PKA phosphorylation redirects ERα to promoters of a unique gene set to induce tamoxifen resistance

Protein kinase A (PKA)-induced estrogen receptor alpha (ER alpha) phosphorylation at serine residue 305 (ER alpha S305-P) can induce tamoxifen (TAM) resistance in breast cancer. How this phospho-modification affects ER alpha specificity and translates into TAM resistance is unclear. Here, we show that S305-P modification of ER alpha reprograms the receptor, redirecting it to new transcriptional start sites, thus modulating the transcriptome. By altering the chromatin-binding pattern, Ser305 phosphorylation of ER alpha translates into a 26-gene expression classifier that identifies breast cancer patients with a poor disease outcome after TAM treatment. MYC-target genes and networks were significantly enriched in this gene classifier that includes a number of selective targets for ER alpha S305-P. The enhanced expression of MYC increased cell proliferation in the presence of TAM. We demonstrate that activation of the PKA signaling pathway alters the transcriptome by redirecting ERa to new transcriptional start sites, resulting in altered transcription and TAM resistance.