Xuri Li

PDGF-C is a new protease-activated ligand for the PDGF alpha-receptor.

Platelet-derived growth factors (PDGFs) are important in many types of mesenchymal cell. Here we identify a new PDGF, PDGF-C, which binds to and activates the PDGF α-receptor. PDGF-C is activated by proteolysis and induces proliferation of fibroblasts when overexpressed in transgenic mice. In situ hybridization analysis in the murine embryonic kidney shows preferential expression of PDGF-C messenger RNA in the metanephric mesenchyme during epithelial conversion. Analysis of kidneys lacking the PDGF α-receptor shows selective loss of mesenchymal cells adjacent to sites of expression of PDGF-C mRNA; this is not found in kidneys from animals lacking PDGF-A or both PDGF-A and PDGF-B, indicating that PDGF-C may have a unique function.

Angiogenesis stimulated by PDGF-CC, a novel member in the PDGF family, involves activation of PDGFR-αα and -αβ receptors

A newly discovered PDGF isoform, PDGF‐CC, is expressed in actively angiogenic tissues such as placenta, some embryonic tissues, and tumors. We test the possibility that PDGF‐CC promotes angiogenesis in vivo. The core domain (mature form) of human PDGF‐CC is sufficiently potent to stimulate neovascularization in the mouse cornea. The corneal angiogenic response induced by PDGF‐CC is robust although the area of neovascularization is smaller than those of FGF‐2‐and VEGF‐stimulated angiogenesis. Similarly, PDGF‐BB and PDGF‐AB induce angiogenic responses virtually indistinguishable from PDGF‐CCstimulated vessels. In contrast, PDGF‐AA displays only a weak angiogenic response in the mouse cornea. Although there was no significant difference in incorporation of mural cells to the newly formed blood vessels induced by PDGF‐BB and ‐CC, the percentage of mural cell positive vessels induced by PDGF‐AA was greater than those induced by FGF‐2, PDGF‐BB, and PDGF‐CC. In the developing chick embryo, PDGF‐CC induced branch sprouts from established blood vessels. In PDGF receptor‐transfected endothelial cells, PDGF‐CC activated the PDGF receptor alpha subunit (PDGFR‐a). PDGF‐CC, but not PDGF‐AA, was able to activate PDGFR‐p receptor in endothelial cells that coexpress both α and β forms of receptors. Thus, the PDGF‐CC‐mediated angiogenic response is most likely transduced by PDGF‐aa and ‐ap receptors. These data demonstrate that the PDGF family is a complex and important group of proangiogenic factors.—Cao, R., Bråkenhielm, E., Li, X., Pietras, K., Widenfalk, J, Östman, A., Eriksson, U., Cao, Y. Angiogenesis stimulated by PDGF‐CC, a novel member in the PDGF family, involves activation of PDGFR‐aa and ‐ap receptors. FASEB J. 16, 1575–1583 (2002)

VEGF-B is dispensable for blood vessel growth but critical for their survival, and VEGF-B targeting inhibits pathological angiogenesis

VEGF-B, a homolog of VEGF discovered a long time ago, has not been considered an important target in antiangiogenic therapy. Instead, it has received little attention from the field. In this study, using different animal models and multiple types of vascular cells, we revealed that although VEGF-B is dispensable for blood vessel growth, it is critical for their survival. Importantly, the survival effect of VEGF-B is not only on vascular endothelial cells, but also on pericytes, smooth muscle cells, and vascular stem/progenitor cells. In vivo, VEGF-B targeting inhibited both choroidal and retinal neovascularization. Mechanistically, we found that the vascular survival effect of VEGF-B is achieved by regulating the expression of many vascular prosurvival genes via both NP-1 and VEGFR-1. Our work thus indicates that the function of VEGF-B in the vascular system is to act as a “survival,” rather than an “angiogenic” factor and that VEGF-B inhibition may offer new therapeutic opportunities to treat neovascular diseases.

Vascular Endothelial Growth Factor-B–Deficient Mice Display an Atrial Conduction Defect

Background—Vascular endothelial growth factors (VEGFs) and their receptors are essential regulators of vasculogenesis and angiogenesis in both embryos and adults. One of the factors with a still unknown physiological function is VEGF-B, which is expressed in many tissues, including the heart. Methods and Results—Mice carrying a targeted deletion in the VEGF-B gene were developed. In VEGF-B−/− animals, no gross abnormalities were observed in organs that normally show high expression of VEGF-B, such as the heart, muscle, and kidney. Analysis of heart function by ECG showed that adult VEGF-B−/− mice have an atrial conduction abnormality characterized by a prolonged PQ interval. VEGF- or basic fibroblast growth factor–induced corneal angiogenesis was similar in normal and VEGF-B−/− mice. Conclusions—VEGF-B seems to be required for normal heart function in adult animals but is not required for proper development of the cardiovascular system either during development or for angiogenesis in adults.

Paracrine Signaling by Platelet-Derived Growth Factor-CC Promotes Tumor Growth by Recruitment of Cancer-Associated Fibroblasts

Cancer results from the concerted performance of malignant cells and stromal cells. Cell types populating the microenvironment are enlisted by the tumor to secrete a host of growth-promoting cues, thus upholding tumor initiation and progression. Platelet-derived growth factors (PDGF) support the formation of a prominent tumor stromal compartment by as of yet unidentified molecular effectors. Whereas PDGF-CC induces fibroblast reactivity and fibrosis in a range of tissues, little is known about the function of PDGF-CC in shaping the tumor-stroma interplay. Herein, we present evidence for a paracrine signaling network involving PDGF-CC and PDGF receptor-alpha in malignant melanoma. Expression of PDGFC in a mouse model accelerated tumor growth through recruitment and activation of different subsets of cancer-associated fibroblasts. In seeking the molecular identity of the supporting factors provided by cancer-associated fibroblasts, we made use of antibody arrays and an in vivo coinjection model to identify osteopontin as the effector of the augmented tumor growth induced by PDGF-CC. In conclusion, we establish paracrine signaling by PDGF-CC as a potential drug target to reduce stromal support in malignant melanoma.

Tissue plasminogen activator is a potent activator of PDGF‐CC

Tissue plasminogen activator (tPA) is a serine protease involved in the degradation of blood clots through the activation of plasminogen to plasmin. Here we report on the identification of tPA as a specific protease able to activate platelet‐derived growth factor C (PDGF‐C). The newly identified PDGF‐C is secreted as a latent dimeric factor (PDGF‐CC) that upon proteolytic removal of the N‐terminal CUB domains becomes a PDGF receptor α agonist. The CUB domains in PDGF‐CC directly interact with tPA, and fibroblasts from tPA‐deficient mice fail to activate latent PDGF‐CC. We further demonstrate that growth of primary fibroblasts in culture is dependent on a tPA‐mediated cleavage of latent PDGF‐CC, generating a growth stimulatory loop. Immunohistochemical analysis showed similar expression patterns of PDGF‐C and tPA in developing mouse embryos and in tumors, indicating both autocrine and paracrine modes of activation of PDGF receptor‐mediated signaling pathways. The identification of tPA as an activator of PDGF signaling establishes a novel role for the protease in normal and pathological tissue growth and maintenance, distinct from its well‐known role in plasminogen activation and fibrinolysis.

Novel PDGF family members: PDGF-C and PDGF-D

Platelet-derived growth factors (PDGFs) were discovered almost two decades ago. The classical PDGF polypeptide chains, PDGF-A and PDGF-B, are well studied and they regulate a number of physiological and pathophysiological processes in many types of mesenchymal cells via two receptor tyrosine kinases, PDGF receptors alpha and beta. Recently, two additional PDGF polypeptide chains were discovered, namely PDGF-C and PDGF-D. The discovery of two additional ligands for the two PDGF receptors suggests that PDGF-mediated signaling is more complex than previously anticipated.

Revascularization of ischemic tissues by PDGF-CC via effects on endothelial cells and their progenitors

The angiogenic mechanism and therapeutic potential of PDGF-CC, a recently discovered member of the VEGF/PDGF superfamily, remain incompletely characterized. Here we report that PDGF-CC mobilized endothelial progenitor cells in ischemic conditions; induced differentiation of bone marrow cells into ECs; and stimulated migration of ECs. Furthermore, PDGF-CC induced the differentiation of bone marrow cells into smooth muscle cells and stimulated their growth during vessel sprouting. Moreover, delivery of PDGF-CC enhanced postischemic revascularization of the heart and limb. Modulating the activity of PDGF-CC may provide novel opportunities for treating ischemic diseases.

PDGF-D is a potent transforming and angiogenic growth factor

Platelet-derived growth factors (PDGFs) are important for normal tissue growth and maintenance. Overexpression of the classical PDGFs, PDGF-A and PDGF-B, has been linked to several diseases, including cancer, fibrotic disease and atherosclerosis. Recently, two novel PDGFs, PDGF-C and PDGF-D, were discovered. It has not yet been established whether PDGF-C and PDGF-D are linked to disease phenotypes like the classical PDGFs. PDGF-B, the cellular homologue of the viral simian sarcoma oncogene v-sis, is known to potently induce cellular transformation through activation of PDGF receptor (PDGFR)-β. In this work, we have determined the transformation efficacy of PDGF-D in comparison with that of PDGF-C and PDGF-B. PDGF-D is a potent transforming growth factor for NIH/3T3 cells, and the transformed cells displayed stress fibre reorganization, increased proliferation rate, anchorage-independent growth in soft agar, ability to induce tumours in nude mice, and upregulation of vascular endothelial growth factor. Morphological analyses of the vasculatures from the PDGF-isoform-expressing tumours revealed marked differences suggesting differential signalling through the two PDGF receptors in tumour vessel development and remodelling. In summary, these results suggest that PDGF-D induce cellular transformation and promote tumour growth by accelerating the proliferation rate of the tumour cells, and by stimulation of tumour neovascularization.

VEGF-B inhibits apoptosis via VEGFR-1–mediated suppression of the expression of BH3-only protein genes in mice and rats

Despite its early discovery and high sequence homology to the other VEGF family members, the biological functions of VEGF-B remain poorly understood. We revealed here a novel function for VEGF-B as a potent inhibitor of apoptosis. Using gene expression profiling of mouse primary aortic smooth muscle cells, and confirming the results by real-time PCR using mouse and rat cell lines, we showed that VEGF-B inhibited the expression of genes encoding the proapoptotic BH3-only proteins and other apoptosis- and cell death-related proteins, including p53 and members of the caspase family, via activation of VEGFR-1. Consistent with this, VEGF-B treatment rescued neurons from apoptosis in the retina and brain in mouse models of ocular neurodegenerative disorders and stroke, respectively. Interestingly, VEGF-B treatment at the dose effective for neuronal survival did not cause retinal neovascularization, suggesting that VEGF-B is the first member of the VEGF family that has a potent antiapoptotic effect while lacking a general angiogenic activity. These findings indicate that VEGF-B may potentially offer a new therapeutic option for the treatment of neurodegenerative diseases.