Effects of Different Conditions of Retinoic Acid Treatment on Expression of MK Gene, which is Transiently Activated during Differentiation of Embryonal Carcinoma Cells1

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Abstract
MK gene was intensely expressed, when aggregates of HM-1 embryonal carcinoma (EC) cells were treated with retinoic acid for 2 days to induce the differentiation to nerve cells, myoblasts and extraembryonic endoderm cells. The conditions inhibiting nerve cell diffrentiation or extraembryonic endoderm cell differentiation affected MK gene expression only slightly. The maximum level of MK RNA was detected 2 days after initiation of retionic acid treatment, when cells were morphologically indistinguishable from undifferentiated EC cells. Thus, MK gene appears to be expressed in differentiating EC cells irrespective of the direction of differentiation. The degree of MK gene expression in sparsely cultured HM-1 cells correlated with the concentration of retinoic acid, especially between 10-8 and 10-7 M. When retinoic acid treatment was terminated after 1 day, the amount of MK RNA started to decrease. These two results are consistent with the view that retionic acid complexed with the receptor is directly involved in expression of MK gene.