Michael J. Cross

FGF and VEGF function in angiogenesis: signalling pathways, biological responses and therapeutic inhibition

Angiogenic growth factors such as fibroblast growth factors (FGFs) and vascular endothelial growth factors (VEGFs) are currently targets of intense efforts to inhibit deregulated blood vessel formation in diseases such as cancer. FGFs and VEGFs exert their effects via specific binding to cell surface-expressed receptors equipped with tyrosine kinase activity. Activation of the receptor kinase activity allows coupling to downstream signal transduction pathways that regulate proliferation, migration and differentiation of endothelial cells. Inhibitors of FGF and VEGF signalling are currently in clinical trials. In this article, the current knowledge of FGF- and VEGF-induced signal transduction that leads to specific biological responses will be summarized. Furthermore, the manner in which this knowledge is being exploited to regulate angiogenesis will be discussed.

VEGF-receptor signal transduction

The family of vascular endothelial growth factors (VEGFs) currently includes VEGF-A, -B, -C, -D, -E, and placenta growth factor (PlGF). Several of these factors, notably VEGF-A, exist as different isoforms, which appear to have unique biological functions. The VEGF family proteins bind in a distinct pattern to three structurally related receptor tyrosine kinases, denoted VEGF receptor-1, -2, and -3. Neuropilins, heparan-sulfated proteoglycans, cadherins, and integrin alphavbeta3 serve as coreceptors for certain but not all VEGF proteins. Moreover, the angiogenic response to VEGF varies between different organs and is dependent on the genetic background of the animal. Inactivation of the genes for VEGF-A and VEGF receptor-2 leads to embryonal death due to the lack of endothelial cells. Inactivation of the gene encoding VEGF receptor-1 leads to an increased number of endothelial cells, which obstruct the vessel lumen. Inactivation of VEGF receptor-3 leads to abnormally organized vessels and cardiac failure. Although VEGF receptor-3 normally is expressed only on lymphatic endothelial cells, it is up-regulated on vascular as well as nonvascular tumors and appears to be involved in the regulation of angiogenesis. A large body of data, such as those on gene inactivation, indicate that VEGF receptor-1 exerts a negative regulatory effect on VEGF receptor-2, at least during embryogenesis. Recent data imply a positive regulatory role for VEGF receptor-1 in pathological angiogenesis. The VEGF proteins are in general poor mitogens, but binding of VEGF-A to VEGF receptor-2 leads to survival, migration, and differentiation of endothelial cells and mediation of vascular permeability. This review outlines the current knowledge about the signal transduction properties of VEGF receptors, with focus on VEGF receptor-2.

The Adaptor Protein Shb Binds to Tyrosine 1175 in Vascular Endothelial Growth Factor (VEGF) Receptor-2 and Regulates VEGF-dependent Cellular Migration

Previous studies have shown that the adaptor protein Shb is involved in receptor tyrosine kinase signaling. In this study, we demonstrate that Shb is phosphorylated in an Src-dependent manner upon vascular endothelial growth factor (VEGF) stimulation using porcine aortic endothelial cells expressing the human VEGF receptor 2 (VEGFR-2) (KDR). In co-immunoprecipitation experiments, we could detect an interaction between Shb and the VEGFR-2 in human telomerase-immortalized microvascular endothelial cells. Furthermore, in a glutathione S-transferase pull-down assay, the Src homology 2 domain of Shb was shown to interact with phosphorylated tyrosine 1175 in the C-terminal tail of VEGFR-2. VEGF-induced Shb phosphorylation was lost in porcine aortic endothelial cells expressing a chimeric murine VEGFR-2 (Flk-1) with a mutation at the corresponding position. Shb expression was specifically decreased by 80%, in a transient manner, by using the short interfering RNA technique. Reduced Shb expression led to a loss of stimulation of phosphatidylinositol 3-kinase, phosphorylation of focal adhesion kinase at tyrosine 576, the generation of focal adhesions, and stress fiber formation in response to VEGF. Furthermore, we show that VEGF-induced migration is inhibited in Shb short interfering RNA-treated cells. Our data demonstrate that Shb is important for VEGF signaling in endothelial cells. This is achieved by Shb binding to tyrosine 1175 in the VEGFR-2, which regulates VEGF-induced formation of focal adhesions and cell migration, of which the latter occurs in a phosphatidylinositol 3-kinase-dependent manner.

How can we improve our understanding of cardiovascular safety liabilities to develop safer medicines

Given that cardiovascular safety liabilities remain a major cause of drug attrition during preclinical and clinical development, adverse drug reactions, and post‐approval withdrawal of medicines, the Medical Research Council Centre for Drug Safety Science hosted a workshop to discuss current challenges in determining, understanding and addressing ‘Cardiovascular Toxicity of Medicines’. This article summarizes the key discussions from the workshop that aimed to address three major questions: (i) what are the key cardiovascular safety liabilities in drug discovery, drug development and clinical practice? (ii) how good are preclinical and clinical strategies for detecting cardiovascular liabilities? and (iii) do we have a mechanistic understanding of these liabilities? It was concluded that in order to understand, address and ultimately reduce cardiovascular safety liabilities of new therapeutic agents there is an urgent need to:

Angiostatin and endostatin inhibit endothelial cell migration in response to FGF and VEGF without interfering with specific intracellular signal transduction pathways.

The anti‐angiogenic agents angiostatin and endostatin have been shown to affect endothelial cell migration in a number of studies. We have examined the effect of these agents on intracellular signalling pathways known to regulate endothelial cell migration and proliferation/survival. Both agents inhibited fibroblast growth factor (FGF)‐, and vascular endothelial growth factor (VEGF)‐mediated migration of primary human microvascular endothelial cells and affected vascular formation in the embryoid body model. However, using phosphospecific antibodies we could not detect any effect of angiostatin or endostatin on phospholipase C‐γ (PLC‐γ), Akt/PKB, p44/42 mitogen‐activated protein kinase (MAPK), p38 MAPK and p21‐activated kinase (PAK) activity. Furthermore, using a glutathione S‐transferase (GST)‐PAK pull‐down assay, we could not detect any effect on Rac activity. We conclude that angiostatin and endostatin inhibit chemotaxis, without affecting intracellular signalling pathways known to regulate endothelial migration and proliferation/survival.

ERK5: structure, regulation and function.

Extracellular signal-regulated kinase 5 (ERK5), also termed big mitogen-activated protein kinase-1 (BMK1), is the most recently identified member of the mitogen-activated protein kinase (MAPK) family and consists of an amino-terminal kinase domain, with a relatively large carboxy-terminal of unique structure and function that makes it distinct from other MAPK members. It is ubiquitously expressed in numerous tissues and is activated by a variety of extracellular stimuli, such as cellular stresses and growth factors, to regulate processes such as cell proliferation and differentiation. Targeted deletion of Erk5 in mice has revealed that the ERK5 signalling cascade plays a critical role in cardiovascular development and vascular integrity. Recent data points to a potential role in pathological conditions such as cancer and tumour angiogenesis. This review focuses on the physiological and pathological role of ERK5, the regulation of this kinase and the recent development of small molecule inhibitors of the ERK5 signalling cascade.

Transcriptional profiling reveals a critical role for tyrosine phosphatase VE-PTP in regulation of VEGFR2 activity and endothelial cell morphogenesis

To define molecular events accompanying formation of the 3‐dimensional (3D) vascular tube, we have characterized gene expression during vascular endothelial growth factor (VEGF)‐induced tubular morphogenesis of endothelial cells. Microarray analyses were performed comparing gene induction in growth‐ arrested, tube‐forming endothelial cells harvested from 3D collagen cultures to that in proliferating endothelial cells cultured on fibronectin. Differentially expressed genes were clustered and analyzed for specific endothelial expression through publicly available datasets. We validated the contribution of one of the identified genes, vascular endothelial protein tyrosine phosphatase (VE‐PTP), to endothelial morphogenesis. Silencing of VE‐PTP expression was accompanied by increased VEGFreceptor‐2 (VEGFR2) tyrosine phosphorylation and activation of downstream signaling pathways. The increased VEGFR2 activity promoted endothelial cell cycle progression, overcoming the G0/G1 arrest associated with organization into tubular structures in the 3D cultures. Proximity ligation showed close association between VEGFR2 and VE‐PTP in resting cells. Activation of VEGFR2 by VEGF led to rapid loss of association, which was resumed with time in parallel with decreased receptor activity. In conclusion, we have identified genes, which may serve critical functions in formation of the vascular tube. One of these, VE‐PTP, regulates VEGFR2 activity thereby modulating the VEGF‐response during angiogenesis.— Mellberg, S., Dimberg, A., Bahram, F., Hayashi, M., Rennel, E., Ameur, A., Westholm, J. O., Larsson, E., Lindahl, P., Cross, M. J., Claesson‐Welsh, L. Transcriptional profiling reveals a critical role for tyrosine phosphatase VE‐PTP in regulation of VEGFR2 activity and endothelial cell morphogenesis. FASEBJ. 23, 1490–1502 (2009)

ERK5 and the regulation of endothelial cell function

ERK5 (extracellular-signal-regulated kinase 5), also termed BMK1 [big MAPK1 (mitogen-activated protein kinase 1)], is the most recently discovered member of the MAPK family. It is expressed in a variety of tissues and is activated by a range of growth factors, cytokines and cellular stresses. Targeted deletion of Erk5 in mice has revealed that the ERK5 signalling cascade is critical for normal cardiovascular development and vascular integrity. In vitro studies have revealed that in endothelial cells, ERK5 is required for preventing apoptosis, mediating shear-stress signalling, regulating hypoxia, tumour angiogenesis and cell migration. This review focuses on our current understanding of the role of ERK5 in regulating endothelial cell function.

Brusatol provokes a rapid and transient inhibition of Nrf2 signaling and sensitizes mammalian cells to chemical toxicity-implications for therapeutic targeting of Nrf2.

The transcription factor Nrf2 regulates the basal and inducible expression of a battery of cytoprotective genes. Whereas numerous Nrf2-inducing small molecules have been reported, very few chemical inhibitors of Nrf2 have been identified to date. The quassinoid brusatol has recently been shown to inhibit Nrf2 and ameliorate chemoresistance in vitro and in vivo. Here, we show that brusatol provokes a rapid and transient depletion of Nrf2 protein, through a posttranscriptional mechanism, in mouse Hepa-1c1c7 hepatoma cells. Importantly, brusatol also inhibits Nrf2 in freshly isolated primary human hepatocytes. In keeping with its ability to inhibit Nrf2 signaling, brusatol sensitizes Hepa-1c1c7 cells to chemical stress provoked by 2,4-dinitrochlorobenzene, iodoacetamide, and N-acetyl-p-benzoquinone imine, the hepatotoxic metabolite of acetaminophen. The inhibitory effect of brusatol toward Nrf2 is shown to be independent of its repressor Keap1, the proteasomal and autophagic protein degradation systems, and protein kinase signaling pathways that are known to modulate Nrf2 activity, implying the involvement of a novel means of Nrf2 regulation. These findings substantiate brusatol as a useful experimental tool for the inhibition of Nrf2 signaling and highlight the potential for therapeutic inhibition of Nrf2 to alter the risk of adverse events by reducing the capacity of nontarget cells to buffer against chemical and oxidative insults. These data will inform a rational assessment of the risk:benefit ratio of inhibiting Nrf2 in relevant therapeutic contexts, which is essential if compounds such as brusatol are to be developed into efficacious and safe drugs.

Endostatin action and intracellular signaling: β-catenin as a potential target?

Abstract Endostatin, the C-terminal part of collagen XVIII, has been shown to inhibit blood vessel formation in different pathological conditions characterized by increased angiogenesis, such as growing tumors. Subcutaneous injection of endostatin in tumor-bearing mice leads to decreased tumor growth, and even in some cases, cure of tumor disease. Endostatin has been tested in a clinical phase I study and shown not to be toxic. Whether the finding in mice that endostatin treatment does not result in development of resistance will hold true in humans is too early to tell. Endostatin binds to a specific motif in heparan sulfate, which may serve a co-receptor function. The structure of a potential primary receptor is not known. The mechanism of action of endostatin in inhibition of angiogenesis and thereby, inhibition of tumor growth, involves apoptosis of tumor cells. The most consistent effect of endostatin on endothelial cells in vitro is inhibition of endothelial cell migration, which may be due to disturbed cell–matrix interactions. An interesting candidate for transducing endostatins effect on apoptosis and cell migration is β-catenin, an intracellular protein which participates both in cell adhesion and in transcriptional regulation.