During the recent years lysine methyltransferase Set7/9 ((Su(var)-3-9, Enhancer-of-Zeste, Trithorax) domain containing protein 7/9) has emerged as an important regulator of different transcription factors. In this study, we report a novel function for Set7/9 as a critical co-activator of E2 promoter-binding factor 1 (E2F1)-dependent transcription in response to DNA damage. By means of various biochemical, cell biology, and bioinformatics approaches, we uncovered that cell-cycle progression through the G1/S checkpoint of tumour cells upon DNA damage is defined by the threshold of expression of both E2F1 and Set7/9. The latter affects the activity of E2F1 by indirectly modulating histone modifications in the promoters of E2F1-dependent genes. Moreover, Set7/9 differentially affects E2F1 transcription targets: it promotes cell proliferation via expression of the CCNE1 gene and represses apoptosis by inhibiting the TP73 gene. Our biochemical screening of the panel of lung tumour cell lines suggests that these two factors are critically important for transcriptional upregulation of the CCNE1 gene product and hence successful progression through cell cycle. These findings identify Set7/9 as a potential biomarker in tumour cells with overexpressed E2F1 activity. Cell Death and Differentiation (2014) 21, 1889-1899; doi:10.1038/cdd.2014.108; published online 15 August 2014Lysine methylation of non-histone proteins has recently emerged as a novel regulatory mechanism to control protein functions. 1-4Set7/9 ((Su(var)-3-9, Enhancer-of-Zeste, Trithorax) domain containing protein 7/9) is a founding member of the family of non-chromatin lysine methyltransferases (KMTases). Set7/9 was initially identified as a monomethylase of histone H3 lysine 4 (H3K4) in vitro. 4,5However, we and others showed that the recombinant Set7/9 failed to target nucleosomes for methylation, [6][7][8] suggesting that Set7/9 functions as a factor-specific KMTase. There have been several non-histone proteins reported as the substrates for Set7/9, including TAF10 (TATA box binding protein (TBP)-associated factor, 30 kDa), 9 oestrogen receptor a (ERa), 10 RelA, 11 PCAF (P300/CBP-associated factor), 12 Stat3,13 Yap, 14 and Suv39h1. 15 However, in most cases the functional significance of this methylation is still not clear. The beststudied targets of Set7/9-mediated methylation are p53 16 and E2 promoter-binding factor 1 (E2F1), 17 transcription factors involved in regulation of DNA damage response (DDR).In response to genotoxic stress cancer cells undergo cell-cycle arrest either in G1/S or G2/M or in both checkpoints. The presence of intact p53 in cancer cells mediates transient G1/S checkpoint arrest, 18,19 which allows cells to repair the damaged DNA before replication or, if the amount of damage is insurmountable, drives cells into apoptosis. 20,21On the contrary, the activity of the E2F family transcription factors, especially E2F1, drives cells from the G1/S block to mitosis. 22,23 Transcriptional activity of E2F1, in turn, is repressed by the retin...

The tumour suppressor p53 is a crucial regulator of cell cycle arrest and apoptosis by acting as a transcription factor to regulate a variety of genes. At least in part, this control is exerted by p53 via regulating expression of numerous microRNAs. We identified two abundantly expressed microRNAs, miR-16 and miR-26a, whose expression is regulated by p53 during the checkpoint arrest induced by the genotoxic drug, doxorubicin. Importantly, among the targets of these miRs are two critical checkpoint kinases, Chk1 and Wee1. The p53-dependent augmentation of miR-16 and miR-26a expression levels led to the cell cycle arrest of tumour cells in G1/S and increased apoptosis. Strikingly, the bioinformatics analysis of survival times for patients with breast and prostate cancers has revealed that co-expression of mir-16 and miR-26a correlated with a better survival outcome. Collectively, our data provide a novel mechanism whereby p53 represses Chk1 and Wee1 expression, at least partially, via upregulation of miR-16 and miR-26a and thus sensitizes tumour cells to genotoxic therapies.