NOXA a member of the pro apoptotic
NOXA, a member of the pro-apoptotic B-cell lymphoma 2 (BCL-2) family, plays a vital role in regulating mitochondrial or inherent apoptotic pathways [7,8]. NOXA binds to MCL-1 with high affinity and interacts with A1 (known as BCL2A1), resulting in BIM release to trigger subsequent apoptosis . In addition, NOXA induction by the activation of oncogenic RAS promotes autophagic cell death through the displacement of MCL-1 from Beclin-1, a key component of the class III PI3K complex required for autophagosome formation . Thus, NOXA induces both apoptotic and autophagic cell death via its interaction with MCL-1; this finding suggests that its levels need to be tightly controlled. It is known that NOXA is regulated at both the transcriptional level and the post-translational level. Specifically, it is transcriptionally modulated by various transcription factors, such as c-MYC, HIF1α and p53, in a stimulus-dependent manner [11,12]. Moreover, NOXA is degraded by the 26S proteasome via K11-linked poly-ubiquitination [13,14] or a ubiquitin-independent pathway . However, whether and how NOXA is degraded during autophagy and whether NOXA serves as a bridge in the crosstalk between autophagy and apoptosis are totally unknown.
Materials and methods
Discussion Autophagy and apoptosis play vital roles in determining cell fate under various physiological and pathological conditions. Accumulating evidence suggests that autophagy mainly serves as a cell survival pathway to suppress apoptosis [, , ]. Thus, inhibiting autophagy may sensitize cancer omecamtiv synthesis to apoptosis inducers or overcome their drug resistance. The combination of anticancer drugs with autophagy inhibitors, such as chloroquine, is an attractive strategy for cancer therapy. However, in some cellular settings, autophagy may degrade cellular components so that the cell eventually undergoes apoptosis; additionally, autophagy may excessively degrade the cytoplasm, leading to autophagic cell death. Apoptotic signaling, in turn, serves to inhibit autophagy. Therefore, the crosstalk between autophagy and apoptosis is complex and sometimes paradoxical [, , ]. Elucidating the mechanisms of their crosstalk could facilitate the control of the balance between cell survival and death, which would help determine the utility of targeting autophagy for treating cancer and other diseases. To investigate the molecular mechanism of the crosstalk between autophagy and apoptosis, we blocked autophagic flux using small molecular autophagy inhibitors; we found that the levels of NOXA, an apoptosis inducer, are obviously increased in dose- and time-dependent manners, and increased NOXA levels are responsible for the induction of apoptosis caused by autophagy inhibition. These findings were proven in all tested human lung cancer cell lines, regardless of their basal levels of NOXA, as well as human colon cell cancer lines; these findings indicate that this phenomenon is general (Fig. 1, Fig. 5, S1, and S2), which needs to be further proved in primary human or mouse cells to extend its applicability. Interestingly, NOXA itself can promote autophagy . Thus, a negative feedback loop might form to avoid excessive autophagy: NOXA is an autophagy inducer as well as an autophagy target. Moreover, NOXA is a well-characterized target gene of p53, which is activated by inhibiting autophagy. Thus, the transcriptional induction of NOXA by p53 enhances the induction of apoptosis (Fig. 5). Autophagy controls cellular quality by catabolizing approximately 1%–1.5% of cellular proteins per hour in nonselective and selective manners. p62 is one of the best-characterized substrates of selective autophagy. Given that p62 contains a ubiquitin-associated (UBA) domain, it is proposed to be a selective autophagy receptor for degrading ubiquitinated proteins [27,29]. Indeed, our mechanistic study revealed that NOXA is hijacked by p62, and the three lysine residues at the C-terminus of NOXA, where ubiquitin molecules may be attached, are required for autophagic degradation (Fig. 3). Further evidence of the following would help to elucidate the mechanism of autophagic NOXA degradation: 1) NOXA binding with p62 is enhanced by the induction of autophagy but abrogated when the three lysine residues at the C-terminus are mutated to arginine residues; and 2) NOXA polyubiquitination is required for its binding to p62.