Role of PLK1 in steroid hormone signaling in breast cancer cells

Abstract

Steroid receptors (SRs) are hormone-induced transcription factors that regulate gene expression by their concerted actions with coregulatory proteins. Performing a quantitative mass spectrometry-based interaction screen in breast cancer cells, we identified Polo-like kinase 1 to interact with progesterone receptor (PR) and estrogen receptor (ER) in presence of their respective hormone stimuli. PLK1 is recruited to chromatin and regulates SR-dependent and –independent gene transcription. Global gene expression analysis let us observe that PLK1-coactivated genes are enriched in developmental functions and linked to good clinical prognosis in breast cancer patients. Using large-scale phosphoproteomics with MCF7 breast cancer cells treated with estradiol in presence or absence of the specific PLK1 inhibitor BI2536, we observed that 28% of estradiol-induced phosphorylation sites depended on PLK1 activity. Among proteins with PLK1-dependent phosphorylation sites, we identified several transcriptional regulators including the ER-associated histone H3K4-methylase MLL2. As PLK1 inhibition reduced the estradiol-mediated induction of H3K4 methylation levels at MLL2 target genes, together these results propose a role of PLK1 in the regulation of MLL2 activity. Finally, in a targeted approach using recombinant histone proteins as substrate of PLK1, we observed a specific phosphorylation of histone H2B at serine 36. In summary, we conclude that PLK1 affects SR-dependent and SR-independent gene transcription by phosphorylating several transcriptional regulators and chromatin components.