Angiogenesis is thought to depend on a precise balance of positive and negative regulation. Angiopoietin-1 is an angiogenic factor that signals through the endothelial cell-specific TIE2 receptor tyrosine kinase (TIE2). Like vascular endothelial growth factor (VEGF), ANGPT1 is essential for normal vascular development in the mouse. By homology screening, Maisonpierre et al. (1997) identified a relative of ANGPT1, termed angiopoietin-2 (ANG2), and showed that it is a naturally occurring antagonist for both ANGPT1 and TIE2.Angiopoietin-2 (ANGPT2) is a naturally occurring antagonist of angiopoietin-1 (ANGPT1; 601667) that competes for binding to the TIE2 receptor (600221) and blocks ANGPT1-induced TIE2 autophosphorylation during vasculogenesis (Kim et al., 2000)Yes-associated protein regulates endothelial cell contact-mediated expression of angiopoietin-2. Choi HJ et al. (2015) Angiogenesis is regulated by the dynamic interaction between endothelial cells (ECs). Hippo-Yes-associated protein (YAP) signalling has emerged as a key pathway that controls organ size and tissue growth by mediating cell contact inhibition. However, the role of YAP in EC has not been defined yet. Here, we show expression of YAP in the developing front of mouse retinal vessels. YAP subcellular localization, phosphorylation and activity are regulated by VE-cadherin-mediated-EC contacts. This VE-cadherin-dependent YAP phosphorylation requires phosphoinositide 3-kinase-Akt activation. We further identify angiopoietin-2 (ANG-2) as a potential transcriptional target of YAP in regulating angiogenic activity of EC in vitro and in vivo. Overexpression of YAP-active form in EC enhances angiogenic sprouting, and this effect is blocked by ANG-2 depletion or soluble Tie-2 treatment. These findings implicate YAP as a critical regulator in angiogenesis and provide new insights into the mechanism coordinating junctional stability and angiogenic activation of ECs.//////////////////
NCBI Summary:
The protein encoded by this gene is an antagonist of angiopoietin 1 (ANGPT1) and endothelial TEK tyrosine kinase (TIE-2, TEK). The encoded protein disrupts the vascular remodeling ability of ANGPT1 and may induce endothelial cell apoptosis. Three transcript variants encoding three different isoforms have been found for this gene. [provided by RefSeq, Jul 2008]
General function
Ligand, Growth factor
Comment
Angiopoietin-2 (ANG2)is a naturally occurring antagonist for both the TIE2 receptor. Xu F, et al reported that local Delivery of Angiopoietin-2 into the Preovulatory Follicle Terminates the Menstrual Cycle in Rhesus Monkeys.
The angiopoietin (ANGPT)-receptor (TEK) system plays a crucial role in blood vessel formation and stability. Since the endogenous agonist ANGPT1, antagonist ANGPT2 and TEK are expressed in the primate ovary, experiments were designed to investigate their role at a critical time during tissue remodeling/angiogenesis in the menstrual cycle, i.e. at midcycle during maturation, ovulation and luteinization of the dominant follicle. Either vehicle, 20 microg of ANGPT1, 2 microg ANGPT2 (low-dose), or 20 microg ANGPT2 (high-dose) was injected directly into the preovulatory follicle of monkeys around the day (-1 to 0) of the midcycle estradiol (E)/LH peak. Ovaries were evaluated on day 3 post-injection for follicle rupture, and serum samples were analyzed for E and progesterone (P) levels. Similar to controls, ANGPT1 treatment was followed by ovulation, and elevated P levels during a luteal phase. In contrast, high-dose ANGPT2 treatment prevented follicle rupture, and P levels never rose above baseline in the subsequent 12 days. However, an E peak typically occurred 12 days post-injection. Laparoscopy detected a preovulatory follicle on the contralateral (non-injected) ovary. P levels subsequently increased above baseline in these animals. Thus, exogenous ANGPT2 disrupted maturation of the preovulatory follicle, preventing its ovulation and conversion into the corpus luteum. ANGPT antagonism eliminated the dominant structure, thereby resetting the ovarian cycle, with selection and maturation of the next preovulatory follicle occurring in a timely manner. The data are consistent with a critical role of the ANGPT-TIE1/TEK system in the ovary, notably at the late stages of follicle maturation during the menstrual cycle.
Cellular localization
Secreted
Comment
Aberrant expression of angiopoietin-like proteins 1 and 2 in cumulus cells is potentially associated with impaired oocyte developmental competence in polycystic ovary syndrome. Liu Z et al. (2016) Polycystic ovary syndrome (PCOS) is a heterogeneous endocrine disorder associated with obesity, insulin resistance, hyperandrogenism, alterations in ovarian angiogenesis and impaired oocyte competence. Emerging evidence demonstrates that angiopoietin-like protein 1 (ANGPTL1) and angiopoietin-like protein 2 (ANGPTL2) have an important influence on angiogenesis, androgen biosynthesis, insulin resistance and adipocytes function. In this study, we set out to determine the potential relationship between ANGPTL1, ANGPTL2 and oocyte competence in PCOS through analyzing the expression levels and dynamic pattern of the two genes in cumulus cells (CCs) during different phases of nuclear maturation of PCOS patients and control groups undergoing controlled ovarian hyperstimulation (COH) for in vitro fertilization and embryo transfer. We found that the relative abundance of ANGPTL1 and ANGPTL2 transcripts in CCs from patients with PCOS showed dynamic changes during oocyte maturation. Specifically, their expressions were increased significantly at the Metaphase II stage. In summary, the present novel evidence indicates that the expression patterns of ANGPTL1 and ANGPTL2 mRNAs are disordered during oocyte maturation in PCOS, which were potentially related to aberrant oocyte quality and developmental potency, at least in part, via pathological angiogenesis and metabolism.//////////////////
Ovarian function
Luteinization
Comment
Effects of Angiopoietin-2 on Transplanted Mouse Ovarian Tissue. Youm HW et al. (2016) Transplantation of ovarian tissue (OT) is currently the only clinical option to restore fertility with cryopreserved OT. However, follicle loss caused by ischemia and slow revascularization occurs in transplanted OT. To shorten the ischemic period and promote angiogenesis, some angiogenic factors have been used. Angiopoietin-2 (Ang2) is one of the major angiogenic factors and has been reported to promote blood vessels and increase vascular permeability in ischemic and/or hypoxic environment. This study was performed to investigate the effects of Ang2 on follicle integrity and revascularization of transplanted mouse OT. Five-week-old B6D2F1 female mice were divided into a control group and two Ang2 groups, followed by ovary collection and vitrification. After warming, the ovaries were autotransplanted into kidney capsules with/without Ang2 injection (50 or 500 ng/kg), and then the mice were sacrificed at days 2, 7, 21, and 42 after transplantation. A total 2,437 follicles in OT grafts were assessed for follicular density, integrity, and classification by using hematoxylin and eosin staining. Apoptosis and revascularization were evaluated by using TUNEL assay and CD31 immunohistochemistry, respectively. Serum follicle-stimulating hormone (FSH) levels were measured by using enzyme-linked immunosorbent assay. Both Ang2 groups showed remarkable increase in morphologically intact follicle ratio across all grafting durations except D21. The numbers of CD31(+) vessels were significantly increased in both Ang2 groups compared with the control group at all durations, except in the 50 ng Ang2 group at D42. However, the mean numbers of follicles of the grafts, apoptosis ratios, and serum FSH levels showed no significant differences among the groups. Our results show that Ang2 treatment significantly increased the intact follicle ratios and the number of blood vessels of the mouse OT grafts. However, further studies performed with large animal or human OT are necessary before clinical application for fertility preservation in cancer patients, and the reliability of the systemic effects of Ang2 should be verified.//////////////////
Angiopoietin-1 and angiopoietin-2 are altered in polycystic ovarian syndrome (PCOS) during controlled ovarian stimulation. Tal R 2013 et al.
Polycystic ovarian syndrome (PCOS) ovaries are characterized by increased angiogenesis and hypervascularity. While angiopoietin-1 (Ang-1) and its antagonist, angiopoietin-2 (Ang-2), are essential for ovarian function and angiogenesis, the levels of Ang-1 and Ang-2 in PCOS are unknown. This was a prospective cohort study of 14 PCOS women and 14 matched controls undergoing controlled ovarian stimulation (COS). Serum was collected on day 3, hCG and retrieval days. Follicular fluid (FF) was collected on retrieval day. Serum Ang-1 and Ang-2 levels were constant throughout COS, but serum Ang-1 levels were increased at all time points in PCOS women compared with controls (p<0.05). No differences between groups were found in serum Ang-2 levels or FF Ang-1 levels. However, FF Ang-2 levels were increased almost 2-fold in PCOS women compared with controls (p<0.01), and correlated positively with number of oocytes retrieved (r = 0.65, p < 0.0001). This study is the first to provide evidence of an alteration in the Ang-1/Ang-2 system in PCOS women. The biological role of Ang-2 in promoting capillary leakage, the increased Ang-2 FF level in PCOS, and its correlation with number of oocytes suggest that Ang-2 may play an important role in the increased risk of ovarian hyperstimulation in PCOS.
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The role of endothelial cell-specific growth factors in the vascularization of the primate peri-ovulatory follicle was examined by Hazzard et al . Experiments were designed firstly to detect expression of vascular endothelial growth factor (VEGF), angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2) in granulosa cells and
secondly, to determine whether gonadotrophins and/or steroids regulate their expression during the peri-ovulatory interval. VEGF, Ang-1 and Ang-2 mRNA were all detected prior to the ovulatory stimulus in punctured granulosa cells. Whereas follicular fluid VEGF concentrations increased 6-fold between 0 and 12 h after hCG treatment, VEGF mRNA values were unchanged and were unaffected by steroid ablation. Ang-1 mRNA decreased from 0 to 12 h, followed by a 30-fold increase at 36 h, while Ang-2 mRNA values were unchanged between 0, 12 and 36 h.
Expression regulated by
FSH, LH, Steroids
Comment
Steroid ablation decreased Ang-1 mRNA at 36 h, and Ang-2 mRNA at 12 h after hCg treatment, while only Ang-1 was restored by progestin replacement Hazzard et al . Pietrowski D, Keck C reported the differential Regulation of ANG2 and VEGF-A in Human Granulosa Lutein Cells by Choriogonadotropin.
Circulating levels of total angiopoietin-2 and the soluble Tie-2 receptor in women during ovarian stimulation and early gestation. Molskness TA et al. Circulating levels of Ang-2 and sTie-2 receptor were detectable but invariant in women during COS cycles. During the postimplantation period, the rise in Ang-2 (but not sTie-2) levels probably reflects placental rather than luteal production.
Ovarian localization
Cumulus, Granulosa, Luteal cells
Comment
Expression and regulation of Ang-2 in murine ovaries during sexual maturation and development of corpus luteum. Guo B et al. The aim of this study was to examine the expression and regulation of angiopoietin-2 (Ang-2) in murine ovaries during sexual maturation, gonadotropin treatment and luteal development by in situ hybridization and RT-PCR. By in situ hybridization Ang-2 mRNA was mainly localized in granulosa cells, thecal cells and corpus luteum, otherwise in oocytes. Moreover, Ang-2 mRNA was highly expressed in corpus luteum and granulosa cells of atretic follicles. According to RT-PCR data, Ang-2 mRNA was lowly expressed on day 10 after birth, then expression levels gradually increased and reached their highest values on day 25 after birth. In the superovulated model of immature mice, Ang-2 expression was strongly induced by equine chorionic gonadotropin (eCG) 48 h post the eCG injection, and was high from 0.5 to 13 h after hCG treatment. In situ hybridization showed that Ang-2 mRNA was highly expressed in corpus luteum from day 2 to 9 post the hCG injection, then the expression levels gradually declined on days 11 and 13 after hCG treatment. According to RT-PCR data, the levels of Ang-2 mRNA expression showed a decline after the hCG injection, with a nadir on day 3, followed by an increase, reaching the highest level on day 9 post-hCG injection. Then again Ang-2 expression gradually declined from day 11 to 15 after hCG injection. These results suggest that Ang-2 may be involved in follicular development, atresia, ovulation, and corpus luteum formation and regression.
Hazzard, et al 2000 reported changes in expression of vascular endothelial growth
factor and angiopoietin-1 and -2 in the macaque corpus
luteum during the menstrual cycle.
Messenger ribonucleic Acid expressions of hepatocyte growth factor, angiopoietins and their receptors during follicular development in gilts Shimizu T, et al .
Angiogenic factors are associated with angiogenesis during follicular development in the mammalian ovary. The aim of the present study was to determine the relationships between the vascular network and mRNA expressions of angiopoietins (Ang)-1, Ang-2 and hepatocyte growth factor (HGF), and their receptors in follicles at different developmental stages during follicular development. Ovaries in gilts were collected 72 h after equine chorionic gonadotropin (eCG, 1250 IU) treatment for histological observation of the capillary network. Granulosa cells and thecal tissues in small (<4 mm), medium (4-5 mm) or large (>5 mm) individual follicles were collected for detection of mRNA expression of HGF, Ang-1 and Ang-2 in granulosa cells, and HGF receptor (HGF-R) and Tie-2 in the theca cells by semi-quantitative RT-PCR. The number of capillaries in the thecal cell layer increased significantly in healthy follicles at all developmental stages in the eCG group compared with those in controls. The expression of Ang-1 mRNA declined in granulosa cells of medium and large follicles and the level of Ang-2 mRNA increased in granulosa cells of small follicles after eCG treatment. The ratio of Ang-2/Ang-1 increased in small, medium and large follicles from ovaries after eCG treatment, but Tie-2 mRNA expression in the theca cells did not change. The level of HGF mRNA increased in granulosa cells of small follicles after eCG treatment but HGF-R in theca cells was not increased by eCG. These data suggested that the angiopoietins might be associated with thecal angiogenesis during follicular development in eCG-treated gilts.
L.K. Christenson3 and R.L. Stouffer1,2,4
Ang-1 and Ang-2 mRNA
expression was low in the early to mid-luteal phase but increased (P < 0.05) at late luteal phase before declining at
menstruation. These data suggest transcriptional control of VEGF, Ang-1 and Ang-2, as well as post-transcriptional control
of VEGF, in macaque corpus luteum. Dynamic expression of angiogenic/angiostatic factors appears critical for development,
maintenance and regression of the luteal microvasculature during the menstrual cycle.
Follicle stages
Antral, Preovulatory, Corpus luteum
Comment
Local Delivery of Angiopoietin-2 into the Preovulatory Follicle Terminates the Menstrual Cycle in Rhesus Monkeys Xu F, Stouffer .
The angiopoietin (ANGPT)-receptor (TEK) system plays a crucial role in blood vessel formation and stability. Since the endogenous agonist ANGPT1, antagonist ANGPT2 and TEK are expressed in the primate ovary, experiments were designed to investigate their role at a critical time during tissue remodeling/angiogenesis in the menstrual cycle, i.e. at midcycle during maturation, ovulation and luteinization of the dominant follicle. Either vehicle, 20 microg of ANGPT1, 2 microg ANGPT2 (low-dose), or 20 microg ANGPT2 (high-dose) was injected directly into the preovulatory follicle of monkeys around the day (-1 to 0) of the midcycle estradiol (E)/LH peak. Ovaries were evaluated on day 3 post-injection for follicle rupture, and serum samples were analyzed for E and progesterone (P) levels. Similar to controls, ANGPT1 treatment was followed by ovulation, and elevated P levels during a luteal phase. In contrast, high-dose ANGPT2 treatment prevented follicle rupture, and P levels never rose above baseline in the subsequent 12 days. However, an E peak typically occurred 12 days post-injection. Laparoscopy detected a preovulatory follicle on the contralateral (non-injected) ovary. P levels subsequently increased above baseline in these animals. Thus, exogenous ANGPT2 disrupted maturation of the preovulatory follicle, preventing its ovulation and conversion into the corpus luteum. ANGPT antagonism eliminated the dominant structure, thereby resetting the ovarian cycle, with selection and maturation of the next preovulatory follicle occurring in a timely manner. The data are consistent with a critical role of the ANGPT-TIE1/TEK system in the ovary, notably at the late stages of follicle maturation during the menstrual cycle.
Angiogenesis in the human corpus luteum: Changes in expression of angiopoietins in the corpus luteum throughout the menstrual cycle and in early pregnancy Sugino N, et al .
Context: Blood vessel stabilization is regulated by angiopoietins and important for angiogenesis in the corpus luteum (CL). Objective: To study angiogenesis and blood vessel stabilization in the human CL, changes in expression of angiopoietin-1 (Ang-1), Ang-2 and their specific receptor, Tie-2 together with the number of blood vessels and pericytes were examined in the CL throughout the menstrual cycle and in early pregnancy. Design: The number of blood vessels and pericytes was determined by immunohistochemistry for CD34 and alpha-smooth muscle actin, respectively. Ang and Tie-2 expression were examined by immunohistochemistry or RT-PCR. Results: The number of blood vessels increased during the early luteal phase whereas the number of pericytes was small in the early luteal phase and increased in the mid-luteal phase, suggesting that angiogenesis is undergoing during the early luteal phase and blood vessels are stabilized in the mid-luteal phase. Blood vessels and pericytes decreased in number during the late luteal phase. The increased number of both blood vessels and pericytes in early pregnancy suggests that angiogenesis is undergoing accompanied by blood vessel stabilization. Ang-2 expression with low Ang-1 expression was found during the early luteal phase. Thereafter, increasing Ang-1 expression during the mid-luteal phase, declining Ang-1 expression with continued Ang-2 expression during the late luteal phase, and relatively high Ang-1 expression in early pregnancy were observed. Conclusions: The change in Ang expression is closely associated with angiogenesis, blood vessel stabilization and blood vessel regression during the divergent phases of luteal formation, luteal regression, and luteal rescue by pregnancy.