MEK1-RSK2 contributes to Hedgehog signaling by stabilizing GLI2 transcription factor and inhibiting ubiquitination

The transcription factor GLI2 has an important role in the transduction of Hedgehog signaling and thereby regulates tumorigenesis in a wide variety of human tumors. However, the mechanisms controlling GLI2 protein expression and stabilization are incompletely understood. In this study, we show that the mitogen-activated protein kinase MEK1 modulates GLI2 both at the mRNA and protein level. Constitutively activated MEK1 prolonged the half-life of GLI2 and increased its nuclear translocation, accompanied by attenuated ubiquitination of GLI2 protein. RSK2, a protein kinase lying downstream of MEK-ERK cascade, mimicked the effect of MEK on GLI2 stabilization. MEK1 and RSK2 failed to augment the half-life of GLI2 lacking GSK-3β phosphorylation sites, indicating that MEK-RSK stabilizes GLI2 by controlling targeting GSK-3β-mediated phosphorylation and ubiquitination of GLI2. The significance of MEK-RSK stabilization was demonstrated in experiments showing that activation of MEK-RSK paralleled higher protein level of GLI2 in several multiple myelomas (MM) cells relative to normal B cells. Moreover, combined treatment with RSK and GLI inhibitors led to an enhanced apoptosis of MM cells. Thus, our results indicate that MEK-RSK cascade positively regulates GLI2 stabilization and represses its degradation via inhibiting GSK-3β-dependent phosphorylation and ubiquitination of GLI2.

Basic helix-loop-helix transcription factor Twist1 inhibits transactivator function of master chondrogenic regulator Sox9

Canonical Wnt signaling strongly inhibits chondrogenesis. Previously, we identified Twist1 as a critical downstream mediator of Wnt in repression of chondrocyte differentiation. However, the mechanistic basis for the antichondrogenic activity of Twist1 has not heretofore been established. Here, we show that Twist1 suppresses cartilage development by directly inhibiting the transcriptional activity of Sox9, the master regulator of chondrogenesis. Twist1, through its carboxyl-terminal Twist-box, binds to the Sox9 high mobility group DNA-binding domain, inhibiting Sox9 transactivation potential. In chondrocyte precursor cells, Twist1, in a Twist-box-dependent manner, inhibits Sox9-dependent activation of chondrocyte marker gene expression by blocking Sox9-enhancer DNA association. These findings identify Twist1 as an inhibitor of Sox9 and further suggest that the balance between Twist1 and Sox9 may determine the earliest steps of chondrogenesis.

N-cadherin adherens junctions mediate osteogenesis through PI3K signaling

During endochondral ossification, the cartilage is surrounded by a layer of cells that constitute the perichondrium. Communication between osteoblasts in the perichondrium via N-cadherin adherens junctions is essential for endochondral bone growth. We observed that adherens junction molecule N-cadherin and its interacting partners p120, β-catenin and PTEN are expressed by cells present in the perichondrium. To study if N-cadherin mediated adherens junctions play a role in mediating signal transduction events during bone development, we utilized MC3T3E1 preosteoblasts plated at sub confluent (low) and confluent (high) densities to mimic adherens junction formation. When MC3T3E1 cells were plated at high density we observed an increase in phosphorylation of AKTSer473 and its downstream target GSK3Ser9, which coincided with an increase in Osterix, Osteomodulin and Osteoglycin gene expression. Using immunofluorescence, we identified N-cadherin, p120 and β-catenin localized at the membrane of MC3T3E1 cells. Treatment of confluent MC3T3E1 cells with an N-cadherin junction inhibitor-EGTA and a PI3K inhibitor LY294002 resulted in reduction of phosphorylation levels of AKT and GSK3 and expression of Osterix, Osteomodulin and Osteoglycin. Furthermore, utilizing an N-cadherin blocking antibody resulted in reduced AKT signaling and Osterix gene expression, suggesting that osteoblast junction formation is linked to activation of PI3K signaling, which leads to osteoblast differentiation. To further explore the strength of this linkage, we utilized a conditional knockout approach using Dermo1cre to delete β-catenin and PTEN, two important proteins known to be essential for adherens junctions and PI3K signaling, respectively. In the absence of β-catenin, we observed a decrease in adherens junctions and AKT signaling in the perichondrium. PTEN deletion, on the other hand, increased the number of cells expressing N-cadherin in the perichondrium. These observations show that N-cadherin mediated junctions between osteoblasts are needed for osteoblast gene transcription.

Conditional ablation of Pten in osteoprogenitors stimulates FGF signaling

Phosphatase and tensin homolog deleted on chromosome ten (PTEN) is a direct antagonist of phosphatidylinositol 3 kinase. Pten is a well recognized tumor suppressor and is one of the most commonly mutated genes in human malignancies. More recent studies of development and stem cell behavior have shown that PTEN regulates the growth and differentiation of progenitor cells. Significantly, PTEN is found in osteoprogenitor cells that give rise to bone-forming osteoblasts; however, the role of PTEN in bone development is incompletely understood. To define how PTEN functions in osteoprogenitors during bone development, we conditionally deleted Pten in mice using the cre-deleter strain Dermo1cre, which targets undifferentiated mesenchyme destined to form bone. Deletion of Pten in osteoprogenitor cells led to increased numbers of osteoblasts and expanded bone matrix. Significantly, osteoblast development and synthesis of osteoid in the nascent bone collar was uncoupled from the usual tight linkage to chondrocyte differentiation in the epiphyseal growth plate. The expansion of osteoblasts and osteoprogenitors was found to be due to augmented FGF signaling as evidenced by (1) increased expression of FGF18, a potent osteoblast mitogen, and (2) decreased expression of SPRY2, a repressor of FGF signaling. The differentiation of osteoblasts was autonomous from the growth plate chondrocytes and was correlated with an increase in the protein levels of GLI2, a transcription factor that is a major mediator of hedgehog signaling. We provide evidence that increased GLI2 activity is also a consequence of increased FGF signaling through downstream events requiring mitogen-activated protein kinases. To test whether FGF signaling is required for the effects of Pten deletion, we deleted one allele of fibroblast growth factor receptor 2 (FGFR2). Significantly, deletion of FGFR2 caused a partial rescue of the Pten-null phenotype. This study identifies activated FGF signaling as the major mediator of Pten deletion in osteoprogenitors.

Inhibition of autoregulated TGFbeta signaling simultaneously enhances proliferation and differentiation of kidney epithelium and promotes repair following renal ischemia

We studied autocrine transforming growth factor (TGF)beta signaling in kidney epithelium. Cultured proximal tubule cells showed regulated signaling that was high during log-phase growth, low during contact-inhibited differentiation, and rapidly increased during regeneration of wounded epithelium. Autoregulation of signaling correlated with TGFbeta receptor and Smad7 levels, but not with active TGFbeta, which was barely measurable in the growth medium. Confluent differentiated cells with low receptor and high Smad7 levels exhibited blunted responses to saturating concentrations of exogenously provided active TGFbeta, suggesting that TGFbeta signaling homeostasis was achieved by cell density-dependent modulation of signaling intermediates. Antagonism of Alk5 kinase, the TGFbeta type I receptor, dramatically accelerated the induction of differentiation in sparse, proliferating cultures and permitted better retention of differentiated features in regenerating cells of wounded, confluent cultures. Alk5 antagonism accelerated the differentiation of cells in proximal tubule primary cultures while simultaneously increasing their proliferation. Consequently, Alk5-inhibited primary cultures formed confluent, differentiated monolayers faster than untreated cultures. Furthermore, treatment with an Alk5 antagonist promoted kidney repair reflected by increased tubule differentiation and decreased tubulo-interstitial pathology during the recovery phase following ischemic injury in vivo. Our results show that autocrine TGFbeta signaling in proliferating proximal tubule cells exceeds the levels that are necessary for physiological regeneration. To that end, TGFbeta signaling is redundant and maladaptive during tubule repair by epithelial regeneration.

Direct interactions of Runx2 and canonical Wnt signaling induce FGF18

Canonical Wnt signaling is clearly required for skeletal development and bone formation. However, the targets of Wnt signaling that convert this signal into bone are unclear. Identification of these targets will yield insight into normal bone physiology and suggest new therapeutics for treatment of bone disease. Here we show that an essential regulator of bone development, FGF18, is a direct target of canonical Wnt signaling. A single DNA binding site for the Wnt-dependent transcription factors TCF/Lef accounted for the stimulation of the fgf18 promoter in response to Wnt signaling. Additionally, targeted disruption of betacat blocked fgf18 expression in vivo. Partially overlapping the TCF/Lef binding site is a Runx2 binding site and experiments showed that Runx2 and TCF/Lef work cooperatively to induce fgf18 expression. RNA interference knockdown of Runx2 inhibited and Runx2 forced expression augmented the induction of fgf18 by canonical Wnt signaling. Significantly, Runx2 formed a complex with Lef1 or TCF4 and this complex bound the composite binding site in the fgf18 promoter. These results demonstrate that two transcription pathways that are essential for bone, physically and functionally converge at the fgf18 promoter.

Glycogen synthase kinase 3 controls endochondral bone development: contribution of fibroblast growth factor 18

Glycogen synthase kinase 3 (GSK3) inhibits signaling pathways that are essential for bone development. To study the requirement for GSK activity during endochondral bone development, we inhibited GSK3 in cultured metatarsal bones with pharmacological antagonists. Interestingly, we find that inhibition of GSK3 strongly repressed chondrocyte and perichondrial osteoblast differentiation. Moreover, chondrocyte proliferation was inhibited, whereas perichondrial cell proliferation was stimulated. These results mirror the effects of fibroblast growth factor signaling (FGF), suggesting the FGF expression is induced. Indeed, we showed that (1) FGF18 expression is stimulated following inhibition of GSK3 and (2) GSK3 regulates FGF18 expression through the control of beta-catenin levels. Stimulation of cultured metatarsal with FGF18 had similar effects on the differentiation and proliferation of chondrocytes and perichondrial cells as GSK3 repression. This suggests that the regulation of FGF18 expression is a major function of GSK3 during endochondral bone development. Consistent with this, we showed that the effect of GSK3 inhibition on chondrocyte proliferation is repressed in tissues lacking a receptor for FGF18, FGF receptor 3.

The Wnt-inducible transcription factor Twist1 inhibits chondrogenesis

Wnt signaling is essential for many developmental processes, including skeletogenesis. To investigate the effects of Wnt signaling during skeletogenesis we studied the effects of Wnt on cultured chondrocytic cells and differentiating limb-bud mesenchyme. We showed that Wnt3a strongly repressed chondrogenesis and chondrocyte gene expression. Canonical Wnt signaling was responsible for the repression of differentiation, as evidenced by results showing that inhibition of glycogen synthase kinase 3 or expression of beta-catenin caused similar repression of differentiation. Significantly, we showed that the transcription repressor Twist1 is induced by canonical Wnt signaling. Expression of Twist1 strongly inhibited chondrocyte gene expression and short hairpin RNA knockdown of Twist1 transcript levels caused increased expression of the chondrocyte-specific genes aggrecan and type II collagen. Interestingly, Twist1 interfered with BMP2-induced expression of aggrecan and type II collagen expression and knockdown of Twist1 augmented BMP2-induced aggrecan and type II collagen expression. These data support the conclusions that Twist1 contributes to the repression of chondrogenesis and chondrocyte gene expression resulting from canonical Wnt signaling and that Twist1 interferes with BMP-dependent signaling.

Regulation of sprouty expression by PLCgamma and calcium-dependent signals

Sprouty, an essential antagonist of fibroblast growth factor receptor signaling, is induced following fibroblast growth factor receptor activation. The signaling pathways that induce sprouty have been incompletely characterized. However, studies show that MAP kinase signaling stimulates sprouty induction in various cell lines. Here we report that activation of sprouty expression by basic fibroblast growth factor required phospholipase Cgamma (PLCgamma) and calcium-dependent signaling. We showed that the induction of sprouty was inhibited by chelation of intracellular or extracellular calcium and that a fibroblast growth factor receptor deficient for PLCgamma signaling only weakly induced sprouty expression. Additionally, inhibition of PLCgamma with a pharmacological antagonist repressed the induction of sprouty by basic fibroblast growth factor. These findings indicate that calcium-dependent signaling regulates sprouty expression and that PLCgamma is vital for this process. This pathway of sprouty induction may be critical at sites such as limb bud mesenchyme where MAP kinases are inactive.

FGF18 represses noggin expression and is induced by calcineurin

Fibroblast growth factors (FGFs) and bone morphogenetic proteins strongly regulate chondrogenesis and chondrocyte gene expression. The interactions of the signaling pathways initiated by these factors are central to the control of chondrocyte differentiation. Here we show that calcium-dependent signals induce expression of FGF18, an essential regulator of bone and cartilage differentiation. The induction of FGF18 expression required the calcium-dependent phosphatase, calcineurin. The activated forms of calcineurin or the calcineurin-dependent transcription factor, NFAT4 (nuclear factor of activated T-cell 4), induced FGF18 expression. FGF18 or a constitutive active FGF receptor suppressed noggin gene induction and thereby increased chondrocyte gene expression and chondrogenesis by facilitating bone morphogenetic protein-dependent signals. These findings reinforce the interdependence of bone morphogenetic protein and FGF signaling and provide a rational explanation for abnormal bone development occurring in humans or mice with constitutively active FGF receptors.

Fibroblast Growth Factor Receptor 3 Gene: Regulation by Serum Response Factor

We have previously identified a cis-acting sequence in the proximal promoter of the fibroblast growth factor receptor 3 (FGFR3) gene that strongly activates transcription in chondrocytic cells. Here we report that the transcriptional activity of this sequence (FRE3) requires serum response factor and its cognate recognition motif, serum response element. Although the FRE3 contains consensus sequence motifs for several transcription factors, the serum response element is paramount for the transcriptional activity of the FRE3. Additionally, the transcriptional activity of the proximal promoter of the FGFR3 gene is suppressed by mutation of the serum response element. Serum response factor binds to the FRE3 as evidenced by gel shift experiments and antibody supershift experiments and expression of a dominant negative form of serum response factor suppresses the activity of FRE3. Additionally, serum response factor binds to the FGFR3 gene in vivo, as demonstrated by chromatin immunoprecipitation. Serum response factor is an important regulator of cardiac, skeletal, and smooth muscle gene expression; these data suggest that serum response factor is also an important determinant of chondrocyte gene expression.

Calcineurin and NFAT4 induce chondrogenesis

Nuclear factor of activated T-cells (NFAT) and calcineurin are essential regulators of immune cell and mesenchymal cell differentiation. Here we show that elevated intracellular calcium induces chondrogenesis through a calcineurin/NFAT signaling axis that activates bone morphogenetic protein (BMP) expression. The calcium ionophore, ionomycin, induced chondrogenesis through activation of calcineurin. The calcineurin substrate, NFAT4, also induced chondrogenesis and chondrocyte gene expression. Significantly, the BMP antagonist, noggin, or dominant negative BMP receptors blocked the effects of elevated intracellular calcium on chondrogenesis. This suggested that calcineurin/NFAT4 activates BMP expression. Consistent with this, BMP2 gene expression was increased by ionomycin and suppressed by the calcineurin inhibitor, cyclosporine A. Furthermore, activated NFAT4 induced BMP2 gene expression. These results have important implications for the effects of NFATs during development and adaptive responses.

Interaction of FGF, Ihh/Pthlh, and BMP signaling integrates chondrocyte proliferation and hypertrophic differentiation

Mutations in fibroblast growth factor (FGF) receptor 3 lead to the human dwarfism syndrome achondroplasia. Using a limb culture system, we have analyzed the role of FGF signaling and its interaction with the Ihh/Pthlh and BMP pathways in regulating chondrocyte differentiation. In contrast to previous suggestions, we demonstrate that FGF signaling accelerates both the onset and the pace of hypertrophic differentiation. We furthermore found that FGF and BMP signaling act in an antagonistic relationship regulating chondrocyte proliferation, Ihh expression, and the process of hypertrophic differentiation. Importantly, BMP signaling rescues the reduced domains of proliferating and hypertrophic chondrocytes in a mouse model for achondroplasia. We propose a model in which the balance of BMP and FGF signaling adjusts the pace of the differentiation process to the proliferation rate.

Expression of FGFR3 with the G380R achondroplasia mutation inhibits proliferation and maturation of CFK2 chondrocytic cells

A G380R substitution in the transmembrane-spanning region of FGFR3 (FGFR3Ach) results in constitutive receptor kinase activity and is the most common cause of achondroplastic dwarfism in humans. The epiphyseal growth plates of affected individuals are disorganized and hypocellular and show aberrant chondrocyte maturation. To examine the molecular basis of these abnormalities, we used a chondrocytic cell line, CFK2, to stably express the b variant of wild-type FGFR3 or the the constitutively active FGFR3Ach. Overexpression of FGFR3 had minimal effects on CFK2 proliferation and maturation compared with the severe growth retardation found in cells expressing FGFR3Ach. Cells expressing the mutant receptor also showed an abnormal apoptotic response to serum deprivation and failed to undergo differentiation under appropriate culture conditions. These changes were associated with altered expression of integrin subunits, which effectively led to a switch in substrate preference of the immature cell from fibronectin to type II collagen. These in vitro observations support those from in vivo studies indicating that FGFR3 mediates an inhibitory influence on chondrocyte proliferation. We now suggest that the mechanism is related to altered integrin expression.

Fibroblast growth factor receptor 3 gene transcription is suppressed by cyclic adenosine 3',5'-monophosphate. Identification of a chondrocytic regulatory element

Signaling through fibroblast growth factor receptors (FGFRs) is critical for the development and patterning of the vertebrate skeleton. Gain-of-function alleles of fgfr2 and fgfr3 have been linked to several dominant skeletal disorders in humans, while null mutations in fgfr3 result in the overgrowth of long bones in a mouse model system. Interestingly, the expression pattern of fgfr3 in growth plate chondrocytes overlaps that of the parathyroid hormone (PTH)-related peptide (PTHrP) receptor, a signaling molecule that also regulates endochondral ossification. The coincident expression of these two receptors suggests that their signaling pathways may also interact. To gain insight into the regulatory mechanism(s) that govern the expression of the fgfr3 gene in chondrocytes, we have identified a cell-specific transcriptional regulatory element (CSRh) by measuring the activity of various promoter fragments in FGFR3-expressing (CFK2) and nonexpressing (RCJ) chondrocyte-like cell lines. Furthermore, we demonstrate that activation of PTH/PTHrP receptors, either by stimulation with PTH or through the introduction of activating mutations, represses CSRh-mediated transcriptional activity. Finally, the transcriptional repression of the CSRh element was mimicked by treatment with forskolin, 8-bromo-cAMP, and 3-isobutyl-1-methylxanthine or by overexpression of the catalytic subunit of protein kinase A. Together, these data suggest that protein kinase A activity is a critical factor that regulates fgfr3 gene expression in the proliferative or prehypertrophic compartment of the epiphyseal growth plate. Furthermore, these results provide a possible link between PTHrP signaling and fgfr3 gene expression during the process of endochondral ossification.

Repression of hedgehog signaling and BMP4 expression in growth plate cartilage by fibroblast growth factor receptor 3

Fibroblast growth factor receptor 3 (FGFR3) is a key regulator of skeletal growth and activating mutations in Fgfr3 cause achondroplasia, the most common genetic form of dwarfism in humans. Little is known about the mechanism by which FGFR3 inhibits bone growth and how FGFR3 signaling interacts with other signaling pathways that regulate endochondral ossification. To understand these mechanisms, we targeted the expression of an activated FGFR3 to growth plate cartilage in mice using regulatory elements from the collagen II gene. As with humans carrying the achondroplasia mutation, the resulting transgenic mice are dwarfed, with axial, appendicular and craniofacial skeletal hypoplasia. We found that FGFR3 inhibited endochondral bone growth by markedly inhibiting chondrocyte proliferation and by slowing chondrocyte differentiation. Significantly, FGFR3 downregulated the Indian hedgehog (Ihh) signaling pathway and Bmp4 expression in both growth plate chondrocytes and in the perichondrium. Conversely, Bmp4 expression is upregulated in the perichondrium of Fgfr3−/− mice. These data support a model in which Fgfr3 is an upstream negative regulator of the hedgehog (Hh) signaling pathway. Additionally, Fgfr3 may coordinate the growth and differentiation of chondrocytes with the growth and differentiation of osteoprogenitor cells by simultaneously modulating Bmp4 and patched expression in both growth plate cartilage and in the perichondrium.

FGF signaling in skeletal development

The fibroblast growth factor receptor family consists of four receptor tyrosine kinases which bind with varying affinity and specificity to a family of at least fifteen polypeptide ligands. The receptors and ligands perform many essential functions during growth, development and repair. Recent discoveries show that a growing number of skeletal abnormalities result from mutations in the fibroblast growth factor receptors. These findings have led to a greater understanding of the role of fibroblast growth factor signaling during skeletogenesis and have focused research interests on the effects of fibroblast growth factors on endochondral and intramembranous bone development.

Graded activation of fibroblast growth factor receptor 3 by mutations causing achondroplasia and thanatophoric dysplasia

The longitudinal growth of the skeleton arises from the continuous process of endochondral ossification occurring at the ends of growing long bones. Dwarfism results when this process is disrupted, as in the autosomal dominant human skeletal diseases hypochondroplasia (HCH), achondroplasia (ACH) and thanatophoric dysplasia (TD). Interestingly, these disorders display a graded spectrum of phenotypic severity and are the result of distinct missense mutations in the fibroblast growth factor receptor 3 gene (FGFR3). TD, characterized by neonatal lethality and profound dwarfism, is the result of FGFR3 mutations, including an R248C substitution in the extracellular domain or a K650E substitution in the tyrosine kinase (TK) domain. ACH, which is non-lethal and presents less severe dwarfism, results almost exclusively from a G380R substitution in the transmembrane domain. Homozygous achondroplasia resembles the phenotype of TD. In this report the effect of the ACH and TD mutations on the activity and regulation of FGFR3 are analysed. We showed that each of the mutations constitutively activate the receptor, as evidenced by ligand-independent receptor tyrosine phosphorylation and cell proliferation. Moreover, the mutations that are responsible for TD were more strongly activating than the mutation causing ACH, providing a biochemical explanation for the observation that the phenotype of TD is more severe than that of ACH.

Kinetics of inactivation of alpha-thrombin by plasminogen activator inhibitor-1. Comparison of the effects of native and urea-treated forms of vitronectin

Kinetic studies are presented which show that native human vitronectin, but not urea-treated vitronectin, accelerates the inactivation of human alpha-thrombin by human plasminogen activator inhibitor-1 (PAI-1). We demonstrate that although urea-treated vitronectin binds PAI-1 with an affinity greater than that of native vitronectin, it does not accelerate the rate of inactivation of alpha-thrombin by PAI-1. We present evidence to suggest that the inability of urea-treated vitronectin to accelerate the reaction between alpha-thrombin and PAI-1 results at least in part from the inability of urea-treated vitronectin to bind to alpha-thrombin. The accelerated reaction between PAI-1 and alpha-thrombin can be accounted for by the formation of a tight complex between native vitronectin and PAI-1 that reacts in a saturable manner (Kd = 75 nM) with alpha-thrombin. The second-order rate constant (kI/Kd) for the reaction of the native vitronectin-PAI-1 complex with alpha-thrombin (1.64 x 10(5) M(-)-1 s-1) is 270-fold greater than the second-order rate constant for the reaction in the absence of vitronectin (610 m-1 s-1). The increase in the second-order rate constant is largely due to an increase in the affinity of alpha-thrombin for the native vitronectin-PAI-1 complex, as reflected by a greater than 25-fold reduction in the dissociation constant (Kd) observed for the vitronectin-PAI-1 complex relative to that of uncomplexed PAI-1.

alpha-Thrombin within fibrin clots: inactivation of thrombin by antithrombin-III

To demonstrate that human alpha-thrombin is effectively inactivated by human antithrombin III (AT) during the production of a fibrin clot we measured the amount of alpha-thrombin activity which can be recovered from a clot generated from purified human proteins. We discovered that 0.05-0.07% of the original alpha-thrombin activity is recovered from a fibrin clot produced from a reaction mixture where the initial concentrations of AT and alpha-thrombin were chosen at a ratio (17.5) to allow complete conversion of fibrinogen to fibrin. These results indicated that alpha-thrombin is successfully inactivated by AT during the production of a fibrin clot. Further, when an amount of alpha-thrombin equal to that recovered from a fibrin clot is introduced into a solution of fibrinogen and AT identical to that utilized to produce the clot only 4% of the fibrinogen is converted to fibrin. These results suggest that i) when a fibrin clot is dissolved during fibrinolytic therapy little active alpha-thrombin should be released from the clot and ii) this amount of thrombin is insufficient to catalyze rethrombosis without proposing de novo production of thrombin. The action on factors XI, VIII, and V of the small amount of thrombin released upon thrombolysis, however, may provide the stimulus for de novo production of sufficient thrombin to catalyze rethrombosis.

A kinetic model for the alpha-thrombin-catalyzed conversion of plasma levels of fibrinogen to fibrin in the presence of antithrombin III

In this study we report a kinetic model for the alpha-thrombin-catalyzed production of fibrin I and fibrin II at pH 7.4, 37 degrees C, gamma/2 0.17. The fibrin is produced by the action of human alpha-thrombin on plasma levels of human fibrinogen in the presence of the major inhibitor of alpha-thrombin in plasma, antithrombin III (AT). This model quantitatively accounts for the time dependence of alpha-thrombin-catalyzed release of fibrinopeptides A and B concurrent with the inactivation of alpha-thrombin by AT and delineates the concerted interactions of alpha-thrombin, fibrin(ogen), and AT during the production of a fibrin clot. The model also provides a method for estimating the concentration of alpha-thrombin required to produce a clot of known composition and predicts a direct relationship between the plasma concentration of fibrinogen and the amount of fibrin produced by a bolus of alpha-thrombin. The predicted relationship between the concentration of fibrinogen and the amount of fibrin produced in plasma provides a plausible explanation for the observed linkage between plasma concentrations of fibrinogen and the risk for ischemic heart disease.

Characterization of the kinetic pathway for fibrin promotion of alpha-thrombin-catalyzed activation of plasma factor XIII

Kinetic and thermodynamic studies are presented showing that the cofactor activity of fibrin I (polymerized des-A fibrinogen) in the alpha-thrombin-catalyzed proteolysis of activation peptide (AP) from plasma factor XIII can be attributed to formation of a fibrin I-plasma factor XIII complex (Kd = 65 nM), which is processed by alpha-thrombin more efficiently (kcat/Km = 1.2 x 10(7) M-1 s-1) than free, uncomplexed plasma factor XIII (kcat/Km = 1.4 x 10(5) M-1 s-1). The increase in the specificity constant (kcat/Km) is shown to be largely due to an increase in the apparent affinity of alpha-thrombin for the complex of plasma factor XIII and fibrin I, as reflected by the 30-fold decrease in the Michaelis constant observed for fibrin I bound plasma factor XIII relative to that for uncomplexed plasma factor XIII. Analysis of the initial rates of alpha-thrombin-catalyzed hydrolysis of fibrinopeptide B (FPB) from fibrin I polymer in the presence of plasma factor XIII indicated that alpha-thrombin bound to fibrin I in the ternary complex of alpha-thrombin, plasma factor XIII, and fibrin I polymer is competent to catalyze cleavage of both FPB from fibrin I and AP from plasma factor XIII. This observation is consistent with the view that alpha-thrombin within the ternary complex is anchored to fibrin I polymer through a binding site distinct from the active site (an exosite) and that the active site is alternatively complexed with the AP moiety of plasma factor XIII or the FPB moiety of fibrin I. This conclusion is supported by the observation that a 12-residue peptide, which binds to an exosite of alpha-thrombin and blocks the interaction of alpha-thrombin with fibrinogen and fibrin, competitively inhibits alpha-thrombin-catalyzed release of both FPB and AP from the fibrin I-plasma factor XIII complex.

The COOH-terminal domain of hirudin. An exosite-directed competitive inhibitor of the action of alpha-thrombin on fibrinogen

Hirudin, a potent 65-residue polypeptide inhibitor of alpha-thrombin found in the saliva of the leech Hirudo medicinalis, and fragments thereof are potentially useful as antithrombotic agents. Hirugen, the synthetic N-acetylated COOH-terminal dodecapeptide (Ac-Asn-Gly-Asp-Phe-Glu-Glu-Ile-Pro-Glu-Glu-Tyr(SO3)-Leu) of hirudin was shown in the present study to behave as a pure competitive inhibitor (Ki = 0.54 microM) of human alpha-thrombin-catalyzed release of fibrinopeptide A from human fibrinogen. In contrast to this inhibitory activity, hirugen slightly enhanced (increased kcat/Km 1.6-fold) alpha-thrombin-catalyzed hydrolysis of the fluorogenic tripeptide substrate N-p-Tosyl-Gly-Pro-Arg-7-amino-4-methylcoumarin. These observations indicate that hirugen binds to alpha-thrombin at an exosite distinct from the active site, and that interaction with this exosite is a major determinant of the competence of alpha-thrombin to bind fibrinogen. Consistent with this view, hirugen blocked binding of fibrin II to alpha-thrombin. Studies of the effect of hirugen on the rate of inactivation of alpha-thrombin by antithrombin III (AT), the major plasma inhibitor of alpha-thrombin, indicated that binding of hirugen to alpha-thrombin results in less than a 2.5-fold decrease in the rate of inactivation of alpha-thrombin by AT, both in the absence and presence of heparin. This behavior is distinct from that of active site-directed competitive inhibitors of alpha-thrombin which bind to alpha-thrombin and block both conversion of fibrinogen to fibrin and inactivation of alpha-thrombin by AT. Hirugen, an exosite-directed competitive inhibitor, blocks the interaction of alpha-thrombin with fibrinogen while leaving alpha-thrombin competent to react with AT. Thus, unlike active site-directed competitive inhibitors, hirugen should act in concert with AT and heparin to reduce the amount of fibrinogen that is processed during the lifetime of alpha-thrombin in plasma.

Alpha-thrombin-catalyzed hydrolysis of fibrin I. Alternative binding modes and the accessibility of the active site in fibrin I-bound alpha-thrombin

Steady-state kinetic parameters were determined for the action of human alpha-thrombin on human fibrin I polymer, an intermediate in the alpha-thrombin-catalyzed conversion of fibrinogen to the fibrin matrix of blood clots during the terminal phase of the blood clotting cascade. Values of 49 s-1 and 7.5 microM were determined (at 37 degrees C, pH 7.4, gamma/2 0.17) for kcat and Km, respectively. Studies of the effect of fibrin I on alpha-thrombin-catalyzed hydrolysis of the fluorogenic substrate N-p-Tos-Gly-L-Pro-L-Arg-7-amido-4-methylcoumarin (tos-GPR-amc) and the effect of fibrin I on the reaction of alpha-thrombin with antithrombin III (AT) were presented which indicate that the active site of alpha-thrombin is accessible while it is bound to its substrate fibrin I. Fibrin I inhibited alpha-thrombin-catalyzed hydrolysis of tos-GPR-amc in a manner inconsistent with the pure competitive inhibition expected for an alternative substrate, whereas fibrinogen, an alpha-thrombin substrate, behaved as a pure competitive inhibitor of the alpha-thrombin-catalyzed hydrolysis of tos-GPR-amc. The effect of fibrin I on alpha-thrombin-catalyzed hydrolysis of tos-GPR-amc was shown to be consistent with alpha-thrombin binding to fibrin I in alternative orientations. In one orientation both the active site and a site distinct from the active site (an exosite) of alpha-thrombin are occupied by fibrin I. In the other orientation only the exosite of alpha-thrombin is occupied and the active site is freely accessible to other substrates. The values of both kcat (21 s-1) and Km (less than 0.23 microM) determined for fibrin I-bound alpha-thrombin acting on tos-GPR-amc were decreased relative to the values of kcat (180 s-1) and Km (7.3 microM) observed for the action of uncomplexed alpha-thrombin on tos-GPR-amc. This observation suggests that the active site of alpha-thrombin is altered in fibrin I-bound alpha-thrombin. Studies of the effect of fibrin I on the reaction of AT with alpha-thrombin (at 37 degrees C, pH 7.4, gamma/2 0.17) indicated that when alpha-thrombin is bound to fibrin I in an orientation where the active site of alpha-thrombin is accessible, AT reacts with alpha-thrombin with a rate constant (greater than 4.2 x 10(4) M-1 s-1) that is greater than the rate constant (1.5 x 10(4) M-1 s-1) for reaction of AT with the free enzyme.(ABSTRACT TRUNCATED AT 400 WORDS)