Autoradiographic analysis of the distribution of 125I-tyramine-cellobiose-LDL in atherosclerotic lesions of the WHHL rabbit.

Abstract

It is well established that plasma lipoproteins enter the artery wall and play a role in the atherogenic process. However, it is still unclear where within developing atherosclerotic lesions lipoproteins accumulate and which arterial cells participate in the metabolism of these lipoproteins. For this reason, light and electron microscopic autoradiograms were prepared from sections of lesioned aortas of Watanabe heritable hyperlipidemic (WHHL) rabbits 44 hours after injection of 125I-tyramine cellobiose-low density lipoprotein (TC-LDL). After uptake of 125I-TC-LDL and intracellular degradation of the LDL protein, the nondegradable TC ligand remains trapped and thus demarcates the cells participating in the degradation of LDL. Results of other studies indicate that 48 hours after injection into WHHL rabbits, about one half of the 125I label present in lesions represents accumulated degradation products while the remaining 125I label is present as intact 125I-TC-LDL. The distribution of autoradiographic silver grains was analyzed at low resolution in fatty streaks, transitional lesions, and advanced atheroma. In all cases, the majority of silver grains were associated with superficially located subendothelial macrophage-derived foam cells. In more advanced lesions, labeling was predominant in foam cells situated within the lateral margins of the lesions. Morphometric quantification of the distribution of silver grains in electron photomicrographs of fatty streaks from two young WHHL rabbits strongly supported the data obtained at the light microscopic level. In early fatty streaks from the aortic arch and the thoracic and abdominal aortas, subendothelial macrophage-derived foam cells contained a high proportion of the silver grains (40-60% of the total) and accounted for between 30% and 40% of the lesion volume. In contrast, smooth muscle cells in the lesions contained only 7-10% of the total silver grains and accounted for approximately 20% of the lesion volume. Endothelial cells contained the most silver grains on a per-unit-volume basis by occupying only 1-2% of the lesion volume. However, the endothelium contained less than 5% of the total grains in lesions. The remaining silver grains (25-45%) were associated with the extracellular matrix, which constituted between 40% and 50% of the lesion volume. These data indicate that in the WHHL rabbit, subendothelial macrophage-derived foam cells avidly accumulate and metabolize LDL despite having few functional LDL receptors.