Signaling Pathways Involved in Nutrient Sensing Control in Cancer Stem Cells: An Overview

Abstract

Cancer cells characteristically have a high proliferation rate. Because tumor growth depends on energy-consuming anabolic processes, including biosynthesis of protein, lipid, and nucleotides, many tumor-associated conditions, including intermittent oxygen deficiency due to insufficient vascularization, oxidative stress, and nutrient deprivation, results from fast growth. To cope with these environmental stressors, cancer cells, including cancer stem cells, must adapt their metabolism to maintain cellular homeostasis. It is well- known that cancer stem cells (CSC) reprogram their metabolism to adapt to live in hypoxic niches. They usually change from oxidative phosphorylation to increased aerobic glycolysis even in the presence of oxygen. However, as opposed to most differentiated cancer cells relying on glycolysis, CSCs can be highly glycolytic or oxidative phosphorylation-dependent, displaying high metabolic plasticity. Although the influence of the metabolic and nutrient-sensing pathways on the maintenance of stemness has been recognized, the molecular mechanisms that link these pathways to stemness are not well known. Here in this review, we describe the most relevant signaling pathways involved in nutrient sensing and cancer cell survival. Among them, Adenosine monophosphate (AMP)-activated protein kinase (AMPK) pathway, mTOR pathway, and Hexosamine Biosynthetic Pathway (HBP) are critical sensors of cellular energy and nutrient status in cancer cells and interact in complex and dynamic ways.

Keywords: adenosine monophosphate-activated protein kinase (AMPK) signaling; cancer stem cells; hexosamine biosynthesis pathway (HBP) pathway; mammalian target of rapamycin (mTOR) signaling; nutrient sensing.

Figure 1

Hexosamine biosynthetic pathway. A fraction (3–5%) of glucose incoming the cell is shunted through the hexosamine biosynthesis pathway. In this pathway, fructose-6-phosphate is converted to glucosamine-6-phosphate by the glutamine/fructose-6-phosphate amidotransferase (GFAT), the gate-keeper enzyme of the route. The main product of the pathway is UDP-GlcNAc, which is the substrate for O-GlcNAc transferase.

Figure 2

mTOR is activated by growth factors via PI3K-Akt and RAS/MAPK pathways. The phosphorylation of phosphatidylinositol 4,5 biphosphate (PIP2) is catalyzed by PI3K producing PIP3. Once PIP3 is formed, it induces the recruitment of proteins with PH domain such as PDK1, Akt, and mTORC2 complex, facilitating Akt-Thr308 and Akt-Ser473 phosphorylation by PDK1 and mTORC2, respectively. Activated Akt inhibits the TSC complex and promotes mTORC1 activation by Rheb-binding. The Ras/AMPK pathway can regulate both mTOR complexes via the Ras-Raf-MEK-ERK signaling cascade. Activated ERK inhibits the TSC complex by direct phosphorylation, and Ras-GTP can bind to mTORC2, increasing its kinase activity. mTORC1 activation in response to amino acids can be dependent or independent on Rag GTPases. Also, mTORC1 can control its own activation and mTORC2 activity. mTORC2 is negatively regulated by mTORC1-S6K1 that phosphorylates mSN1 and Rictor. The arrows indicate: →, activation signals; ┴, inhibition signals; → pathway activators.

Figure 3

mTOR in cancer stem cells (CSCs). (A) The mTORC1 can modulate cell metabolism, cell survival, proliferation, and stem cell maintenance mostly through protein synthesis activation of transcription factors that induce the expression of genes coding proteins involved in these functions. Also, mTORC1 can promote Gli1 (downstream effector of the Hedgehog pathway), nuclear localization through its effector S6K1 that induces Gli1 releasing from its endogenous inhibitor, SuFu, and inhibits GSK3-mediated its degradation. Besides, mTORC1 under mitogenic signals and amino acid availability controls GSK3 nuclear import and, in turn, its nuclear functions as mediating c-Myc degradation. (B) The role of the mTORC2 complex in CSCs is mainly mediated by its effector Akt. This protein can phosphorylate many substrates, including OCT4 and SOX2, transcription factors that regulate stem cell self-renewal and promote pluripotency. Akt directly phosphorylates these transcription factors increasing its stability and triggering its nuclear import. Also, Akt inhibits GSK3, leading to GSK3 substrates stabilization such as β-catenin and Snail implicated in epithelial-mesenchymal transition (EMT) and Gli2 that increases SOX2, OCT4, and Nanog expression in CSCs. FoxO3 inhibition by mTORC2 signaling through Akt activation and HDACs inhibition avoid FoxO3 nuclear localization and FoxO3 deacetylation, respectively. It might release FoxO3-induced c-Myc repression, promoting upregulation of glycolytic metabolism. 4E-BP1, eukaryotic translation initiation factor 4E binding protein 1; FoxO3, forkhead box O3; GSK3, Glycogen synthase kinase; Gli1/2, Glioma-associated oncogene; HIF1 α, hypoxia-inducible factor 1 subunit alpha; S6K1, ribosomal protein S6 kinase; OCT4, octamer-binding transcription factor 4; SOX2, RY-box transcription factor 2. SuFu, SUFU negative regulator of hedgehog signaling. The arrows indicate: →, activation signals; ┴, inhibition signals.

Figure 4

The AMPK pathway activation and energy homeostasis. Under energy stress, AMPK is phosphorylated at Thr 172 by LKB1 in response to variations in AMP: ADP/ATP ratios. Other upstream kinases such as calmodulin-dependent protein kinase kinase 2 (CAMKK2) activated by intracellular calcium and transforming growth factor-β-activated kinase (TAK1) represent alternative AMPK activation forms. In this context, AMPK-activated can repress anabolic processes and increase catabolism to restore energy balance. AMPK suppresses the ATP-consuming anabolic pathways by direct phosphorylation and inhibition of several proteins: mTORC1, acetyl-CoA carboxylase (ACC1), SREBP (sterol response coactivator), HMGCoA reductase (HMGCR), which play critical roles in protein, fatty acid, sterol, and cholesterol synthesis, respectively. AMPK prevents glycogen storage by inhibitory phosphorylation of the glycogen synthases (GYS1 and GYS2). In addition, AMPK also stimulates the catabolic pathways to produce ATP by several mechanisms. First, increasing glucose utilization by phosphorylation and inactivation of domain family member 1 (TBC1D1) and thioredoxin-interacting protein (TXNIP), which control the translocation of glucose transporters GLUT4 and GLUT1 to the plasmatic membrane, respectively. Second, AMPK increases glucose flux along the glycolytic pathway by PFKFB3 (6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3) phosphorylation, which affects the PFK1 activity, a rate-limiting enzyme in glycolysis. AMPK indirectly stimulates fatty acids transport into the mitochondria by ACC2 inhibition, in turn promoting fatty oxidation. On the other hand, AMPK induces autophagy directly by ULK1 phosphorylation, a kinase essential for autophagy, and indirectly by mTORC1 inactivation. The arrows indicate: →, activation signals; ┴, inhibition signals.

Figure 5

The interplay between HBP, mTOR, and AMPK signaling pathways. The HBP pathway senses glucose, glutamine, and nucleotide levels to produce UDP-GlcNAc, the primary metabolite for protein O-GlcNAcylation, via the enzyme OGT. GAFT1 and OGT suppress the AMPK activity, which is a master energy stress sensing enzyme. AMPK activated by phosphorylation favors the processes that produce ATP over the biosynthesis of molecules. Therefore, AMPK functions as a negative regulator of mTORC1, which induces translation and promotes cell growth when there are high levels of nutrients. Furthermore, AMPK activates TSC1/2, which ensures complete suppression of mTORC1 activity. Also, AMPK and mTORC1 pathways feedback take over the ULK1 activation, a protein necessary for the induction of autophagy. On the other hand, growth factors activate the AKT and MAPK kinase pathways, which convergence in the same way in the inactivation of the TSC1/2 complex, the negative regulators of mTORC1. On the other hand, the mTORC2 complex activity is controlled by glucose and acetate levels through acetyl-CoA, an intermediary metabolite in glycolysis, fatty acid catabolism, and the HBP pathways. mTORC2 also converges with the HBP pathway in the stimulation of GFAT1. In consequence, the interplay between these signaling pathways is involved in nutrient sensing as a means of regulating cell activity and growth and, more importantly, in reacting to changes in the microenvironment of the tumor. The arrows indicate: →, activation signals; ┴, inhibition signals.

Figure 6

HBP, mTOR, and AMPK signaling pathways interaction is crucial to stemness maintenance. Using nutrient-sensing pathways such as HBP, mTOR, and AMPK, stem cells respond to nutritional cues, and the crosstalk between them is key to maintaining stemness. Thus, to regulate the maintenance of stem cells in the tumor microenvironment, conditions such as the availability of nutrients, growth factors, and oxygen can modulate energy maintenance through the activation and inhibition of master proteins of these pathways. The self-renewal of stem cells has been shown by an increase in the expression of stem cell markers like CD44 or CD133 and an increase in the ability to resist chemotherapeutic drugs, which maintains the survival of these cells within the tumor. In the blue circle, the main proteins that allow the interaction between these pathways are highlighted.