Comparative morphometric and immunohistological assessment of the development of restenosis after arterial injury and a cholesterol-rich diet in apolipoprotein E -/-mice and C57BL/6 control mice

A promising carbon-11-labeled sphingosine-1-phosphate receptor 1-specific PET tracer for imaging vascular injury

BACKGROUND

Sphingosine-1-phosphate receptor 1 (S1PR1) is highly expressed in vascular smooth muscle cells from intimal lesions. PET imaging using S1PR1 as a biomarker would increase our understanding of its role in vascular pathologies including in-stent restenosis.

METHODS

The S1PR1 compound TZ3321 was synthesized for in vitro characterization and labeled with Carbon-11 for in vivo studies. The biodistribution of [11C]TZ3321 was evaluated in normal mice; microPET and immunohistochemistry (IHC) studies were performed using a murine femoral artery wire-injury model of restenosis.

RESULTS

The high potency of TZ3321 for S1PR1 (IC 50 = 2.13 ± 1.63 nM), and high selectivity (>1000 nM) for S1PR1 over S1PR2 and S1PR3 were confirmed. Biodistribution data revealed prolonged retention of [11C]TZ3321 in S1PR1-enriched tissues. MicroPET imaging of [11C]TZ3321 showed higher uptake in the wire-injured arteries of ApoE-/- mice than in injured arteries of wild-type mice (SUV 0.40 ± 0.06 vs 0.28 ± 0.04, n = 6, P < .001); FDG-PET showed no difference (SUV 0.98 ± 0.04 vs 0.94 ± 0.01, n = 6, P > .05). Post-PET autoradiography showed >4-fold higher [11C]TZ3321 retention in the injured artery of ApoE-/- mice than in wild-type mice. Subsequent IHC staining confirmed higher expression of S1PR1 in the neointima of the injured artery of ApoE-/- mice than in wild-type mice.

CONCLUSIONS

This preliminary study supports the potential use of PET for quantification of the S1PR1 expression as a biomarker of neointimal hyperplasia.

Early hypercholesterolemia contributes to vasomotor dysfunction and injury associated atherogenesis that can be inhibited by nitric oxide

OBJECTIVE

Atherosclerosis results in vasomotor dysfunction, in part, through impairment of nitric oxide (NO) dependent vasodilation. It is unclear whether blood vessels are dysfunctional in an early environment of hypercholesterolemia alone and if this contributes to the vascular injury response. We hypothesize that early hypercholesterolemia, prior to gross vascular changes, contributes to vasomotor dysfunction and the vascular injury response. The efficacy of NO therapy to protect against the injury response in this setting was also assessed.

METHODS

The effect of oxidized low density lipoprotein (oxLDL) and inducible NO synthase (iNOS) gene transfer on rat aortic smooth muscle cell (SMC) proliferation was measured with (3)H-thymidine incorporation. Common carotid arteries (CCA) from wild-type C57BL6 (WT or C57) and apolipoprotein E deficient (ApoE KO) mice fed normal or Western diets for 6 to 8 weeks were tested for vasomotor function using an arteriograph system. Studies were repeated after CCA injury. The effect of iNOS gene transfer on morphometry by histology and vasomotor responses in injured CCAs in ApoE KO was examined.

RESULTS

OxLDL increased SMC proliferation by >50%. In SMC expressing iNOS, NO production was unaffected by oxLDL and reduced oxLDL and still inhibited SMC proliferation. Endothelium dependent vasorelaxation was reduced in uninjured CCAs from ApoE KO and C57 mice on the Western vs normal diet (ApoE 39% ± 2% vs 55% ± 13%; C57 50% ± 13% vs 76% ± 5%, P < .001) and was increased with longer durations of hypercholesterolemia. Endothelium-dependent and independent vasodilator responses were severely disrupted in C57 and ApoE KO mice 2 weeks following CCA injury but both recovered by 4 weeks. CCA injury in ApoE KO mice resulted in the formation of atheromatous lesions while C57 mice showed no change (intima 27,795 ± 1829 vs 237 ± 28 μm(2); media 46,306 ± 2448 vs 11,714 ± 392 μm(2), respectively; P < .001). This structural change in the ApoE KO reduced distensibility and increased stiffness. Finally, iNOS gene transfer to injured CCA in ApoE KO mice dramatically reduced atheromatous neointimal lesion formation.

CONCLUSIONS

Early hypercholesterolemia impairs endothelial function, with severity being related to duration and magnitude of hypercholesterolemia. Severe hypercholesterolemia leads to atheromatous lesion formation following injury and stresses the role of vascular injury in atherogenesis and suggests different mechanisms are involved in endothelial dysfunction and the injury response. Despite these changes, iNOS gene transfer still effectively inhibits atheroma formation. These findings support early correction of hypercholesterolemia and emphasize the potential role for NO based therapies in disease states.

CX 3 CR1 Deficiency Confers Protection From Intimal Hyperplasia After Arterial Injury

Objective— A functional polymorphism in the chemokine receptor CX 3 CR1 is associated with protection from vascular diseases including coronary artery disease and internal carotid artery occlusive disease. We investigated the mechanisms by which CX 3 CR1 may be involved by evaluating the inflammatory response to arterial injury in CX 3 CR1-deficient animals.

Methods and Results— Femoral arteries of CX 3 CR1 −/− and wild-type (WT) mice were injured with an angioplasty guide wire. After 1, 5, 14, and 28 days, arteries were harvested and evaluated by histology, morphometry, and immunohistochemistry. Arterial injury upregulated the CX 3 CR1 ligand CX 3 CL1. In CX 3 CR1 −/− compared with WT animals, the incidence of neointima formation was 58% lower ( P =0.0017), accompanied by no difference in the area of platelet accumulation at day 1 ( P =0.48) but a significant decrease in intimal monocyte infiltration at day 5 ( P =0.006), vascular smooth muscle cell (VSMC) proliferation at days 5 and 14, and intimal area at day 28 ( P =0.009).

Conclusions— In an endothelial denudation injury model, CX 3 CR1 deficiency protects animals from developing intimal hyperplasia as a result of decreased monocyte trafficking to the lesion. CX 3 CR1 deficiency decreases VSMC proliferation and intimal accumulation either directly or indirectly as a result of defective monocyte infiltration.