Intracisternal A-particles in genetically diabetic mice: identification in pancreas and induction in cultured beta cells

Diabetes mellitus due to viruses--some recent developments

Many different viruses belonging to several genera have the potential to damage beta cells. The mechanisms they employ are varied, and infection may result in either a direct destruction of islets and rapid insulin deficiency, or in a more gradual loss of functioning islets with the onset of diabetes many years later. Several case histories involving extensive cytolysis of beta cells can be directly linked to viral infection, whilst an example of diabetes occurring many years after viral infection is found in individuals who had a congenital infection with rubella virus. Here, the virus induces an autoimmune reaction against beta cells. Autoimmune phenomena have also been observed in islets following infections with viruses other than rubella, and thus activation of autoimmune mechanisms leading to beta-cell destruction may be a relatively frequent occurrence. Recent evidence shows that picornaviruses are not exclusively lytic, and can induce more subtle, long-term changes in beta cells, which may be important in the aetiology of diabetes. The exact mechanisms involved are not known, but it is clear that several viruses can directly inhibit insulin synthesis and induce the expression of other proteins such as interferons, and the HLA antigens. Strain differences in viruses are important since not all variants are tropic for the beta cells. Several laboratories are in the process of identifying the genetic determinants of tropism and diabetogenicity, especially amongst the Coxsackie B (CB) virus group. The sequence of one such diabetogenic CB4 strain virus has been determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic susceptibility to diabetes in inbred strains of mice: measurements of proinsulin mRNA and response to dexamethasone

The insulin resistance produced by the recessive db mutation has led to more severe diabetes in C57BL/KsJ mice relative to that in C57BL/6J mice, suggesting genetic differences between the two strains affecting insulin production or insulin action. To assess these parameters blood glucose, serum insulin, pancreatic insulin, and proinsulin mRNA were measured in both normal and diabetic (db/db) KsJ and 6J strains. The mice were compared at 5 weeks of age, prior to the development of insulin lack known to occur with age in KsJ db/db mice. As a further provocation to insulin production, another group of the normal and db/db mice were given dexamethasone for 4 days. In normal mice there were no strain differences in blood glucose, serum insulin, pancreatic insulin, or proinsulin mRNA. Dexamethasone, presumably by augmenting insulin resistance, induced increases in serum insulin and proinsulin mRNA to the same extent in KsJ and 6J mice. In db/db mice, while blood glucose, serum insulin, and proinsulin mRNA were considerably higher than in normal mice, there were no strain differences observed. After dexamethasone the db/db mice exhibited strain differences which included higher blood glucose and higher serum insulin levels in KsJ mice. These findings were compatible with greater insulin resistance in KsJ than in 6J db/db mice. While dexamethasone treatment increased serum insulin in KsJ db/db mice, there was no augmentation of proinsulin mRNA in either strain, suggesting a limit to the insulin synthesis. Analysis of serum insulin/glucose and proinsulin mRNA/glucose ratios demonstrated a dexamethasone-induced increase in serum insulin/glucose in normal and diabetic mice of both strains.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose induces intracisternal type A retroviral gene transcription and translation in pancreatic beta cells

C57BL/KsJ (BKs) and CBA/J, but not C57BL/6J (B6) mice are susceptible to the diabetogenic action of the obesity gene, "diabetes" (db). BKs and CBA/J, but not B6 mice, constitutively express intracisternal type A particles (IAP), an endogenous class of retrovirus, in beta cells and in cortical thymocytes. IAP genetic expression in these cell types included production of the group-specific antigen, p73, as well as higher-molecular mass p73-related antigens (p114-120). We used islet culture techniques to show that both transcription and translation of IAP genomes in beta cells in enhanced by glucose. Maintenance of CBA/J islets for 48 h in 16.5 mM glucose-containing medium effected a fivefold induction of IAP protein synthesis in comparison to islets cultured in low- (5.5 mM) glucose medium. Analysis of RNA from 16.5 mM glucose-cultured islets revealed induction of 7.2 and 5.4 kbp transcripts known to code for p73 and the p114-120 polypeptides, respectively. This induction in CBA/J islets was 10-15-fold on a tissue basis, and 5-7-fold on an RNA basis. Glucose induction of preproinsulin mRNA levels was also analyzed in the same samples. Islets cultured in 16.5 mM glucose showed an eightfold higher level on a tissue basis, and a fourfold increase in terms of total recovered RNA. Comparison of these glucose-inducible parameters in islets isolated from the diabetes-susceptible BKs strain vs. the resistant B6 strain revealed that expression of the group-specific retroviral p73 antigen was limited to BKs beta cells. This inbred strain control of p73 expression was also found in cortical thymocytes, with B6 thymocytes producing a 117 kD component to the exclusion of p73, while both components were expressed in thymocytes from normal BKs mice. In comparison to normal BKs males, thymocytes from four week-old genetically diabetic (db/db) BKs males showed no change in labeling of p117, but showed a sharply diminished incorporation into p73. This suggested that accelerated thymic involution characteristic of db/db mice may entail selective elimination of p73-producing cells. The possibility that glucose-stressed BKs pancreatic beta cells are marked for autoimmune elimination by the elaboration of p73 or other IAP-related proteins is discussed.

A new mutation (db3J) at the diabetes locus in strain 129/J mice. II. Studies of pancreatic alpha cell function in culture

Monolayer cell cultures from pancreatic islets of aging 129/J strain diabetes (db3J/db3J) and lean littermate control mice were tested for differences in glucagon and insulin secretion in either serum-free Eagle's minimal essential medium (MEM) or Dulbecco's modified minimal essential medium (DMEM). There was a highly significant (p less than 0.0001) main effect of genotype and type of culture medium on glucagon secretion with time. Thus, although numbers of A-cells were not demonstrably increased in db3J/db3J cultures in DMEM, mean medium glucagon levels increased 2.7-, 18-, and 32-fold above littermate normal culture levels at days 4, 6, and 8 respectively. In MEM, the two populations could not be discriminated on the basis of glucagon secretion. By contrast, insulin secretion over culture days showed a highly significant (p less than 0.0001) dependence on genotype, but not type of medium, with the B-cell enriched db3J/db3J preparations secreting between 20 and 30 times as much insulin as controls in both medida. Analysis revealed that the heightened secretory responsiveness of mutatn A-cells in DMEM as compared to MEM was primarily elicited by the elevated DMEM amino acid concentration and specifically lysine (0.8 mmol/l in DMEM versus 0.4 mmol/l in MEM). In pulse-chase experiments using 14 day db3J/db3J cultures, incorporation of 3H-tryptophan into protein that eluted from Biogel P-10 columns in the native glucagon peak indicates that DMEM stimulated glucagon biosynthesis as well as secretion. This study reveals an augmented sensitivity of db3J/db3J A-cells to stimulation by basic amino acids in long-term culture.