Transgenic expression of Angiopoietin 1 in the liver leads to changes in lymphatic and blood vessel architecture

Tie2-dependent neovascularization of the ischemic hindlimb is mediated by angiopoietin-2

The angiopoietins (ANGPT) are ligands for the endothelial cell (EC) receptor tyrosine kinase, Tie2. Angpt-1 is a Tie2 agonist that promotes vascular maturation and stabilization, whereas Angpt-2 is a partial agonist/antagonist involved in the initiation of postnatal angiogenesis. Therefore, we hypothesized that overexpression of Angpt-2 would be more effective than Angpt-1 for enhancing the perfusion recovery in the ischemic hindlimb. Perfusion recovery was markedly impaired in Tie2-deficient animals at day 35 in a model of chronic hindlimb ischemia. Injections of Angpt-2 or VEGFA plasmid at 7 days post femoral artery resection enhanced recovery and improved arteriogenesis as assessed by angiographic scores, whereas Angpt-1 or null plasmid had no effect. In addition, Angpt-2 together with VEGF resulted in greater improvement in perfusion and collateral vessel formation than VEGF alone. Similarly, conditional overexpression of Angpt-2 in mice improved ischemic limb blood flow recovery, while Angpt-1 overexpression was ineffective. These data from Tie2 heterozygote deficient mice demonstrate, for the first time, the importance of the Tie2 pathway in spontaneous neovascularization in response to chronic hindlimb ischemia. Moreover, they show that overexpression of the partial agonist, Angpt-2, but not Angpt-1, enhanced ischemic hind limb perfusion recovery and collateralization, suggesting that a coordinated sequence antagonist and agonist activity is required for effective therapeutic revascularization.

How normal is the liver in which the inflammatory type hepatocellular adenoma develops?

The inflammatory type hepatocellular adenoma (IHCA) is a subtype of HCA which is a benign liver tumor, predominantly occurring in young women in an otherwise normal liver. IHCA contains either a mutation of gp130 or STAT3. Both mutations lead to a similar morphologic phenotype and to increased expression of C-reactive protein (CRP) and/or serum amyloid-A (SAA). IHCA comprised about 40% of all HCAs and is associated with obesity. We investigated the histomorphological and immunophenotypical changes of the nontumorous liver of 32 resected IHCA specimens. Similar types of changes are present in samples taken adjacent to tumor and distant ones. The lobular architecture is well preserved. Mild/moderate steatosis is found in a high frequency which is in accordance with the median BMI of 32 in our cases. Of note are the regular findings of sinusoidal dilatation, single arteries, and minute CRP foci which are all features of HCA. These distinct CRP foci are mostly found in cases of multiple IHCA which indicates that the remnant liver may also contain IHCA foci. These findings show that the nonlesional liver in IHCA does contain abnormalities, and this may have consequences for the followup, especially since it is known that obesity may stimulate malignant growth.

Enhanced expression of VEGF-A in β cells increases endothelial cell number but impairs islet morphogenesis and β cell proliferation

There is a reciprocal interaction between pancreatic islet cells and vascular endothelial cells (EC) in which EC-derived signals promote islet cell differentiation and islet development while islet cell-derived angiogenic factors promote EC recruitment and extensive islet vascularization. To examine the role of angiogenic factors in the coordinated development of islets and their associated vessels, we used a "tet-on" inducible system (mice expressing rat insulin promoter-reverse tetracycline activator transgene and a tet-operon-angiogenic factor transgene) to increase the β cell production of vascular endothelial growth factor-A (VEGF-A), angiopoietin-1 (Ang1), or angiopoietin-2 (Ang2) during islet cell differentiation and islet development. In VEGF-A overexpressing embryos, ECs began to accumulate around epithelial tubes residing in the central region of the developing pancreas (associated with endocrine cells) as early as embryonic day 12.5 (E12.5) and increased dramatically by E16.5. While α and β cells formed islet cell clusters in control embryos at E16.5, the increased EC population perturbed endocrine cell differentiation and islet cell clustering in VEGF-A overexpressing embryos. With continued overexpression of VEGF-A, α and β cells became scattered, remained adjacent to ductal structures, and never coalesced into islets, resulting in a reduction in β cell proliferation and β cell mass at postnatal day 1. A similar impact on islet morphology was observed when VEGF-A was overexpressed in β cells during the postnatal period. In contrast, increased expression of Ang1 or Ang2 in β cells in developing or adult islets did not alter islet differentiation, development, or morphology, but altered islet EC ultrastructure. These data indicate that (1) increased EC number does not promote, but actually impairs β cell proliferation and islet formation; (2) the level of VEGF-A production by islet endocrine cells is critical for islet vascularization during development and postnatally; (3) angiopoietin-Tie2 signaling in endothelial cells does not have a crucial role in the development or maintenance of islet vascularization.

Molecular characterization of the vascular features of focal nodular hyperplasia and hepatocellular adenoma: a role for angiopoietin-1

Focal nodular hyperplasia (FNH) and hepatocellular adenoma (HCA) are two hepatic nodular lesions of different etiologies. FNH, a polyclonal lesion, is assumed to be a regenerative reaction following a vascular injury, whereas HCA is a monoclonal, benign neoplastic lesion. In addition to features that are predominantly found in either FNH or HCA (e.g., dystrophic vessels in FNH and single arteries in HCA), FNH and HCA share morphological vascular abnormalities such as dilated sinusoids. We hypothesized that these anomalous vascular features are associated with altered expression of growth factors involved in vascular remodeling. This was based on reports of morphologically abnormal hepatic vasculature and nodular lesions in transgenic models of hepatocytic overexpression of angiopoietin-1 (Ang-1), a member of the angiopoietin family, which is crucially involved in vascular morphogenesis and homeostasis. We investigated gene and protein expression of members of the angiopoietin system and vascular endothelial growth factor A (VEGF-A) and its receptors in 9 FNH samples, 13 HCA samples, and 9 histologically normal livers. In comparison with normal samples, a significant increase in Ang-1 was found in FNH (P < 0.01) and HCA (P < 0.05), whereas no significant changes in Ang-2, receptor tyrosine kinase with immunoglobulin-like and EGF-like domains 2, VEGF-A, or vascular endothelial growth factor receptor 2 (VEGFR-2) were observed.

CONCLUSION

Because of the different etiological contexts of a preceding vascular injury in FNH and a neoplastic growth in HCA, Ang-1 might exert different effects on the vasculature in these lesions. In FNH, it could predominantly stimulate recruitment of myofibroblasts and result in dystrophic vessels, whereas in HCA, it may drive vascular remodeling that produces enlarged vessels and arterial sprouting that generates single arteries.

Osteoblast-specific Angiopoietin 1 overexpression increases bone mass

Although osteoblasts express the angiogenic protein Angiopoietin 1 (Ang1), the role of Ang1 in bone formation remains largely unknown. Here we report that Ang1 overexpression in osteoblasts driven by the osteoblast-specific 2.3 kb alpha 1 type 1 collagen promoter results in increased bone mass in vivo. In Ang1-transgenic mice (Ang1-Tg), bone volume and bone parameters increased significantly compared with wild-type littermates, although the Ang1 receptor, Tie2 was not expressed in osteoblasts. Tie2 is primarily expressed in vascular endothelial cells, and Ang1-Tie2 signaling is reportedly crucial for angiogenesis. We found that the number of vascular endothelial cells was significantly elevated in Ang1-Tg mice compared with that of wild-type littermates, an increase accompanied by increased alkaline-phosphatase activity, a marker of osteoblast activation. The number of osteoclasts in the bone of Ang1-Tg mice did not differ from wild-type littermates. These results indicate that angiogenesis induced by Ang1 expressed in osteoblasts is coupled with osteogenesis.