Early steps in insulin action

The biological effects of insulin are initiated by the binding of insulin to the insulin receptor. Insulin binds to the extracellular domain of the insulin receptor and induces conformational changes in the receptor, leading to autophosphorylation of the receptor on intracellular tyrosine residues. These phosphorylated tyrosine residues act as binding sites for proteins which subsequently may be phosphorylated by the insulin receptor. As a result, yet other proteins can be recruited to form larger complexes and, in the case of enzymes, changes in their activity may take place. By a combination of these processes, the activated insulin receptor initiates cascades of biochemical events which are regulated mainly by specific phosphorylation or dephosphorylation reactions. Intermediates which are involved in the normal insulin signalling pathway are subjects of expanding research.

Unconventional rapid Erk1,2 activation is indispensable for proliferation of the growth factor-independent myeloid leukemic cell line KG1

While normal hematopoietic progenitor cells are dependent on colony-stimulating factors for in vitro proliferation, myeloid leukemic cells are frequently factor-independent. In this study we investigated several signalling intermediates of the Ras-Er1,2 pathway which may be involved in the development of growth factor independence. In the growth factor independent cell line KG1, an extremely short activation pattern of Erk1,2 with a maximum at 30 s was observed in response to FBS. In contrast, stimulation of the IL-3 receptor in AML193 cells resulted in a transient Erk activation peaking at 5 min and returning to base levels after 15 min. Although the Erk activation in KG1 cells is short-lived, using the MEK inhibitor PD98059, we demonstrated that Erk phosphorylation is essential for proliferation of these cell lines. We also detected major differences in Shc phosphorylation between factor-dependent and -independent cells. These data suggest that Erk activation is essential for proliferation of growth factor-dependent and -independent leukemic cells. The minimal Erk activation observed in KG1 cells in response to serum is sufficient for mitogenesis of these cells.

Osmotic swelling-induced activation of the extracellular-signal-regulated protein kinases Erk-1 and Erk-2 in intestine 407 cells involves the Ras/Raf-signalling pathway

Human Intestine 407 cells respond to hypo-osmotic stress with a rapid stimulation of compensatory ionic conductances accompanied by a transient increase in the activity of the extracellular-signal-regulated protein kinases Erk-1 and Erk-2. In this study, we examined the upstream regulators of hypotonicity-induced Erk-1/Erk-2 activation and their possible role in cell-volume regulation. The hypotonicity-provoked Erk-1/Erk-2 activation was greatly reduced in cells pretreated with the specific mitogen-activated/Erk-activating kinase inhibitor PD098059 and was preceded by a transient stimulation of Raf-1. Pretreatment of the cells with PMA, GF109203X, wortmannin or Clostridium botulinum C3 exoenzyme did not appreciably affect the hypotonicity-provoked Erk-1/Erk-2 stimulation, suggesting the osmosensitive signalling pathway to be largely independent of protein kinase C and p21(rho). In contrast, expression of dominant negative RasN17 completely abolished the hypotonicity-induced Erk-1/Erk-2 activation. Stimulation of the swelling-induced ion efflux was independent of activation of these mitogen-activated protein kinases, as revealed by hypotonicity-provoked isotope efflux from 125I-- and 86Rb+-loaded cells after pretreatment with PD098059 and after expression of RasN17. In addition, the epidermal-growth-factor-induced potentiation of the hypotonicity-provoked anionic response did not depend on the increase in Erk-1/Erk-2 activity but, instead, was found to depend on Ca2+ influx. Taken together, these results indicate that hypotonic stress induces Erk-1/Erk-2 activation through the Ras/Raf-signalling pathway, and argue against a direct role for this pathway in cell-volume control.

[A new subtype of diabetes mellitus: maternally inherited diabetes and deafness (MIDD)]

Diabetes mellitus comprises many subtypes, the pathogenesis of each of which involves a combination of inherited and environmental factors. Recently a new subtype of diabetes mellitus was recognized in a Dutch pedigree, designated as 'maternally inherited diabetes and deafness' (MIDD). Impaired hearing is an associated phenomenon of the disease. Approximately 1.3% of all diabetic cases in the Netherlands exhibit the MIDD subtype. MIDD shows a strictly maternal heredity. In MIDD there is a guanine-for-adenine substitution at position 3243 in mitochondrial DNA. Mitochondria carrying this mutation exhibit a decreased functionality. In carriers of the MIDD mutation the insulin secretion by the pancreas in response to stimulation by glucose is impaired.

Expression, enzyme activity, and subcellular localization of mammalian target of rapamycin in insulin-responsive cells

The role of the mammalian target of rapamycin (mTOR) was investigated in insulin responsive cell lines. mTOR was expressed at high levels in insulin responsive cell types and in 3T3-L1 cells mTOR expression levels increased dramatically as cells differentiated from fibroblasts into insulin responsive adipocytes. mTOR localized to membrane fractions in all cells tested and in 3T3-L1 adipocytes mTOR was specifically localized to microsomal membranes. Protein kinase activity directed towards mTOR was tightly associated with mTOR immunoprecipitates and this kinase activity was inhibited by FKBP12-rapamycin indicating it was due to an autokinase activity present in mTOR. The mTOR autokinase and the protein kinase activity of the p110 alpha isoform of PI 3-kinase shared several notable similarities; (a) both were maximally active in the presence of Mn2+ but also showed significant activity in the presence of Mg2+ (b) neither were inhibited by the presence of non-ionic detergent and (c) both were inhibited by wortmannin and LY294002, known inhibitors of the PI 3-kinase lipid kinase activity. These data taken together indicate the autokinase activity lay in the PI 3-kinase homology domain. In summary mTOR is a membrane anchored protein kinase that is active in conditions encountered in vivo and the fact it is highly expressed in insulin responsive cell types is consistent with a role in insulin signalling.

Two naturally occurring insulin receptor tyrosine kinase domain mutants provide evidence that phosphoinositide 3-kinase activation alone is not sufficient for the mediation of insulin's metabolic and mitogenic effects

We have recently reported (1) that two naturally occurring mutants of the insulin receptor tyrosine kinase domain, Arg-1174 --> Gln and Pro-1178 --> Leu (Gln-1174 and Leu1178, respectively), both found in patients with inherited severe insulin resistance, markedly impaired receptor tyrosine autophosphorylation, with both mutant receptors being unable to mediate the stimulation of glycogen synthesis or mitogenesis by insulin when expressed in Chinese hamster ovary cells. However, these mutations did not fully prevent IRS-1 phosphorylation in response to insulin in these cells, suggesting that IRS-1 alone may not be sufficient to mediate insulin's metabolic and mitogenic effects. In the present study, we have demonstrated that these mutations also impair the ability of the insulin receptor to activate the transcription factor Elk-1 and promote GLUT4 translocation to the plasma membrane. Although at low concentrations of insulin, the mutant receptors were impaired in their ability to stimulate the tyrosine phosphorylation of IRS-1, at higher insulin concentrations we confirmed that the cells expressing the mutant receptors showed significantly increased tyrosine phosphorylation of IRS-1 compared with parental nontransfected cells. In addition, at comparable insulin concentrations, the association of the p85alpha subunit of phosphoinositide 3-kinase (PI3-kinase) with IRS-1 and the enzymatic activity of IRS-1-associated PI3-kinase were significantly enhanced in cells expressing the mutant receptors. In contrast, no significant stimulation of the tyrosine phosphorylation of Shc, GTP loading of Ras, or mitogen-activated protein kinase phosphorylation was seen in cell lines expressing these mutant receptors. Thus, no activation of any measurable mitogenic or metabolic response was detectable, despite significant insulin-induced phosphorylation of IRS-1 and its association with PI3-kinase in cells stably expressing the mutant insulin receptors. These findings suggest that PI3-kinase activation alone may be insufficient to mediate a wide range of the metabolic and mitogenic effects of insulin. Additionally, the data provide support for the notion that insulin activation of Ras is more closely linked with Shc, and not IRS-1, phosphorylation.

Changes in the signalling status of the small GTP-binding proteins Rac and Rho do not influence insulin-stimulated hexose transport

Post-receptor signalling molecules that convey the signal from the activated insulin receptor to the actual process of Glut4 translocation and hexose uptake are poorly understood. Various studies have suggested a requirement of the lipid kinase phosphatidylinositol-3 kinase (PI3-kinase) in this process. PI3kinase regulates the activation status of the small GTP-binding protein Rac which, in turn, is able to activate another G-protein Rho. Rac and Rho are known to regulate the structure of the membrane- and cytoplasmic actin-cytoskeleton. We have examined whether Rac and Rho transfer the signals generated by PI3kinase towards insulin-stimulated hexose uptake. For that purpose, we expressed in 3T3-L1 adipocytes the dominant-negative mutant of RacN17 using vaccinia virus-mediated gene transfer. The expression levels of the RacN17 protein were monitored by Western blotting. The abrogation of endogenous Rac signalling by expression of RacN17 was inferred from the observed loss of arachidonic acid release in response to insulin. Basal and insulin-stimulated hexose transport were not affected by expression of the RacN17 mutant. A possible contribution of Rho.GTP to stimulation of hexose uptake was examined by pre-incubation of adipocytes with lysophosphatidic acid (LPA). We observed a profound effect of LPA on the structure of the cytoskeleton and on the phosphorylation of Focal Adhesion Kinase (p125FAK), indicating that 3T3-L1 adipocytes respond to LPA and that Rho was activated by LPA. However, no effect was detected on the basal or on the insulin-stimulated hexose transport. We conclude that Rac and Rho are unlikely to be involved in insulin-stimulated hexose transport, suggesting a possible contribution of other signalling pathways, downstream of PI3kinase to this process.

Mutation in mitochondrial tRNA(Leu(UUR)) gene associated with progressive kidney disease

Several studies show an association of a guanine for adenine substitution (A-->G) at position 3243 in mitochondrial DNA (mtDNA) with a recently recognized diabetic subtype designated maternally inherited diabetes and deafness (MIDD). This mutation shows heterogeneity in its phenotypic expression as is apparent from its association with several other syndromes. Screening for the 3243A-->G mutation in mtDNA was performed in those diabetic patients attending the Leiden University Hospital diabetics clinic who had a history of maternally inherited diabetes, sensorineural hearing loss, or both. Four individuals from three unrelated families were identified who suffered from progressive nondiabetic kidney disease in association with diabetes mellitus and hearing loss. The mode of inheritance suggested maternal transmission. The combination of renal failure and hearing loss had been misdiagnosed as Alport syndrome in three of the four individuals. Therefore, in addition to these three families, another 63 unrelated patients with possible Alport syndrome were selected at random. DNA from peripheral blood and other tissues from members of the three families and from the 63 additional Alport syndrome patients was examined for the presence of the 3243A-->G mutation in mtDNA. The mutation was detected in heteroplasmic form in the four patients and their maternal relatives. Also, one of the 63 suspected Alport syndrome patients showed heteroplasmy for the 3243 mutation. These data show the existence of a kidney disease that is characterized by the presence of the A-->G mutation at position 3243 in the mtDNA.

8-Bromo-cAMP induces a proliferative response in an IL-3 dependent leukemic cell line and activates Erk 1,2 via a Shc-independent pathway

In a number of cell types, elevation of intracellular cAMP concentrations antagonizes growth factor-induced mitogenesis by abrogating the downstream signaling of RasGTP to extracellular-signal-regulated kinases (Erk 1,2). We studied the effect of elevation of cAMP concentrations on the IL-3-induced mitogenic response in the leukemic cell line AML193. We observed that 8-bromo-cAMP (8-Br-cAMP) had no inhibitory effect on the magnitude of this response. On the contrary. 8-Br-cAMP alone induced a proliferative response in these cells. 8-Br-cAMP activated Erk 1,2 in these cells without involvement of Shc phosphorylation. These findings suggest the presence of a novel cAMP-dependent signaling pathway in AML193 cells, which activates Erk 1,2 via a Shc-independent pathway and leads to the generation of a mitogenic response.

Maternally inherited diabetes and deafness: a diabetic subtype associated with a mutation in mitochondrial DNA

Diabetes mellitus is a common disease with variations in its clinical expression and different modes of pathogenesis. The purpose of this review is to discuss a recently identified diabetic subtype. Based on the triad diabetes, maternal inheritance and impaired hearing in this subtype we have proposed the name Maternally Inherited Diabetes and Deafness (MIDD). This diabetic subtype associates in the vast majority of cases with a single mutation in mitochondrial DNA, at position 3243. The clinical presentation of MIDD which can be IDDM-like or NIDDM-like, its frequency of occurrence, possible pathogenic mechanisms and the contribution of other mitochondrial DNA mutations to the development of diabetes will be discussed.

Detection of mitochondrial DNA deletions in human skin fibroblasts of patients with Pearson's syndrome by two-color fluorescence in situ hybridization

Pearson's marrow/pancreas syndrome is a disease associated with a large mitochondrial DNA (mtDNA) deletion. The various tissues of a patient contain heteroplasmic populations of wild-type (WT) and deleted mtDNA molecules. The clinical phenotype of Pearson's syndrome is variable and is not correlated with the size and position of the deletion. The histo- and cytological distribution of WT and deleted mtDNA molecules may be factors that correlate with the phenotypical expression of the disease. Here we introduce a new application of two-color FISH to visualize WT and deleted mtDNA simultaneously in a cell population of in vitro cultured skin fibroblasts of two patients with Pearson's syndrome. At the third passage of culturing, fibroblasts showed a remarkable heterogeneity of WT and deleted mtDNA: about 90% of the cells contained almost 100% WT mtDNA, and 10% of the cells contained predominantly deleted mtDNA. At the tenth passage of culturing, fibroblasts showed a reduction of intercellular heteroplasmy from 10% to 1%, while intracellular heteroplasmy was maintained. This new approach enables detailed analysis of distribution patterns of WT and deleted mtDNA molecules at the inter- and intracellular levels in clinical samples, and may contribute to a better understanding of genotype-phenotype relationships in patients with mitochondrial diseases.

Association of the insulin-receptor variant Met-985 with hyperglycemia and non-insulin-dependent diabetes mellitus in the Netherlands: a population-based study

One of the characteristics of non-insulin-dependent diabetes mellitus (NIDDM) is the presence of insulin resistance. Most NIDDM patients have a normal sequence of the insulin receptor, indicating that, if insulin-receptor mutations contribute to the development of NIDDM, they will be present only in a minor fraction of the NIDDM population. The goal of the present study was to examine whether insulin-receptor mutations contribute to the development of NIDDM. We examined 161 individuals with NIDDM and 538 healthy controls from the population-based Rotterdam study for the presence of mutations in the insulin-receptor gene by SSCP. A heterozygous mutation changing valine-985 into methionine was detected in 5.6% of diabetic subjects and in 1.3% of individuals with normal oral glucose tolerance test. Adjusted for age, gender, and body-mass index, this revealed a relative risk for diabetes of 4.49 (95% confidence interval 1.59-12.25) for Met-985 carriers. When the total study group was analyzed, the prevalence of the mutation increased with increasing serum glucose levels (test for trend P < .005). We conclude that the Met-985 insulin-receptor variant associates with hyperglycemia and represents a risk factor for NIDDM.

Linkage of Wolfram syndrome to chromosome 4p16.1 and evidence for heterogeneity

Wolfram syndrome (DIDMOAD syndrome; MIM 222300) is an autosomal recessive neurodegenerative disorder characterized by juvenile-onset diabetes mellitus and bilateral optic atrophy. Previous linkage analysis of multiply affected families indicated that the gene for Wolfram syndrome is on chromosome 4p, and it produced no evidence for locus heterogeneity. We have investigated 12 U.K. families with Wolfram syndrome, and we report confirmation of linkage to chromosome 4p, with a maximum two-point LOD score of 4.6 with DRD5, assuming homogeneity, and of 5.1, assuming heterogeneity. Overlapping multipoint analysis using six markers at a time produced definite evidence for locus heterogeneity: the maximum multipoint LOD score under homogeneity was <2, whereas when heterogeneity was allowed for an admixture a LOD of 6.2 was obtained in the interval between D4S432 and D4S431, with the peak close to the marker D4S3023. One family with an atypical phenotype was definitely unlinked to the region. Haplotype inspection of the remaining 11 families, which appear linked to chromosome 4p and had typical phenotypes, revealed crossover events during meiosis, which also placed the gene in the interval D4S432 and D4S431. In these families no recombinants were detected with the marker D4S3023, which maps within the same interval.

Insulin-induced tyrosine dephosphorylation of paxillin and focal adhesion kinase requires active phosphotyrosine phosphatase 1D

Insulin stimulation of fibroblasts rapidly induces the tyrosine dephosphorylation of proteins of 68 kDa and 125 kDa, in addition to the tyrosine phosphorylation of the insulin receptor beta-chain, insulin receptor substrates 1 and 2, and Shc. Using specific antibodies, the 68 kDa and 125 kDa proteins were identified as paxillin and focal adhesion kinase (pp125FAK) respectively. We have examined whether dephosphorylation of paxillin and pp125FAK requires interaction of the cells with the extracellular matrix. For this, cells were grown on poly(L-lysine) plates, and the tyrosine phosphorylation of pp125FAK and paxillin was increased by addition of lysophosphatidic acid. Under these conditions, insulin still induced the complete dephosphorylation of pp125FAK and paxillin, indicating that this process can occur independently of the interaction of integrins with extracellular matrix proteins. We also studied whether dephosphorylation of pp125FAK and paxillin results from the action of a phosphotyrosine phosphatase. It was found that phenylarsine oxide, a phosphotyrosine phosphatase inhibitor, prevented the insulin-induced dephosphorylation of pp125FAK and paxillin. Furthermore, this insulin-induced dephosphorylation was also impaired in cells expressing a dominant-negative mutant of phosphotyrosine phosphatase 1D (PTP 1D). Thus we have identified paxillin as a target for dephosphorylation by insulin. In addition, we have obtained evidence that the insulin-mediated dephosphorylation of paxillin and pp125FAK requires active PTP 1D.

Replacement of the conserved tyrosine 1210 by phenylalanine in the insulin receptor affects insulin-induced dephosphorylation of focal adhesion kinase but leaves other responses intact

The families of tyrosine and serine/threonine kinases exhibit shared clusters of conserved amino acid residues. Some conserved residues are confined to the family of tyrosine kinases (TKs), like Tyr at position 1210 in the insulin receptor. Nearly all TKs have at this position Tyr, whereas Ser/Thr kinases generally have Phe at this site. The three-dimensional structure of the insulin receptor TK domain shows Tyr1210 to be located in the cleft, below bound ATP, in a region which potentially contributes to substrate binding. We have examined whether this specific Tyr residue contributes to the generation of TK-specific responses, such as Tyr phosphorylation of Shc, activation of Ras and Erk1,2, and stimulation of DNA synthesis. In addition, we have examined the contribution of Tyr1210 to insulin receptor-specific responses as Tyr phosphorylation of IRS1, stimulation of glycogen synthesis, and dephosphorylation of focal adhesion kinase (FAK). Wild-type and a mutant insulin receptor, in which Tyr1210 was replaced by Phe, were stably expressed in CHO cells, and clones expressing similar numbers of insulin receptors were selected. It was found that replacement of Tyr1210 by Phe resulted in a receptor which was nearly inactive in inducing dephosphorylation of FAK. The mutant receptor was able to induce RasGTP formation, glycogen synthesis, and activation of phosphatidylinositol 3-kinase, though the magnitude of stimulation of some responses was decreased. These findings indicate that Tyr1210 is not essential for the induction of tyrosine kinase-specific responses, such as activation of the Shc/Ras/Erk1,2 pathway and mitogenicity. On the other hand, the abrogation of insulin-induced FAK dephosphorylation indicates that Tyr1210 is involved in coupling of the activated receptor to some downstream targets. Thus, Tyr1210 may fine tune the signal generated by the activated insulin receptor.

An insulin receptor mutant (Asp707 --> Ala), involved in leprechaunism, is processed and transported to the cell surface but unable to bind insulin

We have identified a homozygous mutation near the carboxyl terminus of the insulin receptor (IR) alpha subunit from a leprechaun patient, changing Asp707 into Ala. Fibroblasts from this patient had no high affinity insulin binding sites. To examine the effect of the mutation on IR properties, the mutant IR was stably expressed in Chinese hamster ovary cells. Western blot analysis and metabolic labeling showed a normal processing of the mutant receptor to alpha and beta subunits. No increase in high affinity insulin binding sites was observed on Chinese hamster ovary cells expressing the mutant receptor, and also, affinity cross-linking of 125I-labeled insulin by disuccinimidyl suberate to these cells failed to label the mutant alpha subunit. Biotinylation of cell surface proteins by biotin succinimidyl ester resulted in efficient biotinylation of the mutant IR alpha and beta subunits, showing its presence on the cell surface. On solubilization of the mutant insulin receptor in Triton X-100-containing buffers, 125I-insulin was efficiently cross-linked to the receptor alpha subunit by disuccinimidyl suberate. These studies demonstrate that Ala707 IR is normally processed and transported to the cell surface and that the mutation distorts the insulin binding site. Detergent restores this site. This is an example of a naturally occurring mutation in the insulin receptor that affects insulin binding without affecting receptor transport and processing. This mutation points to a major contribution of the alpha subunit carboxyl terminus to insulin binding.

Evaluation of the importance of maternal history of diabetes and of mitochondrial variation in the development of NIDDM

In 79 South Indian nuclear pedigrees ascertained via probands with NIDDM and both parents living, parental diabetic status was established through previously diagnosed NIDDM (n = 97) or oral glucose tolerance testing (n = 61). There was no significant difference between diabetes prevalence in mothers and fathers (60 vs 53 (76% vs 67%), respectively, p = 0.22). 'Age at diabetes diagnosis' survival curves did differ according to parental gender (p = 0.02) but this may reflect gender differences in health provision rather than pathophysiology. No maternal excess effects of the magnitude evident in previous studies were detected, suggesting either ethnic differences or overestimation of the maternal effect when reported histories of parental diabetes have been used. The tRNA(Leu(UUR) gene region was studied for diabetes-associated variation given the role of mutations in this gene in some pedigrees displaying maternal transmission of NIDDM. None of 142 unrelated South Indian NIDDM subjects displayed the MELAS mutation at nt3243. However, sequencing identified two variants of potential importance: (a) at nt3290 in the tRNA(Leu(UUR) gene, seen in 7/142 diabetic and 1/85 control subjects (p = 0.11), (b) at nt3316 in the ND1 gene (4/142 vs 1/85 subjects, respectively (p = 0.51)). Further studies are needed to determine the relevance of these variants to the development of NIDDM.

Expression of a dominant-negative Ras mutant does not affect stimulation of glucose uptake and glycogen synthesis by insulin

It has previously been shown that insulin-induced stimulation of glucose uptake and glycogen synthesis requires activation of phosphatidylinositol-3-kinase (PI3kinase). Insulin also induces formation of RasGTP in cells and various studies have yielded inconsistent data with respect to the contribution of signalling pathways activated by RasGTP, to insulin-stimulated glucose uptake and glycogen synthesis. We have examined the requirement of RasGTP-mediated signalling for these insulin responses by expression of a dominant negative mutant of Ras (RasN17) in cells by vaccinia virus mediated gene transfer. This Ras-mutant abrogates the signalling pathways mediated by endogenous RasGTP. Subsequently, the ability of insulin to stimulate 2-deoxyglucose uptake and glycogen was examined. We observed that expression of RasN17 in 3T3L1 adipocytes did not affect the stimulation of hexose uptake by insulin. Similarly, expression of RasN17 in A14 cells, an NIH 3T3-derived cell line with high expression of insulin receptors, did not affect insulin-induced stimulation of glycogen synthesis. In both cell lines, insulin-induced phosphorylation of Mapkinase (Erk1,2) was abrogated after expression of RasN17, demonstrating the functional interference by RasN17 with signalling mediated by endogenous RasGTP. Wortmannin, an inhibitor of PI3kinase, abolished dose-dependently the insulin-induced stimulation of hexose uptake and glycogen synthesis without an effect on RasGTP levels in both cell types. We conclude that stimulation of glucose transport and glycogen synthesis by insulin occurs independently of RasGTP-mediated signalling.

Maternally inherited diabetes and deafness: a new diabetes subtype

Diabetes mellitus is a common disease with many forms of clinical expression. In addition, the development of diabetic complications is not only dependent on glycaemic control but also on individual factors which may be related to genetic heterogeneity. At present, multiple genetic factors are being recognized as contributing to the development of diabetes or possibly modulating its clinical expression. The purpose of this review is to give an overview of our current knowledge on a subtype of diabetes which is apparently caused by a single mutation in the mitochondrial DNA.

Heteroplasmy levels of a mitochondrial gene mutation associated with diabetes mellitus decrease in leucocyte DNA upon aging

We showed previously that a mutation in the mitochondrial tRNALeu(UUR) gene at position 3243 associates with maternally inherited diabetes and deafness (MIDD). This mutation shows heteroplasmy in DNA from peripheral blood and other tissues. To examine whether heteroplasmy levels in peripheral blood DNA change upon aging, heteroplasmy levels were determined in DNA samples from peripheral blood, collected recently and 1.5-6 years ago, from 18 individuals carrying the 3243 mutation. It was found that 17 out of 18 carriers showed a decrease upon aging (P = 0.001), the average change being -0.69 +/- 0.61% per year. These data indicate a continuous selection against haematopoietic (precursor) cells carrying high levels of the 3243 mutation. Moreover, they imply that heteroplasmy levels may decrease below the detection limit if DNA from peripheral blood is analyzed from elderly individuals. DNA from oral mucosa cells was found to be a good alternative as heteroplasmy levels for the 3243 mutation are on the average 1.7 fold higher than in DNA from peripheral blood.

Mitochondrial diabetes mellitus: a review

We review the relationship between various types of mitochondrial DNA mutations and the prevalence as well as the pathobiochemical and clinical features of mitochondrial diabetes mellitus. An A to G transversion mutation in the tRNA(Leu(UUR)) gene is associated with diabetes in about 1.5% of the diabetic population in different countries and races. Phenotypically this type of mitochondrial diabetes is combined with deafness in more than 60% and is clinically distinguishable with respect to several characteristics from the two idiopathic forms of diabetes. The underlying pathomechanism is probably a delayed insulin secretion due to an impaired mitochondrial ATP production in consequence of the mtDNA defect.

Tissue-related changes in insulin receptor number and autophosphorylation induced by starvation and diabetes in rats

Insulin action is subject to regulation at the level of the insulin receptor and at postreceptor levels. Starvation and diabetes are often associated with insulin resistance for glucose metabolism in various tissues. In muscle, fat, and liver, we examined whether changes in the functionality of the insulin receptor correlated with changes in insulin action in the starved and diabetic state. Insulin-stimulated receptor autophosphorylation reflects an early physiologic step in transmission of the insulin signal, and for that reason, changes in autophosphorylation activity of the insulin receptor were used as a marker to determine the functionality of the insulin receptor. Glycoprotein fractions prepared from skeletal muscle, diaphragm, epididymal fat, and liver of control, 3-day starved, short-term 3-day (S) diabetic (streptozotocin, 70 mg/kg intravenously), and long-term 6-month (L) diabetic (neonatal streptozotocin 100 micrograms/g intraperitoneally) rats were used in this study. Receptor activity was monitored by measuring insulin-stimulated [gamma-32P]adenosine triphosphate (ATP) receptor autophosphorylation. In addition, to obtain information about whether changes in receptor autophosphorylation are related to changes in receptor number, relative numbers of high-affinity insulin receptors were determined by affinity cross-linking of [125I]insulin to the receptor alpha-chain and quantitation of the yield of labeled receptor alpha-chain. Control, starved, S diabetic, and L diabetic rats had plasma insulin and glucose levels of 294 +/- 42, 90 +/- 24, 48 +/- 12, and 216 +/- 30 pmol/L and 6.7 +/- 0.2, 4.1 +/- 0.2, 23.3 +/- 0.7, and 21.6 +/- 2.9 mmol/L, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Maternally inherited diabetes and deafness (MIDD): a distinct subtype of diabetes associated with a mitochondrial tRNA(Leu)(UUR) gene point mutation

We have recently described a mitochondrial DNA (mtDNA) point mutation at np 3243 in the tRNA(Leu)(UUR) gene in a large Dutch pedigree with maternally inherited diabetes mellitus and deafness (MIDD) illustrating the importance of mitochondrial function in maintenance of a proper glucose homeostasis. In this review we will focus on the prevalence of the mtDNA mutation at np 3243 in diabetic populations, as well as postulate some working models for its pathogenicity.