Course 2 signal transduction cell signal.pptx

signal transduction cell signal

1. BIO 467/567 Signal Transduction Course 2: Extracellular signals: hormones, growth factors, cytokines Ozgur Kutuk, MD, PhD Molecular Biology, Genetics and Bioengineering Program

2. -One of the basic tenets of cell signalling is that two cells need to communicate with each other. They may be a long way apart, or they may be neighbors, but still, they often need to signal to each other. Many types of molecules are used as signals, and often the way they are classified appears to be a little arbitrary, but here the main groups are discussed. In animals, hormones are commonly used as intercellular signals, and plants use hormones too. -The group of peptides known as the cytokines is used by animals for short-range signalling, and new members of this group of signals seem to be discovered on a regular basis—their use in plants also has been reported, but here the phrase phytocytokine has been suggested. Some of this group were thought to be elixirs with which to cure many diseases, but their full potential has yet to be uncovered. -Uncovering the multitude of intercellular signals used in nature will aid understanding of the messages sent between cells, giving an insight into how cells respond and survive. Often it is the dysfunction of these messages that leads to disease, and a greater understanding may also lead to ways to modulate such messages. Such manipulation may allow organisms to survive in new conditions; for example, perhaps new plants can be developed that require less water, or new drugs can be developed to alleviate the symptoms of a disease, or even cure the disease.

3. -Introduction -One of the most significant ways in which cells communicate with each other is by release and detection of extracellular signalling molecules, such as hormones, cytokines and growth factors. Such molecules often are released some considerable distance from their point of action, and in general have an effect over a relatively long time-scale with an unspecific transport to the site of action. For example, a hormone might be released and carried by the bloodstream where it is transported to all parts of the body and washes around many types of cells, some of which will detect its presence, but many of which will not. Many cells will be unaffected by such a release of hormone, the specificity of effect being determined by the presence of specific receptor molecules at the detecting cells. -Receptors that perceive the presence of these extracellular signals may be at the cell surface, in the cytoplasm or even in the nucleus, depending partly on the properties of the extracellular signal. Many hydrophobic signals have receptors inside the cell, while some hydrophobic signals and hydrophilic signals rely on cell surface receptors.

4. -Many of these extracellular signalling molecules are involved and have been implicated in disease states, a defect either being the cause of the disease, or the presence of the molecule propagating manifestation of the disease. One of the most well studied and understood is the role of insulin in diabetes. This multifactorial disease may involve aberrations in synthesis of the hormone, so relaying no message to the target cells, or may involve lack of detection by the target cells. The role of hormones has been most well studied in animals, although such extracellular signals are widely used in the plant kingdom. A plant hormone, or at least its effects, was studied by Darwin well over a hundred years ago. Hormones also have been found to be important for unicellular eukaryotes, where the release of hormone-like molecules is seen in transfer of messages between individual organisms, in a similar manner to that used by mammals between tissues. Although the distinction among hormones, cytokines and growth factors appears to be somewhat loose, a rough division is given here and they are considered separately.

5. - Hormones Several diverse types of molecule are grouped under the broad name of hormones, a term taken from the Greek meaning “to arouse” or “to excite”. Although the definition of a hormone can be somewhat vague, here the term is used to describe substances released into the extracellular medium by the cells of one tissue, to be carried to a new site of action, such as a different tissue, where they provoke specific responses. They can be split into several broad classes: small water soluble molecules; peptide hormones; lipophilic molecules that are detected by surface receptors; and lipophilic molecules that are detected by intracellular receptors. However, common to all hormones is that they have specificity, being released only when required, and detected only by cells that need to respond to them. -Small water soluble molecules These hormones share common characteristics in that they are all water soluble and cannot cross the plasma membrane of cells, usually because they are too large or carry a charge at physiological pH, and as such their detection requires the presence of specific receptors on the cell surface of the responding cell. They are usually released into the vascular system of the organism, to be carried freely to their sites of perception. Relatively small organic molecules and hormones fall into this category, including histamine and epinephrine (adrenaline).

6. -Histamine is produced in mast cells and is responsible for control of blood vessel dilation. Topical drugs, for example for treatment of bee stings, often contain anti-histamines, which are used to counteract the release of histamine during an inflammatory response. -Epinephrine, or as it used to be known, adrenaline, was classically known as the hormone released at times of panic, and has been referred to as the “fright, fight and flight” hormone. It is produced by the adrenal medulla and causes an increase in blood pressure and pulse rate, contraction of smooth muscles, an increase in glycogen breakdown in muscles and the liver, and an increase in lipid hydrolysis in adipose tissue, all factors that ready the body for a situation in which greater energy requirement is envisaged, such as in fight and flight. In most cases, the effects of epinephrine are fast, and, importantly, quickly reversed. An organism would not wish to remain in a state of panic for too long.

7. -Peptide hormones Like histamine and epinephrine, the peptide hormones are water soluble and are carried to their site of action by the vascular system. Usually their release is rapid, as they are often stored within vacuoles in the cell, ready to be evicted from the cell by exocytosis. Such vacuoles will remain dormant in the cell until such time as the cell is stimulated, involving a signal transduction pathway (often involving a rise in calcium ion concentrations), at which time the vacuoles will be moved to the plasma membrane. Fusion of the vacuole membrane with the plasma membrane will cause the hormone to be released to the extracellular fluid. The breakdown of peptide hormones is also rapid, usually by proteases in the blood and tissues. The two that have probably been most extensively studied are glucagon and insulin, which in fact have antagonistic effects. -Insulin is produced by the β cells of the pancreas as a single pre-proinsulin polypeptide, which is converted to a single proinsulin polypeptide. This folds into its correct secondary and tertiary structure aided and stabilized by the presence of disulfide bridges. Once correctly folded, active insulin is produced by proteolytic cleavage in two places causing removal of a substantial part of the middle of the polypeptide. The result is that insulin contains two polypeptide chains, an A chain of 21 amino acids and a B chain of 30 amino acids, held together by disulfide bonds and electrostatic forces.

8. -Once released, insulin stimulates uptake of glucose by muscle and fat cells, increases lipid synthesis in the adipose tissues and causes a general increase in cell proliferation and protein synthesis. Insulin is detected by the presence on the surface of target cells of a multipolypeptide receptor (actually a tetramer), which contains intrinsic tyrosine kinase activity, and its activation leads to a cascade of events including activation of phosphatidylinositol 3-kinase, activation of G proteins and stimulation of a MAP kinase pathway. -Glucagon, like insulin and all peptide hormones, is also made from a precursor polypeptide. Active glucagon contains a single polypeptide chain of 29 amino acids. It is released from the α cells of the pancreas and causes glycogen breakdown and lipid hydrolysis, allowing an increase in glycolysis and respiratory rates. -Other hormones in this class include follicle stimulating hormone (FSH) and luteinizing hormone (LH). Both, like insulin, have two polypeptide chains; however, they are much bigger, with FSH having an α chain of 92 amino acids and a β chain of 118 amino acids. Both hormones are produced by the anterior pituitary. FSH stimulates the growth of ovarian follicles and oocytes, and also increases production of oestrogen . LH controls the maturation of oocytes and increases release of oestrogen and progesterone. -Hormones often influence release or action of other hormones, and here, release of LH is under the control of another peptide hormone, this time a single polypeptide hormone called LH-releasing hormone, produced by the hypothalamus and neurons. This type of interaction of different hormones and other extracellular signals controlling each other’s synthesis and secretion is not unusual. Interactions of this nature are exemplified by the action of cytokines and chemokines.

9. -Lipophilic molecules detected by cell surface receptors As well as detection of the water-soluble hormones, some receptors on the surface of cells also detect the presence of a group of hydrophobic, or lipophilic, molecules that act like hormones. The main group of signalling molecules here is prostaglandins and related compounds, which often are grouped together as eicosanoids. Despite their hydrophobic nature, such compounds do not cross the plasma membrane to be detected, which is in contrast to the steroid hormones. -Prostaglandins are synthesized from arachidonic acid, a 20-carbon fatty acid. Arachidonic acid often is found covalently attached to the glycerol backbone of phospholipids, which constitute the plasma membrane. The arachidonic acid is attached to the middle carbon of the backbone, but can be hydrolytically released from lipids, for example, by the action of phospholipase A2. -As the prostaglandins are derived from a molecule embedded in a lipid environment, it is not surprising that they are inherently hydrophobic in nature. Although prostaglandins are a relatively large group of chemicals, they can be divided roughly into nine different classes. Although originally named after the organ where they were thought to be made, prostaglandins are produced by most cells, with their action usually being local. Their synthesis can be inhibited by several anti-inflammatory drugs, including aspirin, and their effects range from control of platelet aggregation to the induction of uterine contraction.

10. -Lipophilic molecules that are detected by intracellular receptors: Not all hormones are perceived by cell surface receptors. Many are recognized by receptors inside the cell, and therefore such hormones must be lipophilic, or hydrophobic, to penetrate the plasma membrane before perception. This class of hormones encompasses the steroid hormones, which include, for example, oestrogen , progesterone, androgens and glucocorticoids, as well as the thyroid hormones and retinoids. -Steroid hormones are all derived from cholesterol and are synthesized and secreted by endocrine cells. Progesterone is synthesized by the ovaries and placenta, and is involved in the development of the uterus in readiness for implantation of the new embryo, as well as for the stabilization of early pregnancy and development of the mammary glands. Oestrogens , such as oestradiol , also are involved in development of female sexual characteristics such as uterus differentiation and mammary gland function. Similarly, testosterone, which is produced by the testis, is responsible for the development and functioning of the male sex organs, as well as development of less obviously useful male characteristics such as hair growth..

11. -Mammals are not the only animals that use such hormones. For example, in insects and crustaceans a similar role in the development of sexual characteristics is fulfilled by a steroid-like compound called α- ecdysone. -Other steroid hormones include cortisol and vitamin D. Cortisol, a glucocorticoid, is produced by the cells of the cortex of the adrenal gland and controls the metabolic rates of many cells. It is formed from progesterone by three hydroxylation steps, at carbons 11, 17 and 21. Vitamin D is synthesized in skin exposed to sunlight. 7-Dehydrocholesterol (provitamin D3) in the skin is lysed by ultraviolet light to previtamin D3, an inactive form, which is activated by hydroxylation in the liver and kidneys with its conversion to calcitrol (1,25 dihydroxycholecalcitrol ). Calcitrol is responsible for the control of Ca2+ uptake in the gut and lowering Ca2+ excretion by the kidneys. -Lack of vitamin D during a child’s development can lead to the condition known as rickets. Here, the cartilage and bone fail to calcify properly, leading to malformation of the bones and this often leaves the long bones of the patient bent. This is most noticeable in the legs of patients and was seen when children were forced to work long hours indoors or underground away from adequate sunlight, a situation fortunately not common now. Dietary vitamin D, such as derived from fish oils, can overcome lack of de novo synthesis in the skin and prevent the associated symptoms. Interestingly, and rather disturbingly, recent news reports suggest that rickets is becoming more common, probably because of a combination of social activity and diet.

12. -Thyroid hormones that have effects on the metabolism of many cells, including the increase in heat production and production of polypeptides involved in metabolic pathways, are derived from the amino acid tyrosine. A good example is thyroxine, otherwise known as tetraiodothyronine, which is produced by the thyroid gland. -Another vitamin involved in the synthesis of lipophilic hormones is vitamin A (also called retinol), from which the retinoids are synthesized. For example, retinoic acid is formed by oxidation of the alcohol group of retinol to a carboxyl group. The effects of retinoic acid include an alteration of gene expression profiles in the receptive cell. -Transport of these lipophilic hormones between cells is not as simple as that seen for the water soluble ones, as once released these lipophilic molecules are inherently insoluble in water. Therefore, they need to be stabilized in the bloodstream, which is achieved by association with specific carrier proteins. Dissociation from these carriers must occur before the hormones cross the plasma membrane to enter the cells.

13. -Cytokines: The cytokines are a group of peptide molecules produced by many cell types, but they have their effect on other cells within a short distance, or often even on the cells that produce them. Hence, cytokine effects tend to be local, where they are referred to as being involved in paracrine or autocrine function. Under this definition come those molecules principally responsible for coordination of the immune response of higher animals, and include the interleukin series, tumour necrosis factors and interferons. , -With the recent explosion in the number of cytokines identified, classification into groups has been suggested. At present, there are more than 80 peptides that have been classed as cytokines. Classification is not fixed, as there are various useful ways to do it. The peptides can be grouped using their functionality, by the primary cells from which they are released, by the kinetics of their action and, more recently, it has been suggested that they should be characterized using their peptide structures. Here a classification has been followed that puts them into 16 families (I–XVI), although even then classification is not easy for all the peptides. Some appear to have characteristics common to more than one class, whereas others are not easy to classify at all. -Many of the cytokines have historic names, in that they were classified as part of a growing family, for example the interleukin series, whereas others are named in a logical way that denotes a function or characteristic, for example, FGF-a, which stands for fibroblast growth factor acidic. Often such naming leads to acronyms, which at least allows for easy memorization of their names. Excellent examples here include Trail (TNF-related apoptosis inducing ligand), Trance (TNF-related activation induced cytokine) and April (a proliferation-inducing ligand).

14. -The sizes of the cytokines are variable, with some being relatively small, weighing in at less than 10kDa. Others are much larger, with individual subunits as large as 60–70 kDa . However, the majority fall between 15 and 40 kDa . Interferon- γ appears to be between 20 and 25 kDa , for example. In most cases, the receptors that perceive the cytokines have been identified, and the intracellular cascades that are invoked are being unravelled . -Interleukins The interleukin series has, at the present time, 35 members identified, designated IL-1 to IL-35. Although IL-1 was named as one, it exists in two distinct forms, IL-1 α and IL-1 β, coded for by separate genes. However, both are produced as larger precursor molecules and a cleavage event produces the active extracellular form. IL-1 α is produced as a 271 amino acid, which is cleaved to 159 amino acids, whereas IL-1 β is a 153 amino acid peptide derived from a 269 amino acid precursor. Therefore, similar to the peptide hormones, such as insulin, a major cleavage event has occurred, removing a substantial proportion of the polypeptide. Such cleavage events are common in synthesis of the interleukin family. In the case of IL-1, the uncleaved cell associated form seems to retain biological activity. Several of the cytokine family appear to have membrane bound or membrane associated forms, and are not released freely from the cell. -Several of the genes for cytokines, for example IL-3, IL-4, IL-5, IL-13 along with GM-CSF, are grouped together on the same region of chromosome 5 in humans, or chromosome 11 in mice, suggesting that they arose originally through gene duplication events. This is quite commonly seen where families of proteins exist. Evolution has allowed the copying of a successful protein, and then its subsequent subtle alteration to fulfil a new function or role, unable to be undertaken by the original protein. Such a process is then repeated, building up a family of subtly different, but related, proteins, each with slightly, but significantly, different roles.

15. -Most of the interleukins are active in the monomeric state, but interleukin 5 exists as a homodimer of two 115 amino acid polypeptides, whereas interleukin 12 exists as a heterodimer of variably sized polypeptides. Interleukin 8, otherwise known as neutrophil activating protein 1 (NAP-1), is also a homodimer, but here the subunits are of variable length, 72 amino acids up to 77 amino acids, because of truncation of the N-terminal end of the polypeptides. It is interesting that the IL-8 molecules with the shorter versions of the subunits appear in some cases to be more potent than those with the longer polypeptides. -Cytokines are produced by a variety of blood cells and cells involved in the immune response. IL-2 and IL-3 are, for example, produced mainly by helper T lymphocytes, whereas IL-6 is produced by a variety of cells including T cells, macrophages and fibroblasts. The biological responses of the IL family are varied also. IL-13, formerly known as P600, seems to induce growth and differentiation of B cells and inhibits cytokine production of macrophages and their precursor cells, monocytes. IL-5 leads to activation of eosinophil function, including chemotaxis and eosinophil differentiation. Some of the individual cytokines have a diverse range of biological activities, a good example being IL-6. It stimulates the differentiation of myeloid cell lines, acts as a growth factor on B cells, modulates the responses of stem cells to other cytokines and has effects on non- haematopoietic cells, including affecting the development of nerve cells.

16. -These cytokines all work in a concerted way to coordinate the whole response. They can exert coordinating responses on the cells themselves or regulate the synthesis of each other. For example, IL-5 enhances the IL-4 effect on B cells, enhancing the IL-4 induced synthesis of IgE and the expression of CD23, whereas IL-11 has synergistic effects with both IL-3 and IL-4. On the other hand, IL-6 can induce production of IL-2 in T cells, but IL-10 inhibits the synthesis of several other cytokines, including IL-1, IL-6, IL-8, IL-10 and IL-12. Some cytokines produce similar responses. In the case of IL-13 and IL-4, no additive effect is seen if both of these cytokines are added together, and it is possible that they share a common receptor, or share the same signal transduction pathway. Likewise, IL-15 and IL-2 may share common elements in their receptors.

17. -Interferons The interferons (IFNs) fall into two main groups. Type I includes IFN- α, IFN- β and IFN- Ω, whereas type II includes interferon- γ ( IFN- γ). IFN- γ rose to prominence in the press as an anti-cancer agent and was branded as “a wonder drug”. Although the interferons do have profound effects in some conditions, they are not widely distributed by the medical profession, perhaps in some cases because of their terrific cost. -Like the interleukins, interferon- γ is a peptide. Here, the polypeptide is 143 amino acids long and the interferon exists as a dimer in the active form, or even multimers. However, the peptide is made from expression of a single gene on the long arm of chromosome 12 in humans, the sequence including a signal sequence of 23 amino acids. The α/β forms on the other hand are monomers, coded for by genes on chromosome 9. -Produced by T cells among others, activity of interferon- γ includes induction of expression of the class II histocompatibility antigens on epithelial, endothelial and connective tissue. It also acts as a macrophage activating factor, causing specific gene expression in these cells. This leads to the enhanced ability of these cells to be cytotoxic to tumours and to kill parasites.

18. - Tumour necrosis factors Tumour necrosis factor- α ( TNF- α), originally known as cachectin , is coded for by a single gene, which in humans is found on chromosome 6, but in the soluble active form it is a homotrimer of 157 amino acid subunits (17.5 kDa form). There is also a 26 kDa form that remains membrane anchored. -TNF is produced by monocytes and macrophages. Its activity is closely coupled to that of interferon- γ, and it has been shown to cause necrosis of tumours , hence its name, and to be cytotoxic to transformed cells in vitro. -TNF- β ( also known as lymphotoxin α, LT- α) is also a homotrimer and is encoded for by a gene that is close to the TNF- α gene. They both have similar biological activity and even bind to the same receptors, TNF RI and TNF RII. However, TNF- β can also form membrane surface trimers with LT- β, and bind to the membrane protein herpes virus entry mediator.

19. -Growth factors: The term growth factor is used here to define those compounds that have been shown to have specific functions in regulation of the growth and differentiation of cells. At present, there are over 50 known proteins that possess growth factor-like activity, with at least 14 different receptor families involved in their detection. -Platelet-derived growth factors Platelet-derived growth factors (PDGF) are dimeric proteins, which may contain two related polypeptides, A or B. The active growth factor has a molecular weight of between 25 and 29 kDa , and is made up of either two of the same subunits, for example AA or BB, or it may be a heterodimeric protein, AB. The individual subunits have been shown to have a molecular weight of 12–18 kDa , and are coded for by two separate genes. They are held together by disulfide bonds, and all the cysteine residues in the polypeptides are involved in either inter- or intra-molecular bonding. Interestingly, the receptors for PDGF also are dimeric, composed of two identical subunits, αα , or ββ , or heterodimeric αβ , and the different forms of the receptor have different binding properties to the different forms of PDGF. -PDGF-BB, with two B subunits, can bind and activate all the receptor forms, whereas PDGF-AA, with two A subunits, can activate only the αα receptor. The heterodimeric PDGF-AB has an intermediate activity. PDGF has been shown to induce both cell migration and cell proliferation, and has been implicated in several disease states including fibrosis and arteriosclerosis.

20. -A larger group of growth factors comes under the heading of epidermal growth factors (EGF). These include EGF itself, transforming growth factor- α ( TGF- α), betacellulin and heparin binding EGF. They are very small peptides, rat EGF being only 5.2 kDa , although their precursors are very large. The EGF precursor is a transmembrane protein of 1168 amino acids, of which only 53 are cleaved off to create the active EGF molecule. Likewise, TGF- α is only 6 kDa (50 amino acids), although there are larger members of the family—sensory and motor neuron-derived factor (SMDF) being 296 amino acids. -These growth factors are characterized by the presence of at least one domain containing six cysteine residues, which are involved in the formation of three disulfide bonds. The growth factors also appear to have several aromatic residues, which are exposed to the aqueous medium. This is unusual in proteins, which, in general, try to hide these hydrophobic side chains in the interior of the polypeptide folding structure. It has been proposed that there are interactions between these aromatic groups and the protein surface may contain an aromatic domain, which is vital for its function. Proteins with domains related in sequence to EGF have been found in Drosophila and in sea urchin embryos.

21. -Fibroblast growth factor (FGF) represents a family of molecules involved in the regulation of proliferation, differentiation and cell mobility. In mammals the family comprises many members, including FGF-a, FGF-b and FGF 3–19, although care needs to be taken when assuming different species are the same, even if those species are closely related. In general mammals are grouped together and assumed to be similar to each other, but here it should be noted that there appears to be no FGF-15 in humans. It is erroneous to assume that all mammals are the same, and caution is required in interpretation of data. This is often an argument used to reduce the use of mammals such as rats and mice in biomedical research, as it is hard to simply translate the data across to humans. The FGFs are structurally related proteins of 20–30 kDa , with the genes encoding them probably arising from gene duplication of an ancestral gene. However, as with the expression of many genes, further isoforms can arise from alternative splicing or the use of alternative initiation codons for translation. Alternative post-translational modification leads to further diversification of the molecules within the family, where some are glycosylated, phosphorylated or even methylated or cleaved. Some of the FGF family have been identified as proto-oncogenes, where the oncogene variants lead to the transformation of cells, where proliferation is uncontrolled resulting in the formation of tumours .

22. -Neurotransmitters In the axon termini of the presynaptic cells of the nervous system there are storage vesicles called synaptic vesicles, which contain neurotransmitters. Voltage-gated Ca2+ channels are opened on arrival of an action potential, leading to a sharp increase in concentration of intracellular Ca2+. This, in turn, leads to exocytosis from the synaptic vesicles, releasing the neurotransmitters into the space between the nerve cells, with receptors on the post-synaptic cells detecting the presence of these compounds, leading to propagation of the signal or the response. These transmitters fall into two main groups. The first group is made up of small molecules and contains neurotransmitters including acetylcholine, GABA or γ- aminobutyric acid, and dopamine. This group also includes some molecules already discussed, such as epinephrine and histamine. -Also included here are amino acids that can act as signals, such as glycine and glutamate, and derivatives of amino acids. For example, dopamine is derived from tyrosine, serotonin is a derivative of tryptophan and GABA is derived from glutamate. Also included are nucleotide-derived compounds such as ATP and adenosine. -The second group are the neuropeptides. These include among their number substance P, β- endorphin and vasopressin, with vasopressin also being classed among the hormones. The effects of the neurotransmitters are local, acting on the post-synaptic cell (across the synaptic cleft), or at least within a short distance. The receptors on the post-synaptic cell themselves fall mainly into two classes: G protein coupled receptors or ligand-gated ion channels.

23. -Summary •  Cells commonly communicate by release and detection of signalling molecules, often over relatively large distances. •  Released compounds often are referred to as hormones, taken from the Greek meaning “to arouse” or “to excite”. •  Other released molecules are now categorized separately and are known as cytokines, chemokines or growth factors. •  Such classification does not appear to follow hard and fast rules, allowing the same compounds to be classified differently, a problem that might lead to confusion. •  It is important to remember that, like all signals, extracellular signals must relay a specific message to specific targets at the correct time. That might require their transportation between cells within an organism, or even between organisms. Either way, their recognition and their ability to cause a defined response is key to their success as messengers. •  Hormones are a diverse group of molecules, which may be small and water soluble peptides or lipophilic molecules. •  Hormones are detected either by a cell surface receptor or an intracellular receptor, as seen with steroid hormones. •  Plant hormones are a diverse group of chemicals, including compounds derived from amino acids, lipids or even gases, such as ethylene or nitric oxide. •  Cytokines are a group of peptides that include the interleukins, currently having 35 members in the group, the interferons and tumour necrosis factors. •  Cytokines can be classified into many functional or structural groups, with such molecules being important in orchestration of the immune response and the development of the cells used in animal host defence .

24. -Summary •  Chemokines are a large group of extracellular signals, often involved in control of the immune system. •  Chemokines are grouped into four families: XC (with one cysteine); CC (with two sequential cysteines); CXC (with two cysteines separated by one amino acid); and CX3C (with two cysteines separated by three amino acids). •  Growth factors are molecules involved in regulation of growth and differentiation of cells. •  The growth factor family is known to contain at least 50 proteins, but as with the cytokines and chemokines, undoubtedly more are yet to be discovered.