The influence of high-density lipoprotein (HDL) and HDL subfractions on insulin secretion and cholesterol efflux in pancreatic derived β-cells

BACKGROUND

High-density lipoprotein (HDL) is considered a complex plasma-circulating particle with subfractions that vary in function, size, and chemical composition. We sought to test the effects of HDL, and HDL subfractions on insulin secretion and cholesterol efflux in the β-cell line MIN-6.

METHODS

We used total HDL and HDL subfractions 2a, 2b, 3a, 3b, and 3c, isolated from human plasma, to test insulin secretion under different glucose concentrations as well as insulin content and cholesterol efflux in the insulinoma MIN-6 cell line.

RESULTS

Incubation of MIN-6 cells with low glucose and total HDL increased insulin release two-fold. Meanwhile, when high glucose and HDL were used, insulin release increased more than five times. HDL subfractions 2a, 2b, 3a, 3b, and 3c elicited higher insulin secretion and cholesterol efflux than their respective controls, at both low and high glucose concentrations. The insulin content of the MIN-6 cells incubated with low glucose and any of the five HDL subclasses had a modest reduction compared with their controls. However, there were no statistically significant differences between each HDL subfraction on their capacity of eliciting insulin secretion, insulin content, or cholesterol efflux.

CONCLUSIONS

HDL can trigger insulin secretion under low, normal, and high glucose conditions. We found that all HDL subfractions exhibit very similar capacity to increase insulin secretion and cholesterol efflux. This is the first report demonstrating that HDL subfractions act both as insulin secretagogues (under low glucose) and insulin secretion enhancers (under high glucose) in the MIN-6 cell line.

R230C but not - 565C/T variant of the ABCA1 gene is associated with type 2 diabetes in Mexicans through an effect on lowering HDL-cholesterol levels

PURPOSE

Type 2 diabetes (T2D) and low serum concentration of high-density lipoprotein cholesterol (HDL-c) are common coexisting metabolic disorders. ABCA1 variants have been shown to be associated to these conditions. We sought to test the combined effect of two ABCA1 gene common variants, rs2422493 (- 565C > T) and rs9282541 (R230C) on HDL-c levels and T2D risk.

METHODS

Path analysis was conducted in 3,303 Mexican-mestizos to assess the specific contributions of rs2422493 and rs9282541 ABCA1 variants, insulin resistance, waist-to-height ratio (WHtR), and age on HDL-c levels and T2D risk. Participants were classified into four groups according to their ABCA1 variants carrier status: (i) the reference group carried wild type alleles for both ABCA1 variants (-/-), (ii)  +/-  were carriers of rs2422493 but non-carriers of rs9282541, (iii) -/+ for carriers of rs9282541 but not carriers of rs2422493 and (iv) carriers of minor alleles for both SNPs (+/+). Principal components from two previous genome-wide association studies were used to control for ethnicity.

RESULTS

We identified significant indirect effects on T2D risk mediated by HDL-c in groups -/+ and +/+ (β = 0.04; p = 0.03 and β = 0.06; p < 0.01, respectively) in comparison to the -/- reference group. Low concentrations of HDL-c were directly and significantly associated with increased T2D risk (β = -0.70; p < 0.01). WHtR, male gender, age, and insulin resistance were also associated with T2D risk (p < 0.05). There was no significant direct effect for any of the ABCA1 groups on T2D risk: p = 0.99, p = 0.58, and p = 0.91 for groups +/-, -/+, and +/+ respectively.

CONCLUSIONS

The ABCA1 rs9282541 (R230C) allele is associated with T2D in Mexicans through its effect on lowering HDL-c levels. This is the first report demonstrating that HDL-c levels act as an intermediate factor between an ABCA1 variant and T2D.

rs3918242 MMP9 gene polymorphism is associated with myocardial infarction in Mexican patients

Several studies have demonstrated that matrix metalloproteinases (MMPs) play a major role in atherosclerotic plaque disruption and lead to myocardial infarction (MI). We investigated the association between the MMP1 -1607 1G/2G (rs1799750), MMP3 -1612 5A/6A (rs3025058), and MMP9 -1562 C/T (rs3918242) polymorphisms and the risk of developing MI in a Mexican mestizo cohort. The genotype analysis was performed using the restriction fragment length polymorphism-polymerase chain reaction technique in a group of 236 patients with a history of MI and 285 healthy controls. Similar distributions of rs1799750 and rs3025058 were observed in both groups; however, the MMP9 rs3918242 T allele and the CT genotype were associated with the risk of developing MI (OR = 2.32, pC = 0.02 and OR = 2.40, pC = 0.02, respectively). Multiple logistic analysis was performed between MI patients and controls to estimate the risk, and after adjusting for identified risk factors, the CT + TT genotypes of MMP9 rs3918242 were found to be significantly associated with increased risk of developing MI than those with the CC genotype (OR = 2.88, P < 0.01). In summary, our results reveal that the rs3918242 polymorphism of the MMP9 gene plays a major role in the risk of developing MI.

Purinergic receptors in tubulointerstitial inflammatory cells: a pathophysiological mechanism of salt-sensitive hypertension

Recent studies have suggested that both the tubulointerstitial inflammatory cells and the activation of purinergic receptors integrate common mechanisms that result in salt-sensitive hypertension. The basis of this hypothesis is that renal endothelial cells release ATP in response to shear stress in the setting of hypertension. It has been demonstrated that the over-expression and activation of the P2X7, P2Y12 and P2X1 receptors favour the elevation of blood pressure induced by high-salt intake. In addition, the release of interleukins and inflammatory mediators in the tubulointerstitial area appears to be related to the activation of these receptors. Renal vasoconstriction and tubulointerstitial injury develop as a result, which increase sodium reabsorption by epithelial cells. Consistent with these effects, the reduction of tubulointerstitial inflammation caused by immunosuppressants, such as mycophenolate mofetil, prevents the development of salt-sensitive hypertension. Also, P2X7-receptor knockout mice develop minor renal injury when hypertension is induced via the administration of deoxycorticosterone acetate and a high-salt diet. In the setting of angiotensin II-induced hypertension, which is an early stage in the development of salt-sensitive hypertension, an acute blockade with the specific, non-selective P2 antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid prevented the renal vasoconstriction induced by angiotensin II. In addition, it normalized glomerular haemodynamics and restored sodium excretion to control values. These findings suggest that chronic administration of P2 purinergic antagonists may prevent the deleterious effects of purinergic receptors during the development of salt-sensitive hypertension.

Pioglitazone improves the cardiovascular profile in patients with uncomplicated systemic lupus erythematosus: a double-blind randomized clinical trial

OBJECTIVE

We studied the effect of pioglitazone on insulin levels, inflammation markers, high-density lipoprotein (HDL) composition and subclasses distribution, in young women with uncomplicated systemic lupus erythematosus (SLE).

METHODS

This double-blind trial included 30 premenopausal women (30 ±8 years old) with SLE, who were randomized to pioglitazone (30 mg/day) or placebo treatment for 3 months. Plasma and HDL lipids were determined by colorimetric enzymatic assays, insulin by radioimmunometric assay, inflammation by immunonephelometry and HDL size and subclasses distribution by a native 4-30% polyacrylamide gradient gel electrophoresis.

RESULTS

Compared with placebo, pioglitazone significantly increased HDL-cholesterol plasma levels (14.2%), reduced fasting insulin plasma levels (23.6%) and the homeostasis model assessment-insulin resistance (31.7%). C-reactive protein (70.9%) and serum amyloid A (34.9%) were also significantly reduced with the pioglitazone use, whereas the HDL particle size was increased (8.80 nm vs. 8.95 nm; p = 0.044) by changes in the distribution of HDL(2b), HDL(3b), and HDL(3c) subclasses. The change in HDL size correlated with a rise in free and cholesterol-ester content in the HDL particles.

CONCLUSION

Pioglitazone significantly enhanced insulin sensitivity, reduced inflammation, and modified HDL characteristics, suggesting a potential beneficial effect of this drug in patients with SLE with a risk to develop cardiovascular disease.

TRIAL REGISTRATION

This trial is registered at ClinicalTrials.gov Protocol Registration System, with the number NCT01322308.

Maternal exposure to floricultural work during pregnancy, PON1 Q192R polymorphisms and the risk of low birth weight

BACKGROUND

Although there is evidence from animal studies of impaired reproductive function by exposure to organophosphates (OP), the effects on birth weight have not been sufficiently evaluated in epidemiological studies. Paraoxonase (PON1) detoxifies organophosphates by cleavage of active oxons. Some PON1 gene polymorphisms could reduce the enzyme activity and increase susceptibility to OP toxicity.

OBJECTIVE

To assess the association between maternal exposure to floriculture during pregnancy and the risk of low birth weight (<2500 g) in their offspring, as well as to evaluate the interaction between this exposure and maternal genotype for PON1 Q192R polymorphisms.

MATERIALS AND METHODS

A cross sectional study was carried out in two Mexican states (States of Mexico and Morelos) with high frequencies of greenhouse activity. We interviewed and collected blood samples from 264 females (floriculturists or partners of floricultural workers) who became pregnant during the 10 years prior to the interview. The questionnaire measured socioeconomic characteristics, tobacco and alcohol consumption, diseases and occupational and reproductive history. We also applied a food frequency questionnaire. Information was obtained pertaining to 467 pregnancies. DNA was extracted from white cells, and PON1 genotype was determined by Restriction Fragment Length Polymorphism for Q192R polymorphisms. Results were analyzed with generalized estimating equations models.

RESULTS

After adjusting for potential confounders, we detected a statistically significant interaction between maternal exposure to flower growing work during pregnancy and PON1 Q192R polymorphisms on risk of low birth weight. The risk of having a baby with LBW is nearly six times higher if a mother is a floriculture worker during pregnancy and has PON1 192RR genotype (OR 5.93, 95% CI 1.28, 27.5).

CONCLUSION

These results suggest that the interaction between maternal floriculture work during pregnancy and 192RR PON1 genotype increases the probability of having children with LBW.

Contribution of the hydrophobicity gradient of an amphipathic peptide to its mode of association with lipids

A class of peptides that associate with lipids, known as oblique-orientated peptides, was recently described [Brasseur R., Pillot, T., Lins, L., Vandekerckhove, J. & Rosseneu, M. (1997) Trends Biochem. Sci. 22, 167-171]. Due to an asymmetric distribution of hydrophobic residues along the axis of the alpha-helix, such peptides adopt an oblique orientation which can destabilise membranes or lipid cores. Variants of these oblique peptides, designed to have an homogeneous distribution of hydrophobic and hydrophilic residues along the helical axis, are classified as regular amphipathic peptides. These peptides are expected to lie parallel to the polar/apolar interface with their hydrophobic residues directed towards the apolar and their hydrophilic residues towards the polar phase. An hydrophobic, oblique-orientated peptide was identified at residues 56-68 in the sequence of the lecithin-cholesterol acyltransferase (LCAT), enzyme. This peptide is predicted to penetrate a lipid bilayer at an angle of 40 degrees through its more hydrophobic C-terminal end and thereby induce the destabilisation of a membrane or a lipid core. The LCAT-(56-68) wild-type peptide was synthesised together with the LCAT-(56-68, 0 degrees) variant, in which the hydrophobicity gradient was abolished through residue permutations. In two other variants, designed to keep their oblique orientation, the W61 residue was shifted either towards the more hydrophilic N-terminal at residue 57, or to position 68 at the hydrophobic C-terminal end of the peptide. Peptide-induced vesicle fusion was demonstrated by fluorescence measurements using pyrene-labeled vesicles and by monitoring of vesicle size by gel filtration. The interaction between peptides and lipids was monitored by measurement of the intrinsic tryptophan fluorescence emission of the peptides. Fluorescence polarisation measurements, using diphenyl hexatriene, were carried out to follow changes in the lipid fluidity. The LCAT-(56-68) wild-type peptide and the two oblique variants, induced fusion of unilamellar dimyristoylglycerophosphocholine vesicles. Tryptophan fluorescence emission measurements showed a 12-14 nm blue shift upon addition of the wild-type peptide and of the W61-->68 variant to lipids, whereas the fluorescence of the W61-->57 variant did not change significantly. This observation supports the insertion of the more hydrophobic C-terminal residues into the lipid phase, as predicted by the theoretical calculations. In contrast, the 0 degrees variant peptide had no fusogenic activity, and it associated with lipids to form small discoidal lipid/peptide complexes. The phospholipid transition temperature was decreased after addition of the wild-type, the W61-->68 and W61-->57 fusogenic peptides, whereas the opposite effect was observed with the 0 degrees variant. The behaviour of the wild-type and variant LCAT-(56-68) peptides stresses the contribution of the hydrophobicity gradient along the axis of an amphipathic peptide to the mode of association of this peptide with lipids. This parameter consequently influences the structural modifications occurring to lipids upon association with amphipathic peptides.