Transcription factor 7-like 2 gene links increased in vivo insulin synthesis to type 2 diabetes

Transcription factor 7-like 2 (TCF7L2) is the main susceptibility gene for type 2 diabetes, primarily through impairing the insulin secretion by pancreatic β cells. However, the exact in vivo mechanisms remain poorly understood. We performed a family study and determined if the T risk allele of the rs7903146 in the TCF7L2 gene increases the risk of type 2 diabetes based on real-time stable isotope measurements of insulin synthesis during an Oral Glucose Tolerance Test. In addition, we performed oral minimal model (OMM) analyses to assess insulin sensitivity and β cell function indices. Compared to unaffected relatives, individuals with type 2 diabetes had lower OMM indices and a higher level of insulin synthesis. We found a T allele-dosage effect on insulin synthesis and on glucose tolerance status, therefore insulin synthesis was higher among T-allele carriers with type 2 diabetes than in wild-type individuals. These results suggest that hyperinsulinemia is not only an adaptation to insulin resistance, but also a direct cause of type 2 diabetes.

A stable isotope method for in vivo assessment of human insulin synthesis and secretion

AIMS

In vitro, beta cells immediately secrete stored but readily releasable insulin in response to a rise of glucose. During a prolonged insulin response, this is followed by newly synthesized insulin. Our aim was to develop an in vivo test to determine the ratio between readily available and newly synthesized insulin after a stimulus in humans by labelling newly synthesized insulin.

METHODS

A stable isotope tracer of 1.0 g ¹³C leucine with C-peptide as target peptide was administered 45 min prior to 75 g glucose load of a frequently blood sampled 210-min oral glucose tolerance test (OGTT). Our OGTT also encompassed collection of urine, which has a high content of C-peptide. Prior, the optimal conditions under which the tracer ¹³C leucine was administered for enrichment of (pre) proinsulin were established. Also, techniques to obtain urinary C-peptide under highly purified circumstances were set up. Our main outcome measure was the stable isotope enrichment of de novo C-peptide, which we related to early plasma insulin and glucose AUC. Twelve healthy Caucasian individuals (M4F8, age 41.8 ± 2.3, BMI 28.3 ± 1.7) with normal glucose tolerance underwent our OGTT.

RESULTS

We found that during a 75-g OGTT, newly synthesized insulin contributed approximately 20 % of total insulin secretion. The pattern of isotope enrichment obtained by collecting multiple urine voids was suggestive that the newly synthesized insulin contributes to the late phase of insulin secretion. De novo C-peptide correlated negatively with both early plasma insulin AUC (r = -0.629, P = 0.028) and early plasma glucose AUC (r = -0.605, P = 0.037).

CONCLUSIONS

With stable isotope technique added to OGTT, we were able to measure newly synthesized insulin in healthy individuals. This new technique holds the promise that it is feasible to develop a direct in vivo beta cell function test.

Post-glucose-load urinary C-peptide and glucose concentration obtained during OGTT do not affect oral minimal model-based plasma indices

The purpose of this study was to investigate how renal loss of both C-peptide and glucose during oral glucose tolerance test (OGTT) relate to and affect plasma-derived oral minimal model (OMM) indices. All individuals were recruited during family screening between August 2007 and January 2011 and underwent a 3.5-h OGTT, collecting nine plasma samples and urine during OGTT. We obtained the following three subgroups: normoglycemic, at risk, and T2D. We recruited South Asian and Caucasian families, and we report separate analyses if differences occurred. Plasma glucose, insulin, and C-peptide concentrations were analyzed as AUCs during OGTT, OMM estimate of renal C-peptide secretion, and OMM beta-cell and insulin sensitivity indices were calculated to obtain disposition indices. Post-glucose load glucose and C-peptide in urine were measured and related to plasma-based indices. Urinary glucose corresponded well with plasma glucose AUC (Cau r = 0.64, P < 0.01; SA r = 0.69, P < 0.01), S I (Cau r = -0.51, P < 0.01; SA r = -0.41, P < 0.01), Φ dynamic (Cau r = -0.41, P < 0.01; SA r = -0.57, P < 0.01), and Φ oral (Cau r = -0.61, P < 0.01; SA r = -0.73, P < 0.01). Urinary C-peptide corresponded well to plasma C-peptide AUC (Cau r = 0.45, P < 0.01; SA r = 0.33, P < 0.05) and OMM estimate of renal C-peptide secretion (r = 0.42, P < 0.01). In general, glucose excretion plasma threshold for the presence of glucose in urine was ~10-10.5 mmol L(-1) in non-T2D individuals, but not measurable in T2D individuals. Renal glucose secretion during OGTT did not influence OMM indices in general nor in T2D patients (renal clearance range 0-2.1 %, with median 0.2 % of plasma glucose AUC). C-indices of urinary glucose to detect various stages of glucose intolerance were excellent (Cau 0.83-0.98; SA 0.75-0.89). The limited role of renal glucose secretion validates the neglecting of urinary glucose secretion in kinetic models of glucose homeostasis using plasma glucose concentrations. Both C-peptide and glucose in urine collected during OGTT might be used as non-invasive measures for endogenous insulin secretion and glucose tolerance state.

Failing beta-cell adaptation in South Asian families with a high risk of type 2 diabetes

We performed an extended oral glucose tolerance test (OGTT) to investigate the relationship between early and late beta-cell response and type 2 diabetes (T2D) in families of South Asian origin and indigenous Dutch, burdened by T2D. Based on the OGTT, 22 individuals were normoglycemic, 12 glucose intolerant and 23 had T2D in the South Asian families; these numbers were 34, 12 and 18 in the Caucasian families, respectively. The OGTT had 11 blood samplings in 3.5 h for glucose, insulin and C-peptide measurements. Through early and late insulin secretion rate (ISR), the above basal glucose area-under-the-curve after glucose load (glucose disposal) and insulin sensitivity index (ISI), we obtained early and late disposition indices (DI). South Asians on average had lower ISI than Caucasians (3.8 ± 2.9 vs. 6.5 ± 4.7, respectively, P < 0.001), with rapid decline of their early and late DI between normal glucose tolerance versus impaired fasting glucose/impaired glucose tolerance (late DI; P < 0.0001). Adjusted for ISI, age, gender and waist-to-hip ratio, early ISR was significantly associated with glucose disposal in South Asians (β = 0.55[0.186; 0.920]), but not in Caucasians (β = 0.09[-0.257; 0.441]). Similarly, early ISR was strongly associated with late ISR (β = 0.71[0.291; 1.123]; R (2) = 45.5 %) in South Asians, but not in Caucasians (β = 0.27[-0.035; 0.576]; R (2) = 17.4 %), with significant interaction between ethnicity and early ISR (β = 0.341[0.018; 0.664]). Ordinal regression analyses confirmed that all South Asian OGTT subgroups were homogenously resistant to insulin and solely predicted by early ISR (β = -0.782[-1.922; 0.359], β = -0.020[-0.037; -0.002], respectively), while in Caucasian families both ISI and early ISR were related to glucose tolerance state (β = -0.603[-1.105; -0.101], β = -0.066[-0.105; -0.027], respectively). In South Asian individuals, rapid beta-cell deterioration might occur under insulin resistant conditions. As their early insulin response correlates strongly with both glucose disposal and late insulin response, alterations in beta-cell dynamics may give an explanation to their extreme early onset of T2D, although larger prospective studies are required.

Myeloperoxidase is not associated with scintigraphic myocardial perfusion abnormalities in type 2 diabetic patients with mild stable anginal complaints

BACKGROUND

MPO, an enzyme of the innate immune system, exhibits pro-atherogenic effects. These include oxidative damage to LDL- and HDL-cholesterol, and promotion of endothelial dysfunction. Recent studies revealed that MPO independently predicts adverse outcomes in patients with chest pain or suspected acute coronary syndrome. We evaluated whether plasma myeloperoxidase (MPO) levels are associated with scintigraphic myocardial perfusion abnormalities, in type 2 diabetic patients with mild anginal complaints.

METHODS

MPO was measured in plasma samples of 267 patients with diabetes mellitus type 2 and stable angina pectoris complaints (Canadian Cardiovascular Society class I-II/IV) prior to myocardial perfusion scintigraphy (MPS).

RESULTS

The median plasma level of MPO was 141 pmol/L (IQR 115-171 pmol/L). One-hundred-ninety patients (71%) had perfusion abnormalities on MPS and of these, 138 patients had myocardial ischemia. No relation was found between plasma MPO levels and the scintigraphic myocardial perfusion abnormalities. Even in combination with known other cardiovascular risk factors MPO failed to predict scintigraphic myocardial perfusion abnormalities.

CONCLUSIONS

MPO levels are not associated with scintigraphic myocardial perfusion abnormalities in type 2 diabetic patients with mild anginal complaints. Therefore, in type 2 diabetic patients MPO is not a useful biomarker to predict hemodynamically significant coronary artery disease.

Diabetes mellitus type 2 is associated with higher levels of myeloperoxidase

BACKGROUND

Diabetes mellitus type 2 is linked to augmented endothelial dysfunction and accelerated atherosclerosis. Myeloperoxidase plays an important role in the initiation, progression, and the complications of atherosclerosis. We investigated whether myeloperoxidase levels are increased in diabetic patients.

MATERIAL/METHODS

We compared baseline plasma myeloperoxidase levels in diabetic and nondiabetic patients with mild, stable anginal complaints (Canadian Cardiovascular Society I-II/IV) and performed multivariate linear regression analyses to adjust for possible confounding factors.

RESULTS

A total of 440 patients were recruited from the outpatient clinic of cardiology, 268 patients with and 172 without diabetes mellitus type 2. Levels of myeloperoxidase were significantly higher in diabetic patients (median, 141 pM; interquartile range, 115-171 pM) than in nondiabetic patients (median, 126 pM; interquartile range, 105-167 pM) (P=0.01). Diabetes mellitus type 2, age in years, current smoking status, presence of hypercholesterolemia, and use of calcium antagonists and ACE inhibitors were associated with logarithmically transformed myeloperoxidase levels. Of these variables, diabetes mellitus type 2 (beta 0.096, SE 0.038, P=0.01); age (beta 0.01, SE 0.002, P<0.001), and current smoking (beta 0.166, SE 0.05, P=0.001) remained independently associated with myeloperoxidase levels in multivariate analysis. The linear regression coefficient of diabetes mellitus type 2 in relation to myeloperoxidase was 0.092 in univariate linear regression and 0.078 after adjusting for age, current smoking, and use of ACE inhibitors and calcium antagonists.

CONCLUSIONS

Diabetes mellitus type 2 is associated with mildly increased levels of myeloperoxidase, independent of other clinical variables. This association may contribute to the accelerated progression of atherosclerosis in diabetics.

Myeloperoxidase levels are not associated with carotid atherosclerosis progression in patients with familial hypercholesterolemia

INTRODUCTION

Myeloperoxidase (MPO), an antimicrobial enzyme of the innate immune system, has been proposed to exert a wide array of pro-atherogenic effects throughout all stages of the atherosclerotic process. In view of the potent anti-inflammatory effects of statins in vitro, we evaluated the impact of statin therapy on plasma MPO levels in patients with heterozygous familial hypercholesterolemia (FH), treated with either intensive or conventional lipid-lowering therapy. Furthermore, we evaluated the relation between MPO levels and atherosclerosis progression, as determined by intima media thickness (IMT).

METHODS

We measured plasma MPO levels, lipoprotein profiles, high sensitivity-C-reactive protein (hs-CRP) as well as IMT of carotid artery segments in 122 FH patients at baseline and after 2-year treatment with atorvastatin 80 mg or simvastatin 40 mg QD.

RESULTS

Baseline median MPO values were 147pM (interquartile range (IQR) 122-217) and 144pM (IQR 118-216) and these increased significantly to 221pM (IQR 144-290) and 255pM (IQR 152-324) during 2-year follow-up in both the atorvastatin 80 mg and simvastatin 40 mg group, respectively. There was no correlation between MPO levels and IMT progression, change in lipoproteins or hs-CRP.

CONCLUSION

In FH patients, statins do not prevent an increase in MPO levels during follow-up. Moreover, MPO levels are not associated with atherosclerosis progression in these patients.

Serum lipoprotein lipase concentration and risk for future coronary artery disease: the EPIC-Norfolk prospective population study

BACKGROUND

Lipoprotein lipase (LPL) is associated with coronary artery disease (CAD) risk, but prospective population data are lacking. This is mainly because of the need for cumbersome heparin injections, which are necessary for LPL measurements. Recent retrospective studies, however, indicate that LPL concentration can be reliably measured in serum that enabled evaluation of the prospective association between LPL and future CAD.

METHODS AND RESULTS

LPL concentration was determined in serum samples of men and women in the EPIC-Norfolk population cohort who developed fatal or nonfatal CAD during 7 years of follow-up. For each case (n=1006), 2 controls, matched for age, sex, and enrollment time, were identified. Serum LPL concentration was lower in cases compared with controls (median and interquartile range: 61 [43-85] versus 66 [46-92] ng/mL; P<0.0001). Those in the highest LPL concentration quartile had a 34% lower risk for future CAD compared with those in the lowest quartile (odds ratio [OR] 0.66; confidence interval [CI], 0.53 to 0.83; P<0.0001). This effect remained significant after adjustment for blood pressure, diabetes, smoking, body mass index, and low-density lipoprotein (LDL) cholesterol (OR, 0.77; CI, 0.60-0.99; P=0.02). As expected from LPL biology, additional adjustments for either high-density lipoprotein cholesterol (HDL-C) or triglyceride (TG) levels rendered loss of statistical significance. Of interest, serum LPL concentration was positively linear correlated with HDL and LDL size.

CONCLUSIONS

Reduced levels of serum LPL are associated with an increased risk for future CAD. The data suggest that high LPL concentrations may be atheroprotective through decreasing TG levels and increasing HDL-C levels.

Characterization of the final step in the conversion of phytol into phytanic acid

Phytol is a branched-chain fatty alcohol that is a naturally occurring precursor of phytanic acid, a fatty acid involved in the pathogenesis of Refsum disease. The conversion of phytol into phytanic acid is generally believed to take place via three enzymatic steps that involve 1) oxidation to its aldehyde, 2) further oxidation to phytenic acid, and 3) reduction of the double bond at the 2,3 position, yielding phytanic acid. Our recent investigations of this mechanism have elucidated the enzymatic steps leading to phytenic acid production, but the final step of the pathway has not been investigated so far. In this study, we describe the characterization of phytenic acid reduction in rat liver. NADPH-dependent conversion of phytenic acid into phytanic acid was detected, although at a slow rate. However, it was shown that phytenic acid can be activated to its CoA ester and that reduction of phytenoyl-CoA is much more efficient than that of phytenic acid. Furthermore, in rat hepatocytes cultured in the presence of phytol, phytenoyl-CoA could be detected, showing that it is a bona fide intermediate of phytol degradation. Subcellular fractionation experiments revealed that phytenoyl-CoA reductase activity is present in peroxisomes and mitochondria. With these findings, we have accomplished the full elucidation of the mechanism by which phytol is converted into phytanic acid.

Identification of fatty aldehyde dehydrogenase in the breakdown of phytol to phytanic acid

Phytol is a branched chain fatty alcohol, which is abundantly present in nature as part of the chlorophyll molecule. In its free form, phytol is metabolized to phytanic acid, which accumulates in patients suffering from a variety of peroxisomal disorders, including Refsum disease. The breakdown of phytol to phytanic acid takes place in three steps, in which first, the alcohol is converted to the aldehyde, second the aldehyde is converted to phytenic acid, and finally the double bond is reduced to yield phytanic acid. By culturing fibroblasts in the presence of phytol, increases in the levels of phytenic and phytanic acid were detected. Interestingly, fibroblasts derived from patients affected by Sjögren Larsson syndrome (SLS), known to be deficient in microsomal fatty aldehyde dehydrogenase (FALDH) were found to be deficient in this. In addition, fibroblast homogenates of these patients, incubated with phytol in the presence of NAD+ did not produce any phytenic acid. This indicates that FALDH is involved in the breakdown of phytol.