Hormonal induction of mouse selenocysteine transfer ribonucleic acid (tRNA) gene transcription-activating factor and its functional importance in the selenocysteine tRNA gene transcription in mouse mammary gland.

Mouse selenocysteine transfer RNA (tRNA) gene transcription-activating factor (mStaf) is a transcriptional activator that enhances RNA polymerase III-dependent mouse selenocysteine tRNA (tRNA(Sec)) gene transcription. The DNA-binding activity of mStaf in mouse mammary gland undergoes developmental changes, reaching a maximal level during the period of lactation. In this study, we employed an organ culture system to examine the hormonal regulation of mStaf binding and its role in the tRNA(Sec) transcription in the mammary gland. The results showed that mStaf binding in mammary explants was stimulated by treatment with the lactogenic hormones, PRL, insulin, and hydrocortisone and that a specific MEK inhibitor, PD98059, inhibited the hormonal stimulation of mStaf binding. Other kinase inhibitors, such as a Janus kinase inhibitor and a calmodulin kinase inhibitor, had no apparent effect. Northern and Western blot analyses revealed that the level of both mStaf messenger RNA and protein was enhanced by the lactogenic hormones and was reduced by the concomitant treatment with PD98059. The mitogen-activated protein kinase activity in cultured explants was rapidly induced and maintained at high levels by the lactogenic hormones. We also found that the lactogenic hormones increased the amount of tRNA(Sec) in a time-dependent manner, which followed the increase in mStaf binding in cultured mammary explants. These results support the view that mStaf plays a key role in the hormonal stimulation of tRNA(Sec) transcription in the mammary gland.