HER2 signaling drives DNA anabolism and proliferation through SRC-3 phosphorylation and E2F1-regulated genes

Approximately 20% of early-stage breast cancers display amplification or overexpression of the ErbB2/HER2 oncogene, conferring poor prognosis and resistance to endocrine therapy. Targeting HER2+ tumors with trastuzumab or the receptor tyrosine kinase (RTK) inhibitor lapatinib significantly improves survival, yet tumor resistance and progression of metastatic disease can develop over time. While the mechanisms of cytosolic HER2 signaling are well studied, nuclear signaling components and gene regulatory networks that bestow therapeutic resistance and limitless proliferative potential are incompletely understood. Here, we use biochemical and bioinformatics approaches to identify effectors and targets of HER2 transcriptional signaling in human breast cancer. Phosphorylation and activity of the Steroid Receptor Coactivator-3 (SRC-3) is reduced upon HER2 inhibition, and recruitment of SRC-3 to regulatory elements of endogenous genes is altered. Transcripts regulated by HER2 signaling are highly enriched with E2F1 binding sites and define a gene signature associated with proliferative breast tumor subtypes, cell cycle progression, and G1 to S phase transition. We show that HER2 signaling drives proliferation in breast cancer cells through regulation of E2F1-driven DNA metabolism and replication genes together with phosphorylation and activity of the transcriptional coactivator SRC-3. Furthermore, our analyses identified a cyclin dependent kinase (CDK) signaling node that, when targeted using the CDK4/6 inhibitor Palbociclib, defines cooperative signaling pathways for expression of tumorigenic gene networks. Our findings suggest this proliferative gene signature is amendable to pharmacological targeting. These results have implications for rational discovery of pharmacological combinations in pre-clinical models of adjuvant treatment and therapeutic resistance

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Nikolai BC, Lanz RB, York B, Dasgupta S, Mitsiades N, Creighton CJ, Tsimelzon A, Hilsenbeck SG, Lonard DM, Smith CL, O'Malley BW