Changes in thrombospondin-1 levels in the endothelial cells of the anterior pituitary during estrogen-induced prolactin-secreting pituitary tumors

Carbon monoxide induces chromatin remodelling to facilitate endothelial cell migration

Vascular injury to vessel endothelial cells (EC), caused by either mechanical damage or chronic inflammation, is still awaiting effective therapies. In the present study we hypothesised that carbon monoxide (CO) acts on the nuclear receptor Rev-erbα to induce chromatin modification and endothelial cell migration. We demonstrate that administration of low, safe doses of exogenous CO enhances endothelial cell (EC) migration, which occurs in part through chromatin remodelling and histone H3 acetylation. Further, we show that the effects of CO are dependent on inhibition of phosphorylation of glycogen synthase kinase-3 β (GSK3β), activation of haem synthesis, and increased expression of Rev-erbα. Rev-erbα is a haem-containing transcription factor which in response to CO binds to target DNA, recruits the Histone Deacetylase/nuclear Receptor Corepressor (HDAC/N-CoR) complex, and regulates transcription of genes responsible for endothelial cell migration and angiogenesis. Decreased levels of Rev-erbα in chimeric mice after bone marrow transplant from Rev-erbα following bone marrow transplantation from rev-erb+/− mice resulted in loss of protective effects of CO against neointima formation after wire injury. Collectively, CO modifies chromatin structure through enhanced acetylation of histone H3 via a GSK3β-Rev-erbα-mediated pathway to increase EC migration. We propose that CO enhances vessel repair following injury in part by regulating EPC/EC motility via Rev-erbα. Thus, inhaled CO may be beneficial in the treatment of vascular syndromes associated with dysregulated thrombosis, wound healing, and angiogenesis.

Thrombospondin-1 (TSP-1), a new bone morphogenetic protein-2 and -4 (BMP-2/4) antagonist identified in pituitary cells

Bone morphogenetic proteins (BMPs) regulate diverse cellular responses during embryogenesis and in adulthood including cell differentiation, proliferation, and death in various tissues. In the adult pituitary, BMPs participate in the control of hormone secretion and cell proliferation, suggesting a potential endocrine/paracrine role for BMPs, but some of the mechanisms are unclear. Here, using a bioactivity test based on embryonic cells (C3H10T1/2) transfected with a BMP-responsive element, we sought to determine whether pituitary cells secrete BMPs or BMP antagonists. Interestingly, we found that pituitary-conditioned medium contains a factor that inhibits action of BMP-2 and -4. Combining surface plasmon resonance and high-resolution mass spectrometry helped pinpoint this factor as thrombospondin-1 (TSP-1). Surface plasmon resonance and co-immunoprecipitation confirmed that recombinant human TSP-1 can bind BMP-2 and -4 and antagonize their effects on C3H10T1/2 cells. Moreover, TSP-1 inhibited the action of serum BMPs. We also report that the von Willebrand type C domain of TSP-1 is likely responsible for this BMP-2/4-binding activity, an assertion based on sequence similarity that TSP-1 shares with the von Willebrand type C domain of Crossveinless 2 (CV-2), a BMP antagonist and member of the chordin family. In summary, we identified for the first time TSP-1 as a BMP-2/-4 antagonist and presented a structural basis for the physical interaction between TSP-1 and BMP-4. We propose that TSP-1 could regulate bioavailability of BMPs, either produced locally or reaching the pituitary via blood circulation. In conclusion, our findings provide new insights into the involvement of TSP-1 in the BMP-2/-4 mechanisms of action.

Thrombospondin-1 is a multifaceted player in tumor progression

Thrombospondins are a family of extracellular matrix (ECM) proteins. Thrombospondin-1 (TSP1) was the first member to be identified and is a main player in tumor microenvironment. The diverse functions of TSP1 depend on the interactions between its structural domains and multiple cell surface molecules. TSP1 acts as an angiogenesis inhibitor by stimulating endothelial cell apoptosis, inhibiting endothelial cell migration and proliferation, and regulating vascular endothelial growth factor bioavailability and activity. In addition to angiogenesis modulation, TSP1 also affects tumor cell adhesion, invasion, migration, proliferation, apoptosis and tumor immunity. This review discusses the multifaceted and sometimes opposite effects of TSP1 on tumor progression depending on the molecular and cellular composition of the microenvironment. Clinical implications of TSP1-related compounds are also discussed.

The pituitary TGFβ1 system as a novel target for the treatment of resistant prolactinomas

Prolactinomas are the most frequently observed pituitary adenomas and most of them respond well to conventional treatment with dopamine agonists (DAs). However, a subset of prolactinomas fails to respond to such therapies and is considered as DA-resistant prolactinomas (DARPs). New therapeutic approaches are necessary for these tumors. Transforming growth factor β1 (TGFβ1) is a known inhibitor of lactotroph cell proliferation and prolactin secretion, and it partly mediates dopamine inhibitory action. TGFβ1 is secreted to the extracellular matrix as an inactive latent complex, and its bioavailability is tightly regulated by different components of the TGFβ1 system including latent binding proteins, local activators (thrombospondin-1, matrix metalloproteases, integrins, among others), and TGFβ receptors. Pituitary TGFβ1 activity and the expression of different components of the TGFβ1 system are regulated by dopamine and estradiol. Prolactinomas (animal models and humans) present reduced TGFβ1 activity as well as reduced expression of several components of the TGFβ1 system. Therefore, restoration of TGFβ1 inhibitory activity represents a novel therapeutic approach to bypass dopamine action in DARPs. The aim of this review is to summarize the large literature supporting TGFβ1 important role as a local modulator of pituitary lactotroph function and to provide recent evidence of the restoration of TGFβ1 activity as an effective treatment in experimental prolactinomas.

Angiogenesis in pituitary adenomas: human studies and new mutant mouse models

The role of angiogenesis in pituitary tumor development has been questioned, as pituitary tumors have been usually found to be less vascularized than the normal pituitary tissue. Nevertheless, a significantly higher degree of vasculature has been shown in invasive or macropituitary prolactinomas when compared to noninvasive and microprolactinomas. Many growth factors and their receptors are involved in pituitary tumor development. For example, VEGF, FGF-2, FGFR1, and PTTG, which give a particular vascular phenotype, are modified in human and experimental pituitary adenomas of different histotypes. In particular, vascular endothelial growth factor, VEGF, the central mediator of angiogenesis in endocrine glands, was encountered in experimental and human pituitary tumors at different levels of expression and, in particular, was higher in dopamine agonist resistant prolactinomas. Furthermore, several anti-VEGF techniques lowered tumor burden in human and experimental pituitary adenomas. Therefore, even though the role of angiogenesis in pituitary adenomas is contentious, VEGF, making permeable pituitary endothelia, might contribute to adequate temporal vascular supply and mechanisms other than endothelial cell proliferation. The study of angiogenic factor expression in aggressive prolactinomas with resistance to dopamine agonists will yield important data in the search of therapeutical alternatives.

Sex differences in the pituitary transforming growth factor-β1 system: studies in a model of resistant prolactinomas

Dopamine and estradiol interact in the regulation of lactotroph cell proliferation and prolactin secretion. Ablation of the dopamine D2 receptor gene (Drd2(-/-)) in mice leads to a sexually dimorphic phenotype of hyperprolactinemia and pituitary hyperplasia, which is stronger in females. TGF-β1 is a known inhibitor of lactotroph proliferation. TGF-β1 is regulated by dopamine and estradiol, and it is usually down-regulated in prolactinoma experimental models. To understand the role of TGF-β1 in the gender-specific development of prolactinomas in Drd2(-/-) mice, we compared the expression of different components of the pituitary TGF-β1 system, including active cytokine content, latent TGF-β-binding protein isoforms, and possible local TGF-β1 activators, in males and females in this model. Furthermore, we evaluated the effects of dopamine and estradiol administration to elucidate their role in TGF-β1 system regulation. The expression of active TGF-β1, latent TGF-β-binding protein isoforms, and several putative TGF-β1 activators evaluated was higher in male than in female mouse pituitary glands. However, Drd2(-/-) female mice were more sensitive to the decrease in active TGF-β1 content, as reflected by the down-regulation of TGF-β1 target genes. Estrogen and dopamine caused differential regulation of several components of the TGF-β1 system. In particular, we found sex- and genotype- dependent regulation of active TGF-β1 content and a similar expression pattern for 2 of the putative TGF-β1 activators, thrombospondin-1 and kallikrein-1, suggesting that these proteins could mediate TGF-β1 activation elicited by dopamine and estradiol. Our results indicate that (1) the loss of dopaminergic tone affects the pituitary TGF-β1 system more strongly in females than in males, (2) males express higher levels of pituitary TGF-β1 system components including active cytokine, and (3) estradiol negatively controls most of the components of the system. Because TGF-β1 inhibits lactotroph proliferation, we propose that the higher levels of the TGF-β1 system in males could protect or delay the development of prolactinomas in Drd2(-/-) male mice.

Thrombospondin-1 (TSP-1) analogs ABT-510 and ABT-898 inhibit prolactinoma growth and recover active pituitary transforming growth factor-β1 (TGF-β1)

Prolactinomas are the most prevalent type of secreting pituitary tumors in humans and generally respond well to a medical therapy with dopamine agonists. However, for patients exhibiting resistance to dopaminergic drugs, alternative treatments are desired. Antiangiogenic strategies might represent a potential therapy for these tumors. Thrombospondin 1 (TSP-1) is a large multifunctional glycoprotein involved in multiple biological processes including angiogenesis, apoptosis, and activation of TGF-β1. Because tumors that overexpress TSP-1 grow more slowly, have fewer metastases, and have decreased angiogenesis, TSP-1 provides a novel target for cancer treatment. ABT-510 and ABT-898 are TSP-1 synthetic analogs that mimic its antiangiogenic action. In the present study, we explored the potential effect of ABT-510 and ABT-898 on experimental prolactinomas induced by chronic diethylstilbestrol (DES) treatment in female rats. We demonstrated that a 2-wk treatment with ABT-510 and ABT-898 counteracted the increase in pituitary size and serum prolactin levels as well as the pituitary proliferation rate induced by DES. These inhibitory effects on tumor growth could be mediated by the antiangiogenic properties of the drugs. We also demonstrated that ABT-510 and ABT-898, in addition to their described antiangiogenic effects, increased active TGF-β1 level in the tumors. We postulate that the recovery of the local cytokine activation participates in the inhibition of lactotrope function. These results place these synthetic TSP-1 analogs as potential alternative or complementary treatments in dopamine agonist-resistant prolactinomas.

Active and total transforming growth factor-β1 are differentially regulated by dopamine and estradiol in the pituitary

Dopamine, acting through the dopamine type 2 receptor (Drd2), is the main inhibitor of pituitary prolactin (PRL) secretion and lactotroph proliferation. TGF-β1 is involved, at least in part, in mediating these actions. It was described that TGF-β1 synthesis in rat pituitary lactotrophs is up-regulated by dopamine and down-regulated by estradiol. TGF-β1 is secreted as a large latent complex. The local regulation of cytokine activation in the pituitary has not yet been explored. In this work, we studied pituitary active and total TGF-β1 content, as well as TGF-β1 mRNA, and the in vivo role of dopamine and estradiol on pituitary TGF-β1 levels. Adult female mice (wild type), and female mice with a null mutation in the Drd2 (Drd2(-/-)), were used. The loss of dopaminergic tone induced a decrease in TGF-β1 mRNA expression, in active and total cytokine content, and in TGF-β type II receptor expression. Dopamine regulation of pituitary TGF-β1 activation process was inferred by the inhibition of active cytokine by in vivo sulpiride treatment. Interestingly, in the absence of dopaminergic tone, estradiol induced a strong increase in active TGF-β1. PRL secretion correlated with active, but not total cytokine. TGF-β1 inhibitory action on lactotroph proliferation and PRL secretion was decreased in Drd2(-/-) pituitary cells, in correlation with decreased TGF-β type II receptor. The study of the TGF-β1 activation process and its regulation is essential to understand the cytokine activity. As an intermediary of dopamine inhibition of lactotroph function, TGF-β1 and local activators may be important targets in the treatment of dopamine agonist-resistant prolactinomas.

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.

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.

Paracrinicity: the story of 30 years of cellular pituitary crosstalk

Living organisms represent, in essence, dynamic interactions of high complexity between membrane-separated compartments that cannot exist on their own, but reach behaviour in co-ordination. In multicellular organisms, there must be communication and co-ordination between individual cells and cell groups to achieve appropriate behaviour of the system. Depending on the mode of signal transportation and the target, intercellular communication is neuronal, hormonal, paracrine or juxtacrine. Cell signalling can also be self-targeting or autocrine. Although the notion of paracrine and autocrine signalling was already suggested more than 100 years ago, it is only during the last 30 years that these mechanisms have been characterised. In the anterior pituitary, paracrine communication and autocrine loops that operate during fetal and postnatal development in mammals and lower vertebrates have been shown in all hormonal cell types and in folliculo-stellate cells. More than 100 compounds have been identified that have, or may have, paracrine or autocrine actions. They include the neurotransmitters acetylcholine and gamma-aminobutyric acid, peptides such as vasoactive intestinal peptide, galanin, endothelins, calcitonin, neuromedin B and melanocortins, growth factors of the epidermal growth factor, fibroblast growth factor, nerve growth factor and transforming growth factor-beta families, cytokines, tissue factors such as annexin-1 and follistatin, hormones, nitric oxide, purines, retinoids and fatty acid derivatives. In addition, connective tissue cells, endothelial cells and vascular pericytes may influence paracrinicity by delivering growth factors, cytokines, heparan sulphate proteoglycans and proteases. Basement membranes may influence paracrine signalling through the binding of signalling molecules to heparan sulphate proteoglycans. Paracrine/autocrine actions are highly context-dependent. They are turned on/off when hormonal outputs need to be adapted to changing demands of the organism, such as during reproduction, stress, inflammation, starvation and circadian rhythms. Specificity and selectivity in autocrine/paracrine interactions may rely on microanatomical specialisations, functional compartmentalisation in receptor-ligand distribution and the non-equilibrium dynamics of the receptor-ligand interactions in the loops.