Decrease in phospho-PRAS40 plays a role in the synergy between erlotinib and crizotinib in an EGFR and cMET wild-type squamous non-small cell lung cancer cell line.

INTRODUCTION: Lung squamous cell carcinomas (SCC) typically harbor a strong activation of epidermal growth factor receptor (EGFR) pathway. Since one of the most common resistance mechanisms against EGFR inhibition relies on the activation of cMET parallel signaling, we investigated the efficacy of a dual blockade with erlotinib and crizotinib in EGFR and cMET wild-type lung SCC cell lines.

METHODS: Drug sensitivity assays were performed on LUDLU, SKMES-1, H1703, Calu1 and H520 cells. Further studies included analysis of cell cycle, apoptosis, spheroids, migration and Pathscan intracellular signaling array. Expression of emerging proteins was validated by Western blot and evaluated by immunohistochemistry in tissue-microarrays from lung cancer patients.

RESULTS: Erlotinib and crizotinib showed additive interaction in Calu1, H520 and SKMES-1, and strong synergism in the LUDLU cells (Combination Index: 0.387), associated to G2/M phase arrest, increased apoptosis, spheroid size reduction and inhibition of migration. Remarkably, this combination decreased the phosphorylation of downstream targets of MAPK and PI3K/Akt/mTOR pathways, with the largest decrease observed for PRAS40 Thr246. Moreover, it reduced the expression of both p-Her3 and p-PRAS40 in the synergistic LUDLU cells. Tissue specimens showed a higher expression of both proteins in SCC compared to adenocarcinoma histology.

CONCLUSIONS: Combining erlotinib and crizotinib led to an additive/synergistic interaction in 4 out of 5 SCC cells. By combining both inhibitors, MAPK and PI3K/Akt/mTOR pathways were strongly inhibited, leading to increased cell death. p-Her3 and p-PRAS40 might be used as markers for determining the synergistic effect and for selecting potential candidates for the combination treatment.