WIXLIB

Adult (Somatic) Stem CellsIn the adult organism, stem cells are present in tissues that continuouslydivide such as the bone marrow, the skin, and the lining of the GI tract.Stem cells may also be present in organs such as liver, pancreas,and adipose tissue, in which, under normal conditions, they donot actively produce differentiated cell lineages.Regardless of their proliferative activity, somatic stem cellsgenerate rapidly dividing cells known as transit amplifying cells.These cells lose the capacity of self-perpetuation, and give riseto cells with restricted developmental potential known asprogenitor cells.

A change in the differentiation of a cell from one typeto another is known as transdifferentiation, and thecapacity of a cell to transdifferentiate into diverselineages is referred to as developmental plasticity.Hemopoietic stem cells (HSCs) maintained inculture have been shown to transdifferentiate intoother cell types, such as hepatocytes and neurons.

Stem Cells in Tissue HomeostasisBone marrow.The bone marrow contains HSCs and stromal cells (also known asmultipotent stromal cells, mesenchymal stem cells or MSCs).Hematopoietic Stem Cells (HSCs) generate all of the blood cell lineages,can reconstitute the bone marrow after depletion caused by diseaseor irradiation, and are widely used for the treatment ofhematologic diseases. They can be collected directly from the bonemarrow, from umbilical cord blood, and from the peripheralblood of individuals receiving cytokines such as granulocytemacrophage colony-stimulating factor, which mobilize HSCs.

Marrow Stromal Cells. MSCs are multipotent.They have potentially important therapeutic applications,because they can generate chondrocytes, osteoblasts,adipocytes, myoblasts, and endothelial cellprecursors depending on the tissue to which they migrate.MSCs migrate to injured tissues and generate stromalcells or other cell lineages, but do not seem toparticipate in normal tissue homeostasis.

LiverThe liver contains stem cells/progenitor cells in the canals ofHering, the junction between the biliary ductularsystem and parenchymal hepatocytes. Cells locatedin this niche can give rise to a population ofprecursor cells known as oval cells, which arebipotential progenitors, capable of differentiatinginto hepatocytes and biliary cells.canals of Hering (thick arrow), structures thatconnect bile ductules (thin arrow) withparenchymal hepatocytes

Brain.Neurogenesis from neural stem cells (NSCs) occurs in the brainof adult rodents and humans. Thus, the long-establisheddogma that no new neurons are generated in thebrain of normal adult mammals is now known to beincorrect. NSCs (also known as neural precursor cells),capable of generating neurons, astrocytes, andoligodendrocytes, have been identified in two areas ofadult brains, the subventricular zone (SVZ) and thedentate gyrus of the hippocampus

Skin.Stem cells are located in three different areas of the epidermis:the hair follicle bulge, interfollicular areas of the surfaceepidermis, and sebaceous glands. The bulge area of the hairfollicle constitutes a niche for stem cells that produce allof the cell lineages of the hair follicle.Interfollicular stem cells arescattered individually in theepidermis and are notcontained in niches.

Intestinal epitheliumIn the small intestine, crypts are monoclonal structures derived fromsingle stem cells: the villus is a differentiated compartment thatcontains cells from multiple crypts. Stem cells in small intestinecrypts regenerate the crypt in 3 to 5 days.Stem cells may be located immediately above Paneth cells inthe small intestine, or at the base of the crypt, as is the casein the colon.

Skeletal and cardiac muscleSkeletal muscle myocytes do not divide, even after injury;growth and regeneration of injured skeletal muscle occur byreplication of satellite cells. These cells, locatedbeneath the myocyte basal lamina, constitute areserve pool of stem cells that can generatedifferentiated myocytes after injury. Active Notchsignaling, triggered by up-regulation of delta-like(Dll) ligands, stimulates the proliferation of satellitecells (Notch signaling is discussed later in “Mechanisms of Angiogenesis”).

CorneaThe transparency of the cornea depends on the integrity of theoutermost corneal epithelium, which is maintained by limbalstem cells (LSCs). These cells are located at the junctionbetween the epithelium of the cornea and the conjunctiva.Hereditary or acquired conditions that result in LSCdeficiency and corneal opacification can be treated by limbaltransplantation or LSC grafting.

Cell Cycle and the Regulation of CellReplicationThe replication of cells is stimulated by growth factors or by signalingfrom ECM components through integrins.The cell cycle consists of:G1 (presynthetic),S (DNA synthesis),G2 (Premitotic), andM (mitotic) phases.Quiescent cells that have not entered the cell cycle are in theG 0 state.

The figure shows the cell cycle phases (G0,G1,G2,S and M) the location of theG1 restriction point, and the G1/S and G2/M cell cycle checkpoints.Cells from labile tissues such as the epidermis and the GI tract may cyclecontinuously; stable cells such as hepatocytes are quiescent but can enterthe cell cycle; permanent cells such as neurons and cardiac myocytes havelost the capacity to proliferate.

Because of its central role in maintaining tissue homeostasisand regulating physiologic growth processes such asregeneration and repair, the cell cycle has multiplecontrols and redundancies, particularly during thetransition between the G1 and S phases.These controlsincludeactivators andinhibitors, aswell as sensorsthat areresponsible forcheckpoints

Cells can enter G1 either from G0 (quiescent cells) or aftercompleting mitosis (continuously replicating cells).Quiescent cells first must go through the transition fromG0 to G1, the first decision step, which functions as agateway to the cell cycle.

This transition involves the transcriptional activation of alarge set of genes, including various proto-oncogenes andgenes required for ribosome synthesis and protein.translation.Cells in G1progress throughthe cycle andreach a criticalstage at theG1/S transition,known as arestriction point,a rate limitingstep forreplication

Upon passing this restriction point, normal cells becomeirreversibly committed to DNA replication. Progressionthrough the cell cycle, particularly at the G1/S transition, istightly regulated by proteins called cyclins and associatedenzymes called cyclin-dependent kinases (CDKs).

CDKs acquire catalytic activity by binding to and formingcomplexes with the cyclins.Activated CDKs in these complexes drive the cell cycle byphosphorylating proteins that are critical for cell cycletransitions.CDKs work by promoting DNA replication, the mitoticprocess, and are required for the cell cycle progression.CDKs are suppressed during G1 by multiple mechanisms,and a major action of GFs is to overcome cell cyclecheckpoint controls by releasing the suppression of CDKactivity. Once cells enter the S phase, the DNA isreplicated & the cell progresses through G2 & mitosis.

Example: One such protein is the retinoblastomasusceptibility (RB) protein, which normallyprevents cells from replicating by forming a tight,inactive complex with the transcription factor E2F.Phosphorylation of RB causes its release, whichactivates E2F and allows it to stimulate transcription ofgenes whose products drive cells through the cycle.E2F is a group of genes that codifies a family of transcription factor (TF).Three of them are activators: E2F1, 2 and E2F3a. Six others act as suppressors:E2F3b, E2F4-8. All of them are involved in the cell cycle regulation andsynthesis of DNA in mammalian cells.

The activity of cyclin-CDK complexes is tightly regulated byCDK inhibitors. Some growth factors shut off production ofthese inhibitors. Embedded in the cell cycle are surveillancemechanisms that are geared primarily at sensing damage toDNA and chromosomes. These quality control checks arecalled checkpoints; they ensure that cells with damaged DNAor chromosomes do not complete replication.

The G1/S checkpoint monitors the integrity of DNA beforereplication, whereas the G2/M checkpoint checks DNAafter replication and monitors whether the cell can safelyenter mitosis.When cells sense DNA damage, checkpoint activation delaysthe cell cycle and triggers DNA repair mechanisms.If DNA damage is too severe to be repaired, the cells areeliminated by apoptosis, or enter a nonreplicative statecalled senescence, primarily through p53-dependentmechanisms.

Growth FactorsThe proliferation of many cell types is driven by polypeptides known asgrowth factors. These factors, may also promote cell survival,locomotion, contractility, differentiation, and angiogenesis,activities that may be as important as their growthpromoting effects.All growth factors function as ligands that bind tospecific receptors, which deliver signals to thetarget cells.These signals stimulate the transcription of genes that may besilent in resting cells, including genes that control cell cycleentry and progression.

Epidermal Growth Factor (EGF) andTransforming Growth Factor α (TGF-α).EGF & TGF- α : share a common R (epidermal growthfactor receptor, or EGFR) with intrinsic tyrosine kinaseactivity.The EGFR is actually a family of receptors that respond toEGF,TGF-α, & other ligands of the EGF family.Both EGF/TGF-α are mitogenic for hepatocytes andmost epithelial cells, including keratinocytes.In cutaneous wound healing, EGF is produced bykeratinocytes, macrophages, & other inflammatory cells.

The main EGFR called EGFR1 or ERB B1, whichfrequently over expressed in lung & brain tumors,and is an important therapeutic target for thetreatment of these tumors.ERB B2 (also known as HER-2/NEU) hasreceived great attention, because of its overexpression in breast cancers, in which, it is atarget for effective cancer control.

Hepatocyte Growth Factor (HGF).The factor is often referred to as HGF/SF (scatter factor), butin this chapter we will use the simpler notation, HGF.HGF has mitogenic effects on hepatocytes and most epithelialcells, including cells of the biliary epithelium, and epithelialcells of the lungs, kidney, mammary gland, and skin.It is produced by fibroblasts and most mesenchymal cells,endothelial cells, and liver non-parenchymal cells.It is produced as an inactive single-chain form (pro-HGF) thatis activated by serine proteases released in damaged tissues.

The receptor for HGF, c-MET, is often highlyexpressed or mutated in human tumors, especiallyin renal and thyroid papillary carcinomas.Several HGF and c-MET inhibitors are presently beingevaluated in cancer therapy clinical trials.

Platelet-Derived Growth Factor (PDGF).is a family of several closely related proteins, each consistingof two chains. Three isoforms of PDGF (AA, AB, and BB)are secreted as biologically active molecules. The morerecently identified isoforms PDGF-CC and PDGF-DDrequire extracellular proteolytic cleavage to release theactive growth factor.All PDGF isoforms exert their effects by binding to two cellsurface receptors, designated PDGFR α and β, which havedifferent ligand specificities.

PDGF is stored in platelet granules and is released onplatelet activation. It is produced by a variety ofcells, including activated macrophages, endothelialcells, smooth muscle cells, and many tumor cells.

Vascular Endothelial Growth Factor (VEGF).VEGFs are a family of homodimeric proteins that includeVEGF-A (referred throughout as VEGF), VEGF-B, VEGF-C,VEGF-D, and PIGF (placental growth factor).VEGF is a potent inducer of blood vessel formation in earlydevelopment (vasculogenesis) and has a central role in thegrowth of new blood vessels (angiogenesis) in adults.It promotes angiogenesis in chronic inflammation,healing of wounds, and in tumors.

VEGF family members signal through three tyrosine kinasereceptors: VEGFR-1, VEGFR-2, and VEGFR-3.VEGFR-2, located in endothelial cells and many other celltypes, is the main receptor for the vasculogenic andangiogenic effects of VEGF.The role of VEGFR-1 is less well understood, but it mayfacilitate the mobilization of endothelial stem cells and has arole in inflammation.VEGF-C and VEGF-D bind to VEGFR-3 and act on lymphaticendothelial cells to induce the production of lymphaticvessels (lymphangiogenesis).

Fibroblast Growth Factor (FGF).This is a family of growth factors containing more than20 members, of which acidic FGF (aFGF, or FGF-1)and basic FGF (bFGF, or FGF-2) are the bestcharacterized.FGFs transduce signals through four tyrosine kinasereceptors (FGFRs 1–4). FGF-1 binds to allreceptors; FGF-7 is referred to as keratinocyte growthfactor or KGF.

FGFs contribute to wound healing responses, hematopoiesis,angiogenesis, development, and other processes throughseveral functions:1- Wound repair: FGF-2 and KGF (FGF-7) .2- New blood vessel formation (angiogenesis): FGF-2, in particular.3- Hematopoiesis: FGFs have been implicated in the differentiationof specific lineages of blood cells and development of bonemarrow stroma.4- Development: FGFs play a role in skeletal and cardiac muscledevelopment, lung maturation, and the specification of theliver from endodermal cells.

Transforming Growth Factor β (TGF-β) and RelatedGrowth Factors.TGF-β belongs to a superfamily of about 30 members thatincludes three TGF-β isoforms (TGF-β1, TGF-β2, TGFβ3) and factors with wide-ranging functions, such as BMPs,activins, inhibins, and müllerian inhibiting substance.TGF-β1 has the most widespread distribution in mammalsand will be referred to as TGF-β.It is a homodimeric protein produced by a variety of differentcell types, including platelets, endothelial cells,lymphocytes, and macrophages.

1- TGF-β is a growth inhibitor for most epithelial cells. It blocks the cell cycleby increasing the expression of cell cycle inhibitors2- TGF-β is a potent fibrogenic agent that stimulates fibroblast chemotaxisand enhances the production of collagen, fibronectin, andproteoglycans. It inhibits collagen degradation by decreasing matrixproteases and increasing protease inhibitor activities.3- TGF-β has a strong anti-inflammatory effect but may enhance some immunefunctions. Knockout mice lacking the TGF-β1 gene in T cells havedefects in regulatory T cells leading to widespread inflammation withabundant T-cell proliferation and CD4 differentiation into TH1 andTH2 helper cells.

CytokinesSome of these proteins can also be considered asgrowth factors, because they have growth-promotingactivities for a variety of cells.Tumor necrosis factor (TNF) and IL-1 participate inwound healing reactions, and TNF and IL-6 areinvolved in the initiation of liver regeneration.

SIGNALING MECHANISMS IN CELL GROWTHAccording to the source of the ligand and the location of itsreceptors (i.e., in the same, adjacent, or distant cells),three general modes of signaling, named autocrine,paracrine, and endocrine, can be distinguished.The binding of a ligand to its Receptor triggers a series ofevents, by which extracellular signals are transduced intothe cell, leading to the stimulation or repression.Signaling may occur (1) directly, in the same cell, (2) betweenadjacent cells, or (3) over greater distances

Autocrine signaling, in which a soluble mediator actspredominantly on the cell that secretes it.This pathway is important in the immune response (e.glymphocyte proliferation induced by some cytokines),and in compensatory epithelial hyperplasia (e.g liver regeneration).Paracrine signaling, in which, a substance affect cells in theimmediate vicinity of the cell that released the agent. Thispathway is important for (1) recruiting inflammatory cellsto the site of infection, & for (2) wound healing.Endocrine signaling, in which a regulatory substance, such as ahormone, is released into the blood stream & acts on targetcells at a distance.

Patterns ofextracellularsignaling.

Receptors and Signal Transduction PathwaysThe binding of a ligand to its receptor triggers aseries of events by which extracellular signalsare transduced into the cell resulting in changesin gene expression.Receptors are generally located on the surface of thetarget cell but can also be found in the cytoplasm ornucleus.

Receptors with intrinsic tyrosine kinase activity.The ligands for receptors with tyrosine kinase activity includemost growth factors such as EGF, TGF-α, HGF, PDGF,VEGF, FGF, c-KIT ligand, and insulin.Binding of the ligand induces dimerization of the receptor,tyrosine phosphorylation, and activation of the receptortyrosine kinase.

The active kinase then phosphorylates, and thereby activates,many downstream effector molecules (molecules that mediatethe effects of receptor engagement with a ligand).

A prototypical adapter protein is GRB-2, which binds a guanosinetriphosphate–guanosine diphosphate (GTP-GDP) exchange factor calledSOS. SOS acts on the GTP-binding (G) protein RAS and catalyzes theformation of RAS-GTP, which triggers the mitogen-activated proteinkinase (MAP kinase) cascade.

Other effector molecules activated by receptors with intrinsictyrosine kinase activity include phospholipase Cγ (PLCγ) andphosphatidyl inositol-3 kinase (PI3K).

phospholipase Cγ (PLCγ) catalyzes the breakdown ofmembrane inositol phospholipids into inositol 1,4,5triphosphate (IP3), which functions to increaseconcentrations of calcium, an important effectormolecule, and diacylglycerol, which activates the serinethreonine kinase protein kinase C that in turn activatesvarious transcription factors.PI3K phosphorylates a membrane phospholipid, generatingproducts that activate the kinase Akt (also referred to asprotein kinase B), which is involved in cell proliferationand cell survival through inhibition of apoptosis.

Receptors lacking intrinsic tyrosine kinase activity thatrecruit kinases.Ligands for these receptors include many cytokines, such asIL-2, IL-3, and other interleukins; interferons α, β, and γ;erythropoietin; granulocyte colony-stimulating factor;growth hormone; and prolactin.These receptors transmit extracellular signals to the nucleusby activating members of the JAK (Janus kinase) family o

contain the damage and initiate the healing process, which can be broadly separated into regeneration and repair. Healing after acute injury can occur by regeneration that restores normal tissue structure or by repair with scar formation. Healing in chronic injury involves scar formation and fibrosis.