Recruitment of coregulator G9a by Runx2 for selective enhancement or suppression of transcription

Runx2, best known for its role in regulating osteoblast-specific gene expression, also plays an increasingly recognized role in prostate and breast cancer metastasis. Using the C4-2B/Rx2(dox) prostate cancer cell line that conditionally expressed Runx2 in response to doxycycline treatment, we identified and characterized G9a, a histone methyltransferase, as a novel regulator for Runx2 activity. G9a function was locus-dependent. Whereas depletion of G9a reduced expression of many Runx2 target genes, including MMP9, CSF2, SDF1, and CST7, expression of others, such as MMP13 and PIP, was enhanced. Physical association between G9a and Runx2 was indicated by co-immunoprecipitation, GST-pulldown, immunofluorescence, and fluorescence recovery after photobleaching (FRAP) assays. Since G9a makes repressive histone methylation marks and is primarily known as a corepressor, we further investigated the mechanism by which G9a functioned as a positive regulator for Runx2 target genes. Transient reporter assays indicated that the histone methyltransferase activity of G9a was not required for transcriptional activation by Runx2. Chromatin immunoprecipitation assays for Runx2 and G9a showed that G9a was recruited to endogenous Runx2 binding sites. We conclude that a subset of cancer-related Runx2 target genes require recruitment of G9a for their expression, but do not depend on its histone methyltransferase activity.