Biomedical subjects

H Heimberg

Publications and source records attributed to H Heimberg.

29 records · Page 2Linked to original sources

PubMed1996

The glucose sensor protein glucokinase is expressed in glucagon-producing alpha-cells.

Expression of glucokinase in hepatocytes and pancreatic 6-cells is of major physiologic importance to mammalian glucose homeostasis. Liver glucokinase catalyzes the first committed step in the disposal of glucose, and beta-cell glucokinase catalyzes a rate-limiting step required for glucose-regulated insulin release. The present study reports the expression of glucokinase in rat glucagon-producing alpha-cells, which are negatively regulated by glucose. Purified rat alpha-cells express glucokinase mRNA and protein with the same transcript length, nucleotide sequence, and immunoreactivity as the beta-cell isoform. Glucokinase activity accounts for more than 50% of glucose phosphorylation in extracts of alpha-cells and for more than 90% of glucose utilization in intact cells. The glucagon-producing tumor MSL-G-AN also contained glucokinase mRNA, protein, and enzymatic activity. These data indicate that glucokinase may serve as a metabolic glucose sensor in pancreatic alpha-cells and, hence, mediate a mechanism for direct regulation of glucagon release by extracellular glucose. Since these cells do not express Glut2, we suggest that glucose sensing does not necessarily require the coexpression of Glut2 and glucokinase.

1-Methyl-3-isobutylxanthine↗

PubMed1996

Expression and functional activity of glucagon, glucagon-like peptide I, and glucose-dependent insulinotropic peptide receptors in rat pancreatic islet cells.

Rat pancreatic alpha- and beta-cells are critically dependent on hormonal signals generating cyclic AMP (cAMP) as a synergistic messenger for nutrient-induced hormone release. Several peptides of the glucagon-secretin family have been proposed as physiological ligands for cAMP production in beta-cells, but their relative importance for islet function is still unknown. The present study shows expression at the RNA level in beta-cells of receptors for glucagon, glucose-dependent insulinotropic polypeptide (GIP), and glucagon-like peptide I(7-36) amide (GLP-I), while RNA from islet alpha-cells hybridized only with GIP receptor cDNA. Western blots confirmed that GLP-I receptors were expressed in beta-cells and not in alpha-cells. Receptor activity, measured as cellular cAMP production after exposing islet beta-cells for 15 min to a range of peptide concentrations, was already detected using 10 pmol/l GLP-I and 50 pmol/l GIP but required 1 nmol/l glucagon. EC50 values of GLP-I- and GIP-induced cAMP formation were comparable (0.2 nmol/l) and 45-fold lower than the EC50 of glucagon (9 nmol/l). Maximal stimulation of cAMP production was comparable for the three peptides. In purified alpha-cells, 1 nmol/l GLP-I failed to increase cAMP levels, while 10 pmol/l to 10 nmol/l GIP exerted similar stimulatory effects as in beta-cells. In conclusion, these data show that stimulation of glucagon, GLP-I, and GIP receptors in rat beta-cells causes cAMP production required for insulin release, while adenylate cyclase in alpha-cells is positively regulated by GIP.

Adenylyl Cyclases↗

PubMed1995

Differences in glucose transporter gene expression between rat pancreatic alpha- and beta-cells are correlated to differences in glucose transport but not in glucose utilization.

Glucose exerts inverse effects upon the secretory function of islet alpha- and beta-cells, suppressing glucagon release and increasing insulin release. This diverse action may result from differences in glucose transport and metabolism between the two cell types. The present study compares glucose transport in rat alpha- and beta-cells. beta-Cells transcribed GLUT2 and, to a lesser extent, GLUT 1; alpha-cells contained GLUT1 but no GLUT2 mRNA. No other GLUT-like sequences were found among cDNAs from alpha- or beta-cells. Both cell types expressed 43-kDa GLUT1 protein which was enhanced by culture. The 62-kDa beta-cell GLUT2 protein was converted to a 58-kDa protein after trypsin treatment of the cells without detectable consequences upon glucose transport kinetics. In beta-cells, the rates of glucose transport were 10-fold higher than in alpha-cells. In both cell types, glucose uptake exceeded the rates of glucose utilization by a factor of 10 or more. Glycolytic flux, measured as D-[5(3)H]glucose utilization, was comparable in alpha- and beta-cells between 1 and 10 mmol/liter substrate. In conclusion, differences in glucose transporter gene expression between alpha- and beta-cells can be correlated with differences in glucose transport kinetics but not with different glucose utilization rates.

3-O-Methylglucose↗

PubMed1995

Further definition of the sequence and position requirements of the arginine control element that mediates repression and induction by arginine in Saccharomyces cerevisiae.

Repression or induction of the genes involved in arginine biosynthesis or catabolism, respectively, both require participation of the ArgRp/Mcm1p regulatory complex. Our previous work showed that those opposite effects were mediated by a similar arginine-responsive element of 23 nucleotides (that we now call ARC, for ARginine Control) situated close to the start of transcription in the repressed promoters and far upstream of the TATA-element in the induced promoters. To define more precisely the sequence and position requirements of the ARC element, we have now characterized by mutagenesis the promoter elements of the arginine-repressible ARG1 and ARG8 genes. We also identify a functional ARC in the CPA1 promoter, thereby confirming, in agreement with our previous mRNA pulse-labelling data, the participation of a transcriptional component in the arginine regulation of that gene otherwise submitted to a translational regulation. From the 12 ARC elements now characterized, we have derived a consensus sequence and show that such a synthetic element is able to mediate ArgRp/Mcm1p-dependent arginine regulation. An important new finding illustrated by ARG1 and CPA1, is that contrary to what all the previous data suggested, repression can be mediated by ARC elements located far upstream of the TATA-box. The new data suggest that the arginine repressor might inhibit transcription in an active process.

Arginine↗

PubMed1995

Human and rat beta cells differ in glucose transporter but not in glucokinase gene expression.

Glucose homeostasis is controlled by a glucose sensor in pancreatic beta-cells. Studies on rodent beta-cells have suggested a role for GLUT2 and glucokinase in this control function and in mechanisms leading to diabetes. Little direct evidence exists so far to implicate these two proteins in glucose recognition by human beta-cells. The present in vitro study investigates the role of glucose transport and phosphorylation in beta-cell preparations from nondiabetic human pancreata. Human beta-cells differ from rodent beta-cells in glucose transporter gene expression (predominantly GLUT1 instead of GLUT2), explaining their low Km (3 mmol/liter) and low VMAX (3 mmol/min per liter) for 3-O-methyl glucose transport. The 100-fold lower GLUT2 abundance in human versus rat beta-cells is associated with a 10-fold slower uptake of alloxan, explaining their resistance to this rodent diabetogenic agent. Human and rat beta-cells exhibit comparable glucokinase expression with similar flux-generating influence on total glucose utilization. These data underline the importance of glucokinase but not of GLUT2 in the glucose sensor of human beta-cells.

Animals↗

PubMed1995

Identification of rat pancreatic duct cells by their expression of cytokeratins 7, 19, and 20 in vivo and after isolation and culture.

Cells from the excretory ducts of the pancreas are thought to be capable of differentiating into exocrine and endocrine cells. To study this in rat models, markers must be found to identify the cells under different experimental conditions. We tested antibodies to different cytokeratins (CKs) by immunocytochemical staining on pancreatic tissue sections from normal rats, after partial pancreatectomy, and after isolation and culture of duct fragments. Monoclonal antibodies to human CK7, CK19, and CK20 were found to react specifically on rat pancreas tissue, as shown by Western blotting. CK20 and CK19 were immunocytochemically detected only in cells of the ductal system, from centroacinar cells to main ducts. CK7 was expressed by islets of Langerhans and by duct cells from main, inter-, and intralobular ducts, but not by centroacinar and terminal duct cells. CKs 7, 19, and 20 were also expressed in proliferating duct cells during tissue regeneration and after isolation and different periods of culture. We conclude that CKs 7, 19, and 20 are very useful markers to study the differentiation of rat duct cells under experimental conditions in vivo and in vitro.

Animals↗

PubMed1995

DRB10403 protects against IDDM in Caucasians with the high-risk heterozygous DQA10301-DQB10302/DQA10501-DQB1*0201 genotype. Belgian Diabetes Registry.

The human leukocyte antigen (HLA) class II genotype DQA1*0301-DQB1*0302/DQA1*0501-DQB1*0201 has been identified as a marker strongly predisposing to insulin-dependent diabetes mellitus (IDDM) in Caucasian populations. Its frequency in control populations (1-3%) is still, however, 1 order of magnitude higher than the prevalence of IDDM, suggesting that its penetrance can be modified by protective factors. In this study we searched for such a factor in the DRB1 locus by studying DRB1*04 polymorphism in 174 European Caucasian IDDM patients and 73 nondiabetic control subjects, all sharing the HLA-DR3/DR4 phenotype. Significant protection was encoded by the DRB1*0403 allele, which was observed in 5 of 49 control subjects (10%) and none of 171 IDDM patients (0%) with the DQA1*0301-DQB1*0302/DQA1*0501-DQB1*0201 genotype (RR = 0.02 [0.01-0.18], P < 0.0005). These data support the concept that protective HLA class II genes can overrule the risk caused by HLA-DQ susceptibility dimers. They also contribute to a possible strategy to screen for nondiabetic individuals with increased genetic risk of developing IDDM.

Alleles↗

PubMed1993

Heterogeneity in glucose sensitivity among pancreatic beta-cells is correlated to differences in glucose phosphorylation rather than glucose transport.

Rat beta-cells differ in their individual rates of glucose-induced insulin biosynthesis and release. This functional heterogeneity has been correlated with intercellular differences in metabolic redox responsiveness to glucose. The present study compares glucose metabolism in two beta-cell subpopulations that have been separated on the basis of the presence (high responsive) or absence (low responsive) of a metabolic redox shift at 7.5 mM glucose. Mean rates of glucose utilization and glucose oxidation in high responsive beta-cells were 2- to 4-fold higher than in low responsive beta-cells, whereas their leucine and glutamine oxidation was only 10-50% higher. This heterogeneity in glucose metabolism cannot be attributed to differences in GLUT2 mRNA levels or in glucose transport. In both cell subpopulations, the rates of glucose transport (13-19 pmol/min/10(3) beta-cells) were at least 50-fold higher than corresponding rates of glucose utilization. On the other hand, rates of glucose phosphorylation (0.3-0.7 pmol/min/10(3) beta-cells) ranged within those of total glucose utilization (0.2-0.4 pmol/min/10(3) beta-cells). High responsive beta-cells exhibited a 60% higher glucokinase activity than low responsive beta-cells and their glucokinase mRNA level was 100% higher. Furthermore, glucose phosphorylation via low Km hexokinase was detected only in the high responsive beta-cell subpopulation. Heterogeneity in glucose sensitivity among pancreatic beta-cells can therefore be explained by intercellular differences in glucose phosphorylation rather than in glucose transport.

Amino Acids↗

PubMed1993

5' insulin gene polymorphism confers risk to IDDM independently of HLA class II susceptibility.

The polymorphic variable number of tandem repeats in the 5' upstream region of the human insulin gene is a well-known non-human leukocyte antigen locus contributing to genetic susceptibility to IDDM. Controversy exists about the question as to whether INS susceptibility haplotypes are or are not preferentially inherited together with HLA-DR4 haplotypes. We investigated whether genetic interaction between INS and the HLA complex can be better defined using DQ genotypic and phenotypic markers in addition to DR serology. The 5' INS 1/1 genotype was positively associated with IDDM both in non-DR4 subjects (relative risk = 4.3; 95% confidence interval, 1.6-11.5) and DR4 subjects (relative risk = 4.2; 95% confidence interval, 1.9-9.0). Further subdivision of IDDM patients and matched control subjects according to HLA-DQA1 and HLA-DQB1 genotype or phenotype also failed to show any association between 5' INS and HLA class II genes in diabetic patients. The 5' INS and HLA class II polymorphisms therefore provide independent risk markers, which may both contribute to the genetic screening of a high-risk population among nondiabetic individuals.

Adolescent↗

PubMed1992

Complementation of HLA-DQA and -DQB genes confers susceptibility and protection to insulin-dependent diabetes mellitus.

Lack of an aspartic acid 57 in the HLA-DQ beta chain was introduced as a genetic marker of insulin-dependent diabetes mellitus (IDDM). Because 25% of the control population carries the same marker, we analyzed the DQ locus for the presence of more specific disease susceptibility markers, taking into account a possible role for the polymorphic DQA gene. We thereby identified the DQA3-DQB3.2/DQA4.1-DQB2 (DQA1*0301-DQB1*0302/DQA1*0501-DQB1*0201) genotype which was detected in 30% of the 268 typed IDDM patients and only in 1% of the 331 typed healthy controls, resulting in a relative risk of 35. This genetic marker was more frequent in patients with clinical onset before age 18 years (36%) than in patients diagnosed between age 18 and 40 years (22%) and was not observed in patients with non-IDDM. The new susceptibility genotype DQA3-DQB3.2/DQA4.1-DQB2 (DQA1*0301-DQB1*0302/DQA1*0501-DQB1*0201) may explain the well-known excess of DR3/DR4 heterozygous IDDM patients and is expected to help identify individuals at risk for developing the disease.

Adolescent↗

PubMed1990

Escherichia coli and Saccharomyces cerevisiae acetylornithine aminotransferase: evolutionary relationship with ornithine aminotransferase.

Genes argD and ARG8, encoding the acetylornithine aminotransferase (ACOAT) subunit in Escherichia coli and Saccharomyces cerevisiae, respectively, have been cloned and sequenced. The deduced amino acid sequences show substantial similarity. Moreover, they resemble ornithine aminotransferase (OAT) sequences (i.e., those from yeast, rat and man); the observed similarities are statistically significant, indicating that the enzymes are homologous. However, in contrast to OATs, which appear to be substrate (i.e., ornithine)-specific, S. cerevisiae ACOAT transaminates ornithine about as efficiently as E. coli does. The evolutionary relationship between ACOATs and OATs is discussed in terms of substrate ambiguity.

Amino Acid Sequence↗