Autophagy and cancer therapy

Abstract

Autophagy is an intracellular degradation process that sequesters cytoplasmic components in double-membrane vesicles known as autophagosomes, which are degraded upon fusion with lysosomes. This pathway maintains the integrity of proteins and organelles while providing energy and nutrients to cells, particularly under nutrient deprivation. Deregulation of autophagy can cause genomic instability, low protein quality, and DNA damage, all of which can contribute to cancer. Autophagy can also be overactivated in cancer cells to aid in cancer cell survival and drug resistance. Emerging evidence indicates that autophagy has functions beyond cargo degradation, including roles in tumor immunity and cancer stem cell survival. Additionally, autophagy can also influence the tumor microenvironment. This feature warrants further investigation of the role of autophagy in cancer, in which autophagy manipulation can improve cancer therapies, including cancer immunotherapy. This review discusses recent findings on the regulation of autophagy and its role in cancer therapy and drug resistance.

Keywords: Autophagy; Cancer; Resistance; Therapy.

Figure 1:. An overview of autophagy.

A) Autophagy begins in cells when they are deprived of nutrients. AMPK is activated and inhibits the autophagy inhibitor proteins TSC1/2 and mTOR/RAPTOR. B) Phagophore elongation occurs and ATG13 is activated, forming the ULK1 complex with ATG101, FIP200 and ULK1. C) The ULK1 complex activates the VPS34 complex, which includes VPS34, Beclin-1, AMBRA1, VPS15, ATG14L, and UVRAG. D) The VPS34 complex produces PI3P and recruits WIPI2B and DFCP1 to the phagophore. E) The ATG16L1-ATG5-ATG12 complex then promotes autophagosome maturation. This complex matures further through the interaction of ATG12 with both ATG5 and ATG16L1, which is facilitated by ATG9. F) The ATG8 system promotes the absorption of cargo into the autophagosome as it matures. This mechanism involves LC-I being cleaved by ATG4 into LC-II, which is then cleaved into LC-III by ATG3 and ATG7. G-I) Following that, the autophagosome forms and merges with a lysosome to form the autophagolysosome with the help of various proteins. Among these proteins are p62/SQSTM1, NRB1, RAB7, LCP B/D, LAMP 1/2 and Presenilin. J) Autophagy culminates in cargo degradation.

Figure 2:. Autophagy and tumor immunity.

Autophagy can contribute to tumor immunity. MHC-1 is degraded by autophagy and its expression can be decreased by inhibiting IFN-γ with ATG7 or LKB1 with ULK1. Chemokines such as CCL5 can also help with tumor immunity; however, Beclin-1 can reduce their levels. Autophagy can also upregulate PD-L1. p62/SQSTM1 and LC3 can activate the NF-κB pathway, resulting in increased expression of PD-L1. miRNA-3127-5p can also increase PD-L1 expression by promoting STAT3 phosphorylation and reducing autophagosome formation, while FIP200 does so by decreasing CXCL9 and CXCL10. ATG7 decreases FOXO3, which reduces miR-145 and increases PD-L1. The inhibition of VPS34 inhibition also increases the expression of PD-L1. In contrast, ATG5 and Beclin-1 (not shown) can reduce PD-L1 by promoting the degradation of HDAC proteins. Red indicates autophagy proteins.

Figure 3:. Regulation of autophagy in cancer stem cells.

Autophagy can be activated through various mechanisms, triggering multiple cellular responses. Hypoxia induces HIF-α, which activates ATG4 and BNIP3/BNIP3L. This process leads to the dissociation of Beclin-1 from Bcl-2, thus promoting autophagy. HIF-α also enhances ATG5, facilitating activation of LC3 through NRF-mediated Notch signaling. Reactive oxygen species (ROS) stimulate autophagy by activating LC3, AMPK, and Beclin-1. Additionally, TNF-α promotes autophagy through ERK signaling and mTOR inhibition. mTOR can be inhibited by nutrient deprivation, which activates AMPK. Proteins such as FIP200 and ULK1/2 significantly promote autophagy in cancer stem cells (CSC). Several miRNAs regulate autophagy in CSCs. For example, miR-200b inhibits autophagy by suppressing RAB37-mediated activation of ATG5, while IL-6 can inhibit autophagy by upregulating has-miR-486-3p, targeting and inhibiting ULK1/2. JAK2/STAT3 can upregulate IL-6 after autophagy activation, mediated by ATG7 and Beclin-1. Subsequently, IL-6 may inhibit autophagy by suppressing LC3 or up-regulating ARH-I, resulting in inhibition of Beclin-1. In some contexts, IL-6 can also activate Beclin-1. Other miRNAs, such as miR-24-2 and miR-675, promote autophagy. In particular, miR-24-2 activates Beclin-1, ATG4A, ATG5 and LC3. Following this, LC3 activates Cyclin D, which enhances cell growth and supports autophagy. Ultimately, activation of autophagy in CSCs promotes cell growth while inhibiting apoptosis.

Figure 4:. Relationship between autophagy and the tumor microenvironment.

A) Various factors within the tumor microenvironment, such as hypoxia, reactive oxygen species (ROS), stromal cells, nutrient deprivation, and inflammation, promote autophagy. Key proteins involved in this process include ATG5, secretion of IL-6 and IL-8, and inhibition of mTOR. B) Autophagy can also regulate the immunogenicity of the TME either by initiating or suppressing an immune response. The expression of MHC-1 expression can be reduced by LC3. PD-L1 expression can be reduced by overexpression of ATG7. PD-L1 expression can also be activated by a decrease in FOXO3a/miR-145 expression, p62/SQSTM1 accumulation, and NF-κB activation.

Figure 5:. Targeting autophagy in cancer therapy.

Metformin-mediated activation of AMPK can induce autophagy. However, the initiation of autophagy can be suppressed by mTOR inhibitors such as rapamycin, temsirolimus, everolimus, ridaforolimus, and torin-1. Suppressing the ULK1 and VPS34 complexes can limit the autophagy phagophore elongation step. Drugs targeting the ULK1 complex include SBI-0206965, SBP-7455, MRT67307, MRT68921, PF-03814735, GW837331X, and GW406108X. Drugs that target the VPS34 complex include VPS34-IN1, SB02024, SAR405, and 3-methyladenine. Bafilomycin A1, chloroquine, hydroxychloroquine, lys01, and lys05 can all inhibit the fusion of the lysosome with the autophagosome by acting as lysosomal alkalizers, resulting in less cargo degradation.