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F Liu

Publications and source records attributed to F Liu.

At least 37 records · Page 2Linked to original sources
PubMed1998

Activity-regulated phosphorylation of cAMP response element binding protein in the developing striatum: implications for patterning the neurochemical phenotypes of striatal compartments.

A distinct feature of the striatum is that much of the expression of neurochemicals in this forebrain structure follows a compartmental (striosome/matrix) ordering. How this pattern of neurochemical compartmentation is developed and regulated is still unknown. In the present paper we propose that a key feature of this early striatal plasticity is differential control of the activation kinetics of the cAMP response element binding protein (CREB), which in turn influences striatal expression of genes containing cAMP response elements in their promoter sequences. Specifically, we propose that activity-dependent regulation of the kinetics of CREB phosphorylation, brought about early expressed protein phosphatases and protein phosphatase inhibitors in the developing striatal compartments, helps to set up and to maintain patterning of the neurochemical prototypes characteristic of striatal compartments.

Aging↗
PubMed1998

The insulin-like growth factor axis and growth in children with chronic renal failure: a report of the Southwest Pediatric Nephrology Study Group.

Children with chronic renal failure (CRF) are often growth recarded despite normal serum levels of GH and insulin-like growth factors (IGFs). Recent studies suggest that excess IGF-binding proteins (IGFBPs) in the 35-kDa fractions of CRF serum contribute to CRF growth failure. This report characterizes the relationship between IGFBP-3 and IGF peptides in the serum of growth-retarded CRF children. Size-exclusion chromatography at pH 7.4 found IGFBP-3 and IGFs almost exclusively in the 150-kDa fractions of normal serum, where their molar stoichiometry was approximately 1:1. However, similar chromatography of CRF serum found a molar excess of IGFBP-3 over total IGFs in the 150-kDa fractions and large amounts of IGFs in the 35-kDa fractions. In the 150-kDa fractions of CRF serum, IGFBP-3 was present in normal amounts, but a greater than normal amount was in the form of a 29-kDa IGFBP-3 fragment. Treatment of these CRF children with recombinant human GH increased the molar excess of IGFBP-3 over total IGFs in the 150-kDa fractions, the amount of IGFBP-3 and total IGFs in the 150-kDa fractions, and the amount of IGFs, but not IGFBPs, in the 35-kDa fractions. These data suggest that in untreated CRF children, proteolysis of IGFBP-3 in the 150-kDa fractions releases IGFs to the excess IGFBPs in the 35-kDa fractions, but insufficient IGF is released to overcome the growth-inhibiting effects of these excess IGFBPs. Treatment with recombinant human GH increases levels of IGFs and IGFBP-3 in the 150-kDa fractions, and subsequent IGFBP-3 proteolysis releases sufficient IGF to overcome the growth inhibitory effects of excess IGFBPs in the 35-kDa fractions of CRF serum.

Child↗
PubMed1998

LIF, but not IL-6, regulates osteoprogenitor differentiation in rat calvaria cell cultures: modulation by dexamethasone.

Cytokines of the interleukin 6 (IL-6) subfamily are a group of factors produced by osteoblasts and acting through the same transducing element, membrane protein gp130. We have previously shown that exogenous (added to the culture medium) leukemia inhibitory factor (LIF) inhibits bone nodule formation and expression of osteoblast-associated genes in fetal rat calvaria (RC) cell cultures and that dexamethasone (Dex) increases the ID50 of LIF. To investigate the respective roles of IL-6-related cytokines and receptors in osteprogenitor differentiation, and their regulatory interplay with Dex, we used reverse transcribed polymerase chain reaction, bioassay, and blocking antibody techniques to assess the time courses of LIF, IL-6, LIF transmembrane receptor, IL-6 receptor, and gp130 expression in RC cell cultures grown with and without Dex. The levels of the mRNAs for IL-6, LIF, and gp130 decreased concomitantly with the formation of bone nodules. Dex treatment, which stimulates bone nodule formation, reduced the expression of LIF and IL-6 mRNAs and IL-6 bioactivity in the culture medium. LIF treatment strongly stimulated the expression of IL-6. Incubation with anti-LIF antibodies increased the number of nodules, while an antibody blocking IL-6 activity had little or no effect on nodule numbers and did not antagonize the action of exogenous LIF, indicating that IL-6 does not mediate the action of LIF in this system. Moreover, although exogenously added IL-6 was active in the cultures as noted by a reduction of nodule mineralization, it had no effect on nodule numbers, i.e., on osteoprogenitor differentiation, in the presence or absence of Dex. In conclusion, IL-6, LIF, and their receptors are expressed throughout the time-course of osteogenesis in RC cell cultures. However, only LIF, but not IL-6, appears to play a significant role in autocrine regulation of osteoblastic differentiation in this system. The antagonist action of Dex on the effects of exogenously added LIF, as well as the bone-promoting action of Dex in RC cell cultures, could be exerted partly through the down-regulation of the expression of endogenous LIF.

Animals↗
PubMed1998

Binding of SH2 containing proteins to the insulin receptor: a new way for modulating insulin signalling.

Prior studies have established a role in insulin action for the tyrosine phosphorylation of substrates and their subsequent complexing with SH2 containing proteins. More recently, SH2 proteins have been identified which can tightly bind to the tyrosine phosphorylated insulin receptor. The major protein identified so far (called Grb-IR or Grb10) of this type appears to be present in at least 3 isoforms, varying in the presence of a pleckstrin homology domain and in the sequence of its amino terminus. The binding of this protein to the insulin receptor appears to inhibit signalling by the receptor. The present review will discuss the current knowledge of the structure and function of this protein.

Amino Acid Sequence↗
PubMed1997

Dual role of the Smad4/DPC4 tumor suppressor in TGFbeta-inducible transcriptional complexes.

Upon ligand binding, the receptors of the TGFbeta family phosphorylate Smad proteins, which then move into the nucleus where they activate transcription. To carry out this function, the receptor-activated Smads 1 and 2 require association with the product of deleted in pancreatic carcinoma, locus 4 (DPC4), Smad4. We investigated the step at which Smad4 is required for transcriptional activation. Smad4 is not required for nuclear translocation of Smads 1 or 2, or for association of Smad2 with a DNA binding partner, the winged helix protein FAST-1. Receptor-activated Smad2 takes Smad4 into the nucleus where they form a complex with FAST-1 that requires these three components to activate transcription. Smad4 contributes two functions: Through its amino-terminal domain, Smad4 promotes binding of the Smad2/Smad4/FAST-1 complex to DNA; through its carboxy-terminal domain, Smad4 provides an activation function required for Smad1 or Smad2 to stimulate transcription. The dual function of Smad4 in transcriptional activation underscores its central role in TGFbeta signaling.

Activin Receptors, Type I↗
PubMed1997

Cloning, chromosome localization, expression, and characterization of an Src homology 2 and pleckstrin homology domain-containing insulin receptor binding protein hGrb10gamma.

hGrb10alpha (previously named Grb-IR) is a Src-homology 2 domain-containing protein that binds with high affinity to the tyrosine-phosphorylated insulin receptor and insulin-like growth factor-1 receptor. At least two isoforms of human Grb10, (hGrb10alpha and hGrb10beta), which differ in the pleckstrin homology (PH) domain and the N-terminal sequence, have previously been identified in insulin target tissues such as human skeletal muscle and fat cells. Here we report the cloning of the third isoform of the hGrb10 family (hGrb10gamma) from human skeletal muscle and its localization to human chromosome 7. We have also determined the human chromosome localization of Grb7 to 17q21-q22 and Grb14 to chromosome 2. hGrb10gamma contains an intact PH domain and an N-terminal sequence that is present in hGrb10alpha but absent in hGrb10beta. RNase protection assays and Western blot analysis showed that hGrb10alpha and hGrb10gamma are differentially expressed in insulin target cells including skeletal muscle, liver, and adipocyte cells. hGrb10gamma is also expressed in HeLa cells and various breast cancer cell lines. The protein bound with high affinity to the insulin receptor in cells, and the interaction was dependent on the tyrosine phosphorylation of the receptor. hGrb10gamma also underwent insulin-stimulated membrane translocation and serine phosphorylation. hGrb10gamma phosphorylation was inhibited by PD98059, a specific inhibitor of mitogen-activated protein kinase kinase, and wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase. Taken together, our data suggest that hGrb10 isoforms are potential downstream signaling components of the insulin receptor tyrosine kinase and that the PH domain may play an important role in the involvement of these isoforms in signal transduction pathways initiated by insulin and other growth factors.

Amino Acid Sequence↗
PubMed1997

Protein tyrosine phosphatase 1B interacts with and is tyrosine phosphorylated by the epidermal growth factor receptor.

We used a substrate-trapping technique to search for substrates of protein tyrosine phosphatase (PTP) 1B. A catalytically inactive form of this enzyme forms a stable, phosphotyrosine-dependent complex with epidermal growth factor receptor (EGFR) both in vitro and in cells. PTP1B also interacts with activated platelet-derived growth factor receptor (PDGFR) but not with colony-stimulating factor 1 receptor (CSF-1R). After binding to EGFR, PTP1B becomes tyrosine-phosphorylated at Tyr-66, a site that conforms to the consensus binding sequence for the Src homology 2 (SH2) domains of the adapter protein Grb2. This tyrosine phosphorylation is correlated with a 3-fold increase in PTP catalytic activity. These findings suggest that PTP1B selectively regulates specific activated receptor protein tyrosine kinases (RPTKs) in vivo and might itself be regulated by such receptors.

Animals↗
PubMed1997

The granulocyte colony-stimulating factor receptor is required for the mobilization of murine hematopoietic progenitors into peripheral blood by cyclophosphamide or interleukin-8 but not flt-3 ligand.

Hematopoietic progenitor cells (HPC) can be mobilized from the bone marrow into the peripheral circulation in response to a number of stimuli including hematopoietic growth factors, cytotoxic agents, and certain chemokines. Despite significant differences in their biological activities, these stimuli result in the mobilization of HPC with a similar phenotype, suggesting that a common mechanism for mobilization may exist. In this study, the role of granulocyte colony-stimulating factor (G-CSF) in progenitor mobilization was examined using G-CSF receptor (G-CSFR)-deficient mice. In contrast to wild-type mice, no increase in circulating colony-forming cells (CFU-C), CD34+ lineage- progenitors, or day 12 colony-forming unit-spleen progenitors (CFU-S) was detected in G-CSFR-deficient mice after cyclophosphamide administration. This defect was not due to a failure to regenerate HPC following cyclophosphamide administration as the number of CFU-C in the bone marrow of G-CSFR-deficient mice was increased relative to wild-type mice. Likewise, no increase in circulating CFU-C was detected in G-CSFR-deficient mice following interleukin-8 (IL-8) administration. In contrast, mobilization of HPC in response to flt-3 ligand was nearly normal. These results show that the G-CSFR is required for mobilization in response to cyclophosphamide or IL-8 but not flt-3 ligand and suggest that the G-CSFR may play an important and previously unexpected role in HPC migration.

Animals↗
PubMed1997

Interleukin-6 and the granulocyte colony-stimulating factor receptor are major independent regulators of granulopoiesis in vivo but are not required for lineage commitment or terminal differentiation.

Multiple hematopoietic cytokines can stimulate granulopoiesis; however, their relative importance in vivo and mechanisms of action remain unclear. We recently reported that granulocyte colony-stimulating factor receptor (G-CSFR)-deficient mice have a severe quantitative defect in granulopoiesis despite which phenotypically normal neutrophils were still detected. These results confirmed a role for the G-CSFR as a major regulator of granulopoiesis in vivo, but also indicated that G-CSFR independent mechanisms of granulopoiesis must exist. To explore the role of interleukin-6 (IL-6) in granulopoiesis, we generated IL-6 x G-CSFR doubly deficient mice. The additional loss of IL-6 significantly worsened the neutropenia present in young adult G-CSFR-deficient mice; moreover, exogenous IL-6 stimulated granulopoiesis in vivo in the absence of G-CSFR signals. Near normal numbers of myeloid progenitors were detected in the bone marrow of IL-6 x G-CSFR-deficient mice and their ability to terminally differentiate into mature neutrophils was observed. These results indicate that IL-6 is an independent regulator of granulopoiesis in vivo and show that neither G-CSFR or IL-6 signals are required for the commitment of multipotential progenitors to the myeloid lineage or for their terminal differentiation.

Animals↗
PubMed1997

Characterization of cationic liposome-mediated gene transfer in vivo by intravenous administration.

Physicochemical properties of the cationic liposomes, including structure of the cationic lipid-to-DNA ratio, liposome particle size, and inclusion of the helper lipids, were studied for their effect on the level, site, and duration time of gene expression in vivo by intravenous administration. Using a cytomegalovirus (CMV)-driven gene expression system containing either the luciferase or green fluorescence protein gene as a reporter and two cationic lipids [N-(2,3-dioleoyloxy)propyl-N,N,N-trimethylammonium chloride (DOTMA) and 1,2-dioleoyloxy-3-trimethylammonium propane (DOTAP)], we demonstrated in vivo by a single intravenous injection of DNA/liposome complexes into mice, that cationic liposomes are capable of transfecting cells in organs such as the lung, heart, liver, spleen, and kidney. Transfection efficiency is determined mainly by the structure of the cationic lipid and the ratio of cationic lipid to DNA. Although the presence of cholesterol in DOTAP liposomes did not affect transfection activity, inclusion of dioleoylphosphatidylethanolamine (DOPE) into either DOTAP or DOTMA liposomes significantly decreases liposome transfection activity in vivo. Results form time course show that gene expression in different organs is transient, with a peak level between 4 and 24 hr, dropping to less than 1% of the peak level by day 4. Experiments with repeated injections showed that the peak level of gene expression could be regained by subsequent injection.

Animals↗
PubMed1997

Permeability and metabolic properties of a trophoblast cell line (HRP-1) derived from normal rat placenta.

The HRP-1 cell line is derived from normal rat placenta and appears morphologically similar to and retains characteristic expression of cellular markers of labyrinthine trophoblast cells. In this study, monolayers of HRP-1 cells grown on permeable supports were evaluated as a potential in vitro system to study trophoblast transport and metabolism. The cell line was shown to express and retain functional activity of the predominant placental cytochrome P450 isozyme, CYP1A1. Additionally, the HRP-1 cells retain functional activity of angiotensin I converting enzyme and carboxypeptidase N-like enzyme, peptidases characteristic of the trophoblast. The permeation of several hydrophilic, inert markers across the HRP-1 monolayers was observed to be dependent on effective molecular size and to be passive in nature. Functional asymmetry of the HRP-1 cells was illustrated by the predominant permeation of linoleic acid in the apical-to-basolateral direction across the monolayers. Transferrin passage across HRP-1 monolayers was concentration-dependent, was bidirectional, and could be inhibited by unlabeled transferrin, features typical of the trophoblast transport system for transferrin. Collectively, these properties suggest that the HRP-1 cell line may provide a useful tool for evaluating some of the permeability and metabolic properties of the trophoblast.

Animals↗
PubMed1997

Mechanism of TGFbeta receptor inhibition by FKBP12.

Transforming growth factor-beta (TGFbeta) signaling requires phosphorylation of the type I receptor TbetaR-I by TbetaR-II. Although TGFbeta promotes the association of TbetaR-I with TbetaR-II, these receptor components have affinity for each other which can lead to their ligand-independent activation. The immunophilin FKBP12 binds to TbetaR-I and inhibits its signaling function. We investigated the mechanism and functional significance of this effect. FKBP12 binding to TbetaR-I involves the rapamycin/Leu-Pro binding pocket of FKBP12 and a Leu-Pro sequence located next to the activating phosphorylation sites in TbetaR-I. Mutations in the binding sites of FKBP12 or TbetaR-I abolish the interaction between these proteins, leading to receptor activation in the absence of added ligand. FKBP12 does not inhibit TbetaR-I association with TbetaR-II, but inhibits TbetaR-I phosphorylation by TbetaR-II. Rapamycin, which blocks FKBP12 binding to TbetaR-I, reverses the inhibitory effect of FKBP12 on TbetaR-I phosphorylation. By impeding the activation of TGFbeta receptor complexes formed in the absence of ligand, FKBP12 may provide a safeguard against leaky signaling resulting from the innate tendency of TbetaR-I and TbetaR-II to interact with each other.

Activin Receptors, Type I↗
PubMed1997

[Treatment of a large intracoronary thrombus with urokinase and a chimeric monoclonal platelet aggregation inhibitor].

HISTORY AND CLINICAL FINDINGS: 7 days after an operation for intervertebral disc prolapse a 43-year-old man was referred with the clinical and ECG signs of an acute posterior wall myocardial infarction. INVESTIGATIONS: Creatine kinase (CK) activity was raised to 204 U/I (myocardial-specific isoenzyme CKMB of 23.6 U/I, 11.6% of total) and glutamic-oxalate transferase (GOT) activity to 37 U/I. Emergency cardiac catheterisation, performed 4 hours after renewed onset of precordial pain showed no abnormal findings in the right coronary artery, despite the ECG signs, but a definite filling defect in the anterior interventricular branch, which on intravascular ultrasound was an echo-dense noncalcified structure. TREATMENT AND COURSE: After percutaneous transluminal coronary angioplasty in the area of the obstructing structure a free-floating mass was identified in the proximal part of the anterior interventricular branch, most likely a thrombus. Intercoronary thrombolysis was therefore undertaken with urokinase (bolus of 1 mill. IU) together with the chimeric monoclonal antibody c7E3, which inhibits platelet aggregation by blocking the platelet glycoprotein surface receptor IIb/IIIa. Coronary angiography 12 hours later revealed almost complete dissolution of the previously obstructing mass. CONCLUSION: Combining the platelet aggregation inhibitor c7E3 with a thrombolytic agent is an alternative treatment to the current management of intracoronary thrombi. Intravascular ultrasound is a suitable method for demonstrating angiographically inconspicuous or unclear but pathogenetically significant vessel changes.

Abciximab↗
PubMed1997

The TGF-beta family mediator Smad1 is phosphorylated directly and activated functionally by the BMP receptor kinase.

Bone morphogenetic proteins (BMPs) are members of the TGF-beta family that regulate cell proliferation, apoptosis, and differentiation, and participate in the development of most tissues and organs in vertebrates. Smad proteins function downstream of TGF-beta receptor serine/threonine kinases and undergo serine phosphorylation in response to receptor activation. Smad1 is regulated in this fashion by BMP receptors, and Smad2 and Smad3 by TGF-beta and activin receptors. Here, we report that BMP receptors phosphorylate and activate Smad1 directly. Phosphorylation of Smad1 in vivo involves serines in the carboxy-terminal motif SSXS. These residues are phosphorylated directly by a BMP type I receptor in vitro. Mutation of these carboxy-terminal serines prevents several Smad1 activation events, namely, Smad1 association with the related protein DPC4, accumulation in the nucleus, and gain of transcriptional activity. Similar carboxy-terminal serines in Smad2 are required for its phosphorylation and association with DPC4 in response to TGF-beta, indicating the generality of this process of Smad activation. As a direct physiological substrate of BMP receptors, Smad1 provides a link between receptor serine/threonine kinases and the nucleus.

Activin Receptors↗
PubMed1997

The mature osteoblast phenotype is characterized by extensive plasticity.

While both morphological and biochemical-molecular attributes demarcate differentiation stages in specific cell and tissue types, what constitutes necessary and sufficient expression to define particular cell types is not always known. For example, mature osteoblasts (OBs) are defined morphologically as the cuboidal, biosynthetically active, basophilic cells residing on bone surfaces and responsible for the deposition of osteoid matrix. However, several recent observations suggest that not all mature OBs are identical. To explore further the validity of the hypothesis that heterogeneity of phenotype exists among mature OBs, we grew fetal rat calvaria cells in vitro at low density under conditions in which bone nodules form and mineralize in isolation of other contaminating cell and colony types. Cells resident in mature OB colonies, i.e., those comprising mainly cuboidal cells associated with an osteoid matrix that had begun to mineralize, were analyzed in situ for protein expression by immunocytochemistry with antibodies against collagen type I, alkaline phosphatase, osteopontin, bone sialoprotein, and osteocalcin. Consistent with the expected phenotype of mature OBs, many OBs expressed high levels of all of these markers, but strikingly even adjacent morphologically indistinguishable cuboidal OBs had differences in protein expression, especially in relation to osteopontin, bone sialoprotein, and osteocalcin expression. Double-labeling with Hoechst 33258 and osteocalcin indicated that the variation in antibody labeling intensity/protein expression appeared independent of a variation in cell cycle. To further ascertain the extent of this heterogeneity, 20 single cells were micromanipulated from colonies and subjected to poly(A)-PCR to analyze the simultaneous coexpression profiles of the same five markers analyzed by immunocytochemistry and two other markers, the OB-osteocyte transition marker E11 and the parathyroid hormone/parathyroid hormone-related protein receptor. Notably, the repertoire of genes expressed and their levels of expression varied markedly in individual OBs. The observed heterogeneity suggests that the mature OB phenotype is not a single unique phenotype but rather encompasses a flexible pattern of expression from the repertoire of OB-associated markers.

Alkaline Phosphatase↗
PubMed1997

Novel repeat elements direct rat proenkephalin transcription during spermatogenesis.

The developmental program controlling sperm formation occurs in multiple stages that sequentially involve mitosis, meiosis, and spermiogenesis. The transcriptional mechanisms regulating these distinct phases are poorly understood. In particular, while a required role for the germ cell transcription factor cyclic AMP response element modulator-tau during spermiogenesis has recently been demonstrated, the transcriptional mechanisms leading to early haploid cell formation are unknown. The rat and mouse proenkephalin genes are selectively expressed from an alternate, germ cell-specific promoter in meiotic and early haploid cells. In this study, the minimal rat proenkephalin germ line promoter was localized to a 116-bp region encompassing the transcriptional start site region. Further, a proximal 51-bp sequence located in the 5'-flanking region is absolutely required for germ line promoter activity. This 51 bp sequence corresponds to a previously characterized binding element (GCP1) that forms cell-specific complexes with rat spermatogenic cell nuclear factors distinct from cyclic AMP response element binding proteins. Further, GCP1 contains novel direct repeat sequences required for factor binding and transgene expression in spermatogenic cells. These repeat elements are highly similar to sequences within the active regions of other male germ line promoters expressed during meiosis. GCP1 may therefore contain transcriptional elements that participate more generally during meiosis in the differentiation of spermatocytes and early haploid spermatids.

Animals↗