Randomized noninferiority clinical trial evaluating 3 commercial dry cow mastitis preparations: I. Quarter-level outcomes

The study objective was to compare the efficacy of 3 commercial dry cow mastitis formulations regarding quarter-level prevalence of intramammary infections (IMI) postcalving, cure of preexisting infections over the dry period, prevention of new infections during the dry period, and risk for a clinical mastitis case between calving and 100d in milk (DIM). A total of 1,091 cows (4,364 quarters) from 6 commercial dairy herds in 4 different states (California, Iowa, Minnesota, and Wisconsin) were enrolled and randomized to 1 of the 3 treatments at dry-off: Quartermaster (QT; 1,000,000 IU of procaine penicillin G and 1 g of dihydrostreptomycin; Pfizer Animal Health, New York, NY), Spectramast DC (SP; 500 mg of ceftiofur hydrochloride; Pfizer Animal Health), or ToMorrow Dry Cow (TM; 300mg of cephapirin benzathine; Boehringer Ingelheim Vetmedica Inc., St. Joseph, MO). Quarter milk samples were collected for routine bacteriological culture before dry cow therapy treatment at dry-off, 0 to 6 DIM, and 7 to 13 DIM and an on-farm record-keeping system was used to retrieve data on clinical mastitis cases. Noninferiority analysis was used to evaluate the effect of treatment on the primary outcome, risk for a bacteriological cure during the dry period. Multivariable logistic regression techniques were used to describe the effect of treatment on risk for presence of IMI postcalving and risk of a new IMI during the dry period. Cox proportional hazards regression was used to describe the effect of treatment on the risk and time for quarters to experience an episode of clinical mastitis between calving and 100 DIM. The overall crude quarter-level prevalence of infection at dry-off was 19.2%. The most common pathogen isolated from milk samples at dry-off was coagulase-negative Staphylococcus, followed by Aerococcus spp. and other Streptococcus spp. Noninferiority analysis showed no effect of treatment on risk for a cure between dry-off and calving [least squares means (LSM): QT=93.3%, SP=92.6%, and TM=94.0%] and secondary analysis showed no effect of treatment on risk for presence of an IMI at 0 to 6 DIM (LSM: QT=16.5%, SP=14.1%, and TM=16.0%), risk for development of a new IMI between dry-off and 0 to 6 DIM (LSM: QT=14.8%, SP=12.3%, and TM=14.2%), or risk of experiencing a clinical mastitis event between calving and 100 DIM (LSM: QT=5.3%, SP=3.8%, and TM=4.1%). In conclusion, no difference was observed in efficacy among the 3 products evaluated when assessing the aforementioned quarter-level outcomes.

Consequences of stopping and restarting leptin in an adolescent with lipodystrophy

BACKGROUND/AIMS

Lipodystrophy encompasses a group of rare disorders characterized by deficiency of adipose tissue resulting in hypoleptinemia, and metabolic abnormalities including insulin resistance, diabetes, dyslipidemia, and nonalcoholic steatohepatitis. Leptin replacement effectively ameliorates these metabolic derangements. We report effects of leptin discontinuation and resumption in a child with acquired generalized lipodystrophy.

METHODS

Intermittent treatment with leptin with follow-up over 5 years.

RESULTS

Pretreatment metabolic abnormalities included insulin resistance, hypertriglyceridemia and steatohepatitis. Leptin was started at the age of 10 years. After 2 years, the family requested discontinuation of leptin due to lack of visible physical changes. Nine months later, worsened metabolic abnormalities and arrest of pubertal development were observed. Leptin was restarted, followed by improvements in metabolic parameters. Laboratory changes (before vs. 6 months after restarting leptin) were: fasting glucose from 232 to 85 mg/dl, insulin from 232 to 38.9 µU/ml, HbA(1c) from 7.5 to 4.8%, triglycerides from 622 to 96 mg/dl, ALT from 229 to 61 U/l, AST from 91 to 18 U/l, and urine protein:creatinine ratio from 5.4 to 0.3. Progression of puberty was observed 1 year after restarting leptin.

CONCLUSION

Initial leptin therapy likely prevented progression of metabolic abnormalities. Treatment discontinuation led to rapid metabolic decomposition and pubertal arrest. Reintroduction of leptin reversed metabolic abnormalities and allowed normal pubertal progression.

Founder effect in the Horn of Africa for an insulin receptor mutation that may impair receptor recycling

AIMS/HYPOTHESIS

Genetic insulin receptoropathies are a rare cause of severe insulin resistance. We identified the Ile119Met missense mutation in the insulin receptor INSR gene, previously reported in a Yemeni kindred, in four unrelated patients with Somali ancestry. We aimed to investigate a possible genetic founder effect, and to study the mechanism of loss of function of the mutant receptor.

METHODS

Biochemical profiling and DNA haplotype analysis of affected patients were performed. Insulin receptor expression in lymphoblastoid cells from a homozygous p.Ile119Met INSR patient, and in cells heterologously expressing the mutant receptor, was examined. Insulin binding, insulin-stimulated receptor autophosphorylation, and cooperativity and pH dependency of insulin dissociation were also assessed.

RESULTS

All patients had biochemical profiles pathognomonic of insulin receptoropathy, while haplotype analysis revealed the putative shared region around the INSR mutant to be no larger than 28 kb. An increased insulin proreceptor to β subunit ratio was seen in patient-derived cells. Steady state insulin binding and insulin-stimulated autophosphorylation of the mutant receptor was normal; however it exhibited decreased insulin dissociation rates with preserved cooperativity, a difference accentuated at low pH.

CONCLUSIONS/INTERPRETATION

The p.Ile119Met INSR appears to have arisen around the Horn of Africa, and should be sought first in severely insulin resistant patients with ancestry from this region. Despite collectively compelling genetic, clinical and biochemical evidence for its pathogenicity, loss of function in conventional in vitro assays is subtle, suggesting mildly impaired receptor recycling only.

Treatment of type B insulin resistance: a novel approach to reduce insulin receptor autoantibodies

BACKGROUND

Type B insulin resistance belongs to a class of diseases caused by an autoantibody to a cell surface receptor. Blockade of insulin action results in hyperglycemia, hypercatabolism, severe acanthosis nigricans, and hyperandrogenism in women. This rare autoimmune disorder has been treated with various forms of immunosuppression with mixed success.

METHODS

We describe 14 patients with type B insulin resistance referred to the National Institutes of Health, adding to an existing cohort of 24 patients. This report focuses on seven patients who were treated with an intensive combination protocol of rituximab, cyclophosphamide, and pulse corticosteroids aimed at control of pathogenic autoantibody production. Hematological, metabolic, and endocrine parameters, including fasting glucose, glycated hemoglobin, insulin dose, lipids, and testosterone, were monitored before and after treatment.

RESULTS

All seven treated patients achieved remission, defined as amelioration of hyperglycemia, discontinuation of insulin therapy, and resolution of hyperandrogenism. Glycated hemoglobin has normalized in all seven treated patients. Remission was achieved on average in 8 months from initiation of treatment. The medication regimen was well tolerated, with no serious adverse events.

CONCLUSIONS

In seven patients with type B insulin resistance, standardized treatment with rituximab, cyclophosphamide, and pulse steroids results in remission of the disease. Future studies will determine whether this treatment protocol can be applied to other autoantibody/cell surface receptor disease states.

Efficacy of leptin therapy in the different forms of human lipodystrophy

AIMS/HYPOTHESIS

Lipodystrophy is a rare disorder characterised by loss of adipose tissue, hypoleptinaemia, severe insulin resistance, diabetes and dyslipidaemia. The aims of this study were to determine whether leptin replacement in lipodystrophy patients ameliorates their metabolic abnormalities over an extended period of time and whether leptin therapy is effective in the different forms of lipodystrophy.

METHODS

We conducted an open-label prospective study of patients with acquired forms of lipodystrophy and inherited forms of lipodystrophy secondary to mutations in the AGPAT2, SEIPIN (also known as BSCL2), LMNA and PPARgamma (also known as PPARG) genes. Between July 2000 and November 2008, 48 patients with lipodystrophy were treated with s.c. recombinant methionyl human leptin.

RESULTS

Serum triacylglycerol and HbA(1c) levels declined dramatically with leptin therapy. Among 35 patients with data at baseline and 12 months, serum triacylglycerol fell by 59% (from 10.18 +/- 2.67 mmol/l to 4.16 +/- 0.99 mmol/l [means +/- SE]; p = 0.008) and HbA(1c) decreased by 1.5 percentage points (from 8.4 +/- 0.3% to 6.9 +/- 0.3%; p < 0.001). A significant reduction was seen in total cholesterol and a trend towards reduction was observed in LDL-cholesterol at 12 months. HDL-cholesterol was unchanged. Among generalised lipodystrophy patients, proteinuria diminished with leptin replacement. Patients with both acquired and inherited forms of lipodystrophy experienced decreases in serum triacylglycerol and HbA(1c) levels.

CONCLUSIONS/INTERPRETATION

Leptin replacement in lipodystrophy patients leads to significant and sustained improvements in glycaemic control and dyslipidaemia. Leptin is effective in the various forms of lipodystrophy, whether they are acquired or inherited, generalised or partial.

TRIAL REGISTRATION

ClinicalTrials.gov ID NCT00025883

FUNDING

This work was supported by intramural research funding from the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) of the National Institutes of Health (NIH).

Elevated plasma adiponectin in humans with genetically defective insulin receptors

CONTEXT

Adiponectin has been suggested to play a role in the etiopathogenesis of at least some forms of insulin resistance, in part based on a strong correlation between plasma levels of adiponectin and measures of insulin sensitivity.

OBJECTIVE

The objective of the study was to establish whether this relationship is maintained at extreme levels of insulin resistance.

DESIGN/SETTING

This was a cross-sectional study in a university teaching hospital of subjects recruited from the United Kingdom and the United States.

PARTICIPANTS

Participants included 75 subjects with a range of syndromes of severe insulin resistance and 872 nondiabetic controls.

OUTCOME MEASURES

Fasting plasma insulin, adiponectin, and leptin were measured.

RESULTS

Unexpectedly, subjects with mutations in the insulin receptor, despite having the most severe degree of insulin resistance, had elevated plasma adiponectin [median 24.4 mg/liter; range 6.6-36.6 (normal adult range for body mass index 20 kg/m(2) = 3-19 mg/liter)], whereas all other subjects had low adiponectin levels (median 2.0 mg/liter; range 0.12-11.2). Plasma leptin in all but one subject with an insulin receptoropathy was low or undetectable [median 0.5 ng/ml; range 0-16: normal adult range for body mass index of < 25 kg/m(2) = 2.4-24.4 (female) and 0.4-8.3 ng/ml (male)].

CONCLUSIONS

We conclude that the relationship between plasma adiponectin and insulin sensitivity is complex and dependent on the precise etiology of defective insulin action and that the combination of high plasma adiponectin with low leptin may have clinical utility in patients with severe insulin resistance as a marker of the presence of a genetic defect in the insulin receptor.

The clinical utility of leptin therapy in metabolic dysfunction

Obesity and diabetes are major public health threats worldwide. Insulin resistance appears to be a significant factor in this global epidemic. In this present review, we have focused on a human model of insulin resistance which embodies many of the metabolic abnormalities that are associated with the morbidity of diabetes and obesity. Lipodystrophy in rodents and humans is a severe model of insulin resistance, and we use a novel therapeutic approach with the administration of the newly discovered leptin to ameliorate many of these metabolic abnormalities. The ability to study the administration of leptin in this setting of severe insulin resistance allows us perform a coveted look into a human condition where metabolic dysfunction can be reversed or controlled.

Repaglinide-induced factitious hypoglycemia

We report the first case of repaglinide-induced factitious hypoglycemia in a young male. This case posed a challenging diagnostic dilemma because commercial assays for repaglinide are not available. Furthermore, the patient had a series of positive diagnostic tests such as high proinsulin and localizing intra-arterial calcium stimulation suggestive of insulinoma. This case, again, demonstrates the importance of pure clinical judgment in the face of often-conflicting laboratory data in making a correct diagnosis and the requirement of definitive data for an appropriate therapeutic resolution.

Forty-eight-hour fast: the diagnostic test for insulinoma

Insulinoma causes fasting hypoglycemia due to inappropriate insulin secretion. Its diagnosis is based on demonstrating Whipple's triad during a supervised 72-h fast. For 75 yr, the 72-h fast has been the cornerstone for the diagnosis; however, it has never been critically assessed using newer assays for insulin, C peptide, and proinsulin. Thus, the aim of the current study is to assess the need for a full 72-h fast for the diagnosis of insulinoma. Patients with suspected hypoglycemia with documented glucose concentrations below 45 mg/dL were admitted to the NIH. Data obtained during the supervised fast of patients with pathologically proven insulinoma over a 30-yr period (1970-2000) were reviewed. We identified 127 patients with insulinoma. The average age of patients was 42.7 +/- 15.9 yr, with a predominance of females (62%). 107 patients had a benign tumor, 20 had malignant insulinoma, and 15 patients had multiple endocrine neoplasia type 1. The fast was terminated due to hypoglycemia in 44 patients (42.5%) by 12 h, 85 patients (66.9%) by 24 h, and 120 (94.5%) by 48 h. Seven patients fasted beyond 48 h despite subtle neuroglycopenic symptoms and glucose and insulin concentrations diagnostic of insulinoma. Immunoreactive proinsulin was elevated at the beginning of the fast in 90% of 42 patients. Proinsulin in noninsulinoma, in contrast to insulinoma, patients is usually suppressible; therefore, samples taken in the suppressed state have the greatest diagnostic value. We conclude that with the current available insulin and proinsulin assays, the diagnosis of insulinoma can be made within 48 h. Thus, the 48-h fast should replace the 72-h fast in textbooks and hospital protocols as the new diagnostic standard.

Forty-eight-hour fast: the diagnostic test for insulinoma

Insulinoma causes fasting hypoglycemia due to inappropriate insulin secretion. Its diagnosis is based on demonstrating Whipple's triad during a supervised 72-h fast. For 75 yr, the 72-h fast has been the cornerstone for the diagnosis; however, it has never been critically assessed using newer assays for insulin, C peptide, and proinsulin. Thus, the aim of the current study is to assess the need for a full 72-h fast for the diagnosis of insulinoma. Patients with suspected hypoglycemia with documented glucose concentrations below 45 mg/dL were admitted to the NIH. Data obtained during the supervised fast of patients with pathologically proven insulinoma over a 30-yr period (1970-2000) were reviewed. We identified 127 patients with insulinoma. The average age of patients was 42.7 +/- 15.9 yr, with a predominance of females (62%). 107 patients had a benign tumor, 20 had malignant insulinoma, and 15 patients had multiple endocrine neoplasia type 1. The fast was terminated due to hypoglycemia in 44 patients (42.5%) by 12 h, 85 patients (66.9%) by 24 h, and 120 (94.5%) by 48 h. Seven patients fasted beyond 48 h despite subtle neuroglycopenic symptoms and glucose and insulin concentrations diagnostic of insulinoma. Immunoreactive proinsulin was elevated at the beginning of the fast in 90% of 42 patients. Proinsulin in noninsulinoma, in contrast to insulinoma, patients is usually suppressible; therefore, samples taken in the suppressed state have the greatest diagnostic value. We conclude that with the current available insulin and proinsulin assays, the diagnosis of insulinoma can be made within 48 h. Thus, the 48-h fast should replace the 72-h fast in textbooks and hospital protocols as the new diagnostic standard.

Operative management of islet-cell tumors arising in the head of the pancreas

BACKGROUND

Pancreatic islet cell tumors (ICTs) can be treated by enucleation or pancreatic resection. We reviewed our experience with ICTs in the head of the pancreas to define indications for enucleation versus pancreaticoduodenectomy.

METHODS

Between January 1982 and December 1997, 48 patients underwent surgical resection for presumed ICTs of the pancreatic head. Of these, 18 were found on pathologic examination to be disease in a lymph node. Thirty patients had 32 true pancreatic head ICTs. We reviewed the operative results and postoperative courses in these patients.

RESULTS

Mean diameter of the ICTs was 15 mm. Twenty-seven patients (90%) underwent successful enucleations. Three patients failed enucleation and underwent pancreaticoduodenectomy. There was no operative mortality. The median time to regular diet was 7 days. The median time to removal of all drains was 22 days. The most common complication was pancreatic fistula (15%). No patient required reoperation for treatment of a fistula.

CONCLUSION

Most ICTs of the pancreatic head can be removed with enucleation, preserving pancreatic tissue and avoiding the morbidity of pancreaticoduodenectomy. The primary indication for pancreaticoduodenectomy is not the size of the lesion but its proximity to the pancreatic duct.

Long-term medical treatment of ectopic ACTH syndrome

BACKGROUND

The morbidity of hypercortisolemia due to ectopic production of ACTH by various tumors may be greater than the morbidity of the tumor itself.

METHODS

We report three cases of long-term treatment of ectopic ACTH syndrome due to metastatic bronchial carcinoid, islet cell carcinoma, and malignant thymoma tumors. Clinical and biochemical eucortisolemia was achieved in each case and was sustained from 24 to 55 months. We review the therapeutic options and their reported efficacy.

RESULTS

Cessation of therapy resulted in recurrence of hypercortisolemia in each case, showing the effectiveness of therapy.

CONCLUSION

Long-term treatment of ectopic ACTH-induced hypercortisolemia by blocking adrenal steroidogenesis is clinically effective and well tolerated.

Effects of proteasomal inhibitors on the maturation of the insulin proreceptor: an anatomical paradox

Inhibitors of proteasomal functions Carbobenzoxy-L-leucyl-L-leucyl-L-leucinal (MG132) and Carbobenzoxy-L-isoleucyl-gamma-t-butyl-L-alanyl-L-leucinal (PSI) were found to inhibit the conversion of the Insulin proreceptor to its mature alpha and beta subunits. By contrast no effect of these inhibitors was found on 125-I insulin binding, internalization and degradation. Since the insulin proreceptor is an integral membrane protein that is compartmentally separated from the cytoplasmic 26S proteasome, the inhibition of the normal biosynthetic processing of the insulin proreceptor presents an anatomical paradox.

Intraarterial calcium stimulation and intraoperative ultrasonography in the localization and resection of insulinomas

BACKGROUND

Standard imaging studies (computed tomography, magnetic resonance imaging, somatostatin receptor scintigraphy, ultrasonography, and angiography) correctly localize insulinomas in less than 50% of patients and provide no information about the feasibility of enucleation based on proximity of tumor to pancreatic duct. We reviewed our experience with intraarterial calcium stimulation (Ca-Stim) and intraoperative ultrasonography (IOUS) to localize and guide management of insulinomas.

METHODS

Thirty-six patients (14 men, 22 women, median age 44 years) with insulinomas were treated between August 1989 and June 1996. Preoperative imaging studies were obtained. Patients underwent abdominal exploration with IOUS. Fourteen were evaluated by a surgeon blinded to preoperative imaging results.

RESULTS

Tumors (4 to 50 mm) were resected by enucleation (67%) or partial pancreatectomy (33%); all were cured. Sensitivities of computed tomography, magnetic resonance imaging, somatostatin receptor scintigraphy, ultrasonography, angiography, and Ca-Stim in localizing insulinomas were 24%, 45%, 17%, 13%, 43%, and 94%, respectively. Tumors were identified by blinded surgical exploration with IOUS in 12 of 14 patients (86%).

CONCLUSIONS

All insulinomas were identified before operation; however sensitivity of individual noninvasive tests was low (less than 50%). In contrast, Ca-Stim was correct in 94% of cases, thus allowing a focused pancreatic exploration and obviating use of blind distal pancreatectomy. IOUS can then be used to guide safe enucleation.

Non-insulin dependent diabetes--the past, present and future

Diabetes, known since antiquity, has been defined by glycosuria. In 1886, when Minkowski demonstrated that pancreatectomized dogs developed diabetes, the islets of Langerhans became a focus of the search for an active principle culminating in the discovery and the isolation of insulin in 1921 by Banting, Best and Collip. In 1959, the radioimmunoassay of Yalow and Berson solidified the concept of insulin resistance in non-insulin dependent diabetes (NIDDM). In 1971, the insulin receptor was defined as a cell surface protein that initiated the insulin signal transduction cascade. Today, we know that NIDDM accounts for at least 90% of all diabetes worldwide and involves approximately 100 million people. The microvascular complications of NIDDM are the same as for insulin dependent diabetes (IDDM) and are related to the intensity and duration of hyperglycaemia. Further, it is clear from the Diabetes Control and Complications Trial (DCCT) that all microvascular complications can be reduced with intensive control of the blood glucose. Macrovascular disease is also accelerated in NIDDM, including both hypertension and dyslipidemia. The major risk factor for NIDDM are age, obesity, physical inactivity, and genetic background. The earliest features seen in individuals destined to develop NIDDM is insulin resistance, but for hyperglycaemia to ensure there must be a defect in insulin secretion. Thus, insulin resistance defines the prehyperglycaemic phase of NIDDM, but varying degrees of insulin secretory deficiency define the hyperglycaemic phase. Macrovascular risk occurs throughout the lifetime of the individual, whereas microvascular risk ensues with the inception of hyperglycaemia. Tomorrow, we will understand more clearly whether lifestyle changes, such as diet and exercise, or new classes of drugs, can delay or prevent NIDDM. Clinical trials are now beginning to test whether impaired glucose tolerance (IGT) can be delayed or prevented from moving to overt NIDDM. The genetics of NIDDM are under intense study. Mutations in the insulin receptor lead to NIDDM in a small number of patients, and mutations in the glucokinase gene lead to maturity onset diabetes of the young (MODY). Work is now underway to study other candidate genes as well as work on positional cloning techniques to identify diabetes genetic loci. The hormone Leptin has just been discovered and is a major regulator of body weight. In summary, the most important new emphasis on the treatment of NIDDM is the recognition of the importance of hyperglycaemia and our ability to both treat and possibly prevent this metabolic perturbation. This joins the longer-term emphasis on cardiovascular risk reduction from both treatment and prevention of hypertension and dyslipidemia.

Receptor-mediated internalization of insulin. Potential role of pp120/HA4, a substrate of the insulin receptor kinase

pp120/HA4 is a hepatocyte membrane glycoprotein phosphorylated by the insulin receptor tyrosine kinase. In this study, we have investigated the role of pp120/HA4 in insulin action. Transfection of antisense pp120/HA4 cDNA in H35 hepatoma cells resulted in inhibition of pp120/HA4 expression and was associated with a 2-3-fold decrease in the rate of insulin internalization. Furthermore, insulin internalization in NIH 3T3 fibroblasts co-transfected with insulin receptors and pp120/HA4 was increased 2-fold compared with cells expressing insulin receptors alone. In contrast, no effect on internalization was observed in cells overexpressing a naturally occurring splice variant of pp120/HA4 that lacks the phosphorylation sites in the intracellular domain. Insulin internalization was also unaffected in cells expressing three site-directed mutants of pp120/HA4 in which the sites of phosphorylation by the insulin receptor kinase had been removed (Y488F, Y488F/Y513F, and S503A). Our data suggest that pp120/HA4 is part of a complex of proteins required for receptor-mediated internalization of insulin. It is possible that this function is regulated by insulin-induced phosphorylation of the intracellular domain of pp120/HA4.

Localization of insulinomas to regions of the pancreas by intra-arterial stimulation with calcium

OBJECTIVE

To determine the sensitivity of calcium injected into pancreatic arteries in localizing insulin-secreting tumors to regions of the pancreas.

DESIGN AND PATIENTS

To stimulate the release of insulin, 25 patients with surgically proven insulinomas (average diameter, 15 mm) had calcium gluconate (0.025 mEq Ca++/kg body weight) injected before surgery into the arteries supplying the pancreatic head (gastroduodenal and superior mesenteric arteries) and the body and tail (splenic artery) of the pancreas.

SETTING

Tertiary referral hospital.

MEASUREMENTS

Insulin levels were measured in samples taken from the right and left hepatic veins before and 30, 60, and 120 seconds after calcium injection. A twofold increase in insulin level in the sample taken from the right hepatic vein 30 or 60 seconds after injection localized the insulinoma to the segment of the pancreas supplied by the selectively injected artery. Localization done using calcium stimulation was compared with localization done using transcutaneous ultrasonography (n = 22), computed tomography (n = 23), magnetic resonance imaging (n = 21), arteriography (n = 25), and portal venous sampling (n = 9).

RESULTS

Calcium stimulation localized 22 of 25 insulinomas (sensitivity, 88% [95% CI, 68% to 97%]) to the correct region of the pancreas. The sensitivities of the other imaging methods were 9% for ultrasonography (CI, 1% to 23%), 17% for computed tomography (CI, 5% to 39%), 43% for magnetic resonance imaging (CI, 22% to 66%), 36% for arteriography (CI, 18% to 57%), and 67% for portal venous sampling (CI, 30% to 93%). Calcium stimulation added only a few minutes to the time needed for pancreatic arteriography and caused no morbid conditions.

CONCLUSION

Intra-arterial calcium stimulation with right hepatic vein sampling for insulin gradients is the most sensitive preoperative test for localizing insulinomas.

Insulin-induced activation of phosphatidyl inositol 3-kinase. Demonstration that the p85 subunit binds directly to the COOH terminus of the insulin receptor in intact cells

Insulin activates the insulin receptor tyrosine kinase to phosphorylate signaling molecules such as insulin receptor substrate-1 (IRS-1). Phosphorylated IRS-1 binds to SH2 domains in the p85 regulatory subunit of phosphatidyl inositol (PI) 3-kinase, thereby stimulating the catalytic activity of PI 3-kinase. For most growth factor receptor tyrosine kinases (including receptors for epidermal growth factor and platelet-derived growth factor), the p85 regulatory subunit of PI 3-kinase binds directly to phosphorylated YXXM motifs contained in the cytoplasmic domain of the receptor itself. Previous studies in cell-free systems have shown that the phosphorylated YHTM sequence (amino acid residues 1322-1325) in the COOH terminus of the insulin receptor has the ability to bind to the p85 subunit of PI 3-kinase, thereby activating the enzyme. In this investigation, we demonstrate the occurrence of the same direct binding interaction in intact cells. Subsequent to insulin-stimulated phosphorylation of the insulin receptor, a complex is formed that contains the insulin receptor and PI 3-kinase. This complex can be immunoprecipitated by antibodies directed against either the insulin receptor or the p85 subunit of PI 3-kinase. The delta 43 mutant insulin receptor that lacks 43 amino acids at the COOH terminus does not bind p85. In addition, the delta 43 truncation impairs the ability of the receptor to mediate the activation of PI 3-kinase. Thus, by binding directly to p85, the phosphorylated YHTM motif in the COOH terminus of the insulin receptor contributes partially to mediating the effect of insulin to activate PI 3-kinase.

Rescue and activation of a binding-deficient insulin receptor. Evidence for intermolecular transphosphorylation

Binding of insulin to the alpha subunit of the insulin receptor (IR) leads to autophosphorylation of the beta subunit. The reaction proceeds as intramolecular transphosphorylation between alpha beta half-receptors of the heterotetrameric receptor dimer (alpha 2 beta 2). Since IRs are mobile in the plane of the plasma membrane, it is also possible that transphosphorylation may occur between adjacent holoreceptors (alpha 2 beta 2) by an intermolecular reaction. To address this question, we cotransfected NIH-3T3 cells with two IR cDNA constructs: a truncated but functionally normal IR lacking the C-terminal 43 amino acids (delta 43) and a full-length Leu323 mutant receptor that is expressed on the cell surface but that does not bind insulin. A clonal cell line was selected from cells cotransfected with a 1/5 ratio of delta 43 cDNA/Leu323 cDNA. The two homodimers (Leu323 and delta 43) were expressed without detectable formation of hybrid receptors. By using specific antibodies, we demonstrate that in cells coexpressing both homodimers, the Leu323 mutant receptor was phosphorylated in vivo by the delta 43 IR in an insulin-dependent manner. However, when the Leu323 mutant receptor was expressed alone, no phosphorylation was detected. In addition, we demonstrate the association of the phosphorylated Leu323 mutant receptor with insulin receptor substrate-1 and with phosphatidylinositol 3-kinase. These findings indicate that insulin binding is not required for phosphorylation of the Leu323 mutant receptor, that the phosphorylation of the Leu323 mutant receptor occurs by an intermolecular transphosphorylation mechanism, and, finally, that the Leu323 mutant receptor, once phosphorylated, can associate with insulin receptor substrate-1 and phosphatidylinositol 3-kinase.

A novel human insulin receptor gene mutation uniquely inhibits insulin binding without impairing posttranslational processing

The precise nature of the insulin-binding site of the insulin receptor (IR) has not been determined, although the importance of several regions of the alpha-subunit in insulin binding has been demonstrated. A naturally occurring mutation in a patient with severe insulin resistance that changes the Ser323 codon in the alpha-subunit of the IR to a leucine codon is associated with markedly impaired insulin binding to cells from the patient and to transfected cells expressing the mutant receptor. However, unlike other IR alpha-subunit mutations associated with decreased insulin binding, this mutation does not lead to a defect in posttranslational processing or cell-surface expression of IRs. Thus, the defect in insulin binding associated with the Leu323 mutant IR is a direct result of an alteration in the insulin-binding site. No natural IR mutation described thus far is associated with both decreased insulin binding and normal cell-surface expression of the mutant receptor. This study demonstrates the critical role that Ser323 of the IR alpha-subunit plays in insulin binding, either by forming part of the binding site or by stabilizing its conformation.

Structural basis by which a recessive mutation in the alpha-subunit of the insulin receptor affects insulin binding

Recently, a mutation substituting Leu for Ser323 in the alpha-subunit of the human insulin receptor has been identified in an insulin-resistant patient. The Leu323 mutation leads to a severe impairment in insulin binding without significantly altering the processing or cell surface expression of the receptor. In order to study how alpha beta half-receptors interact to form the insulin-binding site, we cotransfected NIH-3T3 cells with two insulin receptor cDNA constructs: a truncated insulin receptor lacking the C-terminal 43 amino acids (delta 43) and the full-length Leu323 mutant receptor. A clonal cell line from cotransfected cells expresses a hybrid receptor consisting of a Leu323 half-receptor and a delta 43 half-receptor. We demonstrate that the Leu323-delta 43 hybrid receptor binds insulin with high affinity. Furthermore, by cross-linking 125I-insulin to immobilized hybrid receptors, we show that only the alpha beta delta half of the hybrid receptor binds insulin. Since the isolated half-insulin receptor has low affinity for insulin, this suggests that the addition of even a non-binding alpha-subunit can result in high affinity binding to the holoreceptor (alpha alpha mut beta delta beta). Both beta and beta delta-subunits of the Leu323-delta 43 hybrid receptor are phosphorylated in vivo and in vitro in an insulin-dependent manner, suggesting an intramolecular transphosphorylation mechanism and that the presence of the Leu323 mutant receptor that lacks an intrinsic high affinity binding site does not prevent the associated beta-subunit from functioning either as a tyrosine kinase or as a phosphate acceptor in the hybrid insulin receptor molecule (alpha alpha mut beta delta beta). Furthermore, we show that the hybrid receptor can phosphorylate insulin receptor substrate-1 (IRS-1) in response to insulin and can be coimmunoprecipitated together with IRS-1 by anti-IRS-1 antibody.

Mutational analysis of the NH2-terminal glycosylation sites of the insulin receptor alpha-subunit

The insulin receptor is synthesized as a single chain of 190 kiloDaltons, which is processed to disulfide-linked mature alpha- and beta- subunits, containing N- and O-linked oligosaccharides and fatty acids. Previously (Collier E, Carpentier J-L, Beitz L, Caro LHP, Taylor SI, Gorden P: Biochemistry 32:7818-23, 1993), site directed mutagenesis of the asparagine in the first four sites of N-linked glycosylation to glutamine resulted in a receptor that was retained in the endoplasmic reticulum and not processed past the proreceptor form. In this study, mutation of these sites individually and in various combinations is studied. Mutation in the first or second glycosylation site does not significantly impair processing of the receptor; the receptor is found on the cell surface and binds insulin normally. If both the first and second sites are mutated, a significant reduction occurs in the amount of receptor found on the cell surface and in insulin binding. There is some processing of the receptor in cells expressing this mutant compared with the four-part mutant. If only the third and fourth sites are mutated, processing is impaired less than in the mutant with the first and second sites mutated. However, the amount of receptor found on the cell surface is less than in the mutant of only the first or only the second site. In all of these glycosylation mutants, the amount of receptor on the cell surface correlates with the level of 125I-labeled insulin binding on the cell surface.(ABSTRACT TRUNCATED AT 250 WORDS)

Endocytosis is inhibited in hepatocytes from diabetic rats

The consequences of type I diabetes on cellular endocytosis were investigated by comparing [125I]insulin, [125I]alpha 2-macroglobulin, and Lucifer yellow uptake in hepatocytes freshly isolated from control and STZ-induced diabetic rats. In addition to the previously described reversible inhibition of ligand-induced internalization of the insulin receptor, we report a decrease in the constitutive receptor-mediated endocytosis of alpha 2-macroglobulin and a near abolition of fluid-phase endocytosis of Lucifer yellow in cells from diabetic animals. Despite decreased receptor autophosphorylation and internalization, the ligand-induced surface redistribution of the insulin receptor was normal in the diabetic cell population. By contrast, the insulin receptor association with clathrin-coated pits was impaired in diabetic cells as a result of a decreased concentration of these specialized invaginations on the nonvillous cell surface. The morphology and diameter of clathrin-coated pits were similar in both conditions under study. These results demonstrate a general impairment of endocytosis in hypoinsulinemic diabetes: receptor-mediated endocytosis was less affected than fluid-phase endocytosis. Impaired endocytosis of specific ligands or other macromolecules could be an important mechanism underlying the accumulation of extracellular matrix or even blood cholesterol removal in diabetes.

Specific glycosylation site mutations of the insulin receptor alpha subunit impair intracellular transport

The insulin receptor is a transmembrane protein found on multiple cell types. This receptor is synthesized as a 190-kDa proreceptor which is cleaved to produce mature alpha and beta subunits. The proreceptor contains 18 potential sites for N-linked glycosylation: 14 on the alpha subunit and 4 on the beta subunit. The codons for asparagine in the first four sites at the amino terminus of the alpha subunit were mutated to code for glutamine. This mutant receptor cDNA was stably transfected into NIH 3T3 cells. The insulin receptor produced in these cells remained in the proreceptor form; no mature alpha and beta subunits were produced. The proreceptor was slightly smaller on SDS-PAGE gels than the wild-type proreceptor and contained four less oligosaccharide chains by tryptic peptide mapping. The carbohydrate chains on the mutant proreceptor remained endoglycosidase H sensitive. However, in the presence of brefeldin A, these oligosaccharide chains could be processed to endoglycosidase H resistant chains. By immunofluorescence, the mutant proreceptor was shown to be localized to the endoplasmic reticulum. No insulin receptors could be found on the cell-surface either with cell surface labeling with biotin or with 125I-insulin binding. Thus, glycosylation of the first four N-linked glycosylation sites of the insulin receptor is necessary for the proper processing and intracellular transport of the receptor. This is in contrast to glycosylation at the four sites on the beta subunit which appear not to be important for processing but necessary for signal transduction. Therefore, N-linked glycosylation of the insulin receptor at specific sites has multiple distinctive roles.

Lessening the burden of diabetes. Intervention strategies

OBJECTIVE

To evaluate the impact of primary and secondary interventions on the development of complications from diabetes, we modeled the effects of primary and secondary interventions for diabetes on a single well-studied complication, diabetic retinopathy.

RESEARCH DESIGN AND METHODS

A model was developed to predict cumulative incidence of retinopathy in IDDM and NIDDM. Risk functions are based on duration of diabetes. The effects of intervention strategies were simulated by altering the retinopathy risk. The effects of the simulations were assessed using cumulative incidence.

RESULTS

Simulations of delaying the onset of IDDM from 2 to 8 yr and decreasing the retinopathy rates by 20-80% were performed for each type of retinopathy. Simulating primary prevention shifted the cumulative incidence curves to the right, and simulating secondary intervention shifted the curves downward. Primary prevention was less effective than secondary prevention. This difference was more apparent for IDDM than for NIDDM, where disease duration and exposure to retinopathy risk were shorter. All interventions shifted the development of retinopathy to later in life.

CONCLUSIONS

The greatest effect on cumulative incidence of all forms of retinopathy occurs when primary and secondary interventions are combined.

Intraarterial calcium stimulation test for detection of insulinomas

Occult insulinomas are adenomas not visualized by routine preoperative imaging studies, including ultrasonography (US), computed tomography (CT), magnetic resonance imaging (MRI), and arteriography. Reliance for detecting occult insulinomas has been placed on portal venous sampling (PVS), a sensitive (> 75%) but invasive procedure that requires considerable experience to enable sampling of small veins about the pancreatic head. For the past 2 years we have attempted to localize insulinomas by stimulating the release of insulin with a selective intraarterial injection of calcium gluconate and sampling for insulin gradients in the hepatic veins. This study replaces PVS by indicating the region of the pancreas within which the tumor lies, although, like PVS, it does not directly visualize the tumor. Our experience in 9 patients with surgically proved insulinomas is reviewed, comparing the results with PVS as well as with US, CT, MRI, and arteriography.

Substitution of glutamic acid for alanine 1135 in the putative "catalytic loop" of the tyrosine kinase domain of the human insulin receptor. A mutation that impairs proteolytic processing into subunits and inhibits receptor tyrosine kinase activity

The intracellular domain of the insulin receptor possesses activity as a tyrosine-specific protein kinase which is stimulated by insulin binding to the extracellular domain of the receptor. We have identified a patient with a genetic form of insulin resistance who is heterozygous for a mutation substituting Glu for Ala1135 in the putative "catalytic loop" of the tyrosine kinase domain of the receptor. In this investigation, the Glu1135 mutant receptor was expressed by transfection of mutant cDNA into NIH-3T3 cells. Like previously described mutations in the tyrosine kinase domain, the Glu1135 mutation impairs receptor tyrosine kinase activity and inhibits the ability of insulin to stimulate thymidine incorporation and receptor endocytosis. These data support the hypothesis that the receptor tyrosine kinase activity plays a necessary role in the ability of the receptor to mediate insulin action in vitro and in vivo. However, unlike previously described mutations in the intracellular domain of the receptor, the Glu1135 mutation impairs proteolytic cleavage of the proreceptor into separate subunits and impairs the transport of the receptor to the cell surface. These latter defects provide an explanation for the decrease in the number of receptors on the cell surface observed in the patient's circulating monocytes despite the fact that the mutant receptor is resistant to endocytosis and insulin-induced down-regulation.

Radiation therapy of acromegaly

Conventional megavoltage irradiation of GH-secreting tumors has predictable effects on tumor mass, GH, and pituitary function. 1. Further growth of the tumor is prevented in more than 99% of patients, with only a fraction of a percent of patients requiring subsequent surgery for tumor mass effects. 2. GH falls predictably with time. By 2 years GH falls by about 50% from the baseline level, and by 5 years by about 75% from the baseline level. The initial GH elevation and the size and erosive features of the sella turcica do not affect the percent decrease in GH from the baseline elevation. 3. With prolonged follow-up, further decrease in GH is seen at 10 and 15 years, with the fraction of surviving patients achieving GH levels less than 5 ng/mL approaching 90% after 15 years in our experience. Gender, previous surgery, and hyperprolactinemia do not seem to affect the response to treatment. Patients with initial GH greater than 100 ng/mL are significantly less likely to achieve GH values less than 5 ng/mL during long-term follow-up. 4. Hypopituitarism is a predictable outcome of treatment, is delayed, and may be more likely in patients who have had surgery prior to irradiation. There is no evidence that this complication is more common in patients with acromegaly than in patients with other pituitary adenomas receiving similar treatment. 5. Vision loss due to megavoltage irradiation--using modern techniques and limiting the total dose to 4680 rad given in 25 fractions over 35 days, with individual fractions not exceeding 180 rad--is extremely rare. The reported cases have occurred almost entirely in patients who have received larger doses or higher fractional doses. The theory that patients with acromegaly are prone to radiation-induced injury to the CNS and optic nerves and chiasm because of small vessel disease is not supported by a review of the reported cases. 6. Brain necrosis and secondary neoplasms induced by irradiation are extremely rare. 7. Although anecdotal evidence raises the question of changes in intellectual function following irradiation, this has not been studied in adults receiving pituitary irradiation.

Epidermal growth factor receptor internalization and biosynthesis in the diabetic rat

The number of surface EGF receptors as well as their internalization rate and biosynthesis were analyzed in hepatocytes freshly isolated from control, streptozotocin-diabetic, and insulin-treated diabetic rats. All three parameters were decreased in diabetic animals and values were corrected by insulin treatment. Moreover, the inhibition of synthesis was specific for the EGF receptor since the other biosynthetically labeled proteins were not affected. These data demonstrate that the reduced number of hepatocyte surface EGF receptors results from an inhibition of EGF-receptor synthesis which is not compensated by a reduced internalization rate.

A prospective examination of octreotide-induced gall-bladder changes in acromegaly

OBJECTIVE

We wished to determine the effects of octreotide acetate, a somatostatin analogue, on gall-bladder function during treatment of acromegaly.

DESIGN

We used a prospective, open label trial of somatostatin analogue.

PATIENTS

Seventeen patients with acromegaly took part.

MEASUREMENTS

Ultrasonographic evaluation of gall-bladder contents were performed pretreatment, after 1 month, and subsequently at intervals of 3-6 months.

RESULTS

Non-shadowing floating echogenic particles were observed in the gall-bladder in 12 of 17 patients after (mean +/- SEM) 2.5 +/- 0.6 months of treatment. During long-term treatment (mean 20.8 +/- 4.3, median 13, range 1-59 months), ultrasound evidence for cholelithiasis was observed in four patients after 20 +/- 4 months (range 4.2-43) months of octreotide therapy. No symptoms of biliary tract disease have been observed. Duration of acromegaly, average GH, average IGF-I, gender, age at entry, dose of analogue, and concurrent use of non-steroidal anti-inflammatory drugs did not affect the occurrence of sludge or gallstones.

CONCLUSIONS

Formation of non-shadowing, floating echogenic particles occurs commonly during the first 6 months of treatment with octreotide acetate. Cholelithiasis is a risk of long-term treatment.

Characterization of purified autoantibodies to the insulin receptor from six patients with type B insulin resistance

Anti-insulin-receptor autoantibodies are present in the serum of patients with the type B syndrome of extreme insulin resistance. Sera from six patients with this syndrome were purified over protein-A agarose to remove insulin and other serum factors and obtain an immunoglobulin fraction. These purified fractions were used to quantitatively determine the antibodies' activity in three separate assays. The ability to inhibit insulin binding was determined in an assay using fibroblasts that overexpress the human insulin receptor; the ability to immunoprecipitate the receptor was determined in an assay using biosynthetically labeled insulin receptors rather than insulin cross-linked receptors; and the ability to stimulate glucose oxidation was determined in isolated adipocytes. We show that the ability of these antibodies to inhibit insulin binding is tightly coupled to their ability to immunoprecipitate the biosynthetically labeled receptor, but neither assay predicts the bioactivity of these immunoglobulins. We suggest that the inability to show this tight coupling in the past may be due to methodological differences. We find no evidence that these antibodies are anti-idiotypic antibodies.

Calcium ions are required for the intracellular routing of insulin and its receptor

We have studied the role of the cytosolic-free calcium concentration ([Ca2+]i) on the early and later internalization steps of insulin and its receptor. As before, we find that the rate of 125I-insulin internalization in HL60 cells remains normal when [Ca2+]i is lowered 10 times below normal resting level by the use of an intracellular Ca2+ chelator. By contrast, the subsequent intracellular steps, i.e. insulin receptor recycling and insulin degradation, are inhibited in calcium-depleted cells. Under low [Ca2+]i conditions, the association of 125I-insulin with late endosomes and lysosomes is also reduced. This suggests that calcium ions are required for fusion processes occurring at the endosomal or postendosomal stage of internalization. Thus, by regulating insulin receptor recycling and by controlling insulin degradation, Ca2+ ions play a key role in the regulation of insulin action.

Insulin-induced surface redistribution regulates internalization of the insulin receptor and requires its autophosphorylation

The role of insulin-induced receptor autophosphorylation in its internalization was analyzed by comparing 125I-labeled insulin (125I-insulin) internalization in Chinese hamster ovary (CHO) cell lines transfected with normal (CHO.T) or mutated insulin receptors. In four cell lines with a defect of insulin-induced autophosphorylation, 125I-insulin internalization was impaired. By contrast, in CHO.T cells and in two other CHO cell lines with amino acid deletions or insertions that do not perturb autophosphorylation, 125I-insulin internalization was not affected. A morphological analysis showed that the inhibition is linked to the ligand-specific surface redistribution in which the insulin-receptor complexes leave microvilli and concentrate on nonvillous segments of the membrane where endocytosis occurs.

Persistent or recurrent acromegaly. Long-term endocrinologic efficacy and neurologic safety of postsurgical radiation therapy

As in other clinics, pituitary surgery was definitive treatment in less than 50% of cases of acromegaly treated at one institution over several decades. From 1965 to 1989, 24 acromegalic patients who had noncurative pituitary surgery received radiation therapy at the National Institutes of Health, with a basal human growth hormone level of greater than 5 ng/ml as the criterion for active disease. Using megavoltage irradiation, more than 60% of these patients stabilized at a normal hormonal range, and the overwhelming majority had decreasing growth hormone levels with time. No major side effects of irradiation were encountered except panhypopituitarism of varying degrees. The authors evolved a policy of surgery as the first option, followed by irradiation for patients with postoperative growth hormone levels more than 5 ng/ml.

Results of a prospective strategy to diagnose, localize, and resect insulinomas

Since 1982, 25 consecutive patients with benign sporadic (non-multiple endocrine neoplasia type I) insulinomas have been studied. Most were referred because either the tumor was not identified at the referring institution or the diagnosis was unclear. Each patient suffered severe neuroglycopenic symptoms for a median of 24 months before diagnosis of insulinoma, and 32% had hypoglycemic seizures. Eighteen patients (72%) had a confirmed weight gain. Each patient underwent a supervised fast until 72 hours or the onset of significant neuroglycopenic symptoms (median duration 16 hours), with serum levels of glucose (median 35 mg/dl; range 24 to 46 mg/dl), insulin (median 21 microU/ml; range 11 to 230 microU/ml), C-peptide (median 2.5 ng/ml; range 1.0 to 7.2 ng/ml), and proinsulin fraction (median 55%; range 14% to 86%) measured at the termination of the fast. Preoperative imaging with ultrasonography, computed tomography, magnetic resonance, and angiography visualized tumor in a minority of patients (26%, 17%, 25%, and 35%, respectively); in 48% of patients one or more imaging study results was positive. Selective portal venous sampling for insulin was the most informative localizing test (77% positive; no false-positive results). Tumor was resected for cure in 24 of 25 patients. Intraoperative ultrasonography identified nonpalpable tumor in seven patients and was crucial to the achievement of this high rate of surgical cure. We conclude that the diagnosis of insulinoma can be made by the results of a supervised fast, portal venous sampling is the most sensitive preoperative test for localizing insulinomas, and intraoperative ultrasonography is essential for intraoperative detection of insulinomas.

A mutation in the tyrosine kinase domain of the insulin receptor associated with insulin resistance in an obese woman

Insulin resistance is frequently associated with acanthosis nigricans and hyperandrogenism. In patients with type A insulin resistance, this has been shown to be due to genetic defects in insulin receptor function. However, other patients with a similar clinical syndrome have been reported to have a variant of this syndrome, in which assays of insulin receptor function were normal. We have sequenced a portion of the insulin receptor gene in one such patient, a 29-yr-old woman with obesity and insulin resistance. The patient is heterozygous for a mutation substituting isoleucine for methionine at position 1153. Met1153 is located in the intracellular domain of the receptor near the cluster of tyrosine phosphorylation sites at positions 1158, 1162, and 1163. Studies of the mutant receptor expressed in NIH-3T3 cells demonstrated that the Ile1153-mutation impairs the ability of insulin to stimulate autophosphorylation of solubilized insulin receptors. In addition, the mutation impairs the ability of insulin to stimulate receptor tyrosine kinase activity to phosphorylate an artificial substrate [poly(Glu-Tyr)]. It seems likely that this defect in receptor tyrosine kinase activity explains the defect in the ability of the patient's insulin receptors to mediate insulin action in vivo. Furthermore, this patient provides a paradigm in which genetic factors act in concert with other risk factors, such as obesity, to cause clinically important insulin resistance.

The relationship of ligand receptor mobility to internalization of polypeptide hormones and growth factors

Polypeptide hormones and growth factors bind to cell surface receptors and are internalized by receptor-mediated endocytosis. Both [125I]insulin and [125I]epidermal growth factor (EGF) are internalized to a much greater extent than [125I]glucagon in freshly isolated rat hepatocytes. All three ligands bind initially and preferentially to the microvillous surface of the hepatocyte, but only [125I]insulin and [125I]EGF undergo significant redistribution to the nonvillous surface of the cell. Thus, the degree of lateral mobility of the ligand receptor complex is strongly correlated with the extent of internalization of the ligand. Since the beta-subunit of the insulin and the EGF receptors span the plasma membrane only once and both receptors are autophosphorylated, it is possible that these are important determinants of the receptor mobility.

Cystic fibrosis research

In Virginia Morell's Research News article "New light on writing in the Americas" (18 Jan., p. 268), the illustrations on page 268 should have been credited to George Stuart of the National Geographic Society.

O-linked oligosaccharides on insulin receptor

The insulin receptor, an integral membrane glycoprotein, is synthesized as a single-chain precursor that is cleaved to produce two mature subunits, both of which contain N-linked oligosaccharide chains and covalently linked fatty acids. We report that the beta-subunit also contains O-linked oligosaccharides. The proreceptor, alpha-subunit, and beta-subunit were labeled with [3H]mannose and [3H]galactose in the presence or absence of an inhibitor of O-linked glycosylation. Tryptic peptides from each component were separated by reverse-phase high-performance liquid chromatography. N- and O-linked oligosaccharide chains were identified on these peptides by specific enzymatic digestions. The proreceptor and alpha-subunit contained only N-linked oligosaccharides, whereas the beta-subunit contained both N- and O-linked oligosaccharides. The O-linked oligosaccharide chains were attached to a single tryptic fraction of the beta-subunit, which also contained N-linked chains. This fraction was further localized to the NH2-terminal tryptic peptide of the beta-subunit by specific immunoprecipitation with an anti-peptide antibody with specificity for this region. Binding of insulin and autophosphorylation of the beta-subunit were not dependent on O-linked glycosylation, because cells grown in the presence of the inhibitor exhibited a normal dose response to insulin. Therefore, the insulin receptor contains O-linked oligosaccharides on the NH2-terminal tryptic peptide of the beta-subunit, and these O-linked oligosaccharides are not necessary to the binding or autophosphorylation function of the receptor.