Mutations in the insulin receptor gene in patients with genetic syndromes of insulin resistance

Insulin Resistance in Mitochondrial Diabetes

Mitochondrial diabetes (MD) is generally classified as a genetic defect of β-cells. The main pathophysiology is insulin secretion failure in pancreatic β-cells due to impaired mitochondrial ATP production. However, several reports have mentioned the presence of insulin resistance (IR) as a clinical feature of MD. As mitochondrial dysfunction is one of the important factors causing IR, we need to focus on IR as another pathophysiology of MD. In this special issue, we first briefly summarized the insulin signaling and molecular mechanisms of IR. Second, we overviewed currently confirmed pathogenic mitochondrial DNA (mtDNA) mutations from the MITOMAP database. The variants causing diabetes were mostly point mutations in the transfer RNA (tRNA) of the mitochondrial genome. Third, we focused on these variants leading to the recently described "tRNA modopathies" and reviewed the clinical features of patients with diabetes. Finally, we discussed the pathophysiology of MD caused by mtDNA mutations and explored the possible mechanism underlying the development of IR. This review should be beneficial to all clinicians involved in diagnostics and therapeutics related to diabetes and mitochondrial diseases.

Proteolytic Cleavage of Receptor Tyrosine Kinases

The receptor tyrosine kinases (RTKs) are a large family of cell-surface receptors, which are essential components of signal transduction pathways. There are more than fifty human RTKs that can be grouped into multiple RTK subfamilies. RTKs mediate cellular signaling transduction, and they play important roles in the regulation of numerous cellular processes. The dysregulation of RTK signaling is related to various human diseases, including cancers. The proteolytic cleavage phenomenon has frequently been found among multiple receptor tyrosine kinases. More and more information about proteolytic cleavage in RTKs has been discovered, providing rich insight. In this review, we summarize research about different aspects of RTK cleavage, including its relation to cancer, to better elucidate this phenomenon. This review also presents proteolytic cleavage in various members of the RTKs.

Insulin receptor signaling in normal and insulin-resistant states

In the wake of the worldwide increase in type-2 diabetes, a major focus of research is understanding the signaling pathways impacting this disease. Insulin signaling regulates glucose, lipid, and energy homeostasis, predominantly via action on liver, skeletal muscle, and adipose tissue. Precise modulation of this pathway is vital for adaption as the individual moves from the fed to the fasted state. The positive and negative modulators acting on different steps of the signaling pathway, as well as the diversity of protein isoform interaction, ensure a proper and coordinated biological response to insulin in different tissues. Whereas genetic mutations are causes of rare and severe insulin resistance, obesity can lead to insulin resistance through a variety of mechanisms. Understanding these pathways is essential for development of new drugs to treat diabetes, metabolic syndrome, and their complications.

Phenotypical variety of insulin resistance in a family with a novel mutation of the insulin receptor gene

A novel mutation of insulin receptor gene (INSR gene) was identified in a three generation family with phenotypical variety. Proband was a 12-year-old Japanese girl with type A insulin resistance. She showed diabetes mellitus with severe acanthosis nigricans and hyperinsulinemia without obesity. Using direct sequencing, a heterozygous nonsense mutation causing premature termination at amino acid 331 in the alpha subunit of INSR gene (R331X) was identified. Her father, 40 years old, was not obese but showed impaired glucose tolerance. Her paternal grandmother, 66 years old, has been suffered from diabetes mellitus for 15 years. Interestingly, they had the same mutation. One case of leprechaunism bearing homozygous mutation at codon 331 was identified. These findings led to the hypothesis that R331X may contribute to the variation of DM in the general population in Japan. An extensive search was done in 272 participants in a group medical examination that included 92 healthy cases of normoglycemia and 180 cases already diagnosed type 2 DM or detected hyperglycemia. The search, however, failed to detect any R331X mutation in this local population. In addition, the proband showed low level C-peptide/insulin molar ratio, indicating that this ratio is considered to be a useful index for identifying patients with genetic insulin resistance. In conclusion, a nonsense mutation causing premature termination after amino acid 331 in the alpha subunit of the insulin receptor was identified in Japanese diabetes patients. Further investigations are called for to address the molecular mechanism.

Two novel mutations in the insulin binding subunit of the insulin receptor gene without insulin binding impairment in a patient with Rabson-Mendenhall syndrome

Homozygous or compound heterozygous mutations within the insulin binding domain of the human insulin receptor (INSR) are usually associated with severe impairment of insulin binding leading to Donohue syndrome ("Leprechaunism"), which is characterized by excessive hyperglycemia with hyperinsulinism, pre- and postnatal growth retardation, distinct dysmorphism and early death. Missense mutations in the beta subunits are commonly associated with a milder impairment of insulin binding and milder phenotype with prolonged survival and less dysmorphism, the so called Rabson-Mendenhall syndrome. We report on a 13-year-old girl with Donohue syndrome like dysmorphism, hyperinsulinism and prolonged survival due to two novel INSR missense mutations within the insulin binding domain. Unexpectedly, insulin binding assays and investigations of activation of central insulin signaling pathways in fibroblasts revealed no significant alterations. Instead, immunofluorescence studies showed abnormal perinuclear distribution of the INSR alpha and beta subunits. Our data indicate that the quality of insulin binding activity is correlated with survival, not with the dysmorphic phenotype, and it is not always a valid parameter for predicting INSR mutations as proposed.

Acanthosis nigricans as a risk factor for non-insulin dependent diabetes mellitus

The prevalences of obesity and of non-insulin dependent diabetes mellitus (NIDDM) have increased in the United States population over the past two decades, and thus diabetes prevention has become a major concern of public health agencies such as the National Institutes of Health. Identification of individuals at risk for diabetes is an essential first step in designing and implementing intervention programs. Insulin resistance is the hallmark of the pathophysiology of NIDDM. Subjects with hyperinsulinemia, impaired glucose tolerance, or gestational diabetes are well accepted as being at high risk for diabetes. We propose that the easily identifiable skin lesion, acanthosis nigricans, is common in the major minority groups in the United States and that its presence is a surrogate for laboratory-determined hyperinsulinemia.

Insulin resistance is mediated by a proteolytic fragment of the insulin receptor

Insulin resistance is a common clinical feature of obesity and non-insulin-dependent diabetes mellitus, and is characterized by elevated serum levels of glucose, insulin, and lipids. The mechanism by which insulin resistance is acquired is unknown. We have previously demonstrated that upon chronic treatment of fibroblasts with insulin, conditions that mimic the hyperinsulinemia associated with insulin resistance, the membrane-associated insulin receptor beta subunit is proteolytically cleaved, resulting in the generation of a cytosolic fragment of the beta subunit, beta', and that the generation of beta' is inhibited by the thiol protease inhibitor E64 (Knutson, V. P. (1991) J. Biol. Chem. 266, 15656-15662). In this report, we demonstrate that in 3T3-L1 adipocytes: 1) cytosolic beta' is generated by chronic insulin administration to the cells, and that E64 inhibits the production of beta'; 2) chronic administration of insulin to the adipocytes leads to an insulin-resistant state, as measured by lipogenesis and glycogen synthesis, and E64 totally prevents the generation of this insulin-induced cellular insulin resistance; 3) E64 has no effect on the insulin-induced down-regulation of insulin receptor substrate-1, and therefore insulin resistance is not mediated by the down-regulation of insulin receptor substrate-1; 4) under in vitro conditions, partially purified beta' stoichiometrically inhibits the insulin-induced autophosphorylation of the insulin receptor beta subunit; and 5) administration of E64 to obese Zucker fatty rats improves the insulin resistance of the rats compared to saline-treated animals. These data indicate that beta' is a mediator of insulin resistance, and the mechanism of action of beta' is the inhibition of the insulin-induced autophosphorylation of the beta subunit of the insulin receptor.