Phorbol ester regulation of terminal deoxynucleotidyl transferase, proliferation, and TcR α in a pre-T cell line

Terminal deoxynucleotidyl transferase (TdT) is a template‐independent DNA polymerase that is transiently expressed during the normal development of T and B lymphocytes. Phorbol 12‐myristate 13‐acetate (PMA) has been reported to induce maturation‐like changes, including the loss of TdT, in many leukemic cell lines. We investigated the mechanism of TdT repression by PMA in an early thymocyte‐like cell line, RPMI 8402. At a concentration of 8 nM, PMA caused both repression of TdT synthesis and arrest of proliferation. At greater concentrations of PMA, these same changes initially occurred, but then cell proliferation resumed, and TdT was reexpressed. At both 8 and 160 nM PMA, TdT biosynthesis and TdT mRNA became undetectable within 8 hours, while cell proliferation and DNA synthesis were not significantly reduced until 16 hours. Growth arrest induced by serum starvation did not result in a similar reduction of TdT RNA even after 48 hours. With 160 nM PMA, TdT mRNA could be detected again by 24 hours, and proliferation resumed. Transcription run‐off assays indicated that TdT RNA synthesis ceased within 1 hour after exposure to both 8 and 160 nM PMA. T cell receptor α (TcR α) RNA was induced when TdT RNA was repressed. TcR β RNA levels were unchanged, and TcR γ RNA was up‐regulated. TdT gene repression and modulation of cell proliferation as well as induction of TcR gene expression are normal events during intrathymic T cell maturation. This cell model provides a system for analyzing the molecular regulation of these significant developmental events.