PAK1-mediated phosphorylation of ARGLU1 promotes tumorigenic potential of breast cancer.

P21-activated kinase1 (PAK1) is an oncogenic kinase that is often hyper-activated or overexpressed in multiple cancers including breast cancer (BC). By phosphorylating a diverse range of substrates, PAK1 promotes tumor progression through multiple signaling pathways. Herein, we report arginine and glutamate rich-protein 1 (ARGLU1), a protein that is overexpressed in BC, as a novel substrate of PAK1. Through mass spectrometry, PhosTag, and metabolic labeling of cells with P32-orthophosphoric acid, we identified and confirmed that PAK1 phosphorylated ARGLU1 at Ser 77. Furthermore, to investigate the impact of this phosphorylation, we created BC cells with ARGLU1 Ser 77 to Ala mutation that resulted in lower E-box promoter activity, lower expression of MCM genes, and higher G2/M fraction and this was rescued by the phosphomimic mutant (Ser 77 to Asp). Furthermore, BC cells that stably expressed the phosphor inactive form of ARGLU1 displayed lower proliferation rates along with diminished ability to form colonies, migrate, invade, and form spheroids indicating the significance of the phosphorylation. Furthermore, ARGLU1 was found to be overexpressed in BC samples and clinical data from publicly available databases also implied that pARGLU1 S77 was not only elevated in BC, but also was associated with advanced stages and poor overall survival. The present study provides evidence that ARGLU1 is a novel substrate of PAK1 and its phosphorylation at S77 contributes to BC pathogenesis.