ESTROGEN UP-REGULATES APOLIPOPROTEIN-E (APOE) GENE-EXPRESSION BY INCREASING APOE MESSENGER-RNA IN THE TRANSLATING POOL VIA THE ESTROGEN-RECEPTOR ALPHA-MEDIATED PATHWAY

The antiatherogenic property of estrogens is mediated via at least two mechanisms: first by affecting plasma lipoprotein profiles, and secon d by affecting the components of the vessel wall. Raising plasma apoli poprotein E (apoE) in mice protects them against diet-induced atherosc lerosis (Shimano, H., Yamada, N., Katsuki, M., Gotoda, T., Harada, K., Murase, T., Fukuzawa, C., Takaku, F., and Yazaka, Y. (1992) Proc. Nat l. Acad. Sci. U.S.A. 89, 1750-1754). It is possible that estrogen may be antiatherogenic at least in part by increasing plasma apoE levels. Therefore, we studied the regulation of apoE by estrogen. A survey of 15 inbred strains of mice showed that some mouse strains responded to injections or subcutaneously implanted pellets of estradiol by raising their apoB and apoE levels and some did not. We performed detailed st udies in two ''responder'' strains, C57L and C57BL, and two ''non-resp onder'' strains, C3H and BALBc. Responders increased their plasma apoE levels 2.5-fold. Non-responders' levels were altered +/-10%. In the r esponders the distribution of apoE among the plasma lipoproteins shift ed from high density lipoprotein toward the apoB-containing lipoprotei n fractions. In nonresponders the shift was toward high density lipopr otein. Hepatic apoE mRNA levels and relative rates of apoE mRNA transc ription were unchanged in all strains, suggesting that apoE regulation occurred at posttranscriptional loci. Therefore, we measured apoE syn thesis in fresh liver slices and on isolated hepatic polysomes. Two-fo ld increases were noted but only in responders accompanied by selectiv e 1.5-fold increases in polysomal apoE mRNA levels. Similar increases in apoE synthesis were also observed in castrated C57BL mice given eit her physiological or pharmacological replacement doses of estradiol, b ut not testosterone, suggesting that the effect of estradiol was speci fic on the distribution of apoE mRNA in the translationally active pol ysomal pool. Next, we examined whether the effects of estrogen on apoE translation were mediated by estrogen receptors (ER). ER-alpha knock- out mice and their wild-type littermates were administered estradiol. As expected, apoE levels and hepatic apoE synthesis increased more tha n 2-fold in the wildtype littermates, but only 20% increases in the pl asma apoE and hepatic synthesis were observed in the ER knock-out mice . Hepatic apoE mRNA levels did not change in either the wild-type or t he ER knock-out mice. Thus, estradiol up-regulates apoE gene expressio n by increasing levels of apoE mRNA in the polysomal translating pool. Furthermore, the increased polysomal recruitment of apoE mRNA is larg ely mediated by estrogen receptors.