MYC Upregulation Confers Resistance to Everolimus and Establishes Vulnerability to Cyclin-Dependent Kinase Inhibitors in Pancreatic Neuroendocrine Neoplasm Cells.

Dysregulation of the mechanistic target of rapamycin complex 1 (mTORC1)-dependent pathways in pancreatic neuroendocrine neoplasms (PanNENs) underlies the introduction of the mTORC1 inhibitor everolimus as treatment of advanced progressive PanNENs. Although everolimus significantly increases progression-free survival, most patients acquire secondary resistance to the drug. This study aimed at identifying mechanisms involved in acquisition of resistance to everolimus. BON-1 and everolimus-resistant (ER) BON-1 cells were used as in vitro system of sensitivity and acquired resistance. Transcriptome changes occurring in BON-1 and ER-BON-1 were investigated by RNA sequencing and validated by quantitative PCR analysis. RNA extracted from patients' biopsies was used to validate MYC upregulation. Drug screening and functional assays were performed using ER-BON-1 cells. Cell cycle progression was evaluated by FACS analysis. Our results show that MYC overexpression is a key event in the development of secondary resistance to everolimus in PanNEN cell lines and in metastatic lesions from neuroendocrine neoplasm patients. MYC knockdown restored ER-BON-1 sensitivity to everolimus. Pharmacological inhibition of MYC mediated by the cyclin-dependent kinase inhibitor dinaciclib strongly reduced viability of ER-BON-1. Dinaciclib synergized with everolimus and inhibited ER-BON-1 cell cycle progression. Our findings suggest that MYC upregulation drives the development of secondary resistance to everolimus in PanNENs and that its inhibition is an exploitable vulnerability. Indeed, our results indicate that combined treatments with cyclin-dependent kinase and mTOR inhibitors may counteract secondary resistance to everolimus in PanNENs and may pave the ground for new therapeutic regimens for these tumors.

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