Genetic Events That Limit the Efficacy of MEK and RTK Inhibitor Therapies in a Mouse Model of KRAS-Driven Pancreatic Cancer

Abstract

Mutated KRAS (KRAS*) is a fundamental driver in the majority of pancreatic ductal adenocarcinomas (PDAC). Using an inducible mouse model of KRAS*-driven PDAC, we compared KRAS* genetic extinction to pharmacological inhibition of MEK1 in tumor spheres and in vivo. KRAS* ablation blocked proliferation and induced apoptosis while MEK1 inhibition exerted cytostatic effects. Proteomic analysis evidenced that MEK1 inhibition was accompanied by a sustained activation of the PI3K-AKT-MTOR pathway and by the activation of AXL, PDGFRa, and HER1-2 receptor tyrosine kinases (RTKs) expressed in a large proportion of human PDAC samples analyzed. While single inhibition of each RTK alone or plus MEK1 inhibitors was ineffective, a combination of inhibitors targeting all three co-activated RTKs and MEK1 was needed to inhibit proliferation and induce apoptosis in both mouse and human low-passage PDAC cultures. Importantly, constitutive AKT activation, which may mimic the fraction of AKT2-amplified PDAC, was able to bypass the induction of apoptosis caused by KRAS* ablation, highlighting a potential inherent resistance mechanism that may inform the clinical application of MEK inhibitor therapy. This study suggests that combinatorial targeted therapies for pancreatic cancer must be informed by the activation state of each putative driver in a given treatment context. Additionally, our work may offer explanative and predictive power in understanding why inhibitors of EGFR signaling fail in PDAC treatment and how drug resistance mechanisms may arise in strategies to directly target KRAS.

RAS signaling and anti-RAS therapy: lessons learned from genetically engineered mouse models, human cancer cells, and patient-related studies

“…The same results were observed in lung tumors [88] and pancreatic tumors [89,90] -mediated tumors [88]. Although the p53 gene or Ink4a/Arf deficiencies dramatically accelerate tumor initiation and progression, removal of doxycycline caused a rapid regression of tumor burdens, implying that continued production of mutant Kras is necessary to maintain the viability of tumor cells, regardless of the presence of other cancer drivers.…”

“…2) alone induced only partial tumor growth suppression, which did not significantly prolong overall survival (OS). Treatment with a MEK1 inhibitor resulted in cytostatic effects accompanied by sustained activation of the PI3K/AKT/mTOR pathway and receptor tyrosine kinases EGFR, HER2, PDGFR, and AXL [90]. Similar results were observed in Kras…”

“…Treatment with a dual pan-PI3K and mTOR inhibitor BEZ235 is able to substantially suppress the growth of Pik3ca 1047R -induced lung tumors but not Kras 12D -driven lung tumors [91], suggesting that PI3K may be required for Kras-induced tumorigenesis but less crucial for tumor maintenance. Nevertheless, simultaneously targeting both MEK and PI3K pathways led to marked synergy in shrinking these Kras mutant cancers, resulting in a significant survival advantage when compared with controls [90,92,93]. Partial inhibition of Kras 12D -driven lung tumorigenesis was also observed in treatment with the MEK inhibitor selumetinib (AZD6244) and the TBK1/IKK/JAK inhibitor CYT387 [74].…”

Resolution of Novel Pancreatic Ductal Adenocarcinoma Subtypes by Global Phosphotyrosine Profiling

“…Consequently, although subtype 3 cell lines exhibit a greater inhibition of proliferation upon treatment with the EGFR TKI erlotinib than those from other subtypes, and the subtype 3 cell lines HPAC and PL45 are sensitive to knockdown of MET and AXL, respectively (42), it remains possible that combined targeting of subtype 3-enriched RTKs will achieve a greater efficacy. This possibility is supported by a recent study demonstrating that the efficacy of MEK1 inhibition in PDAC is limited by compensatory signaling by multiple RTKs to the PI3K pathway, and that drug resistance can be overcome by combined targeting of EGFR, ERBB2, PDGFRA, and AXL (12). Importantly, the detailed information provided by our study will assist the design of therapeutic approaches directed at effective RTK blockade, and also fuel further studies that characterize crosstalk between particular RTKs in PDAC.…”

Targeted Inhibition of Phosphoinositide 3-Kinase/Mammalian Target of Rapamycin Sensitizes Pancreatic Cancer Cells to Doxorubicin without Exacerbating Cardiac Toxicity

“…Due to the high degree of cross-talk as well as overexpression of one or more receptor tyrosine kinases, there is also a high prevalence of enhanced activation of the PI3K/mTOR pathway (Edling et al, 2010). Likewise, AKT2 amplification, which occurs in 10-20% of PDAC, can lead to constitutive activation of AKT and its downstream targets (Ruggeri et al, 1998;Ying et al, 2011;Pettazzoni et al, 2015).…”