Infantile hemangiomas (IHs) are the most common vascular tumor and arise from a hemangioma stem cell (HemSC). Propranolol has proved efficacious for problematic IHs. Propranolol is a nonselective β-adrenergic receptor (βAR) antagonist that can lower cAMP levels and activate the mitogen-activated protein kinase (MAPK) pathway downstream of βARs. We found that HemSCs express β1AR and β2AR in proliferating IHs and determined the role of these βARs and the downstream pathways in mediating propranolol's effects. In isolated HemSCs, propranolol suppressed cAMP levels and activated extracellular signal-regulated kinase (ERK)1/2 in a dose-dependent fashion. Propranolol, used at doses of <10−4 M, reduced cAMP levels and decreased HemSC proliferation and viability. Propranolol at ≥10−5 M reduced cAMP levels and activated ERK1/2, and this correlated with HemSC apoptosis and cytotoxicity at ≥10−4 M. Stimulation with a βAR agonist, isoprenaline, promoted HemSC proliferation and rescued the antiproliferative effects of propranolol, suggesting that propranolol inhibits βAR signaling in HemSCs. Treatment with a cAMP analog or a MAPK inhibitor partially rescued the HemSC cell viability suppressed by propranolol. A selective β2AR antagonist mirrored propranolol's effects on HemSCs in a dose-dependent fashion, and a selective β1AR antagonist had no effect, supporting a role for β2AR signaling in IH pathobiology. In a mouse model of IH, propranolol reduced the vessel caliber and blood flow assessed by ultrasound Doppler and increased activation of ERK1/2 in IH cells. We have thus demonstrated that propranolol acts on HemSCs in IH to suppress proliferation and promote apoptosis in a dose-dependent fashion via β2AR perturbation, resulting in reduced cAMP and MAPK activation.Significance: The present study investigated the action of propranolol in infantile hemangiomas (IHs). IHs are the most common vascular tumor in children and have been proposed to arise from a hemangioma stem cell (HemSC). Propranolol, a nonselective β-adrenergic receptor (βAR) antagonist, has proven efficacy; however, understanding of its mechanism of action on HemSCs is limited. The presented data demonstrate that propranolol, via βAR perturbation, dose dependently suppresses cAMP levels and activated extracellular signal-regulated kinase 1/2. Furthermore, propranolol acts via perturbation of β2AR, and not β1AR, although both receptors are expressed in HemSCs. These results provide important insight into propranolol's action in IHs and can be used to guide the development of more targeted therapy.

Background: Propranolol, a nonselective β-adrenergic receptor antagonist, is approved by the U.S. Food and Drug Administration to treat problematic infantile hemangiomas, but a subset of patients experience treatment complications. Parents wary of long-term use and side effects consult plastic surgeons on surgical options or as a second opinion. Understanding the mechanism(s) of action of propranolol will allow plastic surgeons to better inform parents. Methods: A systemic literature search was performed to query published translational and basic science studies on propranolol effects on infantile hemangiomas and cells derived from these lesions. Results: In experimental studies, propranolol was antiproliferative and cytotoxic against hemangioma endothelial and stem cells and affected infantile hemangioma perivascular cell contractility. Propranolol inhibited migration, network formation, vascular endothelial growth factor A production, and vascular endothelial growth factor receptor 2 activation and down-regulated PI3K/AKT and mitogen-activated protein kinase signaling in hemangioma endothelial cells, but it increased ERK1/2 activity in hemangioma stem cells. At effective clinical doses, measured propranolol plasma concentration is 100 times higher than necessary for complete β-adrenergic receptor blockade, yet was 10 to 100 times less than required to induce hemangioma stem cell death. Conclusions: Propranolol targets multiple cell types in infantile hemangiomas by means of β-adrenergic receptor–dependent and –independent mechanisms. Plasma concentration played a significant role. At clinically relevant doses, incomplete infantile hemangioma suppression may explain the rebound phenomenon and worsening ulceration, and propranolol off target effects may lead to commonly reported adverse effects, such as sleep and gastrointestinal disturbances. Propranolol limitations and complications underscore the importance of surgical treatment options in cases of rebound and severe adverse effects. Surgical intervention remains an important treatment choice when parents are hesitant to use propranolol.

Infantile hemangioma (IH) is a vascular tumor that begins with rapid vascular proliferation shortly after birth, followed by vascular involution in early childhood. We have found that NOTCH3, a critical regulator of mural cell differentiation and maturation, is expressed in hemangioma stem cells (HemSCs), suggesting that NOTCH3 may function in HemSC-to–mural cell differentiation and pathological vessel stabilization. Here, we demonstrate that NOTCH3 is expressed in NG2 ⁺ PDGFRβ ⁺ perivascular HemSCs and CD31 ⁺ GLUT1 ⁺ hemangioma endothelial cells (HemECs) in proliferating IHs and becomes mostly restricted to the αSMA ⁺ NG2 ^lo PDGFRβ ^lo mural cells in involuting IHs. NOTCH3 knockdown in HemSCs inhibited in vitro mural cell differentiation and perturbed αSMA expression. In a mouse model of IH, NOTCH3 knockdown or systemic expression of the NOTCH3 inhibitor, NOTCH3 Decoy, significantly decreased IH blood flow, vessel caliber, and αSMA ⁺ perivascular cell coverage. Thus, NOTCH3 is necessary for HemSC-to–mural cell differentiation, and adequate perivascular cell coverage of IH vessels is required for IH vessel stability.

A formulation for analysis of dextrous workspaces of manipulators is presented, including mobility-limited joints, full-range dexterity requirements, full-range input requirements, and planar and spatial closed-loop kinematic structures. A terminology for mechanism and manipulator models that permits specification of input, output, and dexterity requirements is proposed and demonstrated to yield broadly applicable conditions necessary for boundaries of dextrous workspaces. Row-rank deficiency of constraint sub-Jacobian matrices is shown to yield necessary conditions for boundaries of accessible output sets and dextrous accessible output sets that characterize the workspace and dextrous workspace, respectively, of both planar and spatial manipulators. Three closed-loop planar manipulators are analyzed in detail and a three-dimensional, closed-loop Stewart platform is formulated to illustrate applicability of necessary conditions

Infantile hemangiomas (IHs) are the most common benign tumor of infancy. As our understanding of their pathobiology has evolved, treatment has become more focused and tailored to specifically treat IH while minimizing adverse effects. Propranolol has gained FDA approval as the first medical therapy for a traditionally surgical disease. This review provides readers with an overview of IH, treatment modalities, and addresses specific considerations in IH disease management.