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Cell Proliferation
Cell Proliferation N Yang and SD Ray, Manchester University, Fort Wayne, IN, USA K Krafts, University of Minnesota, Minneapolis, MN, USA Ó 2014 Elsevier Inc. All rights reserved. This article is a revision of the previous edition article by Sanjay Chanda and Harihara M. Mehendale, volume 1, pp 495–498, Ó 2005, Elsevier Inc.
Introduction Unicellular organisms like yeasts, bacteria, or protozoa have a strong selective pressure to grow and divide as rapidly as possible. The rate of cell division in these cases is limited only by the rate at which nutrients can be taken from the medium and converted into cellular materials. Multicellular organisms, in contrast, are made up of many different types of cells performing a variety of functions. The primary drive is the survival of the organism as a whole rather than the survival or proliferation of a single individual type of cell. Therefore, different cells proliferate at different times, depending on the changing needs of the organism. Some tissues contain cells that, once terminally differentiated, are incapable of reentering the cell cycle. These tissues are called nondividing tissues; an example is neural tissue. Other tissues, such as liver tissue, contain cells that normally reside outside the cell cycle, but which can be stimulated to proliferate when necessary. These tissues are called stable tissues. A third type of tissue, termed continuously dividing tissue, is continuously being replaced due to frequent cell sloughing or cell death. Skin and bone marrow are examples of this type of cell. Replacement in these tissues is accomplished through native stem cell populations. For multicellular organisms to survive, some cells must refrain from dividing even when nutrients are plentiful. In tissues capable of cell proliferation (stable and continuously dividing tissues), when the need arises for new cells, as in the case of tissue injury, cells are replaced either by cell division or by replenishment from stem cell reserves.
replaced promptly to restore tissue function. The phases of cell division and the transitions between the phases are orchestrated by an intricate series of signaling mechanisms. When the lost tissue is replaced, the cells return to the normal resting state, thereby reestablishing the cellular, organ, and tissue homeostatic mechanisms.
Genetic Control of Cell Structure and Function during and after Embryonic Development Multicellular animals are clones of cells descended from a single original cell, the fertilized egg. The cells in the body, as a rule, are genetically alike. However, they are phenotypically different; some are muscle cells, others neurons, still others hepatocytes, and so on. The different cell types are arranged into precisely organized tissues and organs, and the entire structure has a well-defined shape. All of these features are determined by the DNA sequence of the genome, which is reproduced in every cell. Each cell must act according to the same genetic instructions, but it must interpret them with due regard to time and circumstance so as to play a proper part in multicellular organization. The development of vertebrates can be divided into three phases. In the first phase, the fertilized egg divides into many smaller cells that become organized into epithelium. Following a complex series of gastrulation and neurulation movements, a rudimentary gut cavity and neural tube are formed. In the second or organogenesis phase, the various organs, such as limbs, eyes, and heart, are formed. In the third
Division Cycle of Cells Adult multicellular animals must produce millions of new cells in order to replace dead cells. Cells undertake the process of division by progressing through a highly regulated process known as the cell cycle (Figure 1), the end product being a duplication of the contents of the mother cell into two daughter cells. In an adult animal, most cells are stable cells, and reside in the G0 (gap) phase of cell cycle. When division is necessary, cells that are capable of doing so enter the G1 phase of the cell cycle. In most cells the DNA in the nucleus is replicated during only a limited portion of the cell cycle called the S (synthesis) phase of the cell cycle. After the S phase, the cells enter a second gap phase called the G2 phase. Finally, in the M (mitosis) phase, the contents of the nucleus condense to form visible chromosomes, which through an elaborately orchestrated series of movements are pulled apart into two equal sets. The cell itself then splits into two daughter cells. Upon loss of tissues due to injury, or due to normal cell sloughing, the division cycle of cells is stimulated in tissue- or organ-specific fashion so that the lost tissues can be
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Figure 1 In adult organisms normally cells are in resting phase (G0) of the cell division cycle. Upon appropriate stimulus the cells enter the division cycle, which is characterized by G1, S, G2, and M phases. After division, the daughter cells (D) may either reenter the division cycle or enter the resting phase, depending on the stimulus.
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phase, the generated structures grow to their adult size. These phases are not sharply distinct but overlap considerably in time.
Terminal Differentiation and Cell Division After embryonic development, cells in the normal adult human body divide at very different rates. Some, such as neurons and skeletal muscle cells, do not divide at all; others, such as liver cells, normally divide once every year or two; and certain epithelial cells in the gut divide more than twice a day so as to provide constant renewal of the gut lining. Most cells in the vertebrates fall somewhere between these extremes: they are able to divide, but normally do so infrequently. Almost all the variation in division rate lies in the time cells spend between mitosis and the S phase; slowly dividing cells remain arrested after mitosis for weeks or even years. By contrast, the time taken for a cell to progress from the beginning of S phase through mitosis is brief (typically 12–24 h in mammals) and remarkably constant, irrespective of the interval from one division to the next. The time cells spend in a nonproliferative state (G0 in the cell cycle) varies according to both the cell type and the circumstances under which division is stimulated. Hepatocytes, for example, exist mostly in a resting state unless liver damage provokes proliferation. In contrast, uterine lining cells enter the cell cycle for a few days each month. A fraction of all hematopoietic precursors are always dividing to compensate for normal cell loss; this fraction increases after an episode of blood loss. Delicately adjusted and highly specific controls govern the proliferation of each class of cells in the body in each situation.
Role of Growth Factors and Cytokines in Cell Division When put into an artificial culture medium completely devoid of serum, vertebrate cells normally do not pass through the G1/S restriction point, even though all the requisite nutrients are present in the medium. Rather, they halt their progress
Table 1
through the cell cycle. In order to complete the cycle of cell growth and division, cells require highly specific growth factors and cytokines, usually present in very small concentrations (109–1011 mol l1) in the serum. Different cells require different combinations of growth factors and cytokines. Some directly stimulate cell division and are called complete mitogens. Others control cell division by directly inhibiting cell cycle progression; these are called growth inhibitors. Yet others cause cell cycle progression in an indirect way and are called growth triggers. Table 1 provides examples of growth factors and cytokines involved in cell division along with their particular functions. Growth factors and cytokines often interact with cell-surface receptors in order to carry out their particular functions. For example, epidermal growth factor (EGF) binds to a receptor tyrosine kinase (RTK) (a cell-surface molecule present on many different kinds of cells). This triggers phosphorylation of tyrosine residues and dimerization of the receptors. An adaptor protein called Grb2 (growth factor receptor-bound protein 2) is then recruited to promote the formation of Ras-GTP. Active Ras-GTP binds to and activates a protein kinase termed Raf. Raf further phosphorylates and activates MEK (MAP kinase kinase). The downstream target is MAP kinase (MAPK). Active MAPK phosphorylates various proteins, including transcription factors that regulate the expression of cell cycle proteins to induce cell proliferation.
Cell Senescence and Reluctance to Divide Most normal mammalian cells show a striking inability to proliferate indefinitely. Fibroblasts taken from a normal human fetus, for example, undergo approximately 50 population doublings when cultured in a standard growth medium. Toward the end of this time, proliferation slows down and after spending some time in a quiescent state, the cells die. Similar cells taken from a 40-year-old person stop dividing after approximately 40 doublings, while cells from an 80-year-old stop after approximately 30 doublings. Fibroblasts from animals with shorter life spans stop after a smaller number of division cycles in culture. Because of the
Example of growth factors and cytokines known to regulate cell proliferation
Factor
Representative functions
Platelet-derived growth factor (PDGF)
Stimulates proliferation of connective tissue cells and neuroglial cells Stimulates proliferation of many cell types Work with PDGF and EGF to stimulate fat cell proliferation Stimulates proliferation of many cell types including fibroblasts, endothelial cells, and myoblasts Stimulates proliferation of T lymphocytes Inhibits cell cycle progression of different cell types Inhibits proliferation of hepatocytes and other cells types Inhibits hepatocyte proliferation Promotes axon growth and survival of sympathetic and some sensory and CNS neurons Promote division of different blood cells and various other types of cells
Epidermal growth factor (EGF) Insulin like growth factors I and II (IGF-I and -II) Fibroblast growth factor (FGF) Interleukin-2 (IL-2) Transforming growth factor b (TGF-b) Interleukin-1 (IL-1) Hepatocyte proliferation inhibitor Nerve growth factor (NGF) Hematopoietic cell growth factors (IL-3, GM-CSF, M-CSF, G-CSF, and erythropoietin)
Cell Proliferation
correspondence with aging of the body as a whole, this phenomenon is called cell senescence. According to one theory, cell senescence is the result of a catastrophic accumulation of self-propagating errors in a cell’s biosynthetic machinery. An alternative theory suggests that cell senescence is the result of a mechanism that has evolved to protect us from cancer by limiting the growth of tumors. It has been reported that telomeres play an essential role in chromosome capping affecting the cell proliferation. Telomere DNA undergoes progressive shortening of 50 and 200 bp per round of DNA replication. And then, telomere dysfunction leads to disruption of the telomere structure, resulting in end-to-end chromosome fusions and genomic instability. It can activate DNA damageinduced pathways that trigger cell cycle arrest or apoptosis.
Cell Proliferation as a Compensatory Response to Toxic Tissue Injury Human beings are exposed to numerous toxic insults every day. Fortunately, the body has several defense mechanisms to combat toxicants. Some toxicants are prevented from entering the body by virtue of their particle size. Toxicants that do enter the body are metabolized or conjugated in an attempt to safely carry out their excretion. When these first lines of defense are overcome, toxic substances may cause severe cell injury or even cell death. At this point, the tissue may respond by stimulating its healthy cells to divide and restore tissue structure and function. The ability of the tissue to undergo repair depends on the type of tissue damaged and the extent of the damage. Damage occurring in tissues that are unable to proliferate (for example, cardiac muscle) will not result in replacement of parenchymal cells, and a scar will take place of the dead cells. Likewise, damage that disrupts the supporting structure of the tissue – the basement membrane scaffolding upon which the cells reside – will likewise involve scarring rather than complete resolution to the normal, pre-damaged state. The process of tissue repair stops at a precise, preordained point. For example, liver regeneration ends when the functional mass of the liver is restored. At low to moderate doses of a particular toxicant, the process functions well, and repair is usually adequate. At high doses of a toxicant, however, the ability of the cells to progress through the cell cycle is inhibited, leading to two consequences. First, dead cells are not replaced, which may lead to organ failure and death. Second, in the absence of compensatory cell division, which normally serves to contain the toxic injury, tissue injury can progress in an unrestrained manner. The ability of cells to enter and progress through the cell cycle following toxic injury decreases with age, a finding which explains, in part, why an 80-year-old person may be more susceptible to the same dose of a toxicant as a 40-year-old. Table 2 shows a list of drugs and chemicals that affect cell proliferation in a variety of model systems.
Stem Cells and Terminally Differentiated Cells Many of the tissues in the body undergoing constant renewal, such as skin and the lining of the intestine, accomplish this task
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by means of a small population of tissue stem cells. The defining properties of a stem cell are (1) the ability to divide virtually without limit throughout the lifetime of the organism and (2) the ability to divide symmetrically (leading to two terminally differentiated cells) or asymmetrically (giving rise to one stem cell and one terminally differentiated cell). Stem cells are required wherever there is a recurring need to replace nondividing, terminally differentiated cells. Some terminally differentiated cells, such as mature mammalian red blood cells and the cells in the outermost layer of the skin, lack a cell nucleus and are therefore unable to divide. Others contain cytoplasmic structures (such as the myofibrils of striated muscle cells) that hinder cell duplication. And in some terminally differentiated cells, the chemistry of differentiation may simply be incompatible with cell division. The job of the stem cell is not to carry out the function of the differentiated cell, but rather to produce the cells that will carry out those functions. Stem cells that give rise to only one type of differentiated cell are called unipotent; these are capable of differentiating along only one lineage. Also, adult stem cells in many differentiated undamaged tissues are typically unipotent and give rise to just one cell type under normal conditions. Additionally, a small number of cell types are called oligopotent. These stem cells have the ability to differentiate into just a few types of cells. A lymphoid stem cell is an example of an oligopotent stem cell. These stem cells cannot develop into any type of blood cell as bone marrow stem cells can. They give rise to only blood cells of the lymphatic system, such as T cells. Cells that give rise to many cell types are called pluripotent or totipotent. Yet, there is another type of stem cell that is called multipotent. These cells are progenitor cells that have the genetic potential to differentiate into multiple, but limited cell types.
Cell Proliferation and Cancer Cells within a tissue exert an inhibitory effect on each other’s growth. This restraining force is called social control of cell division, and it is mediated by a set of genes called social control genes. A cell that acquires a DNA mutation that disrupts this social restraint will divide without regard to the needs of the organism as a whole, and its progeny may become tumor cells. Approximately 1016 cell divisions take place in a human body in the course of a lifetime. Even in an environment that is free of mutagens, mutations occur spontaneously at an estimated rate of about 106 mutations per gene per cell division, a value set by fundamental limitations on the accuracy of DNA replication and repair. Thus, in an average person’s lifetime, every single gene is likely to have undergone mutations on about 1010 separate occasions. Some mutated genes – if not repaired by the cell’s DNA repair mechanisms – are involved in the regulation of cell division. Consequently, the affected cell, now lacking the normal ‘brakes’ on cell growth, may continuously progress through the cell cycle. To transform into a malignant cell, however, a cell must acquire not just one but a number of DNA mutations to escape the multiple controls on cell division. Further mutations endow the cell with the capacity for invasion
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Table 2
Summary of some toxicants that may affect cell proliferation
Compound
Origin
Specific mechanism
Source reference
Natural food toxicants Ochratoxin A
Fungi
Citrinin
Fungi
Okadaic acid
Algae
Glucosinolates
Plants
Psoralen
Plants
Pyrrolizidine alkaloid
Plants
Induces cell proliferation by activating Cyclin-D and Cox-2. Inhibits cell proliferation by increasing p53 and p21. Inhibits cell proliferation by inducing c-Myc. Inhibits cell proliferation by G2/M arrest. Inhibits cell proliferation by BMP signaling. Inhibits cell proliferation by activating p38.
Kumar R., et al., 2013. Carcinogenesis. 34:647. Chang C.H., et al., 2011. Toxicol. Sci. 119:84. Zhang L., et al., 2007. Cancer Res. 67:10198. Liang H et al., 2008. J. Nat. Prod. 71:1911. Tang D.Z., et al., 2011. Biochem. Biophys. Res. Commun. 405:256. Ji L.L., et al., 2002. Toxicon. 40:1685.
Environmental toxicants Nonmetals (As, S)
Soils, air
Inhibit cell proliferation by DNA damage.
Metals (Cd2þ, Hg2þ, Pb2þ)
Water
Inhibit cell proliferation by their interaction with DNA metabolism.
1,3-Butadiene
Rubber, plastics
Polycyclic aromatic hydrocarbons (PAHs) Dichlorodiphenyl-trichloroethane (DDT) Polychlorinated biphenyls (PCBs) Lithium
Tobacco smoke
Increases cell proliferation by DNA modification. Increases cell proliferation by increasing ER. Increases cell proliferation by chromosomal alterations. Induces cell proliferation of collecting ducts in kidneys. Induces cell proliferation in the liver.
Dimethylnitrosamine
Food and environment
Inhibits cell proliferation in the liver
Doxorubicin þ caffeine
Anticancer drug
Increases in hematopoietic cell proliferation
TCDD
Environmental toxin
Wu J.Z., et al., 2006. Eur. J. Pharm. Sci. 29:35. Costa M., et al., 1982. Res. Commun. Chem. Pathol. Pharmacol. 38:405. Melnick R.L., et al., 2001. Chem. Biol. Interact. 135:27. Plísková M., et al., 2005. Toxicol. Sci. 83:246. Uppala P.T., et al., 2005. Environ. Mol. Mutagen. 46:43. Chaudhuri L., et al., 2010. Free Radic. Biol. Med. 49:40. Gao Y., et al., 2013. Am. J. Physiol. Renal. Physiol. July 24 2013. (Epub ahead of print) Syed, I., et al., 2012. Mol. Cell. Biochem. 365(1–2): 351–361. Motegi, T., et al., 2013. Res. Vet. Sci., July 18 2013. pii: S0034-5288(13) 00210-5. doi: 10.1016/j.rvsc.2013.06.011. (Epub ahead of print) National Toxicology Program. Natl. Toxicol. Program Tech. Rep. Ser. November 2010; (558):1–206.
Pesticides Pesticides Nephrogenic diabetes insipidus
and metastasis. Statistically, it is estimated that somewhere between three and seven independent random events, each of low probability, are typically required to turn a normal cell into a cancer cell; the smaller numbers apply to leukemia and the larger numbers to carcinomas. Proto-oncogenes are normal genes encoding proteins involved in cell proliferation. Like any gene, a proto-oncogene may undergo mutation. When a mutation in a proto-oncogene confers a gain of function, the new mutant gene (now called an oncogene) will dramatically stimulate cell division. Tumor suppressor genes, in contrast, are normal genes that encode proteins that inhibit cell proliferation. If a tumor suppressor gene is mutated in such a way as to inactivate the gene, this removal of inhibition may make the cell grow continuously. Mutations may be spontaneous, or they may result from
exposure to chemical carcinogens or radiation. Mutations in tumor suppressor genes generally need to occur on both alleles for a tumor to arise, whereas most proto-oncogenes need only one mutated allele (one oncogene) to contribute to malignancy.
Importance of Understanding the Mechanisms in Control of Cell Proliferation An understanding of the mechanisms in control of cell proliferation is critical in the development of new tissue restoration therapies. Current clinical treatments for patients with drug overdoses or chemical poisoning are aimed primarily at preventing additional injury, either by blocking further formation
Cell Proliferation
of toxic metabolites or by increasing clearance of the toxin from the body. While these strategies are useful, the survival of the tissue – and sometimes the patient – is heavily dependent on tissue repair, the success of which is in turn contingent on the ability of cells to proliferate. In cases of toxic exposure in which tissue repair is delayed, either due to the massivity of the exposure or to a delay in treatment, organ loss and even death may occur because the damage compromises the regenerating ability of the cells, thereby paving the way for unrestrained progression of injury. If cellular regeneration after massive tissue damage could be actively stimulated, it might be possible to prevent organ loss and death. Animal experiments provide concrete examples of how modification of tissue repair can directly influence survival. Animals given ordinarily lethal doses of toxins are able to survive – even when there is massive liver injury – when tissue repair in the liver is stimulated. Conversely, animals receiving otherwise nonlethal doses of toxins develop liver failure and die if cell division (and therefore tissue repair) is blocked by antimitotic agents. Perhaps carefully induced suppression of pathways involved in cell death and stimulation of pathways involved in cell division could stop the progression of toxic injury and
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restore organ structure and function. With the advent of gene therapy, specific genes could, one day, be delivered directly to the damaged organ to induce expression/suppression of the appropriate factors needed for recovery. Although this may require time, with technological advances this route of therapy appears to be doable.
See also: Cell Cycle.
Further Reading Alberts, B., Johnson, A., Lewis, J., Raff, M. (Eds.), 2007. Molecular Biology of the Cell, fifth ed. Garland, New York. Chanda, S., Mehendale, H.M., 1996. Hepatic cell division and tissue repair: a key to survival after liver injury. Mol. Med. Today 2, 82–89. Gallagher, E.J., LeRoith, D., 2011. Diabetes, cancer, and metformin: connections of metabolism and cell proliferation. Ann. N. Y. Acad. Sci. 1243, 54–68. González-Mariscal, L., Lechuga, S., Garay, E., 2007. Role of tight junctions in cell proliferation and cancer. Prog. Histochem. Cytochem. 42, 1–57. Lodish, H., Berk, A., Kaiser, C.A., et al. (Eds.), 2007. Molecular Cell Biology, sixth ed. Scientific American Books (Distributed by Freeman New York), New York. Mehendale, H.M., 1995. Injury and repair as opposing forces in risk assessment. Toxicol. Lett. 82–83, 891–899. Reinehr, R., Häussinger, D., 2009. Epidermal growth factor receptor signaling in liver cell proliferation and apoptosis. Biol. Chem. 390, 1033–1037.
Introduction Unicellular organisms like yeasts, bacteria, or protozoa have a strong selective pressure to grow and divide as rapidly as possible. The rate of cell division in these cases is limited only by the rate at which nutrients can be taken from the medium and converted into cellular materials. Multicellular organisms, in contrast, are made up of many different types of cells performing a variety of functions. The primary drive is the survival of the organism as a whole rather than the survival or proliferation of a single individual type of cell. Therefore, different cells proliferate at different times, depending on the changing needs of the organism. Some tissues contain cells that, once terminally differentiated, are incapable of reentering the cell cycle. These tissues are called nondividing tissues; an example is neural tissue. Other tissues, such as liver tissue, contain cells that normally reside outside the cell cycle, but which can be stimulated to proliferate when necessary. These tissues are called stable tissues. A third type of tissue, termed continuously dividing tissue, is continuously being replaced due to frequent cell sloughing or cell death. Skin and bone marrow are examples of this type of cell. Replacement in these tissues is accomplished through native stem cell populations. For multicellular organisms to survive, some cells must refrain from dividing even when nutrients are plentiful. In tissues capable of cell proliferation (stable and continuously dividing tissues), when the need arises for new cells, as in the case of tissue injury, cells are replaced either by cell division or by replenishment from stem cell reserves.
replaced promptly to restore tissue function. The phases of cell division and the transitions between the phases are orchestrated by an intricate series of signaling mechanisms. When the lost tissue is replaced, the cells return to the normal resting state, thereby reestablishing the cellular, organ, and tissue homeostatic mechanisms.
Genetic Control of Cell Structure and Function during and after Embryonic Development Multicellular animals are clones of cells descended from a single original cell, the fertilized egg. The cells in the body, as a rule, are genetically alike. However, they are phenotypically different; some are muscle cells, others neurons, still others hepatocytes, and so on. The different cell types are arranged into precisely organized tissues and organs, and the entire structure has a well-defined shape. All of these features are determined by the DNA sequence of the genome, which is reproduced in every cell. Each cell must act according to the same genetic instructions, but it must interpret them with due regard to time and circumstance so as to play a proper part in multicellular organization. The development of vertebrates can be divided into three phases. In the first phase, the fertilized egg divides into many smaller cells that become organized into epithelium. Following a complex series of gastrulation and neurulation movements, a rudimentary gut cavity and neural tube are formed. In the second or organogenesis phase, the various organs, such as limbs, eyes, and heart, are formed. In the third
Division Cycle of Cells Adult multicellular animals must produce millions of new cells in order to replace dead cells. Cells undertake the process of division by progressing through a highly regulated process known as the cell cycle (Figure 1), the end product being a duplication of the contents of the mother cell into two daughter cells. In an adult animal, most cells are stable cells, and reside in the G0 (gap) phase of cell cycle. When division is necessary, cells that are capable of doing so enter the G1 phase of the cell cycle. In most cells the DNA in the nucleus is replicated during only a limited portion of the cell cycle called the S (synthesis) phase of the cell cycle. After the S phase, the cells enter a second gap phase called the G2 phase. Finally, in the M (mitosis) phase, the contents of the nucleus condense to form visible chromosomes, which through an elaborately orchestrated series of movements are pulled apart into two equal sets. The cell itself then splits into two daughter cells. Upon loss of tissues due to injury, or due to normal cell sloughing, the division cycle of cells is stimulated in tissue- or organ-specific fashion so that the lost tissues can be
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Figure 1 In adult organisms normally cells are in resting phase (G0) of the cell division cycle. Upon appropriate stimulus the cells enter the division cycle, which is characterized by G1, S, G2, and M phases. After division, the daughter cells (D) may either reenter the division cycle or enter the resting phase, depending on the stimulus.
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phase, the generated structures grow to their adult size. These phases are not sharply distinct but overlap considerably in time.
Terminal Differentiation and Cell Division After embryonic development, cells in the normal adult human body divide at very different rates. Some, such as neurons and skeletal muscle cells, do not divide at all; others, such as liver cells, normally divide once every year or two; and certain epithelial cells in the gut divide more than twice a day so as to provide constant renewal of the gut lining. Most cells in the vertebrates fall somewhere between these extremes: they are able to divide, but normally do so infrequently. Almost all the variation in division rate lies in the time cells spend between mitosis and the S phase; slowly dividing cells remain arrested after mitosis for weeks or even years. By contrast, the time taken for a cell to progress from the beginning of S phase through mitosis is brief (typically 12–24 h in mammals) and remarkably constant, irrespective of the interval from one division to the next. The time cells spend in a nonproliferative state (G0 in the cell cycle) varies according to both the cell type and the circumstances under which division is stimulated. Hepatocytes, for example, exist mostly in a resting state unless liver damage provokes proliferation. In contrast, uterine lining cells enter the cell cycle for a few days each month. A fraction of all hematopoietic precursors are always dividing to compensate for normal cell loss; this fraction increases after an episode of blood loss. Delicately adjusted and highly specific controls govern the proliferation of each class of cells in the body in each situation.
Role of Growth Factors and Cytokines in Cell Division When put into an artificial culture medium completely devoid of serum, vertebrate cells normally do not pass through the G1/S restriction point, even though all the requisite nutrients are present in the medium. Rather, they halt their progress
Table 1
through the cell cycle. In order to complete the cycle of cell growth and division, cells require highly specific growth factors and cytokines, usually present in very small concentrations (109–1011 mol l1) in the serum. Different cells require different combinations of growth factors and cytokines. Some directly stimulate cell division and are called complete mitogens. Others control cell division by directly inhibiting cell cycle progression; these are called growth inhibitors. Yet others cause cell cycle progression in an indirect way and are called growth triggers. Table 1 provides examples of growth factors and cytokines involved in cell division along with their particular functions. Growth factors and cytokines often interact with cell-surface receptors in order to carry out their particular functions. For example, epidermal growth factor (EGF) binds to a receptor tyrosine kinase (RTK) (a cell-surface molecule present on many different kinds of cells). This triggers phosphorylation of tyrosine residues and dimerization of the receptors. An adaptor protein called Grb2 (growth factor receptor-bound protein 2) is then recruited to promote the formation of Ras-GTP. Active Ras-GTP binds to and activates a protein kinase termed Raf. Raf further phosphorylates and activates MEK (MAP kinase kinase). The downstream target is MAP kinase (MAPK). Active MAPK phosphorylates various proteins, including transcription factors that regulate the expression of cell cycle proteins to induce cell proliferation.
Cell Senescence and Reluctance to Divide Most normal mammalian cells show a striking inability to proliferate indefinitely. Fibroblasts taken from a normal human fetus, for example, undergo approximately 50 population doublings when cultured in a standard growth medium. Toward the end of this time, proliferation slows down and after spending some time in a quiescent state, the cells die. Similar cells taken from a 40-year-old person stop dividing after approximately 40 doublings, while cells from an 80-year-old stop after approximately 30 doublings. Fibroblasts from animals with shorter life spans stop after a smaller number of division cycles in culture. Because of the
Example of growth factors and cytokines known to regulate cell proliferation
Factor
Representative functions
Platelet-derived growth factor (PDGF)
Stimulates proliferation of connective tissue cells and neuroglial cells Stimulates proliferation of many cell types Work with PDGF and EGF to stimulate fat cell proliferation Stimulates proliferation of many cell types including fibroblasts, endothelial cells, and myoblasts Stimulates proliferation of T lymphocytes Inhibits cell cycle progression of different cell types Inhibits proliferation of hepatocytes and other cells types Inhibits hepatocyte proliferation Promotes axon growth and survival of sympathetic and some sensory and CNS neurons Promote division of different blood cells and various other types of cells
Epidermal growth factor (EGF) Insulin like growth factors I and II (IGF-I and -II) Fibroblast growth factor (FGF) Interleukin-2 (IL-2) Transforming growth factor b (TGF-b) Interleukin-1 (IL-1) Hepatocyte proliferation inhibitor Nerve growth factor (NGF) Hematopoietic cell growth factors (IL-3, GM-CSF, M-CSF, G-CSF, and erythropoietin)
Cell Proliferation
correspondence with aging of the body as a whole, this phenomenon is called cell senescence. According to one theory, cell senescence is the result of a catastrophic accumulation of self-propagating errors in a cell’s biosynthetic machinery. An alternative theory suggests that cell senescence is the result of a mechanism that has evolved to protect us from cancer by limiting the growth of tumors. It has been reported that telomeres play an essential role in chromosome capping affecting the cell proliferation. Telomere DNA undergoes progressive shortening of 50 and 200 bp per round of DNA replication. And then, telomere dysfunction leads to disruption of the telomere structure, resulting in end-to-end chromosome fusions and genomic instability. It can activate DNA damageinduced pathways that trigger cell cycle arrest or apoptosis.
Cell Proliferation as a Compensatory Response to Toxic Tissue Injury Human beings are exposed to numerous toxic insults every day. Fortunately, the body has several defense mechanisms to combat toxicants. Some toxicants are prevented from entering the body by virtue of their particle size. Toxicants that do enter the body are metabolized or conjugated in an attempt to safely carry out their excretion. When these first lines of defense are overcome, toxic substances may cause severe cell injury or even cell death. At this point, the tissue may respond by stimulating its healthy cells to divide and restore tissue structure and function. The ability of the tissue to undergo repair depends on the type of tissue damaged and the extent of the damage. Damage occurring in tissues that are unable to proliferate (for example, cardiac muscle) will not result in replacement of parenchymal cells, and a scar will take place of the dead cells. Likewise, damage that disrupts the supporting structure of the tissue – the basement membrane scaffolding upon which the cells reside – will likewise involve scarring rather than complete resolution to the normal, pre-damaged state. The process of tissue repair stops at a precise, preordained point. For example, liver regeneration ends when the functional mass of the liver is restored. At low to moderate doses of a particular toxicant, the process functions well, and repair is usually adequate. At high doses of a toxicant, however, the ability of the cells to progress through the cell cycle is inhibited, leading to two consequences. First, dead cells are not replaced, which may lead to organ failure and death. Second, in the absence of compensatory cell division, which normally serves to contain the toxic injury, tissue injury can progress in an unrestrained manner. The ability of cells to enter and progress through the cell cycle following toxic injury decreases with age, a finding which explains, in part, why an 80-year-old person may be more susceptible to the same dose of a toxicant as a 40-year-old. Table 2 shows a list of drugs and chemicals that affect cell proliferation in a variety of model systems.
Stem Cells and Terminally Differentiated Cells Many of the tissues in the body undergoing constant renewal, such as skin and the lining of the intestine, accomplish this task
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by means of a small population of tissue stem cells. The defining properties of a stem cell are (1) the ability to divide virtually without limit throughout the lifetime of the organism and (2) the ability to divide symmetrically (leading to two terminally differentiated cells) or asymmetrically (giving rise to one stem cell and one terminally differentiated cell). Stem cells are required wherever there is a recurring need to replace nondividing, terminally differentiated cells. Some terminally differentiated cells, such as mature mammalian red blood cells and the cells in the outermost layer of the skin, lack a cell nucleus and are therefore unable to divide. Others contain cytoplasmic structures (such as the myofibrils of striated muscle cells) that hinder cell duplication. And in some terminally differentiated cells, the chemistry of differentiation may simply be incompatible with cell division. The job of the stem cell is not to carry out the function of the differentiated cell, but rather to produce the cells that will carry out those functions. Stem cells that give rise to only one type of differentiated cell are called unipotent; these are capable of differentiating along only one lineage. Also, adult stem cells in many differentiated undamaged tissues are typically unipotent and give rise to just one cell type under normal conditions. Additionally, a small number of cell types are called oligopotent. These stem cells have the ability to differentiate into just a few types of cells. A lymphoid stem cell is an example of an oligopotent stem cell. These stem cells cannot develop into any type of blood cell as bone marrow stem cells can. They give rise to only blood cells of the lymphatic system, such as T cells. Cells that give rise to many cell types are called pluripotent or totipotent. Yet, there is another type of stem cell that is called multipotent. These cells are progenitor cells that have the genetic potential to differentiate into multiple, but limited cell types.
Cell Proliferation and Cancer Cells within a tissue exert an inhibitory effect on each other’s growth. This restraining force is called social control of cell division, and it is mediated by a set of genes called social control genes. A cell that acquires a DNA mutation that disrupts this social restraint will divide without regard to the needs of the organism as a whole, and its progeny may become tumor cells. Approximately 1016 cell divisions take place in a human body in the course of a lifetime. Even in an environment that is free of mutagens, mutations occur spontaneously at an estimated rate of about 106 mutations per gene per cell division, a value set by fundamental limitations on the accuracy of DNA replication and repair. Thus, in an average person’s lifetime, every single gene is likely to have undergone mutations on about 1010 separate occasions. Some mutated genes – if not repaired by the cell’s DNA repair mechanisms – are involved in the regulation of cell division. Consequently, the affected cell, now lacking the normal ‘brakes’ on cell growth, may continuously progress through the cell cycle. To transform into a malignant cell, however, a cell must acquire not just one but a number of DNA mutations to escape the multiple controls on cell division. Further mutations endow the cell with the capacity for invasion
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Cell Proliferation
Table 2
Summary of some toxicants that may affect cell proliferation
Compound
Origin
Specific mechanism
Source reference
Natural food toxicants Ochratoxin A
Fungi
Citrinin
Fungi
Okadaic acid
Algae
Glucosinolates
Plants
Psoralen
Plants
Pyrrolizidine alkaloid
Plants
Induces cell proliferation by activating Cyclin-D and Cox-2. Inhibits cell proliferation by increasing p53 and p21. Inhibits cell proliferation by inducing c-Myc. Inhibits cell proliferation by G2/M arrest. Inhibits cell proliferation by BMP signaling. Inhibits cell proliferation by activating p38.
Kumar R., et al., 2013. Carcinogenesis. 34:647. Chang C.H., et al., 2011. Toxicol. Sci. 119:84. Zhang L., et al., 2007. Cancer Res. 67:10198. Liang H et al., 2008. J. Nat. Prod. 71:1911. Tang D.Z., et al., 2011. Biochem. Biophys. Res. Commun. 405:256. Ji L.L., et al., 2002. Toxicon. 40:1685.
Environmental toxicants Nonmetals (As, S)
Soils, air
Inhibit cell proliferation by DNA damage.
Metals (Cd2þ, Hg2þ, Pb2þ)
Water
Inhibit cell proliferation by their interaction with DNA metabolism.
1,3-Butadiene
Rubber, plastics
Polycyclic aromatic hydrocarbons (PAHs) Dichlorodiphenyl-trichloroethane (DDT) Polychlorinated biphenyls (PCBs) Lithium
Tobacco smoke
Increases cell proliferation by DNA modification. Increases cell proliferation by increasing ER. Increases cell proliferation by chromosomal alterations. Induces cell proliferation of collecting ducts in kidneys. Induces cell proliferation in the liver.
Dimethylnitrosamine
Food and environment
Inhibits cell proliferation in the liver
Doxorubicin þ caffeine
Anticancer drug
Increases in hematopoietic cell proliferation
TCDD
Environmental toxin
Wu J.Z., et al., 2006. Eur. J. Pharm. Sci. 29:35. Costa M., et al., 1982. Res. Commun. Chem. Pathol. Pharmacol. 38:405. Melnick R.L., et al., 2001. Chem. Biol. Interact. 135:27. Plísková M., et al., 2005. Toxicol. Sci. 83:246. Uppala P.T., et al., 2005. Environ. Mol. Mutagen. 46:43. Chaudhuri L., et al., 2010. Free Radic. Biol. Med. 49:40. Gao Y., et al., 2013. Am. J. Physiol. Renal. Physiol. July 24 2013. (Epub ahead of print) Syed, I., et al., 2012. Mol. Cell. Biochem. 365(1–2): 351–361. Motegi, T., et al., 2013. Res. Vet. Sci., July 18 2013. pii: S0034-5288(13) 00210-5. doi: 10.1016/j.rvsc.2013.06.011. (Epub ahead of print) National Toxicology Program. Natl. Toxicol. Program Tech. Rep. Ser. November 2010; (558):1–206.
Pesticides Pesticides Nephrogenic diabetes insipidus
and metastasis. Statistically, it is estimated that somewhere between three and seven independent random events, each of low probability, are typically required to turn a normal cell into a cancer cell; the smaller numbers apply to leukemia and the larger numbers to carcinomas. Proto-oncogenes are normal genes encoding proteins involved in cell proliferation. Like any gene, a proto-oncogene may undergo mutation. When a mutation in a proto-oncogene confers a gain of function, the new mutant gene (now called an oncogene) will dramatically stimulate cell division. Tumor suppressor genes, in contrast, are normal genes that encode proteins that inhibit cell proliferation. If a tumor suppressor gene is mutated in such a way as to inactivate the gene, this removal of inhibition may make the cell grow continuously. Mutations may be spontaneous, or they may result from
exposure to chemical carcinogens or radiation. Mutations in tumor suppressor genes generally need to occur on both alleles for a tumor to arise, whereas most proto-oncogenes need only one mutated allele (one oncogene) to contribute to malignancy.
Importance of Understanding the Mechanisms in Control of Cell Proliferation An understanding of the mechanisms in control of cell proliferation is critical in the development of new tissue restoration therapies. Current clinical treatments for patients with drug overdoses or chemical poisoning are aimed primarily at preventing additional injury, either by blocking further formation
Cell Proliferation
of toxic metabolites or by increasing clearance of the toxin from the body. While these strategies are useful, the survival of the tissue – and sometimes the patient – is heavily dependent on tissue repair, the success of which is in turn contingent on the ability of cells to proliferate. In cases of toxic exposure in which tissue repair is delayed, either due to the massivity of the exposure or to a delay in treatment, organ loss and even death may occur because the damage compromises the regenerating ability of the cells, thereby paving the way for unrestrained progression of injury. If cellular regeneration after massive tissue damage could be actively stimulated, it might be possible to prevent organ loss and death. Animal experiments provide concrete examples of how modification of tissue repair can directly influence survival. Animals given ordinarily lethal doses of toxins are able to survive – even when there is massive liver injury – when tissue repair in the liver is stimulated. Conversely, animals receiving otherwise nonlethal doses of toxins develop liver failure and die if cell division (and therefore tissue repair) is blocked by antimitotic agents. Perhaps carefully induced suppression of pathways involved in cell death and stimulation of pathways involved in cell division could stop the progression of toxic injury and
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restore organ structure and function. With the advent of gene therapy, specific genes could, one day, be delivered directly to the damaged organ to induce expression/suppression of the appropriate factors needed for recovery. Although this may require time, with technological advances this route of therapy appears to be doable.
See also: Cell Cycle.
Further Reading Alberts, B., Johnson, A., Lewis, J., Raff, M. (Eds.), 2007. Molecular Biology of the Cell, fifth ed. Garland, New York. Chanda, S., Mehendale, H.M., 1996. Hepatic cell division and tissue repair: a key to survival after liver injury. Mol. Med. Today 2, 82–89. Gallagher, E.J., LeRoith, D., 2011. Diabetes, cancer, and metformin: connections of metabolism and cell proliferation. Ann. N. Y. Acad. Sci. 1243, 54–68. González-Mariscal, L., Lechuga, S., Garay, E., 2007. Role of tight junctions in cell proliferation and cancer. Prog. Histochem. Cytochem. 42, 1–57. Lodish, H., Berk, A., Kaiser, C.A., et al. (Eds.), 2007. Molecular Cell Biology, sixth ed. Scientific American Books (Distributed by Freeman New York), New York. Mehendale, H.M., 1995. Injury and repair as opposing forces in risk assessment. Toxicol. Lett. 82–83, 891–899. Reinehr, R., Häussinger, D., 2009. Epidermal growth factor receptor signaling in liver cell proliferation and apoptosis. Biol. Chem. 390, 1033–1037.