Regulation of pancreatic beta-cell mass and proliferation by SOCS-3

Growth hormone and prolactin are important growth factors for pancreatic beta-cells. The effects exerted by these hormones on proliferation and on insulin synthesis and secretion in beta-cells are largely mediated through the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling pathway. Suppressors of cytokine signaling (SOCS) proteins are specific inhibitors of the JAK/STAT pathway acting through a negative-feedback loop. To investigate in vivo effects of SOCS-3 in growth hormone (GH)/prolactin signaling in beta-cells we generated transgenic mice with beta-cell-specific overexpression of SOCS-3. The relative beta-cell proliferation and volume in the mice were measured by morphometry. Beta-cell volume of transgenic female mice was reduced by over 30% compared with beta-cell volume in wild-type female mice. Stimulation of transgenic islets in vitro with GH showed a reduced tyrosine phosphorylation of STAT-5 when compared with wild-type islets. Transduction of primary islet cultures with adenoviruses expressing various SOCS proteins followed by stimulation with GH or glucagon-like peptide-1 (GLP-1) revealed that SOCS-3 inhibited GH- but not GLP-1-mediated islet cell proliferation, indicating that the decreased beta-cell volume observed in female transgenic mice could be caused by an inhibition of GH-induced beta-cell proliferation by SOCS-3. In spite of the reduced beta-cell volume the transgenic female mice exhibited enhanced glucose tolerance compared with wild-type littermates following an oral glucose-tolerance test. Together these data suggest that SOCS-3 modulates cytokine signaling in pancreatic beta-cells and therefore potentially could be a candidate target for development of new treatment strategies for diabetes.

Improved metabolic disorders of insulin receptor-deficient mice by transgenic overexpression of glucokinase in the liver

AIMS/HYPOTHESIS

Insulin receptor null mutant mice develop severe diabetes, ketoacidosis and liver steatosis and die within 1 week after birth. Since the liver plays an essential role in the control of glucose homeostasis, we examined in this work whether the metabolic disorders of insulin receptor-deficient mice could be improved upon restoration of hepatic glucose metabolism by transgenic constitutive overexpression of glucokinase selectively in the liver.

METHODS

We first generated transgenic mice overexpressing rat glucokinase cDNA under control of the liver-specific phenylalanine hydroxylase gene promoter. These transgenic mice were crossed with heterozygous insulin-receptor-null mutants to produce homozygous insulin-receptor-null mice overexpressing glucokinase in the liver.

RESULTS

The transgenic mice overexpressing glucokinase in the liver showed improved glucose tolerance and were mildly hypoglycaemic and hyperlipidaemic under starved conditions. The introduction of the glucokinase transgene in insulin receptor null mice did not prevent the development of glycosuria. However, ketoacidosis was delayed by more than 1 week and survival was prolonged to 10 to 16 days in 16% of the pups. In these longer surviving pups, serum glucose and triglyceride concentrations were lowered, hepatic glycogen stores were reconstituted and liver steatosis was absent as compared with the pups which had developed strong ketoacidosis and died earlier.

CONCLUSIONS/INTERPRETATION

These results show that overexpression of hepatic glucokinase can compensate, in part, for the metabolic disorders developed by insulin receptor-deficient mice. This shows the importance of improving hepatic function in diabetes and must revive interest in enhancement of glucokinase activity as a therapeutic strategy for the treatment of diabetes.

Compensatory responses in mice carrying a null mutation for Ins1 or Ins2

Intrauterine growth retardation and postnatal acute diabetes result from insulin deficiency in double homozygous null mutants for Ins1 and Ins2 (Duvillié B, et al., Proc. Natl. Acad. Sci. USA 94:5137-5140, 1997). The characterization of single homozygous null mutants for Ins1 or Ins2 is described here. Neither kind of mutant mice was diabetic. Immunocytochemical analysis of the islets showed normal distribution of the endocrine cells producing insulin, glucagon, somatostatin, or pancreatic polypeptide. Analysis of the expression of the functional insulin gene in Ins1-/- or Ins2-/- mice revealed a dramatic increase of Ins1 transcripts in Ins2-/- mutants. This compensatory response was quantitatively reflected by total pancreatic insulin content similar for both types of mutants and wild-type mice. Moreover, both mutants had normal plasma insulin levels and normal glucose tolerance tests. The determination of beta-cell mass by morphometry indicated beta-cell hyperplasia in the mutant mice. The beta-cell mass in Ins2-/- mice was increased almost threefold, which accounts for the increase of Ins1 transcripts in Ins2-/-mutants. This study thus contributes to evaluate the potential of increasing the beta-cell mass to compensate for low insulin production.

Endocrine pancreas in insulin receptor-deficient mouse pups

Insulin receptor (IR)-deficient pups rapidly become hyperglycemic and hyperinsulinemic and die of diabetic ketoacidosis within a few days. Immunocytochemical analysis of the endocrine pancreas revealed that IR deficiency did not alter islet morphology or the number of beta-, alpha-, delta-, and pancreatic polypeptide (PP) cells. The lack of IR did not result in major changes in the expression of islet hormone genes or of beta-cell-specific marker genes encoding pancreas duodenum homeobox-containing transcription factor-1 (PDX-1), glucokinase (GCK), and GLUT2, as shown by reverse transcriptase-polymerase chain reaction analysis. The serum glucagon levels in IR-deficient and nondiabetic littermates were comparable. Finally, total insulin content in the pancreas of IR-deficient pups was gradually depleted, indicating sustained insulin secretion, not compensated for by increased insulin biosynthesis. These findings are discussed in light of recent results suggesting a role of IR in beta-cell function.

Immunocytochemical localization of the NPY/PYY Y1 receptor in enteric neurons, endothelial cells, and endocrine-like cells of the rat intestinal tract

Neuropeptide Y (NPY) and peptide YY (PYY) are structurally related peptides that are considered to mediate inhibitory actions on gastrointestinal motility, secretion, and blood flow. Several receptor subtypes for these peptides have been identified and the Y1, Y2, Y4/PP1, Y5, and Y5/PP2/Y2b receptors have been cloned. In this article we report the immunocytochemical localization of the Y1 receptor to myenteric and submucosal nerve cell bodies, endothelial cells, and scattered endocrine-like cells of rat intestinal tract. Moreover, double immunofluorescence demonstrates that subpopulations of the Y1 receptor-positive nerve cell bodies are immunopositive for NPY, vasoactive intestinal polypeptide, and nitric oxide synthase. In part, such co-localizations were made possible by use of peroxidase-mediated deposition of tyramide, which permitted use of antisera derived from the same species. Our observations suggest the existence of multiple neuronal, endothelial, and endocrine target sites for NPY and PYY and that some of the actions of these regulatory peptides can be mediated by vasoactive intestinal peptide and nitric oxide synthase.

Immunocytochemical localization of the NPY/PYY Y1 receptor in the developing pancreas

Neuropeptide Y, peptide YY, and pancreatic polypeptide are structurally related peptides that are considered to play a role in the regulation of pancreatic secretion and blood flow. Several receptor subtypes for these peptides have been identified, and the Y1, Y2, Y4/PP1, Y5, and Y5/PP2/Y2b receptors are cloned. We have prepared polyclonal peptide antibodies that recognize the Y1 receptor and now report on its localization in the adult and developing rat pancreas. In the adult pancreas, Y1 receptors were detected both in some centroacinar and intralobular duct cells and in endothelial cells. In the developing pancreas (E12.5-E16.5), Y1 receptor immunoreactivity was observed in numerous nonendocrine epithelial cells. These cells occurred in the immediate vicinity of peptide YY-positive endocrine cells. At E16.5, a fraction of these Y1 receptor-containing cells co-stored amylase. One day later, Y1 receptor immunoreactivity became restricted to pancreatic duct-like cells that occurred in close proximity to peptide YY cells. In fetal rats, intense Y1 receptor staining was also observed in endothelial cells. These observations, together with the finding of early pancreatic peptide YY expression, suggest that peptide YY produced by fetal endocrine cells may exert an action on exocrine cells, duct cells and endothelial cells during development.

A pronounced thymic B cell deficiency in the spontaneously diabetic BB rat

In an attempt to elucidate the origin of the T cell lymphopenia and/or the beta-cell-specific autoimmunity observed in diabetes-prone Bio-Breeding (DP-BB) rats, a thymic cDNA library was subjected to differential screening with thymic cDNA probes of DP-BB rats and nonlymphopenic nondiabetic controls. This approach resulted in the identification of a prominent lack of thymic B cells in DP-BB rats. This deficiency is distinct from a less pronounced peripheral B cell deficiency of different timing. The thymic B cell defect is linked to the lymphopenia trait on chromosome 4 and thereby with susceptibility to diabetes in crosses involving the DP-BB rat. In conclusion, our data suggest that the contribution of thymic B cells to the (negative) selection of thymocytes is inadequate in DP-BB rats, thus providing a plausible explanation for at least some of the spontaneous autoimmune phenomena in this animal model.

A pronounced thymic B cell deficiency in the spontaneously diabetic BB rat

In an attempt to elucidate the origin of the T cell lymphopenia and/or the beta-cell-specific autoimmunity observed in diabetes-prone Bio-Breeding (DP-BB) rats, a thymic cDNA library was subjected to differential screening with thymic cDNA probes of DP-BB rats and nonlymphopenic nondiabetic controls. This approach resulted in the identification of a prominent lack of thymic B cells in DP-BB rats. This deficiency is distinct from a less pronounced peripheral B cell deficiency of different timing. The thymic B cell defect is linked to the lymphopenia trait on chromosome 4 and thereby with susceptibility to diabetes in crosses involving the DP-BB rat. In conclusion, our data suggest that the contribution of thymic B cells to the (negative) selection of thymocytes is inadequate in DP-BB rats, thus providing a plausible explanation for at least some of the spontaneous autoimmune phenomena in this animal model.

Segregation of autoimmune type 1 diabetes in a cross between diabetic BB and brown Norway rats

Diabetes-prone DP-BB rats spontaneously develop insulin-dependent diabetes mellitus resembling type 1 diabetes mellitus in man. Expression of T cell lymphopenia and presence of at least one class II major histocompatibility complex (MHC) RT1u haplotype are required for development of diabetes. Diabetes segregation was studied in lymphopenic backcross (BC) offspring from a cross between male DP-BB/HRI and female BN/Mol rats. Diabetes occurred in 75% of BC rats with genotype RT1u/u and in 18% of those being RT1n/u in genotype. The latter developed diabetes significantly later than MHC homozygotes and parental DP-BBs. Our data further point to the existence of additional genes of minor importance for development of IDDM. One of these seemed to be positioned on the X chromosome. The recently published linkage to chromosome 18 could not be confirmed however. Finally, the BN-derived non-albino allele of the C gene was associated with higher diabetes incidence. This points to the existence of minor susceptibility genes in other strains of rats.

PYY in developing murine islet cells: comparisons to development of islet hormones, NPY, and BrdU incorporation

Exhaustive characterizations of antisera to the structurally related peptides pancreatic polypeptide (PP), neuropeptide Y (NPY), and peptide YY (PYY) enabled us to establish the developmental pattern of these peptides in rat and mouse pancreas. PYY was the earliest detectable peptide and was present in all early appearing endocrine cell types. NPY appeared later and occurred exclusively in a subpopulation of insulin cells, whereas PP cells arose latest. At the earliest stage studied, all endocrine cells stored PYY. Most of these cells also contained glucagon. Subsequently, the endocrine cells comprised glucagon+PYY cells and glucagon+PYY+insulin cells. Later, cells storing either only insulin or insulin+PYY appeared. Quantitations of the relative numbers of these cell populations during development were consistent with a precursor role of triple-positive (insulin+glucagon+PYY) cells. Moreover, bromodeoxyuridine (BrdU) injections at E15.5 showed that a large percentage of triple-positive cells were in S-phase and therefore were actively dividing, whereas almost no pure insulin cells or insulin+PYY cells synthesized DNA at this time. These results suggest that PYY-positive endocrine cells may represent precursors for mature islet cells.