Experimental hypoxia and embryonic angiogenesis

Mechanisms of adaptive angiogenesis to tissue hypoxia

Angiogenesis is mostly an adaptive response to tissue hypoxia, which occurs under a wide variety of situations ranging from embryonic development to tumor growth. In general, angiogenesis is dependent on the accumulation of hypoxia inducible factors (HIFs), which are heterodimeric transcription factors of alpha and beta subunits. Under normoxia, HIF heterodimers are not abundantly present due to oxygen dependent hydroxylation, polyubiquitination, and proteasomal degradation of alpha subunits. Under hypoxia, however, alpha subunits are stabilized and form heterodimers with HIF-1beta which is not subject to oxygen dependent regulation. The accumulation of HIFs under hypoxia allows them to activate the expression of many angiogenic genes and therefore initiates the angiogenic process. In recent years, however, it has become clear that various other mechanisms also participate in fine tuning angiogenesis. In this review, I discuss the relationship between hypoxia and angiogenesis under five topics: (1) regulation of HIF-alpha abundance and activity by oxygen tension and other conditions including oxygen independent mechanisms; (2) hypoxia-regulated expression of angiogenic molecules by HIFs and other transcription factors; (3) responses of vascular cells to hypoxia; (4) angiogenic phenotypes due to altered HIF signaling in mice; and (5) role of the HIF pathway in pathological angiogenesis. Studies discussed under these topics clearly indicate that while mechanisms of oxygen-regulated HIF-alpha stability provide exciting opportunities for the development of angiogenesis or anti-angiogenesis therapies, it is also highly important to consider various other mechanisms for the optimization of these procedures.

Hyperbaric oxygen results in increased vascular endothelial growth factor (VEGF) protein expression in rabbit calvarial critical-sized defects

BACKGROUND

Hyperbaric oxygen therapy (HBO) promotes osseous healing, however the mechanism by which this occurs has not been elucidated. HBO may promote angiogenesis, which is vital for bone healing. Vascular endothelial growth factor (VEGF) is one of the key factors that stimulates angiogenesis.

OBJECTIVE

The objective of this study was to investigate whether HBO altered VEGF expression during bone healing.

METHODS AND MATERIALS

Archived samples from calvarial defects of rabbits exposed to HBO (2.4 ATA, 90 minutes a day, 5 days a week for 4 weeks) and normobaric oxygen controls (NBO) were analyzed by immunohistochemistry.

RESULTS

VEGF expression in 6-week HBO samples was elevated compared to NBO (P = .012). Staining of the 12-week HBO samples was reduced compared to 6-week HBO (P = .008) and was similar to 6- and 12-week NBO control samples.

CONCLUSION

HBO therapy resulted in increased VEGF expression in the defects even 2 weeks after the termination of treatment (6 weeks postsurgery).

The effect of hypoxia in development

There is increasing evidence that the oxygen supply to the human embryo in the first trimester is tightly controlled, suggesting that too much oxygen may interfere with development. The use of hypoxia probes in mammalian embryos during the organogenic period indicates that the embryo is normally in a state of partial hypoxia, and this may be essential to control cardiovascular development, perhaps under the control of hypoxia-inducible factor (HIF). A consequence of this state of partial hypoxia is that disturbances in the oxygen supply can more easily lead to a damaging degree of hypoxia. Experimental mammalian embryos show a surprising degree of resilience to hypoxia, with many organogenic stage embryos able to survive 30-60 min of anoxia. However, in some embryos this degree of hypoxia causes abnormal development, particularly transverse limb reduction defects. These abnormalities are preceded by hemorrhage/edema and tissue necrosis. Other parts of the embryo are also susceptible to this hypoxia-induced damage and include the genital tubercle, the developing nose, the tail, and the central nervous system. Other frequently observed defects in animal models of prenatal hypoxia include cleft lip, maxillary hypoplasia, and heart defects. Animal studies indicate that hypoxic episodes in the first trimester of human pregnancy could occur by temporary constriction of the uterine arteries. This could be a consequence of exposure to cocaine, misoprostol, or severe shock, and there is evidence that these exposures have resulted in hypoxia-related malformations in the human. Exposure to drugs that block the potassium current (IKr) can cause severe slowing and arrhythmia of the mammalian embryonic heart and consequently hypoxia in the embryo. These drugs are highly teratogenic in experimental animals. There is evidence that drugs with IKr blockade as a side effect, for example phenytoin, may cause birth defects in the human by causing periods of embryonic hypoxia. The strongest evidence of hypoxia causing birth defects in the human comes from studies of fetuses lacking hemoglobin (Hb) F. These fetuses are thought to be hypoxic from about the middle of the first trimester and show a range of birth defects, particularly transverse limb reduction defects.

Possible roles of neuropeptide Y Y3-receptor subtype in rat aortic endothelial cell proliferation under hypoxia, and its specific signal transduction

The present study was undertaken to determine whether neuropeptide Y (NPY) induces proliferation of rat aortic endothelial cells (RAECs). Since NPY increased the permeability of RAEC monolayers to large molecules via the NPY Y3 receptor, RAEC proliferation has been evaluated in terms of NPY-receptor subtypes and also intracellular mechanisms. RAECs were incubated with gases containing 20, 15, or 10% O2 and a certain amount of N2, depending on the O2 content in 5% CO2 incubators. NPY (10−9–10−6 M) increased the RAEC numbers under hypoxic conditions, such as 15 or 10% O2. Peptide YY elicited no proliferative effect on RAEC, and NPY-(18-36) inhibited the NPY-induced increase in cell number, suggesting that NPY increases the RAEC count through the NPY Y3 receptor. Pertussis toxin, U-73122, GF-109203X, myristorylated autocamtide-2-related inhibitory peptide, and wortmannin inhibited the NPY-induced proliferation of RAEC concentration dependently. DY9760e little affected the proliferation caused by NPY. ML-9 and imatinib actually enhanced the NPY-induced proliferation of cells. These results indicated that the NPY Y3 receptor is coupled with Gi protein, and that NPY-induced increases in RAEC proliferation are mediated by phospholipase C-protein kinase C and/or phosphatidylinositol 3-kinase pathways. In intracellular Ca2+-calmodulin-dependent pathways, calmodulin-dependent protein kinase II partly participates in the NPY-induced cell proliferation. Regarding the previously reported effect of NPY on the permeability of RAEC monolayers to large molecules, it is probable that protein kinase C and phosphatidylinositol 3-kinase pathways are activated for both permeability and cell proliferation induced by NPY under hypoxia, relevant to new insights into the roles of NPY in ischemia-hypoxia.

Tubular sprouting as a mode of vascular formation in a colonial ascidian (Tunicata)

Although phylogenetically related to vertebrates, invertebrate chordate tunicates possess an open circulatory system, with blood flowing in lacunae among organs. However, the colonial circulatory system (CCS) of the ascidian Botryllus schlosseri runs in the common tunic and forms an anastomized network of vessels, defined by simple epithelium, connected to the open circulatory system of the zooids. The CCS originates from epidermal evagination, grows, and increases its network accompanying colony propagation. New vessels are formed by means of mechanisms of tubular sprouting which, in their morphogenesis and molecular regulation, are very similar to those occurring in other metazoans, particularly during vertebrate angiogenesis. From the apex of new vessels, epithelial cells detach and migrate into the tunic, while exploring filopodia extend toward the tunic and possibly guide vessel growth. Immunohistology showed that growth factors fibroblast growth factor-2 and vascular endothelial growth factor and the receptor vascular endothelial growth factor receptor-1 participate in sprouting, associated with cell proliferation. As in vertebrates, these factors may regulate cell migration, proliferation, sprouting, and tube formation. Our data indicate that similar, conserved signals were co-opted in the sprouting processes of two nonhomologous circulatory systems, that of ascidian CCS, and vertebrate circulatory systems, by recruitment of the same signaling pathway.

Expression and hypoxic regulation of hif1alpha and hif2alpha during early blood and endothelial cell differentiation in chick

HIF1 and HIF2 are major mediators for hypoxia sensing and response. Their roles in early differentiation of two key cell types involved in oxygen supply in amniotes, the primitive blood cells and endothelial cells, are unclear. We show that, in pre-circulation avian embryos, hif1alpha and hif2alpha are expressed in embryonic and extraembryonic tissues, respectively. hif2alpha, first identified as epas1, is not present in endothelial cells at any pre-circulation stage under either normoxia or hypoxia conditions. Differentiating blood cells express low levels of hif2alpha under normoxia, but show a strong and rapid upregulation under hypoxia. Blood cell differentiation, however, is not affected under either hypoxia or hyperoxia conditions.

Vascular response to hypoxic preconditioning in the immature brain

We hypothesized that hypoxic preconditioning (PC) modifies the microvasculature in the immature brain and thereby affects the cerebral blood flow (CBF) during a subsequent hypoxic-ischemic (HI) insult. On postnatal day 6 rats were exposed to hypoxia (36 degrees C, 8.0% O2) or normoxia for 3 h. Unilateral HI (unilateral carotid ligation and 8% hypoxia) was induced 24 h later. Cortical CBF was measured with the 14C-iodoantipyrine technique (at the end of HI) or with laser Doppler flowmetry (Perimed PF5001) before and during HI. At 0, 2, 8, and 24 h cerebral cortex was sampled and analyzed with gene arrays (Affymetrix 230 2.0). L-nitroarginine or vehicle was administrated before hypoxic PC or 30 mins before HI followed by CBF measurement (laser Doppler) during subsequent HI. Twenty-four hours after PC animals were perfusion-fixed and brains immunolabeled for von Willebrand factor and vascular density was determined by stereological quantification. The decrease in CBF during HI was attenuated significantly in PC versus control animals (P<0.01), as detected by both techniques. Several vascular genes (Angpt2, Adm, Apln, Vegf, Flt1, Kdr, Pdgfra, Agtrap, Adora2a, Ednra, serpine1, caveolin, Id1, Prrx1, Ero1l, Acvrl1, Egfl7, Nudt6, Angptl4, Anxa2, and NOS3) were upregulated and a few (Csrp2, Adora2b) were downregulated after PC. A significant increase in vascular density (P<0.05) was seen after PC. Nitric oxide synthase inhibition did not affect CBF during HI after PC. In conclusion, hypoxic PC upregulates vascular genes, increases vascular density and attenuates the decrease of CBF during a subsequent HI, which could contribute to tolerance.

Bisphenol A causes malformation of the head region in embryos of Xenopus laevis and decreases the expression of the ESR-1 gene mediated by Notch signaling

Bisphenol A (BpA) is widely used in industry and dentistry. Its effects on the embryonic development of Xenopus laevis were investigated. Xenopus embryos at stage 10.5 were treated with BpA. Developmental abnormalities were observed at stage 35; malformation of the head region including eyes and scoliosis. The expression of several markers of embryonic development was investigated by reverse transcription-polymerase chain reaction (RT-PCR). The pan-neural marker SOX-2, the neural stem cell marker nrp-1, the mesodermal marker MyoD, and the endodermal marker sox17alpha, were used. Although the expression of marker genes was not changed by treatment with BpA, that of Pax-6, a key regulator of the morphogenesis of the eyes, was decreased by BpA. Pax-6 is a downstream factor of Notch signaling. So, the expression of a typical Notch-dependent factor, ESR-1, was investigated in the presence of BpA. The expression of ESR-1 was efficiently suppressed by BpA. In whole mount in situ hybridization (WISH), Pax-6 was expressed in the central nervous system and eyes. The expression was lost completely on treatment with BpA. The expression of ESR-1 in the central nervous system and eyes also disappeared with BpA treatment. Injection of the intracellular domain of Notch efficiently recovered ESR-1 expression in the presence of BpA although injection of a ligand for notch, Delta, did not. These results suggest that BpA decreased the expression of ESR-1 by disrupting the Notch signal.

Vascular Endothelial Growth Factor Mediates Hypoxic Stimulated Embryonic Bladder Growth in Organ Culture

PURPOSE

Tissue hypoxia enhances embryonic angiogenesis at least in part by up-regulating vascular endothelial growth factor. Additionally, exogenous vascular endothelial growth factor-A enhances embryonic bladder explant growth. We hypothesized that developing bladders are hypoxic in vivo and oxygen tensions modulate explanted bladder growth by altering vascular endothelial growth factor-A expression.

MATERIALS AND METHODS

Embryonic day 14 mouse bladders were cultured in 20% O(2) or 3% O(2) atmospheres. Some cultures were supplemented with a vascular endothelial growth factor receptor 1/Fc chimera to block vascular endothelial growth factor bioactivity. After 6 days explant areas, DNA, protein, total cell numbers, and proportions expressing endothelial and smooth muscle markers were measured. Pimonidazole was administered to pregnant mice and hypoxia was sought in embryonic tissues by immunohistochemistry.

RESULTS

In vivo pimonidazole adducts and vascular endothelial growth factor-A immunolocalized to embryonic urothelium and bladders up-regulated total vascular endothelial growth factor-A between embryonic days 14 and 18. All growth parameters and vascular endothelial growth factor-A protein levels were enhanced in hypoxic vs normoxic culture. Addition of vascular endothelial growth factor receptor 1/Fc prevented this accelerated growth.

CONCLUSIONS

In vivo embryonic bladders are hypoxic and express vascular endothelial growth factor-A. In vitro, when oxygen tensions are manipulated, vascular endothelial growth factor-A protein positively correlates with the growth of whole explants as well as endothelium. Normal embryonic bladder development may be driven at least in part by hypoxic up-regulation of vascular endothelial growth factor-A.