IN-VIVO REGULATION OF LOW-DENSITY-LIPOPROTEIN RECEPTORS BY ESTROGEN DIFFERS AT THE POSTTRANSCRIPTIONAL LEVEL IN RAT AND MOUSE

Rats and mice are frequently used in studies of the regulation of lipoprotein metabolism. Although the species are closely related, they differ dramatically in the responses of their lipoproteins to estrogen administration. In rats, estrogens produce profound decreases in the levels of all plasma lipoproteins and this is attributed largely to estrogen-induced increases of hepatic low-density lipoprotein receptor (LDL-receptor) activity. Estrogens affect mouse plasma lipoproteins to a much lesser extent. Therefore, one of our aims was to compare the regulation of LDL-receptor gene expression in rats and mice at several potential loci of regulation. To assess the specificity of the estrogen effect, we also compared the responses of apolipoprotein AI (apoAI), apolipoprotein B (apoB), and beta-actin to the response of the LDL-receptor. In male Sprague Dawley rats given 17beta-estradiol or 17alpha-ethinyl estradiol at supraphysiological doses of 5 mug/g body mass/day, plasma total cholesterol and triacylglycerols fell to almost-equal-to 5% and almost-equal-to 50%, and, plasma apoAI and apoB fell to almost-equal-to 12% and almost-equal-to 16% of controls, respectively. By contrast, in male C3H/HeJ mice the above parameters dropped only to almost-equal-to 65% of controls and apoB concentrations rose to almost-equal-to 200% of controls. In rats, relative rates of LDL-receptor mRNA transcription (nuclear 'run-off' assay) and total hepatic, nuclear and polysomal LDL-receptor mRNA levels (RNase protection assay) increased by 1.5-2-fold, while synthesis of LDL-receptor protein on hepatic polysomes (in a wheat-germ translation system) increased 8-fold and LDL-receptor protein mass in hepatic plasma membranes increased 10-fold (by immunoblotting). In mouse liver, too, LDL-receptor mRNA levels increased 1.5-fold and the LDL-receptor mRNA transcription start sites in rat and mouse were found to be the same, but mouse LDL-receptor protein mass did not change, i.e. LDL-receptors of mice were similar to rat with respect to transcriptional regulation, but differed in their post-transcriptional control mechanisms. In rats, estrogen administration increased apoAI mRNA transcription rates 1.6-fold and also apoAI mRNA levels in total liver homogenates, nuclei and polysomes, (2-fold for each) consistent with transcriptional regulation. However, apoAI synthesis on total RNA increased less than apoAI mRNA, indicating that apoAI translational control mechanisms, at least in part, also regulate hepatic rates of apoAI production. ApoB mRNA transcription rates and levels showed small increases following estrogen administration. Hepatic beta-actin mRNA transcription and levels did not change. These changes in apoAI and apoB in rats were similar to those previously reported for mice, i.e. estrogen regulates apoAI and apoB gene expression both at the transcriptional and translational levels with rats and mice responding similarly. Thus, in rat liver, estrogen upregulates LDL-receptor production by transcriptional and even more so by translational mechanisms while mouse LDL-receptor remains unchanged. These interspecies differences in the responsiveness of mouse and rat LDL-receptor proteins may explain some of the differences in the responses of plasma lipoproteins in these two species.