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Biomedical subjects

T A Stewart

Publications and source records attributed to T A Stewart.

At least 37 records · Page 2Linked to original sources
PubMed1995

Membrane glycoprotein PC-1 and insulin resistance in non-insulin-dependent diabetes mellitus.

Most patients with non-insulin-dependent diabetes mellitus are resistant to both endogenous and exogenous insulin. Insulin resistance precedes the onset of this disease, suggesting that it may be an initial abnormality. Insulin-receptor kinase activity is impaired in muscle, fibroblasts and other tissues of many patients with non-insulin-dependent diabetes mellitus, but abnormalities in the insulin-receptor gene do not appear to be the cause of this decreased kinase activity. Skin fibroblasts from certain insulin-resistant patients contain an inhibitor of insulin-receptor tyrosine kinase. Here we show that this inhibitor is a membrane glycoprotein, termed PC-1 (refs 10, 11). We find that PC-1 activity is increased in fibroblasts from seven of nine patients with typical non-insulin-dependent diabetes mellitus. In addition, overexpression of PC-1 in transfected cultured cells reduces insulin-stimulated tyrosine kinase activity. These studies raise the possibility that PC-1 has a role in the insulin resistance of non-insulin-dependent diabetes mellitus.

Adult↗
PubMed1995

Interferon expression in the pancreases of patients with type I diabetes.

We have used a reverse transcriptase-polymerase chain reaction (RT-PCR) protocol to examine the expression of cytokines in the pancreases and islets of patients with type I diabetes. We detect a significant increase in the level of expression of interferon (IFN)-alpha in the pancreases of the diabetic patients as compared with the control pancreases. In contrast, IFN-beta was detected at comparable levels in both groups, while IFN-gamma was detected in three of four control pancreases and one of four pancreases from the diabetic individuals. The IFN-alpha cDNAs generated by the RT-PCR were cloned and sequenced to determine which alpha-subtypes were being expressed. We found that the repertoire of subtypes was quite limited in any one individual (diabetic or not), although each individual was different with respect to the pattern of subtypes expressed. We also examined these pancreases for the expression of tumor necrosis factor (TNF)-alpha, interleukin (IL)-1 beta, IL-2, IL-4, and IL-6. We found no detectable expression of TNF-alpha or IL-2 in any pancreases, and the expression of the other cytokines was variable, with no pattern emerging from the comparison of the diabetic and nondiabetic individuals. We conclude that, of the cytokines examined, only IFN-alpha was significantly increased in the diabetic patients, a result that is consistent with the possibility that this cytokine is directly involved in the development of type I diabetes.

Adolescent↗
PubMed1994

Islet expression of interferon-alpha precedes diabetes in both the BB rat and streptozotocin-treated mice.

The mechanism(s) leading to beta cell dysfunction in type I diabetes has not been defined. We have investigated whether islet expression of IFN alpha could be a cause of the lesions that are hallmarks of type I diabetes. Streptozotocin induces the expression of interferon-alpha by pancreatic islets prior to the diabetes induced by streptozotocin. Increased IFN alpha, induced by poly I/C or expressed from a transgene will exacerbate the diabetogenic effects of streptozotocin. In another rodent model of type I diabetes (the BB rat), islet expression of IFN alpha precedes lymphocytic infiltration and diabetes. As in the streptozotocin model, in the BB rats poly I/C will induce islet expression of IFN alpha and accelerate the onset of diabetes. These results are consistent with the hypothesis that islet expression of IFN alpha participates in causing type I diabetes.

Animals↗
PubMed1993

Disseminated tuberculosis in interferon gamma gene-disrupted mice.

The expression of protective immunity to Mycobacterium tuberculosis in mice is mediated by T lymphocytes that secrete cytokines. These molecules then mediate a variety of roles, including the activation of parasitized host macrophages, and the recruitment of other mononuclear phagocytes to the site of the infection in order to initiate granuloma formation. Among these cytokines, interferon gamma (IFN-gamma) is believed to play a key role is these events. In confirmation of this hypothesis, we show in this study that mice in which the IFN-gamma gene has been disrupted were unable to contain or control a normally sublethal dose of M. tuberculosis, delivered either intravenously or aerogenically. In such mice, a progressive and widespread tissue destruction and necrosis, associated with very high numbers of acid-fast bacilli, was observed. In contrast, despite the lack of protective immunity, some DTH-like reactivity could still be elicited. These data, therefore, indicate that although IFN-gamma may not be needed for DTH expression, it plays a pivotal and essential role in protective cellular immunity to tuberculosis infection.

Animals↗
PubMed1993

An essential role for interferon gamma in resistance to Mycobacterium tuberculosis infection.

Tuberculosis, a major health problem in developing countries, has reemerged in recent years in many industrialized countries. The increased susceptibility of immunocompromised individuals to tuberculosis, and many experimental studies indicate that T cell-mediated immunity plays an important role in resistance. The lymphokine interferon gamma (IFN-gamma) is thought to be a principal mediator of macrophage activation and resistance to intracellular pathogens. Mice have been developed which fail to produce IFN-gamma (gko), because of a targeted disruption of the gene for IFN-gamma. Upon infection with Mycobacterium tuberculosis, although they develop granulomas, gko mice fail to produce reactive nitrogen intermediates and are unable to restrict the growth of the bacilli. In contrast to control mice, gko mice exhibit heightened tissue necrosis and succumb to a rapid and fatal course of tuberculosis that could be delayed, but not prevented, by treatment with exogenous recombinant IFN-gamma.

Amino Acid Oxidoreductases↗
PubMed1993

Response to influenza infection in mice with a targeted disruption in the interferon gamma gene.

Interferon gamma (IFN-gamma) is a pleiotropic cytokine secreted by T lymphocytes and natural killer (NK) cells and has been noted to be a first line of host defense in the control of viral infections. To examine further the role of this cytokine in the control of viral infections, mice with a targeted mutation in the IFN-gamma gene were infected with influenza virus, and the in vivo antibody and cell-mediated immune response to viral infection were examined. In addition, cell lines and clones were derived from the immunized animals and the in vitro cytokine production and cytotoxic T lymphocyte (CTL) response were analyzed. The absence of IFN-gamma led to increased production of influenza-specific IgG1, IL-4, and IL-5 as compared to wild-type littermate control animals. In contrast, there was no difference noted in the development of an effective CTL response between IFN-gamma-deficient and wild-type animals. In this model of experimental influenza infection, IFN-gamma is not necessary for the development of an effective humoral or cellular immune response to challenge with this respiratory virus.

Animals↗
PubMed1993

Induction of type I diabetes by interferon-alpha in transgenic mice.

Type I diabetes is an autoimmune disease involving an interaction between an epigenetic event (possibly a viral infection), the pancreatic beta cells, and the immune system in a genetically susceptible host. The possibility that the type I interferons could mediate this interaction was tested with transgenic mice in which the insulin-producing beta cells expressed an interferon-alpha. These mice developed a hypoinsulinemic diabetes associated with a mixed inflammation centered on the islets. The inflammation and the diabetes were prevented with a neutralizing antibody to the interferon-alpha. Thus, the expression of interferon-alpha by the beta cells could be causal in the development of type I diabetes, which suggests a therapeutic approach to this disease.

Animals↗
PubMed1993

Multiple defects of immune cell function in mice with disrupted interferon-gamma genes.

Interferon-gamma (IFN-gamma) is a pleiotrophic cytokine with immunomodulatory effects on a variety of immune cells. Mice with a targeted disruption of the IFN-gamma gene were generated. These mice developed normally and were healthy in the absence of pathogens. However, mice deficient in IFN-gamma had impaired production of macrophage antimicrobial products and reduced expression of macrophage major histocompatibility complex class II antigens. IFN-gamma-deficient mice were killed by a sublethal dose of the intracellular pathogen Mycobacterium bovis. Splenocytes exhibited uncontrolled proliferation in response to mitogen and alloantigen. After a mixed lymphocyte reaction, T cell cytolytic activity was enhanced against allogeneic target cells. Resting splenic natural killer cell activity was reduced in IFN-gamma-deficient mice. Thus, IFN-gamma is essential for the function of several cell types of the murine immune system.

Animals↗
PubMed1993

An evaluation of mechanisms by which tolerance to organ-specific antigens is lost using a transgenic mouse model.

Patients with type I diabetes lose immunologic tolerance to beta cell Ag and produce anti-self (beta cell) antibodies. We have examined mechanisms by which this self tolerance is lost using transgenic mice in which islet cells express human CD4. These mice are immunologically tolerant to this human protein; the tolerance appears to be characterized by the presence of self reactive B cells and nonresponsive T cells. Autoantibodies can be induced by immunization with CD4 in CFA; this autoantibody response is accompanied by minor peri-islet inflammation but diabetes is not induced. Tolerance can also be broken by provision of an independent covalently linked T cell epitope. Although this mimic can lead to the appearance of an autoantibody response, the autoimmune response can not be maintained by the endogenous Ag. We have also demonstrated that an inflammatory lesion leading to death of the CD4-containing beta cells may not be sufficient to break tolerance. These results have implications for potential mechanisms relating to the appearance of autoantibodies in patients with type I diabetes.

Animals↗
PubMed1993

IGF-I is required for normal embryonic growth in mice.

IGF-I is a pleiotropic hormone reported to affect linear growth, glucose metabolism, organ homeostasis, and the immune and neurologic systems. In contrast to IGF-II, IGF-I is expressed at low levels embryonically and has been thought to be more important for postnatal growth and development. To investigate the role of IGF-I in normal development we generated mice with an inactive IGF-I gene by homologous recombination in ES cells. Heterozygous mice are healthy and fertile, but they are 10-20% smaller than wild-type littermates and have lower than normal levels of IGF-I. The size reduction is attributable to a decrease in organs and muscle and bone mass. However, all tissues appear histologically normal. At birth homozygous mutant mice (IGF-I-/-) are < 60% body weight of wild type. Greater than 95% of IGF-I-/- pups die perinatally. Histopathology is characterized by underdevelopment of muscle tissue. Lungs of late embryonic and neonates also appeared less organized with ill-defined alveolae. IGF-I appears to be essential for correct embryonic development in mice.

Animals↗
PubMed1992

Osteoblast-specific expression of growth hormone stimulates bone growth in transgenic mice.

Growth hormone (GH) is an important regulator of postnatal growth, acting on a wide variety of target tissues. Here, we show that local production of GH in osteoblasts is able to stimulate bone growth directly without significant systemic effects. Mice were made transgenic by microinjection of an osteocalcin-human GH (osteocalcin-hGH) gene construct in which approximately 1,800 bp of the rat osteocalcin promoter was fused to the hGH gene. Five lines of transgenic mice, each with measurable amounts of serum hGH (ranging from 1 to 1,000 ng/ml), were analyzed. Northern (RNA) blot hybridization showed that the hGH transcript was detectable only in the bone. Further characterization of hGH mRNA distribution by in situ hybridization revealed that in neonates the most intense signal was found in periosteal osteoblasts, while in adults, trabecular and endosteal osteoblasts were favored. In one transgenic line (992-1), hGH was expressed at a much lower level and had minimal systemic effects; however, the local concentrations of hGH in bone were sufficient to stimulate bone growth in these animals.

Animals↗
PubMed1992

An evaluation of the functions of the 22-kilodalton (kDa), the 20-kDa, and the N-terminal polypeptide forms of human growth hormone using transgenic mice.

Multiple peptide hormones can be derived from the single human GH gene. In addition to the full-length 191-amino acid 22-kilodalton (kDa) form, a 20-kDa variant can be produced by alternative splicing, and a 5-kDa variant can be produced by posttranslational cleavage. To more fully appreciate the physiological roles of these proteins, we have made a comparison of transgenic mice that constitutively overexpress one or another of these variants. We have found that both the 22-kDa and the 20-kDa forms of human GH stimulate linear growth and liver hypertrophy. The increase in linear growth in 22-kDa transgenic mice does not, however, correlate with an increase in circulating IGF-I; rather, the increase in IGF-I that does finally occur correlates with marked liver pathology. Both groups of mice also develop glomerulosclerosis and suffer from hyperinsulinemia. Although there are histologically obvious lesions in the livers of both the 22-kDa and the 20-kDa transgenic mice, only the former exhibit hyperalbuminemia and hypercholesterolemia. Both forms of GH lead to anemia, which is normocytic in the 20-kDa transgenic mice and macrocytic in the 22-kDa transgenic mice. Despite the presence of high levels of the 5-kDa N-terminal form of human GH, the transgenic mice that express this protein are indistinguishable from their nontransgenic littermates.

Animals↗
PubMed1990

Male-specific beta-cell dysfunction and diabetes resulting from increased expression of a syngeneic MHC class I protein in the pancreata of transgenic mice.

It is well established that insulin-dependent diabetes (IDDM) is an autoimmune disease with a strong genetic link to the HLA locus. It is less well understood, however, how the destruction of the insulin-producing beta cells is effected and why neighboring non-beta islet cells are spared. Also incompletely explained are the observations that, unlike other autoimmune diseases such as multiple sclerosis, IDDM does not preferentially affect females, the incidence of the disease is highest among young adults, and there are temporal correlations between the onset of the disease and emotional trauma. We have addressed some of these questions by using transgenic mice that constitutively express the MHC class I antigen Dd in the beta cells of the pancreas. Although both male and female Ins.Dd mice expressed equivalent amounts of the Dd protein only the males developed diabetes. The diabetes in the males could be reversed by castration, and the normoglycemic females became diabetic following either ovariectomy and the implantation of a slow-release pellet containing testosterone or the inclusion of dexamethasone in the drinking water. In contrast, transgenic mice that expressed the herpes simplex virus type 1 glycoprotein D in the pancreatic beta cells were normoglycemic and showed no obvious histopathological consequences. The observation that the beta-cell dysfunction by the increased expression of the MHC class I protein Dd cannot be induced by the herpes viral protein suggests that the cellular damage is related to a specific structure or function of the MHC proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

The gene encoding GnRH and its associated peptide GAP: some insights into hypogonadism.

The hypogonadal (hpg) mouse represents a unique animal model for hypogonadism. In this mutant the truncation of the gene encoding GnRH and its associated peptide GAP leads to drastically lowered gonadotropin levels and increased circulating prolactin. This deficiency in turn leads to a failure of testes and ovaries to develop normally. Using gene therapy we have restored the reproductive functions of the hpg mouse. The success of this therapy uniquely underscores the importance of the gene encoding the GnRH precursor and lends credence to the hypothesis that no other gene in mammals can replace it. As a consequence, defects in the control and/or structural properties of the human GnRH are expected to result in hypogonadism in humans.

Amino Acid Sequence↗
PubMed1989

A dominant phenocopy of hypopituitarism in transgenic mice resulting from central nervous system synthesis of human growth hormone.

We have produced a line of transgenic mice in which expression of human GH has been detected only in the cerebral cortex. Both male and female transgenic mice are growth inhibited with respect to their nontransgenic littermates. Mouse GH mRNA and insulin-like growth factor-I mRNA levels in the pituitary and liver, respectively, are reduced, and circulating insulin-like growth factor-I levels are lower in these mice. Within the hypothalamus somatostatin mRNA levels are increased and GH-releasing factor mRNA levels are reduced compared to those in nontransgenic littermates. We suggest that the growth retardation in these mice is a consequence of the ectopic human GH disturbing the normal controls that regulate mouse GH synthesis and release from the pituitary. These mice provide a resource for analysis of the regulation of GH production and demonstrate that a dominant phenocopy can be made by producing transgenic mice that have local production of an extra-cellular hormone.

Aging↗