KINASES AND CANCER
What causes cancer? There are many kinds of cancer and thus the molecular causes can be varied. One common cause of cancer is a mutation in genes for enzymes called kinases. These kinases add phosphate groups to other proteins and this can serve as the “on/off switch” which activates a diversity of cellular processes including cell division.
ABL1 is a cytoplasmic and nuclear kinase involved in differentiation, division, adhesion, and stress responses. It is widely expressed with higher expression in bone and cartilage. Mutations can cause leukemia (the blood from a leukemia patient is pictured below).
ERBB2 is a kinase gene that can be involved in several cancers including breast and prostate cancer. Increased expression of this gene causes resistance to a drug used in chemotherapy (taxol) and thus chemoresistance in cancer patients. One polymorphism (Val655to ile) increases the risk of breast cancer. African populations have a lower frequency of this allele and thus a lower risk of breast cancer. (Cells of a breast cancer are pictured below.)
These are only two examples of the hundreds of kinase genes which are in the human genome and kinases represent one of the largest human gene families. Human cells depend on these kinases to regulate the various processes which occur within them. Are human cells unique in this regard? No.
Although the primary kinase enzymes in bacteria are thus different from those in eukaryotes, the eukaryotic type of kinases do exist in some prokaryotes and seem to have preceded the evolution of the eukaryotes (Ogawara, 1999). Yeast possess more than 100 protein kinase genes, representing about 2% of their genome. This includes members of a number of the subfamilies of kinases found in humans (Hunter, 1997). As of 2001, there are more than 9000 known plant receptor-like kinases (RLKs), a gene family of kinases which includes three human genes (Shiu, 2001).
One family of kinases, the receptor protein kinases, are especially important in the signaling pathways of animals. One group of protists closely related to animals, the choanoflagellates, are the only organisms to possess such kinases (King, 2001). Sponges, the most primitive animals, have a receptor protein kinase which possesses an immunoglobulin domain (Schacke, 1994)..
Of just under
20,000 genes in the worm C.elegans, about are 500 kinases, making it the second most abundant protein family
in the worm genome. There are a number of kinase
signaling cascades which exist in both worms and humans.
In conclusion, the human kinase genes which regulate so many cellular processes, and whose misexpression is involved in so many cancers are not unique to humans. The kinase gene family has gradually enlarged and diversified since its origin in bacteria and its members perform vital functions in all eukaryotes, especially in animals.
2. ABC TRANSPORTERS
In bacteria (such as those pictured above), ABC transporters can be used in the transport of a variety of substances and this gene family is the largest family of proteins in the E. coli genome. Most ABC transporters are composed of 5 separate protein domains and in prokaryotes, most of these domains are encoded by their own separate genes. Throughout evolution, many of these separate genes have fused so that these transporters are now encoded by two “half” ABC genes (whose channels only function if two “half” proteins form a complex together) or even one “full” ABC gene.
Three ABC transporters: the HisQMP2 transporter of E.coli made of 4 subunits encoded by 3 genes; the
Drosophila eye pigment transporter formed by the products of 2 genes (each
encoding half a transporter), and the chloride ion transporter responsible
for cystic fibrosis (one gene)
Three ABC transporters: the HisQMP2 transporter of E.coli made of 4 subunits encoded by 3 genes; the Drosophila eye pigment transporter formed by the products of 2 genes (each encoding half a transporter), and the chloride ion transporter responsible for cystic fibrosis (one gene)
ABC transporters exist in higher organisms as well. There are more than 100 ABC transporters known in eukaryotes but their substrates are often unknown. At least 29 are known in yeast and almost all human ABC transporters have homologs in yeast. The well known mutation white in Drosophila is caused by a mutation in an ABC transporter.
Most mutations in the CFTR gene (the cystic fibrosis transductance regulator) cause cystic fibrosis; others cause the congenital aplasia of the vas deferens in males (CBAVD; with or without CF), nasal polyps (with CF), and the increase of chloride secreted in sweat (without CF). Although the protein is itself a chloride channel, it also seems to influence the function of other transport proteins.
Other members of this gene family in the human genome include the genes ABCB11 (a bile salt export pump whose mutations cause progressive intrahepatic cholestasis), ABCA4 (a retina specific transporter whose mutations cause a variety of eye disorders), and ABCB1 (a transporter whose expression can cause multidrug resistance in cancer patients and, in pregnant women, may vary the concentration of drugs that fetuses are exposed to).
The first challenge which faced multicellular organisms was how to be multicellular. In multicellular organisms, cells must adhere and interact with each other, and with the matrix around them (as in the images of the hydra body wall, the endostyle of a primitive chordate, or human skin above). This is just as true in human cells as it is in those of simpler animals. Not surprisingly, human cells interact with their surroundings using members of the same gene families which are utilized by the cells throughout the animal kingdom: cadherins, selectins, fibronectins, and immunoglobulins.
The integrin gene family in animals functions in cell adhesion, cell signaling, and (in vertebrates) specialized functions in embryonic development, platelet plug formation, and leukocyte migration. Most are receptors for proteins of the matrix surrounding cells but some interact with other cell membrane proteins. Integrins are composed of two subunits known as a (alpha) and b (beta) subunits. Given the number of a and b genes, over 100 different different combinations are possible. Both a and b subunits are known throughout the animal kingdom, including in sponges and there evidence that integrin-like molecules exist in plants and fungi (Brower, 1997). Sponges, the most primitive animals, possess integrins (Peterson, 2002).
One of the first problems that animals had to solve was how to be multicellular. In a multicellular organism, cells must interact with each other. Once cells began to express the immunoglobulin domain on their cell surfaces, other cells expressing the same (or slightly different) domains could interact with them. As vertebrate animals began to live longer (and put off reproduction until later in life), the issue of distinguishing between one’s own cells and foreign cells became ever more important and important families of immunoglobulins within the superfamily evolved such as MHC proteins (which distinguish between “self” and “nonself”), antibodies, and T-Cell receptors.
A very early duplication of ancestral immunoglobulin/fibronectin molecules produced separate fibronectin and immunoglobulins families. The functional domains of each of these molecules became incorporated into a variety of multi-domain proteins which can have single or multiple fibronectin domains and single or multiple immunoglobulin domains. The following illustration illustrates how the genes of different cell surface molecules consist of a variable number of immunoglobulin domains (in green).
(after Darnell, p. 1033).
The fibronectin/immunoglobulin superfamily is one of the largest groups of genes in the human genome. Although these proteins are expressed all throughout the body, special attention is given to the diversity of immunoglobulins which can be expressed on the surface of white blood cells, such as those in a lymph node in the following image.
Sponges can distinguish between self and non-self in that a sponge can reject a graft made from another sponge. In starfish and higher animals, cells exist in the circulatory system which perform phagocytosis.
Invertebrates do not have a lymphatic system or any aspects of acquired immunity, but certain parts of their immune systems do display characteristics observed in vertebrates. Earthworms possess cells whose function is similar to lymphocytes. Although invertebrates do not possess antibodies, there are molecules called lectins (found in bacteria, plants, and all animals) which can cause cells to clump by binding to sugar groups. In invertebrates, lectins bind to foreign particles and facilitate phagocytosis (as do antibodies in vertebrates). The human protein collectin (related to the lectins) also coats foreign particles before phagocytosis
A number of local hormones are involved in
immune reactions. Starfish possess
molecules similar to interleukin-1.
Acquired immunity is only found in vertebrates and depends on MHC proteins, B cell receptors, and T cell receptors. Jawless fish have no organized lymphatic tissue and lack both a spleen and thymus. Although hagfish are not known to possess antibodies and T cell receptors, they do possess serum heterodimeric proteins which resemble both antibodies and T cell receptors (Varner, 1991).
In sharks, B cells make 4 classes of antibody, including IgM, which humans also produce. Shark antibodies can be secreted and can function on the cell membrane (Hohman, 1993). Cartilaginous fish have 3 types of MHC molecule (MHCIa, MHCIIa, and MHCIIb) despite the fact that they do not have T cells which, in higher vertebrates, interact with MHC proteins. One type of immunoglobulin in sharks, IgW displays characteristics which might be expected from a primordial immunoglobulin (Berstein, 1996). Both B and T cells are known in bony fishes.
5. IONS AND ELECTRICITY
Potassium channels are ancient proteins, and are conserved between prokaryotes and eukaryotes. Prokaryotic potassium channels are homologous to those found in eukaryotes. Eukaryotic channels retain their function when the eukaryotic channel pore is replaced by the pore from prokaryotic channels (Lu, 2001). As pictured below, the potassium channel is not only the simplest of the voltage regulated ion channels, its 6-transmembrane region structure (with the fourth unit being the voltage-regulated portion) is the prototype for the more complex sodium and calcium channels which are composed of four separate homologous regions. The potassium, sodium, and calcium voltage regulated channels are pictured below (after Darnell, p.782).
The ion channels for potassium, sodium, and calcium belong to the same ion channel family. Potassium channels have been isolated from archebacteria (Jiang, 2002a and 2002b). Voltage gated potassium channels represent the prototypical channel of the family of voltage regulated channels. The simplest channels form tetramers using four subunits of the same gene. (Yellen, 2002).
While eukaryotic sodium channels are large proteins, composed of four homologous domains of a potassium-channel like region, a simpler sodium channel is known in bacteria. It has a single domain, homologous to potassium channels (Catterall, 2001).
Mutations in one of the many potassium and sodium channel genes can cause a variety of disorders such as epilepsy, deafness, febrile seizures, and heart arrhythmias.
The cells of the human body (such as human neurons pictured above) depend on receiving signals—they need to know when to store glucose, when to divide, when muscles should contract, when neurons should fire, etc. The molecules that send these signals typically do not enter the cell. How can a hormone, or a neurotransmitter, or a growth factor, or whatever stimulate cells that they do not enter? They activate a second messenger system which involve activating G-protein coupled receptors (GPCRs
In the following illustration, the G-protein coupled receptor has bound its ligand, which activates the G protein, which stimulates adenylate cyclase to produce cAMP from ATP. The cAMP then initiates changes in the cell.
G-protein coupled receptors belong to a giant gene superfamily--the largest gene family in the human body. The cells of the body react to different stimuli because different cells express different GPCRs. In the following three illustrations, three different GPCRs in three different cells bind three different ligands to activate these cells.
G PROTEIN COUPLED RECEPTORS
This superfamily of proteins share a set of 7 regions which pass through the cell membrane connected by alternating intracellular or extracellular loops. As illustrated below, a receptor for a neurotransmitter in the nervous system, an olfactory receptor in the nose, and an opsin in the retina of the eye all share the same basic structure.
b adrenergic receptor
After Darnell, p. 748
After Fuchs, 2001
Common basic structure of 4 human visual opsins
After Nathans, 1986
This is a very old family found even in bacteria; the bacterial light-detecting protein bacteriorhodopsin is homologous to GPCRs. Gene duplication had produced many of the subfamilies of the G proteins before the lineages leading to modern sponges separated from those leading to higher animals (Suga, 1999). In animals (including humans), GPCRs are the receptors for most of the body’s major hormones (such as calcitonin and follicle stimulating hormone), local hormones (such as histamine and interleukin), neurotransmitters (such as dopamine and serotonin), and neuropeptides (such as neuropeptide Y and cannibinoid receptors). Some GPCRs serve as the receptors which allow animals to taste and smell. In humans, most of the large number of olfactory receptor genes appear to be nonfunctional. This is expected because in the evolution of apes and humans in particular, the brain areas involved in processing smell have been greatly reduced.
One group of GPCRs, the opsins, allow organisms ranging from bacteria to humans to detect light. A variety of opsins are expressed in the brain itself, particularly in the pineal gland. Many fossil vertebrates (and some modern ones) possessed an opening in the skull (the pineal foramen) which allowed light to enter the skull to measure the length of daylight (and thus coordinate a number of biological activities).
Opsins expressed in the retina (pictured below) allow us to see.
The ability of many mammal groups to distinguish between red and green (and the ability of many humans to have a much greater sensitivity to color) has resulted from gene duplications of one ancestral opsin gene on the X chromosome.
Most non-primate mammals and more primitive primates (prosimians and New World Monkeys) have one opsin for long wavelengths of light. Old world monkeys and apes have two genes
resulting from gene duplication: red and green. Some people have multiple copies of red
(up to four) and green opsins (up to seven).
Most non-primate mammals and more primitive primates (prosimians and New World Monkeys) have one opsin for long wavelengths of light. Old world monkeys and apes have two genes resulting from gene duplication: red and green. Some people have multiple copies of red (up to four) and green opsins (up to seven).
In conclusion, the gene family of G-protein coupled receptors, coordinates the responses of body cells to hormones, brain cells to neurotransmitters, and sensory cells to smells, tastes, and light. These genes existed in bacteria. A large number of gene duplications followed by subsequent modifications have produced the enormous diversity of GPCRs which exist in organisms and in the human genome today.