The degradation of 125I-labeled low density lipoprotein (125I-LDL) was examined in both lymphocytes and monocyte-derived macrophages which were obtained from the same blood samples. As for lymphocytes, after the separation from the blood and the 3-days' culture with 10% lipoprotein deficient serum (LPDS), 10μg/ml of 125I-LDL were added to the medium in the presence or the absence of 250μg/ml nonlabeled LDL, and then the lymphocytes were incubated for 5 hours. The radioactivity of the trichloroacetic acid-soluble fraction of the medium was measured and the degradation per cell protein was calculated according to the well-known methods.
The receptor-mediated degradation of LDL by lymphocytes was from 0.8% to 15.8% in homozygotes of familial hypercholesterolemia (FH) as compared with that of normal controls, and in heterozygotes the degradation was intermediate between homozygotes and controls. These data coincide well with those of Goldstein et al.
The 4-5 days' culture with 20% human serum altered monocytes to macrophage-like figures and the assay of the degradation was made on day 10. The degradation of LDL by monocyte-macrophages from normal subjects showed the saturable course when the concentration of LDL in the medium was increased, suggesting that those cells metabolised LDL with the high affinity process, namely the LDL receptor-mediated pathway.
The macrophages from a female homozygote of FH, in which the capacity of her lymphocytes to degrade LDL by the receptor-mediated pathway was only 0.8% of that of the normal control, degraded two and a half times more LDL than the macrophages from controls and the competitive study revealed that most of the LDL had been degraded by the receptor-independent pathway.
The importance of the interaction between the denatured or altered LDL and macrophages has been pointed out in terms of atherogenecity, but our data suggest that there may be some differences in macrophages themselves between normal subjects and the atherosclerotic patients as well as FH and that these difference may contribute to the development of atherosclerosis.
