Dougherty MI, et al. Mol Cancer Res 2020.
Epidermal growth factor receptor (EGFR) inhibitors have shown poor efficacy in head and neck squamous cell carcinoma (HNSCC) with demonstrated involvement of the insulin-like growth factor-1 receptor (IGF1R) in resistance to EGFR inhibition. IGF1R activates the phosphoinositide-3-kinase(PI3K)-Akt pathway which phosphorylates proline-rich Akt substrate of 40 kDa (PRAS40) to cease mechanistic target of rapamycin (mTOR) inhibition resulting in increased mTOR signaling. Proliferation assays
separated 6 HNSCC cell lines into 2 groups: sensitive to EGFR inhibition or resistant; all sensitive cell lines demonstrated reduced sensitivity to EGFR inhibition upon IGF1R activation. RPPA analysis and immunoblot identified a correlation between increased PRAS40 phosphorylation and IGFR mediated resistance to EGFR inhibition. In sensitive cell lines, PRAS40 phosphorylation decreased 44-80% with EGFR inhibition and was restored to 98-196% of control by IGF1R activation while phosphorylation was unaffected in resistant cell lines. Possible involvement of mTOR in this resistance mechanism was demonstrated through a similar pattern of p70S6K phosphorylation. However, addition of temsirolimus, an mTORC1 inhibitor, was insufficient to overcome IGF1R-mediated resistance and suggested an alternative mechanism. Forkhead box O3a (FOXO3a), which has been reported to complex with PRAS40 in the cytoplasm, demonstrated a 6-fold increase in nuclear to cytoplasmic ratio upon EGFR inhibition that was eliminated with concurrent IGF1R activation. Transcription of FOXO3a-regulated TNF-related apoptosis-inducing ligand (TRAIL) and PTEN-induced putative kinase-1 (PINK1) was increased with EGFR inhibition in sensitive cell lines; this effect was diminished with IGF1R stimulation. Implications: These data suggest PRAS40 may play an important role in IGF1R-based therapeutic resistance to EGFR inhibition, and this likely occurs via inhibition of FOXO3a-mediated pro-apoptotic gene transcription.