Molecular cloning of a tumor promoter‐inducible mRNA found in JB6 mouse epidermal cells: Induction is stable at high, but not at low, cell densities

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Abstract
From the mouse JB6 epidermal cell line C122 we have isolated a cDNA clone representing a 1.6‐kilobase mRNA, called 2ar, that exhibits a biphasic induction in response to 12‐O‐tetradecanoyl‐phorbol‐13‐acetate (TPA). The first phase of induction in subconfluent cells is transient, peaking at 6 h after the addition of TPA and returning to noninduced levels by 24 h. When the cells reach plateau density, in the continued presence of TPA, this mRNA is reinduced and remains so upon continued exposure to the tumor promoter. Serum and certain growth factors also induce 2ar mRNA in serum‐deprived quiescent fibroblasts. In vitro nuclear “run‐on” transcription experiments indicate that the induction of 2ar mRNA is controlled at the transcriptional level.