Nuclear moonlighting of the secreted growth factor heregulin drives endocrine-resistant breast cancer independently of HER2/HER3 signaling.

The HER3/4 ligand heregulin-β2 (HRG) is a secreted growth factor that transactivates the ligand-less receptor HER2 to promote aggressive phenotypes in breast cancer. HRG can also localize to the nucleus of breast cancer cells, but both the nuclear translocation mechanism and the physiological role of nuclear HRG remain elusive. Here we show that nucleolin-driven nuclear moonlighting of HRG uncouples its role as a driver of endocrine resistance from its canonical HER network-activating role in breast cancer. Tandem affinity purification coupled to mass spectrometry identified the intracellular transporter nucleolin as a major HRG-binding protein. HRG interacts with nucleolin via a nuclear localization signal motif located at the N-terminal extracellular domain of HRG. Nucleolin interacts with HRG via aspartate/glutamate-rich acidic stretches located at the N-terminal domain of nucleolin. Depletion of nucleolin abolishes HRG nuclear translocation and decreases HRG mRNA and protein expression. Isolated deficiency of nuclear HRG abolishes the HRG-driven endocrine resistance phenotype