The suppression of cholesterol synthesis by dietary cholesterol which occurs in the livers of normal animals is absent in hepatomas. This abnormality has been reported to occur in the livers of animals fed hepatocarcinogens, even before there is any histologic evidence of malignancy (premalignant liver). We have proposed, in an earlier publication, that the deletion of feedback inhibition of cholesterol synthesis in malignancy is due, at least in part, to the loss of receptors which bind chylomicron remnants, the lipoprotein particles that transport dietary cholesterol to the liver. This hypothesis was further tested in the premalignant liver model. Rats were fed a diet containing 0.25% of a known hepatic carcinogen, ethionine. After 3 to 5 weeks on this diet, the liver had no histologic evidence of malignancy; the rate of [14C]acetate incorporation into cholesterol by liver homogenates was elevated as compared to that of controls (5.13 ± 0.70 vs. 0.65 ± 0.14 nmoles cholesterol per gm per hr), and in contrast to control animals, this was not reduced by the inclusion of 5% cholesterol in the diet for 48 hr before killing. The serum (44.4 ± 6.3 vs. 51.4 ± 3.8 mg per 100 ml) and hepatic (15.8 ± 0.2 vs. 17.0 ± 0.4 μg per mg protein) cholesterol contents were not substantially different in ethionine-fed as compared to control-fed rats. Hepatic cholesterol content increased when cholesterol was included in the diet (15.8 ± 0.2 to 25.8 ± 7.3 μg per mg protein and 17.0 ± 0.4 to 36 ± 3.7 μg per mg protein in ethionine-fed and control-fed animals, respectively). The binding of chylomicron remnants by liver membranes from ethioninefed rats was about one-fourth that to liver membranes from control rats. This reduction was due to a decrease in the maximum number of binding sites (0.6 vs. 2.5 μg [125I]chylomicron remnants bound per mg protein) without a significant change in their affinity. When [125I]chylomicron remnants were injected into rats fed the ethionine-containing diet, the T1/2 of the rapid phase of their disappearance from plasma was increased compared to the rate in control-fed animals (2.5 vs. 1.6 min). Analysis of tissues for [125I] content revealed that the chylomicron remnant-specific tissue space in the livers of ethionine-fed animals was reduced to less than 50% that of control-fed animals (236 ± 36 vs. 562 ± 54 μl per gm) and that there were increases in the chylomicron remnant tissue spaces for adrenal and spleen. For comparison, similar experiments were conducted with [125I]asialoorosomucoid, a ligand for the liver-specific asialoglycoprotein receptor. Ethionine-fed animals had a reduction in liver membrane asialoglycoprotein receptors (0.10 vs. 0.19 μg asialoorosomucoid bound per mg protein) and a delayed clearance of asialoglycoprotein (uniphasic T1/2 = 2.9 vs. 1.1 min).
These results provide support for the hypothesis that the deletion of feedback inhibition of cholesterol synthesis, which occurs in malignant premalignant liver, is due, at least in part, to a loss of receptors which can recognize chylomicron remnants.