectrometry: identification of vav-2 as a substrate of the epidermal and platelet-derived growth factor receptors

Oligomerization of receptor protein tyrosine kinases such as the epidermal growth factor receptor (EGFR) by their cognate ligands leads to activation of the receptor. Transphosphorylation of the receptor subunits is followed by the recruitment of signaling molecules containing src homology 2 (SH2) or phosphotyrosine interaction domains (PID). Additionally, several cytoplasmic proteins that may or may not associate with the receptor undergo tyrosine phosphorylation. To identify several components of the EGFR signaling pathway in a single step, we have immunoprecipitated molecules that are tyrosine phosphorylated in response to EGF and analyzed them by one-dimensional gel electrophoresis followed by mass spectrometry. Combining matrix-assisted laser desorption/ionization (MALDI) and nanoelectrospray tandem mass spectrometry (MS/MS) led to the identification of nine signaling molecules, seven of which had previously been implicated in EGFR signaling. Several of these molecules were identified from low femtomole levels of protein loaded onto the gel. We identified Vav-2, a recently discovered guanosine nucleotide exchange factor that is expressed ubiquitously, as a substrate of the EGFR. We demonstrate that Vav-2 is phosphorylated on tyrosine residues in response to EGF and associates with the EGFR in vivo. Binding of Vav-2 to the EGFR is mediated by the SH2 domain of Vav-2. In keeping with its ubiquitous expression, Vav-2 seems to be a general signaling molecule, since it also associates with the platelet-derived growth factor (PDGF) receptor and undergoes tyrosine phosphorylation in fibroblasts upon PDGF stimulation. The strategy suggested here can be used for routine identification of downstream components of cell surface receptors in mammalian cells.

Specificity in transforming growth factor beta-induced transcription of the plasminogen activator inhibitor-1 gene: interactions of promoter DNA, transcription factor muE3, and Smad proteins

Transforming growth factor beta (TGF-beta) regulates a broad range of biological processes, including cell growth, development, differentiation, and immunity. TGF-beta signals through its cell surface receptor serine kinases that phosphorylate Smad2 or Smad3 proteins. Because Smad3 and its partner Smad4 bind to only 4-bp Smad binding elements (SBEs) in DNA, a central question is how specificity of TGF-beta-induced transcription is achieved. We show that Smad3 selectively binds to two of the three SBEs in PE2.1, a TGF-beta-inducible fragment of the plasminogen activator inhibitor-1 promoter, to mediate TGF-beta-induced transcription; moreover, a precise 3-bp spacer between one SBE and the E-box, a binding site for transcription factor muE3 (TFE3), is essential for TGF-beta-induced transcription. Whereas an isolated Smad3 MH1 domain binds to TFE3, TGF-beta receptor-mediated phosphorylation of full-length Smad3 enhances its binding to TFE3. Together, these studies elucidate an important mechanism for specificity in TGF-beta-induced transcription of the plasminogen activator inhibitor-1 gene.

BCR-ABL and v-SRC tyrosine kinase oncoproteins support normal erythroid development in erythropoietin receptor-deficient progenitor cells

Erythropoietin (Epo)-independent differentiation of erythroid progenitors is a major characteristic of myeloproliferative disorders, including chronic myeloid leukemia. Epo receptor (EpoR) signaling is crucial for normal erythroid development, as evidenced by the properties of Epo(-/-) and EpoR(-/-) mice, which contain a normal number of fetal liver erythroid progenitors but die in utero from a severe anemia attributable to the absence of red cell maturation. Here we show that two constitutively active cytoplasmic protein tyrosine kinases, P210(BCR-ABL) and v-SRC, can functionally replace the EpoR and support full proliferation, differentiation, and maturation of fetal liver erythroid progenitors from EpoR(-/-) mice. These protein tyrosine kinases can also partially complement the myeloid growth factors IL-3, IL-6, and Steel factor, which are normally required in addition to Epo for erythroid development. Additionally, BCR-ABL mutants that lack residues necessary for transformation of fibroblasts or bone marrow cells can fully support normal erythroid development. These results demonstrate that activated tyrosine kinase oncoproteins implicated in tumorigenesis and human leukemia can functionally complement for cytokine receptor signaling pathways to support normal erythropoiesis in EpoR-deficient cells. Moreover, terminal differentiation of erythroid cells requires generic signals provided by activated protein tyrosine kinases and does not require a specific signal unique to a cytokine receptor.

SnoN and Ski protooncoproteins are rapidly degraded in response to transforming growth factor beta signaling

Transforming growth factor beta (TGF-beta) regulates a variety of physiologic processes, including growth inhibition, differentiation, and induction of apoptosis. Some TGF-beta-initiated signals are conveyed through Smad3; TGF-beta binding to its receptors induces phosphorylation of Smad3, which then migrates to the nucleus where it functions as a transcription factor. We describe here the association of Smad3 with the nuclear protooncogene protein SnoN. Overexpression of SnoN represses transcriptional activation by Smad3. Activation of TGF-beta signaling leads to rapid degradation of SnoN and, to a lesser extent, of the related Ski protein, and this degradation is likely mediated by cellular proteasomes. These results demonstrate the existence of a cascade of the TGF-beta signaling pathway, which, upon TGF-beta stimulation, leads to the destruction of protooncoproteins that antagonize the activation of the TGF-beta signaling.

A deletion in the gene for transforming growth factor beta type I receptor abolishes growth regulation by transforming growth factor beta in a cutaneous T-cell lymphoma

Spontaneous regression of skin lesions is characteristic of lymphomatoid papulosis (LyP), a clonal cutaneous lymphoproliferative disorder. A minority of LyP patients progress to anaplastic large cell lymphoma (ALCL) in which skin lesions no longer regress and extracutaneous dissemination often occurs. In 1 such case, we developed a tumor cell line, JK cells, and show that these cells are resistant to the growth inhibitory effects of transforming growth factor beta (TGF-beta) due to the loss of cell surface expression of the TGF-beta type I receptor (TbetaR-I). Reverse transcriptase-polymerase chain reaction (RT-PCR) and sequencing of JK cell TbetaR-I cDNA clones identified a deletion that spanned the last 178 bp of exon 1, including the initiating methionine. Hybridization of a radiolabeled fragment internal to the deletion was detected in the genomes of TGF-beta-responsive cells, but not in JK cells, indicating that they contain no wild-type TbetaR-I gene. PCR primers that flanked the deleted TbetaR-I region amplified a single band from JK cell genomic DNA that lacked the last 178 bp of exon 1 and all of the approximately 5 kb of intron 1. This JK cell-specific genomic TbetaR-I PCR product was distinct from products amplified from TGF-beta-responsive cells and was also readily detected in tumor biopsies obtained before the establishment of the JK cell line. Our results identify the first inactivating mutation in TbetaR-I gene in a human lymphoma that renders it insensitive to growth inhibition by TGF-beta.

Interaction of the Ski oncoprotein with Smad3 regulates TGF-beta signaling

TGF-beta treatment of cells induces a variety of physiologic responses, including growth inhibition, differentiation, and induction of apoptosis. TGF-beta induces phosphorylation and nuclear translocation of Smad3. We describe here the association of Smad3 with the nuclear protooncogene protein Ski in response to the activation of TGF-beta signaling. Association with Ski represses transcriptional activation by Smad3, and overexpression of Ski renders cells resistant to the growth-inhibitory effects of TGF-beta. The transcriptional repression as well as the growth resistance to TGF-beta by overexpression of Ski can be overcome by overexpression of Smad3. These results demonstrate that Ski is a novel component of the TGF-beta signaling pathway and shed light on the mechanism of action of the Ski oncoprotein.

Identification of the major intestinal fatty acid transport protein

While intestinal transport systems for metabolites such as carbohydrates have been well characterized, the molecular mechanisms of fatty acid (FA) transport across the apical plasmalemma of enterocytes have remained largely unclear. Here, we show that FATP4, a member of a large family of FA transport proteins (FATPs), is expressed at high levels on the apical side of mature enterocytes in the small intestine. Further, overexpression of FATP4 in 293 cells facilitates uptake of long chain FAs with the same specificity as enterocytes, while reduction of FATP4 expression in primary enterocytes by antisense oligonucleotides inhibits FA uptake by 50%. This suggests that FATP4 is the principal fatty acid transporter in enterocytes and may constitute a novel target for antiobesity therapy.

Growth factor independence and BCR/ABL transformation: promise and pitfalls of murine model systems and assays

The expression of the BCR-ABL fusion oncoprotein in primitive hematopoietic cells results in chronic myeloid leukemia. Over the past decade studies of several in vitro and in vivo cell systems revealed multiple signal transduction pathways activated by BCR-ABL. However, the precise function of BCR-ABL in the pathogenesis of CML is still unclear. The goal of this review is to synthesize data on intracellular signaling in the context of the diverse murine assay systems employed. We emphasize the importance of in vivo assays and assays using primary cells in understanding the biology of CML and the molecular mechanisms by which BCR-ABL exerts its effects.

Fetal anemia and apoptosis of red cell progenitors in Stat5a-/-5b-/- mice: a direct role for Stat5 in Bcl-X(L) induction

The erythropoietin receptor (EpoR) is essential for production of red blood cells; a principal function of EpoR is to rescue committed erythroid progenitors from apoptosis. Stat5 is rapidly activated following EpoR stimulation, but its function in erythropoiesis has been unclear since adult Stat5a-/-5b-/- mice have normal steady-state hematocrit. Here we show that Stat5 is essential for the high erythropoietic rate during fetal development. Stat5a-/-5b-/- embryos are severely anemic; erythroid progenitors are present in low numbers, show higher levels of apoptosis, and are less responsive to Epo. These findings are explained by a crucial role for Stat5 in EpoR's antiapoptotic signaling: it mediates the immediate-early induction of Bcl-X(L) in erythroid cells through direct binding to the Bcl-X promoter.

Activation of the erythropoietin receptor by the gp55-P viral envelope protein is determined by a single amino acid in its transmembrane domain

The spleen focus forming virus (SFFV) gp55-P envelope glycoprotein specifically binds to and activates murine erythropoietin receptors (EpoRs) coexpressed in the same cell, triggering proliferation of erythroid progenitors and inducing erythroleukemia. Here we demonstrate specific interactions between the single transmembrane domains of the two proteins that are essential for receptor activation. The human EpoR is not activated by gp55-P but by mutation of a single amino acid, L238, in its transmembrane sequence to its murine counterpart serine, resulting in its ability to be activated. The converse mutation in the murine EpoR (S238L) abolishes activation by gp55-P. Computational searches of interactions between the membrane-spanning segments of murine EpoR and gp55-P provide a possible explanation: the face of the EpoR transmembrane domain containing S238 is predicted to interact specifically with gp55-P but not gp55-A, a variant which is much less effective in activating the murine EpoR. Mutational studies on gp55-P M390, which is predicted to interact with S238, provide additional support for this model. Mutation of M390 to isoleucine, the corresponding residue in gp55-A, abolishes activation, but the gp55-P M390L mutation is fully functional. gp55-P is thought to activate signaling by the EpoR by inducing receptor oligomerization through interactions involving specific transmembrane residues.

Transforming growth factor-beta induces formation of a dithiothreitol-resistant type I/Type II receptor complex in live cells

Transforming growth factor-beta (TGF-beta) binds to and signals via two serine-threonine kinase receptors, the type I (TbetaRI) and type II (TbetaRII) receptors. We have used different and complementary techniques to study the physical nature and ligand dependence of the complex formed by TbetaRI and TbetaRII. Velocity centrifugation of endogenous receptors suggests that ligand-bound TbetaRI and TbetaRII form a heteromeric complex that is most likely a heterotetramer. Antibody-mediated immunofluorescence co-patching of epitope-tagged receptors provides the first evidence in live cells that TbetaRI. TbetaRII complex formation occurs at a low but measurable degree in the absence of ligand, increasing significantly after TGF-beta binding. In addition, we demonstrate that pretreatment of cells with dithiothreitol, which inhibits the binding of TGF-beta to TbetaRI, does not prevent formation of the TbetaRI.TbetaRII complex, but increases its sensitivity to detergent and prevents TGF-beta-activated TbetaRI from phosphorylating Smad3 in vitro. This indicates that either a specific conformation of the TbetaRI. TbetaRII complex, disrupted by dithiothreitol, or direct binding of TGF-beta to TbetaRI is required for signaling.

Expression cloning of LDLB, a gene essential for normal Golgi function and assembly of the ldlCp complex

The Chinese hamster ovary (CHO) cell mutants ldlC and ldlB, which exhibit almost identical phenotypes, define two genes required for multiple steps in the normal medial and trans Golgi-associated processing of glycoconjugates. The LDLC gene encodes ldlCp, an approximately 80-kDa protein, which in wild-type, but not ldlB, cells associates reversibly with the cytoplasmic surface of the Golgi apparatus. Here, we have used a retrovirus-based expression cloning system to clone a murine cDNA, LDLB, that corrects the pleiotropic mutant phenotypes of ldlB cells. The corresponding mRNA was not detected in ldlB mutants. LDLB encodes an approximately 110-kDa protein, ldlBp, which lacks homology to known proteins and contains no common structural motifs. Database searches identified short segments of homology to sequences from Drosophila melanogaster, Arabidopsis thaliana, and Caenorhabditis elegans, and the essentially full-length homologous human sequence (82% identity); however, as was the case for ldlCp, no homologue was identified in Saccharomyces cerevisiae. We have found that in wild-type cell cytosols, ldlCp is a component of an approximately 950-kDa "ldlCp complex," which is smaller, approximately 700 kDa, in ldlB cytosols. Normal assembly of this complex is ldlBp-dependent and may be required for Golgi association of ldlCp and for the normal activities of multiple luminal Golgi processes.

Zinc is essential for binding of p56(lck) to CD4 and CD8alpha

Binding of the protein tyrosine kinase p56(lck) to T-cell co-receptors CD4 and CD8alpha is necessary for T-lymphocyte development and activation. Association of p56(lck) with CD4 requires two conserved cysteine residues in the cytosolic domain of CD4 and two in the amino terminus of p56(lck), consistent with the notion that these four residues coordinate a single metal atom (1-5). Here we demonstrate that Zn2+ is essential for complex formation. In an in vitro binding reaction, Zn2+ mediates p56(lck) association with a glutathione S-transferase (GST) fusion protein containing the cytosolic domains of CD4 or CD8alpha; no other metals tested support binding. Treatment of preformed GST-CD4.p56(lck) dimers with the Zn2+ chelators 1,10-O-phenanthroline or 8-hydroxyquinoline-5-sulfonic acid results in dissociation of GST-CD4 from p56(lck), consistent with the finding of Huse et al. (5) that Zn2+ is contained within similar complexes. Furthermore, we show that, within live cells, CD4.p56(lck) and CD8alpha.p56(lck) interactions occur in a zinc-dependent fashion. Specifically, pretreatment of the human Jurkat T-cell line with membrane permeable zinc chelators disrupts CD4.p56(lck) complexes, and treatment of COS cells co-expressing CD8alpha and p56(lck) with such chelators likewise leads to dissociation of CD8alpha.p56(lck) complexes. CD4. p56(lck) and CD8alpha.p56(lck) represent the first examples of intracellular proteins that require zinc as a bridge for heterodimerization.

Synergistic cooperation of TFE3 and smad proteins in TGF-beta-induced transcription of the plasminogen activator inhibitor-1 gene

Members of the TGF-beta superfamily influence a broad range of biological activities including stimulation of wound healing and inhibition of cell growth. TGF-beta signals through type I and II receptor serine/ threonine kinases and induces transcription of many genes including plasminogen activator inhibitor-1 (PAI-1). To identify proteins that participate in TGF-beta-induced gene expression, we developed a novel retrovirus-mediated expression cloning strategy; and using this approach, we established that transcription factor microE3 (TFE3) is involved in TGF-beta-induced activation of the PAI-1 promoter. We showed that TFE3 binds to an E-box sequence in PE2, a 56-bp promoter fragment of the PAI-1 promoter, and that mutation of this sequence abolishes both TFE3 binding as well as TGF-beta-dependent activation. TFE3 and Smad3 synergistically activate the PE2 promoter and phosphorylated Smad3 and Smad4 bind to a sequence adjacent to the TFE3-binding site in this promoter. Binding of both TFE3 and the Smad proteins to their cognate sequences is indispensable for TGF-beta-inducible activation of the PE2 promoter. Hence, TFE3 is an important transcription factor in at least one TGF-beta-activated signal transduction pathway.

The prolactin receptor rescues EpoR-/- erythroid progenitors and replaces EpoR in a synergistic interaction with c-kit

We recently showed that a retrovirally transduced prolactin receptor (PrlR) efficiently supports the differentiation of wild-type burst-forming unit erythroid (BFU-e) and colony-forming unit erythroid (CFU-e) progenitors in response to prolactin and in the absence of erythropoietin (Epo). To examine directly whether the Epo receptor (EpoR) expressed by wild-type erythroid progenitors was essential for their terminal differentiation, we infected EpoR-/- progenitors with retroviral constructs encoding either the PrlR or a chimeric receptor containing the extracellular domain of the PrlR and intracellular domain of EpoR. In response to prolactin, both receptors were equally efficient in supporting full differentiation of the EpoR-/- progenitors into erythroid colonies in vitro. Therefore, there is no requirement for an EpoR-unique signal in erythroid differentiation; EpoR signaling has no instructive role in red blood cell differentiation. A synergistic interaction between EpoR and c-kit is essential for the production of normal numbers of red blood cells, as demonstrated by the severe anemia of mice mutant for either c-kit or its ligand, stem cell factor. We show that the addition of stem cell factor potentiates the ability of the PrlR to support differentiation of both EpoR-/- and wild-type CFU-e progenitors. This synergism is quantitatively equivalent to that observed between c-kit and EpoR. Therefore, there is no requirement for an EpoR-unique signal in the synergistic interaction between c-kit and EpoR.

A family of fatty acid transporters conserved from mycobacterium to man

Long chain fatty acids (LCFAs) are an important source of energy for most organisms. They also function as blood hormones, regulating key metabolic functions such as hepatic glucose production. Although LCFAs can diffuse through the hydrophobic core of the plasma membrane into cells, this nonspecific transport cannot account for the high affinity and specific transport of LCFAs exhibited by cells such as cardiac muscle, hepatocytes, and adipocytes. Transport of LCFAs across the plasma membrane is facilitated by fatty acid transport protein (FATP), a plasma membrane protein that increases LCFA uptake when expressed in cultured mammalian cells [Schaffer, J. E. & Lodish, H. F. (1994) Cell 79, 427-436]. Here, we report the identification of four novel murine FATPs, one of which is expressed exclusively in liver and another only in liver and kidney. Both genes increase fatty acid uptake when expressed in mammalian cells. All five murine FATPs have homologues in humans in addition to a sixth FATP gene. FATPs are found in such diverse organisms as Fugu rubripes, Caenorhabditis elegans, Drosophila melanogaster, Saccharomyces cerevisiae, and Mycobacterium tuberculosis. The function of the FATP gene family is conserved throughout evolution as the C. elegans and mycobacterial FATPs facilitate LCFA uptake when overexpressed in COS cells or Escherichia coli, respectively. The identification of this evolutionary conserved fatty acid transporter family will allow us to gain a better understanding of the mechanisms whereby LCFAs traverse the lipid bilayer as well as yield insight into the control of energy homeostasis and its dysregulation in diseases such as diabetes and obesity.