In 1935, Hans Krebs proposed the famous tricarboxylic acid cycle (TCA), pointing out the importance of glutamine metabolism in animals. Subsequent studies have shown that glutamine plays an important role in the growth of normal cells and cancer cells.
In view of the key role that glutamine plays in energy production and macromolecule synthesis, related drugs developed for glutamine have great potential in inhibiting tumors. Next, we will introduce the physiological effects of glutamine and the clinical progress of inhibitors.
Glutamine metabolism
High levels of glutamine in the blood provide a ready carbon and nitrogen source to support cancer cell biosynthesis, energy metabolism, and homeostasis to promote tumor growth.
Glutamine delivers quasi-transport protein SLC1A5(the solute carrier family 1 neutral amino acid transporter member 5) to cells.
Under nutrient-deficient conditions, cancer cells can obtain glutamine by breaking down macromolecules.
Oncogene RAS promotes pinocytosis, in which cancer cells remove extracellular proteins and break them down into amino acids to provide nutrients for cancer cells.
Cancer cells absorb a large amount of glucose, but most of the carbon source generates lactic acid through aerobic glycolysis, rather than being used in the TCA cycle.
Tumor cells that over-activate PI3K, Akt, mTOR, KRAS genes, or MYC pathways stimulate the metabolism of glutamate to produce α-ketoglutarate through the catalysis of glutaminase (GLUD) or transaminase. α-ketoglutarate enters The tricarboxylic acid (TCA) cycle can provide energy for cells.
Synthesis of glutamine in nucleic acids, lipids, and proteins
Glutamine can be used as a raw material for cell growth and biosynthesis during division. The carbon from glutamine can be used for the synthesis of amino acids and fatty acids, and the nitrogen from glutamine directly acts on the biosynthesis of purines and pyrimidines.
Nucleic acid synthesis
Aspartic acid produced by the TCA cycle and transamination is the key carbon source for purine and pyrimidine synthesis. Cancer cells that lack glutamine will stall in the cell cycle and cannot be used through TCA circular intermediates such as oxaloacetate for nucleic acid synthesis. However, supplemented exogenous nucleotides or aspartic acid can alleviate cell cycle arrest caused by glutamine deficiency.
In addition, the glutamine-dependent mTOR signal can activate the enzymes carbamoyl phosphate synthase 2, aspartate transferase, and carbamoyl aspartate dehydratase (CAD). And it catalyzes the glutamine-derived nitrogen into the pyrimidine front Body synthesis.
Lipid synthesis
Glutamine is catalyzed by glutaminase (GLS or GLS2) to generate glutaminase and then catalyzed by glutaminase (GLUD) or transaminase to generate α-ketoglutarate. α-ketoglutarate is passed Catalyzes the reverse generation of acetyl-CoA, which can be used for direct lipid synthesis.
Protein synthesis
Besides the carbon in glutamine used for amino acid synthesis, glutamine also plays a key role in protein synthesis. The lack of glutamine can lead to incorrect protein folding and endoplasmic reticulum stress response.
Glutamine can be synthesized by uridine diphosphate acetylglucosamine (UDP-GlcNAc). UDP-GlcNAc is a substrate of β-O-acetyltransferase (OGT), which plays an important role in folding proteins in the endoplasmic reticulum The role of.
GCN2, a serine-threonine kinase, regulates the domain fragment and is similar to histidine -tRNA synthase. The combination of glutamine and histidine-tRNA synthetase inhibits the activity of the GCN2 enzyme. The latter plays an important role in the comprehensive stress responses.
Glutamine and autophagy
The relationship between autophagy and glutamine is intricate, which is also reflected in the role of autophagy in tumor development.
The most contradictory of autophagy in tumors: In some cases, it causes chromosomal instability and inhibits tumor development by inhibiting oxidative stress. Autophagy can also support the survival of cancer cells through stress pathways such as promoting pinocytosis and inhibiting p53.
Glutamine inhibits the activation of GCN2 and comprehensive stress. The ammonia generated from glutamine can promote the development of autophagy in both autocrine and paracrine ways.
ROS can induce autophagy as a stress response, but it will be neutralized by glutathione and NADPH produced by glutamine metabolism. Glutamine can also indirectly stimulate mTOR, which in turn inhibits autophagy through complex mechanisms.
Glutamine and ROS
Cell signals mediated by reactive oxygen species (ROS) can promote tumor development at a certain physiological level, but when the level is too high, reactive oxygen species can cause great damage to the macromolecules in the cell. ROS is produced in several ways, where the mitochondrial electron transport chain will produce superoxide (O2−) anions.
Tumors can control the level of ROS through the products generated by the glutamine metabolic pathway to prevent high levels of ROS from causing chromosomal instability. Among them, the most important way for glutamine to control reactive oxygen species is the synthesis of glutathione. Glutathione is A tripeptide that can be used to neutralize peroxy free radicals.
Glutamine can also affect the balance of reactive oxygen species through NADPH. The malic acid produced by glutamine through a series of reactions is catalyzed by the malic enzyme to generate NADPH, which is used to adjust the balance of ROS.
Clinical of glutaminase inhibitors
The dependence of tumor cells on glutamine metabolism makes it a potential anti-cancer target. Many compounds for glutamine metabolism, from the initial transportation to the subsequent conversion to α-ketoglutarate, have become a research hotspot.
Although most of them are still in the preclinical “tool synthesis” stage or limited by compound toxicity, allosteric inhibitors of glutaminase (GLS) have shown great potential in preclinical cancer models. A very active compound CB- 839, has entered clinical trials.
There are two main types of glutaminase in the human body: kidney-type glutaminase (GLS) and liver-type glutaminase (GLS2).
Tumor cells excessively activate renal glutaminase (GLS), and GLS2 mainly acts on non-cancer cells to catalyze the metabolism of glutamine.
The pleiotropic effects of glutamine in cellular functions, such as energy synthesis, macromolecule synthesis, GLS2 activation, and reactive oxygen species balance, make GLS inhibitors play a synergistic effect in combination therapy.
Inhibition of the glutaminase gene can prevent the transformation of epithelial cells to mesenchymal cells. This step is a key step for tumor cell invasion and final metastasis. Therefore, prevention of metastasis may be an important role for GLS inhibitors to exert anticancer effects in combination therapy with inhibition of glutamine metabolism.
Nowadays, tumor immunity has also become the most promising treatment, such as by blocking immune checkpoint PD-antibodies or using engineered chimeric antigen receptor (CAR) T cells.
These methods require immune cells to play a role in the tumor microenvironment, and metabolic inhibitors in the body may also widely affect the immune function. Recent studies have shown that immune cells compete with cancer cells for glucose, and glutamine may also have a similar mechanism.
In fact, glutamine metabolism plays an important role in the activation of T cells and the regulation of the transformation of CD4+ T cells to inflammatory subtypes.
Glutamine is critical to the activation process of cancer-killer T cells. By blocking the glutamine pathway in cancer cells, increase the content of amino acids in the tumor microenvironment and enhances the killing effect of immune cells.
Conclusion
Ninety years ago, Warburg discovered that many animal and human tumors have a very high affinity for glucose, breaking down a large amount of glucose into lactic acid. He also pointed out that cancer is caused by metabolic changes and loss of mitochondrial function.
People have rediscovered the importance of the physiological mitochondrial oxidation function of cancer. And glutamine also plays an important role in the growth of tumor cells. These arbitrary views have been replaced and perfected in the past few decades.
The pleiotropic role of glutamine in cell functions, such as energy synthesis, macromolecule synthesis, mTOR activation, and reactive oxygen species balance.
Tumor cells over-activate renal glutaminase (GLS), while normal cells catalyze the metabolism of glutamine is hepatic glutaminase (GLS2). It is possible to selectively develop GLS inhibitors clinically.
Targeted inhibition of some oncogenes makes tumor cells dependent on glutamine, so the combination of targeted inhibitors and glutamine metabolism has a synthetic lethal effect.
Due to the complexity of tumor pathogenesis, the physiological mechanism of glutamine in the human body is unclear. For example, in 13 years, Professor Yigong Shi of Tsinghua University pointed out that the main role of glutamine metabolism is to use the generated amine to fight the acidic environment of tumors. Therefore, the combination of GLS inhibitors and other targets has become a trend of development.