p53-independent Noxa induction by cisplatin is regulated by ATF3/ATF4 in head and neck squamous cell carcinoma cells.

The platinum-based DNA damaging agent cisplatin is used as a standard therapy for locally advanced head and neck squamous cell carcinoma (HNSCC). However, the mechanisms underpinning the cytotoxic effects of this compound are not entirely elucidated. Cisplatin produces anticancer effects primarily via activation of the DNA damage response, followed by inducing BCL-2 family dependent mitochondrial apoptosis. We have previously demonstrated that cisplatin induces the expression of proapoptotic BCL-2 family protein, Noxa, that can bind to the prosurvival BCL-2 family protein, MCL-1, to inactivate its function and induce cell death. Here, we show that the upregulation of Noxa is critical for cisplatin-induced apoptosis in p53-null HNSCC cells. This induction is regulated at the transcriptional level. With a series of Noxa promoter-luciferase reporter assays, we find that the CRE (cAMP response element) in the promoter is critical for the Noxa induction by cisplatin treatment. Among the CREB/ATF transcription factors, ATF3 and ATF4 are induced by cisplatin, and downregulation of ATF3 or ATF4 reduced cisplatin-induced Noxa. ATF3 and ATF4 bind to and cooperatively activate the Noxa promoter. Furthermore, ERK1 is involved in cisplatin-induced ATF4 and Noxa induction. In conclusion, ATF3 and ATF4 are important regulators that induce Noxa by cisplatin treatment in a p53-independent manner.

DNA damaging agent-induced apoptosis is regulated by MCL-1 phosphorylation and degradation mediated by the Noxa/MCL-1/CDK2 complex.

Noxa, a BH3-only pro-apoptotic BCL-2 family protein, causes apoptosis by specifically interacting with the anti-apoptotic protein MCL-1 to induce its proteasome-mediated degradation. We show here that the DNA damaging agents cisplatin and etoposide upregulate Noxa expression, which is required for the phosphorylation of MCL-1 at Ser64/Thr70 sites, proteasome-dependent degradation, and apoptosis. Noxa-induced MCL-1 phosphorylation at these sites occurs at the mitochondria and is primarily regulated by CDK2. MCL-1 and CDK2 form a stable complex and Noxa binds to this complex to facilitate the phosphorylation of MCL-1. When Ser64 and Thr70 of MCL-1 are substituted with alanine, the mutated MCL-1 is neither phosphorylated nor ubiquitinated, and becomes more stable than the wild-type protein. As a consequence, this mutant can inhibit apoptosis induced by Noxa overexpression or cisplatin treatment. These results indicate that Noxa-mediated MCL-1 phosphorylation followed by MCL-1 degradation is critical for apoptosis induced by DNA damaging agents through regulation of the Noxa/MCL-1/CDK2 complex.