Targeting autophagy as a therapeutic strategy against pancreatic cancer

Abstract

Macroautophagy (hereafter autophagy) is a catabolic process through which cytosolic components are captured in the autophagosome and degraded in the lysosome. Autophagy plays two major roles: nutrient recycling under starvation or stress conditions and maintenance of cellular homeostasis by removing the damaged organelles or protein aggregates. In established cancer cells, autophagy-mediated nutrient recycling promotes tumor progression, whereas in normal/premalignant cells, autophagy suppresses tumor initiation by eliminating the oncogenic/harmful molecules. Pancreatic ductal adenocarcinoma (PDAC) is a deadly disease that is refractory to most currently available treatment modalities, including immune checkpoint blockade and molecular-targeted therapy. One prominent feature of PDAC is its constitutively active and elevated autophagy-lysosome function, which enables PDAC to thrive in its nutrient-scarce tumor microenvironment. In addition to metabolic support, autophagy promotes PDAC progression in a metabolism-independent manner by conferring resistance to therapeutic treatment or facilitating immune evasion. Besides to cell-autonomous autophagy in cancer cells, host autophagy (autophagy in non-cancer cells) supports PDAC progression, further highlighting autophagy as a promising therapeutic target in PDAC. Based on a growing list of compelling preclinical evidence, there are numerous ongoing clinical trials targeting the autophagy-lysosome pathway in PDAC. Given the multifaceted and context-dependent roles of autophagy in both cancer cells and normal host cells, a deeper understanding of the mechanisms underlying the tumor-promoting roles of autophagy as well as of the consequences of autophagy inhibition is necessary for the development of autophagy inhibition-based therapies against PDAC.

Keywords: Anti-tumor immunity; Autophagy; Host autophagy; Lysosome; PDAC.

Fig. 1

Pancreatic ductal adenocarcinoma (PDAC) relies on autophagy and macropinocytosis for nutrient scavenging. PDAC cells show elevated autophagy and macropinocytosis. Autophagy targets intracellular constituents, such as protein aggregates, damaged organelles, and lipids, whereas macropinocytosis enables bulk uptake of extracellular proteins, such as serum albumin or collagen, in the tumor microenvironment. Autophagy captures its cargo with double-membrane vesicles, termed autophagosomes, while macropinocytosis engulfs a portion of extracellular fluids and materials via invagination of the plasma membrane and formation of single-membrane vesicles, termed macropinosomes. Both autophagosomes and macropinosomes are fused with lysosomes for the degradation of cargo and recycling of nutrients. Inhibitors of these pathways are shown. EIPA 5'-(N-ethyl-N-isopropyl)amiloride, CQ chloroquine, HCQ hydroxychloroquine, BafA1 bafilomycin A1

Fig. 2

Overview of the general autophagy pathway in mammalian cells. (Bottom) Autophagy can be divided into five major steps: (1) Initiation and nucleation of the double-membrane phagophore, (2) elongation and (3) closure of the phagophore to form the autophagosome, (4) autophagosome-lysosome fusion, and (5) lysosomal degradation and nutrient recycling. (Top left) Autophagy induction is primarily mediated by the ULK1 complex, which is regulated by AMPK and mTORC1. Upon activation, the ULK1 complex activates the class III phosphatidylinositol 3-kinase (PI3K) complex through phosphorylation of beclin 1 (BECN1) and VPS34. The Class III PI3K complex generates PI3P at the site of nucleation of phagophore from ER. (Top middle) ProLC3B is converted to LC3B-I via the cleavage by ATG4B. LC3B-I is conjugated with PE through ubiquitin-like conjugation systems that include ATG7 (E1 ligase), ATG3 (E2 ligase), and ATG12, ATG5, and ATG16L (the E3 ligase complex). The resulting PE-conjugated LC3, which is called LC3B-II (shown as small green circles), is inserted on the phagophore membranes, where it facilitates phagophore elongation and closure. (Pale blue frames) Inhibitors of ULK1/2 (SBI-0206965 [139], MRT68921, MRT 67,307 [140], ULK101 [141]), VPS34 (VPS34-In1 [142], PIK-III [143], SAR405 [144], Compound 31 [145], Spautin1 [146]), ATG4B (S130 [147], FMK-9a [148], NSC185058 [149]), and the lysosome [CQ/HCQ, BafA1, Lys05 [150]]) are shown. ULK Unc-51-like kinase, PI3K phosphatidylinositol 3-kinase, PI3P phosphatidylinositol 3-phosphate, ER endoplasmic reticulum, WIPI WD-repeat protein interacting with phosphoinositide, mTORC1 mammalian target of rapamycin complex 1, AMPK 5' AMP-activated protein kinase, PE phosphatidylethanolamine, p62/SQSTM1 sequestosome 1, NBR1 neighbor of BRCA1

Fig. 3

Mechanisms of constitutively high autophagy-lysosome activity in PDAC. The MiT/TFE family transcription factors are critical regulators of autophagy-lysosome genes. (Left) Under nutrient-rich conditions, MiT/TFE proteins are repressed by mTORC1 via phosphorylation and remain in the cytosol. (Middle) Upon starvation, mTORC1 is inactivated and MiT/TFE proteins enter the nucleus, where they activate autophagy-lysosome gene transcription. (Right) PDAC cells overexpress the nucleocytoplasmic transporter, importin (IPO)-7/8, to facilitate the nuclear translocation of MiT/TFE, thereby decoupling MiT/TFE from mTORC1-mediated suppression and upregulating autophagy-lysosome gene expression. Consequently, PDAC maintains constitutively high basal autophagy/lysosome activity under sustained mTORC1 activity, thus simultaneously employing the catabolic process mediated by the autophagy/lysosomal pathway and the anabolic process driven by mTOR signaling to maximize their proliferation [64]

Fig. 4

Autophagy in non-cancer cells supports unique metabolic properties in PDAC. In response to stimuli from PDAC cells, PSCs become activated and secrete alanine in an autophagy-dependent manner. This PSC-derived alanine is taken up by PDAC cells and used to fuel the TCA cycle and biosynthesis of free fatty acids (FFAs)(red), allowing PDAC cells to use glucose and glutamine primarily to generate nucleic acids [69](blue) and NADPH for redox control [70](green). Ac-CoA acetyl-CoA, Ala alanine, aKG α-ketoglutarate, Asp aspartate, Cit citrate, GLUD1 glutamate dehydrogenase 1, GLS glutaminase, GLUT1/3 glucose transporter 1/3, GOT1/2 aspartate aminotransferase ½, GPT1/2 alanine aminotransferase ½, HBP hexosamine biosynthesis pathway, HK1/2 hexokinase ½, LDHA lactate dehydrogenase A, MDH1 malate dehydrogenase 1, ME1 malic enzyme 1, Mal malate, Non-ox PPP oxidative branch of pentose phosphate pathway, OAA oxaloacetic acid, ox-PPP oxidative branch of pentose phosphate pathway, PFK1 phosphofructokinase 1, Pyr pyruvate, RPE ribulose-phosphate 3-epimerase, RPIA ribose-5-phosphate isomerase, SLC1A5 solute carrier family 1 member 5, TCA tricarboxylic acid, UDP-GlcNAc uridine diphosphate N-acetylglucosamine

Fig. 5

Selective autophagy of MHC-I promotes immune evasion of PDAC. (Left) In normal cells, major histocompatibility complex class I (MHC-I) is localized on the plasma membrane, where it presents endogenous antigens to CD8.⁺ T cells. (Middle) In PDAC cells, MHC-I is actively targeted for lysosomal degradation through NBR1-mediated selective autophagy, leading to reduced MHC-I levels on the plasma membrane, thereby facilitating immune evasion. (Right) Importantly, autophagy or lysosome inhibition restores MHC-I expression, leading to enhanced anti-tumor T cell immunity and improved response to ICB [101]. ER endoplasmic reticulum