Expression of the murine apolipoprotein E gene is coupled to the differentiated state of F9 embryonal carcinoma cells.

Apolipoprotein E (apoE) expression was studied in F9 embryonal carcinoma cells that differentiate in response to retinoic acid into cells resembling either parietal endoderm or visceral endoderm of the parietal or visceral yolk sac. F9 cells secreted newly synthesized apoE when incubated with radiolabeled amino acid. Upon differentiation to parietal endoderm-like cells, apoE synthesis and secretion were markedly down-regulated. In contrast, apoE secretion was up-regulated upon differentiation to visceral endoderm-like cells. These changes in apoE expression were due, at least in part, to regulation of apoE mRNA abundance since visceral endoderm-like cells contained 5- to 7-fold more apoE mRNA than parietal endoderm-like cells. The apoE phenotype reflected the differentiated state of the F9 cell since either the up-regulated or down-regulated pattern was stable to the removal of the retinoic acid inducer. To determine how well apoE regulation in F9 cells reflects regulation in visceral and parietal yolk sacs, apoE mRNA was measured in yolk sac tissues of the 12-day mouse embryo. Visceral yolk sac contained 109 +/- 11 pg of apoE mRNA per micrograms of RNA while parietal yolk sac contained 14.3 +/- 3.1 pg/micrograms of RNA. This 7- to 8-fold difference is similar to the difference in the apoE mRNA contents of visceral endoderm-like and parietal endoderm-like F9 cells. RNA gel blot analysis showed that apoE mRNA is the same size in F9 cells as in yolk sac tissues and fetal or adult liver. In addition, primer-extension analysis showed that transcription is initiated at or near the same site on the apoE gene in F9 cells and mouse tissues. These data suggest that both quantitative and qualitative features of apoE gene expression in development are retained in the F9 cell. The F9 cell should provide a useful system to study the developmental activation of endogenous apolipoprotein genes as well as exogenous apolipoprotein genes introduced by transfection.