Previous studies have demonstrated that miR-34a is a direct transcriptional target of tumor suppressor p53 and plays a crucial role in p53-mediated biological processes, such as cell cycle arrest, apoptosis and senescence. However, the role of p53 phosphorylation at Ser15 and histone modifications in ionizing radiation (IR)-induced miR-34a transcription in human mammary epithelial cells remains unknown. The present study showed that IR triggers miR-34a induction in rat mammary gland tissue and human mammary epithelial cells in a dose- and time-dependent fashion. Gene copy number and CpG methylation exhibit no effect on IR-inducible miR-34a expression, while the levels of phosphorylated p53 at Ser15 are markedly elevated in human mammary epithelial cells 96 h post-IR, which correlates with IR-inducible miR-34a transcription and the p38 MAPK pathway. Conversely, suppression of p38 MAPK with SB239063 inhibits IR-induced p53 phosphorylation at Ser15 and miR-34a expression in a dose-dependent manner. Our study found that wild-type p53 is enriched at miR-34a promoter, and luciferase activity of miR-34a promoter reporter is attenuated by either mutant p53 (Ser15Ala) or mutant miR-34a promoter. Furthermore, IR also triggers phosphorylation, tri-methylation and acetylation of histone H3 and acetylation of histone H4, which correlates with IR-inducible miR-34a transcription, while SAHA potentiates IR-inducible miR-34a expression. Moreover, acetyl-histone H3 is significantly enriched at miR-34a promoter in IR-exposed HMEC cells. Yet, we show that there is no correlation between IR-inducible miR-34a expression and IR-induced rapid and transient G 2/M arrest. In sum, our novel data for the first time demonstrate that IR-induced p53 Ser15 phosphorylation via p38 MAPK is essential for its functional regulation of IR-inducible miR-34a transcription in human mammary epithelial cells, and that histone modifications may also play a key role in IR-inducible miR-34a expression.
Keywords: histone modification; ionizing radiation; mammary epithelial cells; miR-34a transcription; p53 phosphorylation.