Emerging Role of Autophagy in Governing Cellular Dormancy, Metabolic Functions, and Therapeutic Responses of Cancer Stem Cells

Abstract

Tumors are composed of heterogeneous populations of dysregulated cells that grow in specialized niches that support their growth and maintain their properties. Tumor heterogeneity and metastasis are among the major hindrances that exist while treating cancer patients, leading to poor clinical outcomes. Although the factors that determine tumor complexity remain largely unknown, several genotypic and phenotypic changes, including DNA mutations and metabolic reprograming provide cancer cells with a survival advantage over host cells and resistance to therapeutics. Furthermore, the presence of a specific population of cells within the tumor mass, commonly known as cancer stem cells (CSCs), is thought to initiate tumor formation, maintenance, resistance, and recurrence. Therefore, these CSCs have been investigated in detail recently as potential targets to treat cancer and prevent recurrence. Understanding the molecular mechanisms involved in CSC proliferation, self-renewal, and dormancy may provide important clues for developing effective therapeutic strategies. Autophagy, a catabolic process, has long been recognized to regulate various physiological and pathological processes. In addition to regulating cancer cells, recent studies have identified a critical role for autophagy in regulating CSC functions. Autophagy is activated under various adverse conditions and promotes cellular maintenance, survival, and even cell death. Thus, it is intriguing to address whether autophagy promotes or inhibits CSC functions and whether autophagy modulation can be used to regulate CSC functions, either alone or in combination. This review describes the roles of autophagy in the regulation of metabolic functions, proliferation and quiescence of CSCs, and its role during therapeutic stress. The review further highlights the autophagy-associated pathways that could be used to regulate CSCs. Overall, the present review will help to rationalize various translational approaches that involve autophagy-mediated modulation of CSCs in controlling cancer progression, metastasis, and recurrence.

Keywords: autophagy; cancer stem cells; chemotherapy; metabolic functions; mitophagy; quiescence; radiotherapy.

Figure 1

Diagrammatic representation of various models leading to carcinogenesis. (A). The stochastic or clonal evolution model, which suggests that normal cells undergo a series of mutations and transform into cancer cells that possess the capacity to form a bulk tumor. (B). The hierarchy model suggests that tumors originate from CSCs that are pluripotent and self-renewing. (C). The plasticity model suggests that differentiated cells can dedifferentiate into CSCs and that there is a dynamic interconversion between CSCs and non-CSCs, leading to tumor heterogeneity. (The template for figures was adapted from Biorender.com).

Figure 2

An overview of cancer progression. Stem cells or differentiated cells may undergo various types of mutations, including genetic mutations such as the p53 mutation, mitochondrial reprogramming, and/or autophagy/mitophagy alterations, to transform into cancer cells. Among these heterogeneous cancer subpopulations, CSCs are considered to be the main cause of tumor recurrence and drug resistance. (The template for figures was adapted from Biorender.com).

Figure 3

Types of autophagy based on substrates and regulatory proteins. (A). Macroautophagy, in which cellular components are degraded via the fusion of autophagosomes with lysosomes to form autolysosomes. (B). Microautophagy, in which cytosolic components are directly engulfed without the formation of vesicles. (C). Chaperone-mediated autophagy, in which a selectively targeted protein is attached to a specific KERQ sequence, recognized by Hsc70, and transported to the lumen of the lysosome by LAMP-2A. (Template for figures was adapted from Biorender.com).

Figure 4

Regulation of autophagy and its mechanisms. (A) Regulation—Autophagy is tightly regulated by several key signaling pathways involving mTOR, AMPK, and mTORC2. In conditions of energy depletion, AMPK becomes activated, leading to the phosphorylation of ULK1 at Ser-317 and Ser-777, thereby promoting autophagy. Conversely, when nutrients are abundant, mTOR inhibits autophagy by phosphorylating ULK1 at the Ser-757 residue, which hinders the interaction between ULK1 and AMPK. (B) Mechanism of regulation—The process of autophagosome formation begins with the activation of ULK1, which forms a preinitiation complex comprising ATG13 and FIP200. This complex subsequently recruits PI3KIII, initiating the formation of the Omegasome complex, which includes Beclin-1, UV-RAG, ATG14, and Ambra1 at the endoplasmic reticulum (ER), facilitated by ATG9. PI3KIII catalyzes the conversion of phosphatidylinositol to phosphatidylinositol-3-phosphate (PI3P), altering the lipid composition of the membrane. PI3P then recruits WIPI2B, facilitating autophagosome elongation. Autophagosome elongation involves two distinct ubiquitin-like conjugation pathways. ATG12 and ATG5 form a complex with the E1 ligase ATG7 and the E2 ligase ATG10, leading to the formation of the E3 complex. This E3 complex associates with the outer membrane of the phagophore. LC3-I, generated by the cleavage of LC3 by ATG4, is then conjugated to phosphatidylethanolamine (PE) to form LC3-II, a process mediated by the E1 ligase ATG7 and the E3 complex. LC3-II is recruited to both the outer and inner membranes of the autophagosome. As autophagy progresses, the autophagosome membrane expands, engulfing cytoplasmic contents, including proteins tagged with ubiquitin, such as sequestosome1 (SQSTM1)/p62, which bind to LC3. The autophagosome closes, facilitated by STX17, detaching from the ER membrane. This allows the autophagosome to move closer to lysosomes, where fusion occurs, mediated by the HOPS (homotypic fusion and protein sorting) complex. RAB7 adaptors facilitate fusion by binding Q-SNARE with STX17. The fusion of the autophagosome and lysosome forms the autophagolysosome complex, where lysosomal degradation of the contents occurs, releasing degraded material back into the cytoplasm. (The template for figures was adapted from Biorender.com).

Figure 5

Targeting tumor dormancy through autophagy modulation for therapeutic potential. Tumor relapse can be addressed by targeting tumor dormancy, which can be approached in two ways: the first approach is by inhibiting proliferating CSCs from entering into the quiescent state; another approach is by inhibiting the re-entry of quiescent CSCs into the proliferating state of the cell cycle. Autophagy plays a crucial role in quiescent (G0) CSCs by providing basic requirements for CSC survival, such as amino acids, ATP production, and preventing energetic catastrophe/apoptotic escape. Thus, targeting autophagy can sensitize dormant tumors to mitogens and chemotherapy and address tumor relapse. (Template for figures was adapted from Biorender.com).

Figure 6

Autophagy in the regulation of cancer stem cells and its therapeutic potential. Cancer stem cells maintain a higher level of autophagy, which plays a role in metabolic reprograming and regulates stemness, differentiation, drug resistance, survival, and epithelial–mesenchymal transition. Metabolic reprograming mediated by autophagy also controls various properties of CSCs, such as epithelial–mesenchymal transition, which promotes migration, drug resistance, stemness, and differentiation. (The template for the figures was adapted from Biorender.com).