Receptor tyrosine kinase cross talk affects tumor cell sensitivity to RTK inhibitors
Authors: Helen Zou, Qiuhua Li, Joseph Lee, Kristina Burgess, Mark Ozeck, Max Parker, Adam Pavlicek, Jingrong Cui, Steve Bender, James Christensen
Journal: Cancer Research (2008)

AACR Annual Meeting-- Apr 12-16, 2008; San Diego, CA

LB-198 

Oncogene addiction is defined as thedependence of a given neoplasm on a single oncogenic pathway for itssustained growth and survival. This phenomenon may be best illustrated by the success of targeting genetically altered receptor tyrosine kinases (RTKs) in selected cancers. To further understand mechanisms underlying Oncogene Addiction, selective c-Met (PF-4254644) and EGFR (erlotinib) inhibitors were used to study RTK cross-talk (heterodimerization) and downstream signaling. In this report, phosphorylation events were assessed by a phosphotyrosine sandwich ELISA under non-denaturing conditions under which RTK-signaling complexes were captured. In Met highly amplified GTL-16 gastric carcinoma cells, PF-4254644 inhibited not only c-Met phosphorylation (IC50 = 8 nM) but also other co-activated RTKs (i.e. Her1-3, RON, Mer, IGF1R, EphA2, Tie2, TrkA and Ret (4 - 16 nM) in cells and xenograft tumors. Complete inhibition of GTL-16 cell or xenograft growth was observed over a comparable range of concentrations and was accompanied by apoptosis and inhibition of downstream AKT/S6K activities. PF-4254644 did not inhibit any of the co-activated RTKs in engineered NIH3T3 cell or enzyme assays indicating no direct inhibition of the aforementioned RTKs. In contrast, erlotinib lost activity against EGFR phosphorylation (> 2 µM) in GTL-16 cells and no growth inhibition was observed. Stimulating the cells with EGF did not restore erlotinib inhibition of EGFR. In EGFR amplified A431 epidermoid carcinoma cells, erlotinib inhibited EGFR (23 nM) and the activities of co-activated Her2-3 and IGF1R as well as downstream AKT signaling in serum starved conditions resulting in tumor growth inhibition. EGF stimulation activated multiple RTKs in A431 cells and erlotinib suppressed the phosphorylation of most of these RTKs (i.e. c-Met, Ron, EGFR, Her3, FGFR3, IGF1R, M-CSFR and TrkA; 4 - 27 nM). In contrast, stimulating A431 cells with HGF or MSP markedly shifted erlotinib potency for inhibiting EGFR (174 nM), Met, Ron and Her3 (each > 2 µM) activities and resulted in loss of AKT inhibition. Similarly, PF-04254644 potently (4 nM) and completely inhibited c-Met in HGF-treated A431 cells but only partially inhibited c-Met phosphorylation (40%) in EGF-treated cells. This pattern of RTK potency shift with the c-Met or EGFR inhibitors was also observed in a panel of tumor cell lines. Collectively, these data suggest that A) RTK cross-talk is common among RTKs in selected tumor lines, B) amplified c-Met or EGFR can act as dominant trans-activators of other RTKs and inhibiting this dominant trans-activator may completely block RTK signaling resulting in tumor cell stasis and apoptosis, C) presence of RTK ligands can shift the pattern of RTK complex formation as well as potency of RTK inhibitors. These results provide insight towards RTK net working and tumor cell sensitivity to RTK inhibitors under selected conditions including RTK pathway dependence.