Apoliprotein A-I Mimetic Peptide and Sickle Vasculopathy: Mouse Model Study of Acute Administration.

Abstract 1521

Poster Board I-544

Epidemiologic observations associate low levels of Apolipoprotein A-1 (ApoA-I) and low high density lipoprotein (HDL) with greater prevalence of pulmonary hypertension in sickle cell patients. Sickle cell mice with knockout of apoliproteins have greater pulmonary hypertension and endothelial dysfunction than sickle cell mice with wild-type levels of ApoA-I, and they have further worsening of the global dysregulation of the nitric oxide axis (NO) seen in sickle vasculopathy. Ou et al. (Circulation 2003; 107(18):2337-41) showed that chronic administration (3-4 weeks) of the ApoA-I mimetic L-4F improves vasodilation in hypercholesterolemic and sickle mice. Remaley et al. (Circulation 2006;114:II_23) showed atheroprotective effects after a single (1mg/mouse) intravenous administration of the ApoA-I mimetic peptide 5A as a result of increased reverse cholesterol transfer. These preclinical data provide a mechanistic basis for the effects of ApoA-I in sickle cell disease and suggest that ApoA-I mimetics deserve further study as a potential therapy for sickle cell disease. However, it is unknown whether acute administration of an ApoA-I mimetic has any efficacy for sickle cell disease, and whether all ApoA-I mimetic peptides will be helpful. We hypothesized that acute administration of ApoA-I mimetic could improve endothelial dysfunction in sickle cell mouse models. Bone marrow harvested from Berkeley sickle cell mouse donors and hemizygote non-sickling control mice were used to generate three groups of mice after myeloablative irradiation of wild-type C57BL6 recipient mice. Three million cells whole marrow was injected per mouse. Mice were fully engrafted by 3 months. Hemoglobin analysis showed one group produced only sickle RBC, another group only hemizygote RBC, and a third group were mixed chimeras with 30% sickle RBC and 70% hemizygote RBC (intended to model a sickle cell patient on chronic transfusion). Pulse-wave velocities, a measure of arterial stiffness, showed no improvement after IV injection of the ApoA-I mimetic peptide, 5A-POPC (170 mcl of 8.6 mg/ml). Aortic rings prepared from mice of all three groups showed blunted relaxation response to acetylcholine and sodium nitroprusside, as expected for abnormal nitric oxide bioavailability - these vasorelaxation responses were not significantly different in the absence vs. presence of 5A-POPC incubation in vitro. These results agree with our previous observation that sickle bone marrow transplanted into transgenic mice overexpressing ApoA-I had no difference in vasculopathy from sickle bone marrow transplanted into wild-type mice We conclude that acute exposure to this ApoA-I mimetic, either in vivo or in vitro, is not sufficient to relieve the vasculopathy of sickle cell mouse models. Higher doses or long term therapy may be necessary for benefit. An alternative explanation is that ApoA-I mimetic peptides differ in their benefit for sickle cell vasculopathy, and that the key property is the high level of oxidant scavenging by L-4F rather than cholesterol removal by 5A-POPC. Future studies can determine whether the cardiovascular benefits of ApoA-I mimetics correlate with their antioxidant function.