The ERK-dependent proteome is enriched in the mouse PDAC KPC RAS-dependent phosphoproteome

Mutational activation of the KRAS oncogene is a major genetic driver of pancreatic ductal adenocarcinoma (PDAC) growth. KRAS-dependent PDAC growth is mediated primarily through persistent activation of the RAF-MEK-ERK mitogen-activated protein kinase (MAPK) cascade, one of the most extensively studied cancer signaling networks. While substrates of RAF and MEK kinases are highly restricted, ERK1/2 has been attributed to over 1,000 substrates. In this study, we used the highly selective ERK1/2 inhibitor, SCH772984, and proteomic and phosphoproteomic analyses to extend the repertoire of ERK-dependent phosphosites and phosphoproteins in PDAC. We validated the specificity of SCH772984 in our cell lines using multiplexed inhibitor beads coupled with mass spectrometry (MIB/MS). We then performed phosphoproteomics and global proteomics in a panel of PDAC cell lines and identified 5,117 ERK-dependent phosphosites on 2,252 proteins, of which 88% and 67%, respectively, were not previously associated with ERK. We then utilized our recently annotated serine/threonine kinome motif database to dissect the phosphoproteome and reveal an expansive ERK-regulated kinase network. We found that ERK- and immediate downstream kinase RSK-substrate motifs predominated after one hour of ERK inhibition, whereas cell cycle regulatory cyclin-dependent kinase motifs predominated by 24 h, reflecting a highly dynamic ERK-dependent phosphoproteome. We find compensatory activation of HIPK, CLK, PKN, PAK, and DYRK family kinases. Finally, using the genome-wide CRISPR-Cas9 dataset in the Cancer Dependency Map portal (DepMap), we determined that approximately 18% of ERK dependent phosphoproteins are essential for pancreatic cancer growth, and these are enriched in nuclear proteins. Together, our findings provide a system-wide profile of the mechanistic basis for ERK-driven pancreatic cancer growth.