Plasma oxidized low-density lipoprotein levels and arterial stiffness in older adults: the health, aging, and body composition study

Serum paraoxonase 1 activity predicts arterial stiffness in renal transplant recipients

AIM

Paraoxanase 1 (PON 1) has been shown to protect against atherosclerosis by modifying lipoproteins. Its activity decreases in dialysis patients but is restored after transplantation. Whether it affects arterial stiffness is unclear. In this study we aimed to investigate the effects of PON 1 on arterial stiffness in renal transplant patients.

METHODS

Seventy renal transplant recipients were enrolled. Arterial stiffness was measured using a Syphmocor device. PON-1 activity was assessed from the rate of enzymatic hydrolysis of paraoxon to p-nitrophenol.

RESULTS

Mean age was 39.0 ± 9.6 years and 5.7% of the patients were diabetic. Post-transplant follow-up time was 46.7 ± 37.9 months. Eighty-five percent received anti-hypertensive and 12.9% anti-hyperlipidemic medication. Mean PON1 activity was 75.9 ± 52.4 U/L. PON1 activity was negatively correlated with systolic and diastolic blood pressure, mean arterial pressure, LDL-cholesterol and carotid-femoral pulse wave velocity (cf-PWV). Mean c-f PWV was 8.10 ± 1.39 m/s. Cf-PWV was positively correlated with age, systolic and diastolic blood pressure, mean arterial pressure, proteinuria and negatively correlated with PON1, PON1/HDL ratio and creatinine clearance. In linear regression analysis, PON1 was a predictor of cf-PWV in a model that included age, gender, diabetes, mean arterial pressure, urine protein level, creatinine clearance and PON 1.

CONCLUSIONS

Reduced PON1 activity is significantly associated with increased arterial stiffness. The results of this study show the possible role of PON1 for arterial stiffening in renal transplant recipients.

Role of liver sinusoidal endothelial cells and stabilins in elimination of oxidized low-density lipoproteins

Atherogenesis is associated with elevated levels of low-density lipoprotein (LDL) and its oxidized form (oxLDL) in the blood. The liver is an important scavenger organ for circulating oxLDLs. The present study aimed to examine endocytosis of mildly oxLDL (the major circulating form of oxLDLs) in liver sinusoidal endothelial cells (LSECs) and the involvement of the scavenger receptors stabilin-1 and stabilin-2 in this process. Freshly isolated LSECs, Kupffer cells (KCs), and stabilin-1- and stabilin-2-transfected human embryonic kidney cells were incubated with fluorescently labeled or radiolabeled oxLDLs [oxidized for 3 h (oxLDL(3)), 6 h, or 24 h (oxLDL(24))] to measure endocytosis. The intracellular localization of oxLDLs and stabilins in LSECs was examined by immunofluorescence and immunogold electron microscopy. Whereas oxLDL(24) was endocytosed both by LSECs and KCs, oxLDL(3) (mildly oxLDL) was taken up by LSECs only. The LSEC uptake of oxLDLs was significantly inhibited by the scavenger receptor ligand formaldehyde-treated serum albumin. Uptake of all modified LDLs was high in stabilin-1-transfected cells, whereas stabilin-2-transfected cells preferentially took up oxLDL(24), suggesting that stabilin-1 is a more important receptor for mildly oxLDLs than stabilin-2. Double immunogold labeling experiments in LSECs indicated interactions of stabilin-1 and stabilin-2 with oxLDL(3) on the cell surface, in coated pits, and endocytic vesicles. LSECs but not KCs endocytosed mildly oxLDL. Both stabilin-1 and stabilin-2 were involved in the LSEC endocytosis of oxLDLs, but experiments with stabilin-transfected cells pointed to stabilin-1 as the most important receptor for mildly oxLDL.

Contributions of the Ah receptor to bilirubin homeostasis and its antioxidative and atheroprotective functions

The homeostasis and atheroprotective function of bilirubin could be an appealing model to investigate one of the many physiologic functions of the human aryl hydrocarbon receptor (AhR). Several clinical and epidemiological studies have been carried out on key enzymes generating and eliminating bilirubin (heme oxygenase-1 and UDP-glucuronosyltransferase UGT1A1, respectively) and their regulation by the AhR. Studies with AhR-deficient mice strongly suggest a role of the AhR in vascular biology. Atherosclerosis, a major cause of premature death, is initiated by pro-oxidative insults of the vascular endothelium. The strong antioxidant and activator of AhR bilirubin is generated in vascular endothelial cells, smooth muscles and macrophages. It acts mostly in the lipid environment, thereby complementing other antioxidants such as glutathione which act mostly on water-soluble proteins. In conclusion, the atheroprotective functions of bilirubin might not only provide models to study physiologic functions of the human AhR but also provide opportunities to improve prevention and treatment of a major life-threatening disease.

The effect of smoking on arterial stiffness

A systematic literature review was conducted using PubMed, Embase and the Cochrane Library to determine the effect of acute, chronic and passive smoking on arterial stiffness and to determine whether these effects are reversible after smoking cessation. A total of 39 relevant studies were identified and included. Acute smoking was found to cause an acute increase in arterial stiffness. Similarly, passive smoking increased arterial stiffness acutely and chronically. The majority of studies identified chronic smoking as a risk factor for increasing arterial stiffness. However, some studies found no statistical difference in arterial stiffness between nonsmokers and long-term smokers, although chronic smoking seems to sensitize the arterial response to acute smoking. In addition, whether arterial stiffness is reversed after smoking cessation and the timeline in which this may occur could not be determined from the identified literature. The effect of smoking discontinuation on arterial stiffness remains to be established by prospective smoking cessation trials.