Thrombospondin-1 inhibits VEGF levels in the ovary directly by binding and internalization via the low density lipoprotein receptor-related protein-1 (LRP-1)

Angiogenesis in chronic liver disease and its complications

Nowadays, liver cancer, cirrhosis and other liver-related diseases are the fifth most common cause of mortality in the UK. Furthermore, chronic liver diseases (CLDs) are one of the major causes of death, which are still increasing year-on-year. Therefore, knowledge about the pathophysiology of CLDs and its complications is of uttermost importance. The goal of this review is to clarify the role of angiogenesis in the disease progression of various liver diseases. Looking closer at the pathophysiology of portal hypertension (PH), fibrosis, cirrhosis, non-alcoholic steatohepatitis (NASH) and hepatocellular carcinoma (HCC), we find that angiogenesis is a recurring factor in the disease progression. In PH, several factors involved in its pathogenesis, such as hypoxia, oxidative stress, inflammation and shear stress are potential mediators for the angiogenic response. The progression from fibrosis to cirrhosis, the end-point of CLDs, is distinguished by a prolonged inflammatory and fibrogenic process that leads to an abnormal angioarchitecture distinctive for cirrhosis. In several stages of NASH, a link might be made between the disease progression and hepatic microvasculature changes. HCC is one of the most vascular solid tumours in which angiogenesis plays an important role in its development, progression and metastasis. The close relationship between the progression of CLDs and angiogenesis emphasises the need for anti-angiogenic therapy as a tool for blocking or slowing down the disease progression. The fact that angiogenesis plays a pivotal role in CLDs gives rise to new opportunities for treating CLDs and its complications.

Expression of angiogenic basic fibroblast growth factor, platelet derived growth factor, thrombospondin-1 and their receptors at the porcine maternal-fetal interface

BACKGROUND

Commercial swine breeds in North America undergo two waves of spontaneous fetal loss; one during peri-attachment and another during mid-gestation. Although an exact mechanism for this loss is not known, deficits in vasculature at the attachment sites appear to be a major cause. We hypothesized that a balance between pro-angiogenic and anti-angiogenic factors is needed at the maternal-fetal interface for successful conceptus development. Six selected members of the pro-angiogenic fibroblast growth factor (FGF) and platelet derived growth factor (PDGF) families and anti-angiogenic factor thrombospondin-1 (TSP-1) and its receptor CD36 were quantified and localized at the porcine maternal-fetal interface at early and midgestation time points.

METHODS

Mesometrial endometrium was collected from non-pregnant gilts (n = 8). Endometrial and chorioallantoic membrane samples were collected from healthy and arresting conceptus attachment sites at gestation day (gd) 20 (n = 8) and gd 50 (n = 8). At gd20 arresting conceptus attachment sites were distinguished by decreased vasculature of the placental membranes and decreased conceptus size. At gd50 arresting conceptuses attachment sites were identified by smaller conceptus length and weight measurements. Quantitative real time PCR was used to determine relative transcript levels of genes of interest, and cellular localization was determined by immunohistochemistry in paraffin embedded endometrial sections.

RESULTS

At gd20, endometrial samples from arresting conceptuses had elevated transcripts for bFGF, and PDGF-bb than healthy sites (p < 0.05). At gd50, bFGF, FGFR2, and CD36 were more abundant at arresting than at healthy conceptus attachment sites (p < 0.05). Chorioallantoic membrane from arresting conceptus attachment sites at gd20 had elevated transcripts for bFGF, FGFR1, FGFR2 and CD36 compared with healthy sites (p < 0.05). FGFR2 transcripts were more abundant in chorioallantoic membrane from arresting conceptuses at gd 50 (p < 0.05). Immunohistochemical localization of selected pro- and anti-angiogenic factors and receptors revealed their abundance in the luminal epithelium, uterine glands and perivascular areas of endometrium at gd20 and gd50.

CONCLUSIONS

We provide comprehensive analysis of pro and anti-angiogenic factors at the porcine maternal fetal interface during early and mid-pregnancy. At mRNA levels, the majority of pro-angiogenic factors investigated were elevated at the sites of fetal arrest. These observations contrast with our previous findings of decreased Vascular Endothelial Growth Factor (VEGF) family members at arresting sites, and suggest that the bFGF family functions as a compensatory survival mechanism when major angiogenic proteins are decreasing at the sites of fetal arrest.

A thrombospondin-mimetic peptide, ABT-898, suppresses angiogenesis and promotes follicular atresia in pre- and early-antral follicles in vivo

Using a novel in vitro angiogenesis assay, we previously showed that thrombospondin (TSP)-1 has antiangiogenic effects on rat follicles and induces apoptosis in granulosa cells in vitro. ABT-898 is an octapeptide mimetic of TSP-1 closely related to ABT-510. Here, we demonstrate the inhibitory effects of ABT-898 on follicular angiogenesis and its proapoptotic effect on granulosa cells. To investigate the potential of this peptide to inhibit follicular angiogenesis in vivo, marmoset monkeys were treated with 2.5 mg/kg ABT-898 twice daily throughout the follicular phase of the cycle. Although treatment did not block emergence of dominant follicles, angiogenesis was reduced in preantral and early-antral follicles. Furthermore, the incidence of atresia at these follicle stages was increased. To investigate whether treatment with ABT-898 would interfere with the timing or duration of the normal ovulatory rise in plasma progesterone, marmosets were treated with a depot formulation containing 25 mg ABT-898 at the start of the follicular phase, with a second injection after 2 wk. Despite active concentrations of peptide being maintained in the circulation, no apparent effects on the ovulatory cycle were observed. Taken together, these results indicate that ABT-898 is capable of having a dual effect by inhibiting follicular angiogenesis and promoting atresia of antral follicles in vivo but does not prevent ovulation or induce luteolysis, as has been observed with direct vascular endothelial growth factor inhibitors. These results suggest that ABT-898 could be a novel therapeutic to inhibit abnormal angiogenesis and induce atresia of accumulated follicles in polycystic ovary syndrome.

E2F-1 directly regulates thrombospondin 1 expression

Thrombospondin 1 (TSP1) has been shown to play a critical role in inhibiting angiogenesis, resulting in inhibition of tumor growth and metastases. To figure out TSP1's regulators will lead to reveal its biological function mechanistically. In this study, we show that E2F-1 could activate the transcription of TSP1 by both promoter assays and Northern blot. Analysis of various TSP1 promoter mutant constructs showed that a sequence located −144/−137 up-stream of the transcriptional initiation site, related to the consensus E2F-responsive sequence, is necessary for the activation. In consistence with up-regulation of TSP-1 activity by over-expression of E2F-1, the knockdown of endogenous E2F-1 inhibited TSP-1 promoter activity significantly, implying that E2F-1 mediated regulation of TSP-1 is relevant in vivo. In addition, E2F-1 could also directly bind to the TSP1 promoter region covering −144/−137 region as revealed by ChIP assays. Furthermore, the E2F-1-induced activation of TSP1 gene transcription is suppressed by pRB1 in a dose-dependent manner. Taken together, the results demonstrate that TSP1 is a novel target for E2F1, which might imply that E2F-1 can affect angiogenesis by modulating TSP1 expression.

Thrombospondin-1 inhibits VEGF receptor-2 signaling by disrupting its association with CD47

Thrombospondin-1 (TSP1) can inhibit angiogenic responses directly by interacting with VEGF and indirectly by engaging several endothelial cell TSP1 receptors. We now describe a more potent mechanism by which TSP1 inhibits VEGF receptor-2 (VEGFR2) activation through engaging its receptor CD47. CD47 ligation is known to inhibit downstream signaling targets of VEGFR2, including endothelial nitric-oxide synthase and soluble guanylate cyclase, but direct effects on VEGFR2 have not been examined. Based on FRET and co-immunoprecipitation, CD47 constitutively associated with VEGFR2. Ligation of CD47 by TSP1 abolished resonance energy transfer with VEGFR2 and inhibited phosphorylation of VEGFR2 and its downstream target Akt without inhibiting VEGF binding to VEGFR2. The inhibitory activity of TSP1 in large vessel and microvascular endothelial cells was replicated by a recombinant domain of the protein containing its CD47-binding site and by a CD47-binding peptide derived from this domain but not by the CD36-binding domain of TSP1. Inhibition of VEGFR2 phosphorylation was lost when CD47 expression was suppressed in human endothelial cells and in murine CD47-null cells. These results reveal that anti-angiogenic signaling through CD47 is highly redundant and extends beyond inhibition of nitric oxide signaling to global inhibition of VEGFR2 signaling.

Extracellular matrix proteins and tumor angiogenesis

Tumor development is a complex process that relies on interaction and communication between a number of cellular compartments. Much of the mass of a solid tumor is comprised of the stroma which is richly invested with extracellular matrix. Within this matrix are a host of matricellular proteins that regulate the expression and function of a myriad of proteins that regulate tumorigenic processes. One of the processes that is vital to tumor growth and progression is angiogenesis, or the formation of new blood vessels from preexisting vasculature. Within the extracellular matrix are structural proteins, a host of proteases, and resident pro- and antiangiogenic factors that control tumor angiogenesis in a tightly regulated fashion. This paper discusses the role that the extracellular matrix and ECM proteins play in the regulation of tumor angiogenesis.

Thrombospondin-1 as a Paradigm for the Development of Antiangiogenic Agents Endowed with Multiple Mechanisms of Action

Uncontrolled neovascularization occurs in several angiogenesis-dependent diseases, including cancer. Neovascularization is tightly controlled by the balance between angiogenic growth factors and antiangiogenic agents. The various natural angiogenesis inhibitors identified so far affect neovascularization by different mechanisms of action. Thrombospondin-1 (TSP-1) is a matricellular modular glycoprotein that acts as a powerful endogenous inhibitor of angiogenesis. It acts both indirectly, by sequestering angiogenic growth factors and effectors in the extracellular environment, and directly, by inducing an antiangiogenic program in endothelial cells following engagement of specific receptors including CD36, CD47, integrins and proteoglycans (all involved in angiogenesis ). In view of its central, multifaceted role in angiogenesis, TSP-1 has served as a source of antiangiogenic tools, including TSP-1 fragments, synthetic peptides and peptidomimetics, gene therapy strategies, and agents that up-regulate TSP-1 expression. This review discusses TSP-1-based inhibitors of angiogenesis, their mechanisms of action and therapeutic potential, drawing our experience with angiogenic growth factor-interacting TSP-1 peptides, and the possibility of exploiting them to design novel antiangiogenic agents.

Thrombospondin-1 inhibits angiogenesis and promotes follicular atresia in a novel in vitro angiogenesis assay

Thrombospondin-1 (TSP-1) is a putative antiangiogenic factor, but its role in regulating physiological angiogenesis is unclear. We have developed a novel in vitro angiogenesis assay to study the effect of TSP-1 on follicular angiogenesis and development. Intact preantral/early antral follicles dissected from 21-d-old rat ovaries were cultured for 6 d in the presence or absence of TSP-1. At the end of the culture period, angiogenic sprouting from the follicles was quantified using image analysis. Follicles were fixed and sectioned, and follicular apoptosis was assessed by immunohistochemistry for activated caspase-3 in granulosa cells. The results showed that TSP-1 inhibited follicular angiogenesis (P < 0.01) and promoted follicular apoptosis (P < 0.001) in a dose-dependent manner. To determine whether the proapoptotic activity of TSP-1 is mediated by direct effects on granulosa cells, isolated granulosa cells were cultured with TSP-1 (0, 10, 100, and 1000 ng/ml) for 48 h. Apoptosis was quantified using a luminescent caspase-3/7 assay. TSP-1 promoted apoptosis of granulosa cells in a dose-dependent manner (P < 0.05), suggesting that TSP-1 can act independently of the angiogenesis pathway to promote follicular apoptosis. These results show that TSP-1 can both inhibit follicular angiogenesis and directly induce apoptosis of granulosa cells. As such, it may have potential as a therapeutic for abnormal ovarian angiogenesis and could facilitate the destruction of abnormal follicles observed in polycystic ovary syndrome.

Extracellular matrix, inflammation, and the angiogenic response

Inflammation and angiogenesis are frequently coupled in pathological situations such as atherosclerosis, diabetes, and arthritis. The inflammatory response increases capillary permeability and induces endothelial activation, which, when persistent, results in capillary sprouting. This inflammation-induced angiogenesis and the subsequent remodelling steps are in large part mediated by extracellular matrix (ECM) proteins and proteases. The focal increase in capillary permeability is an early consequence of inflammation, and results in the deposition of a provisional fibrin matrix. Subsequently, ECM turnover by proteases permits an invasive program by specialized endothelial cells whose phenotype can be regulated by inflammatory stimuli. ECM activity also provides specific mechanical forces, exposes cryptic adhesion sites, and releases biologically active fragments (matrikines) and matrix-sequestered growth factors, all of which are critical for vascular morphogenesis. Further matrix remodelling and vascular regression contribute to the resolution of the inflammatory response and facilitate tissue repair.

Angiogenesis in cutaneous disease: part I

Angiogenesis is an important process in normal physiology and disease pathogenesis. Angiogenesis is controlled in a healthy body by a system of angiogenic growth factors and angiogenesis inhibitors. When angiogenic growth factors are predominantly expressed, blood vessel growth occurs and disease may result. Successful therapies have been developed that target growth factors, their receptors, or the cascade pathways that are activated by growth factor/receptor interactions. There is good evidence that angiogenesis plays an important role in a wide range of cutaneous maladies, and angiogenesis-targeting therapies are playing an increasing role in the management of dermatologic disease. Cutaneous angiogenesis offers an exciting new arena for targeted dermatologic therapeutics.

LEARNING OBJECTIVES

After completing this learning activity, participants should be able to distinguish angiogenic growth factors and inhibitors, recognize angiogenic mediating agents and compare their mechanisms of action, and apply the use of angiogenic mediating agents in clinical and research situations.

The role of thrombospondins in wound healing, ischemia, and the foreign body reaction

Thrombospondin (TSP) 1 and TSP2 have been implicated in the regulation of several processes during tissue repair. Due to their matricellular nature, these proteins are thought to modulate cell-matrix interactions through a variety of mechanisms specific to the spatio-temporal context of their expression. Most notably, TSP1 and TSP2 appear to play distinct, non-overlapping roles in the healing of skin wounds. In contrast, both proteins have been implicated as regulators of ischemia-induced angiogenesis. Moreover, TSP2 has been shown to be a critical regulator of angiogenesis in the foreign body response (FBR). In this review, we discuss the role of TSPs in tissue repair and examine the mechanistic data regarding the ability of the thrombospondins to modulate cell-matrix interactions in this context.

The interaction of Thrombospondins with extracellular matrix proteins

The thrombospondins (TSPs) are a family of five matricellular proteins that appear to function as adapter molecules to guide extracellular matrix synthesis and tissue remodeling in a variety of normal and disease settings. Various TSPs have been shown to bind to fibronectin, laminin, matrilins, collagens and other extracellular matrix (ECM) proteins. The importance of TSP-1 in this context is underscored by the fact that it is rapidly deposited at the sites of tissue damage by platelets. An association of TSPs with collagens has been known for over 25 years. The observation that the disruption of the TSP-2 gene in mice leads to collagen fibril abnormalities provided important in vivo evidence that these interactions are physiologically important. Recent biochemical studies have shown that TSP-5 promotes collagen fibril assembly and structural studies suggest that TSPs may interact with collagens through a highly conserved potential metal ion dependent adhesion site (MIDAS). These interactions are critical for normal tissue homeostasis, tumor progression and the etiology of skeletal dysplasias.

Thrombospondins function as regulators of angiogenesis

Thrombospondins (TSPs) -1 and -2 were among the first protein inhibitors of angiogenesis to be identified, a property that was subsequently attributed to the interactions of sequences in their type I repeats with endothelial cell-surface receptors. The interactions of TSPs-1 and -2 with cell-surface receptors, proteases, growth factors, and other bioactive molecules, coupled with the absence of direct structural functions that can be attributed to these matrix proteins, qualify them for inclusion in the category of ‘matricellular proteins’. The phenotypes of TSP-1, TSP-2, and double TSP-1/2-null mice confirm the roles that these proteins play in the regulation of angiogenesis, and provide clues to some of the other important functions of these multi-domain proteins. One of these functions is the ability of TSP-1 to activate the latent TGFβ1 complex, a property that is not shared by TSP-2. A major pathway by which TSP1 or TSP2 inhibits angiogenesis involves an interaction with CD 36 on endothelial cells, which leads to apoptosis of both the liganded and adjacent cells. However a homeostatic mechanism, which inhibits endothelial cell proliferation, and may be physiologically preferable under some circumstances, has also been elucidated, and involves interaction with the very low density lipoprotein receptor (VLDLR). The interaction of TSP1with its receptor, CD47, further inhibits angiogenesis by antagonizing nitric oxide signaling in endothelial and vascular smooth muscle cells. Paradoxically, there is also evidence that TSP-1 can function to promote angiogenesis. This apparent contradiction can be explained by the presence of sequences in different domains of the protein that interact with different receptors on endothelial cells. The anti-angiogenic function of TSPs has spurred interest in their use as anti-tumor agents. Currently, peptide mimetics, based on sequences in the type I repeats of TSPs that have been shown to have anti-angiogenic properties, are undergoing clinical testing.

The mammalian ovary from genesis to revelation

Two major functions of the mammalian ovary are the production of germ cells (oocytes), which allow continuation of the species, and the generation of bioactive molecules, primarily steroids (mainly estrogens and progestins) and peptide growth factors, which are critical for ovarian function, regulation of the hypothalamic-pituitary-ovarian axis, and development of secondary sex characteristics. The female germline is created during embryogenesis when the precursors of primordial germ cells differentiate from somatic lineages of the embryo and take a unique route to reach the urogenital ridge. This undifferentiated gonad will differentiate along a female pathway, and the newly formed oocytes will proliferate and subsequently enter meiosis. At this point, the oocyte has two alternative fates: die, a common destiny of millions of oocytes, or be fertilized, a fate of at most approximately 100 oocytes, depending on the species. At every step from germline development and ovary formation to oogenesis and ovarian development and differentiation, there are coordinated interactions of hundreds of proteins and small RNAs. These studies have helped reproductive biologists to understand not only the normal functioning of the ovary but also the pathophysiology and genetics of diseases such as infertility and ovarian cancer. Over the last two decades, parallel progress has been made in the assisted reproductive technology clinic including better hormonal preparations, prenatal genetic testing, and optimal oocyte and embryo analysis and cryopreservation. Clearly, we have learned much about the mammalian ovary and manipulating its most important cargo, the oocyte, since the birth of Louise Brown over 30 yr ago.

Effects of rosiglitazone on ovarian function and fertility in animals with reduced fertility following fetal and neonatal exposure to nicotine

We have previously shown that in utero nicotine exposure causes impaired fertility, follicle immaturity, and ovarian dysfunction in adult female rat offspring. These characteristics overtly resemble the clinical profile of polycystic ovarian syndrome (PCOS) and recent studies have shown that thiazolidinediones such as rosiglitazone improve fertility in women with PCOS but the mechanism is not well defined. Our goal was to examine whether rosiglitazone would (1) ameliorate the altered ovarian physiology that occurs following fetal and neonatal exposure to nicotine and (2) to examine whether this could be due to normalization of ovarian vascularization. At weaning, offspring of nicotine-exposed dams were given either vehicle (NV) or rosiglitazone (3 mg kg(-1) day(-1); NR). Offspring of saline-exposed dams received vehicle (SV). Tissues were collected when the female offspring reached 26 weeks of age. NV animals had reduced granulosa cell proliferation and increased ovarian cell apoptosis. Treatment with rosiglitazone increased proliferation, and decreased apoptosis, compared NV animals. NV animals had decreased ovarian vascularity relative to controls, whereas NR animals had an intermediate level of ovarian vessel density. Moreover, ovaries from NV animals had decreased levels of the pro-angiogenic growth factors vascular endothelial growth factor (VEGF) and endocrine gland-derived VEGF both of which were increased with rosiglitazone treatment. Rosiglitazone reversed some of the nicotine effects in the ovary and increased ovarian vascularization, follicle maturation and improved oocyte competence. Rosiglitazone may be an important treatment option for PCOS and the present study provides a potential mechanism by which rosiglitazone may have beneficial effects on fertility in these patients.

Thrombospondin-1 modulates vascular endothelial growth factor activity at the receptor level

Vascular endothelial growth factor (VEGF) is a well-established stimulator of vascular permeability and angiogenesis, whereas thrombospondin-1 (TSP-1) is a potent angiogenic inhibitor. In this study, we have found that the TSP-1 receptors CD36 and beta1 integrin associate with the VEGF receptor 2 (VEGFR2). The coclustering of receptors that regulate angiogenesis may provide the endothelial cell with a platform for integration of positive and negative signals in the plane of the membrane. Thus, this complex may represent a molecular switch that regulates angiogenesis and determines endothelial cell behavior. In this context, physiological levels of TSP-1 appear to support VEGFR2 function on both the cellular and tissue level, because phosphorylation of VEGFR2 and vascular permeability in response to VEGF are decreased in TSP-1-null mice and isolated endothelial cells. A therapeutic agent based on the antiangiogenic domain of TSP-1, designated 3TSR (for three TSP-1 type 1 repeats), has significant antiangiogenic and antitumor efficacy. Systemic treatment of wild-type mice with 3TSR significantly decreased VEGF-induced permeability. Consistent with this result, VEGF-stimulated phosphorylation of VEGFR2 was also significantly decreased in lung extracts from 3TSR-treated mice. Moreover, 3TSR significantly decreased VEGF-stimulated VEGFR2 phosphorylation in human dermal microvascular endothelial cells in culture. Taken together, the results indicate that TSP-1 and 3TSR modulate the function of VEGFR2.

Duplication and distinct expression patterns of two thrombospondin-1 isoforms in teleost fishes

Two types of thrombospondin-1 (named TSP-1a and TSP-1b) were cloned from two species of teleosts, the Nile tilapia and medaka. Phylogenetic analysis of these TSP-1 sequences, together with those available from other vertebrates further demonstrated that two types of TSP-1 exist only in teleosts, extending the finding in fugu and tetraodon to two additional fish species. The expression of both genes was examined using tilapia at various developmental stages. Tilapia TSP-1a and TSP-1b were each expressed in a wide range of tissues examined. The early expression of TSP-1b in both XX and XY gonads from 5dah (day after hatching) onwards suggested an important role in the formation of gonads, while the expression of TSP-1a only in ovaries during later stages of development (from 120dah onwards) may suggest that TSP-1a is involved in oogenesis. During the 14-day spawning cycle, the expression of both types of TSP-1 exhibited distinct peaks at day 5 (peak of vitellogenesis) and day 12 (oocyte maturation). In situ hybridization analyses revealed differential expression, with TSP-1a occurring in granulosa cells and TSP-1b in theca cells. Furthermore, both TSP-1a and -1b were expressed in skeletal tissues but with clear temporal and spatial differences. In contrast, only TSP-1b was found in the myosepta. The positive signals of both TSP-1a and TSP-1b were also detected in the heart and spleen, and TSP-1a in brain and intestine by both RT-PCR and in situ hybridization.

SRB Reproduction, Fertility and Development Award Lecture 2008. Regulation and manipulation of angiogenesis in the ovary and endometrium

The marked cyclical physiological angiogenesis in the developing follicle, corpus luteum and endometrium implies a critical role in health and disease. Our approach to understanding its regulation has been to localise and quantify the temporal changes in putative angiogenic factors, and their receptors, in human and non-human primate tissue and to use antagonists to dissect their role by specific inhibition at defined periods during the ovulatory cycle in non-human primates in vivo. The course of angiogenesis throughout the cycle and the cellular and molecular effects of inhibitory treatments have been investigated in the marmoset ovary and uterus, whereas consequences on pituitary-ovarian function have been monitored in macaques. Inhibition of vascular endothelial growth factor (VEGF) at the time of follicle recruitment or selection prevents endothelial cell proliferation, leading to inhibition of follicular development. VEGF inhibition during the early luteal phase prevents angiogenesis and restricts development of the luteal microvasculature. Inhibition of angiogenesis at all stages of the cycle leads to profound suppression of ovarian function. Even during the 'post-angiogenic' period of the luteal phase, inhibition of VEGF precipitates a suppression of progesterone secretion, pointing to additional roles for VEGF in the ovary. In the endometrium, oestrogen drives endometrial angiogenesis through VEGF. Thus, oestrogen can restore angiogenesis after ovariectomy, but not in the presence of VEGF inhibitors. These investigations enhance our understanding of the regulation of angiogenesis in the ovary and uterus and inform studies on conditions with abnormal vascularisation, such as polycystic ovary syndrome, endometriosis, uterine fibroids and menstrual dysfunction.

Microarray analysis of insulin-like growth factor-I-induced changes in messenger ribonucleic acid expression in cultured porcine granulosa cells: possible role of insulin-like growth factor-I in angiogenesis

Insulin-like growth factor-I in conjunction with gonadotropins are important stimulators of mitosis and ovarian steroid production by granulosa and thecal cells, which are required for normal oocyte development and hormonal feedback signaling to the hypothalamus and pituitary. However, a comprehensive evaluation of the changes in gene expression induced by IGF-I has not been conducted. Our objective was to characterize granulosa cell gene expression in response to IGF-I treatment. Porcine granulosa cells were pooled in 4 biological replicates and treated with FSH (baseline) or FSH+IGF-I for 24 h in vitro. The RNA was collected and hybridized to 8 Affymetrix Porcine GeneChips (Affymetrix, Santa Clara, CA) in a paired design. Differentially regulated gene sequence element sets (P < 0.01) were used as queries in the UniGene database searching for annotated genes. Abundance of messenger RNA (mRNA) for genes differentially expressed in the microarray analysis was determined through multiplex assays of one-step real-time reverse transcription-PCR and further analyzed under a statistical model including the fixed effect of treatment. A total of 388 gene sequence element sets were differentially expressed, and 42 matched annotated genes in the UniGene database. Of the 3 upregulated target genes selected for further quantitative reverse transcription-PCR analysis, only FGF receptor 2 III c (FGFR2IIIc) mRNA abundance was significantly increased by IGF-I. Of the 3 downregulated target genes selected for further analysis, only thrombospondin-1 (THBS1) mRNA abundance was significantly decreased by IGF-I. Further study revealed that neither FSH nor estradiol affected the IGF-I-induced suppression of THBS1 mRNA abundance. These results provide the first comprehensive assessment of IGF-I-induced gene expression in granulosa cells and will contribute to a better understanding of the molecular mechanisms of IGF-I regulation of follicular development. Involvement of FGFR2IIIc and THBS1 in mediating IGF-I-induced granulosa cell steroidogenesis and proliferation during follicular development is novel, but their specific roles will require further elucidation.

ABT-510 induces tumor cell apoptosis and inhibits ovarian tumor growth in an orthotopic, syngeneic model of epithelial ovarian cancer

Epithelial ovarian cancer (EOC) is the fifth most common cancer in women and is characterized by a low 5-year survival rate. One strategy that can potentially improve the overall survival rate in ovarian cancer is the use of antitumor agents such as ABT-510. ABT-510 is a small mimetic peptide of the naturally occurring antiangiogenic compound thrombospondin-1 and has been shown to significantly reduce tumor growth and burden in preclinical mouse models and in naturally occurring tumors in dogs. This is the first evaluation of ABT-510 in a preclinical model of human EOC. Tumorigenic mouse surface epithelial cells were injected into the bursa of C57BL/6 mice that were treated with either 100 mg/kg ABT-510 or an equivalent amount of PBS. ABT-510 caused a significant reduction in tumor size, ascites fluid volume, and secondary lesion dissemination when compared with PBS controls. Analysis of the vasculature of ABT-510-treated mice revealed vascular remodeling with smaller diameter vessels and lower overall area, increased number of mature vessels, and decreased tissue hypoxia. Tumors of ABT-510-treated mice had a significantly higher proportion of apoptotic tumor cells compared with the PBS-treated controls. Immunoblot analysis of cell lysates revealed a reduction in vascular endothelial growth factor, vascular endothelial growth factor receptor-2, and proliferating cell nuclear antigen protein expression as well as expression of members of the phosphatidylinositol 3-kinase and mitogen-activated protein kinase survival pathways. In vitro, ABT-510 induced tumor cell apoptosis in mouse and human ovarian cancer cells. This study shows ABT-510 as a promising candidate for inhibiting tumor growth and ascites formation in human EOC.

Enhanced angiogenesis and reduced contraction in thrombospondin-2-null wounds is associated with increased levels of matrix metalloproteinases-2 and -9, and soluble VEGF

Thrombospondin-2 (TSP2) is an inhibitor of angiogenesis with pro-apoptotic and anti-proliferative effects on endothelial cells. Mice deficient in this matricellular protein display improved recovery from ischemia and accelerated wound healing associated with alterations in angiogenesis and extracellular matrix remodeling. In this study, we probed the function of TSP2 by performing a detailed analysis of dermal wounds and wound-derived fibroblasts. Specifically, we analyzed incisional wounds by tensiometry and found no differences in strength recovery between wild-type and TSP2-null mice. In addition, analysis of full-thickness excisional wounds by terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate nick-end labeling stain and MIB-5 immunohistochemistry revealed similar numbers of apoptotic and proliferating cells, respectively. In contrast, the levels of matrix metalloproteinase (MMP)-2, MMP-9, tissue inhibitors of metalloproteinase (TIMP)-1, TIMP-2, and soluble vascular endothelial growth factor were increased in wounds of TSP2-null mice. Evaluation of the ability of TSP2-null wound fibroblasts to contract collagen gels revealed that it was compromised, even though TSP2-null wounds displayed normal myofibroblast content. Therefore, we conclude that the lack of TSP2 leads to aberrant extracellular matrix remodeling, increased neovascularization, and reduced contraction due in part to elevated levels of MMP-2 and MMP-9. These observations provide in vivo supporting evidence for a newly proposed function of TSP2 as a modulator of extracellular matrix remodeling.

LDL receptor-related protein 1: unique tissue-specific functions revealed by selective gene knockout studies

The LDL receptor-related protein (originally called LRP, but now referred to as LRP1) is a large endocytic receptor that is widely expressed in several tissues. LRP1 is a member of the LDL receptor family that plays diverse roles in various biological processes including lipoprotein metabolism, degradation of proteases, activation of lysosomal enzymes, and cellular entry of bacterial toxins and viruses. Deletion of the LRP1 gene leads to lethality in mice, revealing a critical, but as of yet, undefined role in development. Tissue-specific gene deletion studies reveal an important contribution of LRP1 in the vasculature, central nervous system, macrophages, and adipocytes. Three important properties of LRP1 dictate its diverse role in physiology: 1) its ability to recognize more than 30 distinct ligands, 2) its ability to bind a large number of cytoplasmic adaptor proteins via determinants located on its cytoplasmic domain in a phosphorylation-specific manner, and 3) its ability to associate with and modulate the activity of other transmembrane receptors such as integrins and receptor tyrosine kinases.

Thrombospondins in cancer

The thrombospondins (TSPs) are a family of five proteins that are involved in the tissue remodeling that is associated with embryonic development, wound healing, synaptogenesis, and neoplasia. These proteins mediate the interaction of normal and neoplastic cells with the extracellular matrix and surrounding tissue. In the tumor microenvironment, TSP-1 has been shown to suppress tumor growth by inhibiting angiogenesis and by activating transforming growth factor beta. TSP-1 inhibits angiogenesis through direct effects on endothelial cell migration and survival, and through effects on vascular endothelial cell growth factor bioavailability. In addition, TSP-1 may affect tumor cell function through interaction with cell surface receptors and regulation of extracellular proteases. Whereas the role of TSP-1 in the tumor microenvironment is the best characterized, the other TSPs may have similar functions. (Part of a Multi-author Review).

Thrombospondin-1 and thrombospondin-2 mRNA and TSP-1 and TSP-2 protein expression in uterine fibroids and correlation to the genes COL1A1 and COL3A1 and to the collagen cross-link hydroxyproline

Uterine fibroids are composed of altered collagen fibrils and represent an arrested response to injury-initiating fibrosis. In many tissues, TSP-1 is secreted by adult macrophages and monocytes upon wounding and is involved in the activation of transforming growth factor beta. In the absence of TSP-1, the orchestrated process of wound healing is impaired. The authors obtained tissue from the edge and center of fibroids at the time of hysterectomy and compared them with adjacent myometrium. The pattern of TSP-1 and TSP-2 expression was correlated to that of COL1A1 and COL3A1. Collagen and hydroxyproline were increased in fibroids. Thrombospondin-1 was consistently underexpressed in both the edge and center of the fibroids, while COL1A1 and COL3A1 were consistently overexpressed. However, TSP-2 was inconsistently expressed. These findings lead to the conclusion that the underexpression of TSP-1 may contribute to the overall development of uterine fibroids.

Thrombospondin-1 expression is increased during follicular atresia in the primate ovary

Thrombospondin (TSP)-1 is an antiangiogenic extracellular matrix glycoprotein that modulates several aspects of cellular function. The aim of this study was to determine the pattern of TSP-1 mRNA and protein expression as well as expression of its receptor CD36 in the marmoset ovary and to investigate the effects of inhibition of gonadotropins or VEGF activity on TSP-1 and CD36 expression in vivo. GnRH antagonist or VEGF Trap, a soluble decoy receptor, was administered on d 0 of the follicular phase of the cycle, and ovaries were collected at the end of the follicular phase (d 10). TSP-1 mRNA and protein were present in granulosa cells of preantral and antral follicles, with the highest staining at the late secondary and tertiary stages. Moreover, expression of TSP-1 mRNA and protein was significantly increased in tertiary follicles undergoing atresia. CD36 protein was detected in granulosa cells of preantral and antral follicles as well as in endothelial cells of large vessels. Inhibition of gonadotropin secretion or VEGF activity had no effect on TSP-1 expression; however, expression of CD36 protein was inhibited by the VEGF Trap. In conclusion, TSP-1 may be involved in the cessation of angiogenesis in follicles undergoing atresia; alternatively, TSP-1 may act on granulosa and/or endothelial cells to promote follicular atresia in the ovary. Angiogenesis is likely to involve a balance between pro- and antiangiogenic factors. Our results suggest that loss of VEGF activity does not regulate TSP-1 expression directly but may influence TSP-1 activity via down-regulation of the CD36 receptor.

Heparin-induced cis- and trans-dimerization modes of the thrombospondin-1 N-terminal domain

Through its interactions with proteins and proteoglycans, thrombospondin-1 (TSP-1) functions at the interface of the cell membrane and the extracellular matrix to regulate matrix structure and cellular phenotype. We have previously determined the structure of the high affinity heparin-binding domain of TSP-1, designated TSPN-1, in association with the synthetic heparin, Arixtra. To establish that the binding of TSPN-1 to Arixtra is representative of the association with naturally occurring heparins, we have determined the structures of TSPN-1 in complex with heparin oligosaccharides containing eight (dp8) and ten (dp10) subunits, by x-ray crystallography. We have found that dp8 and dp10 bind to TSPN-1 in a manner similar to Arixtra and that dp8 and dp10 induce the formation of trans and cis TSPN-1 dimers, respectively. In silico docking calculations partnered with our crystal structures support the importance of arginine residues in positions 29, 42, and 77 in binding sulfate groups of the dp8 and dp10 forms of heparin. The ability of several TSPN-1 domains to bind to glycosaminoglycans simultaneously probably increases the affinity of binding through multivalent interactions. The formation of cis and trans dimers of the TSPN-1 domain with relatively short segments of heparin further enhances the ability of TSP-1 to participate in high affinity binding to glycosaminoglycans. Dimer formation may also involve TSPN-1 domains from two separate TSP-1 molecules. This association would enable glycosaminoglycans to cluster TSP-1.

Thrombospondins use the VLDL receptor and a nonapoptotic pathway to inhibit cell division in microvascular endothelial cells

TSPs 1 and 2 function as endogenous inhibitors of angiogenesis. Although thrombospondins (TSPs) have been shown to induce apoptosis in HMVECs, we reasoned that a homeostatic mechanism would also be needed to inhibit EC growth without causing cell death, e.g., in the maintenance of a normal vascular endothelium. HMVECs, cultured in low serum, responded to VEGF with an increase in [(3)H]thymidine incorporation that was inhibited by TSPs and was accompanied by decreases in the phosphorylation of Akt and MAPK, without an increase in apoptosis. RAP, an inhibitor of the low-density lipoprotein (LDL) family of endocytic receptors, and blocking antibodies to VLDLR were as effective as TSPs in the inhibition of thymidine uptake in response to VEGF, and the effects of these agents were not additive. Supportive evidence for the role of the VLDLR in mediating this inhibition was provided by the demonstration of a high-affinity interaction between TSPs and the VLDLR. We propose that TSP1 and TSP2, together with the VLDLR, initiate a nonapoptotic pathway for maintenance of the normal adult vascular endothelium in a quiescent state, similar to that invoked for the regulation of mitogenesis by PDGF, but involving signaling via the VLDLR rather than LRP1.

Thrombospondin-based antiangiogenic therapy

Thrombospondins (TSPs) are a family of extracellular matrix proteins that regulate tissue genesis and remodeling. TSP-1 plays a pivotal role in the regulation of both physiological and pathological angiogenesis. The inhibitory effects of TSP-1 on angiogenesis have been established in numerous experimental models. Among other TSP members, TSP-2 has equivalent domain structure as TSP-1 and shares most functions of TSP-1. The mechanisms by which TSP-1 and -2 inhibit angiogenesis can be broadly characterized as direct effects on vascular endothelial cells and indirect effects on the various angiogenic regulators. The fact that TSP-1 and -2 are potent endogenous angiogenic inhibitors has prompted studies to explore their therapeutic applications, and detailed understanding of the mechanisms of action of TSP-1 and -2 has facilitated the design of therapeutic strategies to optimize these activities. The therapeutic effects can be achieved by up-regulation of endogenous TSPs, or by the delivery of recombinant proteins or synthetic peptides that contain sequences from the angiogenic domain of TSP-1. In this article, we review the progress in thrombospondin-based antiangiogenic therapy and discuss the perspectives on the significant challenges that remain.

Boswellic acid acetate induces apoptosis through caspase-mediated pathways in myeloid leukemia cells

The mechanism of the cytotoxic effect of boswellic acid acetate, a 1:1 mixture of alpha-boswellic acid acetate and beta-boswellic acid acetate, isolated from Boswellia carterri Birdw on myeloid leukemia cells was investigated in six human myeloid leukemia cell lines (NB4, SKNO-1, K562, U937, ML-1, and HL-60 cells). Morphologic and DNA fragmentation assays indicated that the cytotoxic effect of boswellic acid acetate was mediated by induction of apoptosis. More than 50% of the cells underwent apoptosis after treatment with 20 mug/mL boswellic acid for 24 hours. This apoptotic process was p53 independent. The levels of apoptosis-related proteins Bcl-2, Bax, and Bcl-XL were not modulated by boswellic acid acetate. Boswellic acid acetate induced Bid cleavage and decreased mitochondrial membrane potential without production of hydrogen peroxide. A general caspase inhibitor (Z-VAD-FMK) and a specific caspase-8 inhibitor II (Z-IETD-FMK) blocked boswellic acid acetate-induced apoptosis. The mRNAs of death receptors 4 and 5 (DR4 and DR5) were induced in leukemia cells undergoing apoptosis after boswellic acid acetate treatment. These data taken together suggest that boswellic acid acetate induces myeloid leukemia cell apoptosis through activation of caspase-8 by induced expression of DR4 and DR5, and that the activated caspase-8 either directly activates caspase-3 by cleavage or indirectly by cleaving Bid, which in turn decreases mitochondria membrane potential.