Calcification of arteries, especially in the coronaries (CAC), progresses with age and is associated with increased risk. However, evidence shows that CAC progression is promoted by statins, leading some to believe that CAC is protective. This leaves a clinical dilemma for the elderly with CAC, from being told: 1) to start statins to reduce their high-risk CAC and 2) that statins reduce risk by increasing CAC. We posit that calcification reduces risk only in older individuals with high coronary calcium. Clinically, numerous small deposits are associated with higher risk than large, contiguous deposits of the same volume. The link between CAC and risk is most likely plaque rupture due to compliance mismatch at the interface between compliant vascular tissue and rigid calcium mineral. Theoretically, surfaces of calcium deposits that face toward mechanical stress have increased rupture risk, which should increase as mineral surface area (SA) increases. Thus, SA is predicted to increase initially with progression of calcification, but, eventually, when deposits coalesce they may reduce SA and presumably risk. This results in a roughly bell-shaped curve for the relationship of risk to calcification. Older individuals, who generally have more calcification, may be on the downslope of this curve. Therefore, SA may be a useful indicator of vulnerable plaque. In support of this, clinical PET imaging with fluoride (18F-NaF), which preferentially labels the surface of calcium deposits, was shown to identify vulnerable plaque. Based on these studies, we have developed a novel way to quantitatively assess changes in SA of aortic calcium deposits in mice and found that the opposite trends in 18F-NaF incorporation by microPET vs. in mineral content by microCT would suggest coalescence of calcium deposits. We propose the novel hypothesis that calcification reduces risk only in older individuals, who have enough calcium content for progression to lead to coalescence. We propose 2 aims.
AIM 1. DETERMINE IF AGING CAUSES PARADOXICAL CHANGES IN AORTIC CALCIUM CONTENT VS. MINERAL SURFACE AREA IN MICE. We will assess changes over 2 years in mineral SA vs. calcium content in hyperlipidemic (Apoe-/- and Ldlr-/-) mice by serial, 18F-microPET/microCT imaging and quantitative histochemical analyses using automated edge- detection.
AIM 2. DETERMINE IF MORPHOLOGIC RUPTURE RISK OF AORTIC CALCIUM DEPOSITS CHANGES DIFFERENTIALLY IN OLD VS. YOUNG MICE ON STATINS. Aortas of hyperlipidemic mice on and off statins for 5 months will be subjected to the imaging and histological analysis, as in Aim1. In addition, after euthanasia, we will determine plaque rupture threshold from stress-strain recordings and assess high resolution, 3D, laser light-sheet fluorescence microscopy to overcome any sampling limitation of histology. Results will reveal whether aging transforms the risk of calcification progression through coalescence and reduction of surface area, compliance mismatch, and mechanical rupture stress. If progression is found to reduce risk, it would have a profound impact on clinical management in the aging population.
In the vast majority of older adults, the arteries become stiff, scarred and constricted, leading to high morbidity and mortality, with no known medical treatment. Since lipid lowering medications (statin) have been shown to reduce cardiovascular events but increase calcification of arteries, we will determine if calcification in the elderly is uniquely protective unlike that in younger adults.