Previous work has shown that the dnmt1 promoter of mouse is independently activated by Sp1/Sp3 [4
] and E2F [5
] transcription factors, and that human DNMT1 is activated by STAT3 [7
]. A recent study demonstrated that p53 is a negative regulator of Dnmt1 promoter activity [26
]. However, no information is available whether Dnmt1 gene transcription is regulated by nuclear receptors.
In this study, we identified nuclear receptor SHP as a transcriptional repressor of both mouse dnmt1 and human DNMT1 expression by inhibiting the activity of ERRγ. Such regulation was observed in several cancer cell lines including Hela and Huh7 cells, but was stronger in Hela cells, suggesting that this regulatory mechanism may play an important role in controlling DNMT1 function in cervical cancer. However, it should be noted that the expression of Dnmt1 was only partially decreased by ERRγ knockdown, indicating that in addition to the ERRγ/SHP pathway, other mechanisms may exist to control the expression of Dnmt1.
Another observation was that the ERRγ protein was moderately repressed by SHP, which is distinct from the complete repression of its mRNA by SHP. We also observed that SHP overexpression itself did not inhibit DNMT1 mRNA and protein (not shown) in Hela cells. This could be in part attributed to the moderate inhibitory effect of SHP on the ERRγ protein. Nevertheless, our results provide strong evidence for a novel role of SHP in repressing gene transcription of Dnmt1 via ERRγ.
Importantly, the co-recruitment of SHP with ERRγ to the dnmt1/DNMT1 promoters significantly decreased the enrichment of ERRγ. In addition, binding of ERRγ to the dnmt1 promoter induces transcriptionally active configuration of the promoter, which is reversed by co-expression of SHP. This suggests that the major mechanism for SHP inhibition of dnmt1 promoter activity is through alteration of local chromatin structure by ERRγ. SHP-mediated repression is not limited to the action of ERRγ, because SHP also decreases H4Ac occupancy which is not significantly affected by ERRγ. The results suggest that SHP may have a general function to keep the local chromatin configuration of its target genes in a repressive mode, which is consistent with its role as a universal transcriptional repressor [27
Over sixty chromatin modifying enzymes have been identified [28
], and many of them are associated with post-translational modification of H3Ac, H4Ac, H3K4Me2, and H3K9Me2. It remains to be determined in future studies whether SHP and ERRγ are co-recruited to a multiprotein complex including proteins governing the post-translational modification of histones, and how this would regulate the gene transcription of Dnmt1.
Overall, our findings are important because they link SHP function to DNA methylation at the molecular levels. Considering the importance of Dnmt1 in DNA methylation associated silencing of tumor suppressors [2
], and the critical role of SHP in liver cancer progression [13
], the identification of the regulatory role of SHP in controlling Dnmt1 expression improves our understanding of the epigenetic mechanisms governing the development of liver cancer.