Background. Triple negative breast cancer (TNBC) is a very aggressive form of breast cancer which is characterized by a poor survival rate and high incidence of metastases. The integrated stress response (ISR) is activated under stress conditions (hypoxia, nutrient deprivation, or endoplasmic reticulum stress) and reduces the protein synthesis through phospho-eIF2α/ATF4 (activating transcription factor 4) to regulate cell fate. ATF4, which is overexpressed in breast cancer including TNBC, regulates tumor growth, autophagy, drug resistance, and metastasis during ISR through PERK and GCN2 pathways. We have reported enhanced ATF4 expression in unstressed MCF10A cells treated with TGFβ1. Here, we investigate the potential TGFβ-mediated stress-independent control of ATF4 activity and its impact on the TNBC-associated metastasis.
Methods. SUM159PT and BT549 TNBC cell lines were treated with human recombinant TGFβ1 (10 ng/ml) and the TGFBRI kinase inhibitor LY2157299 (5µM) for 72h. Effects of TGFβ on ATF4 expression were assessed by silencing SMAD2/3 and SMAD4 with TGFβ1 for 24h in SUM159PT and BT549 cells. Changes in ATF4 expression by IRS were assessed after PERK, GCN2, PKR, HRI, and eIF2α inhibition with TGFβ1 for 72h in SUM159PT, BT549 and MDA-MB-231 cell lines. ATF4 expression levels were determined by RT-PCR and/or western blot. Effects on TGFβ-induced metastasis were analyzed by ATF4 knockdown for 48h by using two different siRNA sequences (#1 and #2) following treatment with TGFβ1 for 24h. Migration and invasion were performed by wound healing and transwell assays, respectively. Results were compared to a scrambled siRNA as negative control (SCR).
Results. Our results show that ATF4 expression (at RNA and protein levels) was abrogated by LY2157299 upon TGFβ activation, suggesting that ATF4 is active downstream of TGFBRI. This result was supported by further SMAD2/3 and SMAD4 knockdown following treatment with TGFβ1, which was correlated with decreased ATF4 expression. Additionally, we assessed whether ATF4 inhibition can reduce the TGFβ-induced metastatic properties of TNCB cell lines. We found that ATF4 depletion inhibited migration and invasiveness in the three cell lines tested. ATF4 is known to exert a pro-metastatic role downstream of ISR through PERK and GCN2 pathways. Our results demonstrate that, upon treatment with TGFβ1, knockdown of the IRS mediators PERK, PKR, GCN2, HRI, and eIF2α did not correlate with a consistent decrease of ATF4 levels in the three cell lines.
Conclusion. In conclusion, our results show for the first time that ATF4 is a downstream target of the canonical TGFβ/SMAD pathway in a ISR-independent fashion, and its depletion correlates with an inhibition of the TGFβ1-mediated migration and invasion of TNBC cell lines. Therefore, ATF4 may represent a therapeutic target in TNBC patients with active TGFβ signaling pathway.
Citation Format: Jenny C. Chang, Alberto Ramirez, Maria P. Molina, Francisca E. Cara, Wei Qian, Wen Chen, Anthony J. Kozielski, Roberto R. Rosato, Juan A. Marchal, Jose A. Lorente, Pedro Sanchez-Rovira, Sergio Granados-Principal. Knockdown of TGFβ-activated ATF4 inhibits triple negative breast cancer metastases independently of cellular stress [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 2035.