Structural Biochemistry/Nucleic Acid/Biology of Cancer
Cancer is a set of diseases in which cells escape from the control mechanisms normally limiting their growth. Cancer cells ignore the normal signals that operate the cell cycle and affect the body by dividing uncontrollably and invading surrounding tissues. The gene regulation systems that go wrong during cancer turn out to be the very same systems that play important roles in embryonic development, the immune response, and many other biological processes.The genes that normally regulate cell growth and division during the cell cycle include genes for growth factors, their receptors, and the intracellular molecules of signaling pathways. mutations that alter any of these genes in somatic cells can lead to cancer.
Nearly all cancers are caused by abnormalities in the genetic material of the transformed cells. These abnormalities may be due to the effects of carcinogens, such as tobacco smoke, radiation, chemicals, or infectious agents. Other cancer-promoting genetic abnormalities may be randomly acquired through errors in DNA replication, or are inherited, and thus present in all cells from birth. The heritability of cancers is usually affected by complex interactions between carcinogens and the host's genome. New aspects of the genetics of cancer pathogenesis, such as DNA methylation, and microRNAs are increasingly recognized as important.
Cancer is simply the gain of oncogenes and the loss of tumor suppressor genes. Genetic abnormalities found in cancer typically affect two general classes of genes. Cancer-promoting oncogenes are typically activated in cancer cells, giving those cells new properties, such as hyperactive growth and division, protection against programmed cell death, loss of respect for normal tissue boundaries, and the ability to become established in diverse tissue environments. Activated oncogenes result form different types of mutations including point mutations, chromosomal translocations, promoter translocations, and amplifications. Point mutations increase proteins' activity and prevent it from being turned off. Chromosomal translocations encode fusion proteins that are hyperactivated or inappropriately regulated or localized. Promoter translocations drive abnormally high levels of expression. Amplifications increase the copy number and expression of a gene. Tumor suppressor genes (TSG) are then inactivated in cancer cells, resulting in the loss of normal functions in those cells, such as accurate DNA replication, control over the cell cycle, orientation and adhesion within tissues, and interaction with protective cells of the immune system. Tumor suppressor genes can be inactivated by missense mutations that decrease the protein's activity, deletions, frameshifts, and promoter methylation.
Diagnosis usually requires the histologic examination of a tissue biopsy specimen by a pathologist, although the initial indication of malignancy can be symptoms or radiographic imaging abnormalities. Most cancers can be treated and some cured, depending on the specific type, location, and stage. Once diagnosed, cancer is usually treated with a combination of surgery, chemotherapy and radiotherapy. As research develops, treatments are becoming more specific for different varieties of cancer. There has been significant progress in the development of targeted therapy drugs that act specifically on detectable molecular abnormalities in certain tumors, and which minimize damage to normal cells. The prognosis of cancer patients is most influenced by the type of cancer, as well as the stage, or extent of the disease. In addition, histologic grading and the presence of specific molecular markers can also be useful in establishing prognosis, as well as in determining individual treatments.
History of Cancer
The word cancer comes from the Greek physician Hippocrates. He used the Greek words "carcinos" and "carcinoma", meaning crab in Greek, to describe cancer because he believed that tumors resembled crabs. The two words were combined into one: "karkinos".
The oldest record of cancer is from the Egyptians around 1500 B.C. The Edwin Smith Papyrus documents 8 cases of breast tumors treated with cauterization, a method of destroying tissue with a hot instrument called a "fire drill". Records indicate that there were no treatments for the disease. In addition, archaeological finding has discovered mummies with fossilized bone tumors suggestive of bone cancer. Also in the Papyrus, it is suggested that Egyptian physicians could differentiate malignant tumors from benign tumors.
Later, the city of Constantinople became the Medical Center of the World. During this time period, the cause of cancer was attributed to an excess of black bile. This was based on Hippocrates's idea that the human body was composed of four different types of bodily fluids: blood, phlegm, yellow bile and black bile. Any excess or deficiencies in either fluid would cause disease, and in this case, excess of black bile was believed to cause cancer. This was the knowledge for the next 1400 years, through the Middle Ages, and went unchallenged since religious superstition at the time prohibited doctors from performing autopsies.
In the 15th Century Renaissance, physicians began to acquire more knowledge on human physiology. Giovanni Morgangni, in the 16th century, began to do autopsies on the bodies of the deceased in order to discover a pathological relationship between disease and death. Also, during this time, a Scottish physician named John Hunter suggested that cancerous tumors may be removed by surgical means. In the 17th Century, the Black Bile theory was replaced with another theory called the Lymph theory. The idea was that cancer was caused by degenerating and fermating lymph.
In the 19th Century, Rudolf Virchow, who is considered the father of cellular pathology, linked the clinical course of illnesses with pathological findings. This discovery allowed physicians to not only assess the damage of a particular cancer on the body, but also laid the foundations of cancer surgery. Tissues removed from the infected area could rapidly and quickly assessed to determine the type of cancer and also to elucidate whether or not the tumor was completed excised.
The 20th Century saw a rise in cancer knowledge. Research on carcinogens, radiation therapy, and better means of identification were discovered. There was tremendous scientific improvement on the understanding of cell growth and division mechanisms.
Types of Cancer
There are many different kind of cancer diverse by the type of cell that made up the tumor and therefore the tumor is known as the origin of cancer. A benign tumor is a lump of abnormal cells that do not spread to other cells or other parts of the body, but rather remains at the original location that it was formed. However, a malignant tumor is a more dangerous form because it is able to invade and damage the functions of other cells and organs.
- When cancers are derived from the epithelial cells whose genome has altered or damaged, we called it Carcinoma. It is the most common type of cancer occurring in humans and it begins in a tissue that lines the inner or outer surfaces of the body. The cells will begin to exhibit abnormal malignant properties. The most common example is the breast, lung, prostate cancers.
- The second type is called Sarcoma which the cancers are arising from the connective tissue. It develops develop from cells originating in mesenchymal cells outside the bone marrow. Therefore Sarcoma includes tumors of bone (osteosacrcoma), fat (liposacrcoma), muscle (leiomyosarcoma) and vascular. There are two classes of cancer arise from the blood forming cells.
- Leukemia is a type of cancer of the blood or bone marrow characterized by an abnormal increase of white blood cell. The cause of such cancer is also by mutation in the DNA that can trigger leukemia by activating oncogenes or deactivating tumor suppressor genes.
- Lymphoma is a cancer that develops in the immune system. It usually presents to be a solid tumor in the lymphoid cells. The malignant cells often originate in the lymph node and presents as an enlargement of a tumor. It will also affect other organs such as skin, brain, bowels and bone.
- Cancer also appears in the pluripotent cell in the testicle or the ovary. This is called the germ cell tumor. There are two types of germ cell tumor either cancerous or non-cancerous. It may cause by error in development of the embryo.
- The last type is the Blastoma which is developed from immature embryonic tissue. This is most common in children.
Hallmarks of Cancer
- Uncontrolled Cell Division Cancer cells ignore signals that would regulate cell growth and division in the body. Since the cells grow and divide uncontrollably, it leads to the production of more and more cancer cells within the body.
- Evading Apoptosis Cancer cells are able to avoid the process of apoptosis for normal cells. Apoptosis is the process of programmed death carried out in normal cells, but cancer cells are able to evade the process and can therefore continue to progress.
- Independent from Growth Regulation Cancer cells are self-sufficient and do not require external signals to regulate its growth and division. Cancer cells are also capable of ignoring negative signals from its neighbors and can therefore continue to grow and divide on its own.
- Angiogenic The cancer cells that make up the tumor need a system to provide nutrients and dispose of waste. Therefore cancer cells have become angiogenic, in which the tumor attracts blood vessels to grow into the tumor mass and nourish the cancer cells.
- Immortality Cancer cells have developed the ability to proliferate indefinitely. By doing so, cancer cells have become immortalized and are capable of indefinite growth and cell division.
- Invasion and Metastasis Cancer cells are capable of entering the stage of metastasis and invade surrounding cells and tissues. By developing the ability invade and metastasize, cancer cells are able to spread and establish the disease in other areas of the body and not just the original location.
In 1910, an American researcher, Peyton Rous, discovered that the Rous sarcoma virus (Rous virus)could cause cancer and he studied how this cancer could spread in chickens. He performed three experiment to test how this cancer spread.
- He took out cancerous cells from one chicken and injected into a healthy chicken. He observed that the healthy chicken became afflicted with cancer.
- He purified the cell to create cell-free extracts. He then injected the extract into a healthy chicken. Again, the chicken contracted cancer.
- He used a special filter, that had holes too small for viruses to pass through, to filter the cells. He inject this extract into healthy chickens and discovered that they did not contract cancer, and remained healthy.
From his experiments, he concluded that viruses can cause cancer. Later, scientists discovered that the Rous virus is a retro-virus. A retro-virus can insert its gene into a host DNA and uses host protein mechanisms to reproduce. The viral DNA inside of the host cell causes uncontrollable cell division, cell growth, etc.
Although there are several different systems for classifying the oncogenes, these classifications are yet to be accepted standard. Some are grouped spatially or chronologically. The first category, growth factors (mitogen), include cancer genes such as fibrosarcomas, osteosarcomas, breast carcinomas, and melanomas. These genes induce cell proliferation. The growth factors from specific cells trigger cell proliferation not only in themselves but also nearby and distant cells. The Receptor tyrosine kinases cause breast cancer, non-small-cell lung cancer and pancreatic cancer. These receptor tyrosine kinases genes transduce signals for cell growth. The receptor tyrosine kinases add phosphate groups to other proteins. This can either turn the proteins permanently on or off. The cytoplasmic tyrosine kinases category include breast cancers, melanomas, ovarian cancers, head and neck cancers, blood cancers and brain cancers. These genes are in charge of the responses back and forth from active receptors of the cells that mediate proliferation, migration, differentiation and survival. The cytoplasmic serine/threonine kinases are responsible for malignant melanoma, colorectal cancer, and ovarian cancer. These genes are involves in cell cycle regulation, cell proliferation, cell survival, and apoptosis. More than 125 of the human protein kinases are serine/threonine kinases. The regulatory GTPases genes are involved in leading pathways to cell proliferation. An example of the regulatory GTPases include the Ras protein. The Ras hydrolyses GTP into GDP and phosphate and is responsible for adenocarcinomas of the pancreas and colon, thyroid tumors, and myeloid leukemia. Lastly, the transcription factors regulate the transcription of genes that trigger cell proliferation. For example, the myc gene can cause cancers such as small cell lung cancer, breast cancer, acute myeloid leukemia and malignant T-cell lymphomas.
Cancer cannot be diagnosed accurately by one single test. A complete and thorough history and physical examination along with several diagnostic tests must be performed in order to evaluate whether the patient has cancer or other conditions are being misinterpreted as symptoms of cancer.
An effective procedure of testing can be used to confirm or exclude the presence of cancer, determine the disease process and preliminary plan for treatments. Tests needed to be repeated if the patient’s symptoms have changed or the testing sample is not qualified or the test results show abnormality.
Diagnostic procedures for cancer may include imaging, laboratory tests, tumor biopsy, and endoscopic examination.
Diagnostic imaging is the process of obtaining pictures of body structures and organs. It is used to detect tumors and other abnormalities and their extent. It is also the most applicable way to determine the effectiveness of the treatments.
- Transmission imaging In transmission imaging, a beam of photons with high energy is generated and allowed to pass through the body structure being examined.
- X-ray X-rays use electromagnetic energy beams to produce images of internal tissues, bones, and organs on film. Based on the images, tumor or cancer cells can be located.
- Computed Tomography scan (CT scan or CAT scan) A CT scan is more detailed than general X-ray scan. By combining x-rays and computer technology, images of bones, muscles, fat, and organs in the body are showed with more details.
- Bone scan Bone scans are pictures of X-ray or CT scan taken of the bone after a dye has been injected to bone tissue. These scans are used to detect tumors and abnormalities in the bone structure.
- Lymphangiogram(LAG) Lymphangiogram is an image that can detect cancer cells or abnormalities in the lymphatic system and structures after a dye is injected into the lymph system.
- Mammogram A mammogram is an x-ray image of the breast. It is used to detect and diagnose breast disease in women by locating abnormal area. A biopsy is required for further diagnosis.
- Reflection Imaging In Reflection Imaging, images are produced by high-frequency sounds bouncing off of the surface of body tissues and structures at varying speeds, depending on the density of the tissues present. The bounced sound waves are then analyzed by a computer and a visual image is produced.
- Emission Imaging MRI combine a large magnet and a computer to produce detailed images of the heart, brain, liver, pancreas, male and female reproductive organs, and other soft tissues.
- Blood Tests Blood tests are used to check the levels of substances that are indicative of how healthy the body is and whether infection is present. Other tests check for the presence of electrolytes such as sodium and potassium that are critical to the body's healthy functioning.
- Urinalysis Urinalysis breaks down the components of urine to check for the presence of drugs, blood, protein, and other substances.
- Tumor markers Tumor markers are substances released by cancer cells or substances created by the body in response to cancer cells.
Prostate-specific Antigen (PSA) An elevated PSA level in the blood may indicate prostate cancer, but other conditions such as benign prostatic hyperplasia (BPH) and prostatitis can also raise PSA levels.
CA 125 Ovarian cancer is the most common cause of elevated CA 125, but cancers of the uterus, cervix, pancreas, liver, colon, breast, lung, and digestive tract can also raise CA 125 levels.
Prostatic acid phosphatase (PAP) In addition to prostate cancer, elevated levels of PAP may indicate testicular cancer, leukemia, and non-Hodgkin’s lymphoma, as well as some noncancerous conditions.
Human chorionic gonadotropin (HCG) If pregnancy is ruled out, HCG may indicate cancer in the testis, ovary, liver, stomach, pancreas, and lung.
Carcinoembryonic Antigen (CEA) Colorectal cancer is the most common cancer that raises this tumor marker. Several other cancers can also raise levels of carcinoembryonic antigen.
Alpha-fetoprotein (AFP) In men, and in women who are not pregnant, an elevated level of AFP may indicate liver cancer or cancer of the ovary or testicle. Noncancerous conditions may also cause elevated AFP levels.
CA 19-9 Elevated levels of CA 19-9 may indicate advanced cancer in the pancreas, but it is also associated with noncancerous conditions, including gallstones, pancreatitis, cirrhosis of the liver.
CA 27-29 Cancers of the colon, stomach, kidney, lung, ovary, pancreas, uterus, and liver may also raise CA 27-29 levels. Noncancerous conditions associated with this substance are first trimester pregnancy, endometriosis, ovarian cysts, benign breast disease, kidney disease, and liver disease.
CA 15-3 Elevated levels of CA 15-3 are also associated with cancers of the ovary, lung, and prostate, as well as noncancerous conditions such as benign breast or ovarian disease, endometriosis, pelvic inflammatory disease, and hepatitis. Pregnancy and lactation also can raise CA 15-3 levels.
Neuron-specific enolase (NSE) NSE is associated with several cancers, but it is used most often to monitor treatment in patients with neuroblastoma or small cell lung cancer.
A biopsy is the removal of tissues or cells from the patient’s body for examination under a microscope. Biopsies are usually performed to determine whether a tumor is cancerous or just an infection or inflammation.
- Endoscopic biopsy This type of biopsy is performed through a fiberoptic endoscope (a long, thin tube that has a close-focusing telescope at the end for closeup observation) through a natural body orifice or a small incision. The endoscope is used to view the organ in question for abnormal or suspicious areas, in order to obtain a small amount of tissue for study.
- Bone marrow biopsy This type of biopsy is performed either from the sternum or the iliac crest hipbone. A needle is inserted into the marrow, and cells are taken for study.
- Excisional biopsy This type of biopsy is often used when a wider or deeper portion of the skin is needed. Using a scalpel, a full thickness of skin is removed for further examination.
- Fine needle aspiration (FNA) biopsy This type of biopsy involves using a thin needle to remove very small pieces from a tumor. FNA is not used for diagnosis of a suspicious mole, but may be used to biopsy large lymph nodes near a melanoma to see if the melanoma has metastasized (spread).
- Punch biopsy Punch biopsies involve taking a deeper sample of skin with a biopsy instrument that removes a short cylinder of tissue.
- Skin biopsy Skin biopsies involve removing a sample of skin for examination under the microscope to determine if melanoma is present.
An endoscope is a small, flexible tube with a light and a lens on the end used to look into the esophagus, stomach, duodenum, colon, or rectum. It can also be used to take tissue from the body for testing or to take color photographs of the inside of the body.
Colonscopy involves inserting a colonoscope, which is a long, flexible, lighted tube, in through the rectum up into the colon. It allows the physician to view the entire length of the large intestine, and can often help identify abnormal growths as well as inflamed tissue, ulcers, and bleeding. The physician can also remove tissue for further examination.
- Endoscopic retrograde cholangiopancreatography (ERCP)
ERCP is a procedure that allows the physician to diagnose and treat problems in the liver, gallbladder, bile ducts, and pancreas. The procedure combines x-ray and the use of an endoscope. The scope is inserted through the patient's mouth and throat, then through the esophagus, stomach, and duodenum.
- Esophagogastroduodenoscopy (EGD)
An EGD is a procedure that allows the physician to examine the inside of the esophagus, stomach, and duodenum. An endoscope is guided into the mouth and throat, then into the esophagus, stomach, and duodenum. The endoscope allows the physician to view the inside of this area of the body, as well as to insert instruments through a scope for the removal of a sample of tissue for biopsy.
A sigmoidoscopy is a diagnostic procedure that allows the physician to examine the inside of a portion of the large intestine, and is helpful in identifying the causes of diarrhea, abdominal pain, constipation, abnormal growths, and bleeding. A short, flexible, lighted tube, called a sigmoidoscope, is inserted into the intestine through the rectum. The scope blows air into the intestine to inflate it to have a better view of the inside.
An examination in which a scope is inserted through the urethra to examine the bladder and urinary tract for structural abnormalities or obstructions, such as tumors or stones.
Surgery is one of the initial treatments used to operate on cancer cells. Cancer surgery attempts to prevent the spread of cancer cells by locating the tumor and removing it along with any possible lymph nodes that are near the source location. This method is commonly used for cases with benign tumors because the mass of cancer cells are typically located in one area. Surgery may be the only treatment needed for some cancer cases, but if a few cancer cells have broken off or get left behind, then it would only be a matter of time before the disease returns and more treatments are needed.
Chemotherapy is another treatment used in addition to surgery if cancer cells still remain in the body. Chemotherapy is used to treat cancer cells that have entered the stage of metastasis, in which the cells have spread from their original location. The treatment utilizes drugs that are toxic to interfere and kill cells that divide. The goal is to kill the cancer cells faster than the normal dividing cells in the body because cancer cells divide more rapidly. Even though chemotherapy has been proven to be effective, the side effects include hair loss and nausea because the treatment is blasting every cell being divided including the normal cells.
Radiation therapy for cancer utilizes a beam of high-energy particles to kill off the cancer cells located in the body. This treatment targets the cancer areas by marking the skin, thus enabling the beam of high-energy particles to directly hit and destroy the cancer cells. The radiation from this therapy can shrink the tumors and also relieve the symptoms caused by the cancer. Although this therapy benefits cancer patients, it still has side effects and risks of causing new problems for these patients. The side effects caused by the radiation depends on the area of the body that undergoes the therapy; and this area is also the main location of the side effects.The radiation focuses on the tumor, but since the ray of high-energy particles targets the skin as well, a common side effect caused from radiation therapy is that the skin that was marked turns red and gradually may look more dark or tan over the years. The skin may also dry and flake, like a burn to the skin, during the period of recovery after the radiation therapy.
Monoclonal antibodies serve as a method in cancer therapy to enforce the immune system and aid in diagnosis. Monoclonal antibodies are created by injecting human cancer cells into mice so that they are able to produce antibodies against the foreign antigens invading their immune system. From there, the murine cells producing the antibodies are then removed and combined with laboratory-grown cells. This combination creates hybrid cells called hybridomas, which in turn is capable of producing large quantities of these pure antibodies so that the human body is able to process and use these antibodies to fight off the cancer cells.
Tumor Suppressor Genes
Tumor suppressor proteins protect cells from being cancerous.
Using Minicells to Treat Tumors
What are Minicells
Minicells are small non-chromosomal "cells" that result from abnormalities during cell division in prokaryotes. They do not contain any of the original DNA present in its larger sister, but may contain proteins and copies of transformed plasmids that were present in the are in the original cell when the minicell formed. First discovered over 70 years ago, minicells are becoming of interest to researchers in their potential as anti-tumor agents1. In order to understand how minicells form, it is important to understand how most prokaryotes mediate cell division. During the cytokinesis stage of cell division, a framework of proteins form at the site of division. The main protein involved in this process, FtsZ, forms what is called a Z-Ring around the septum of division. FtsZ is "tethered" to the membrane by a series of other proteins involved in the process of cytokinesis. The Z-Ring usually aligns itself at the middle of the enlarged cell as a systems of proteins inhibit the formation of the Z-Ring at the poles of the cell. Abnormalities in this regulatory system for cytokinesis results in the formation of the Z-Ring near the poles of the cell. This results in two dimorphic daughter cells: one larger with all of the bacterial DNA, and another smaller one with no chromosomal DNA. It is important to note that not all prokaryotes divide in cytokinesis by the use of a Z-Ring, and the mechanism by which they divide is still unknown. Minicells can form under certain growth conditions or from mutations. E. coli is usually used to make minicells.
Using Minicells to Treat Tumors
Brahmbhatt et al. reported that minicells can be used in treating drug resistant tumors by using siRNA and cytotoxic drugs2, what follows is a summary of their findings. The goal of the research was to demonstrate that minicells could be targeted to tumor cells, and release cytotoxic drug and siRNA specific to drug resistant genes. It is known the RNA interference caould be a powerful tool in fighting cancer, however there is a need to find an efficient method to transfer siRNA or microRNA to cancerous cell in vivo without them being degraded or causing adverse effects. It was observed that minicells had the ability to uptake cytotoxic drugs and siRNA. The minicells could also contain plasmids encoding siRNA by harvesting them from bacteria (E. coli) transformed with that specific plasmid. Minicells have the ability to uptake siRNA and to contain it for long periods of time. Specific antibodies (BsAB) could also be incorporated into the minicell to help target tumor cells. They had showed previously that minicells could bind to the tumor cells via specific antibodies, and initiate endocytosis via receptors on the cell surface. Once inside the cell, the drug contained in the minicells could be released. The same idea was again applied to siRNA with the ability to interfere with drug resistance genes in the tumor cells. The minicells would contain siRNA, or siRNA encoding plasmids, and would use the same antibody to bind to the receptors of tumor cells. It was shown that in vitro, the siRNA delivery via minicells could diminish drug resistance in cells containing a drug resistance gene.
This was done by delivering the minicells full of siRNA to a type of colon cancer cell overexpressing MDR1 (multi drug resistance gene), and then treating the cells with drugs in order to measure toxicity of the drug to the cell. This resulted in a highly significant increase of toxicity. In vivo experiments were done on human cancer xenografts in mice and showed that the tumor cells treated with a wave of siRNA containing minicells, experienced a knockdown of the drug resistance gene which was targeted. The results were confirmed by performing a Western Blot on protein in the cells.
The next step was to show the efficacy of first treating mice with a wave of siRNA and the cytotoxic drug using minicells. Mice with the Caco-2/MDR1 (colon cncer cells with multi drug resistance gene) were treated siRNA containing mincells to reverse drug resistance by knocking out the MDR1 and thereafter treatment with drug containing minicells. Their analysis showed that all the mice which had under gone this dual treatment exhibited a significant inhibition of cancerous growth and complete survival. This result also showed that after the initial minicell treatment with siRNA, the tumor cells were still able to process the next wave of minicells (no adverse effects on the cellular machinery). However, the mechanism by which siRNA is loaded with minicells or possible effects on the immune system are not yet known, especially since they originated from bacteria. No adverse effects were reported in the in vivo studies, however. It was also concluded that overall, using this sequential method of treating the tumors with minicells, a smaller amount of drug is needed than for the case in which the drug is administered without the siRNA containing minicells.
PKM2 Regulator in cancer
PKM2 is an enigmatic enzyme that catalyzes the last step of glycolysis, converting phosphoenolpyruvate to pyruvate and phosphorylating ADP to Atp. There are four types of PK in mammals, all encoded by these two genes: PKLR and PKM2. PK has not much evolved during evolution, as the four isoforms in mammals are very similar in sequence. In cancer cells, PKM2 takesover the PK isoform until it becomes the main isoform, which supports the idea of tumorigenesis. A study was then in which PKM2 was replaced with PKM1, and tumor growth was delayed, supporting the hypothesis. However, tumor cells can also switch the expression of PKL/R to PKM isoforms, but this hypothesis has not been scientifically validated. So the specific PKM isoform in primary tumors still need to be identified. PKM2 can jump between two states: a tetrameric form and a dimer. The dimer is present in cancer cells. The tetramer may in fact be a dimer of the dimer. Though PKM2 is highly present in cancer cells, it may not be the only PK isoform in cancer, and it may not be cancer specific. Other isoforms are allosterically regulated. In PKM2, it is the main PK isoform, and the ratio between the active and inactive forms mold the usage of glucose for energy production or anabolic precursors. PKM2 is complex regulator. It has been shown to also be phosphorylated on tyrosine residues in the low activity state. In the high activity state, PKM2 also forms complexes with other glycolytic enzymes such as GADPDH, PGAM, and LDH. It can also control pyruvate and induce lactate production. Tyrosine phophorylated polypeptides can also inhibit PKM2 binding, releasing activator FBP. These events are seen often in cancer and can control glucose metabolism. There are many mechanisms to inhibit PKM2, which is useful for survival under cell stress and can explain why PKM2 is the main enzyme in cancer cells. PKM2 also is regulated by nutrients and other growth factors, which allows it to meet the demands metabolcity of cancer cells. PKM2 is also present in the nucleus, due to a nuclear localization signal in the C0terminal domain. The functions of nuclear PKM2 are not clear. It is required for cell survival after interleukin stimulation, but is also necessary for agoptosis. Nuclear PKM2 also can interact with transcription factors and epidermal growth factors. PKM2 binds to phophorylated beta catenin and promotes transcription in the nucleus. It is activated by HIF-1. However, in cancer cells, a regulatory feedback mechanism seems to occur where instead, PKM2 activates HIF-1 instead, leading to activation of genes for glucose transporters and glycolytic enzymes. In cancer treatment, PKM2 is a target, so an understanding of its regulation can improve pharmaceutical drugs aimed to counteract or mimic its effects. PKM2 is highly expressed in cancer cells, but many post-translational modifications also suggest that inhibiting PKM2 may promote cell proliferation. Therefore, both PKM2 inhibiting and activating drugs have been synthesized to target tumor cells. A study suggested that replacing PKM2 isoform with PKM1 may also result in a significant reduction of cancerous cell growth, but no evidence of PKM2 activators providing a efficient treatment have yet been performed. Thus, many challenges remain in this field of research today.
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