We are searching data for your request:
Upon completion, a link will appear to access the found materials.
I am searching for source, that providing information about enzymesproteins, in different types of cancers, that their amount in cell is significantly higher - comparing to normal, healthy cell.
Some pathways that exist in cancer in much more higher amounts.
For example - Telomerase.
Is there some sort of database for this information?
Yes. The dbDEPC 2.0 database for example. If you want to extend your search to pathways the best database I know of is the KEGG.
There are multiple databases out there. You might want to look at this wikipedia page.
By the way what you are looking for is called a cancer proteomic signature.
The gluconeogenic enzyme PCK1 phosphorylates INSIG1/2 for lipogenesis
Cancer cells increase lipogenesis for their proliferation and the activation of sterol regulatory element-binding proteins (SREBPs) has a central role in this process. SREBPs are inhibited by a complex composed of INSIG proteins, SREBP cleavage-activating protein (SCAP) and sterols in the endoplasmic reticulum. Regulation of the interaction between INSIG proteins and SCAP by sterol levels is critical for the dissociation of the SCAP-SREBP complex from the endoplasmic reticulum and the activation of SREBPs 1,2 . However, whether this protein interaction is regulated by a mechanism other than the abundance of sterol-and in particular, whether oncogenic signalling has a role-is unclear. Here we show that activated AKT in human hepatocellular carcinoma (HCC) cells phosphorylates cytosolic phosphoenolpyruvate carboxykinase 1 (PCK1), the rate-limiting enzyme in gluconeogenesis, at Ser90. Phosphorylated PCK1 translocates to the endoplasmic reticulum, where it uses GTP as a phosphate donor to phosphorylate INSIG1 at Ser207 and INSIG2 at Ser151. This phosphorylation reduces the binding of sterols to INSIG1 and INSIG2 and disrupts the interaction between INSIG proteins and SCAP, leading to the translocation of the SCAP-SREBP complex to the Golgi apparatus, the activation of SREBP proteins (SREBP1 or SREBP2) and the transcription of downstream lipogenesis-related genes, proliferation of tumour cells, and tumorigenesis in mice. In addition, phosphorylation of PCK1 at Ser90, INSIG1 at Ser207 and INSIG2 at Ser151 is not only positively correlated with the nuclear accumulation of SREBP1 in samples from patients with HCC, but also associated with poor HCC prognosis. Our findings highlight the importance of the protein kinase activity of PCK1 in the activation of SREBPs, lipogenesis and the development of HCC.
Up to 45 grams per day &ndash 160 caps daily
40 caps 2-3 hrs later (at least 2 hrs before/after food)
This article is for educational purposes only and due to FTC regulations we are unable to give specific medical advice. With this said, if you want help with your health, we offer long-distance functional health coaching programs. We are unable to treat cancer, but we can put together programs to help support your overall health and provide guidance on various health strategies you are interested in applying on your health journey.
Sources For This Article Include:
1. Beard J. THE ACTION OF TRYPSIN UPON THE LIVING CELLS OF JENSEN&rsquoS MOUSE-TUMOUR: A Preliminary Note upon a Research made (with a Grant from the Carnegie Trust). British Medical Journal. 19061(2351):140-141.
2. Conrozier T, Mathieu P, Bonjean M, Marc JF, Renevier JL, Balblanc JC. A complex of three natural anti-inflammatory agents provides relief of osteoarthritis pain. Altern Ther Health Med. 2014 Winter20 Suppl 1:32-7. PMID: 24473984
3. Hale LP, Chichlowski M, Trinh CT, Greer PK. Dietary supplementation with fresh pineapple juice decreases inflammation and colonic neoplasia in IL-10-deficient mice with colitis. Inflamm Bowel Dis. 2010 Dec16(12):2012-21. PMID: 20848493
4. Li M, Bolduc AR, Hoda MN, Gamble DN, Dolisca SB, Bolduc AK, Hoang K, Ashley C, McCall D, Rojiani AM, Maria BL, Rixe O, MacDonald TJ, Heeger PS, Mellor AL, Munn DH, Johnson TS. The indoleamine 2,3-dioxygenase pathway controls complement-dependent enhancement of chemo-radiation therapy against murine glioblastoma. J Immunother Cancer. 2014 Jul 72:21. PMID: 25054064
5. NYU Lagone Cancers, Tumors & Blood Disorders Link Here
6. Kamenícek V, Holán P, Franĕk P. [Systemic enzyme therapy in the treatment and prevention of post-traumatic and postoperative swelling]. Acta Chir Orthop Traumatol Cech. 200168(1):45-9. Czech. PMID: 11706714
7. Sophie Lanone , Robert M. Senior, Jack A. Overlapping and enzyme-specific contributions of matrix metalloproteinases-9 and -12 in IL-13&ndashinduced inflammation and remodeling. Elias Published August 15, 2002 . Citation Information: J Clin Invest. 2002110(4):463-474.
8. Science-Based Medicine: Systemic Enzyme Therapy Link Here
9. Leipner J, Saller R. Systemic Enzyme Therapy in Oncology Link Here
10. Ween MP, Oehler MK, Ricciardelli C. Role of Versican, Hyaluronan and CD44 in Ovarian Cancer Metastasis. International Journal of Molecular Sciences. 201112(2):1009-1029.
11. Wassmann S, Wassmann K, Nickenig G. Modulation of Oxidant and Antioxidant Enzyme Expression and Function in Vascular Cells. Link Here
Dr David Jockers is passionate about seeing people reach their health potential in mind, body and spirit. He is the host of the popular “Dr Jockers Functional Nutrition” podcast and the author of the best selling books, “The Keto Metabolic Breakthrough” and “The Fasting Transformation.
Let's Improve Your Health Today!
Get instant access to 2 FREE eBooks when you subscribe to Dr. Jockers’ newsletter.
"Join my tribe today to discover hidden strategies to improve your energy, brain, digestion & metabolism."— Dr. David Jockers
Proteins labeled for destruction
Degradation needs no energy – or does it?
While great attention and much research have been spent on understanding how the cell controls the synthesis of a certain protein – at least five Nobel Prizes have been awarded in this area – the reverse, the degradation of proteins, has long been considered less important. A number of simple protein-degrading enzymes were already known. One example is trypsin, which in the small intestine breaks down proteins in our food to amino acids. Likewise, a type of cell organelle, the lysosome, in which proteins absorbed from outside are broken down, had long been studied. Common to these processes is that they do not require energy in order to function.
Experiments as long ago as the 1950s showed, however, that the breakdown of the cell’s own proteins does require energy. This long puzzled researchers, and it is precisely this paradox that underlies this year’s Nobel Prize in Chemistry: that the breakdown of proteins within the cell requires energy while other protein degradation takes place without added energy. A first step towards an explanation of this energy-dependent protein degradation was taken by Goldberg and his co-workers who in 1977 produced a cell-free extract from immature red blood cells, reticulocytes, which catalyze the breakdown of abnormal proteins in an ATP-dependent manner (ATP = adenosine triphosphate – the cell’s energy currency).
Using such an extract Aaron Ciechanover, Avram Hershko and Irwin Rose, in a series of epoch-making biochemical studies in the late 1970s and early 1980s, succeeded in showing that protein degradation in cells takes place in a series of step-wise reactions that result in the proteins to be destroyed being labeled with the polypeptide ubiquitin. This process enables the cell to break down unwanted proteins with high specificity, and it is this regulation that requires energy. As distinct from reversible protein modifications such as phosphorylation (Nobel Prize in Physiology or Medicine 1992), regulation through polyubiquitination is often irreversible since the target protein is destroyed. Much of the work was done during a series of sabbatical leaves that Avram Hershko and Aaron Ciechanover of the Technion (Israel Institute of Technology) spent with Irwin Rose at the Fox Chase Cancer Center in Philadelphia, USA.
The molecule that would later prove to be the label that marks out a protein for degradation was isolated as early as 1975. This 76-amino-acid-long polypeptide was isolated from calf sweetbread and was assumed to participate in the maturation of white blood cells. Since the molecule was subsequently found in numerous different tissues and organisms – but not in bacteria – it was given the name ubiquitin (from Latin ubique, “everywhere”) (fig. 1).
The discovery of ubiquitin-mediated protein degradation
After taking his doctorate, Avram Hershko had studied energy-dependent protein degradation in liver cells, but decided in 1977 to transfer to the reticulocyte extract described above. This extract contained large quantities of hemoglobin, which upset the experiments. In their attempts to remove the hemoglobin using chromatography, Aaron Ciechanover and Avram Hershko discovered that the extract could be divided into two fractions, each inactive on its own. But it turned out that as soon as the two fractions were recombined, the ATP-dependent protein degradation restarted. In 1978 the researchers reported that the active component of one fraction was a heat-stable polypeptide with a molecular weight of only 9000 which they termed APF-1 (active principle in fraction 1). This protein later proved to be ubiquitin.
The decisive breakthrough in the research was reported in two works that Ciechanover, Hershko and Rose published in 1980. Until that time the function of APF-1 was entirely unknown. In the first work it was shown that APF-1 was bound covalently, i.e. with a very stable chemical bond, to various proteins in the extract.
In the second work it was further shown that many APF-1 molecules could be bound to the same target protein the latter phenomenon was termed polyubiquitination. We now know that this polyubiquitination of substrate proteins is the triggering signal that leads to degradation of the protein in the proteasome. It is this reaction that constitutes the actual labeling, the “kiss of death” if you will.
At a stroke, these entirely unanticipated discoveries changed the conditions for future work: it now became possible to concentrate on identifying the enzyme system that binds ubiquitin to its target proteins. Since ubiquitin occurs so generally in various tissues and organisms, it was quickly realized that ubiquitin-mediated protein degradation must be of general significance for the cell. In addition, the researchers guessed that the energy requirement in the form of ATP enabled the cell to control the specificity of the process.
The field was now open and between 1981 and 1983 Ciechanover, Hershko, Rose and their post docs and students developed “the multistep ubiquitin-tagging hypothesis” based on three newly-discovered enzyme activities they termed E1, E2 and E3 (fig. 2). We now know that a typical mammalian cell contains one or a few different E1 enzymes, some tens of E2 enzymes and several hundred different E3 enzymes. It is the specificity of the E3 enzyme that determines which proteins in the cell are to be marked for destruction in the proteasomes.
- The E1 enzyme activates the ubiquitin molecule. This reaction requires energy in the form of ATP.
- The ubiquitin molecule is transferred to a different enzyme, E2.
- The E3 enzyme can recognize the protein target which is to be destroyed. The E2-ubiquitin complex binds so near to the protein target that the actual ubiquitin label can be transferred from E2 to the target.
- The E3 enzyme now releases the ubiquitin-labeled protein.
- This last step is repeated until the protein has a short chain of ubiquitin molecules attached to itself.
- This ubiquitin chain is recognized in the opening of the proteasome. The ubiquitin label is disconnected and the protein is admitted and chopped into small pieces.
All the studies up to this point had been done in cell-free systems. To be able to study the physiological function of ubiquitin-mediated protein degradation as well, Avram Hershko and his co-workers developed an immunochemical method. By using antibodies to ubiquitin, ubiquitin-protein-conjugate could be isolated from cells where the cell proteins had been pulse-labeled with a radioactive amino acid not present in ubiquitin. The results showed that cells really break down faulty proteins using the ubiquitin system, and we now know that up to 30% of the newly-synthesized proteins in a cell are broken down via the proteasomes since they do not pass the cell’s rigorous quality control.
The proteasome – the cell’s waste disposer
What is a proteasome? A human cell contains about 30,000 proteasomes: these barrel-formed structures can break down practically all proteins to 7-9-amino-acid-long peptides. The active surface of the proteasome is within the barrel where it is shielded from the rest of the cell. The only way in to the active surface is via the “lock”, which recognizes polyubiquitinated proteins, denatures them with ATP energy and admits them to the barrel for disassembly once the ubiquitin label has been removed. The peptides formed are released from the other end of the proteasome. Thus the proteasome itself cannot choose proteins it is chiefly the E3 enzyme that does this by ubiquitin-labeling the right protein for breakdown (fig. 3).
While the biochemical mechanisms underlying ubiquitin-labeled protein degradation were laid bare around 1983 its physiological significance had not yet been fully understood. That it is of importance in destroying defective intracellular proteins was known but, to proceed, a mutated cell was needed in the ubiquitin system. By studying in detail how the mutated cell differs from a normal cell under various growth conditions, it was hoped to gain a better idea of what reactions in the cell depend on the ubiquitin system.
A mutated mouse cell had been isolated in 1980 by a research group in Tokyo. Their mouse-cell mutant contained a protein that, because of the mutation, was sensitive to temperature. At lower temperatures the protein functioned as it should, but not at higher. Cells cultured at the higher temperature stopped growing. In addition, they showed defective DNA synthesis and other erroneous functions at the higher temperature. Researchers in Boston quickly showed that the heat-sensitive protein in the mutant mouse cell was the ubiquitin-activating enzyme E1. Obviously, ubiquitin activation was necessary for the cell to function and reproduce itself at all. Controlled protein breakdown was not only important for degrading incorrect proteins in the cell but it probably also took part in control of the cell cycle, DNA replication and chromosome structure.
Since the late 1980s a number of physiologically important substrates for ubiquitin-mediated protein breakdown have been identified. Only a few of the most important will be mentioned here.
Prevention of self-pollination in plants
Most plants are bisexual, hermaphroditic. Self-pollination leads to a gradual decline in genetic diversity which in the long run can cause the whole species to die out. To prevent this, plants use ubiquitin-mediated degradation to reject “own” pollen. The exact mechanism has not yet been clarified but the E3 enzyme has been encountered and when proteasome inhibitors have been introduced, the rejection has been impaired.
When a cell is to make a copy of itself, many chemical reactions are involved. In a human being, six thousand million base pairs must be duplicated in DNA. These are gathered in 23 chromosome pairs that must be copied. Ordinary cell division, mitosis, and the formation of sex cells, meiosis, have many points of contact with the subjects of this year’s Nobel Prize. The E3 enzyme responsible, a protein complex termed the “anaphase-promoting complex” (APC) checks that the cell goes out of mitosis. This enzyme complex has also proved to play an important role in the separation of the chromosomes during mitosis and meiosis. A different protein complex acts like a rope around the chromosome pair, holding it together. At a given signal, the APC labels an inhibitor of a certain protein-degrading enzyme, whereupon the inhibitor is carried to the proteasome and destroyed. The enzyme is released, is activated and cuts the rope around the chromosome pair. Once the rope is gone, the chromosome pair can be separated. Incorrect chromosome division during meiosis is the commonest cause of spontaneous miscarriage during pregnancy, and an extra chromosome 21 in humans leads to Down’s syndrome. Most malignant tumors have cells with changed numbers of chromosomes as a result of incorrect chromosome division during mitosis.
DNA repair, cancer and programmed cell death
Protein p53 has been dubbed “the guardian of the genome” and it is a tumor-suppressor gene. This means that as long as a cell can produce p53 the development of cancer is hampered. Sure enough, the protein is mutated in at least 50% of all human cancer. The amount of protein p53 in a normal cell is low in consequence of continual production and breakdown. The breakdown is regulated through ubiquitination and the E3 enzyme responsible forms a complex with protein p53. Following DNA injury, protein p53 is phosphorylated and can no longer bind to its E3 enzyme. The breakdown stops and the quantity of p53 in the cell rises rapidly. Protein p53 acts as a transcription factor, i.e. a protein that controls the expression of a certain gene. Protein p53 binds to and controls genes that regulate DNA repair and programmed cell death. Raised levels of protein p53 lead first to interruption of the cell cycle to allow time for repair of DNA damage. If the damage is too extensive the cell triggers programmed cell death and “commits suicide”.
Infection with human papilloma virus correlates strongly to the occurrence of cervical cancer. The virus avoids the protein p53 control function through one of its proteins activating and changing the recognition pattern of a certain cellular E3 enzyme, E6-AP, which is tricked into ubiquitinating the protein p53, which is totally destroyed. In consequence of this the infected cell can no longer repair DNA damage in a normal manner or trigger programmed cell death. The DNA mutations increase in number and this can ultimately lead to the development of cancer.
Immune and inflammatory reactions
A certain transcription factor regulates many of the genes in the cell that are important for immune defense and inflammatory reactions. This protein, the transcription factor, occurs bound to an inhibitor protein in the cytoplasm of the cell, and the bound form of the transcription factor lacks activity. When cells are exposed to bacteria or various signal substances, the inhibitor protein is phosphorylated, and this results in its being ubiquitinated and broken down in the proteasome. The released transcription factor is transported to the cell nucleus where it binds to, and activates the expression of, specific genes.
The ubiquitin-proteasome system also produces the peptides that are presented by the immune defense on the surface of a virus-infected cell by breaking down virus proteins to suitable sizes. T lymphocytes recognize these peptides and attack the cell as an important part of our defense against virus infections.
The hereditary disease cystic fibrosis, CF, is caused by a non-functioning plasma membrane chloride channel called CFTR, the “cystic fibrosis transmembrane conductance regulator”. Most CF patients have one and the same genetic damage, loss of the amino acid phenylalanine in the CFTR protein. The mutation causes faulty folding of the protein and this in turn leads to the protein being retained in the cell’s control system for protein quality. This system ensures that the incorrectly folded protein is destroyed through ubiquitin-mediated protein breakdown instead of being transported out to the cell wall. A cell with no functioning chloride channel can no longer transport chloride ions through its wall. This affects secretion in, among other organs, the lungs and leads to the accretion of thick phlegm in the lungs which impairs their function, greatly increasing the risk of infection.
The ubiquitin system has become an interesting area of research for medicines against various diseases. Such preparations can be aimed at components of the ubiquitin-mediated breakdown system to prevent the degradation of specific proteins. They can also be designed to cause the system to destroy unwanted proteins. A medicine already being tested clinically is the proteasome inhibitor Velcade (PS341) which is used against multiple myeloma, a cancer disease that affects the body’s antigen-producing cells.
This year’s Laureates have explained the molecular background to a protein regulation system of great importance for all higher cells. New cell functions controlled by ubiquitin-mediated protein degradation are being discovered all the time and this research is being conducted in numerous laboratories all over the world.
Hundreds, if not thousands, of uncharacterized enzymes currently populate the human proteome. Assembly of these proteins into the metabolic and signaling pathways that govern cell physiology and pathology constitutes a grand experimental challenge. Here, we address this problem by using a multidimensional profiling strategy that combines activity-based proteomics and metabolomics. This approach determined that KIAA1363, an uncharacterized enzyme highly elevated in aggressive cancer cells, serves as a central node in an ether lipid signaling network that bridges platelet-activating factor and lysophosphatidic acid. Biochemical studies confirmed that KIAA1363 regulates this pathway by hydrolyzing the metabolic intermediate 2-acetyl monoalkylglycerol. Inactivation of KIAA1363 disrupted ether lipid metabolism in cancer cells and impaired cell migration and tumor growth in vivo. The integrated molecular profiling method described herein should facilitate the functional annotation of metabolic enzymes in any living system.
These authors contributed equally to this work.
Tay Sachs Disease
Tay-Sachs disease is a rare inherited disorder that progressively destroys nerve cells ( neurons ) in the brain and spinal cord .
The most common form of Tay-Sachs disease becomes apparent in infancy. Infants with this disorder typically appear normal until the age of 3 to 6 months, when their development slows and muscles used for movement weaken. Affected infants lose motor skills such as turning over, sitting, and crawling. They also develop an exaggerated startle reaction to loud noises. As the disease progresses, children with Tay-Sachs disease experience seizures, vision and hearing loss, intellectual disability, and paralysis. An eye abnormality called a cherry-red spot, which can be identified with an eye examination, is characteristic of this disorder. Children with this severe infantile form of Tay-Sachs disease usually live only into early childhood.
Mutations in the HEXA gene cause Tay-Sachs disease . The HEXA gene provides instructions for making part of an enzyme called beta-hexosaminidase A, which plays a critical role in the brain and spinal cord . This enzyme is located in lysosomes (remember that these are the organelles that break down toxic substances and act as recycling centers). Within lysosomes, beta-hexosaminidase A helps break down a fatty substance called GM2 ganglioside.
Mutations in the HEXA gene disrupt the activity of beta-hexosaminidase A, which prevents the enzyme from breaking down GM2 ganglioside. As a result, this substance accumulates to toxic levels, particularly in neurons in the brain and spinal cord. Progressive damage caused by the buildup of GM2 ganglioside leads to the destruction of these neurons, which causes the signs and symptoms of Tay-Sachs disease . Because Tay-Sachs disease impairs the function of a lysosomal enzyme, this condition is sometimes referred to as a lysosomal storage disorder.
Protein and Enzyme Activity Assays
Glycosidase enzymes exhibit high selectivity for hydrolysis of their preferred sugars. We offer assays for a wide range of glycosidases and related enzymes including amylase, cellulase, neuraminidase and many more.
We offer the widest and most comprehensive breadth of technologies available to screen for kinase enzyme activity. This portfolio consists of technologies that range from homogenous assays utilizing peptide substrates to antibody specific assays.
Phopholipases play an important part in cellular signaling processes via the generation of second messengers such as diacylglycerols, arachidonate, and inositol 1,4,5-triphosphate. Lipases generally include glycerol ester hydrolases and cholesterol esterases. We offer fluorogenic and fluorescent substrates and kits for detecting phospholipase-A, -C, and -D, as well as phosphatidyl inositol.
Oxidases, the most useful of which is undoubtedly horseradish peroxidase (HRP), are important enzymes that are used in a wide variety of bioassays. Peroxidase activity is also present in many cells. We offer reagents for quantitating peroxidase and the activity of a variety of other oxidases.
Enzymes play an important role in almost all cellular processes, including signaling pathways, metabolism, and gene expression, making them significant targets in drug and therapeutic development. We offer a broad range of reagents and assays for detecting enzyme activity by absorbance, fluorescence, or chemiluminescence.
Regulation of Enzyme Production in Eukaryotic Cells (With Diagram)
As might be expected, the regulation of enzyme production in eukaryotic cells is much more complex than in prokaryotic cells.
Eukaryotic cells contain a number of chromosomes instead of the single chromosome found in prokaryotes. Moreover, eukaryotic cell chromosomes are diploid and at times polyploid.
The DNA of the chromosomes is usually supercoiled and highly folded and the chromosomes themselves are physically separated from the cytoplasmic ribosomes by the nuclear envelope.
Undoubtedly, these factors make control of enzyme synthesis more complex, even though transcription and translation involve what is basically the same mechanism as in prokaryotes.
However, structural genes for a group of functionally related enzymes that might constitute an inducible component such as an operon are not found adjacent to one another on a chromosome and may, in fact, be distributed among different chromosomes.
Enzyme induction does occur in primitive eukaryotic organisms such as yeast and Neurospora, but operons are either few or nonexistent in higher eukaryotes. In all but a few cases, the mRNAs of higher eukaryotes contain the coding sequence of only one structural gene (i.e., they are monogenic).
The induction process in the fungi is slower, and the change in concentration of enzymes is not as great as in the prokaryotes. In yeast, the induction of β-galactosidase takes much longer than in E. coli also, the increase in enzyme activity is 10-fold and not a 1000-fold. The enzyme tryptophan-2,3-oxygenase found in the liver cells of vertebrates is inducible but induction requires many hours.
In addition to the B form of the DNA double helix, there is a Z form. In Z-DNA, the double helix is left-handed, each polynucleotide containing sequences of alternating purines and pyrimidines. A number of observations suggest that Z-DNA may play a role in the regulation of gene expression. For example, Z-DNA is formed in the transcriptionally active macronucleus of Stylonychia mytilis (a protozoan) and in the interband regions of Drosophila salivary chromosomes. Proteins associated with polytene chromosomes of Drosophila bind antibodies specific for Z-DNA. Some regulatory proteins appear to bind to Z-DNA but not B-DNA, including the catabolite activator protein described earlier.
2. Calcium Ions and Calmodulin:
In the past few years, it has become quite clear that calcium ions play a very important role in the regulation of certain activities in eukaryotic cells. For example, such diverse phenomena as cell motility, muscle cell contraction, chromosome movement, endocytosis, exocytosis, cyclic nucleotide metabolism, phosphorylation of proteins, and glycogen metabolism are influenced by the level of Ca 2+ in the cytosol. Some of the effects of calcium ions are mediated through a specific calcium-binding protein called calmodulin (Fig. 11- 16) found in nearly all cells.
Calmodulin is a small protein (its molecular weight is 16,720) consisting of a single polypeptide chain containing 149 amino acids. Especially interesting is the finding that its primary structure is essentially the same in all of the species that have been studied so far, indicating that the protein has undergone little change in the course of evolution.
Each molecule of calmodulin binds four calcium ions and its common occurrence in eukaryotes suggests that it has a universal regulatory function along with Ca 2 + . When Ca 2+ enters the cytosol, it is bound by calmodulin to form a complex that then influences cellular activity by interacting with certain other proteins. Some of the proteins affected by the Ca 2 + – calmodulin complex are structural proteins but most appear to be enzymes.
For example, the Ca 2 + -calmodulin complex serves to directly activate a family of enzymes called protein kinases. Which protein kinases are activated may depend on the amount of Ca 2+ in the cytosol. This is because calmodulin can bind up to four calcium ions and a complex containing only one Ca 2 + may activate a different enzyme than a complex containing two Ca 2+ (and so on). The effect of the Ca 2+ – calmodulin complex on cell metabolism may be indirect.
For example, the Ca 2 + -calmodulin complex acts to trigger adenylate cyclase activity in the plasma membranes of cells and these results in the production of cAMP. cAMP’s role as a “second messenger” influencing cell metabolism is discussed below. Additional examples of the manner in which calmodulin and Ca 2 + regulate cell metabolism are given in Table 11-4.
3. Enzyme Induction by Hormones:
Enzyme induction in prokaryotic cells is usually triggered by a potential metabolite (such as the induction of β-galactosidase by lactose). As noted above, this form of enzyme induction occurs in some eukaryotic cells as well however, in higher animals there also is a highly developed control system in which hormones act as messengers that coordinate the biosynthetic activities of cells.
This coordination may take the form of activating (or deactivating) enzymes that already exist in the cell or the effect may be on gene expression, resulting in the production of additional enzymes. The term “messenger” is appropriate, because the hormones are produced and secreted by one cell (or tissue) and have an effect on a different cell (or tissue).
The two tissues producing and responding to the messenger may be in widely separated parts of the body, the messenger traveling from one site to the other via the bloodstream. Sometimes, the effect of the messenger is to cause the “target” cell to produce and secrete a different hormone (Fig. 11-17a).
From a chemical point of view, hormones are quite diverse. Some hormones (e.g., thyroxin and epinephrine) are small molecules derived from amino acids and others are proteins (e.g., insulin and erythropoietin) or steroids (e.g., progesterone and Cortisol). Hormones regulate the production of enzymes through a variety of mechanisms. For example, the hormone may be the first messenger among a series of messengers that regulate a metabolic response (Figs. 11-17b and 11-17c). Such hormones attach to receptor sites on the plasma membrane of the “target” cell. The receptor sites consist of specific proteins having high affinities for the hormones.
Hormone binding at a receptor site serves to initiate a response by the cell. Different tissues may be affected by the same hormone. The cells in these tissues may contain similar hormone-binding receptors, but the interaction between receptor and hormone is followed by a different cellular response.
In some cases it appears that the hormone receptor of the target cell is associated with an enzyme in the plasma membrane. Hormone binding by the receptor changes the receptor-enzyme complex and activates the enzyme. Once activated, the enzyme catalyzes a reaction that forms a second messenger that passes into the cytosol and triggers further responses in the target cell.
The most common second messenger is cyclic AMP. It is formed by the enzyme adenylate cyclase, a membrane-bound enzyme associated with hormone receptor sites. cAMP produced by adenylate cyclase acts as a second messenger in metabolic pathways associated with the breakdown of glycogen and triglycerides (Fig. 11-18). cAMP appears also to be involved in cellular reactions that produce and secrete hormones.
The cellular response induced when cAMP acts as a second messenger continues for only a brief period because of the presence of phosphodiesterases in the cytosol. These enzymes degrade cAMP, producing the inactive, qoncylic mononucleotide 5′-AMP. Calcium ions also act as second messengers. Some surface receptors are associated with channels through the plasma membrane. When the receptor binds the first messenger (i.e., the hormone) the channel is opened temporarily and Ca 2+ enters the cell. Once inside the cell, the Ca 2+ is bound by specific proteins such as calmodulin.
The activation of enzymes by calmodulin was described earlier. It turns i out that many of the enzymes activated by cAMP are also activated by Ca 2 + -calmodulin and Ca 2 + – calmodulin affects the activities of enzymes that form and degrade cAMP. In turn, cAMP influences the plasma membrane channels through which Ca 2 + passes by phosphorylating the proteins that line these channels.
Cyclic GMP (cGMP), like cAMP, activates certain protein kinases. However, cGMP does not appear in response to the binding of hormones by plasma membrane receptors. Rather, the level and activity of cGMP appear to increase as the intracellular level of free Ca 2 + increases.
“Double” Second Messengers:
Many hormone receptors are associated with a group of plasma membrane phospholipids called polyphosphoinositides. Binding of the first messenger (hormone) to the receptor initiates the breakdown of the membrane-bound polyphosphoinositides, thereby forming diacylglycerol and inositol triphosphate.
The diacylglycerol enters the cytosol where it acts as a second messenger, activating a protein kinase (Fig. 11-19). (The protein kinase activated by diacylglycerol has different properties than the protein kinase activated by cAMP and Ca 2 + -calmodulin.) The inositol triphosphate produced in the plasma membrane also enters the cytosol and acts as a second messenger. When inositol triphosphate reaches the intracellular membranes, it induces the release of Ca 2 + . Ca 2 + released into the cytosol then activates certain proteins such as calmodulin.
4. Effects of Hormones on Gene Expression:
The binding of certain hormones to plasma membrane receptors is followed by internalization. Internalization takes the form of an infolding of that portion of the plasma membrane containing the receptor- hormone complex, thereby forming a vesicle.
The vesicle subsequently fuses with a Golgi body or lysosome, the enzymes of which presumably alter the receptor-hormone complex. Ultimately, a product is formed that enters the cell nucleus where it interacts with the genome, thus altering gene expression.
Steroid hormones (e.g., estrogen and progesterone) (first messenger) enter the target cell by passing directly through the plasma membrane and into the cytosol. Within the cytosol, the hormones combine with receptor molecules and the complexes then migrate to the cell nucleus, where they have a direct impact on the expression of certain genes (Fig. 11-20).
In vitro studies using chick oviduct cells have shown that the hormone estrogen enters the cytosol to form a hormone-receptor complex that migrates to the cell nucleus and induces the increased rate of transcription of the genes for lysozyme and ovalbumin.
5. Protein Phosphorylation and Metabolic Control:
Phosphorylation by protein kinases appears to be one of the major mechanisms for activating and inactivating a great number of enzymes and thereby controlling many processes in eukaryotic cells. Generally speaking, many enzymes in degradative (catabolic) pathways are activated by phosphorylation, whereas the enzymes in synthetic (anabolic) pathways are inactivated by phosphorylation.
Protein kinases activated by Ca 2 + -calmodulin, cAMP, and diacylglycerol phosphorylate key enzymes of glycolysis and other bio- degradative pathways and thereby activate these pathways. The protein kinases also phosphorylate key enzymes in glycogen metabolism, gluconeogenesis, fatty acid synthesis, cholesterol synthesis, and protein synthesis and thus inactivate these pathways. Protein kinases catalyze protein phosphorylation using ATP as the source of phosphate.
Protein phosphatases are enzymes that catalyze the removal of phosphate from phosphorylated enzymes (or other proteins). It has been proposed that protein phosphatases also function in metabolic control because they can dephosphorylate glycolytic enzymes and thereby inactivate them and can dephosphorylate glycogen-synthesizing enzymes and thereby activate them.
Although the evidence is not yet available, it is likely that protein phosphatases dephosphorylate the enzymes of gluconeogenesis, fatty acid synthesis, cholesterol synthesis, and protein synthesis and therefore activate these enzymes as well.
6. Amplification of Signals:
Signals to cells in the form of a few molecules of a hormone or other first messenger are greatly enhanced by mechanisms that use second messengers, double messengers, or even third messengers. Although only a few molecules of the first messenger may bind to the surface receptors, they activate enzymes each of which produces many copies of the second messenger. If additional messengers are produced by enzymes activated by second messengers, thousands of signals will be produced in the cytosol within a very short time.
7. Repression of the Genome in Eukaryotes:
Unlike prokaryotic cells, the cells of higher animals and plants undergo extensive differentiation, forming ‘ tissues that have specific and limited physiological roles. Yet most differentiated cells of higher plants and animals contain complete genomes.
This implies that large segments of the genome are repressed and go unexpressed. RNA-DNA saturation hybridization experiments indicate that only 6- 30% of the genome is expressed. In some cases, the repression is reversible.
For example, when differentiated carrot root cells are isolated and grown in tissue culture, new differentiated cells are produced, including cells that are characteristic of vascular tissue, storage tissue, and epidermal tissue. However, in many differentiated cells, such as nerve and muscle cells of higher animals, large portions of the genome are permanently repressed.
The mechanism of gene repression of eukaryotic cells is not yet clear. The chromosomes of eukaryotic cells contain large quantities of proteins and RNA in addition to DNA (Table 11-5), and over the years these have been considered potential repressors. However, their chemical properties do not adequately support such a contention.
For example, Stedman and Stedman proposed as long ago as the 1940s that the histones might act as gene repressors. However, there are only five major classes of histones in eukaryotic cells and they occur in about equal amounts, with little variation between different tissues of an organism or between species.
It would therefore appear that his- tone function is more fundamental. If the histones do have repressor activity, then the effect is nonspecific. Electrophoretic analysis of the non-histone proteins in chromatin reveals that they occur in much greater variety than the histones. However, their diversity is insufficient to support the contention that they play a role in gene repression.
8. The Britten-Davidson Model of Gene Regulation in Eukaryotes:
Although several models have been proposed to explain gene regulation in eukaryotes, none has been documented with evidence that would give it the degree of certainty that surrounds the Jacob-Monod operon model for prokaryotic cells. However, the model proposed by R. Britten and E. Davidson in 1969 has attracted a great deal of attention.
According to this model (Fig. 11-21), the nuclear chromosomes contain DNA sequences called sensor genes that recognize various cellular substances such as metabolic inducers (substrates), hormone-receptor complexes, or regulatory nucleotides (e.g., ppGpp).
When the inducer enters the nucleus, it binds to the sensor and promotes the transcription of an adjacent integrator gene whose product is a specific activator RNA. The activator RNA can attach to appropriate DNA sequences that constitute receptor sites on either the same or a different chromosome. Presumably, the function of the activator would be analogous to that of the cAMP-CAP for prokaryotes.
The binding of the activator to a receptor site promotes transcription of adjacent structural genes. After undergoing some modification called “processing” the mRNA transcript leaves the nucleus and is translated into protein.
A number of elaborate modifications of the basic model can explain the variations in gene expression and differentiation in eukaryotes. For example (Fig. 11-22a), a number of different sensor genes (S1, S2, and S3) on binding different inducers (I1, I2, and I3) promote the formation of different activator RNA molecules (a, b, and c) by their companion integrator genes.
The presence of multiple receptor sites for each structural gene (i.e., L, M, and N) would imply that different combinations of structural genes would be transcribed, depending on binding of the various activators to their respective receptor sites. The binding of an activator to any one receptor would trigger transcription of the adjacent structural gene.
In an alternative model (Fig. 11-22b), transcription of structural genes in various combinations results from the binding of a specific inducer. In this variation, the sensors initiate activator synthesis in a number of adjacent integrator genes, and each activator then associates with one receptor.
The models have a number of interesting possibilities and are supported by the observation that a large proportion of the DNA in eukaryotic cells (e.g., 40% in calf thymus gland cells) consists of repeated nucleotide sequences that are too small to be structural genes these, perhaps, are receptor sequences.
A final regulatory mechanism in eukaryotic cells is the physical separation and isolation of groups of enzymes within membranous boundaries, that is, specific groups of enzymes are compartmentalized within the cellular organelles. For example, in both plant and animal cells the enzymes of the tricarboxylic acid or Krebs cycle are physically separated from those of glycolysis because the former are confined within mitochondria and the latter are present in the cytosol.
In plant cells, the enzymes of the dark reactions of photosynthesis are physically isolated within chloroplasts. The enzymes of the glyoxylate cycle are compartmentalized in micro- bodies, whereas many of the cell’s powerful hydrolytic enzymes are restricted within the lysosomes.
Many of these compartmentalized enzymes act on substrates or employ cofactors that are produced by enzymes that are restricted to other parts of the cell. Regulation of the transport of these compounds across cellular membranes from one cell compartment to another affords yet another level at which the control at metabolsim can be exercised.
Enzyme with high potential for new cancer treatment identified
A team of researchers from the Biology department at the TU Darmstadt has identified an enzyme that separates DNA replication from repair. This discovery could be of tremendous significance in the treatment of tumours. The scientists have now published the results of their research in the renowned research journal Molecular Cell.
Several essential processes take place in separate compartments within a cell. The cell's most important asset, DNA, is packed inside the cell nucleus, and a veritable armada of proteins is responsible for the organisation, replication and protection of the DNA. Many of these proteins perform a number of tasks, for instance possessing functions at the replication fork and repairing damage to DNA. Most of the mechanisms for the individual functions of these proteins are understood, but there has as yet been little research into the coordination and regulation between the various processes.
Biologists at the TU Darmstadt under Prof. Dr. Markus Löbrich and Dr. Julian Spies have collaborated with their colleagues at the University of California in Davis and identified a protein kinase called Nek1 that promotes the repair of DNA double-strand breaks and separates this repair from replication. Nek1 switches on the motor protein Rad54 only after the completion of replication in order to finalize the repair process. This is of physiological relevance because during replication, Rad54 possesses additional functions at the replication fork, and premature activation of Rad54 results in a major disturbance of the replication process, report the Darmstadt-based researchers in the renowned research journal Molecular Cell.
Use in cancer treatment
This discovery has a very high potential for use in the development of entirely new kinds of cancer treatments. Finding inhibitors that block the function of Nek1 would lead to a loss in the repair function. Tumour cells in particular would suffer from this loss of function in Nek1, since they experience a tremendous amount of DNA damage during their uncontrolled growth. The scientists suspect that the inhibition of Nek1 could be associated with an accumulation of unrepaired DNA damage in these cells that could cause the tumour cells to die. The team of researchers plans to continue to investigate these assumptions over the coming years.
Nutrition Needs for Cancer Survivors
After completion of cancer treatment, and assuming good health, calorie needs may be the same as other healthy adults without a history of cancer. According to the U.S. Dietary Guidelines 2015-2020, healthy women generally require 1,600 to 2,000 calories daily to maintain weight, and men require 2,000 to 3,000 per day.
Of course, individual calorie requirements can be more personalized and based on activity level, weight status, age and health status. The Recommended Dietary Allowance for protein in healthy adults is a modest 0.8 grams per kilogram of body weight, or 56 grams daily for a person weighing 154 pounds and 72 grams daily for someone weighing 198 pounds.
For long-term health after cancer treatment, a high-quality diet matters and may extend life in cancer survivors, according to research published in the December 2016 issue of Nutrition Reviews.
For people with a history of cancer, the Academy of Nutrition and Dietetics recommends a diet that emphasizes fruits, vegetables, whole grains, beans, lentils and nuts, and limits refined grains, added sugars, red meat and alcohol. This diet pattern is also heart-healthy and is linked to reducing the risk of other health problems. Since more research is needed in the area of nutrition and cancer survivorship, talk to your doctor and dietitian about a long-term eating plan that is right for you.