ATF6: A Double-Edged Sword in Lysosomal Regulation and Mutant TP53 Degradation in Cancer Cells

The relationship between autophagy, the unfolded protein response (UPR), and cancer is a fascinating area of research, especially when you think about how these processes keep cells functioning under stress. Cancer cells are under constant pressure because they multiply so quickly, pushing their cellular machinery—like the endoplasmic reticulum (ER)—to the brink. This kind of stress leads to a buildup of misfolded proteins, which then activates the UPR. The UPR is basically the cell’s way of dealing with this chaos, trying to restore balance or, if that fails, signaling the cell to self-destruct. Interestingly, while the UPR generally helps cells survive tough conditions, in cancer cells, it can be a weak spot that scientists are eager to target for treatments. Among the UPR’s three main branches, ATF6 (activating transcription factor 6) has drawn attention as an underexplored player, especially because of its role in regulating lysosomes and autophagy. Autophagy is like the cell’s cleanup crew, it breaks down and recycle unwanted materials to maintain balance. Cancer cells, being the opportunists they are, hijack this process to survive the harsh environments they create. Two major types of autophagy—macroautophagy and chaperone-mediated autophagy (CMA)—both rely on lysosomes, which are essentially the cell’s waste disposal units. These lysosomes need to function properly for autophagy to work, but we still do not know enough about how they are kept in good shape, especially under ER stress. ATF6 seems to play a big role here which make it an exciting focus for deeper investigation. One of the biggest challenges in cancer research is figuring out how to handle mutant TP53 (MUT TP53), a gene mutation found in many cancers. Normally, TP53 acts as a tumor suppressor, but when mutated, it not only loses this ability but also gains harmful properties that help tumors grow and resist treatment. Recent studies have hinted that lysosomes rather than proteasomes are key to breaking down MUT TP53. However, the exact mechanisms and how the UPR influences this process are still unclear. In a new study published in Autophagy Journal and conducted by Rossella Benedetti, Maria Anele Romeo, Andrea Arena, Maria Saveria Gilardini Montani, & Dr. Mara Cirone from Sapienza” University of Rome alongside Gabriella D’Orazi from the University “G. D’Annunzio” in Italy investigated how ATF6 supports lysosomal function during ER stress and affects MUT TP53 degradation. Their findings are critical for designing therapies, as targeting autophagy or UPR pathways could unintentionally stabilize MUT TP53, potentially doing more harm than good.

The research team started by testing two well-known ER stressors, thapsigargin (TG) and tunicamycin (TN), on colon cancer cell lines carrying common TP53 mutations. These stressors are known to activate the UPR. After treating the cells, they noticed a significant drop in the levels of MUT TP53 protein, suggesting that ER stress plays a role in breaking it down. To figure out how this happens, the researchers blocked each of the three UPR pathways individually. They discovered that only when ATF6 was inhibited did MUT TP53 levels stop decreasing, highlighting ATF6’s importance in this process. Moreover, the authors found that lysosomal activity, not proteasomal pathways, was central to clearing MUT TP53 and they confirmed this by using ammonium chloride which disrupts lysosome function. When they treated cells with TG and ammonium chloride together, the degradation of MUT TP53 was halted. Further tests using fluorescent dyes to measure lysosomal acidity revealed that ATF6 inhibition reduced the acidity of lysosomes. Since acidity is critical for their function, this showed that ATF6 helps lysosomes work properly during ER stress, allowing MUT TP53 to be degraded. To explore how autophagy contributes, the team focused on two pathways: macroautophagy and CMA. By silencing genes essential to these processes—ATG5 for macroautophagy and LAMP2A for CMA—they demonstrated that both played roles in degrading MUT TP53 during TN treatment. However, during TG treatment, macroautophagy was suppressed, leaving CMA as the dominant pathway for breaking down MUT TP53. This revealed how cancer cells adapt to stress by using different autophagic mechanisms based on the situation. One of the study’s key findings was the effect of ATF6 inhibition on lysosomal regulators. Western blot analysis showed that blocking ATF6 led to a decrease in TFEB, a master regulator of lysosome function. At the same time, mTOR, a kinase that suppresses autophagy, became more active. These changes created a feedback loop where lysosomal dysfunction stabilized MUT TP53, further disrupting autophagic processes. In cells with MUT TP53, this vicious cycle could make cancer cells even harder to treat. Furthermore, the researchers demonstrated how ATF6 directly supports lysosomal health. Overexpressing a cleaved form of ATF6 improved lysosomal function and sped up MUT TP53 breakdown, while silencing ATF6 had the opposite effect. Immunofluorescence imaging confirmed that MUT TP53 localized to lysosomes in TG-treated cells, providing clear visual evidence of its lysosomal degradation pathway.

In conclusion, the Italian scientists revealed the complex relationship between ATF6, lysosomal function, and autophagy in cancer, especially when it comes to breaking down MUT TP53. Moreover, the authors highlighted that while ATF6 inhibitors might initially seem like a good way to disrupt cancer cell survival, their findings also showed blocking ATF6 could stabilize MUT TP53 and by this potentially activate pathways that help tumors survive. This paradoxical effect could undermine the very treatments meant to kill the cancer. As a result, any therapy targeting ATF6 needs to be carefully adapted to the tumor’s genetic makeup, especially regarding its TP53 mutation status. The dual role of autophagy in cancer is another important point raised. On one hand, autophagy helps cancer cells survive by cleaning up cellular debris during stress. On the other, it can be used to break down harmful proteins like MUT TP53. By uncovering how macroautophagy and CMA contribute to this process, the study opens the door for more precise therapies. For example, enhancing CMA in specific cases might lower MUT TP53 levels without accidentally helping the tumor in other ways. Additionally, the role of ATF6 in balancing these outcomes teach us the need for therapies that hit the right targets without unintentionally aiding cancer survival.