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The SMAD Pathway
The DAXX Pathway

Transforming growth factor beta (TGF-β) is a protein that controls proliferation, cellular differentiation, and other functions in most cells. It plays a role in immunity, cancer, heart disease, diabetes, and Marfan syndrome. TGF-beta acts as an antiproliferative factor in normal epithelial cells and at early stages of oncogenesis.[1]

Some cells secrete TGF-β, and also have receptors for TGF-β. This is known as autocrine signalling. Cancerous cells increase their production of TGF-β, which also acts on surrounding cells.

TGF-β is a secreted protein that exists in three isoforms called TGF-β1, TGF-β2 and TGF-β3. It was also the original name for TGF-β1, which was the founding member of this family. The TGF-β family is part of a superfamily of proteins known as the transforming growth factor beta superfamily, which includes inhibins, activin, anti-müllerian hormone, bone morphogenetic protein, decapentaplegic and Vg-1.

Contents

The Structure of TGF-β

The peptide structures of the three members of the TGF-β family are highly similar. They are all encoded as large protein precursors; TGF-β1 contains 390 amino acids and TGF-β2 and TGF-β3 each contain 412 amino acids. They each have an N-terminal signal peptide of 20-30 amino acids that they require for secretion from a cell, a pro-region (called latency associated peptide or LAP), and a 112-114 amino acid C-terminal region that becomes the mature TGF-β molecule following its release from the pro-region by proteolytic cleavage.[2] The mature TGF-β protein dimerizes to produce a 25 KDa active molecule with many conserved structural motifs.[3] TGF-β has nine cysteine residues that are conserved among its family; eight form disulfide bonds within the molecule to create a cysteine knot structure characteristic of the TGF-β superfamily while the ninth cysteine forms a bond with the ninth cysteine of another TGF-β molecule to produce the dimer.[4] Many other conserved residues in TGF-β are thought to form secondary structure through hydrophobic interactions. The region between the fifth and sixth conserved cysteines houses the most divergent area of TGF-β molecules that is exposed at the surface of the molecule and is implicated in receptor binding and specificity of TGF-β.

Function

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Apoptosis

Cells can die in two ways: Through programmed cell death (including apoptosis and autophagy), when the cell self-destructs as a result of "death signals", and through necrosis, which is death from other causes, such as lack of oxygen or toxins.

TGF-β induces apoptosis in numerous cell types. TGF-β can induce apoptosis in two ways: through the SMAD pathway or the DAXX pathway.

SMAD pathway

The SMAD pathway is the canonical signaling pathway that TGF-β family members signal through. In this pathway, TGF-β dimers bind to a type II receptor which recruits and phosphorylates a type I receptor. The type I receptor then recruits and phosphorylates a receptor regulated SMAD (R-SMAD). SMAD3, an R-SMAD, has been implicated in inducing apoptosis. The R-SMAD then binds to the common SMAD (coSMAD) SMAD4 and forms a heterodimeric complex. This complex then enters the cell nucleus where it acts as a transcription factor for various genes, including those to activate the mitogen-activated protein kinase 8 pathway, which triggers apoptosis.

DAXX pathway

TGF-β may also trigger apoptosis via the death associated protein 6 (DAXX adapter protein).

DAXX has been shown to associate with and bind to the type II TGF-β receptor kinase.

Cell cycle

TGF-β plays a crucial role in the regulation of the cell cycle.

TGF-β causes synthesis of p15 and p21 proteins, which block the cyclin:CDK complex responsible for Retinoblastoma protein (Rb) phosphorylation. Thus TGF-β blocks advance through the G1 phase of the cycle.[5]

TGF-β suppresses expression of c-myc gene which is involved in G1 cell cycle progression.[5]

Immune System

TGF-β is believed to be important in regulation of the immune system by CD25+ Regulatory T cell and the development of both CD25+ Regulatory T cell and Th17 cells. TGF-β appears to block the activation of lymphocytes and monocyte derived phagocytes.

Clinical significance

Cancer

In normal cells, TGF-β, acting through its signaling pathway, stops the cell cycle at the G1 stage to stop proliferation, induce differentiation, or promote apoptosis. When a cell is transformed into a cancer cell, parts of the TGF-β signaling pathway are mutated, and TGF-β no longer controls the cell. These cancer cells proliferate. The surrounding stromal cells (fibroblasts) also proliferate. Both cells increase their production of TGF-β. This TGF-β acts on the surrounding stromal cells, immune cells, endothelial and smooth-muscle cells. It causes immunosuppression and angiogenesis, which makes the cancer more invasive.[6] TGF-β also converts effector T-cells, which normally attack cancer with an inflammatory (immune) reaction, into regulatory (suppressor) T-cells, which turn off the inflammatory reaction.

Heart disease

A study at the Saint Louis University School of Medicine has found that cholesterol suppresses the responsiveness of cardiovascular cells to TGF-β and its protective qualities, thus allowing atherosclerosis to develop. It was also found that statins, drugs that lower cholesterol levels, enhance the responsiveness of cardiovascular cells to the protective actions of TGF-β, thus helping prevent the development of atherosclerosis and heart disease.[7]

Marfan Syndrome

TGF-β signaling also likely plays an major role in the pathogenesis of Marfan syndrome[8], a disease characterized by disproportionate height, arachnodactyly, ectopia lentis and heart complications such as mitral valve prolapse and aortic enlargement increasing the likelihood of aortic dissection. While the underlying defect in Marfan syndrome is faulty synthesis of the glycoprotein fibrillin I, normally an important component of elastic fibers, it has been shown that the Marfan syndrome phenotype can be relieved by addition of a TGF-β antagonist in affected mice. This suggests that while the symptoms of Marfan syndrome may seem consistent with a connective tissue disorder, the mechanism is more likely related to reduced sequestration of TGF-β by fibrillin.[9]

Types

The primary three are:

TGF-β activation

Although TGF-β is important in regulating crucial cellular activities, only few TGF-β activating pathways are currently known, and yet, the full mechanism behind the suggested activation pathways is not well understood. Some of the known activating pathways are cell or tissue specific, while some are seen in multiple cell types and tissues[10][11]. Proteases, integrins, pH, and reactive oxygen species are just few of the currently know factors that can activate TGF-β[12 ][13 ][14]. It is well known that perturbations of these activating factors can lead to unregulated TGF-β signaling levels that may cause several complications including inflammation, autoimmune disorders, fibrosis, cancer and cataracts[15][16]. In most cases an activated TGF-β ligand will initiate the TGF-β signaling cascade as long as TGF-β receptors are I and II are within reach, this is due to high affinity between TGF-β and its receptors, suggesting why the TGF-β signaling recruits a latency system to mediate its signaling[10].

TGF-β latency (latent TGF-β complex)

All three TGF-βs are synthesized as precursor molecules containing a propeptide region in addition to the TGF-β homodimer[17]. After it is synthesized, the TGF-β homodimer interacts with a Latency Associated Peptide (LAP)[a protein derived from the N-terminal region of the TGF beta gene product] forming a complex called Small Latent Complex (SLC). This complex remains in the cell until it is bound by another protein called Latent TGF-β-Binding Protein (LTBP), forming a larger complex called Large Latent Complex (LLC). It is LLC that get secreted to the ECM [18].

In most cases, before the LLC is secreted, the TGF-β precursor is cleaved from the propeptide but remains attached to it by noncovalent bonds[19]. After its secretion, it remains in the extracellular matrix as an inactivated complex containg both the LTBP and the LAP which need to be further processed in order to release active TGF-β[10]. The attachment of TGF-β to the LTBP is by disulfide bond which allows it to remain inactive by preventing it from binding to its receptors. Because different cellular mechanisms require distinct levels of TGF-β signaling, the inactive complex of this cytokine gives opportunity for a proper mediation of TGF-β signaling[10].

There are four different LAP isoforms known, LAP-1, LAP-2, LAP-3 and LAP-4[20]. Mutation or alteration of LAP or LTBP can result to improper TGF-β signaling. Mice lacking LAP-3 or LAP-4 demonstrate phenotypes consistent to phenotypes seen in mice with altered TGF-β signaling[21]. Furthermore, specific LAP isoforms have a propensity to associate with specific TGF-β isoforms. For example, LAP-4 is reported to bind only to TGF-β1[22], thus, mutation in LAP-4 can lead to TGF-β associated complications which are specific to tissues that predominantly involves TGF-β1. Moreover, the structural differences within the LAP’s provide different latent TGF-β complexes which are selective but to specific stimuli generated by specific activators.

Integrin-independent TGF-β activation

  • Activation by protease and metalloprotease

Plasmin and a number of matrix metalloproteinases (MMP) play a key role in promoting tumor invasion and tissue remodeling by inducing proteolysis of several ECM components[12 ]. The TGF-β activation process involves the release of the LLC from the matrix, followed by further proteolysis of the LAP to release TGF-β to its receptors. MMP-9 and MMP-2 are known to cleave latent TGF-β[15]. The LAP complex contains a protease-sensitive hinge region which can be the potential target for this liberation of TGF-β[16]. Despite the fact that MMPs have been proven to play a key role in activating TGF-β, mice with mutations in MMP-9 and MMP-2 genes can still activate TGF-β and do not show any TGF-β deficiency phenotypes, this may reflect redundancy among the activating enzymes[10] suggesting that other unknown proteases might be involved.

  • Activation by pH

Acidic conditions can denature the LAP. Treatment of the medium with extremes of pH (1.5 or 12) resulted in significant activation of TGF beta as shown by radio-receptor assays, while mild acid treatment (pH 4.5) yielded only 20-30% of the competition achieved by pH 1.5[23].

  • Activation reactive oxygen species (ROS)

The LAP structure is important to maintain its function. Structure modification of the LAP can lead to disturbing the interaction between LAP and TGF-β and thus activating it. Factors that may cause such modification may include hydroxyl radicals from reactive oxygen species (ROS). TGF-β was rapidly activated after in vivo radiation exposure ROS[13 ].

  • Activation by thrombospondin-1

Thrombospondin-1 (TSP-1) is a matricellular glycoprotein found in plasma of healthy patients with levels in the range of 50–250 ng/ml[24]. TSP-1 levels are known to increase in response to injury and during development[25]. TSP-1 activates latent TGF-beta [26] by forming direct interactions with the latent TGF-β complex and induces a conformational rearrangement preventing it from binding to the matured TGF-β[27].

Activation by Alpha(V) containing integrins

The general theme of integrins to participate in latent TGF-β1 activation, arose from studies that examined mutations/knockouts of β6 integrin[28], αV integrin[29], β8 integrin and in LAP. Theses mutations produced phenotypes that were similar to phenotypes seen in TGF-β1 knockout mice[30]. Currently there are two proposed models of how αV containing integrins can activate latent TGF-β1; the first proposed model is by inducing conformational change to the latent TGF-β1 complex and hence releasing the active TGF-β1 and the second model is by a protease-dependent mechanism[31].

  • Conformation change mechanism pathway (without proteolysis)

αVβ6 integrin was the first integrin to be identified as TGF-β1 activator[10]. LAPs contain an RGD motif which is recognized by vast majority of αV containing integrins[32], and αVβ6 integrin can activate TGF-β1 by binding to the RGD motif present in LAP-β1 and LAP-β 3[33]. Upon binding, it induces adhesion-mediated cell forces that are translated into biochemical signals which can lead to liberation/activation of TGFb from its latent complex[34]. This pathway has been demonstrated for activation of TGF-β in epithelial cells and does not associate MMPs[35].

  • Integrin protease-dependent activation mechanism

Because MMP-2 and MMP-9 can activate TGF-β through proteolytic degradation of the latent TGF beta complex[15], αV containing integrins activate TGF-β1 by creating a close connection between the latent TGF-β complex and MMPs. Integrins αVβ6 and αVβ3 are suggested to simultaneously bind the latent TGF-β1 complex and proteinases, simultaneous inducing conformation changes of the LAP and sequestering proteases to close proximity. Regardless of involving MMPs, this mechanism still necessitate the association of intergrins and that makes it a non protolylic pathway[31][36].

Disorders associated with activation of TGF-β signaling

Upregulation of TGF-β has been documented in several inflammatory disorders. Most of these studies proposed that by restoring normal control of TGF-β signaling or sometimes by inhibiting it without impairing its beneficial effects can lead to treatment of chronic inflammatory disorders as well as other TGF-β signaling associated disorders. The following are several reported diseases associated with activation of TGF-β by αV containing integrins that pose as potential therapeutic targets.

Integrin Medical condition Experimental conclusion ref.
αVβ3 1.Inflammation and fibrosis

2.Rheumatoid arthritis

1. αVβ3 and αvβ5 expression induce TGFb that regulate pulmonary inflammation and fibrosis in pulmonary T lymphocytes

2. αVβ3 and TGFb plays a major role in Rheumatoid arthritis; αvβ3 as a target for treatment.

[37]

[38]

αVβ6 1.Inflammatory

2. Carcinomas

3. Fibrosis

4. Cataracts

1. αvβ6 protect against inflammatory periodontal disease through activation of TGF-beta1

2. Blockade of integrin αvβ6 inhibits tumor progression in vivo by a transforming growth factor-beta-regulated mechanism

3. Inhibitors of alphavbeta6 integrin or TGFb for down-regulating fibrosis in the setting of acute or ongoing pulmonary, biliary injury, renal injury

4. αVβ6 is the main activator of TGF-beta1 in the lens capsule and represents a new target for PCO prevention.

[39]

[40]

[41]

[42]

[43]

αVβ8 1.Inflammation

2.Autoimmunity

3.Brain hemorrhage

1. Conditional loss of the TGF-beta-activating integrin αvβ8 on leukocytes causes severe inflammatory bowel

2. Loss of αvβ8 integrin on dendritic cells causes autoimmunity and colitis in mice.

3. Astrocytic αvβ8 acts as a central regulator of brain vessel homeostasis through regulation of TGF-beta activation

[44]

[45]

See also

  • Anita Roberts, a molecular biologist who made pioneering observations of TGF-β

External links

References

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