Caspase-8 as a Candidate Mediator Associated with Everolimus Resistance in Neuroendocrine Tumors

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BACKGROUND: Everolimus, an mTORC1 inhibitor, is approved for treating pancreatic and lung NETs, yet acquired resistance limits its long-term efficacy. The molecular mechanisms underlying resistance and potential strategies to overcome it remain poorly defined. METHODS: RNA sequencing was performed in everolimus-sensitive and everolimus-resistant QGP-1 pancreatic NET cells (QS and QR, respectively) to identify transcriptional changes associated with everolimus resistance. Key differentially expressed genes were validated in QGP-1, H727 lung NET cells, and patient-derived Pa-NET cells using RT-qPCR. The potential functional involvement of caspase-8 (CASP8) was investigated pharmacologically using the preferential CASP8 inhibitor Z-IETD-FMK, alone or in combination with everolimus, by evaluating effects on cell viability and CASP activity. RESULTS: Differential expression analysis identified 292 genes significantly dysregulated in QR versus QS cells, including CASP8, A2M, MGP, PTGS1, and IGFBP5. Enrichment analysis highlighted pathways related to ECM remodeling, neuroactive ligand-receptor interactions, and GPCR signaling. Validation in H727 cells and primary Pa-NET samples showed some conserved transcriptional changes, with CASP8 upregulation emerging as a particularly consistent alteration. CASP8 inhibition reduced viability in both QS and QR cells, while combined Z-IETD-FMK and everolimus treatment produced the greatest decrease in QS viability and was associated with reduced CASP3/7 activity. CONCLUSIONS: Our study identifies CASP8 as a candidate mediator associated with everolimus resistance in NETs, together with alterations in ECM remodeling and receptor signalling. These findings support further investigation of CASP8-targeting strategies, particularly in combination with mTOR inhibition, as a potential approach to enhance therapeutic efficacy.

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