Differential expression of two classes of lck transcripts upon phorbol ester treatment of human leukemic T cells

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Abstract
The human lymphocyte‐specific tyrosine kinase gene, Ick, is transcribed from two distinct promoters, resulting in two classes of transcripts (type I and II) differing in their 5′ untranslated regions. The steady‐state levels of the type I and II Ick transcripts were measured in a variety of lymphoid and non‐lymphoid human tumor cell lines by S1 nuclease mapping and by a sensitive assay system using the polymerase chain reaction. Human thymocytes and all the leukemic T cell lines tested express both type I and II Ick transcripts, albeit at different relative levels. Peripheral blood T cells express mainly type II Ick transcripts, whereas two colonic carcinoma lines, COLO 201 and COLO 205, express exclusively type I Ick transcripts. Treatment of the leukemic T cell lines, P30/OKUBO and Jurkat, by the phorbol esters tetradecanoylphorbol acetate (TPA) or phorbol dibutyrate (PDB) results in the down‐regulation of the type I, and the up‐regulation of the type II, Ick transcript levels. The effect of PDB on the in vitro differentiation of Jurkat cells, and the expression of Ick transcripts, is reversible. The modulation of Ick transcript levels in TPA‐treated Jurkat cells is not due to differential RNA stability, suggesting that the two Ick promoters are utilized differentially during T cell differentiation. The leukemic T cell line, Jurkat, may thus serve as a model for the elucidation of molecular mechanisms that regulate Ick transcription and T cell differentiation.