Abstract 2426: Tyrosine kinase and ERAD inhibitors promote oxidative stress-induced apoptosis through activation of ATF4 / KLF9 axis in medullary thyroid cancer

Medullary thyroid carcinoma (MTC) is a neuroendocrine malignancy, caused by activating mutations of the RET proto-oncogene, with frequent lymph node and distant metastasis at diagnosis. Tyrosine kinase inhibitors (TKIs) that target the RET have proven effective for treatment of MTC, but resistance to TKIs evolves in half of the treated patients. We have previously shown that the expression levels of Activating Transcription Factor 4 (ATF4) which is a stress-induced transcription factor, significantly decreased or lost in half of the MTC tumors. We also demonstrated that forced expression of ATF4 or TKIs-induced level of ATF4 decreases survival of MTC cells by promoting degradation of RET, blocking the activation of RET downstream signaling pathways and subsequently induces apoptosis. More importantly, ATF4 knockdown by shRNA decreased the sensitivity to tyrosine kinase inhibitor-induced apoptosis. Further thyroid gland of Atf4-knockout mice showed C-cell hyperplasia, a precancerous lesion for MTC suggesting a potential tumor suppressor role of ATF4 in MTC. Stress-induced by the accumulation of unfolded proteins in the endoplasmic reticulum (ER) is a feature of specialized secretory cells, including MTC cells that secrete calcitonin. ATF4 promotes the induction of apoptosis under persistent stress conditions, although the mechanism is not clearly understood. We hypothesize that the combination of ATF4 inducer and TKIs causes an excessive cellular oxidative stress resulting in an activation of apoptosis, all of which may prevent resistance to TKIs. Here, we show that the ER-associated protein degradation (ERAD) inhibitor, eeyarestatin sensitizes MTC cells to the TKIs including sunitinib and vandetanib. The combination of eeyarestatin and TKIs causes a synergistic induction of ATF4 expression and its target genes, an accumulation of reactive oxygen species and subsequent cell death. Chromatin immunoprecipitation followed by sequencing assay (ChIP-seq) in MTC cells treated with eeyarestatin and immunoprecipitated with ATF4 and acetylated lysine 9 of histone 3, identified transcription Kruppel-like factor 9 (KLF9) gene as a transcriptional target of ATF4 activation. KLF9 plays a functional role in oxidative stress-induced cell death. Treatment with eeyarestatin and vandetanib alone or in combination increases the occupancy of ATF4 at the promoter of KLF9 gene and stimulates the expression of KLF9. Depletion of ATF4 by shRNA leads to downregulation of KLF9 expression and prevents oxidative stress-induced cell death. These findings suggest that induction of the ATF4/KLF9 axis causes oxidative stress leading to excessive ER stress, and subsequent cell death. Thus, combining TKIs and ERAD inhibitors that promote ATF4 levels could be effective therapeutic strategies for treating MTC and preventing resistance to TKIs. Citation Format: Rozita Bagheri-Yarmand, Krishna M. Sinha, Ling Li, Yue Lu, Robert F. Gagel. Tyrosine kinase and ERAD inhibitors promote oxidative stress-induced apoptosis through activation of ATF4 / KLF9 axis in medullary thyroid cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2426.