Abstract 1201: Haptoglobin Genotype is a Key Regulator of Reverse Cholesterol Transport in Diabetes In Vitro and In Vivo
Introduction: Two common alleles exist at the Haptoglobin (Hp) locus and the Hp 2 allele has been associated with a 5-fold increased incidence of atherosclerotic CVD, specifically in the setting of diabetes mellitus (DM). Hp binds free hemoglobin (Hb) and thereby prevents Hb-induced oxidative reactions. In vitro, the Hp 2 protein is an inferior antioxidant as compared to the Hp 1 protein and these differences are accentuated by hyperglycemia. In vivo, oxidative stress is markedly increased in Hp 2 mice and humans with DM. Apolipoprotein A-I (apoA-I) promotes reverse cholesterol transport (RCT) and it has been shown to be exquisitely sensitive to oxidative modification. In the present study we set out to test the hypothesis that the accelerated atherosclerosis seen in Hp 2 DM patients might be attributed to impaired RCT.
Methods: In vitro using purified Hp and serum from non-DM and DM individuals we measured HDL-mediated cholesterol efflux from 3H-cholesterol-labeled macrophages as a function of Hp type. In vivo, we injected 3H-cholesterol-labeled and cholesterol-loaded macrophages intraperitoneally into non-DM and DM mice with the Hp 1 or Hp 2 allele, and monitored 3H-tracer levels in plasma, liver and feces.
Results: In vitro, we found that the rate of cholesterol efflux was both Hp genotype- and DM-dependent. We observed significantly increased cholesterol efflux from macrophages incubated with serum from DM Hp 1–1 patients as compared to DM Hp 2–1 or Hp 2–2 patients (9.3% ± 2%; 6.8% ± 1.5%; 6.4% ± 1.3% efflux rate, for Hp 1–1, Hp 2–1 and Hp 2–2, respectively; P<0.001; n = 30 for each group). There was no difference between individuals with the different Hp types in the absence of DM. This interaction between Hp type and DM was recapitulated using purified Hp and glycated Hb. In vivo, DM mice loaded with 3H-cholesterol-labeled macrophages had a 45% reduction in 3H-cholesterol level in plasma, liver, and feces as compared to non-DM mice (p<0.01). Specifically, this reduction in RCT associated with DM was Hp type-dependent (32% vs. 58% reduction in plasma; 35% vs. 64% in liver tissue; 37% vs. 61% in feces; for Hp 1 vs. Hp 2 DM mice, respectively; p<0.03; n = 4 for each group).
Conclusion: These studies demonstrate that the Hp genotype modulates RCT, specifically in the setting of DM.