Chronic Hemolysis, Oxidative Stress and Inflammation Contribute to Dysfunctional HDL in Sickle Cell Disease

Introduction

Hypocholesterolemia was observed in sickle cell disease (SCD) patients according to various reports in the literature. It was previously linked to anemia and inflammation by others and to oxidative stress by our group. In this study we investigated the relationship between cholesterol metabolism, hemolysis, inflammation and oxidative stress in SCD.

Methods

The study consisted of pediatric SCD patients (N=34) and healthy controls (N=19). Patients were crisis-free and did not receive transfusion for the last 3 months.

Hemolysis was assessed with the measurement of haptoglobin (Hpg), bilirubin and LDH. Hemin values were accepted as a representation of oxidative stress; while serum amyloid A (SAA) and myeloperoxidase (MPO) levels were measured to determine the level of inflammation.

Hemin, SAA and MPO levels were measured by commercial kits while all other parameters including the lipid panel and liver function tests were measured by autoanalyzers in the clinical chemistry lab. Statistical analyses were performed with IBM SPSS v23.0. The study was approved by the Institutional Review Board of Mersin University.

Results

Mean age of the patients and controls were 13.71±3.79 and 13.47±3.49 years respectively. Patient and control groups were similar in terms of age and sex. All values are given as patients vs. controls and mean±SD.

Among the values representing hemolysis, statistically significant differences between groups were found for Hpg (3.84±16.18 mg/dL vs. 69.98±37.66 mg/dL, p<0.001), total and direct bilirubin (1.93±1.17 mg/dL vs. 0.27±0.14 mg/dL and 0.40±0.21 mg/dL vs. 0.06±0.04 mg/dL, p<0.001), and LDH (540.29±217.32 U/L vs. 202.79±49.46 U/L, p<0.001).

Hemin levels, as an index of oxidative stress, were significantly higher in the patient group (9.52±8.75 mg/L vs. 0.27±0.01 mg/L, p<0.001).

SAA and MPO levels which were accepted to determine the level of chronic inflammation were both significantly higher in the patient group (1.19±0.60 mg/L vs. 0.68±0.52 mg/L, p=0.013 and 76.53±31.03 μg/L vs. 44.56±8.29 μg/L, p=0.001; respectively).

In the lipid panel, significant differences between groups were found in total cholesterol (116.97±17.22 mg/dL vs. 169.84±33.70 mg/dL, p<0.001), HDL (28.31±5.21 mg/dL vs. 50.95±12.95 mg/dL, p<0.001), LDL (70.80±11.96 mg/dL vs. 99.89±22.95 mg/dL, p<0.001), and apolipoprotein AI (149.61±54.88 mg/dL vs. 199.32±33.48 mg/dL, p=0.001); whereas no difference was found in apolipoprotein B (77.65±25.52 mg/dL vs. 81.57±13.96 mg/dL, p=0.541) levels.

Conclusion

Pediatric SCD patients in this study had hypocholesterolemia reflected by low LDL and HDL levels. Apolipoprotein AI levels were also lower in patients suggesting either low production or increased catabolism of the HDL particle.

The drastic decrease in Hpg levels and the increase in bilirubin and LDH values show the overwhelming chronic hemolysis in SCD. Hpg protects vascular endothelium by binding to hemoglobin. Hpg was also shown to protect Apo AI from oxidative damage. However, the consumption of Hpg in our patients may possibly render Apo AI open to the damaging effects of free radicals formed during hemolysis.

Hemin is produced by the oxidation of free heme to the ferric state. Increased hemin levels in SCD patients, compared to the almost-undetectable levels in controls, may cause oxidative damage to plasma proteins and lipids as well as activating neutrophils with its oxidative effect. Apolipoproteins and lipids in the structure of HDL and LDL particles are targets for oxidative damage.

The increase in SAA and MPO levels indicate the chronic inflammation in SCD. SAA is an important acute phase protein secreted by the liver during inflammation. It translocates Apo AI by incorporating into HDL and slows down reverse cholesterol transport by inhibiting cholesterol incorporation into HDL. A previous study reports SAA as a marker of crisis in SCD. Increased MPO release by the leukocytes was shown to oxidize Apo AI and disrupt its function in patients with cardiovascular disease. Therefore, increased MPO levels in our patients may contribute to the oxidative damage on the Apo AI of the HDL particle.

Thus, the cumulative effect of chronic hemolysis, oxidative stress and inflammation in SCD may result with a pathological structural alteration in HDL and contribute to HDL dysfunction. The slowing down of reverse cholesterol transport may be an important cause of hypocholesterolemia in SCD.