TRAIL-induces Src mediated MEK/ERK, SMAD3 and β-catenin signalling in apoptosis resistant NSCLC cells

Tumour-necrosis factor related apoptosis-inducing ligand (TRAIL) receptors (TRAIL-R1 and -R2) are appealing therapeutic targets to eradicate tumours specifically via caspase-dependent apoptosis. However, resistance is often observed and TRAIL-R activation can even activate pro-tumorigenic non-canonical signalling pathways. Previously, we found that TRAIL-induced RIPK1-Src-STAT3 signalling was mediating cell migration and invasion in resistant non-small cell lung cancer (NSCLC). Here, the contribution of Src in TRAIL signalling in NSCLC cell lines was further examined. TRAIL sensitive H460 and resistant A549 NSCLC cells showed distinct time-dependent rhTRAIL-induced Src phosphorylation patterns with early activation in A549 cells. Pharmacological Src inhibition as well as shRNA knockdown or CRISPR/CAS9-dependent knockout of Src expression did not alter sensitivity to rhTRAIL-induced apoptosis in both cell lines. Silencing of secondary complex proteins showed that TRADD, but not TRAF2, FADD nor caspase-8, was required for Src activation in A549 cells. Possible mediators of Src-dependent rhTRAIL signalling were identified by Src co-IP-LC-mass spectrometric analyses. In A549 cells the number of Src-interacting proteins increased after rhTRAIL treatment, whereas protein numbers decreased in H460 cells. In rhTRAIL treated A549 cells, Src biding proteins included components of the RAF-MEK1/2-ERK, Wnt and SMAD3 signalling pathways. Functional analyses showed that Src mediated phosphorylation of MEK1/2 and ERK, prevented phosphorylation of SMAD3 and was required for nuclear translocation of ERK and β-catenin in A549 cells. Clonogenic growth of both Src proficient and deficient A549 cells was not affected by rhTRAIL exposure, although Src depletion and MEK1/2 inhibition reduced colony size and numbers significantly. In conclusion, rhTRAIL-induced and Src dependent MEK/ERK, SMAD3 and β-catenin signalling may contribute to the known pro-tumorigenic effects of rhTRAIL in resistant NSCLC cells. However, this needs to be further examined, as well as the potential therapeutic implications of targeting these pathways when combined with TRAIL receptor agonists. ### Competing Interest Statement The authors have declared no competing interest.