P21-activated kinase group II small compound inhibitor GNE-2861 perturbs estrogen receptor alpha signaling and restores tamoxifen-sensitivity in breast cancer cells

Estrogen receptor alpha (ERa) is highly expressed in most breast cancers. Consequently, ERa modulators, such as tamoxifen, are successful in breast cancer treatment, although tamoxifen resistance is commonly observed. While tamoxifen resistance may be caused by altered ERa signaling, the molecular mechanisms regulating ERa signaling and tamoxifen resistance are not entirely clear. Here, we found that PAK4 expression was consistently correlated to poor patient outcome in endocrine treated and tamoxifen-only treated breast cancer patients. Importantly, while PAK4 overexpression promoted tamoxifen resistance in MCF-7 human breast cancer cells, pharmacological treatment with a group II PAK (PAK4, 5, 6) inhibitor, GNE-2861, sensitized tamoxifen resistant MCF-7/LCC2 breast cancer cells to tamoxifen. Mechanistically, we identified a regulatory positive feedback loop, where ERa bound to the PAK4 gene, thereby promoting PAK4 expression, while PAK4 in turn stabilized the ERa protein, activated ERa transcriptional activity and ERa target gene expression. Further, PAK4 phosphorylated ERa-Ser305, a phosphorylation event needed for the PAK4 activation of ERa-dependent transcription. In conclusion, PAK4 may be a suitable target for perturbing ERa signaling and tamoxifen resistance in breast cancer patients. Overall design: MCF7 cells were serum-starved for 72h and then treated with E2 for 45 min.