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Skin angiogenesis: biologic bases for pathological processes
Skin Angiogenesis: Biologic Bases for Pathological Processes SILVIA MORETTI, MD ADELINA SPALLANZANI, MD CINZIA PINZI, MD
A
ngiogenesis is a complex biologic process characterized by the development of new blood vessels from existing vasculature. It is essential in reproduction, development, and wound repair.1 Under these conditions, angiogenesis is highly regulated (ie, turned on for short periods and then completely inhibited). In contrast, persistent unregulated angiogenesis can drive many diseases, such as growth of primary tumors and metastases, chronic arthropathies, and diabetic retinopathy. Common to all forms of angiogenesis is a general pattern of response by the capillary endothelial cell. Capillary blood vessels consist of endothelial cells and pericytes: these two cell types carry all genetic information to form tubes, branches, and whole capillary networks. Specific stimulatory molecules can initiate the process and specific inhibitory molecules can stop it. These molecules with opposing functions appear to be continuously acting in concert to maintain a quiescent microvasculature in which endothelial cell turnover is slow. Endothelial cells, however, can undergo rapid proliferation (few days’ turnover) during active angiogenesis (eg, in wound healing). The proteins involved in angiogenesis have been discovered in the past decades and their properties are well known, but their interactions with each other are not yet completely clarified. In addition, other entities such as nonvascular cells can modulate angiogenic response. This point may be crucial within a specific microenvironment, such as the skin, where resident cells—in particular, mast cells, macrophages, and fibroblasts—may exert modulation of the process. New blood vessel occurrence in the skin may be observed in several physiological and pathological circumstances, such as wound healing, psoriasis, hemangiomas, and benign and malignant cutaneous neoplasia. From the Second Dermatology Unit, S.M. Nuova Hospital, Azienda Sanitaria di Firenze, and Dermatology Institute, University of Florence, Florence, Italy. Address correspondence to Silvia Moretti, MD, Azienda Sanitaria di Firenze, S.M. Nuova Hospital, Second Dermatology Unit, Firenze, Italy. © 1999 by Elsevier Science Inc. All rights reserved. 655 Avenue of the Americas, New York, NY 10010
Morphological and Functional Bases of Angiogenesis: Vasculature in the Embryo In the embryonal life the first blood vessels appear ex novo as the result of vasculogenesis, that is, the formation of capillaries from endothelial cells differentiating in situ from groups of mesenchymal cells or blood islands (aggregations of mesodermal cells in the yolk sac derived from splanchnopleure), which rapidly increase in number over the whole vascular area. Their peripheral components—angioblasts— give rise to endothelium, whereas the blood cellular elements originate from the central part. Then, endothelial cells divide to form a primitive vascular plexus, which is the source of an arteriovenous network when blood circulation commences. From then forward its vessels are continuously reshaped through growth and regression. In developing organs, including the skin, blood vessel growth is primarily the result of invasion of the buds by capillaries from the primitive vascular plexus.2 In the first stage, this network develops through the sprouting and elongation of vascular islands, structurally consisting of sinusoids rather than true capillaries. These vessels undergo a gradual remodeling in accordance with hemodynamic influences, and most of these subepithelial plexuses become postcapillary venular networks. In the second stage, true capillaries (arteriovenous capillaries) are formed, connecting arteriolar and venular vessels, and capillary loops sprout and project into epithelial indentations (papillae) within epiparenchymal networks. In the third stage, marked capillary proliferation occurs in parallel with rapid parenchymal differentiation, and capillary nets are formed mostly in parallel to existing arteriovenous capillaries.
Angiogenesis in Adult Life In adult angiogenesis, the sequence of events, once initiated, is very similar to that in the embryo after primary vascular histogenesis. These events are represented by (a) vasodilation and congestion of the vascular beds; (b) elongation or buckling of vessels associated with the development sinuous structural change; (c) dissolution of the vessel basement membrane and sprouting of endothelial cells into the surrounding tis0738-081X/99/$–see front matter PII S0738-081X(99)00082-6
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sue; (d) migration of endothelium in solid projection toward the angiogenic stimulus, with proximal mitosis; and finally (e) by lumen formation (through intracellular and intercellular mechanisms), (f) anastomosis with other endothelial sprouts and loop formation, with (g) consequent development of circulation and (h) maturation/evolution of preferential channels with arterial and venous segments. New vessel growth occurs predominantly from venules and venular capillaries, which show ultrastructurally multilaminated basement membranes,3 a feature suggesting rapid exchange of substances and increased endothelial turnover.4
Angiogenic Molecules A number of angiogenic polypeptides have been identified that can affect angiogenesis. The most important are basic fibroblastic growth factors (FGFs), vascular endothelial growth factor (VEGF), platelet-derived endothelial cell growth factor (PD-ECGF), transforming growth factor (TGF-␣ and -), tumor necrosis factor (TNF-␣), and angiogenin.1 Angiogenin is a polypeptide first isolated from a conditioned medium of a human tumor cell line, but is also found in normal organs (eg, the liver),2 which stimulates endotheliocytes to produce diacylglycerol and prostacyclin and also exerts a unique ribonucleolytic activity essential for neovascularization.1,5 Both acidic and basic FGF (bFGF and aFGF) are pleiotropic cytokines that are mitogenic for a variety of cell types, including endothelial cells, smooth muscle cells and fibroblasts, and that stimulate endothelial cells to migrate and form tubes, to increase production of proteases and plasminogen activator, and to act as embryonic inducers.1 They are produced in the skin by proliferating epidermal keratinocytes, dermal fibroblasts, endotheliocytes, and monocytes.6 TGF-␣, a highly mitogenic and transforming peptide, exerting its effects by binding to the epidermal growth factor receptor, is expressed by keratinocytes,7 and TGF- is a multifunctional cytokine produced by most epithelial and mesenchymal cells (keratinocytes, endothelial cells, macrophages, fibroblasts), which can act both as an inhibitor and stimulator of cell replication, and as regulator of the synthesis of many components of the extracellular matrix.8 Its angiogenic activity relies on acceleration of vascular sprouting and potentiation of the angiogenic action of bFGF.9 TNF-␣ induces production of bFGF in endothelial cells and enhances its secretion, and is chemotactic for monocytes and activates macrophages1; in the skin it is mainly found in keratinocytes and macrophages.10 PD-ECGF is biologically active in stimulating endothelial cell DNA synthesis and chemotaxis and in amplifying DNA synthesis activity of FGFs on endotheliocytes. It is produced by endothe-
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lial cells and macrophages.2,11 VEGF, like PDGF, acts mainly as mitogen for vascular endothelial cells, increases vascular permeability, and induces plasminogen activator and plasminogen activator inhibitor in endotheliocytes.12 VEGF is a dimeric glycoprotein that can show different molecular forms as a result of alternative mRNA splicing, and it is secreted by several types of cells, including tumor cells; in the skin, three major splice forms of VEGF are constitutively expressed by keratinocytes.13 VEGF is also expressed on mesenchymal cells such as macrophages and endotheliocytes,14 and it seems the major candidate for the mediation of hypoxia-induced vascular alterations in human skin.15 In addition, several low molecular weight factors are also angiogenic, such as prostaglandins PGE1 and PGE2, nicotinamide, and related compounds such as adenosine, certain degradation products of hyaluronic acid, hydroxyeicosatrienoic acid, and okadaic acid.1
Angiogenesis and Skin Inflammation In pathological inflammatory conditions of the skin an increased angiogenesis has often been detected; in fact, a number of inflammatory and immune diseases are associated with vascular changes. Psoriasis, for example, is a common phlogistic skin condition with dilation of capillaries as an early histological change, and in late psoriatic lesions there is a proliferation of blood vessels and neovascularization. High angiogenic activity has been associated with psoriatic epidermis,16 possibly related to overexpression of VEGF by epidermal cells,17 and increased VEGF amounts were detected in immunomediated bullous diseases associated with subepidermal blister formation, such as bullous pemphigoid, dermatitis herpetiformis, and erythema multiforme.18 In addition, overexpression of PD-ECGF has been demonstrated in lesional psoriatic skin, both as mRNA production and protein immunoreactivity.19 These data indicate a potential role of angiogenic factors in cutaneous inflammatory diseases, and they suggest a role for antiangiogenic therapy. A number of growth factor receptor tyrosine kinases have been implicated in angiogenesis, including epidermal growth factor receptor, FGF receptor, platelet-derived growth factor receptor, and VEGF receptors Flt-1 and Flk-1/ KDR, so that they might be used as a target for antiangiogenic therapy; Flk-1/KDR receptor, in particular, may be an appropriate target, for it is expressed exclusively in endothelial cells.20 Matrix metalloproteinases (MMPs), which are a family of zinc-dependent endopeptidases capable of degrading essentially all extracellular matrix components, are another possible target. These enzymes can be produced by several types of skin cells such as keratinocytes, fibroblasts, endotheliocytes, macrophages, and
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mast cells and can be specifically inhibited by tissue inhibitor metalloproteinases. The MMPs do not seem constitutively expressed in the skin but can be temporarily induced in response to various exogenous signals such as cytokines and growth factors, cell matrix interactions, and altered cell– cell contacts, and they play an important role in proteolytic remodeling of extracellular matrix during angiogenesis (ie, degradation by MMPs of the basement membrane of capillaries).21 Recently, a novel shark cartilage extract showed antiangiogenic properties when tested in vitro for collagenase activity and in vivo using a cutaneous irritation model in humans.22 Finally, these data suggest that new classes of agents can be used to treat cutaneous inflammatory diseases.
References 1. Folkman J, Shing Y. Angiogenesis. J Biol Chem 1992;267: 10931– 4. 2. Ribatti D, Vacca A, Roncali L, et al. Angiogenesis under normal and pathological conditions. Haematologica 1991; 76:311–20. 3. Yen A, Braverman IM. Ultrastructure of the human dermal microcirculaton: The horizontal plexus of the papillary dermis. J Invest Dermatol 1976;66:131– 42. 4. Vracko R, Benditt RP. Basal lamina: The scaffold for regeneration of injured skeletal muscle fibers and capillaries. J Cell Biol 1972;55:406 –19. 5. Folkman J, Klagsburn M. Angiogenic factors. Science 1987;235:442–7. 6. Goldfarb M. The fibroblast growth factor family. Cell Growth Differ 1990;1:439 – 45. 7. Massague J. Transforming growth factor-␣: A model for membrane-anchored growth factors. J Biol Chem 1990; 265:21393– 6. 8. Sporn MB, Roberts AB. Transforming growth factor-: Recent progress and new challenges. J Cell Biol 1992;1119: 1017–21. 9. Gjdusek CM, Luo Z, Mayberg MR. Basic fibroblastic growth factor and transforming growth factor beta-1: Synergistic mediators of angiogenesis in vitro. J Cell Physiol 1993;157:133– 44.
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10. Luger TA, Schwartz T. Epidermal cell-derived cytokines. In: Bos JD, editor. Skin immune system. Boca Raton, Florida: CRC Press, 1989:257– 83. 11. Herlyn M. Molecular and cellular biology of melanoma. Austin, Texas: R.G. Landes, 1993. 12. Ferrara N, Houck K, Jakeman L, et al. Molecular and biologic properties of the vascular endothelial growth factor family of proteins. Endocr Rev 1992;13:18 –32. 13. Ballaun C, Weninger W, Uthman A, et al. Human keratinocytes express the three major splice forms of vascular endothelial growth factor. J Invest Dermatol 1995;104: 7–10. 14. Berse B, Brown LF, Van de Water L, et al. Vascular permeability factor (vascular endothelial growth factor) gene is expressed differentially in normal tissues, macrophages and tumors. Mol Cell Biol 1992;3:211–20. 15. Detmar M, Brown LF, Berse B, et al. Hypoxia regulates the expression of vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) and its receptors in human skin. J Invest Dermatol 1997;108:263– 8. 16. Malhotra R, Stenn KS, Fernandez LA, et al. Angiogenic properties of normal and psoriatic skin associate with epidermis, not dermis. J Invest Dermatol 1989;61:162–5. 17. Detmar M, Brown L, Claffey K, et al. Overexpression of vascular permeability factor and its receptors in psoriasis. J Exp Med 1994;180:1141– 6. 18. Brown LF, Harrist TJ, Yeo KT, et al. Increased expression of vascular permeability factor (VEGF) in bullous pemphigoid, dermatitis herpetiformis and erythema multiforme. J Invest Dermatol 1995;104:744 –9. 19. Creamer D, Jaggar R, Allen M, et al. Overexpression of the angiogenic factor pletelet-derived endothelial cell growth factor/thymidin phosphorylase in psoriatic epidermis. Br J Dermatol 1997;137:851–5. 20. Strawn LM, McMahon G, App H, et al. Flk-1 as a target for tumor growth inhibition. Cancer Res 1996;56:3540 –5. 21. Kahari VM, Saarialho-Kere U. Matrix metalloproteinases in skin. Exp Dermatol 1997;6:199 –213. 22. Dupont E, Savard PE, Jourdan C, et al. Antiangiogenic properties of a novel shark cartilage extract: Potential role in the treatment of psoriasis. J Cutan Med Surg 1998;2: 146 –52.
A
ngiogenesis is a complex biologic process characterized by the development of new blood vessels from existing vasculature. It is essential in reproduction, development, and wound repair.1 Under these conditions, angiogenesis is highly regulated (ie, turned on for short periods and then completely inhibited). In contrast, persistent unregulated angiogenesis can drive many diseases, such as growth of primary tumors and metastases, chronic arthropathies, and diabetic retinopathy. Common to all forms of angiogenesis is a general pattern of response by the capillary endothelial cell. Capillary blood vessels consist of endothelial cells and pericytes: these two cell types carry all genetic information to form tubes, branches, and whole capillary networks. Specific stimulatory molecules can initiate the process and specific inhibitory molecules can stop it. These molecules with opposing functions appear to be continuously acting in concert to maintain a quiescent microvasculature in which endothelial cell turnover is slow. Endothelial cells, however, can undergo rapid proliferation (few days’ turnover) during active angiogenesis (eg, in wound healing). The proteins involved in angiogenesis have been discovered in the past decades and their properties are well known, but their interactions with each other are not yet completely clarified. In addition, other entities such as nonvascular cells can modulate angiogenic response. This point may be crucial within a specific microenvironment, such as the skin, where resident cells—in particular, mast cells, macrophages, and fibroblasts—may exert modulation of the process. New blood vessel occurrence in the skin may be observed in several physiological and pathological circumstances, such as wound healing, psoriasis, hemangiomas, and benign and malignant cutaneous neoplasia. From the Second Dermatology Unit, S.M. Nuova Hospital, Azienda Sanitaria di Firenze, and Dermatology Institute, University of Florence, Florence, Italy. Address correspondence to Silvia Moretti, MD, Azienda Sanitaria di Firenze, S.M. Nuova Hospital, Second Dermatology Unit, Firenze, Italy. © 1999 by Elsevier Science Inc. All rights reserved. 655 Avenue of the Americas, New York, NY 10010
Morphological and Functional Bases of Angiogenesis: Vasculature in the Embryo In the embryonal life the first blood vessels appear ex novo as the result of vasculogenesis, that is, the formation of capillaries from endothelial cells differentiating in situ from groups of mesenchymal cells or blood islands (aggregations of mesodermal cells in the yolk sac derived from splanchnopleure), which rapidly increase in number over the whole vascular area. Their peripheral components—angioblasts— give rise to endothelium, whereas the blood cellular elements originate from the central part. Then, endothelial cells divide to form a primitive vascular plexus, which is the source of an arteriovenous network when blood circulation commences. From then forward its vessels are continuously reshaped through growth and regression. In developing organs, including the skin, blood vessel growth is primarily the result of invasion of the buds by capillaries from the primitive vascular plexus.2 In the first stage, this network develops through the sprouting and elongation of vascular islands, structurally consisting of sinusoids rather than true capillaries. These vessels undergo a gradual remodeling in accordance with hemodynamic influences, and most of these subepithelial plexuses become postcapillary venular networks. In the second stage, true capillaries (arteriovenous capillaries) are formed, connecting arteriolar and venular vessels, and capillary loops sprout and project into epithelial indentations (papillae) within epiparenchymal networks. In the third stage, marked capillary proliferation occurs in parallel with rapid parenchymal differentiation, and capillary nets are formed mostly in parallel to existing arteriovenous capillaries.
Angiogenesis in Adult Life In adult angiogenesis, the sequence of events, once initiated, is very similar to that in the embryo after primary vascular histogenesis. These events are represented by (a) vasodilation and congestion of the vascular beds; (b) elongation or buckling of vessels associated with the development sinuous structural change; (c) dissolution of the vessel basement membrane and sprouting of endothelial cells into the surrounding tis0738-081X/99/$–see front matter PII S0738-081X(99)00082-6
630 MORETTI ET AL.
sue; (d) migration of endothelium in solid projection toward the angiogenic stimulus, with proximal mitosis; and finally (e) by lumen formation (through intracellular and intercellular mechanisms), (f) anastomosis with other endothelial sprouts and loop formation, with (g) consequent development of circulation and (h) maturation/evolution of preferential channels with arterial and venous segments. New vessel growth occurs predominantly from venules and venular capillaries, which show ultrastructurally multilaminated basement membranes,3 a feature suggesting rapid exchange of substances and increased endothelial turnover.4
Angiogenic Molecules A number of angiogenic polypeptides have been identified that can affect angiogenesis. The most important are basic fibroblastic growth factors (FGFs), vascular endothelial growth factor (VEGF), platelet-derived endothelial cell growth factor (PD-ECGF), transforming growth factor (TGF-␣ and -), tumor necrosis factor (TNF-␣), and angiogenin.1 Angiogenin is a polypeptide first isolated from a conditioned medium of a human tumor cell line, but is also found in normal organs (eg, the liver),2 which stimulates endotheliocytes to produce diacylglycerol and prostacyclin and also exerts a unique ribonucleolytic activity essential for neovascularization.1,5 Both acidic and basic FGF (bFGF and aFGF) are pleiotropic cytokines that are mitogenic for a variety of cell types, including endothelial cells, smooth muscle cells and fibroblasts, and that stimulate endothelial cells to migrate and form tubes, to increase production of proteases and plasminogen activator, and to act as embryonic inducers.1 They are produced in the skin by proliferating epidermal keratinocytes, dermal fibroblasts, endotheliocytes, and monocytes.6 TGF-␣, a highly mitogenic and transforming peptide, exerting its effects by binding to the epidermal growth factor receptor, is expressed by keratinocytes,7 and TGF- is a multifunctional cytokine produced by most epithelial and mesenchymal cells (keratinocytes, endothelial cells, macrophages, fibroblasts), which can act both as an inhibitor and stimulator of cell replication, and as regulator of the synthesis of many components of the extracellular matrix.8 Its angiogenic activity relies on acceleration of vascular sprouting and potentiation of the angiogenic action of bFGF.9 TNF-␣ induces production of bFGF in endothelial cells and enhances its secretion, and is chemotactic for monocytes and activates macrophages1; in the skin it is mainly found in keratinocytes and macrophages.10 PD-ECGF is biologically active in stimulating endothelial cell DNA synthesis and chemotaxis and in amplifying DNA synthesis activity of FGFs on endotheliocytes. It is produced by endothe-
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1999;17:629 – 631
lial cells and macrophages.2,11 VEGF, like PDGF, acts mainly as mitogen for vascular endothelial cells, increases vascular permeability, and induces plasminogen activator and plasminogen activator inhibitor in endotheliocytes.12 VEGF is a dimeric glycoprotein that can show different molecular forms as a result of alternative mRNA splicing, and it is secreted by several types of cells, including tumor cells; in the skin, three major splice forms of VEGF are constitutively expressed by keratinocytes.13 VEGF is also expressed on mesenchymal cells such as macrophages and endotheliocytes,14 and it seems the major candidate for the mediation of hypoxia-induced vascular alterations in human skin.15 In addition, several low molecular weight factors are also angiogenic, such as prostaglandins PGE1 and PGE2, nicotinamide, and related compounds such as adenosine, certain degradation products of hyaluronic acid, hydroxyeicosatrienoic acid, and okadaic acid.1
Angiogenesis and Skin Inflammation In pathological inflammatory conditions of the skin an increased angiogenesis has often been detected; in fact, a number of inflammatory and immune diseases are associated with vascular changes. Psoriasis, for example, is a common phlogistic skin condition with dilation of capillaries as an early histological change, and in late psoriatic lesions there is a proliferation of blood vessels and neovascularization. High angiogenic activity has been associated with psoriatic epidermis,16 possibly related to overexpression of VEGF by epidermal cells,17 and increased VEGF amounts were detected in immunomediated bullous diseases associated with subepidermal blister formation, such as bullous pemphigoid, dermatitis herpetiformis, and erythema multiforme.18 In addition, overexpression of PD-ECGF has been demonstrated in lesional psoriatic skin, both as mRNA production and protein immunoreactivity.19 These data indicate a potential role of angiogenic factors in cutaneous inflammatory diseases, and they suggest a role for antiangiogenic therapy. A number of growth factor receptor tyrosine kinases have been implicated in angiogenesis, including epidermal growth factor receptor, FGF receptor, platelet-derived growth factor receptor, and VEGF receptors Flt-1 and Flk-1/ KDR, so that they might be used as a target for antiangiogenic therapy; Flk-1/KDR receptor, in particular, may be an appropriate target, for it is expressed exclusively in endothelial cells.20 Matrix metalloproteinases (MMPs), which are a family of zinc-dependent endopeptidases capable of degrading essentially all extracellular matrix components, are another possible target. These enzymes can be produced by several types of skin cells such as keratinocytes, fibroblasts, endotheliocytes, macrophages, and
Clinics in Dermatology
Y
1999;17:629 – 631
mast cells and can be specifically inhibited by tissue inhibitor metalloproteinases. The MMPs do not seem constitutively expressed in the skin but can be temporarily induced in response to various exogenous signals such as cytokines and growth factors, cell matrix interactions, and altered cell– cell contacts, and they play an important role in proteolytic remodeling of extracellular matrix during angiogenesis (ie, degradation by MMPs of the basement membrane of capillaries).21 Recently, a novel shark cartilage extract showed antiangiogenic properties when tested in vitro for collagenase activity and in vivo using a cutaneous irritation model in humans.22 Finally, these data suggest that new classes of agents can be used to treat cutaneous inflammatory diseases.
References 1. Folkman J, Shing Y. Angiogenesis. J Biol Chem 1992;267: 10931– 4. 2. Ribatti D, Vacca A, Roncali L, et al. Angiogenesis under normal and pathological conditions. Haematologica 1991; 76:311–20. 3. Yen A, Braverman IM. Ultrastructure of the human dermal microcirculaton: The horizontal plexus of the papillary dermis. J Invest Dermatol 1976;66:131– 42. 4. Vracko R, Benditt RP. Basal lamina: The scaffold for regeneration of injured skeletal muscle fibers and capillaries. J Cell Biol 1972;55:406 –19. 5. Folkman J, Klagsburn M. Angiogenic factors. Science 1987;235:442–7. 6. Goldfarb M. The fibroblast growth factor family. Cell Growth Differ 1990;1:439 – 45. 7. Massague J. Transforming growth factor-␣: A model for membrane-anchored growth factors. J Biol Chem 1990; 265:21393– 6. 8. Sporn MB, Roberts AB. Transforming growth factor-: Recent progress and new challenges. J Cell Biol 1992;1119: 1017–21. 9. Gjdusek CM, Luo Z, Mayberg MR. Basic fibroblastic growth factor and transforming growth factor beta-1: Synergistic mediators of angiogenesis in vitro. J Cell Physiol 1993;157:133– 44.
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10. Luger TA, Schwartz T. Epidermal cell-derived cytokines. In: Bos JD, editor. Skin immune system. Boca Raton, Florida: CRC Press, 1989:257– 83. 11. Herlyn M. Molecular and cellular biology of melanoma. Austin, Texas: R.G. Landes, 1993. 12. Ferrara N, Houck K, Jakeman L, et al. Molecular and biologic properties of the vascular endothelial growth factor family of proteins. Endocr Rev 1992;13:18 –32. 13. Ballaun C, Weninger W, Uthman A, et al. Human keratinocytes express the three major splice forms of vascular endothelial growth factor. J Invest Dermatol 1995;104: 7–10. 14. Berse B, Brown LF, Van de Water L, et al. Vascular permeability factor (vascular endothelial growth factor) gene is expressed differentially in normal tissues, macrophages and tumors. Mol Cell Biol 1992;3:211–20. 15. Detmar M, Brown LF, Berse B, et al. Hypoxia regulates the expression of vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) and its receptors in human skin. J Invest Dermatol 1997;108:263– 8. 16. Malhotra R, Stenn KS, Fernandez LA, et al. Angiogenic properties of normal and psoriatic skin associate with epidermis, not dermis. J Invest Dermatol 1989;61:162–5. 17. Detmar M, Brown L, Claffey K, et al. Overexpression of vascular permeability factor and its receptors in psoriasis. J Exp Med 1994;180:1141– 6. 18. Brown LF, Harrist TJ, Yeo KT, et al. Increased expression of vascular permeability factor (VEGF) in bullous pemphigoid, dermatitis herpetiformis and erythema multiforme. J Invest Dermatol 1995;104:744 –9. 19. Creamer D, Jaggar R, Allen M, et al. Overexpression of the angiogenic factor pletelet-derived endothelial cell growth factor/thymidin phosphorylase in psoriatic epidermis. Br J Dermatol 1997;137:851–5. 20. Strawn LM, McMahon G, App H, et al. Flk-1 as a target for tumor growth inhibition. Cancer Res 1996;56:3540 –5. 21. Kahari VM, Saarialho-Kere U. Matrix metalloproteinases in skin. Exp Dermatol 1997;6:199 –213. 22. Dupont E, Savard PE, Jourdan C, et al. Antiangiogenic properties of a novel shark cartilage extract: Potential role in the treatment of psoriasis. J Cutan Med Surg 1998;2: 146 –52.