Autophagy inhibitors for cancer therapy: Small molecules and nanomedicines

Autophagy is a conserved process in which the cytosolic materials are degraded and eventually recycled for cellular metabolism to maintain homeostasis. The dichotomous role of autophagy in pathogenesis is complicated. Accumulating reports have suggested that cytoprotective autophagy is responsible for tumor growth and progression. Autophagy inhibitors, such as chloroquine (CQ) and hydroxychloroquine (HCQ), are promising for treating malignancies or overcoming drug resistance in chemotherapy. With the rapid development of nanotechnology, nanomaterials also show autophagy-inhibitory effects or are reported as the carriers delivering autophagy inhibitors. In this review, we summarize the small-molecule compounds and nanomaterials inhibiting autophagic flux as well as the mechanisms involved. The nanocarrier-based drug delivery systems for autophagy inhibitors and their distinct advantages are also described. The progress of autophagy inhibitors for clinical applications is finally introduced, and their future perspectives are discussed.

Lipophagy deficiency exacerbates ectopic lipid accumulation and tubular cells injury in diabetic nephropathy

Autophagy-mediated lipotoxicity plays a critical role in the progression of diabetic nephropathy (DN), but the precise mechanism is not fully understood. Whether lipophagy, a selective type of autophagy participates in renal ectopic lipid deposition (ELD) and lipotoxicity in the kidney of DN is unknown. Here, decreased lipophagy, increased ELD and lipotoxcity were observed in tubular cells of patients with DN, which were accompanied with reduced expression of AdipoR1 and p-AMPK. Similar results were found in db/db mice, these changes were reversed by AdipoRon, an adiponectin receptor activator that promotes autophagy. Additionally, a significantly decreased level of lipophagy was observed in HK-2 cells, a human proximal tubular cell line treated with high glucose, which was consistent with increased lipid deposition, apoptosis and fibrosis, while were partially alleviated by AdipoRon. However, these effects were abolished by pretreatment with ULK1 inhibitor SBI-0206965, autophagy inhibitor chloroquine and enhanced by AMPK activator AICAR. These data suggested by the first time that autophagy-mediated lipophagy deficiency plays a critical role in the ELD and lipid-related renal injury of DN.

Investigation of the specificity and mechanism of action of the ULK1/AMPK inhibitor SBI-0206965

SBI-0206965, originally identified as an inhibitor of the autophagy initiator kinase ULK1, has recently been reported as a more potent and selective AMP-activated protein kinase (AMPK) inhibitor relative to the widely used, but promiscuous inhibitor Compound C/Dorsomorphin. Here, we studied the effects of SBI-0206965 on AMPK signalling and metabolic readouts in multiple cell types, including hepatocytes, skeletal muscle cells and adipocytes. We observed SBI-0206965 dose dependently attenuated AMPK activator (991)-stimulated ACC phosphorylation and inhibition of lipogenesis in hepatocytes. SBI-0206965 (≥25 μM) modestly inhibited AMPK signalling in C2C12 myotubes, but also inhibited insulin signalling, insulin-mediated/AMPK-independent glucose uptake, and AICA-riboside uptake. We performed an extended screen of SBI-0206965 against a panel of 140 human protein kinases in vitro, which showed SBI-0206965 inhibits several kinases, including members of AMPK-related kinases (NUAK1, MARK3/4), equally or more potently than AMPK or ULK1. This screen, together with molecular modelling, revealed that most SBI-0206965-sensitive kinases contain a large gatekeeper residue with a preference for methionine at this position. We observed that mutation of the gatekeeper methionine to a smaller side chain amino acid (threonine) rendered AMPK and ULK1 resistant to SBI-0206965 inhibition. These results demonstrate that although SBI-0206965 has utility for delineating AMPK or ULK1 signalling and cellular functions, the compound potently inhibits several other kinases and critical cellular functions such as glucose and nucleoside uptake. Our study demonstrates a role for the gatekeeper residue as a determinant of the inhibitor sensitivity and inhibitor-resistant mutant forms could be exploited as potential controls to probe specific cellular effects of SBI-0206965.

Microglial PGC-1α protects against ischemic brain injury by suppressing neuroinflammation

BACKGROUND

Neuroinflammation and immune responses occurring minutes to hours after stroke are associated with brain injury after acute ischemic stroke (AIS). PPARγ coactivator-1α (PGC-1α), as a master coregulator of gene expression in mitochondrial biogenesis, was found to be transiently upregulated in microglia after AIS. However, the role of microglial PGC-1α in poststroke immune modulation remains unknown.

METHODS

PGC-1α expression in microglia from human and mouse brain samples following ischemic stroke was first determined. Subsequently, we employed transgenic mice with microglia-specific overexpression of PGC-1α for middle cerebral artery occlusion (MCAO). The morphology and gene expression profile of microglia with PGC-1α overexpression were evaluated. Downstream inflammatory cytokine production and NLRP3 activation were also determined. ChIP-Seq analysis was performed to detect PGC-1α-binding sites in microglia. Autophagic and mitophagic activity was further monitored by immunofluorescence staining. Unc-51-like autophagy activating kinase 1 (ULK1) expression was evaluated under the PGC-1α interaction with ERRα. Finally, pharmacological inhibition and genomic knockdown of ULK1 were performed to estimate the role of ULK1 in mediating mitophagic activity after ischemic stroke.

RESULTS

PGC-1α expression was shortly increased after ischemic stroke, not only in human brain samples but also in mouse brain samples. Microglia-specific PGC-1α overexpressing mice exhibited significantly decreased neurologic deficits after ischemic injury, with reduced NLRP3 activation and proinflammatory cytokine production. ChIP-Seq analysis and KEGG pathway analysis revealed that mitophagy was significantly enhanced. PGC-1α significantly promoted autophagic flux and induced autolysosome formation. More specifically, the autophagic clearance of mitochondria was enhanced by PGC-1α regulation, indicating the important role of mitophagy. Pharmacological inhibition or knockdown of ULK1 expression impaired autophagic/mitophagic activity, thus abolishing the neuroprotective effects of PGC-1α.

CONCLUSIONS

Mechanistically, in AIS, PGC-1α promotes autophagy and mitophagy through ULK1 and reduces NLRP3 activation. Our findings indicate that microglial PGC-1α may be a promising therapeutic target for AIS.

LAP-like non-canonical autophagy and evolution of endocytic vacuoles in pancreatic acinar cells

Activation of trypsinogen (formation of trypsin) inside the pancreas is an early pathological event in the development of acute pancreatitis. In our previous studies we identified the activation of trypsinogen within endocytic vacuoles (EVs), cellular organelles that appear in pancreatic acinar cells treated with the inducers of acute pancreatitis. EVs are formed as a result of aberrant compound exocytosis and subsequent internalization of post-exocytic structures. These organelles can be up to 12 μm in diameter and can be actinated (i.e. coated with F-actin). Notably, EVs can undergo intracellular rupture and fusion with the plasma membrane, providing trypsin with access to cytoplasmic and extracellular targets. Unraveling the mechanisms involved in cellular processing of EVs is an interesting cell biological challenge with potential benefits for understanding acute pancreatitis. In this study we have investigated autophagy of EVs and discovered that it involves a non-canonical LC3-conjugation mechanism, reminiscent in its properties to LC3-associated phagocytosis (LAP); in both processes LC3 was recruited to single, outer organellar membranes. Trypsinogen activation peptide was observed in approximately 55% of LC3-coated EVs indicating the relevance of the described process to the early cellular events of acute pancreatitis. We also investigated relationships between actination and non-canonical autophagy of EVs and concluded that these processes represent sequential steps in the evolution of EVs. Our study expands the known roles of LAP and indicates that, in addition to its well-established functions in phagocytosis and macropinocytosis, LAP is also involved in the processing of post-exocytic organelles in exocrine secretory cells.

ABBREVIATIONS

AP: acute pancreatitis; CCK: cholecystokinin; CLEM: correlative light and electron microscopy; DPI: diphenyleneiodonium; EV: endocytic vacuole; LAP: LC3-associate phagocytosis; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; PACs: pancreatic acinar cells; PFA: paraformaldehyde; PtdIns3K: phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol 3-phosphate; Res: resveratrol; TAP: trypsinogen activation peptide; TEM: transmission electron microscopy; TLC-S: taurolithocholic acid 3-sulfate; TRD: Dextran Texas Red 3000 MW Neutral; ZGs: zymogen granules.

The ULK1/2 and AMPK Inhibitor SBI-0206965 Blocks AICAR and Insulin-Stimulated Glucose Transport

The small molecule kinase inhibitor SBI-0206965 was originally described as a specific inhibitor of ULK1/2. More recently, it was reported to effectively inhibit AMPK and several studies now report its use as an AMPK inhibitor. Currently, we investigated the specificity of SBI-0206965 in incubated mouse skeletal muscle, measuring the effect on analog 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR)-stimulated AMPK-dependent glucose transport and insulin-stimulated AMPK-independent glucose uptake. Pre-treatment with 10 µM SBI-0206965 for 50 min potently suppressed AICAR-stimulated glucose transport in both the extensor digitorum longus (EDL) and soleus muscle. This was despite only a modest lowering of AICAR-stimulated AMPK activation measured as ACC2 Ser212, while ULK1/2 Ser555 phosphorylation was prevented. Insulin-stimulated glucose transport was also potently inhibited by SBI-0206965 in soleus. No major changes were observed on insulin-stimulated cell signaling. No general effect of SBI-0206965 on intracellular membrane morphology was observed by transmission electron microscopy. As insulin is known to neither activate AMPK nor require AMPK to stimulate glucose transport, and insulin inhibits ULK1/2 activity, these data strongly suggest that SBI-0206965 has a non-specific off-target inhibitory effect on muscle glucose transport. Thus, SBI-0206965 is not a specific inhibitor of the AMPK/ULK-signaling axis in skeletal muscle, and data generated with this inhibitor must be interpreted with caution.

Lipid-modified cell-penetrating peptide-based self-assembly micelles for co-delivery of narciclasine and siULK1 in hepatocellular carcinoma therapy

Hepatocellular carcinoma (HCC) is the most frequent type of primary liver cancer, and one therapeutic approach is to target both the AMPK and autophagy pathways in order to synergistically promote programmed cell death. Here, a series of amphiphilic, lipid-modified cell-penetrating peptides were synthesized and allowed to self-assemble into micelles loaded with the AMPK activator narciclasine (Narc) and short interfering RNA targeting the unc-51-like kinase 1 (siULK1). The size of these micelles, their efficiency of transfection into cells, and their ability to release drug or siRNA cargo in vitro were pH-sensitive, such that drug release was facilitated in the acidic microenvironment of the tumor. Transfecting the micelles into HCC cells significantly inhibited protective autophagy within tumor cells, and delivering the micelles into mice carrying HCC xenografts induced apoptosis, slowed tumor growth, and inhibited autophagy. Our results indicate that co-delivering Narc and siULK1 in biocompatible micelles can safely inhibit tumor growth and protective autophagy, justifying further studies into this promising therapeutic approach against HCC.

STATEMENT OF SIGNIFICANCE

We have focused on the targeted therapy of HCC via synergistically inhibiting the autophagy and inducing apoptosis. The lipid-modified cell-penetrating peptide can not only aggregate into micelles to load natural product narciclasine and ULK1 siRNA simultaneously, but also facilitate uptake and endosome escape with a pH-sensitive manner in HepG2 cells. HepG2 cell treated with siULK1-M-Narc has increased apoptotic levels and declined autophagy via the targeted regulation of AMPK-ULK1 signaling axis. The in vivo studies have confirmed that siULK1-M-Narc efficiently reduce the growth of tumor on HCC xenograft models with good safety. Thus, we suppose the lipid-modified cell-penetrating peptide has good application prospects in the targeted combinational therapy of HCC.

Canonical and noncanonical functions of ULK/Atg1

Mammalian Unc-51-like kinases 1 and 2 (ULK1 and ULK2) belong to the ULK/Atg1 family of serine/threonine kinases, which are conserved from yeast to mammals. Although ULK/Atg1 is best known for regulating flux through the autophagy pathway, it has evolutionarily conserved noncanonical functions in protein trafficking that are essential for maintaining cellular homeostasis. As a direct target of energy- and nutrient-sensing kinases, ULK/Atg1 is positioned to regulate the distribution and use of cellular resources in response to metabolic cues. In this review, we provide an overview of the molecular mechanisms through which ULK/Atg1 carries out its canonical and noncanonical functions and the signaling pathways that link its function to metabolism. We also highlight potential contributions of ULK/Atg1 in human diseases, including cancer and neurodegeneration.

Co-targeting hexokinase 2-mediated Warburg effect and ULK1-dependent autophagy suppresses tumor growth of PTEN- and TP53-deficiency-driven castration-resistant prostate cancer

Currently, no therapeutic options exist for castration-resistant prostate cancer (CRPC) patients who have developed resistance to the second generation anti-androgen receptor (AR) axis therapy. Here we report that co-deletion of Pten and p53 in murine prostate epithelium, often observed in human CRPC, leads to AR-independent CRPC and thus confers de novo resistance to second generation androgen deprivation therapy (ADT) in multiple independent yet complementary preclinical mouse models. In contrast, mechanism-driven co-targeting hexokinase 2 (HK2)-mediated Warburg effect with 2-deoxyglucose (2-DG) and ULK1-dependent autophagy with chloroquine (CQ) selectively kills cancer cells through intrinsic apoptosis to cause tumor regression in xenograft, leads to a near-complete tumor suppression and remarkably extends survival in Pten-/p53-deficiency-driven CRPC mouse model. Mechanistically, 2-DG causes AMPK phosphorylation, which in turn inhibits mTORC1-S6K1 translation signaling to preferentially block anti-apoptotic protein MCL-l synthesis to prime mitochondria-dependent apoptosis while simultaneously activates ULK1-driven autophagy for cell survival to counteract the apoptotic action of anti-Warburg effect. Accordingly, inhibition of autophagy with CQ sensitizes cancer cells to apoptosis upon 2-DG challenge. Given that 2-DG is recommended for phase II clinical trials for prostate cancer and CQ has been clinically used as an anti-malaria drug for many decades, the preclinical results from our proof-of-principle studies in vivo are imminently translatable to clinical trials to evaluate the therapeutic efficacy by the combination modality for a subset of currently incurable CRPC harboring PTEN and TP53 mutations.