Thomsen R, Christensen DB, Rosborg S, Linnet TE, Blechingberg J, Nielsen AL
The three mammalian HP1 proteins, HP1α/CBX5, HP1β/CBX1, and HPγ/CBX3, are involved in chromatin packing and gene regulation. The HP1α protein is down-regulated in invasive compared to non-invasive breast cancer cells and HP1α is a suppressor of cell migration and invasion. In this report, we examined the background for HP1α protein down-regulation in invasive breast cancer cells. We identified a strict correlation between HP1α down-regulation at the protein level and the mRNA level. The HP1α mRNA down-regulation in invasive cancer cells was not caused by mRNA destabilization. Chromatin immunoprecipitation analysis of the HP1α gene showed a decrease in the histone mark for transcriptional activity H3-K36 tri-methylation and RNA polymerase II in invasive breast cancer cells which correlated with a decreased abundance of basal transcription factors at the HP1α promoter. E2F transcription factors regulate HP1α transcription and we identified that E2F5 depletion increased HP1α expression in invasive breast cancer cells. Finally, we have characterized two HP1α mRNA isoforms and both HP1α mRNA isoforms were down-regulated to a similar extend at the transcriptional level in invasive breast cancer cells. Collectively the presented results show that HP1α down-regulation in invasive breast cancer cells is primary a transcriptional effect and demonstrates a novel set of mechanisms involved in HP1α transcriptional regulation. The finding that HP1α is down-regulated primarily at the transcriptional level provides a new insight for the further elucidation of the detailed molecular mechanisms causing the HP1α down-regulation in invasive breast cancer cells.