Expression of the glucose-regulated proteins (GRP94 and GRP78) in differentiated and undifferentiated mouse embryonic cells and the use of the GRP78 promoter as an expression system in embryonic cells.

We examined the expression of GRP94 and GRP78 in embryonal carcinoma (EC) cells and during early mouse development. The GRP78 protein is constitutively expressed in the undifferentiated F9 cells whereas the level of GRP94 is below detection limit. These cells, like adult differentiated cells, exhibit enhancement of the GRP94 and GRP78 transcript levels in response to the calcium ionophore A23187, resulting in an increase of GRP94 and GRP78 proteins after 8 h of treatment. The GRP78 promoter has been identified as containing a strong enhancer. To test whether the GRP78 promoter is effective in directing expression of a heterologous gene in F9 cells, CAT fusion genes containing the GRP78 promoter were transfected into F9 cells. In non-induced F9 cells the activity of the GRP78 promoter was about 5- and 10-fold higher than the RSV promoter and the SV40 early promoter, respectively. Treatment of the cells with A23187 further increased the GRP78 promoter activity 3-fold. Since GRP78 is expressed during mouse embryogenesis and the promoter is also active in F9 cells treated with retinoic acid, the GRP78 promoter may be useful for the efficient expression of heterologous genes in undifferentiated and differentiated mouse embryonal cells.