Atherosclerosis Resistance in Rats Correlates with Lack of Expansion of an Immature Smooth Muscle Cell Population

Abstract

In this study we asked whether the well-known atherosclerosis resistance of rats might be reduced with aging. Two groups of young, adult and aged Wistar rats, one of which was kept on a standard, low-cholesterol (CT) diet, and the other one was fed a 2% CT diet for 2 months were enrolled. Potential modifications in the phenotypic profile of aortic smooth muscle (SM) were assessed by SDS-gel electrophoresis, Western blotting and immunofluorescence procedures using a panel of monoclonal antibodies to myosin isoforms, cytoskeletal and extracellular matrix proteins. With development and aging, the expression of 196-kD non-muscle-type myosin heavy-chain isoform (MyHC), the EIIIA fibronectin variant and keratins was downregulated, whereas that of the 204- and 200-kD SM-type MyHC isoforms, SM-type α-actin and desmin did not change. The levels of hypercholesterolemia achieved in this model did not substantially modify the distribution of the downregulated markers, except for the subendothelial grouping of immature SM cells in aged rats. Morphometric measurements indicated a slight increase of medial cross-sectional area accompanied by a decrease in total SM cell number, both with aging and with hypercholesterolemia. In no circumstance was the presence of atherosclerotic lesions histologically detectable. Bromo-deoxyuridine (BrdU) incorporation analysis revealed a marked age-dependent decline in DNA synthesis and the formation of binucleated cells in aged aortas. This pattern was not influenced by hypercholesterolemia, except in aged rats where BrdU-positive SM cells are almost doubled. Our data indicate that aging and hypercholesterolemia cannot affect the phenotypic stability of rat SM cells and confirm that the change from a fully differentiated to an immature state is a general prerequisite to allow the development of atherosclerotic lesions in mammalian species.

Is the Phenotypic Change in Vascular Smooth Muscle Cells a Prerequisite for the Efficacy of Environmental Cues as Inducers of Arterial Wall Lesions?

“…the rat and related rodents, a large component of the resistance is very likely due to differences in cholesterol me tabolism rather than any unique SMC biolo gy. The results of Amore et al [1] are also consistent with this latter literature.…”

“…Atherosclerotic lesions induced by feed ing Watanabe heritable hyperlipidemic or 'normal' rabbits high cholesterol-containing diets is associated with large increases in ath erogenic particles, particularly the [Emigrat ing very low density lipoproteins (VLDL) carrying up to 70% of the plasma cholesterol compared with I % in the high density lipo protein (HDL) fraction [2], In rats fed high cholesterol diets approximately 22% of the plasma cholesterol is transported in the VLDL fraction and 23% in HDL and in such instances, as in the paper of Amore et al [ 1 ], atherosclerotic lesions are relatively rare [3,4]. Evidence that differences between the species in their susceptibility to atherosclero sis are associated with the level of atherogen ic particles (VLDL remnants, chylomicron remnants or low density lipoproteins) comes from a number of metabolic and recent ge netic studies.…”

“…Amore et al [1] report that aging and hypercholesterolemia in rats, the latter in duced by feeding a diet containing 2% cho lesterol, do not induce a population of im mature-type smooth muscle cells (SMCs) to expand or aortic atherosclerotic lesions to develop. They propose that their results, ob tained from the hypercholesterolcmic ani mals, support the hypotheses that (1) the expansion of such a cell population is impor tant for athcrogenesis in mammalian species and (2) the unique biology of rat vascular SMC compared to those of atherosclerosisprone species such as the rabbits and hu mans is largely responsible for the resistance to atherosclerosis.…”

“…Comments raised by Dr. Bobik, concern ing the paper of Amore et al [ 1 ], deserve fur ther elucidation about the role played by lip id metabolism on smooth muscle cell (SMC) biology in atherosclerosis-prone versus ath erosclerosis-resistant species.…”

Aging Influences Development and Progression of Early Aortic Atherosclerotic Lesions in Cholesterol-Fed Rabbits

“…[22][23][24] In aged rats subjected to a hypercholesterolemic stimulus, aortic intimal cells, compared with underlying medial SMCs, associated phenotypic changes with a double proliferation. 59 Preliminary studies by double immunohistochemistry demonstrated that macrophagic cells mainly proliferate in early as well as in advanced lesions of cholesterol-fed young rabbits, whereas in aged rabbits, myocytic and macrophagic cells proliferate (L.G.S. et al, unpublished data, 1999).…”