Systemwide characterization of the ERK program that drives pancreatic cancer

Over 95% of pancreatic ductal adenocarcinoma (PDAC) tumors harbor a KRAS mutation and nearly half are KRASG12D. Therefore, advancement of KRASG12D and RASMULTI ON inhibitors into the clinic will have a major impact on treating one of the deadliest cancers. However, treatment-associated resistance to currently approved KRAS inhibitors has emerged, with ~60% of relapsed patients acquiring mutations that reactivate the RAF-MEK-ERK and PI3K-AKT effector pathways. We found that ectopic expression of constitutively activated MEK1 kinase (MEK1- DD), ERK1 (ERK1-SD) or ERK2 (ERK2-SD) kinases, but not AKT1 (myr-AKT) kinase, drove near-complete resistance to direct inhibitors of KRAS in PDAC. Consistent with this, we found that treatment with either RASMULTI ON (RMC-7977), KRASG12D (MRTX1133), MEK (trametinib), or ERK (SCH772984) inhibitors induced nearly identical transcriptomic changes in PDAC. These observations prompted us to define the mechanistic basis for ERK-dependent growth in KRAS mutant PDAC. Ectopic expression of either constitutively active ERK1 or ERK2 was sufficient to rescue 92% (ERK1) or 83% (ERK2) of KRAS mediated transcription. Combined with genetic-loss-of function studies, we demonstrated that ERK1 and ERK2 are functionally identical in KRAS-mutant PDAC. Many ERK substrates are characterized by two ERK docking sequences, the DEF and D motifs. We identified 2,123 ERK-dependent phosphoproteins, with ~40% containing DEF and/or D-motifs. To address how each of these docking motifs contributes to mutant KRAS-ERK driven signaling in PDAC, we generated mutations in constitutively active ERK1 and ERK2 that prohibited ERK interaction with either of the two substrate docking motifs. We found that both DEF and D-motif interactions were required to rescue PDAC cell growth following MEK inhibition (trametinib) or KRAS inhibition (MRTX1133). Our studies establish ERK as the key driver of KRAS-dependent cancer growth, regulating a complex transcriptome and phosphoproteome that is reliant on ERK’s ability to recognize two docking motifs.