ATF4 Targets RET for Degradation and Is a Candidate Tumor Suppressor Gene in Medullary Thyroid Cancer.

Context: Medullary thyroid cancer (MTC) is an aggressive tumor that harbors activating mutations of the RET proto-oncogene. We previously reported that RET inhibits transcriptional activity of ATF4, the master regulator of the stress response pathway, to prevent cell death. Objective: We hypothesized that loss of function of ATF4 plays a role in initiation of MTC. Design: Targeted deletion of Atf4 in mice was used to assess ATF4 function in the thyroid gland. ATF4 overexpression was achieved by adenoviral and lentiviral vectors. We used immunohistochemical analysis and western blotting of MTC tumors to determine protein levels of RET and ATF4 and the Kaplan-Meier method to determine their association with clinical outcome. Results: Targeted deletion of Atf4 in mice causes C-cell hyperplasia, a precancerous lesion for MTC. Forced ATF4 expression decreased survival of MTC cells and blocked the activation of RET downstream signaling pathways (phosphorylated ERK, phosphorylated AKT, and p70S6K). ATF4 knockdown decreased sensitivity to tyrosine kinase inhibitor-induced apoptosis. Moreover, ATF4 expression decreased RET protein levels by promoting RET ubiquitination. We found decreased or loss of ATF4 in 52% of MTC tumors (n = 39) compared with normal thyroid follicle cells. A negative correlation was observed between RET and ATF4 protein levels in MTC tumors, and low ATF4 expression was associated with poor overall survival in patients with MTC. Conclusions: ATF4 was identified as a negative regulator of RET, a candidate tumor suppressor gene, and may be a molecular marker that distinguishes patients at high risk of MTC from those with a longer survival prognosis.