VEGF-B: a thing of beauty RESEARCH HIGHLIGHT Cell Research (2010) 20:741-744. © 2010 IBCB, SIBS, CAS All rights reserved 1001-0602/10 $ 32.00 www.nature.com/cr npg Correspondence: Xuri Li E-mail: lixur@nei.nih.gov VEGF-B: a thing of beauty Xuri Li1 1National Institutes of Health/National Eye Institute, Rockville, 20852, Maryland, USA Cell Research (2010) 20:741-744. doi:10.1038/cr.2010.77; published online 8 June 2010 More than a decade ago, when we first embarked on our journey to delin- eate the biological function of vascular endothelial growth factor B (VEGF-B), we had a hard time comprehending why VEGF-B was needed. In mice, genetic deletion of VEGF-B seemed to be harm- less, since the VEGF-B null mice, to a large extent, can still live a fairly normal life [1]. Moreover, overexpression of VEGF-B in different mouse tissues, such as the skin or skeletal muscle, did not seem to result in any obvious pheno- type [2]. Due to these seemingly come- to-nothing findings, many researchers lost their scientific interests in VEGF-B. However, new discoveries on VEGF-B function have recently begun to surprise us – the latest one being that VEGF-B plays an important role in modulating fat utilization. Deeper scrutiny dem- onstrated that VEGF-B deficient mice display greater amount of body fat and weight due to impaired fatty acid (FA) uptake by the endothelium, as shown elegantly in a recent study from Dr Ulf Eriksson’s laboratory [3]. VEGF-B was discovered in 1996 as a VEGF homologue [4]. VEGF-B is produced as a secreted homodimer. Due to alternative splicing, the VEGF-B gene gives rise to two isoforms, VEGF-B167 and VEGF-B186, which are homodimers of about 42 and 60 kDa, respectively. VEGF-B186 can be proteolytically pro- cessed at Arg127 and give rise to a 34 kDa dimer. VEGF-B167 has a heparin- binding domain, so that upon secre- tion, VEGF-B167 binds to cell-surface heparan sulphate proteoglycans. By contrast, VEGF-B186 does not contain the heparin-binding domain and there- fore is more soluble. VEGF-B binds to vascular endothelial growth factor receptor-1 (VEGFR-1) and neuropilin-1 (NRP-1) [5, 6]. VEGF-B is expressed early during fetal development in mice, and remains abundantly expressed in most tissues and organs in adult mice, especially in the cardiac myocytes, skeletal muscles and neuronal tissues [7]. VEGF-B167 is the predominant isoform expressed in most tissues and organs, accounting for more than 80% of the total VEGF-B transcripts, while VEGF-B186 is expressed at lower levels and in a limited number of tissues [7]. For many years, research efforts on VEGF-B have focused on its speculated angiogenic activities, based on its high sequence homology and similar receptor binding pattern to VEGF, a prototype angiogenic factor. However, studies along this line, most of the time, led to inconsistent results [8]. Compared with the other VEGF family members, VEGF-B has received much less atten- tion thus far. Recent years have witnessed several advances in VEGF-B biology. First, dif- ferent groups have shown that VEGF-B is a potent neuroprotective factor [8-10]. Second, it is recently recognized that VEGF-B has an ischemic myocardium- specific angiogenic activity while being minimally angiogenic in most of the other organs [2, 11, 12]. Compared with these findings, the more recent discovery by Dr Eriksson’ group, per- haps is more unexpected and striking. In this study, Hagberg et al. [3] showed that VEGF-B is a critical regulator of energy metabolism by regulating fatty acid uptake. In their recent study, Hagberg et al. [3] provided several lines of evidence at different levels to show that VEGF-B has a unique and critical role in regu- lating fatty acid transportation. First, the authors conducted bioinformatic analysis of published microarray data and found that VEGF-B expression was closely associated with the expression of nuclear-encoded mitochondrial genes under different conditions in mice. This association appears to be specific to VEGF-B, since the other VEGF family members, such as VEGF and PlGF, do not display the same type of association in their expression. These observations at gene expression level thus pointed to a potential role of VEGF-B in energy metabolism. The authors then went on and verified the significance of the above observations using cultured cells, and found that, in endothelial cells, VEGF-B stimulation upregulated the expression of the fatty acid transport proteins (FATPs), which are a family of proteins needed for fatty acid transpor- tation across the endothelium. Indeed, in a two-liquid-compartment endothelial cell culture assay, VEGF-B treatment increased trans-endothelial transfer of 14C-labelled oleic acid from the up- per to the basal liquid compartment, Cell Research | Vol 20 No 7 | July 2010 742 npg Figure 1 A unique role of VEGF-B in fatty acid (FA) uptake (A) VEGF-B expression is tightly correlated with the expression of nuclear-encoded mitochondrial genes, indicating a role of VEGF-B in energy metabolism. In normal mice, tissues with high energy metabolism demand, such as skeletal muscles, cardiac myocytes and brown adipose tissues (BATs), express high levels of VEGF-B. Upon binding to VEGFR-1 and Nrp-1 expressed by the vascular endothelial cells, VEGF-B upregulates the expression of fatty acid transport proteins (FATPs). The FATPs transport the fatty acids from blood stream across the en- dothelium to peripheral tissues for energy production. (B) In the VEGF-B deficient mice, lack of VEGF-B results in decreased FATP expression, and subsequently reduces FA uptake by the endothelium. This in turn leads to decreased FA consumption by the heart, muscle and brown adipose tissue. Importantly, as a result of the impaired FA transport and utilization, the uncon- sumed FA are accumulated in the white adipose tissues (WATs), resulting in greater body fat mass and weight in the VEGF-B null mice. The biological consequences of the impaired FA uptake in the VEGF-B deficient hearts, muscles and BATs remain unclear. It is also unknown whether the reported survival/antiapoptotic effect of VEGF-B is linked to its FA-transport function. In addition, it remains to be studied why VEGF-B deficiency does not blunt FA uptake in the WATs, while this is true in other tissues. FA transportation Metabolic demand Brown adipose tissue (BAT) Wild-type mice White adipose tissue (WAT) FA transportation Apoptosis Brown adipose tissue (BAT) White adipose tissue (WAT) VEGF-B deficient mice VEFG-B VEGFR-1 Nrp-1 FATP Fatty acid Mitochondria Endothelial cell Cardiomyocyte Skeletal muscle cell Brown adipocyte White adipocyte A B Transport STOP STOP STOP GO demonstrating that VEGF-B promotes FA transport across the endothelium by upregulating the expression of the FATPs. In vivo, Hagberg et al. found that VEGF-B deficient mice had a reduced FA uptake, leading to significantly less FA accumulation in their hearts, muscles and brown adipose tissues (BATs). In- stead, due to the impaired FA transport and utilization, the unconsumed FA were shunted to white adipose tissues (WATs) www.cell-research.com | Cell Research 743 npg and resulted in increased amount of body fat and weight in the VEGF-B null mice (Figure 1). Furthermore, as a compensation for the reduced lipid utilization, the VEGF-B deficient mice had an increased glucose uptake and utilization in their hearts as an alterna- tive source of energy. Mechanistically, the authors revealed that the effect of VEGF-B on endothelial FA uptake was mediated by Flt-1 and Nrp-1. This no- tion is supported by the findings that in mice lacking functional Flt-1 and Nrp1, the cardiac expression of FATPs was de- creased. Indeed, the authors also found that the Nrp1 deficient mice displayed a similar defect in fatty acid uptake to peripheral tissues. One common exciting aspect of all breakthroughs is that they always lead to interesting questions. There appears to be an apparent tissue specificity of VEGF-B action in affecting fatty acid metabolism, since VEGF-B deficient mice display an impaired FA uptake in their BATs, but an increased FA uptake in their WATs. Why does VEGF-B deficiency not blunt FA uptake in the WATs, since VEGF-B is a secreted protein and can be found in the blood stream in differ- ent tissues? Are Flt-1 and Nrp-1 not expressed by the endothelium in WAT? Or, does FA uptake in WATs utilize dif- ferent molecules other than VEGF-B? Furthermore, it is known that Flt-1 and Nrp-1 are expressed by many cell types. Does VEGF-B affect FA uptake in other types of cells apart from vascular endothelial cells? It would be interesting to know the direct or indirect biological conse- quences of the impaired FA uptake in the VEGF-B deficient heart, muscle and BAT. VEGF-B has been shown to be a potent survival factor [8, 9]. VEGF-B treatment increased the survival of different cell types, including neurons [9, 10], blood vessels [11], and cardiac myocytes [13, 14]. It would be interest- ing to see whether the survival effect of VEGF-B is linked to its FA-transport function, or, whether they are two sepa- rate pathways. It is noteworthy that do- cosahexaenoic acid (DHA), one major n-3 fatty acid, is specifically required for retinal neuronal survival [15]. This has indicated a link between FA uptake and neuronal survival, in both of which VEGF-B plays an important role. Also, it remains unclear why VEGF- B186, the more diffusible form of VEGF- B, was more effective than the heparin- binding form of VEGF-B, VEGF-B167, in inducing FATP expression. In the study by Hagberg et al., the co-expression of VEGF-B with the mitochondrial protein genes was the initial indication of a role of VEGF-B in energy metabolism. However, VEGF-B186 is secreted and soluble and can be transported freely to different tissues through the blood stream. Instead, one would hypothesize that VEGF-B167, which is heparin bind- ing and less soluble with a greater tis- sue specificity, might be more likely to fulfill the tissue-specific demand of FA uptake to match up with the oxidative capacity of that specific tissue. It is particularly interesting to note that VEGF-B deficiency results in greater body fat mass and weight in mice. Given that obesity is becoming an epidemic in developed and developing countries and causes significant morbid- ity and mortality, it will be interesting to further delve into the expression and functional status of VEGF-B in obese patients, and to verify whether obesity in human may potentially be associated with any functional defect of VEGF-B, and if so, whether fortified VEGF-B expression could help fight against obesity. Moreover, it is known that fatty acid uptake affects numerous biological processes in the body, including car- diovascular, neurological and immune functions. It is therefore reasonable to expect that the discovery of a new critical player in lipid uptake, such as VEGF-B, might open up new therapeu- tic possibilities to tackle pathological lipid accumulation in obesity, diabetes, cardiovascular and other diseases. We await the next surprise from VEGF-B. Acknowledgments The author thanks Dr Fan Zhang (Na- tional Institutes of Health/National Eye Institute) for the artistic work for the fig- ure, and thanks Dr Lijin Dong (National Institutes of Health/National Eye Institute) and the laboratory members for the helpful comments. References 1 Aase K, von Euler G, Li X, et al. Vas- cular endothelial growth factor-B-defi- cient mice display an atrial conduction defect. Circulation 2001; 104:358-364. 2 Li X, Tjwa M, Van Hove I, et al. Re- evaluation of the role of VEGF-B sug- gests a restricted role in the revascular- ization of the ischemic myocardium. Arterioscler Thromb Vasc Biol 2008; 28:1614-1620. 3 Hagberg CE, Falkevall A, Wang X, et al. Vascular endothelial growth factor B controls endothelial fatty acid up- take. Nature 2010; 464:917-921. 4 Olofsson B, Pajusola K, Kaipainen A, et al. Vascular endothelial growth factor B, a novel growth factor for en- dothelial cells. 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