Apolipoprotein E gene expression in various tissues of mouse and regulation by estrogen

Apolipoprotein (apo) E is associated with several classes of lipoproteins and serves as a ligand for the receptor mediated uptake of cholesterol-rich particles by hepatocytes and peripheral tissues. Variant forms of apo E is also associated with dyslipidemia and late-onset of Alzheimer's Disease (AD). We report here expression of apoE in various mouse tissues, and regulation of apoE in liver, kidney, brain and testes by supraphysiological doses of estrogen. ApoE mRNA was quantified by RNase protection assay and translatable apoE mRNA by in vitro translation. As an internal control the levels of beta-actin mRNA were also quantified. Highest levels of apoE were expressed in liver (220-280 pg/mu g RNA) with negligible levels in small intestine. Brain expressed highest levels of total (35-40 pg/mu g RNA) and translatable apoE mRNA next only to liver. Other tissues that expressed relatively higher levels of apoE were adrenals, testes and ovary. ApoE was also found to be expressed in heart, lung, kidney and spleen. Regulation of apoE gene expression by estrogen (3 mu g 17beta-estradiol/ g body weight/ day for 5 consecutive days) was studied in liver, kidney, brain and testes of 4 mouse strains. Hepatic apoE mRNA did not change significantly in any of the mouse strains following estradiol administration. Of note was significant increases in the levels of brain apoE mRNA in the strain C3H. These studies demonstrate that estrogen regulates apoE gene expression in a tissue-specific manner in mice, and increases in apoE mRNA in the brain by estrogen may have implications in late-onset of Alzheimer's Disease.