A glucose molecule in which a hydrogen atom replaces the 2-OH group is termed 2DG. It is a glucose analog but an inhibitor of the glycolytic pathway. 2DG binds the Glut transporters at the place of glucose, enters the cell, and accumulates there. It is converted into 2-deoxy-D-glucose 6 phosphate (2DG6P) by Hexokinase. Then it inhibits the Hexokinase, which is a rate-limiting enzyme of glycolysis. 2DG accumulation also inhibits the N-glycosylation of proteins in cells by interfering with different enzymes and causing cell death.
Mannose is an epimer of glucose at carbon 2. Epimers are molecules that differ with the location of OH groups. Mannose is an Aldo-sugar monomer. It plays a vital role in different metabolic processes in the human body. One of the significant roles of mannose is the Glycosylation of many proteins. Oligosaccharides are attached at the N or C terminal of proteins that change their properties and functionality (Nan et al., 2022).
Mannose metabolism is equally essential in the body; mutations in any enzyme involved in mannose metabolism can lead to several metabolic disorders. Mannose is used to treat many inherited disorders related to metabolic pathways. This is also used in treating syndromes caused by carbohydrate deficiencies of glycoproteins and in preventing Urinary Tract Infections (UTIs) (Nan et al., 2022).
The function of Mannose in N-Glycosylation
The catabolism of mannose in cells is almost like that of glucose. It enters the cells via transporter proteins, and out of all the mannose entering cells, only 2% is used in the glycosylation process. After entering the cell, enzymes convert it into Mannose-6-phosphate and Maannose-1-phosphate, which are later converted into GDP-Mannose.
Different experiments have been performed to check whether N-glycans are derived from mannose or glucose. It is proved by the experimental findings that mannose plays a significant role in adding oligosaccharides to proteins. However, glucose is not involved (Jitsuhara et al., 2002).
N-glycans have high mannose.
All the N-glycans have an oligosaccharide chain attached to them composed of three mannose residues and two GlcNAc molecules. However, N-glycans differ in their oligosaccharide chains other than the core region to form complex structures.
The oligosaccharide chain is first formed on carrier lipids in the cell’s Endoplasmic Reticulum. It undergoes processing in ER and Golgi apparatus and is then transferred to the proteins.
Microheterogeneity is observed in the oligosaccharide chains that are attached to the proteins for the formation of N-glycans. About 100 complex oligosaccharide structures have been discovered in the cell up till now. These are involved in cell signaling processes (Jitsuhara et al., 2002).
N-Acetyl Cysteine (NAC)
N acetylcysteine is found in the extracellular environment. It is the main component in synthesizing glutathione and cysteine within the cell. It has proved to be very effective against DNA damage, cancer, or other disorders caused by genetic mutations. So, it is being used as a drug to treat several disorders. Its safety and efficacy have been proven through many experimental studies.
Mechanism of action of N-acetyl cysteine
N-acetyl cysteine modulates a cell’s metabolism, and it has anti-oxidant activity and nucleophilic effects. It also affects the cell’s mitochondria, helps repair DNA, decreases the concentration of carcinogens within the cell, inhibits the toxic effects of gene mutations that play a role in cellular transformation, and regulates signal transduction processes within cells. It also regulates gene expression in the nucleus, the survival, and death of cells depending upon mutations, prevents angiogenesis and inflammation in cells, and prevents metastasis of cells to other tissues. N-acetyl cysteine also protects normal cells from the harmful effects of chemotherapeutic drugs (De Flora et al., 2001).
Thus, N-acetyl cysteine plays many vital roles in preventing oncogenesis and treating other genetic disorders. It has a significant role in the repair of DNA damage caused in cells.
Experiments on the use of N-acetyl cysteine against cancer
Different experiments have been performed on pre-clinical models and on humans as clinical trials to check the safety and efficacy of N-acetyl cysteine. These experiments have studied the complete mechanism of action of this compound.
NAC scavenges the reactive oxygen species ROS within the cell. It also inhibits many mutated genes within cells to prevent carcinogenesis. It stimulates apoptosis selectively in the malignant cells but not in the normal cells. It activates different signals for apoptosis and attenuates DNA damage and oxidative stress in malignant cells (Karaca et al., 2015).
Effect of 2DG on Glycosylation
2DG is glucose antimetabolite used as a therapeutic modality against cancer cells nowadays. Cancer cells have many hallmarks, one of them being the increased rate of glycolysis. 2DG inhibits the glycolytic pathway in cancer cells, depriving them of energy so they cannot perform their basic survival activities. Moreover, 2DG also plays a role in the inhibition of protein glycosylation. Proteins are glycosylated to perform their proper function. The unfolded, or misfolded proteins are degraded in the cells. Glycosylation of proteins is essential for different functions in the body, like cell-to-cell interaction, migration, immune system response, and invasion. So, controlling the Glycosylation of proteins in cancer cells can be one of the best anti-tumor therapies (Ahadova et al., 2015).
The effect of 2DG on protein glycosylation results from increased Mannose incorporation.
It has been found experimentally that 2DG plays an essential role in inhibiting the Glycosylation of proteins in cancer cells. It does so by increasing the incorporation of mannose in the cells and then inhibiting the binding of oligosaccharides with proteins to prevent N-glycans synthesis. It does not affect the enzymes involved in the Glycosylation but affects the incorporation of mannose.
2DG foe N-acetylcysteine
As mentioned earlier, N-acetyl cysteine scavenges ROS and oxidative stress within the cells. It protects the cells from the harmful effects of chemotherapeutic drugs like Cisplatin. However, in contrast to this, 2DG increases the production of ROS and oxidative stress in cancer cells.
N-acetyl cysteine plays a role in glutathione production, which is an intracellular Redox buffer; on the other hand, 2DG decreases glutathione production in malignant cells. Thus, the effects caused by 2DG are inhibited by NAC and vice versa.
2DG is essential in treating different types of cancers by increasing oxidative stress. 2DG mainly produces oxidative stress and ROS in head and neck cancer patients. This stress makes the survival of malignant cells difficult and halts their primary activities.
So, when 2DG is given to malignant cells, it causes glucose deprivation that leads to a decrease in glutathione levels in cells leading to oxidative stress, which ultimately could be used for the treatment of different types of cancers (Luengo et al., 2017).
As 2DG and NAC play opposite roles in the cells, we can say that 2DG foes N-acetyl cysteine.
Mannose and 2DG
Mannose rescues 2-DG-induced cell viability inhibition and chemosensitization. 2-DG has a dual activity of inhibiting glycolysis and
N-linked Glycosylation. Increasing evidence indicates that interfering with N-linked Glycosylation is crucial in eliciting tumour cell death under normoxia.
To further explore the molecular mechanism, we co-treated cells with mannose to rescue N-linked Glycosylation. The cell apoptosis and death induced by 2-DG were mostly rescued by mannose.
Cancer cells are rapidly proliferating cells that require many nutrients and energy. They have two methods for producing energy: glycolysis and oxidative phosphorylation. The best therapy for cancer treatment and stopping their proliferation and metastasis is to inhibit glycolysis in them. Another method is to prevent the Glycosylation of proteins in cancer cells so that their proteins become nonfunctional and cannot perform the normal function of energy production and other biochemical pathways in these cells.
Mannose is an epimer of glucose at C-2. It is an essential component of protein glycosylation. We can use some glucose antimetabolite like 2DG to reverse mannose’s effects on energy production. So that cancer cells become deprived of nutrients and may die.
N-acetyl cysteine is being used as a drug in cancer therapeutics. It is essential in adding oligosaccharides to proteins and prevents cell oxidative stress. So that proteins can perform their function well. 2DG also acts opposite to N-acetyl cysteine and inhibits protein glycosylation.
Ahadova, A., Gebert, J., Von Knebel Doeberitz, M., Kopitz, J., & Kloor, M. (2015). Dose-dependent effect of 2-deoxy-D-glucose on glycoprotein mannosylation in cancer cells. IUBMB Life, 67(3), 218–226. https://doi.org/10.1002/iub.1364
De Flora, S., Izzotti, A., & D ’agostini, F. (2001). Mechanisms of N-acetylcysteine in the prevention of DNA damage and cancer, with particular reference to smoking-related end-points protective effects of NAC, were observed under various conditions produced by various treatments or imbal. Carcinogenesis, 22(7), 999–1013.
Jitsuhara, Y., Toyoda, T., Itai, T., & Yamaguchi, H. (2002). Chaperone-like functions of high-mannose type and complex-type N-glycans and their molecular basis. Journal of Biochemistry, 132(5), 803–811. https://doi.org/10.1093/oxfordjournals.jbchem.a003290
Karaca, E., Lewicki, J., & Hermanson, O. (2015). Oxygen-dependent acetylation and dimerization of the corepressor CtBP2 in neural stem cells. Experimental Cell Research, 332(1), 128–135. https://doi.org/10.1016/j.yexcr.2014.10.013
Luengo, A., Gui, D. Y., & Vander Heiden, M. G. (2017). Targeting Metabolism for Cancer Therapy. Cell Chemical Biology, 24(9), 1161–1180. https://doi.org/10.1016/j.chembiol.2017.08.028
Nan, F., Sun, Y., Liang, H., Zhou, J., Ma, X., & Zhang, D. (2022). Mannose: A Great Option in the Treatment of Cancer and Inflammation. Frontiers in Pharmacology, 13(May), pp. 1–6. https://doi.org/10.3389/fphar.2022.877543
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