Synergistic effect of local endothelial shear stress and systemic hypercholesterolemia on coronary atherosclerotic plaque progression and composition in pigs

Abstract

Background: Systemic risk factors and local hemodynamic factors both contribute to coronary atherosclerosis, but their possibly synergistic inter-relationship remains unknown. The purpose of this natural history study was to investigate the combined in-vivo effect of varying levels of systemic hypercholesterolemia and local endothelial shear stress (ESS) on subsequent plaque progression and histological composition.

Methods: Diabetic, hyperlipidemic swine with higher systemic total cholesterol (TC) (n=4) and relatively lower TC levels (n=5) underwent three-vessel intravascular ultrasound (IVUS) at 3-5 consecutive time-points in-vivo. ESS was calculated serially using computational fluid dynamics. 3-D reconstructed coronary arteries were divided into 3mm-long segments (n=595), which were stratified according to higher vs. relatively lower TC and low (<1.2Pa) vs. higher local ESS (≥1.2Pa). Arteries were harvested at 9months, and a subset of segments (n=114) underwent histopathologic analyses.

Results: Change of plaque volume (ΔPV) by IVUS over time was most pronounced in low-ESS segments from higher-TC animals. Notably, higher-ESS segments from higher-TC animals had greater ΔPV compared to low-ESS segments from lower-TC animals (p<0.001). The time-averaged ESS in segments that resulted in significant plaque increased with increasing TC levels (slope: 0.24Pa/100mg/dl; r=0.80; p<0.01). At follow-up, low-ESS segments from higher-TC animals had the highest mRNA levels of lipoprotein receptors and inflammatory mediators and, consequently, the greatest lipid accumulation and inflammation.

Conclusions: This study redefines the principle concept that "low" ESS promotes coronary plaque growth and vulnerability by demonstrating that: (i.) the pro-atherogenic threshold of low ESS is not uniform, but cholesterol-dependent; and (ii.) the atherogenic effects of local low ESS are amplified, and the athero-protective effects of higher ESS may be outweighed, by increasing cholesterol levels. Intense hypercholesterolemia and very low ESS are synergistic in favoring rapid atheroma progression and high-risk composition.

Keywords: Coronary atherosclerosis; Endothelial shear stress; Histology; Hypercholesterolemia; Intravascular ultrasound.

Figure 1

(A) Time-normalized change of plaque volume (nΔPV), for all intervals between consecutive time-points (T₂→T₃; T₃→T₄; T₄→T₅), in segments stratified according to low ESS (<1.2Pa) vs. higher ESS (≥1.2Pa) at each time-point and according to higher-TC vs. relatively lower-TC. Note that segments only from the 36-week cohort (n=304) were analyzed at interval T₂→T₃, whereas segments from both the 30-week and 36-week cohort (n=595) were analyzed at intervals T₃→T₄ and T₄→T₅. (B) Representative examples of plaque progression by IVUS between consecutive time-points (T₃→T₄) in segments stratified by local ESS and systemic TC categories.

Figure 2

(A) Time-normalized change of plaque volume (nΔPV) throughout the study period in segments stratified according to low (<1.2Pa) vs. higher time-averaged ESS (≥1.2Pa) and according to higher-TC vs. relatively lower-TC. (B) Time-normalized change of plaque volume (nΔPV) throughout the study period across quartiles of time-averaged ESS. P values next to each line, shown in bold, represent the overall association within the corresponding TC category; p values in each ESS quartile represent the difference of nΔPV between higher TC vs. relatively lower TC segments for the given ESS quartile.

Figure 3

The time-averaged ESS in segments that culminated in significant plaque by IVUS (defined as maxIMT≥0.5mm by IVUS at follow-up) in each animal is related to the total cholesterol (A) and the LDL-cholesterol levels (B) in the corresponding animal. Dashed lines represent 95% confidence intervals for the linear regression lines.

Figure 4

Relative mRNA levels of the LDL-receptor (A), LOX-1 (B), VCAM-1 (C) and MCP-1 (D) in segments with low- vs. higher time-averaged ESS, from higher-TC animals vs. relatively lower-TC animals.

Figure 5

Quantitative analyses of lipid accumulation (A) and CD45-positive leukocyte infiltration (C), and representative examples of Oil-red-O staining (B) and CD45 immunostaining (D) in segments stratified according to time-averaged local ESS (low ESS <1.2Pa vs. higher ESS ≥1.2Pa) and the higher vs. relatively lower cholesterol levels of the corresponding animal.