Okadaic acid is a potent angiogenesis inducer

Pharmacological control of angiogenesis by regulating phosphorylation of myosin light chain 2

BACKGROUND

Control of angiogenesis is widely considered a therapeutic strategy, but reliable control methods are still under development. Phosphorylation of myosin light chain 2 (MLC2), which regulates actin-myosin interaction, is critical to the behavior of vascular endothelial cells (ECs) during angiogenesis. MLC2 is phosphorylated by MLC kinase (MLCK) and dephosphorylated by MLC phosphatase (MLCP) containing a catalytic subunit PP1. We investigated the potential role of MLC2 in the pharmacological control of angiogenesis.

METHODS AND RESULTS

We exposed transgenic zebrafish Tg(fli1a:Myr-mCherry)ncv1 embryos to chemical inhibitors and observed vascular development. PP1 inhibition by tautomycetin increased length of intersegmental vessels (ISVs), whereas MLCK inhibition by ML7 decreased it; these effects were not accompanied by structural dysplasia. ROCK inhibition by Y-27632 also decreased vessel length. An in vitro angiogenesis model of human umbilical vein endothelial cells (HUVECs) showed that tautomycetin increased vascular cord formation, whereas ML7 and Y-27632 decreased it. These effects appear to be influenced by regulation of cell morphology rather than cell viability or motility. Actin co-localized with phosphorylated MLC2 (pMLC2) was abundant in vascular-like elongated-shaped ECs, but poor in non-elongated ECs. pMLC2 was associated with tightly arranged actin, but not with loosely arranged actin. Moreover, knockdown of MYL9 gene encoding MLC2 reduced total MLC2 and pMLC2 protein and inhibited angiogenesis in HUVECs.

CONCLUSION

The present study found that MLC2 is a pivotal regulator of angiogenesis. MLC2 phosphorylation may be involved in the regulation of of cell morphogenesis and cell elongation. The functionally opposite inhibitors positively or negatively control angiogenesis, probably through the regulating EC morphology. These findings may provide a unique therapeutic target for angiogenesis.

FOXO1 promotes endothelial cell elongation and angiogenesis by up-regulating the phosphorylation of myosin light chain 2

The forkhead box O1 (FOXO1) is an important transcription factor related to proliferation, metabolism, and homeostasis, while the major phenotype of FOXO1-null mice is abnormal vascular morphology, such as vessel enlargement and dilation. In in vitro mouse embryonic stem cell (ESC)-differentiation system, Foxo1-/- vascular endothelial cells (ECs) fail to elongate, and mimic the abnormalities of FOXO1-deficiency in vivo. Here, we identified the PPP1R14C gene as the FOXO1 target genes responsible for elongating using transcriptome analyses in ESC-derived ECs (ESC-ECs), and found that the FOXO1-PPP1R14C-myosin light chain 2 (MLC2) axis is required for EC elongation during angiogenesis. MLC2 is phosphorylated by MLC kinase (MLCK) and dephosphorylated by MLC phosphatase (MLCP). PPP1R14C is an inhibitor of PP1, the catalytic subunit of MLCP. The abnormal morphology of Foxo1-/- ESC-ECs was associated with low level of PPP1R14C and loss of MLC2 phosphorylation, which were reversed by PPP1R14C-introduction. Knockdown of either FOXO1 or PPP1R14C suppressed vascular cord formation and reduced MLC2 phosphorylation in human ECs (HUVECs). The mouse and human PPP1R14C locus possesses an enhancer element containing conserved FOXO1-binding motifs. In vivo chemical inhibition of MLC2 phosphorylation caused dilated vascular structures in mouse embryos. Furthermore, foxo1 or ppp1r14c-knockdown zebrafish exhibited vascular malformations, which were also restored by PPP1R14C-introduction. Mechanistically, FOXO1 suppressed MLCP activity by up-regulating PPP1R14C expression, thereby promoting MLC2 phosphorylation and EC elongation, which are necessary for vascular development. Given the importance of MLC2 phosphorylation in cell morphogenesis, this study may provide novel insights into the role of FOXO1 in control of angiogenesis.

Okadaic acid promotes angiogenesis via activation of hypoxia-inducible factor-1

Okadaic acid, a potent tumor promoter and an inhibitor of protein phosphatase 1 and 2A, has also been characterized as an angiogenic inducer in the chorioallantoic membrane of the chick embryo. To elucidate the roles of okadaic acid on angiogenic processes, we conducted in vitro angiogenesis assays. In this study, we report that okadaic acid potently stimulated tube formation, migration, and invasion of human umbilical vein endothelial cells. Moreover, okadaic acid elevated the activities of hypoxia-inducible factor-1 (HIF-1), which is closely related with the expression of vascular endothelial growth factor. Exposure to okadaic acid markedly increased the HIF-1alpha protein level through up-regulation of translation via activation of Akt and mTOR pathway. Taken together, these results demonstrated that okadaic acid promotes angiogenesis through stimulation of Akt mediated HIF-1alpha translation.

Stereologic evaluation of the vasculature in a MX1 xenotransplanted tumour model after combinations of treatment with ifosfamide, hyperthermia and irradiation

The vascularization of tumours is a critical parameter of their growing and metastatic behaviour. However, little is known about the morphologic reactions of the microvasculature, especially the capillary bed of tumours and the adjacent tissue. In this study, the vessels in MX1 xenotransplants in athymic nu/nu nude mice were quantified and the angioarchitecture was visualized with the aim of presenting stereologic parameters of vessels based on a morphometric analysis of post mortem tissue blocks which were processed by standard histological procedures. In order to study changes of the microvasculature of MX1 tumours, the xenotransplanted nude mice were treated by different therapeutic regimens. Standardized hyperthermia, ifosfamide and irradiation therapies were applied. Special interest was focused on early changes of capillaries and of the pre- as well as post-terminal vascular bed. The stereologic evaluation of capillaries and larger vessels immediately after the therapy with ifosfamide and hyperthermia shows an increase of the mean capillary sizes. Furthermore, tumour samples after the 5th day of irradiation (5 x 2 Gy) and combinations of irradiation and chemotherapy treatment have been investigated. After 5 days of irradiation, a significant decrease of the vascular density was found. The results presented here clearly show that the timing and the mode of therapy influence the capillary morphology and periterminal vasculature of xenotransplanted MX1 tumours.

Angiogenesis: models and modulators

Angiogenesis in vivo is distinguished by four stages: subsequent to the transduction of signals to differentiate, stage 1 is defined as an altered proteolytic balance of the cell allowing it to digest through the surrounding matrix. These committed cells then proliferate (stage 2), and migrate (stage 3) to form aligned cords of cells. The final stage is the development of vessel patency (stage 4), generated by a coalescing of intracellular vacuoles. Subsequently, these structures anastamose and the initial flow of blood through the new vessel completes the process. We present and discuss how the available models most closely represent phases of in vivo angiogenesis. The enhancement of angiogenesis by hyaluronic acid fragments, transforming growth factor beta, tumor necrosis factor alpha, angiogenin, okadaic acid, fibroblast growth factor, interleukin 8, vascular endothelial growth factor, haptoglobin, and gangliosides, and the inhibition of the process by hyaluronic acid, estrogen metabolites, genestein, heparin, cyclosporin A, placental RNase inhibitor, steroids, collagen synthesis inhibitors, thrombospondin, fumagellin, and protamine are also discussed.