Insulin propeptides in conditions associated with insulin resistance in humans and their relevance to insulin measurements

Cross-sectional and prospective associations between abdominal adiposity and proinsulin concentration

The objective of this study was to investigate the associations of total and abdominal obesity with variation in proinsulin concentration in a Native Canadian population experiencing an epidemic of type 2 diabetes mellitus (DM). Between 1993 and 1995, 728 members of a Native Canadian community participated in a population-based survey to determine the prevalence and risk factors for type 2 DM. Samples for glucose, C-peptide, and proinsulin were drawn after an overnight fast, and a 75-g oral glucose tolerance test was administered. Type 2 DM and impaired glucose tolerance (IGT) were diagnosed using World Health Organization criteria. Height, weight, waist circumference, and percent body fat were measured. In 1998, 95 individuals who, at baseline, had IGT or normal glucose tolerance with an elevated 2-h glucose level (> or = 7.0 mM) participated in a follow-up evaluation using the same protocol. After adjustment for age, sex, C-peptide concentration, per cent body fat, and waist circumference, proinsulin was found to be significantly elevated in diabetic subjects, relative to subjects with both impaired and normal glucose tolerance (both P < 0.0001); and the concentration in those with IGT was higher, compared with normals (P < 0.0001). Among nondiabetic subjects, proinsulin showed significant univariate associations with percent body fat, body mass index, and waist circumference (r = 0.34, 0.45, 0.41, respectively, all P < 0.0001). After adjustment for body fat and other covariates, waist circumference remained significantly associated with proinsulin concentration in nondiabetic subjects (r = 0.20, P < 0.0001). In prospective analysis, adjusted for covariates (including baseline IGT and follow-up glucose tolerance status), baseline waist circumference was positively associated with both follow-up and change in proinsulin concentration (r = 0.27, P = 0.01; r = 0.24, P = 0.03, respectively). These data highlight the detrimental effects of abdominal obesity on beta-cell function, and support the hypothesis that beta-cell dysfunction occurs early in the natural history of glucose intolerance.

Evidence for a role of tumor necrosis factor alpha in disturbances of triglyceride and glucose metabolism predisposing to coronary heart disease

Elevated plasma levels of triglyceride-rich lipoproteins, a decreased high-density lipoprotein (HDL) cholesterol concentration, hyperinsulinemia, and impaired fibrinolytic function frequently aggregate in patients with premature coronary heart disease (CHD). Experimental studies suggest that the cytokine tumor necrosis factor alpha (TNFalpha) produced by adipocytes plays a part in the regulation of triglyceride and glucose metabolism. The present study examined whether TNFalpha is implicated in these metabolic and fibrinolytic disturbances in young postinfarction patients. TNFalpha levels were determined in two groups of young (age <45 years) male postinfarction patients (n = 92 and 60) and in matched, population-based control subjects (n = 63). Plasma TNFalpha was higher in patients than in controls (4.1 +/- 1.6 v2.5 +/- 0.4 pg/mL, P < .0001). In hyperlipidemic patients, TNFalpha levels correlated significantly with the concentrations of very-low-density lipoprotein (VLDL) triglyceride and cholesterol and negatively with HDL cholesterol. Treatment with bezafibrate decreased VLDL triglycerides and increased HDL cholesterol, but did not affect TNFalpha levels. The TNFalpha concentration also correlated significantly with fasting glucose and proinsulin concentrations, as well as glucose and proinsulin levels after glucose ingestion. In contrast, no relations were found with the insulin level or degree of insulin resistance. The present results provide clinical evidence for a basic role of TNFalpha in hypertriglyceridemia, glucose intolerance, and the etiology of premature CHD.

Third trimester fetal growth and measures of carbohydrate and lipid metabolism in umbilical venous blood at term

AIM

To compare measures of carbohydrate and lipid metabolism in umbilical venous blood after birth at term in pregnancies with normal and retarded fetal growth during the third trimester.

METHODS

Three groups of pregnancies reaching term, in which fetal growth had been prospectively monitored by repeated ultrasound measurements during the third trimester, were studied. Sequential fetal abdominal circumference measurements remained above the 10th centile in 42 (normal size, normal growth group), below the 10th centile but did not depart further than 1.5 SD (small, normal growth group), or below the 10th centile and subsequently fell away by more than 1.5 SD before delivery (small, growth retarded group). Birthweight, neonatal morphometric measures (ponderal index, mid arm:head circumference ratio, subscapular and triceps skinfold thickness), umbilical venous blood concentrations of glucose, insulin, pro-insulin, des 31,32 proinsulin, total cholesterol, free cholesterol, cholesterol ester, triglycerides, lipoprotein (a), apolipoprotein A-1 and apolipoprotein B were measured.

RESULTS

The median birthweight of the three groups was significantly different (3570, 2569, and 2277 g, respectively). Median values of ponderal index and mid arm:head circumference ratio were significantly lower in the small, growth retarded group and did not differ between the small and normal size groups with normal growth. Both groups with small fetuses had significantly lower mean glucose and cholesterol ester concentrations, and higher mean free cholesterol:cholesterol ester ratios, compared with the normal size, normal growth group. The group showing fetal growth retardation had mean total cholesterol and mean cholesterol ester concentrations that were significantly lower than those of both the other two groups. Mean des 31,32 proinsulin concentrations were low in both groups of small fetuses, but only significantly so in the group without fetal growth retardation. Mean insulin, proinsulin, free cholesterol, triglycerides, lipoprotein(a), apolipoprotein A-1, apolipoprotein B concentrations and the ratio of A-1:B were similar in all three groups.

CONCLUSION

The similarity in the umbilical venous blood carbohydrate and lipid profile at term between pregnancies with documented third trimester fetal growth retardation and those with "genetically" small babies argues against a major role for intrauterine nutritional deprivation as a cause for the association between birth-weight and subsequent adult disease.

High-density lipoprotein: relations to metabolic parameters and severity of coronary artery disease

The regulation of plasma high-density lipoprotein (HDL) cholesterol level by the joint influence of plasma lipoprotein lipids, lipoprotein lipase (LPL), hepatic lipase (HL), cholesteryl ester transfer protein (CETP), oral glucose tolerance, and postload plasma insulin and proinsulin levels was investigated in young postinfarction patients and healthy population-based control subjects. In addition, the association between HDL cholesterol and the number and severity of coronary stenoses previously reported in this cohort of young postinfarction patients was further investigated by analyzing the determinants and angiographic relations of HDL subclasses measured by gradient gel electrophoresis. The following parameters showed significant univariate relations with HDL cholesterol level in the patient group: very-low-density lipoprotein (VLDL) cholesterol and triglyceride, low-density lipoprotein (LDL) triglyceride, and postload plasma insulin concentrations, preheparin plasma LPL mass, and postheparin plasma HL activity. In the control group, significant correlations with HDL cholesterol concentration in addition to those noted among the patients were found for body mass index (BMI), LDL cholesterol level, postload plasma intact proinsulin concentration, and LPL activity in postheparin plasma. In contrast to the patients, no significant relations were noted for postload plasma insulin level and preheparin plasma LPL mass. Multiple stepwise regression analysis showed that 42% of the variability of HDL cholesterol in the patients could be accounted for by VLDL cholesterol concentration (29%), LDL triglyceride level (7%), and postheparin plasma HL activity (8%), whereas the corresponding figure in controls was 35% (VLDL cholesterol concentration [9%] and postheparin plasma HL activity [26%]. The strength of the relationships of HDL cholesterol and HDL subclasses to the coronary stenosis score was similar and statistically significant (r = .25 to .36). When the metabolic parameters that correlated with HDL cholesterol and HDL subclass concentrations in univariate analysis were used as covariates, all relations to the coronary stenosis score disappeared. This clearly indicates that the influence of triglyceride-rich lipoproteins and lipolytic enzymes needs to be considered when assessing the association between HDL cholesterol and coronary artery disease (CAD).

Use of specific immunoradiometric assay to determine preterm neonatal insulin-glucose relations

Highly specific immunoradiometric assays were used to measure plasma concentrations of insulin, proinsulin, and 32-33 split proinsulin in neonates (n = 16). Neonatal plasma insulin concentrations were high relative to blood glucose concentrations and compared with adult insulin-glucose relations. Concentrations of proinsulin and 32-33 split proinsulin together accounted for 34-70% of the total concentration of insulin and pro-peptides. This study confirms the need to use a specific assay and neonatal reference data in the diagnosis of neonatal hyperinsulinism, and shows that neonatal pancreatic beta cell function may differ from that of older subjects.

Association of insulin and insulin propeptides with an atherogenic lipoprotein phenotype

A characteristic lipoprotein phenotype, including hypertriglyceridemia, a low high-density lipoprotein (HDL) cholesterol concentration, and a predominance of small, dense low-density lipoprotein (LDL) particles, is linked to insulin resistance and hyperinsulinemia. Individuals with these characteristics are supposed to be at increased risk of developing coronary heart disease (CHD). To address this issue further, relations between basal and postload glucose, insulin and insulin propeptide concentrations and subfractions of apolipoprotein (apo) B-containing lipoproteins were examined in 62 consecutive Swedish nondiabetic men who had experienced a first myocardial infarction before the age of 45. A total of 41 age-matched, population-based healthy men were investigated as controls. Highly specific immunoradiometric assays were used for measuring intact proinsulin and des 31,32proinsulin levels. In all, 39% of the patients were found to be glucose-intolerant, and basal and postload hyper(pro)insulinemia were characteristic features irrespective of glucose tolerance category. Hypertriglyceridemic (HTG) lipoprotein phenotypes with a low HDL cholesterol concentration dominated among the patients, and hyperinsulinemia was linked to hypertriglyceridemia and putatively atherogenic lipoprotein traits, such as increased particle numbers of small very-low-density lipoprotein (VLDL) and intermediate-density lipoprotein (IDL) and triglyceride enrichment of LDL. The corollary of these findings is that insulin resistance is a characteristic feature of young postinfarction patients and is accompanied by a complex atherogenic lipoprotein phenotype, new components of which are an abundance of small cholesteryl ester-rich VLDL and an elevated LDL triglyceride concentration.

Insulin, intact and split proinsulin, and coronary artery disease in young men

BACKGROUND

Glucose intolerance and hyperinsulinemia are common disturbances in nondiabetic men with premature coronary artery disease (CAD). To investigate the relation between insulin-like molecules and severity of coronary atherosclerosis, 62 consecutive nondiabetic men presenting with a first myocardial infarction before the age of 45 were studied along with 41 healthy, age-matched, male, population-based control subjects.

METHODS AND RESULTS

Specific two-site immunoradiometric assays were used to distinguish intact proinsulin, (des 31,32) proinsulin, and "true" insulin in fasting plasma and during an oral glucose tolerance test (OGTT). Global coronary atherosclerosis and number and severity of distinct stenoses were determined in the patients in 15 proximal coronary arterial segments by use of separate semiquantitative classification systems. The patients had a two- to threefold increase in insulin and insulin propeptide concentrations in the fasting state as well as during the OGTT. Severity of coronary atherosclerosis correlated significantly (P < .05 to P < .01) with basal proinsulin (r = .40) and the proinsulin area under the curve (AUC) (r = .34), basal insulin (r = .31), basal C peptide (r = .30), and the glucose AUC (r = .30). In multiple stepwise regression analysis including insulin-like molecules, major plasma lipoproteins, and lipoprotein subfractions, basal proinsulin (increase in R2 = .09) and dense LDL triglycerides (increase in R2 = .10) predicted 19% of the variation of the global coronary atherosclerosis score after adjustment for age, body mass index, fasting insulin concentration, and VLDL triglycerides.

CONCLUSIONS

This study shows that young, nondiabetic, male survivors of myocardial infarction are truly hyperinsulinemic during an OGTT and suggests a close association between proinsulin and coronary atherosclerosis.

Allele-specific increase in basal transcription of the plasminogen-activator inhibitor 1 gene is associated with myocardial infarction

Increased plasminogen-activator inhibitor 1 (PAI-1) activity is a common finding in patients with coronary heart disease. Here we provide evidence for an independent, etiological role of PAI-1 in myocardial infarction. The 4G allele of a recently described common 4/5-guanine-tract (4G/5G) polymorphism in the PAI-1 promoter is associated with higher plasma PAI-1 activity. The prevalence of the 4G allele is significantly higher in patients with myocardial infarction before the age of 45 than in population-based controls (allele frequencies of 0.63 vs. 0.53). Both alleles bind a transcriptional activator, whereas the 5G allele also binds a repressor protein to an overlapping binding site. In the absence of bound repressor, the basal level of PAI-1 transcription is increased.