Autophagy is a lysosome-mediated degradation pathway that enables the degradation and recycling of cytoplasmic components to sustain metabolic homoeostasis. Recently, autophagy has been reported to have an astonishing number of connections to cancer, as tumor cells require proficient autophagy in response to metabolic and therapeutic stresses to sustain cell proliferation. Autophagy-related gene 4 (ATG4) is essential for autophagy by affecting autophagosome formation through processing full-length microtubule-associated protein 1A/1B-light chain 3 (pro-LC3) and lipidated LC3. An increasing amount of evidence suggests that ATG4B expression is elevated in certain types of cancer, implying that ATG4B is a potential anticancer target. In this review, we address the central roles of ATG4B in the autophagy machinery and in targeted cancer therapy. Specifically, we discuss how pharmacologically inhibiting ATG4B can benefit cancer therapies.

Autophagy inhibition has been proposed to be a potential therapeutic strategy for cancer, however, few autophagy inhibitors have been developed. Recent studies have indicated that lysosome and autophagy related 4B cysteine peptidase (ATG4B) are two promising targets in autophagy for cancer therapy. Although some inhibitors of either lysosome or ATG4B were reported, there are limitations in the use of these single target compounds. Considering multi-functional drugs have advantages, such as high efficacy and low toxicity, we first screened and validated a batch of compounds designed and synthesized in our laboratory by combining the screening method of ATG4B inhibitors and the identification method of lysosome inhibitors. ATG4B activity was effectively inhibited in vitro. Moreover, 163N inhibited autophagic flux and caused the accumulation of autolysosomes. Further studies demonstrated that 163N could not affect the autophagosome-lysosome fusion but could cause lysosome dysfunction. In addition, 163N diminished tumor cell viability and impaired the development of colorectal cancer in vivo. The current study findings indicate that the dual effect inhibitor 163N offers an attractive new anti-cancer drug and compounds having a combination of lysosome inhibition and ATG4B inhibition are a promising therapeutic strategy for colorectal cancer therapy.

Autophagy is a highly- evolutionarily-conserved catabolic pathway activated by various cellular stresses. Recently, non-canonical autophagy (NCA), which does not require all of the ATG proteins to form autophagosome or autophagosome-like structures, has been found in various conditions. Moreover, mounting evidence has indicated that non-canonical LC3 lipidation (NCLL) may reflect NCA. We and others have reported that niclosamide (Nic), an anti-helminthic drug approved by the Food and Drug Administration, could induce canonical autophagy via a feedback downregulation of mTOR complex 1. In this study, we found that Nic could also induce NCLL, which is independent of the ULK1 complex and Beclin 1 complex, but dependent on ubiquitin-like conjugation systems. Although bafilomycin A1 and concanamycin A, two known V-ATPase inhibitors, significantly inhibited Nic-induced NCLL, Nic-induced NCLL was demonstrated to be independent of V-ATPase. In addition, the Golgi complex and vimentin were involved in Nic-induced NCLL, which might be a platform or membrane source for Nic-induced LC3-positive structures. These results would be helpful to broaden our understanding of the working mechanisms of Nic and evaluate its pharmacological activities in diseases.

Toosendanin (TSN) is a triterpenoid from the fruit or bark of Melia toosendan Sieb et Zucc, which has clear antitumor and insecticidal activities, but it possesses limiting hepatotoxicity in clinical application. Autophagy is a degradation and recycling mechanism to maintain cellular homeostasis, and it also plays an essential role in TSN-induced hepatotoxicity. Nevertheless, the specific mechanism of TSN on autophagy-related hepatotoxicity is still unknown. The hepatotoxicity of TSN in vivo and in vitro was explored in this study. It was found that TSN induced the upregulation of the autophagy-marker microtubule-associated proteins 1A/1B light chain 3B (LC3B) and P62, the accumulation of autolysosomes, and the inhibition of autophagic flux. The middle and late stages of autophagy were mainly studied. The data showed that TSN did not affect the fusion of autophagosomes and lysosomes but significantly inhibited the acidity, the degradation capacity of lysosomes, and the expression of hydrolase cathepsin B (CTSB). The activation of autophagy could alleviate TSN-induced hepatocyte damage. TSN inhibited the expression of transcription factor EB (TFEB), which is a key transcription factor for many genes of autophagy and lysosomes, such as CTSB, and overexpression of TFEB alleviated the autophagic flux blockade caused by TSN. In summary, TSN caused hepatotoxicity by inhibiting TFEB-lysosome-mediated autophagic flux and activating autophagy by rapamycin (Rapa), which could effectively alleviate TSN-induced hepatotoxicity, indicating that targeting autophagy is a new strategy to intervene in the hepatotoxicity of TSN.

As a structural and catalytic cofactor, copper is involved in many biological pathways and is required for the biochemistry of all living organisms. However, excess intracellular copper can induce cell death due to its potential to catalyze the generation of reactive oxygen species, thus copper homeostasis is strictly regulated. And the deficiency or accumulation of intracellular copper is connected with various pathological conditions. Since the success of platinum-based compounds in the clinical treatment of various types of neoplasias, metal-based drugs have shown encouraging perspectives for drug development. Compared to platinum, copper is an essential intracellular trace element that may have better prospects for drug development than platinum. Recently, the potential therapeutic role of copper-induced autophagy in chronic diseases such as Parkinson’s, Wilson’s, and cardiovascular disease has already been demonstrated. In brief, copper ions, numerous copper complexes, and copper-based nano-preparations could induce autophagy, a lysosome-dependent process that plays an important role in various human diseases. In this review, we not only focus on the current advances in elucidating the mechanisms of copper or copper-based compounds/preparations on the regulation of autophagy but also outline the association between copper-induced autophagy and human diseases.