myospheroid


EVOLUTIONARY HOMOLOGS


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Integrins and Focal adhesion kinase (FAK)

A characteristic feature of certain integrins is their ability to modulate their affinity for extracellular ligands in response to intracellular signals, a process termed "activation" or inside-out signaling". A Ras/Raf-initiated MAP kinase activity suppresses mammalian integrin activation. Using a screen for suppressors of integrin activation, the small GTP-binding protein H-Ras, and its effector kinase, Raf-1 were identified as negative regulators of integrin activation. HRas inhibits the activation of integrins with three distinct alpha and beta subunit cytoplasmic domains. Suppression is not associated with integrin phosphorylation and is independent of both mRNA transcription and protein synthesis. Furthermore, suppression correlates with activation of the ERK MAP kinase pathway. It is possible that the integrin suppression pathway forms a local negative feedback loop for the regulation of integrin function. Ras activation through integrins might occur via the formation of a complex of FAK (Drosophila homolog Focal adhesion kinase-like), GRB-2 and SOS. Cells derived from FAK-deficient mice show enhanced focal adhesion formation, suggesting that these cells may have lost a negative regulator of integrin function. In addition, dominant negative Ras can enhance focal adhesion formation. Further evidence for the existence of a negative feedback loop comes from observations that integrin occupancy or the expression of isolated beta subunit cytoplasmic domains can suppress the function of other integrins. It is likely that a cytoplasmic substrate of a MAP kinase is involved in suppression (Hughes, 1997).

Integrin-mediated adhesion of cells to extracellular matrix proteins triggers a variety of intracellular signaling pathways including a cascade of tyrosine phosphorylations. In many cell types, the cytoplasmic focal adhesion tyrosine kinase, FAK, appears to be the initial protein that becomes tyrosine-phosphorylated in response to adhesion; however, the molecular mechanisms regulating integrin-triggered FAK phosphorylation are not understood. Previous studies have shown that the integrin beta1, beta3, and beta5 subunit cytoplasmic domains all contain sufficient information to trigger FAK phosphorylation when expressed in single-subunit chimeric receptors connected to an extracellular reporter. In the present study, beta3 cytoplasmic domain deletion and substitution mutants were constructed to identify amino acids within the integrin beta3 cytoplasmic domain that regulate its ability to trigger FAK phosphorylation. Cells transiently expressing chimeric receptors containing these mutant cytoplasmic domains were magnetically sorted and assayed for the tyrosine phosphorylation of FAK. Analysis of these mutants indicate that structural information in both the membrane-proximal and C-terminal segments of the beta3 cytoplasmic domain is important for triggering FAK phosphorylation. In the C-terminal segment of the beta3 cytoplasmic domain, the highly conserved NPXY motif is found to be required for the beta3 cytoplasmic domain to trigger FAK phosphorylation. However, the putative FAK binding domain within the N-terminal segment of the beta3 cytoplasmic domain is found to be neither required nor sufficient for this signaling event. The serine 752 to proline mutation, known to cause a variant of Glanzmann's thrombasthenia, inhibits the ability of the beta3 cytoplasmic domain to signal FAK phosphorylation, suggesting that a single mutation in the beta3 cytoplasmic domain can inhibit both "inside-out" and "outside-in" integrin signaling (Tahiliani, 1997).

Focal adhesion kinase (FAK) overexpression enhances ras-dependent integrin signaling to ERK2/mitogen-activated protein kinase through interactions with and activation of c-Src. Focal adhesion kinase associates with integrin receptors, and FN-stimulated phosphorylation of FAK at Tyr-397 and Tyr-925 promotes the binding of Src family protein tyrosine kinases (PTKs) and Grb2, respectively. To investigate the mechanisms by which FAK, c-Src, and Grb2 function in Fibronectin-stimulated signaling events to ERK2, wild type and mutant forms of FAK were expressed in human 293 epithelial cells by transient transfection. FAK overexpression enhances FN-stimulated activation of ERK2 approximately 4-fold. This is blocked by co-expression of the dominant negative Asn-17 mutant Ras, indicating that FN stimulation of ERK2 is Ras-dependent. FN-stimulated c-Src PTK activity is enhanced by wild type FAK expression, whereas FN-stimulated activation of ERK2 is blocked by expression of the c-Src binding site Phe-397 mutant of FAK. Expression of the Grb2 binding site Phe-925 mutant of FAK enhances activation of ERK2, whereas a kinase-inactive Arg-454 mutant FAK does not. Expression of wild type and Phe-925 FAK, but not Phe-397 FAK, enhances p130(Cas) association with FAK, Shc tyrosine phosphorylation, and Grb2 binding to Shc after FN stimulation. FN-induced Grb2-Shc association is another pathway leading to activation of ERK2 via Ras. The inhibitory effects of Tyr-397 FAK expression show that FAK-mediated association and activation of c-Src is essential for maximal signaling to ERK2. Moreover, multiple signaling pathways are activated upon the formation of a FAK.c-Src complex, and several of these can lead to Ras-dependent ERK2 mitogen-activated protein kinase activation (Schlaepfer, 1997).

Src family kinases (SFKs) have been implicated as important regulators of ligand-induced cellular responses including proliferation, survival, adhesion and migration. Analysis of SFK function has been impeded by extensive redundancy between family members. Mouse embryos were generated harboring functional null mutations of the ubiquitously expressed SFKs Src, Yes and Fyn. This triple mutation leads to severe developmental defects and lethality by E9.5. To elucidate the molecular mechanisms underlying this phenotype, SYF cells (deficient for Src, Yes and Fyn) were derived and tested for their ability to respond to growth factors or plating on extracellular matrix. While Src, Yes and Fyn are largely dispensable for platelet-derived growth factor (PDGF)-induced signaling, they are absolutely required to mediate specific functions regulated by extracellular matrix proteins. Fibronectin-induced tyrosine phosphorylation of focal adhesion proteins, including the focal adhesion kinase FAK, is nearly eliminated in the absence of Src, Yes and Fyn. Furthermore, consistent with previous reports demonstrating the importance of FAK for cell migration, SYF cells display reduced motility in vitro. These results demonstrate that SFK activity is essential during embryogenesis and suggest that defects observed in SYF triple mutant embryos may be linked to deficiencies in signaling by extracellular matrix-coupled receptors (Klinghoffer, 1999).

pp125FAK is a tyrosine kinase that appears to regulate the assembly of focal adhesions and thereby promotes cell spreading on the extracellular matrix. In some cells, the C terminus of pp125FAK is expressed as a separate protein, pp41/43FRNK. Overexpression of pp41/43FRNK inhibits tyrosine phosphorylation of pp125FAK and paxillin and, in addition, overexpression delays cell spreading and focal adhesion assembly. Thus, pp41/43FRNK functions as a negative inhibitor of adhesion signaling and provides a tool to dissect the mechanism by which pp125FAK promotes cell spreading. The inhibitory effects of pp41/43FRNK expression can be rescued by the co-overexpression of wild-type pp125FAK and partially rescued by catalytically inactive variants of pp125FAK. However, both coexpression of a pp125FAK mutant for the autophosphorylation site that fails to bind the SH2 domain of pp60c-Src, or of a mutant that fails to bind paxillin, fail to promotes cell spreading. In contrast, expression of pp41/43FRNK and pp60c-Src reconstitute cell spreading and tyrosine phosphorylation of paxillin but do so without inducing tyrosine phosphorylation of pp125FAK. These data provide additional support for a model whereby pp125FAK acts as a "switchable adaptor" that recruits pp60c-Src to phosphorylate paxillin, promoting cell spreading. In addition, these data point to tyrosine phosphorylation of paxillin as being a critical step in focal adhesion assembly (Richardson, 1997).

pp125 focal adhesion kinase (FAK), a cytoplasmic tyrosine kinase transducing signals initiated by integrin engagement and G protein-coupled receptors, is highly expressed in brain. FAK from brain has a higher molecular weight and an increased autophosphorylation activity, as compared to FAK from other tissues. In addition to a 9-base insertion in the 3'-coding region, which defines FAK+, rat striatal FAK mRNAs contained several additional short exons, coding for peptides of 28, 6, and 7 residues, respectively (termed boxes 28, 6, and 7), surrounding the autophosphorylated Tyr-397. In transfected COS 7 cells, the presence of boxes 6 and 7 confer an increased overall tyrosine phosphorylation, a higher phosphorylation of Tyr-397 assessed with a phosphorylation state-specific antibody, and a more active autophosphorylation in immune precipitates. The presence of box 28 does not further alter these parameters. Two-dimensional phosphopeptide maps of hippocampal FAK are identical to those of FAK+6,7. The presence of the various exons does not alter the interaction of FAK with c-Src, n-Src, or Fyn. Thus, several splice isoforms of FAK are preferentially expressed in rat brain, some of which have an increased autophosphorylation activity, suggesting that FAK may have specific properties in neurons (Burgaya, 1997).

Integrin alphaIIbbeta3 functions as the fibrinogen receptor on platelets and mediates platelet aggregation and clot retraction. Among the events that occur during either "inside-out" or "outside-in" signaling through alphaIIbbeta3 is the phosphorylation of focal adhesion kinase [pp125(FAK)] and the association of pp125(FAK) with cytoskeletal components. To examine the role of pp125(FAK) in these integrin-mediated events, pp125(FAK) phosphorylation and association with the cytoskeleton was determined in cells expressing two mutant forms of alphaIIbbeta3: alphaIIbbeta3(D723A/E726A), a constitutively active integrin in which the putative binding site for pp125(FAK) is altered, and alphaIIbbeta3(F727A/K729E/F730A), in which the putative binding site for alpha-actinin is altered. Whereas cells expressing alphaIIbbeta3(D723A/E726A) are able to form focal adhesions and stress fibers upon adherence to fibrinogen, cells expressing alphaIIbbeta3(F727A/K729E/F730A) adhere to fibrinogen, but have reduced focal adhesions and stress fibers. pp125(FAK) is recruited to focal adhesions in adherent cells expressing alphaIIbbeta3(D723A/E726A) and is phosphorylated in adherent cells or in cells in suspension in the presence of fibrinogen. In adherent cells expressing alphaIIbbeta3(F727A/K729E/F730A), pp125(FAK) is phosphorylated despite reduced formation of focal adhesions and stress fibers. It is concluded that activation of pp125(FAK) can be dissociated from two important events in integrin signaling, the assembly of focal adhesions in adherent cells and integrin activation following ligand occupation (Lyman, 1997).

Apoptotic cells undergo characteristic morphological changes that include detachment of cell attachment from the substratum and loss of cell-cell interactions. Attachment of cells to the extracellular matrix and to other cells is mediated by integrins. The interactions of integrins with the extracellular matrix activate focal adhesion kinase (FAK) and suppress apoptosis in diverse cell types. Members of the tumor necrosis family such as Fas and Apo-2L, also known as tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), induce apoptosis in both suspension and adherent cells through the activation of caspases. These caspases, when activated, cleave substrates that are important for the maintenance of nuclear and membrane integrity. FAK is sequentially cleaved into two different fragments early in Apo-2L-induced apoptosis. FAK cleavage is mediated by caspases, and FAK shows unique sensitivity to different caspases. These results suggest that disruption of FAK may contribute to the morphological changes observed in apoptotic suspension and adherent cells (Wen, 1997).

The tumor suppressor PTEN dephosphorylates focal adhesion kinase (FAK) and inhibits integrin-mediated cell spreading and cell migration. Expression of PTEN selectively inhibits activation of the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) pathway. PTEN expression in glioblastoma cells lacking the protein results in inhibition of integrin-mediated MAP kinase activation. Epidermal growth factor (EGF) and platelet-derived growth factor (PDGF)- induced MAPK activation are also blocked. To determine the specific point of inhibition in the Ras/Raf/ MEK/ERK pathway, these components were examined after stimulation by fibronectin or growth factors. Shc phosphorylation and Ras activity are inhibited by expression of PTEN, whereas EGF receptor autophosphorylation is unaffected. The ability of cells to spread at normal rates is partially rescued by coexpression of constitutively activated MEK1, a downstream component of the pathway. In addition, focal contact formation is enhanced as indicated by paxillin staining. The phosphatase domain of PTEN is essential for all of these functions, because PTEN with an inactive phosphatase domain does not suppress MAP kinase or Ras activity. In contrast to its effects on ERK, PTEN expression does not affect c-Jun NH2-terminal kinase (JNK) or PDGF-stimulated Akt. These data suggest that a general function of PTEN is to down-regulate FAK and Shc phosphorylation, Ras activity, downstream MAP kinase activation, and associated focal contact formation and cell spreading (Gu, 1998).

There are contrasting roles for integrin alpha subunits and their cytoplasmic domains in controlling cell cycle withdrawal and the onset of terminal differentiation. Ectopic expression of the integrin alpha5 or alpha6A subunit in primary quail myoblasts either decreases or enhances the probability of cell cycle withdrawal, respectively. The mechanisms by which changes in integrin alpha subunit ratios regulate this decision are addressed. Ectopic expression of truncated alpha5 or alpha6A indicate that the alpha5 cytoplasmic domain is permissive for the proliferative pathway, whereas the COOH-terminal 11 amino acids of alpha6A cytoplasmic domain inhibit proliferation and promote differentiation. The alpha5 and alpha6A cytoplasmic domains do not appear to initiate these signals directly, but instead regulate beta1 signaling. Ectopically expressed IL2R-alpha5 or IL2R-alpha6A have no detectable effect on the myoblast phenotype. However, ectopic expression of the beta1A integrin subunit or IL2R-beta1A, autonomously inhibits differentiation and maintains a proliferative state. Perturbing alpha5 or alpha6A ratios also significantly affects activation of beta1 integrin signaling pathways. Ectopic alpha5 expression enhances expression and activation of paxillin as well as mitogen-activated protein (MAP) kinase with little effect on focal adhesion kinase (FAK). In contrast, ectopic alpha6A expression suppresses FAK and MAP kinase activation with a lesser effect on paxillin. Ectopic expression of wild-type and mutant forms of FAK, paxillin, and MAP/erk kinase (MEK) confirm these correlations. These data demonstrate that (1) proliferative signaling (i.e., inhibition of cell cycle withdrawal and the onset of terminal differentiation) occurs through the beta1A subunit and is modulated by the alpha subunit cytoplasmic domains; (2) perturbing alpha subunit ratios alters paxillin expression and phosphorylation and FAK and MAP kinase activation; (3) quantitative changes in the level of adhesive signaling through integrins and focal adhesion components regulate the decision of myoblasts to withdraw from the cell cycle, in part via MAP kinase (Sastry, 1999).

The extracellular matrix exerts a stringent control on the proliferation of normal cells, suggesting the existence of a mitogenic signaling pathway activated by integrins, but not significantly by growth factor receptors. Evidence has been found that integrins cause a significant and protracted activation of Jun NH2-terminal kinase (JNK), while several growth factors cause more modest or no activation of this enzyme. Integrin-mediated stimulation of JNK requires the association of focal adhesion kinase (FAK) with a Src kinase and p130CAS, the phosphorylation of p130CAS, and subsequently, the recruitment of Crk. Ras and PI-3K are not required. FAK-JNK signaling is necessary for proper progression through the G1 phase of the cell cycle. These findings establish a role for FAK in both the activation of JNK and the control of the cell cycle, and identify a physiological stimulus for JNK signaling that is consistent with the role of Jun in both proliferation and transformation (Oktay, 1999).

What is the mechanism by which FAK activates JNK? Upon activation, FAK undergoes autophosphorylation at tyrosine 397 and combines with the SH2 domain of Src or Fyn. The most prominent substrates of the FAK/Src complex are the docking adaptor proteins p130CAS and paxillin. Both contain tyrosine phosphorylation sites conforming to the consensus for binding to the adaptor protein Crk. However, while paxillin has only two such sites and does not appear to associate efficiently with Crk in response to integrin ligation, p130CAS contains nine Crk-binding motifs and associates well with Crk in cells adhering to fibronectin. The expression of dominant-negative versions of FAK, Src, p130CAS, and Crk suppress the activation of JNK by integrins. These findings provide evidence that integrin-mediated activation of JNK requires the association of FAK with Src (or Fyn) and p130CAS, and the recruitment of Crk. Thus, it appears that the beta1 and alphav integrins activate JNK and ERK via two separate pathways. By contrast, the alpha6beta4 integrin, which is presumably unable to activate FAK because it does not contain the sequences required for its recruitment, is coupled to JNK signaling via the Ras-PI-3K-Rac pathway. The identity of genes regulated by JNK is largely unknown, but they must include genes important for cell proliferation. The evidence for this is several fold: (1) deregulated expression of c-Jun or its mutated viral version v-Jun is sufficient to cause neoplastic transformation of primary avian and mammalian fibroblasts; (2) primary fibroblasts derived from c-Jun minus mice display a severe proliferation defect; and (3) several oncoproteins, including v-Src, activated Ras, v-Crk, Bcr-Abl, and Met, potently activate JNK and there is evidence to suggest that this activation is required to cause neoplastic transformation. Despite the clear requirement for c-Jun transcriptional activity in cell proliferation, it has been difficult to identify a physiological, nonstress stimulus for JNK consistent with its role in the regulation of AP-1 transcription. With the notable exception of EGF, mitogenic neuropeptides, and muscarinic receptor ligands, which indeed activate FAK or the related kinase PYK-2, most growth factors cause a relatively modest activation of JNK. The results indicating that integrin ligation causes a significant activation of JNK and TRE-dependent transcription provide a physiological stimulus for JNK signaling that is consistent with its role in the control of cell proliferation (Oktay, 1999 and references).

The signaling events downstream of integrins that regulate cell attachment and motility are only partially understood. Using osteoclasts and transfected 293 cells, it has been found that a molecular complex comprising Src, Pyk2 (a calcium-dependent tyrosine kinase that is the predominant FAK family member in the adhesion structures of osteoclasts), and Cbl functions to regulate cell adhesion and motility. The activation of integrin alphavß3 induces the [Ca2+]i-dependent phosphorylation of Pyk2 Y402, its association with Src SH2, Src activation, and the Src SH3-dependent recruitment and phosphorylation of c-Cbl. Furthermore, the PTB domain of Cbl is shown to bind to phosphorylated Tyr-416 in the activation loop of Src, the autophosphorylation site of Src, inhibiting Src kinase activity and integrin-mediated adhesion. Deletion of c-Src or c-Cbl leads to a decrease in osteoclast migration. Thus, binding of alphavß3 integrin induces the formation of a Pyk2/Src/Cbl complex in which Cbl is a key regulator of Src kinase activity and of cell adhesion and migration. These findings may explain the osteopetrotic phenotype in the Src-/- mice (Sanjay, 2001).

Cyclic attachment and detachment of individual podosomes is required as cells, particularly highly motile cells such as macrophages and osteoclasts, migrate over a substratum. Pyk2, Src, and Cbl appear to play a pivotal role in these processes. Deletion of Src significantly alters the initial distribution of podosomes in osteoclasts, eventually leading to the formation of focal adhesion-like structures. This shift from podosome to focal adhesion correlates with a decrease in the formation and motility of lamellipodia and in cell migration, although it is not possible to determine which comes first. Adhesion to vitronectin or activation of the alphavß3 receptor induces an increase in intracellular calcium that does not depend on the presence of Src. The formation of a kinase-rich trimolecular complex results in the Pyk2- and Src-dependent phosphorylation of Cbl. Therefore, Pyk2, Src, and Cbl are all involved in the 'outside-in' alphavß3 integrin signaling which results in podosome assembly. In transfected 293-VnR cells, the PTB domain of Cbl binds to Tyr 416 in the activation loop of the Src kinase domain, and this interaction downregulates both Src kinase activity and integrin-mediated adhesion. Thus, Cbl might also be crucial in 'inside-out' signaling, playing a key role in podosome detachment and subsequent disassembly. In agreement with this hypothesis, deletion of the gene encoding Cbl, like the deletion of the gene encoding Src, leads to a significant, albeit smaller, decrease in osteoclast migration. This series of events could form the basis for the cyclic attachment-detachment of single adhesion sites at the leading edge of lamellipodia in motile cells, and thereby participates in the assembly-disassembly of individual podosomes, thereby ensuring cell adhesion while still allowing cell motility (Sanjay, 2001).

alphaß1 integrins have been implicated in the survival, spreading, and migration of cells and tissues. To explore the underlying biology, conditions were identified where primary ß1 null keratinocytes adhere, proliferate, and display robust alphavß6 integrin-induced, peripheral focal contacts associated with elaborate stress fibers. Mechanistically, this appears to be due to reduced FAK and Src and elevated RhoA and Rock activities. Visualization on a genetic background of GFPactin shows that ß1 null keratinocytes spread, but do so aberrantly, and when induced to migrate from skin explants in vitro, the cells are not able to rapidly reorient their actin cytoskeleton toward the polarized movement. As judged by RFPzyxin/GFPactin videomicroscopy, the alphavß6-actin network does not undergo efficient turnover. Without the ability to remodel their integrin-actin network efficiently, alphaß1-deficient keratinocytes cannot respond dynamically to their environment and polarize movements (Raghavan, 2003).

The results underscore a novel and distinct role for alphaß1 integrins in regulating this equilibrium in focal adhesion dynamics. Not surprisingly, three well-known regulators of focal contacts, FAK, RhoA, and Rock, appear to be at the heart of this regulation. As judged by immunofluorescence with purportedly specific phospho-FAK Abs, activated FAK localizes to the focal contacts of ß1 null keratinocytes. By this criterion, the underlying defects in focal contact turnover and in overall FAK and Src activities are not attributable to a defect in targeting FAK to alphavß6 focal contacts, and indeed, ligand-engaged alphavß6 can bind and activate FAK. Rather, in the absence of ß1, alphavß6 appears unable on its own to activate FAK to the threshold levels needed to properly control focal adhesion-actin cytoskeletal dynamics. Irrespective of the precise underlying mechanism, the consequences to this imbalance are excessive adhesion and inefficient spreading (Raghavan, 2003).

Although tyrosine kinase inhibitors can block focal adhesion formation in some situations, a greater role for tyrosine phosphorylation has been found in focal adhesion turnover and cell motility. Thus, activated FAK negatively regulates RhoA activity, and FAK null fibroblasts express robust actin stress-fiber networks that can be dissipated by Rock inhibition. The ability of Rho and Rock inhibitors to disperse both stress fibers and associated focal contacts in ß1 null keratinocytes provides compelling evidence that a FAK-RhoA imbalance is at the root of the focal adhesion-cytoskeletal imbalance in these cells. Although more complicated mechanisms are possible, the data are consistent with a model whereby in the absence of alphaß1 integrins, FAK/Src activation is not fully achieved, thereby diminishing p190RhoGAP phosphorylation, and yielding elevated RhoA/Rock activities (Raghavan, 2003).

FAK independent and other signaling by integrins

Integrins induce the formation of large complexes of cytoskeletal and signaling proteins, which regulate many intracellular processes. The activation and assembly of signaling complexes involving focal adhesion kinase (FAK) occurs late in integrin signaling, downstream from actin polymerization. Integrin-mediated activation of the non-receptor tyrosine kinase Syk in hematopoietic cells is independent of FAK and actin polymerization, and suggests the existence of a distinct signaling pathway regulated by Syk. Multiple proteins are activated by Syk, downstream of engagement of the platelet/megakaryocyte-specific integrin alphaIIbbeta3. The guanine nucleotide exchange factor Vav1 is inducibly phosphorylated in a Syk-dependent manner in cells following their attachment to fibrinogen. Together, Syk and Vav1 trigger lamellipodia formation in fibrinogen-adherent cells; both Syk and Vav1 colocalize with alphaIIbbeta3 in lamellipodia but not in focal adhesions. Additionally, Syk and Vav1 cooperatively induce activation of Jun N-terminal kinase (JNK), extracellular-signal-regulated kinase 2 (ERK2) and the kinase Akt, and phosphorylation of the oncoprotein Cbl in fibrinogen-adherent cells. Activation of all of these proteins by Syk and Vav1 is not dependent on actin polymerization. It is concluded that Syk and Vav1 regulate a unique integrin signaling pathway that differs from the FAK pathway in its proximity to the integrin itself, its localization to lamellipodia, and its activation, which is independent of actin polymerization. This pathway may regulate multiple downstream events in hematopoietic cells, including Rac-induced lamellipodia formation, tyrosine phosphorylation of Cbl, and activation of JNK, ERK2 and the phosphatidylinositol 3'-kinase-regulated kinase Akt (Miranti, 1998).

Integrins are widely expressed plasma membrane adhesion molecules that tether cells to matrix proteins and to one another in cell-cell interactions. Integrins also transmit outside-in signals that regulate functional responses of cells, and are known to influence gene expression by regulating transcription. Platelets, which are naturally occurring anucleate cytoplasts, translate preformed mRNA transcripts when they are activated by outside-in signals. Using strategies that interrupt engagement of integrin alphaIIbbeta3 by fibrinogen and platelets deficient in this integrin, it was found that alphaIIbbeta3 regulates the synthesis of B cell lymphoma 3 (Bcl-3) when platelet aggregation is induced by thrombin. Synthesis of Bcl-3, which occurs via a specialized translation control pathway regulated by mammalian target of rapamycin (mTOR), is induced when platelets adhere to immobilized fibrinogen in the absence of thrombin and when integrin alphaIIbbeta3 is engaged by a conformation-altering antibody against integrin alphaIIbbeta3. Thus, outside-in signals delivered by integrin alphaIIbbeta3 are required for translation of Bcl-3 in thrombin-stimulated aggregated platelets and are sufficient to induce translation of this marker protein in the absence of thrombin. Engagement of integrin alpha2beta1 by collagen also triggers synthesis of Bcl-3. Thus, control of translation may be a general mechanism by which surface adhesion molecules regulate gene expression (Pabla, 1999).


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myospheroid: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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