Phosphatidylinositol 3-kinase and focal adhesion kinase are early signals in the growth factor-like responses to thrombospondin-1 seen in human vascular smooth muscle

MYC-mediated upregulation of PNO1 promotes glioma tumorigenesis by activating THBS1/FAK/Akt signaling

PNO1 has been reported to be involved in tumorigenesis, however, its role in glioma remains unexplored. In the present study, PNO1 expression in glioma from on-line databases, cDNA, and tissue microarrays was upregulated and associated with poor prognosis. PNO1 knockdown inhibits tumor cell growth and invasion both in vitro and in vivo; whereas PNO1 overexpression promoted cell proliferation and invasion in vitro. Notably, PNO1 interacted with THBS1 and the promotion of glioma by PNO1 overexpression could be attenuated or even reversed by simultaneously silencing THBS1. Functionally, PNO1 was involved in activation of FAK/Akt pathway. Moreover, overexpressing MYC increased PNO1 promoter activity. MYC knockdown decreased PNO1 and THBS1 expression, while inhibited cell proliferation and invasion. In conclusion, MYC-mediated upregulation of PNO1 contributes to glioma progression by activating THBS1/FAK/Akt signaling. PNO1 was reported to be a tumor promotor in the development and progression of glioma and may act as a candidate of therapeutic target in glioma treatment.

Synthetic poly(2-acrylamido-2-methylpropanesulfonic acid) gel induces chondrogenic differentiation of ATDC5 cells via a novel protein reservoir function

We previously demonstrated that a synthetic negatively charged poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) gel induced chondrogenic differentiation of ATDC5 cells. In this study, we clarified the underlying molecular mechanism, in particular, focusing on the events that occurred at the interface between the gel and the cells. Gene expression profiling revealed that the expression of extracellular components was enhanced in the ATDC5 cells that were cultured on the PAMPS gel, suggesting that extracellular proteins secreted from the ATDC5 cells might be adsorbed in the PAMPS gel, thereby contributing to the induction of chondrogenic differentiation. Therefore, we created "Treated-PAMPS gel," which adsorbed various proteins secreted from the cultured ATDC5 cells during 7 days. Proteomic analysis identified 27 proteins, including extracellular matrix proteins such as Types I, III, and V collagens and thrombospondin (THBS) in the Treated-PAMPS gel. The Treated-PAMPS gel preferentially induced expression of chondrogenic markers, namely, aggrecan and Type II collagen, in the ATDC5 cells compared with the untreated PAMPS gel. Addition of recombinant THBS1 to the ATDC5 cells significantly enhanced the PAMPS-induced chondrogenic differentiation, whereas knockdown of THBS1 completely abolished this response. In conclusion, we demonstrated that the PAMPS gel has the potential to induce chondrogenic differentiation through novel reservoir functions, and the adsorbed THBS plays a significant role in the induction.

Hippo component YAP promotes focal adhesion and tumour aggressiveness via transcriptionally activating THBS1/FAK signalling in breast cancer

BACKGROUND

Focal adhesion plays an essential role in tumour invasiveness and metastasis. Hippo component YAP has been widely reported to be involved in many aspects of tumour biology. However, its role in focal adhesion regulation in breast cancer remains unexplored.

METHODS

Tissue microarray was used to evaluate YAP expression in clinical breast cancer specimens by immunohistochemical staining. Cell migration and invasion abilities were measured by Transwell assay. A cell adhesion assay was used to measure the ability of cell adhesion to gelatin. The focal adhesion was visualized through immunofluorescence. Phosphorylated FAK and other proteins were detected by Western blot analysis. Gene expression profiling was used to screen differently expressed genes, and gene ontology enrichment was performed using DAVID software. The gene mRNA levels were measured by quantitative real-time PCR. The activity of the THBS1-promoter was evaluated by dual luciferase assay. Chromatin immunoprecipitation (ChIP) was used to verify whether YAP could bind to the THBS1-promoter region. The prediction of potential protein-interaction was performed with the String program. The ChIP sequence data of TEAD was obtained from the ENCODE database and analysed via the ChIP-seek tool. The gene expression dataset (GSE30480) of purified tumour cells from primary breast tumour tissues and metastatic lymph nodes was used in the gene set enrichment analysis. Prognostic analysis of the TCGA dataset was performed by the SurvExpress program. Gene expression correlation of the TCGA dataset was analysed via R2: Genomics Analysis and Visualization Platform.

RESULTS

Our study provides evidence that YAP acts as a promoter of focal adhesion and tumour invasiveness via regulating FAK phosphorylation in breast cancer. Further experiments reveal that YAP could induce FAK phosphorylation through a TEAD-dependent manner. Using gene expression profiling and bioinformatics analysis, we identify the FAK upstream gene, thrombospondin 1, as a direct transcriptional target of YAP-TEAD. Silencing THBS1 could reverse the YAP-induced FAK activation and focal adhesion.

CONCLUSION

Our results unveil a new signal axis, YAP/THBS1/FAK, in the modulation of cell adhesion and invasiveness, and provides new insights into the crosstalk between Hippo signalling and focal adhesion.

Linking phospholipase C isoforms with differentiation function in human vascular smooth muscle cells

The phosphoinositol-phospholipase C (PLC) family of enzymes consists of a number of isoforms, each of which has different cellular functions. PLCγ1 is primarily linked to tyrosine kinase transduction pathways, whereas PLCδ1 has been associated with a number of regulatory proteins, including those controlling the cell cycle. Recent studies have shown a central role of PLC in cell organisation and in regulating a wide array of cellular responses. It is of importance to define the precise role of each isoform, and how this changes the functional outcome of the cell. Here we investigated differences in PLC isoform levels and activity in relation to differentiation of human and rat vascular smooth muscle cells. Using Western blotting and PLC activity assay, we show that PLCδ1 and PLCγ1 are the predominant isoforms in randomly cycling human vascular smooth muscle cells (HVSMCs). Growth arrest of HVSMCs for seven days of serum deprivation was consistently associated with increases in PLCδ1 and SM α-actin, whereas there were no changes in PLCγ1 immuno-reactivity. Organ culture of rat mesenteric arteries in serum free media (SFM), a model of de-differentiation, led to a loss of contractility as well as a loss of contractile proteins (SM α-actin and calponin) and PLCδ1, and no change in PLCγ1 immuno-reactivity. Taken together, these data indicate that PLCδ1 is the predominant PLC isoform in vascular smooth muscle, and confirm that PLCδ1 expression is affected by conditions that affect the cell cycle, differentiation status and contractile function.

A role for thrombospondin-1 deficits in astrocyte-mediated spine and synaptic pathology in Down's syndrome

BACKGROUND

Down's syndrome (DS) is the most common genetic cause of mental retardation. Reduced number and aberrant architecture of dendritic spines are common features of DS neuropathology. However, the mechanisms involved in DS spine alterations are not known. In addition to a relevant role in synapse formation and maintenance, astrocytes can regulate spine dynamics by releasing soluble factors or by physical contact with neurons. We have previously shown impaired mitochondrial function in DS astrocytes leading to metabolic alterations in protein processing and secretion. In this study, we investigated whether deficits in astrocyte function contribute to DS spine pathology.

METHODOLOGY/PRINCIPAL FINDINGS

Using a human astrocyte/rat hippocampal neuron coculture, we found that DS astrocytes are directly involved in the development of spine malformations and reduced synaptic density. We also show that thrombospondin 1 (TSP-1), an astrocyte-secreted protein, possesses a potent modulatory effect on spine number and morphology, and that both DS brains and DS astrocytes exhibit marked deficits in TSP-1 protein expression. Depletion of TSP-1 from normal astrocytes resulted in dramatic changes in spine morphology, while restoration of TSP-1 levels prevented DS astrocyte-mediated spine and synaptic alterations. Astrocyte cultures derived from TSP-1 KO mice exhibited similar deficits to support spine formation and structure than DS astrocytes.

CONCLUSIONS/SIGNIFICANCE

These results indicate that human astrocytes promote spine and synapse formation, identify astrocyte dysfunction as a significant factor of spine and synaptic pathology in the DS brain, and provide a mechanistic rationale for the exploration of TSP-1-based therapies to treat spine and synaptic pathology in DS and other neurological conditions.

Thrombospondin-1-induced migration is functionally dependent upon focal adhesion kinase

Vascular smooth muscle cell migration is important in vascular disease. Previously, we showed thrombospondin-1 activates focal adhesion kinase in these cells. We hypothesized that focal adhesion kinase is important for thrombspondin-1-induced vascular smooth muscle cell migration. Bovine aortic smooth muscle cells were transfected with FAK397, FAK-wild type, pcDNA, or beta-Gal plasmids. Migration was assessed with thrombospondin-1 or serum-free medium in quiescent transfected cells or quiescent cells pretreated with the focal adhesion kinase inhibitor, geldanamycin. Number of cells migrated per 5 fields (x400) were recorded. Antihemagglutinin immunoprecipitation and Western blot were used to examine thrombospondin-1-induced focal adhesion kinase phosphorylation in transfected cells. FAK397 transfection inhibited thrombospondin-1-induced focal adhesion kinase phosphorylation and migration (P < .05). Geldanamycin inhibited thrombospondin-1-induced smooth muscle cell migration (P < .05). In conclusion, vascular smooth muscle cells transfected with FAK397 inhibited thrombosponin-1-induced migration and tyrosine phosphorylation. Further, geldanamycin also inhibited migration. These results suggest focal adhesion kinase is involved in thrombospondin-1-induced vascular smooth muscle cell migration.

Spatial and temporal regulation of focal adhesion kinase activity in living cells

Focal adhesion kinase (FAK) is an essential kinase that regulates developmental processes and functions in the pathology of human disease. An intramolecular autoinhibitory interaction between the FERM and catalytic domains is a major mechanism of regulation. Based upon structural studies, a fluorescence resonance energy transfer (FRET)-based FAK biosensor that discriminates between autoinhibited and active conformations of the kinase was developed. This biosensor was used to probe FAK conformational change in live cells and the mechanism of regulation. The biosensor demonstrates directly that FAK undergoes conformational change in vivo in response to activating stimuli. A conserved FERM domain basic patch is required for this conformational change and for interaction with a novel ligand for FAK, acidic phospholipids. Binding to phosphatidylinositol 4,5-bisphosphate (PIP2)-containing phospholipid vesicles activated and induced conformational change in FAK in vitro, and alteration of PIP2 levels in vivo changed the level of activation of the conformational biosensor. These findings provide direct evidence of conformational regulation of FAK in living cells and novel insight into the mechanism regulating FAK conformation.

FAK association with multiple signal proteins mediates pressure-induced colon cancer cell adhesion via a Src-dependent PI3K/Akt pathway

Cancer cell adhesion is traditionally viewed as random, occurring if the cell's receptors match the substrate. Cancer cells are subjected to pressure and shear during growth against a constraining stroma, surgical manipulation, and passage through the venous and lymphatic system. Cells shed into a cavity such as the abdomen postoperatively also experience increased pressure from postoperative edema. Increased extracellular pressure stimulates integrin-mediated cancer cell adhesion via FAK and Src. PI 3-kinase (PI3K) inhibitors (LY294002 or wortmannin), Akt inhibitors, or Akt1 siRNA blocked adhesion stimulated by 15 mmHg pressure in SW620 or primary human malignant colonocytes. Pressure activated PI3K, tyrosine-phosphorylated and membrane-translocated the p85 subunit, and phosphorylated Akt. PI3K inhibitor (LY294002) prevented pressure-stimulated Akt Ser473 and FAK Tyr397, but not FAK576 or Src416 phosphorylation. PP2 inhibited PI3K activity and Akt phosphorylation. FAK siRNA did not affect pressure-induced PI3K activation but blocked Akt phosphorylation. Pressure also stimulated FAK or FAKY397F mutant translocation to the membrane. Akt inhibitor IV blocked pressure-induced Akt and FAK translocation. Pressure activated Src- and PI3K-dependently induced p85 interaction with FAK, and FAK with beta1 integrin. These results delineate a novel force-activated inside-out Src/PI3K/FAK/Akt pathway by which cancer cells regulate their own adhesion. These signals may be potential targets for inhibition of metastatic adhesion.

Signal transduction in endothelial cells by the angiogenesis inhibitor histidine-rich glycoprotein targets focal adhesions

Histidine-rich glycoprotein (HRGP) is an abundant heparin-binding plasma protein. We have shown that a fragment released from the central histidine/proline-rich (His/Pro-rich) domain of HRGP blocks endothelial cell migration in vitro and vascularization and growth of murine fibrosarcoma in vivo. The minimal active HRGP domain exerting the anti-angiogenic effect was recently narrowed down to a 35 amino acid peptide, HRGP330, derived from the His/Pro-rich domain of HRGP. By use of a signal transduction antibody array representing 400 different signal transduction molecules, we now show that HRGP and the synthetic peptide HRGP330 specifically induce tyrosine phosphorylation of focal adhesion kinase and its downstream substrate paxillin in endothelial cells. HRGP/HRGP330 treatment of endothelial cells induced disruption of actin stress fibers, a process reversed by treatment of cells with the FAK inhibitor geldanamycin. In addition, VEGF-mediated endothelial cell tubular morphogenesis in a three-dimensional collagen matrix was inhibited by HRGP and HRGP330. In contrast, VEGF-induced proliferation was not affected by HRGP or HRGP330, demonstrating the central role of cell migration during tube formation. In conclusion, our data show that HRGP targets focal adhesions in endothelial cells, thereby disrupting the cytoskeletal organization and the ability of endothelial cells to assemble into vessel structures.

Cellular, molecular and immunological mechanisms in the pathophysiology of vein graft intimal hyperplasia

Coronary artery disease, leading to myocardial infarction and ischaemia, affects millions of persons and is one of the leading causes of morbidity and mortality worldwide. Invasive techniques such as coronary artery bypass grafting are used to alleviate the sequelae of arterial occlusion. Unfortunately, restenosis or occlusion of the grafted conduit occurs over a time frame of months to years with a gradual reduction in patency, especially in vein grafts. The events leading to intimal hyperplasia (IH) formation involve numerous cellular and molecular components. Various cellular elements of the vessel wall are involved as are leucocyte-endothelial interactions that trigger the coagulation cascade leading to localized thrombus formation. Subsequent phenotypic modification of the medial smooth muscle cells and their intimal migration is the basis of the lesion formation that is thought to be propagated by an immune-mediated reaction. Despite intense scrutiny, the pathophysiology of IH remains an enigma. Although several growth factors, cytokines and numerous other biomolecules have been implicated and their relationship to prohyperplasia pathways such as the phosphatidyl-inositol 3-kinase (PI3K)-Akt pathway has been established, many pieces of the puzzle are still missing. An in-depth understanding of early vein graft adaptation and progression is necessary to improve the long-term prognosis and develop more effective therapeutic measures. In this review, we have critically evaluated and summarized the literature to elucidate and interlink the numerous established and emerging factors that play a key role in the development of IH leading to vein graft restenosis.

Antiangiogenic peptides and proteins: from experimental tools to clinical drugs

The formation of a 'tumor-associated vasculature', a process referred to as tumor angiogenesis, is a stromal reaction essential for tumor progression. Inhibition of tumor angiogenesis suppresses tumor growth in many experimental models, thereby indicating that tumor-associated vasculature may be a relevant target to inhibit tumor progression. Among the antiangiogenic molecules reported to date many are peptides and proteins. They include cytokines, chemokines, antibodies to vascular growth factors and growth factor receptors, soluble receptors, fragments derived from extracellular matrix proteins and small synthetic peptides. The polypeptide tumor necrosis factor (TNF, Beromun) was the first drug registered for the regional treatment of human cancer, whose mechanisms of action involved selective disruption of the tumor vasculature. More recently, bevacizumab (Avastin), an antibody against vascular endothelial growth factor (VEGF)-A, was approved as the first systemic antiangiogenic drug that had a significant impact on the survival of patients with advanced colorectal cancer, in combination with chemotherapy. Several additional peptides and antibodies with antiangiogenic activity are currently tested in clinical trials for their therapeutic efficacy. Thus, peptides, polypeptides and antibodies are emerging as leading molecules among the plethora of compounds with antiangiogenic activity. In this article, we will review some of these molecules and discuss their mechanism of action and their potential therapeutic use as anticancer agents in humans.

Immunohistochemical expression of thrombospondin-1 in invasive vulvar squamous cell carcinoma

OBJECTIVES

Thrombospondin-1 (TSP-1) is a multifunctional matricellular glycoprotein involved in several mechanisms critical to the formation and progression of solid tumors including cell adhesion, proliferation, migration, invasion, and angiogenesis. The present study was designed to investigate the expression of TSP-1 in invasive vulvar squamous cell carcinoma.

METHODS

A total of 75 invasive vulvar squamous cell carcinomas were evaluated for TSP-1 expression by immunohistochemistry. Results were correlated with the clinicopathologic parameters including tumor stage, groin lymph node status, tumor grade, patient's age, patients' disease-free, and overall survival.

RESULTS

TSP-1 expression was detected in 35/75 (46.7%) specimens of invasive vulvar squamous cell carcinomas. The expression of TSP-1 was generally localized to the cytoplasm and occasionally seen in the nucleus. An increased TSP-1 expression was detected in patients with an advanced tumor stage (P = 0.01) and a positive groin lymph nodes status (P = 0.01). Tumor stage and groin lymph node status were associated with patients' disease-free and overall survival. All other parameters failed to be of prognostic significance.

CONCLUSIONS

We are the first to report on the immunohistochemical expression of TSP-1 in invasive vulvar squamous cell carcinoma. Increased TSP-1 expression was associated with an advanced tumor stage and a positive groin lymph node status, suggesting its pro-angiogenic potential in vulvar carcinogenesis.

Endogenous thrombospondin-1 is not necessary for proliferation but is permissive for vascular smooth muscle cell responses to platelet-derived growth factor

We have reexamined the role of endogenous thrombospondin-1 (TSP1) in growth and motility of vascular smooth muscle cells (SMCs). Based on the ability of aortic-derived SMCs isolated from TSP1 null mice and grown in the absence of exogenous TSP1 to grow at comparable rates and to a slightly higher density than equivalent cells from wild-type mice, TSP1 is not necessary for their growth. Low concentrations of exogenous TSP1 stimulate growth of TSP1 null SMCs, but higher doses of TSP1 or its C-terminal domain are inhibitory. However, SMCs from TSP1 null mice are selectively deficient in chemotactic and proliferative responses to platelet-derived growth factor and in outgrowth in three-dimensional cultures. Recombinant portions of the N- and C-terminal domains of TSP1 stimulate SMC chemotaxis through different integrin receptors. Based on these data, the relative deficiency in SMC outgrowth during an ex vivo angiogenic response of muscle tissue from TSP1 null mice is probably due to restriction of platelet-derived growth factor dependent SMC migration and/or proliferation.

Insulin-like growth factor binding protein-5 (IGFBP-5) interacts with thrombospondin-1 to induce negative regulatory effects on IGF-I actions

Insulin-like growth factor binding protein-5 (IGFBP-5) and thrombospondin-1 (TS-1) are both present in extracellular matrix (ECM). Both proteins have been shown to bind to one another with high affinity. The purpose of these studies was to determine how the interaction between IGFBP-5 and TS-1 modulates IGF-I actions in porcine aortic smooth muscle cells (pSMC) in culture. The addition of increasing concentrations of TS-1 to pSMC cultures enhanced the protein synthesis and cell migration responses to IGF-I; whereas the addition of IGFBP-5 alone resulted in minimal changes. In contrast, the addition of IGFBP-5 to cultures that were also exposed to IGF-I and TS-1 resulted in inhibition of protein synthesis. When the cell migration response was assessed, the response to IGF-I plus TS-1 was also significantly inhibited by the addition of IGFBP-5, whereas 1.0 microg/ml of IGFBP-5 alone had no effect on the response to IGF-I. To determine the molecular mechanism by which this inhibition occurred, a mutant form of IGFBP-5 that does not bind to IGF-I was tested. This mutant was equipotent compared to native IGFBP-5 in its ability to inhibit both protein synthesis and cell migration responses to IGF-I plus TS-1 thus excluding the possibility that IGFBP-5 was inhibiting the response to TS-1 and IGF-I by inhibiting IGF-I binding to the IGF-I receptor. To determine if an interaction between TS-1 and IGFBP-5 was the primary determinant of the inhibitory effect of IGFBP-5, an IGFBP-5 mutant that bound poorly to TS-1 was utilized. The addition of 1.0 microg/ml of this mutant did not inhibit the protein synthesis or cell migration responses to IGF-I plus TS-1. To determine the mechanism by which IGFBP-5 binding to TS-1 inhibited cellular responses to TS-1 plus IGF-I, TS-1 binding to integrin associated protein (IAP) was assessed. The addition of IGFBP-5 (1.0 microg/ml) inhibited TS-1-IAP association. In contrast, a mutant form of IGFBP-5 that bound poorly to TS-1 had a minimal effect on TS-1 binding to IAP. Further analysis showed that IGFBP-5 addition altered the ability of TS-1 to modulate the SHPS-1/IAP interaction. When the IGFBP-5 mutant that did not bind to IGF-I was incubated with TS-1 and IGF-I, it inhibited the capacity of TS-1 to enhance the IGF-I receptor phosphorylation and MAP kinase activation in response to IGF-I. In contrast, the IGFBP-5 mutant that did not bind to TS-1 had no effect on IGF-I stimulated IGF-I receptor phosphorylation or MAP kinase activation. These results indicate that IGFBP-5 inhibits the binding of TS-1 to IAP, and this results in an alteration of the ability of TS-1 to modulate the disruption of the IAP/SHPS-1 interaction which leads to attenuation of the ability of TS-1 to enhance cellular responsiveness to IGF-I.

The role of thrombospondin-1 in human disease

Thrombospondin-1 (TSP-1) is a large matricellular glycoprotein secreted by many cell types. It is a component of the extracellular matrix during active and subacute processes. Due to TSP-1's ability to interact with a variety of matrix proteins and cell-surface receptors, controversy exists about its conflicting functions. In this review, we will discuss the role of TSP-1 in human disease.

Functions of the conserved thrombospondin carboxy-terminal cassette in cell-extracellular matrix interactions and signaling

Thrombospondins (TSPs) are extracellular, multidomain, calcium-binding glycoproteins that function at cell surfaces, in extracellular matrix (ECM) and as bridging molecules in cell-cell interactions. TSPs are multifunctional and modulate cell behavior during development, wound-healing, immune response, tumor growth and in the homeostasis of adult tissues. TSPs are assembled as oligomers that are composed of homologous polypeptides. In all the TSP polypeptides, the most highly-conserved region is the carboxyl-region, which contains a characteristic set of domains comprising EGF domains, TSP type 3 repeats and a globular carboxy-terminal domain. This large region is termed here the thrombospondin carboxy-terminal cassette (TSP-CTC). The strong conservation of the TSP-CTC suggests that it may mediate ancestral functions that are shared by all TSPs. This review summarizes the current knowledge of the TSP-CTC and areas of future interest.

Thrombospondin Induces RhoA Inactivation through FAK-dependent Signaling to Stimulate Focal Adhesion Disassembly

Cells utilize dynamic interactions with the extracellular matrix to adapt to changing environmental conditions. Thrombospondin 1 (TSP1) induces focal adhesion disassembly and cell migration through a sequence (hep I) in its heparin-binding domain signaling through the calreticulin-low density lipoprotein receptor-related protein receptor complex. This involves the Galphai-dependent activation of ERK and phosphoinositide (PI) 3-kinase, both of which are required for focal adhesion disassembly. Focal adhesion kinase (FAK) regulates adhesion dynamics, acting in part by modulating RhoA activity, and FAK is implicated in ERK and PI 3-kinase activation. In this work, we sought to determine the role of FAK in TSP1-induced focal adhesion disassembly. TSP1/hep I does not stimulate focal adhesion disassembly in FAK knockout fibroblasts, whereas re-expressing FAK rescues responsiveness. Inhibiting FAK signaling through FRNK or FAK Y397F expression in endothelial cells also abrogates this response. TSP1/hep I stimulates a transient increase in FAK phosphorylation that requires calreticulin and Galphai, but not ERK or PI 3-kinase. Hep I does not activate ERK or PI 3-kinase in FAK knockout fibroblasts, suggesting activation occurs downstream of FAK. TSP1/hep I stimulates RhoA inactivation with kinetics corresponding to focal adhesion disassembly in a FAK, ERK, and PI 3-kinase-dependent manner. Furthermore, hep I does not stimulate focal adhesion disassembly in cells expressing constitutively active RhoA, suggesting that RhoA inactivation is required for this response. This is the first work to illustrate a connection between FAK phosphorylation in response to a soluble factor and RhoA inactivation, as well as the first report of PI 3-kinase and ERK in FAK regulation of RhoA activity.

Vascular smooth muscle cell migration: current research and clinical implications

Atherosclerosis and intimal hyperplasia are major causes of morbidity and mortality. These processes develop secondary to endothelial injury due to multiple stimuli, including smoking, diabetes mellitus, hypertension, and hyperlipidemia. Once this injury occurs, an essential element in the development of both these processes is vascular smooth muscle cell (VSMC) migration. Understanding the mechanisms involved in VSMC migration and ultimately the development of strategies by which this process can be inhibited, has been a major focus of research. The authors present a review of the extracellular proteins (growth factors, extracellular matrix components, and cell surface receptors) and intracellular signaling pathways involved in VSMC migration, as well as potential therapeutic approaches to inhibit this process.

PDGF stimulates DNA synthesis in human vascular smooth muscle cells via a novel wortmannin-insensitive phosphatidylinositol 3-kinase

The class 1(A) phosphatidylinositol 3-kinase enzymes consist of a number of heterodimeric complexes of regulatory and catalytic subunits and have been implicated in a number of cellular responses. While platelet-derived growth factor (PDGF)-induced chemotaxis of human vascular smooth muscle cells (HVSMC) is inhibited by both wortmannin and LY294002, DNA synthesis is only inhibited by LY294002. Serum-induced DNA synthesis however is inhibited by LY294002, wortmannin and rapamycin. Similarly PDGF-induced protein kinase B (PKB) activation is inhibited by LY294002 but not by wortmannin or rapamycin. In conclusion PDGF-induced DNA synthesis appears to occur through a phosphatidylinositol 3-kinase (PI3-K)-dependent, but wortmannin-insensitive, PKB/Akt pathway.

Thrombospondin-1 differentially induces chemotaxis and DNA synthesis of human venous smooth muscle cells at the receptor-binding level

Thrombospondin-1 is a large matricellular protein that acts as a pleiotropic growth factor for human vascular smooth muscle cells, and may play a role in the progression of vascular disease. Although we have previously demonstrated the dependence of both thrombospondin-1-stimulated cell chemotaxis and proliferation on tyrosine kinases, the receptor mechanisms involved remain obscure. This investigation aims to determine the nature of the receptor(s) involved in the cellular responses to thrombospondin-1. Cellular signals were identified by western blotting following cell stimulation, while cellular responses were assessed by measuring DNA synthesis and chemotaxis. These data demonstrate that thrombospondin-1-induced cell chemotaxis can be inhibited by a peptide containing the Arg-Gly-Asp motif, a function-blocking alpha(v)beta(3) antibody, a function-blocking integrin-associated protein (IAP) antibody and pertussis toxin, while thrombospondin-1-stimulated DNA synthesis is inhibited by a function-blocking alpha(3)beta(1) antibody. Similarly the Arg-Gly-Asp-containing peptide inhibits tyrosine phosphorylation of focal adhesion kinase and the p85 regulatory subunit of phosphatidylinositol 3-kinase, but does not significantly affect tyrosine phosphorylation, or activation, of extracellular-regulated kinase. These data suggest that soluble thrombospondin-1 interacts with human vascular smooth muscle cells via two independent and separable receptor-binding sites, to differentially stimulate cell chemotaxis and DNA synthesis.

Tyrosine phosphorylation of the catalytic subunit p110 of phosphatidylinositol-3 kinase induced by HMG-CoA reductase inhibitor inhibits its kinase activity in L6 myoblasts

Previous studies from this laboratory have shown that 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor (HCRI) causes apoptotic cell death of a muscle cell-derived cell line, L6 myoblasts, by involving the phosphatidylinositol-3 (PI-3) kinase pathway and tyrosine phosphorylation of several cellular proteins, although the relationship between PI-3 kinase pathway and tyrosine phosphorylation responses remained to be elucidated. Here, we show that HCRI induces tyrosine phosphorylation of catalytic subunit p110 of PI-3 kinase as early as 5 min after addition of HCRI into culture medium. We could not detect the tyrosine phosphorylation of the regulatory subunit p85 of PI-3 kinase under the present experimental conditions. Concomitantly, the kinase activity toward PI in p110 immunoprecipitates was decreased with a similar time course. Furthermore, both herbimycin A and genistein, potent inhibitors of tyrosine kinase activity, inhibited HCRI-induced inhibition of PI-3 kinase activity as well as HCRI-induced apoptotic cell death. Once the catalytic subunit p110 becomes tyrosine-phosphorylated, the regulatory subunit p85 appears to be dissociated from the catalytic subunit, because we observed a decreasing amount of p85 regulatory subunits in p110 immunoprecipitates in response to HCRI treatment. These results strongly suggest the novel function of tyrosine phosphorylation of catalytic subunit p110 of PI-3 kinase in the regulation of its kinase activity. The tyrosine phosphorylation of these catalytic subunits may play an important role in the intracellular signal transduction of apoptotic cell death. To our knowledge, this is the first report that tyrosine phosphorylation of p110 catalytic subunit acts as a negative regulator of its kinase activity.

The cell biology of thrombospondin-1

Thrombospondin-1 (TSP-1) is a matricellular protein that regulates cellular phenotype during tissue genesis and repair. It acts as a molecular facilitator by bringing together cytokines, growth factors, matrix components, membrane receptors and extracellular proteases. TSP-1 binds to a wide variety of integrin and non-integrin cell surface receptors. The binding sites for these receptors on TSP-1 are dispersed throughout the molecule, with most domains binding multiple receptors. In some cases, TSP-1 binds to multiple receptors concurrently, and recent data indicate that there is cross-talk between the receptor systems. Thus, TSP-1 may function to direct the clustering of receptors to specialized domains for adhesion and signal transduction.

The functions of thrombospondin-1 and-2

Considerable progress has been made towards understanding the function of thrombospondin-1 and-2. The description of the phenotype of mice with thrombospondin-1 and-2 knocked-out supports in vitro biochemical and cell-biological data and has opened new avenues of research. Recently, our understanding of the roles of thrombospondins in the activation of TGFbeta, inhibition of angiogenesis and the initiation of signal transduction has advanced.

Inhibition of phosphatidylinositol 3-kinase and protein kinase C attenuates extracellular matrix protein-induced vascular smooth muscle cell chemotaxis

PURPOSE

Intimal hyperplasia (IH), a significant cause of vascular reconstructive failure, is characterized by abnormal vascular smooth muscle cell (VSMC) migration, proliferation, and extracellular matrix (ECM) deposition. The ECM proteins, thrombospondin-1 (TSP-1), fibronectin (Fn), and vitronectin (Vn) can induce VSMC migration; however, the cellular signaling pathways involved are not identical for each ECM protein. Phosphatidylinositol 3-kinase (PI3K) and protein kinase C (PKC) are two enzymes that have been associated with VSMC migration. We sought to elucidate the roles of these enzymes in TSP-1-, Fn-, and Vn-stimulated VSMC migration.

METHODS

Chemotaxis assays were performed by using a modified Boyden Chamber. TSP-1, Fn, or Vn (20 microg/mL) or serum-free media (SFM) was placed in the bottom wells of the chamber. Quiescent bovine aortic VSMC were preincubated with LY 294002 (100 micromol/L), a PI3K inhibitor, bisindolylmaleimide I (GF 109203X, 1 micromol/L), a PKC inhibitor, or in SFM alone for 30 minutes. VSMCs (50,000 cells per well) were then placed in the top wells of the chamber, and the assay was conducted for 4 hours at 37 degrees C. Results were recorded as the number of cells migrated per five fields (400x) and analyzed by means of the paired t test, with P value less than.05 considered to be significant (n = 3).

RESULTS

The VSMC migration was significantly increased by TSP-1, Fn, and Vn. LY 294002 inhibited TSP-1-, Fn-, and Vn-stimulated VSMC migration (85% to 89%, P <.05). GF 109203X inhibited only TSP-1-stimulated migration (65%, P <.05).

CONCLUSION

These results suggest that TSP-1-, Fn-, and Vn-stimulated migration is at least partially dependent on PI3K. However, only TSP-1 stimulated migration is at least partially dependent on PKC.