dermatological aspects of angiogenesis

TOPICAL REVIEW

Dermatological aspects of angiogenesis

P . V E L A S C O A N D B . L A N G E - A S S C H E N F E L D TDepartment of Dermatology, University of Kiel, Schittenhelmstraße 7, 24105 Kiel, Germany

Accepted for publication 14 August 2001

Summary Neovascularization is vital for the growth of tumours, providing a lifeline for sustenance and waste

disposal. Tumour vessels can grow by sprouting, intussusception or by incorporating bone marrow-derived endothelial precursor cells into growing vessels. Recent advances in vascular biology have

identified some key factors that control vascular growth, and have led to the hypothesis that in

normal tissues vascular quiescence is maintained by the dominant influence of endogenousangiogenesis inhibitors over angiogenic stimuli. In contrast, increased secretion of angiogenic

factors and the down-regulation of endogenous angiogenesis inhibitors induce tumour

angiogenesis. Vascular quiescence in the skin seems to be primarily maintained by a balancebetween the endogenous angiogenesis inhibitors thrombospondin 1 and thrombospondin 2 and the

potent proangiogenic factor vascular endothelial growth factor A. Inhibiting tumour growth by

controlling angiogenesis is an intriguing approach with great potential for the treatment of vasculartumours such as haemangioma, Kaposi’s sarcoma and solid cutaneous tumours such as squamous

cell carcinoma, melanoma and basal cell carcinoma. In this review, the role of angiogenesis and

more recent topics such as lymphangiogenesis in cutaneous tumour growth, invasion andmetastasis will be discussed.

Key words: angiogenesis inhibitors, angiogenesis, lymphangiogenesis, skin, tumour, vascular

endothelial growth factor

Human skin is a richly vascularized organ consisting of

a superficial vascular plexus that is found directly

beneath the epidermis and a deep vascular plexuslocated at the dermal-subcutaneous border. These

vascular beds are tightly interconnected and play an

important role in vital physiological processes of theskin such as the supply of nutrients, controlling the

influx of immune cells, hair growth and thermoregu-

lation. The development of the blood vasculature is acomplex process that begins when progenitor cells

(angioblasts) differentiate into endothelial cells, co-

alesce and form the first vessels in the embryo(vasculogenesis). These embryonic vascular tubes give

rise to new vessels by branching or sprouting (angio-

genesis) that in turn can elongate, enlarge, mature orregress (vascular remodelling) in response to different

stimuli (reviewed by Carmeliet and Jain1). Together,

these processes are vital in the developing embryo but

are mostly absent in the adult.In normal and diseased skin, the angiogenic stimulus

seems to originate from the epidermis and not the

dermis.2 This makes intuitive sense as the avascularepidermis would require mechanisms designed to meet

the high metabolic demands associated with hyperpro-

liferation in healing wounds, inflammation or neo-plasms. Many potent proangiogenic factors such as

basic fibroblast growth factor (FGF-2)3–5 and chemok-

ines such as interleukin (IL)-86,7 have been described incutaneous angiogenesis. However, vascular endothelial

growth factor (VEGF)-A is regarded as the major

angiogenesis factor in many cutaneous diseases, par-ticularly in tumour growth.8 VEGF-A is a homodimeric

heparin-binding glycoprotein occurring in at least four

isoforms as a result of alternative splicing.9 It binds twotype III tyrosine kinase receptors primarily expressed

Correspondence: Dr. med. Bernhard Lange-Asschenfeldt.

E-mail: [email protected]

British Journal of Dermatology 2002; 147: 841–852.

� 2002 British Association of Dermatologists 841

on vascular endothelial cells, VEGF receptor (VEGFR)-1, also termed fms-like-tyrosine kinase (Flt)-1, and

VEGFR-2,10–12 also known as kinase-insert domain-

containing receptor or fetal liver kinase-1, as well asthe neuropilin receptors.13 Several experimental mod-

els highlight the importance of VEGF-A as a potent

proangiogenic and tumour growth factor (reviewed byFerrara and Gerber14), and VEGF-A up-regulation is

implicated in promoting tumour invasiveness.15–17

VEGF-A secreted by tumour cells can induce the releaseof matrix metalloproteinases (MMPs) by endothelial

cells.18 Elevated levels of membrane type 1 MMP,

MMP-2 and MMP-919 have been demonstrated todegrade the extracellular matrix, paving the way for

the invasion of endothelial cells into the surrounding

tissue. Furthermore, MMPs release sequestered angio-genic factors such as FGF-2 and VEGF-A from extra-

cellular stores, thereby increasing their bioavailability

(Table 1). Conversely, MMP activity is important forthe generation of endogenous angiogenesis inhibitors

such as endostatin, which is a cleavage product of

collagen XVIII.20

Recently, the angiopoietin growth factor family has

been shown to modulate vascular remodelling, leak-

age21 and tumour growth.22 Angiopoietin 1 (Ang-1),normally found on pericytes, binds the tyrosine-protein

kinase receptor (Tie)-2 primarily expressed on endot-

helial cells and stabilizes the blood vessel. Angiopoietin2 (Ang-2) is a competitive antagonist to Ang-1 and

counters this stability. The vessel can undergo angio-

genesis in the presence of a proangiogenic factor, orregress in its absence.23,24 Ang-2 and VEGFR-2 have

been shown to be up-regulated in the very early stages

of tumour growth and to be continuously overex-pressed in the lesions, suggesting that these molecules

work together to induce new vessel growth andremodel the tumour vasculature.25

It is generally believed that in quiescent vessels, a

balance between proangiogenic and antiangiogenicfactors maintains homeostasis. Experimental data sug-

gest that the �angiogenic switch�, the change from a

quiescent tumour cell population to cells that caninduce angiogenesis, is a rate-limiting step in the

progression from a premalignant minimal lesion to a

malignant invasive lesion. It is dependent on theup-regulation of angiogenic factors and the down-

regulation of inhibitors.26 In the skin, aside from

follicular-related angiogenesis in certain phases of thehair cycle,27 the blood vessels remain quiescent and

neovascularization is generally not seen. It has been

suggested that the normal dermal matrix, in particularthe basement membrane surrounding microvascular

endothelial cells, may function as a natural inhibitor of

angiogenesis, thereby ensuring the quiescence of ves-sels in healthy skin. Recent evidence suggests that

thrombospondin (TSP)-1 and TSP-2 are major physio-

logical inhibitors of skin angiogenesis.28,29 TSP-1 andTSP-2 are members of a family of matrix glycoproteins

and are deposited in the dermoepidermal basement

membrane,30 thereby contributing to the antiangio-genic barrier that separates the avascular epidermis

from the vascularized dermis.28 Furthermore, these

endogenous angiogenesis inhibitors,31–34 as well asendostatin,20 angiostatin,35 vasostatin36 and IL-12,37

have been identified as inhibitors of tumour angiogen-

esis and tumour growth in vivo (Table 2).Early metastasis to the lymph nodes as well as

the presence of dilated peritumour lymphatic vessels

in melanoma and other tumours38 led to rising interestin investigations of tumour lymphangiogenesis.

Table 1. Partial list of proangiogenic factors endogenously expressed in cutaneous lesions

Molecule Function H K M S B

FGF-2 EC mitogen, stimulates tube formation and proteinase production • • • • •VEGF-A EC mitogen, vascular permeability factor and leucocyte adhesion promoter • • • •VEGF-C EC and lymphatic EC mitogen • • • •VEGF-D Lymphatic EC mitogen •PlGF Potentiates VEGF-A, recruits bone-marrow-derived cells into growing vessels • •PDGF Recruits pericytes and up-regulates other angiogenic factors • • •IL-8 Induces tumour angiogenesis • • • •MMPs (2, 9) Remodel extracellular matrix, release and activate growth factors • • • • •Ang-2 Antagonist of Ang-1 • • • •Integrins (avb3, a5b1) Important for the formation, survival and maturation of new blood vessels and

potential modulators of EC invasion• • •

H, haemangioma; K, Kaposi’s sarcoma; M, melanoma; S, squamous cell carcinoma; B, basal cell carcinoma; FGF-2, basic fibroblast growth factor;

VEGF, vascular endothelial growth factor; PlGF, placenta-derived growth factor; PDGF, platelet-derived growth factor; IL, interleukin; MMPs,

matrix metalloproteinases; Ang, angiopoietin; EC, endothelial cell.

8 4 2 P . V E L A S C O A N D B . L A N G E - A S S C H E N F E L D T

� 2002 British Association of Dermatologists, British Journal of Dermatology, 147, 841–852

VEGF-C was the first lymphangiogenesis factor des-

cribed.39,40 It binds to VEGFR-3, also known as Flt-4,

normally expressed exclusively on the lymphaticendothelium, and to VEGFR-2.39,40 VEGF-C has re-

cently been demonstrated to promote tumour lymph-

angiogenesis, angiogenesis and metastasis.41–44 Therecent discovery of specific markers for lymphatic

vessels such as the lymphatic vessel endothelial hya-

luronan receptor-145 and the homeobox genePROX146 promises to facilitate studies of lymphangio-

genesis in tumour biology.

Angiogenesis in vascular tumoursof the skin

There are several described benign cutaneous vascular

neoplasms such as pyogenic granuloma and angio-keratoma as well as malignant vascular tumours such

as angiosarcoma, yet most studies concerning angio-

genesis in cutaneous vascular tumours have beenperformed using haemangioma and Kaposi’s sarcoma

(KS) as models.

Haemangioma is the most common neoplasm ofinfancy. This endothelial tumour undergoes a prolifer-

ative phase followed by an involution phase leading to

the spontaneous regression of the lesion (involutedhaemangioma). Normally haemangioma presents no

medical threat but sometimes the fast-growing nature

of this tumour can interfere with normal functions. Themost common complications are circulatory problems

arising from high shunt volumes or vision impairment

resulting from the physical obstruction of the eye.Furthermore, trauma of extensive haemangiomas

can cause pronounced bleeding. In some cases haem-

angiomas can develop into severe diseases such as

Kasabach–Meritt syndrome where dysfunction of the

coagulation system can be lethal.

Proliferating haemangiomas are composed of denselypacked endothelial cells with little connective tissue

and barely discernible vessel lumina. This hyperprolif-

erative endothelial cell population has recently beendemonstrated to be clonal, originating from a progen-

itor cell that could have a mutated angiogenesis

regulation gene. A second group has suggested thatthe clonal cell population could arise from one or a few

placental endothelial cells that have migrated into the

fetal circulation from chorionic villi (reviewed byMarchuk47).

Proliferating haemangiomas overexpress a plethora

of angiogenesis activators (Table 1). Among them areFGF-2,48 monocyte chemoattractant protein-149 and

VEGF-A.5 However, low expression levels of VEGF-A in

haemangioma-derived endothelial cells and in prolifer-ating lesions have recently been reported. This study

identifies the angiopoietins as potential modulators in

the pathological growth and vascular remodellingevident in haemangioma.50 The group reports up-

regulation of Tie-2 expression both in the haemangi-

oma-derived endothelial cells and in the proliferatinglesions.50 Notably, previous studies have linked Tie-2

expression abnormalities to other venous malforma-

tions.51

A small number of thin-walled vessels that resemble

normal capillaries surrounded by regions of fibrofatty

tissue characterizes involuting and involuted haeman-gioma. Interestingly, during this phase the expression

levels of the endogenous angiogenesis suppressors

tissue inhibitor of MMP (TIMP)-1 and interferon(IFN)-b are up-regulated when compared with the

proliferative phase.48,52

Table 2. Partial list of endogenous angiogenesis inhibitors demonstrated successfully to reduce the experimental tumour growth of cutaneous

lesions

Molecule Function H K M S B

Ang-1 Stabilizes blood vessels, inhibits vascular permeability •Angiostatin Inhibits tumour angiogenesis, mechanism is unclear • • • •Endostatin Inhibits EC proliferation and migration as well as apoptosis •Interferon-a Inhibits the production of VEGF-A and FGF-2 • • • •IL-12 Up-regulates interferon-c and IP-10 • • •IP-10 Down-regulates FGF-2 • •Platelet factor 4 Inhibits FGF-2 and VEGF •TSP-1 Inhibits EC growth, migration, tube formation and induces apoptosis • •TSP-2 Inhibits EC migration and tumour angiogenesis •TIMPs (1, 2…) Inhibit MMP activity, tumour growth and metastasis • • •

H, haemangioma; K, Kaposi’s sarcoma; M, melanoma; S, squamous cell carcinoma; B, basal cell carcinoma; Ang, angiopoietin; IL, interleukin; IP,

interferon inducible protein; TSP, thrombospondin; MMP, matrix metalloproteinase; TIMPs, tissue inhibitor of MMPs; EC, endothelial cell; VEGF,vascular endothelial growth factor; FGF-2, basic fibroblast growth factor.

D E R M A T O L O G I C A L A S P E C T S O F A N G I O G E N E S I S 8 4 3


Clinically, corticosteroid therapy and the angiogen-esis inhibitor IFN-a are used as therapeutic agents for

life-threatening haemangioma. However, because of

side-effects and inconsistent success rates, new therap-ies are still necessary. Both endogenous and exogenous

angiogenesis inhibitors have been reported successfully

to reduce the experimental tumour growth of haeman-gioma (Table 2). Angiostatin has been found to inhibit

murine haemangioendothelioma growth.53 TIMP-254

and a VEGF-toxin conjugate,55 as well as syntheticcompounds such as TNP-470 (a derivative of fumagil-

lin)56 and batimastat (a synthetic MMP inhibitor)57

have been reported effectively to reduce experimentaltumour growth.

Understanding the cellular and biochemical mecha-

nisms that control the growth and spontaneousregression of haemangioma would provide critical

insights into aberrant angiogenesis and the mecha-

nisms that exist to override this defect. It seemsimportant to investigate whether at later stages in

childhood, endogenous angiogenesis inhibitors are

naturally up-regulated in order to counteract theangiogenic stimuli necessary for growth during child-

hood. This could explain the spontaneous regression of

haemangioma.KS is an angiogenic tumour affecting mostly the skin

and mucosa. It is found endogenously among several

ethnic groups but is more commonly associated withimmunological disorders arising from organ trans-

plants and human immunodeficiency virus (HIV)

infection.58,59 It is vascular in origin with tumoursconsisting mainly of hyperproliferative spindle-shaped

tumour cells, inflammatory cell infiltrate and numer-

ous blood vessels, and is frequently accompanied byextensive oedema.60

Many factors and cytokines that promote angiogen-

esis, lymphangiogenesis and vascular remodelling areknown to be overexpressed in KS cells and lesions

(Table 1). Among them are VEGF-A,61 FGF-2,3 IL-662

and IL-8.6 Recently, IL-8 has been identified as animportant angiogenesis factor and autocrine growth

factor in KS, as the functional inhibition of this

proinflammatory cytokine using a neutralizing anti-body resulted in a significant inhibition of experimental

tumour growth.6 Additionally, strong overexpressionof Ang-2, Tie-1 and Tie-2 highlights a potential role of

the angiopoietin family in the pathobiology of both KS

and angiosarcoma.63 Lastly, it has been suggested thatVEGF-C acts as a paracrine growth factor for KS

because its receptors VEGFR-2 and VEGFR-3 have been

demonstrated to be overexpressed by KS tumour

cells.64 In vitro, VEGF-C has been shown to stimulatethe migration and proliferation of KS cells.65 As the

expression of VEGF-C and VEGFR-3 is normally con-

fined to the lymphatic endothelium, it has beensuggested that KS cells might be of lymphatic origin.64

There is convincing evidence that the angiogenic

activity of KS cells is linked to viral infection. Humanherpesvirus 8 (HHV-8) expression is found in both

endogenous and induced KS and is thought to stimu-

late the release of inflammatory cytokines that canconvert nascent endothelial cells into KS-like spindle

cells.66 Furthermore, HHV-8-transformed endothelial

cells overexpress VEGF-A, VEGF-C and VEGF-D.67

However, KS is more common and aggressive in

conjunction with HIV infection. In fact, HIV-1-infected

T cells secrete a transactivator of viral gene expression(Tat), a protein that promotes the migration, invasion,

growth and adhesion of the spindle cells and tumour-

related endothelial cells in vitro.68 Tat evokes apronounced angiogenic effect on endothelial cells when

combined with inflammatory cytokines68 such as IL-

1b, tumour necrosis factor-a and IFN-c, and synergizeswith FGF-2 in vivo.69 Evidently, KS is a model for

altered angiogenesis as a result of either viral infection

or immunosuppressive therapy. Additional investiga-tions of this tumour might elucidate this link and reveal

its potential relevance in other cutaneous tumours.

Highly vascularized tumours such as KS are goodcandidates for therapeutic angiogenesis inhibition. In

fact, many experimental approaches have been inves-

tigated to treat KS with both endogenous and syntheticangiogenesis inhibitors (Table 2). TSP-1,70 IFN-a and

IFN-b71 have been shown to inhibit angiogenesis and

diminish tumour growth in KS. These endogenousangiogenesis inhibitors are thought to suppress the

proangiogenic Tat protein.71,72 Furthermore, IFN-aand IFN-b directly suppress the migration of endothel-ial cells and their organization into capillary-like struc-

tures.72 IFN-b also down-regulates the expression of

FGF-2 and lowers the production of MMP-2.71 TSP-1reportedly binds FGF-273 and modulates the production

of MMP-2 and TIMP-2.73 Other natural compounds

such as antineoplastic urinary protein74 have beenshown to diminish experimental KS growth. Further-

more, gene therapy approaches have been createdwhereby IFN- or angiostatin-producing cells are im-

planted into KS-bearing mice, resulting in inhibition of

both angiogenesis and tumour growth.73,75 Recently,the HIV proteinase inhibitors indinavir and saquinavir

have been shown to exert a potent antiangiogenic and

antitumour effect on experimental KS, suggesting that



these molecules could be used in the treatment of otherHIV-related tumours and in non-HIV KS.76 Finally,

several synthetic compounds such as TNP-470,77

IM862,78 N-acetylcysteine,79,80 SU541681 and tha-lidomide82 have been found to act as angiogenesis

inhibitors in KS and angiosarcoma tumour growth.83

Angiogenesis in squamous cell carcinoma

Squamous cell carcinoma (SCC) is a malignant cancer

of the epidermal keratinocytes with a destructive

growth pattern and the ability to metastasize. Itsdevelopment is a multistep process, in which cells

accumulate genetic alterations thereby gradually con-

verting from normal to malignant cells. In humans,60% of cutaneous SCCs develop from actinic keratoses.

These premalignant lesions exhibit up to 20% probab-

ility of becoming malignant84 and carry similar geneticchanges also found in SCCs, such as mutations in the

p53 tumour suppressor gene.85

The VEGF family has been shown to play a funda-mental role in the growth and invasion of SCC. The

experimental overexpression of VEGF-A alone in a non-

aggressive SCC clone could induce tumour growth andinvasion in vivo.17 Moreover, blocking VEGFR-2 func-

tion using a neutralizing antibody resulted in the

inhibition of angiogenesis and invasion of malignantSCC.86 VEGFR-2 expression has been shown to be

transiently up-regulated in actinic keratosis and to be

continuously overexpressed in malignant lesions, sug-gesting a key role of the receptor in SCC tumorigenic-

ity.86 Meanwhile, VEGF-A expression levels in

premalignant and malignant lesions have been repor-ted to be similar.87 In these tumour biopsies, increased

neovascularization has been observed only in the later

stages of the tumour, indicating a late onset of theangiogenic switch.87 Yet, in a transgenic model, the

angiogenic switch in SCC was reported to occur early,

before a malignant lesion was evident.26 Interestingly,a recent report shows that VEGFR-2 up-regulation in

experimental tumours occurs earlier than the up-

regulation of VEGF-A. The group concludes that aslow levels of VEGF-A can normally be found in

tumours, the rate-limiting step for tumour angiogenesis

is the induction of VEGFR-2.25

TSP-1 and TSP-2 expression in the basement mem-

brane separating the dermis from the epidermis is lostin invasive SCC, suggesting that the down-regulation of

TSP-1 and TSP-2 is a requisite step for SCC invasion.29

Interestingly, some SCC cell lines have a deletion inchromosome 15, where the TSP-1 gene has been

mapped.32 Adding a copy of chromosome 15 or TSP-1to one of these cell lines resulted in tumour growth

suppression. These experiments provide convincing

evidence that the aberrant expression of this endog-enous angiogenesis inhibitor is an important aspect of

SCC.32 Furthermore, Streit et al. demonstrated that the

experimental growth of SCC is significantly inhibitedwhen the cells overexpress TSP-1 and ⁄ or TSP-2.33,34

In fact, cell-based antiangiogenic gene therapy of SCC

has resulted in the successful inhibition of experimentaltumour growth using TSP-2-and angiostatin-produ-

cing cells.88

Recently, Ang-1-overexpressing SCC cells wereshown to have diminished tumour growth.89 No major

changes in the vascular density were found in these

tumours, suggesting that other vascular mechanismscould be responsible for the observed inhibition. Inter-

estingly, a higher number of mature vessels composed

of endothelial cells closely associated with pericytes wasreported in these tumours. Previous studies have

shown that mature vessels have a diminished ability

to sprout and form new capillaries90–92 and, further-more, have been found in other tumours subjected to

antiangiogenesis treatment.93

Angiogenesis in melanoma

Cutaneous malignant melanoma is known for its poor

prognosis and high resistance to treatment as well as

its steadily increasing incidence rate. Distinct sequen-tial steps from a common acquired melanocytic naevus

to a dysplastic naevus leading to a radial growth phase

primary melanoma characterize the progression oftumour malignancy. This primary phase is followed

by a vertical growth phase that is linked to high

angiogenic activity and can result in metastasis.94

Melanoma tends to spread via the lymphatic vessels to

the area surrounding the tumour and to the draining

lymph nodes. In later stages, melanoma cells metasta-size distal from the primary tumour mainly to the lung,

cerebrum and liver. Conventional chemotherapy treat-

ments currently available are not very promising andmany research efforts address potential alternative

therapies.

Many potent angiogenesis factors such as VEGF-A,95,96 FGF-2,4 IL-8,7 placenta-derived growth factor97

and Ang-222 have been observed in human melanomacells and tumours (Table 1). VEGF-A-transfected mel-

anomas are characterized by increased angiogenesis

and tumour growth.95 Furthermore, melanoma celllines that develop low metastatic experimental tumours



express low levels of VEGF-A compared with highmetastatic tumour xenografts.15 Importantly, several

receptors previously thought to be exclusively ex-

pressed on endothelial cells such as VEGFR-1, VEGFR-2 and Tie-2 are also expressed on different tumour

cells. Therefore, growth factors formerly described as

specific endothelium modulators are less particularthan previously believed. In fact, in addition to proan-

giogenic roles, VEGF-A, FGF-2 and IL-8 function as

autocrine and paracrine growth factors in melanoma.Moreover, functional blocking studies of these factors

using neutralizing antibodies resulted in the inhibition

of melanoma metastasis.7 Recent experiments impli-cate the Tie-2 signal pathway as an important angi-

ogenesis modulator in melanoma. Blocking the Tie-2

signal using dominant-negative receptor domains98

and using an adenoviral vector to deliver recombinant

soluble Tie-2 resulted in the inhibition of experimental

melanoma growth.99 Several reports indicate that theexpression of the integrin avb3 correlates with the

vertical growth phase of melanoma and could play a

role in the progression of the tumour.100 Functionalblocking studies using antibodies directed against the

avb3 integrin inhibited the experimental growth of

melanoma.101 The integrin family of cell adhesionreceptors mediates cellular interaction with the extra-

cellular matrix and is required for angiogenesis.102

Several experimental approaches to treat melanomausing angiogenesis inhibitors have been reported. The

endogenous inhibitors TSP-1,103 TSP-1 fragments,104

TSP-2,88 IL-12,105 angiostatin,106 endostatin107 andother compounds such as TNP-470,108 thalidomide,109

SU6668,110 SR25989111 and batimastat112 have been

reported effectively to inhibit experimental melanomagrowth. However, Kruger et al. recently found that

batimastat treatment of T-cell lymphoma promoted

liver metastasis, presumably by inhibiting the genera-tion of endostatin by MMPs.113 This report demon-

strates the unexpected side-effects that can result from

the therapeutic use of these novel compounds.The importance of lymphangiogenesis in the spread

of melanoma has recently been reported. VEGF-C is

known to induce lymphatic and vascular endothelialcell proliferation in vivo and is expressed in human

melanoma cells.41,114 Skobe et al. recently reported anincrease in the number of both intratumour lymphatic

and blood vessels using a VEGF-C-overexpressing

melanoma model.41 The authors and other groups42,43

have shown that overexpression of VEGF-C in different

tumour models induced a significant increase of lung

and lymph node metastasis, presumably through the

tumour-associated lymphatic vessels. However, over-expression of VEGF-C did not result in an increase in

the experimental growth of melanoma41 or breast

tumours.42 Finally, VEGF-D has also been implicated ininducing lymphangiogenesis in melanoma. VEGF-D is

expressed in the lesions115 and blocking its activity

diminished experimental metastasis, suggesting thatthis factor might be involved in the spread of melan-

oma.116

Angiogenesis in basal cell carcinoma

Basal cell carcinoma (BCC) is the most common

cutaneous cancer found in humans. This well-vascu-

larized and slow-growing tumour generally follows arelatively benign course, although in some cases BCC

can become aggressive, invade deeper skin structures

and even metastasize. However, few studies haveaddressed the role of angiogenesis in BCC.

VEGF-A is normally absent in BCC. Yet, in contrast to

nascent BCC tumour cells, IL-6-transfected cells dem-onstrated accelerated growth and tended to invade the

area around the xenotransplant in mice. Tumour cells

in these lesions expressed VEGF-A,16 thereby providingfurther evidence that the expression of VEGF-A is

important for tumour growth and tumour invasion.

Macroscopic telangiectasias are a hallmark of BCC.These superficially visible blood vessels allow for a

relatively simple method to observe and quantify blood

vessels during tumour development, making BCC apossible model to study the effects of angiogenesis

inhibition on tumour microcirculation.117 Further

research will clarify the role of angiogenesis in BCCand elucidate whether the onset of the angiogenic

switch is linked to the increasingly aggressive pheno-

type sometimes evident in this tumour.

Angiogenesis in cutaneous lymphomas

Although few studies address the role of angiogenesis

in lymphoproliferative diseases of the skin, a correlationbetween microvascular density (MVD) and growth

dynamics of cutaneous lymphomas has recently been

described. Vacca et al. have shown that the MVD andthe expression of angiogenesis inducers such as MMP-2

and MMP-9 undergo significant up-regulation in rela-

tion to advancing stages of mycosis fungoides.118

Similar results relating vascular density and the

aggressiveness of cutaneous B-cell lymphomashave been reported.119 Interestingly, little is known

about angiogenesis factors or inhibitors in cutaneous



lymphomas. The lack of alternative treatment optionsfor cutaneous lymphoma and the long duration of this

disease call for new therapy concepts. Angiogenesis

inhibitors could be used in the early stages of thedisease in order to keep the tumour in a dormant state,

preventing the onset of the angiogenic switch. To date,

there is no evidence of drug resistance to angiogenesisinhibitors, making them an appealing choice for

long-term treatments particularly in slow-developing

tumours.120

Angiogenesis as a significant prognosticfactor?

In the last decade, several studies have revealed acorrelation between an increase in intratumour MVD

and different parameters of tumour aggressiveness,

metastasis and patient survival.121 However, there arecontradictory reports showing no such correla-

tion.122,123 These discrepancies could be a result of

the methodology selected by different researchers orvariations in the tumour stage of investigated samples.

In the skin, haemangioma and pyogenic granuloma

are examples of extremely well-vascularized fast-grow-ing cutaneous tumours that are nonetheless benign

and non-invasive. Additionally, the MVD in premalig-

nant keratoacanthoma and SCC is similar, showing norelationship with tumour progression.123–125 These

observations question the validity of MVD as a reliable

parameter for the grade of malignancy or prognosis incutaneous tumours. In melanoma, the expression

patterns of factors responsible for angiogenesis and

invasion such as VEGF-A, FGF-2 and IL-8 are a usefulprognostic tool126 that could perhaps improve future

clinical diagnosis for different malignancies. Moreover,

lymphatic vessel density and lymphatic growth factorexpression might some day be a valuable prognostic

tool for patient outcome.127

Angiogenesis inhibitors in clinical trials

There are currently several ongoing clinical trials using

antiangiogenic substances either alone or in combina-

tion with conventional therapies targeting differentcutaneous tumours such as melanoma, KS and T-cell

lymphoma. These trials are based on different experi-

mental strategies for angiogenesis inhibition. Somecompounds interfere with proangiogenic ligands, their

receptors or downstream signals (SU5416). Others up-regulate or deliver endogenous angiogenesis inhibitors

(IL-12) or directly target the tumour vasculature

(thalidomide). Previous trials have reported a partialresponse in the treatment of KS using thalidomide128

and stabilization of melanoma patients using carboxy-

amidotriazole, an inhibitor of calcium-mediated signaltransduction.129,130 Table 3 lists substances currently

undergoing clinical trials. However, most of the com-

pounds listed are in phase II trials and the outcome forcutaneous tumours has not yet been evaluated (more

detailed and up-to-date information is available at

www.nci.nih.gov/clinical_trials).Interestingly, several angiogenesis inhibitors have

proven unsuccessful in clinical trials. These failures

could result from potential problems such as dosage,formulation or drug delivery strategies. Moreover,

during tumour growth a variety of angiogenic mole-

cules is produced by both stroma and tumour cells.Blocking the function of one of these molecules may

not successfully eradicate the tumour as other factors

may be up-regulated and counter the inhibitory effect.Perhaps a mixture of compounds blocking several

angiogenic pathways might be the best therapeutic

approach.Other factors may account for the discrepancy

between experimental results and the outcome of

human clinical trials. Preclinical data are gathered inimmune compromised mice with specially selected fast-

growing tumour cell lines. The short experimental

treatment often commences before or at the time oftumour cell inoculation, whereas oncology treatment

in humans involves drug administration over several

years beginning once the tumour has been detected.

Table 3. List of antiangiogenic compounds currently undergoing

clinical trials for the treatment of different cutaneous tumours

Cutaneous T-cell lymphoma

CC-5013 Thalidomide analogue

Carboxyamidotriazole Inhibits VEGF-A-induced EC proliferationInterferon-a2b Inhibits VEGF-A and FGF-2 production

Thalidomide Inhibits ECs, mechanism is unknown

Melanoma

Bevacizumab Monoclonal anti-VEGF-A antibodyCarboxyamidotriazole Inhibits VEGF-A-induced EC proliferation

Interferon-a Inhibits VEGF-A and FGF-2 production

SU5416 Inhibits VEGF-A receptorThalidomide Inhibits ECs, mechanism is unknown

Kaposi’s sarcoma

BMS-275291 MMP inhibitor

COL-3 MMP inhibitorInterleukin-12 Up-regulates interferon-c and IP-10

VEGF, vascular endothelial growth factor; EC, endothelial cell; FGF,

basic fibroblast growth factor; MMP, matrix metalloproteinase; IP,interferon inducible protein.



Furthermore, patients eligible for clinical trials areoften afflicted with advanced stages of the disease that

include metastases. Novel antiangiogenesis treatments

have been met with great enthusiasm and extremelyhigh expectations of complete tumour eradication. Yet,

little is known about the potential side-effects of long-

term treatments, and extensive toxicity studies mustsoon follow. Potential side-effects, such as impaired

wound healing, remain to be evaluated and recent

studies addressing this question have reported conflict-ing results. Mice treated with TNP-470 exhibited

delayed wound healing,131 while endostatin and vas-

ostatin treatment did not affect cutaneous woundhealing at tumour-inhibiting doses.93,132

Finally, new experimental strategies seek to influence

other vascular parameters such as leakage or thematuration status of blood vessels in order to diminish

the sprouting capability of capillaries. Furthermore,

new approaches have targeted the tumour vasculaturebased on the assumption that most endothelial cells in

tumours express distinct markers (�zip codes�). Identi-

fying this heterogeneous group of surface molecules ondifferent endothelial cell populations133 could be useful

for more precise drug targeting strategies.134

Implications of angiogenesis in dermatology

Many cutaneous diseases could potentially benefit from

antiangiogenic treatments. Potential targets include

the vascular tumours discussed and less commontumours such as angiokeratoma. Moreover, therapeu-

tic neovascularization and its inhibition is not only

restricted to neoplastic skin diseases. Blood vessels areinvolved in a variety of cutaneous diseases and are

often of essential diagnostic value. Different types of

vessel growth can be observed in diseased skin. Forexample, telangiectasias are smaller forms of neovas-

cularization and are a hallmark of certain pathologies

such as rosacea or cutaneous autoimmune disease.Dilated, elongated and tortuous blood vessels are a

typical feature of inflammatory diseases such as

psoriasis. Subsequent investigations will elucidate thepotential value of treating inflammatory disorders of

the skin by blocking angiogenesis. Other potential

targets include fast-growing hypertrophic scars andkeloids that could be equally dependent on a vascular

supply. Furthermore, the administration of angiogen-esis factors on chronically impaired healing wounds

such as diabetic ulcers might stimulate the formation

of granulation tissue and thereby improve the overallwound healing process. It has been recently reported

that angiogenesis plays an important role in ultravi-olet-induced skin ageing135 and hair growth.27 Fur-

ther studies are necessary to reveal whether modulating

angiogenesis can successfully influence these processes.Ongoing and future clinical studies will reveal the

potential of antiangiogenic and proangiogenic therapies

for the treatment of cutaneous pathologies.Skin angiogenesis is easily visualized and facilitates

the use of techniques such as surgically created

windows or intravital microscopy to quantify bloodvessel growth and to study the function of blood vessels

in tumours. Both neovascularization and angiogenesis

inhibition can be directly observed in the skin. Thischaracteristic has made the skin an attractive organ for

the development of experimental assays such as

carcinogenesis and orthotopic tumour growth. Theseexperimental approaches can be combined with thera-

peutic trials to test the effects of substances such as

angiogenesis inhibitors. Dermatologists face the chal-lenge to develop drug delivery systems for locally admin-

istered angiogenesis factors and inhibitors avoiding

systemic side-effects. Lastly, only the dermatologist canidentify the wide range of cutaneous diseases that could

potentially benefit from this novel treatment concept.

Acknowledgments

The authors thank Prof. Enno Christophers, Department

of Dermatology, University of Kiel, Prof. Michael Detmar,

Department of Dermatology, Massachusetts GeneralHospital and Medical School, and Prof. Mihaela Skobe

Derald H. Ruttenberg Cancer Center, Mount Sinai School

of Medicine, New York for critical and helpful advice.

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