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Angiogenesis-Based Dermatology

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Though with some conflicting results, the use of β-blockers has been linked 
to increased survival of patients with solid cancers [56]. In addition, stressful life con-
ditions have been shown to correlate with less favorable prognosis of cancer patients. 
In mouse models of tumors β-adrenergic agonists have also been found accelerate 
tumor progression and metastasis [57, 58]. As expected by the high percentage of 
cAMP responsive genes, many cellular processes in tumors were shown to be modu-
lated by the β-adrenergic system, including expression of pro- inflammatory cytokines, 
of macrophages recruitment, angiogenesis, invasiveness and apoptosis [51, 59].
The study of the mechanism of action of propranolol on IH is much “Younger”. 
Yet, much progress has achieved during recent years. Studies done on human cells 
isolated from IH in vitro and in vivo, on mouse models, revealed an effect on 
three major processes that will be detailed below: vascular tone, angiogenesis and 
 Vascular Tone
Following the administration of propranolol, a rapid change in the tumor consis-
tency and color is typically noticed, especially with deep-seated IH [40, 60]. This 
raises the question of whether propranolol has an effect on the tumor vacular tone 
via vasoconstriction. Indeed, propranolol has been shown to decrease tissue blood 
flow to many organs following single administration [61–63]. Particularly in the 
skin, adrenaline-induced vasoconstriction has been shown to be increased by oral 
propranolol [64]. However, additional mechanisms might be involved in the slower, 
2 Infantile Hemangioma: New Insights on Pathogenesis and Beta Blockers
long-term effect of propranolol. A potential target of this mechanism of action is the 
pericyte. Pericytes are regulators of microvascular tone. Bosclo et al. demonstrated 
that pericytes from proliferating IH have lower density of the cytoskeleton compo-
nent F-actin fiber compared to involuting Hemangioma pericytes and retinal peri-
cytes. Also, these cells exhibit lower contractile capacity compared to normal 
pericytes [36]. In another work from the same group Lee et al. demonstrated that 
Epinephrine-induced relaxation of IH pericytes was prevented by propranolol. 
Using siRNA assay it was demonstrated that both the relaxation and its prevention 
by propranolol were mediated by the β2 receptor [65]. Interestingly, cultured peri-
cytes from other sources have been shown to constrict, not relax, in response to 
catecholamines [66, 67]. Thus, the response of pericytes to NE and its blockers 
might be cell and context dependent.
In addition to direct effect of propranolol on the pericytes to induce contraction, 
it might exert its effect indirectly via the blocking of Nitric oxide release from 
HemEC. The endothelial isoform of nitric-oxide synthase (eNOS), is a key determi-
nant of vascular tone. [68, 69] In different tissues and experimental systems, diverse 
adrenergic receptors subtypes have been shown to modulate of eNOS expression 
and activity, including the β1, 2 and 3 adrenergic receptors [70, 71], eNOS protein 
expression has been demonstrated to be significantly decreased in involuting versus 
proliferating hemangiomas [72]. In patients treated with propranolol, a significant 
decrease in the expression of eNOS in hemangioma tissues was noted compared 
with the age-matched untreated controls [72]. In addition, Wei-li Yuan have shown 
that serum concentrations of eNOS declined gradually during the first 2 months of 
propranolol treatment for IH [73].
Vascular endothelial growth factor (VEGF) has been shown to play a pivotal role 
in the angiogenic process in general and specifically in IH proliferation. The 
VEGF family consists of five ligands VEGF-A, VEGF-B, VEGF-C, VEGF-D 
and placental growth factor (PIGF). These ligands bind to three main subtypes 
of VEGF receptors: VEGFR-1 (Flt-1), VEGFR-2 (KDR/Flk-1)and VEGFR-3 
[74]. Several works showed over-expression of VEGF-A in proliferating hem-
angioma tissue and in serum of patients with IH, compared to healthy controls 
[37, 75–77]. Moreover, we have shown that silencing the expression of VEGF-A 
in HemSC by short hairpin RNA (shRNA) was sufficient to block blood vessel 
formation in vivo [37]. NE enhances VEGF-A expression of both normal and 
tumoral cell types, [78–80]. Propranolol has been shown to reverse this effect via 
the blocking β1 and β2 adrenergic receptors [78–81]. Several lines of evidence 
suggest that propranolol acts at least in part through the blocking of VEGF. First, 
serum VEGF levels were shown to decrease noticeably in 91% of patients after 
a single month of propranolol treatment [82]. Similar results were reported by 
another group following the administration of relatively low doses of propranolol 
[73]. Second, in vitro studies have demonstrated that the NE agonist isoprenaline 
S. Greenberger
increased the expression of VEGF-A and the phosphorylation of VEGFR-2 in 
HemECs in a β-adrenergic receptor- and extracellular-signal-regulated kinase 
(ERK) -dependent manner. This response was blocked by β-adrenergic blockers. 
In HemSC, Ling Zhang et al. showed that propranolol at physiological concen-
trations leads to dose-dependent suppression of VEGF expression, at the mRNA 
and the protein level [83].
An additional major regulator of angiogenesis is Hypoxia-inducible factor 
(HIF)-1α. During hypoxia, HIF-1α binds the regulatory region of the VEGF gene, 
inducing its transcription and initiating its expression [84, 85]. HIF-1α expression 
has shown to be increased in the endothelium of proliferating hemangioma com-
pared with involuting tissues [77]. Recently, it has been shown that HIF-1α was 
upregulated in the serum, urine and tumor tissues of IH, and treatment of proprano-
lol markedly inhibited its expression. In vitro, in HemEC, overexpression of HIF-1α 
blocked the inhibitory effects of propranolol on VEGF expression [86].
Several groups have demonstrated a direct pro-apoptotic or anti proliferative 
effect of propranolol on HemEC. However, a major concern is the high concentra-
tions of propranolol required for these effects. These drug levels are unlikely to be 
present in the tumor’s microenvironment [87]. It is possible, though, that proprano-
lol works by opposing the growth promoting effects of catecholamines. Ji et al. 
showed that HemECs proliferation increased in response to isoprenaline via regula-
tion of thee cell-cycle proteins cyclin D1 and its associated kinases, CDK-4 and 
CDK-6. These effects were reversed by β-adrenergic receptor antagonists. Of note, 
the antagonists had no effect on basal cell proliferation, but significantly decreased 
ISO-induced cell proliferation and cell viability [88]
Vasculogenesis, the creation of blood vessels de-novo from stem/progenitor cells, 
contributes the proliferation of IH. Thus, it is appealing to hypothesize that pro-
pranolol acts as an inhibitor of this process, as was shown for corticosteroids and 
rapamycin [89]. Against this hypothesis is the fact that late regrowth of the heman-
gioma seen in a subset of patients after cessation of the treatment [90]. This might 
mean that propranolol does not target the HemSC or even prevents their terminal 
differentiation or apoptosis. Effect of the adrenergic system on non-hemangioma 
stem/mesenchymal cells has been shown by several works. For example, NE has 
been shown to induce brown adipocyte differentiation of mesenchymal progenitors 
within white adipose tissue [91]. Also, an effect of NE on mesenchymal stem cells 
adipogenesis has been demonstrated in-vitro cell [92]. In work done on HemSC, 
Wong et al. reported that Propranolol-treated cells had a more robust response to 
adipogenic induction when compared to vehicle-treated HemSCs [93]. In a follow-
 up work the same group has shown that this adipogenic differentiation is character-
ized by improper adipogenic gene expression [94]. Similar results were