Propranolol promotes accelerated and dysregulated adipogenesis in hemangioma stem cells

Infantile hemangiomas (IHs) are the most common tumor of infancy, yet there are no Food and Drug Administration-approved therapeutics to date. Recently, the nonselective β-adrenergic-blocker propranolol has been shown to be a safe and effective means of treating IHs, although its mechanism has yet to be elucidated. We have previously demonstrated that propranolol induces early and incomplete adipogenesis in stem cells derived from hemangiomas. We hypothesize that propranolol promotes dysregulated adipogenesis via the improper regulation of adipogenic genes.

Hemangioma stem cells (HemSCs) isolated from resected IH specimens were treated with adipogenic medium for 1 or 4 days in either propranolol or vehicle. Cell death was measured by the incorporation of annexin V and propidium iodide by flow cytometry. Adipogenesis was assessed by visualizing lipid droplet formation by Oil Red O staining. Proadipogenic genes C/EBPα, C/EBPβ, C/EBPδ, PPARδ, PPARγ, RXRα, and RXRγ were analyzed by quantitative reverse transcription and polymerase chain reaction.

Hemangioma stem cells treated with propranolol increased lipid droplet formation compared to vehicle-treated cells indicating increased adipogenesis. Cell death as measured by FACS analysis indicated that the propranolol-treated cells died due to necrosis and not apoptosis. During adipogenesis, transcript levels of PPARδ, PPARγ, C/EBPβ, and C/EBPδ were significantly increased (P<0.01) in propranolol-treated cells relative to control cells. In contrast, RXRα and RXRγ levels were significantly decreased (P<0.05), and C/EBPα, a gene required for terminal adipocyte differentiation, was strongly suppressed by propranolol when compared to vehicle-treated cells (P<0.01).

In HemSCs, propranolol accelerated dysregulated adipogenic differentiation characterized by improper adipogenic gene expression. Consistent with accelerated adipogenesis, propranolol significantly increased the expression of the proadipogenic genes, PPARγ, C/EBPβ, and C/EBPγ compared to control. However, propranolol treatment also led to improper induction of PPARδ and suppression of C/EBPα, RXRα, and RXRγ. Taken together these data indicate that propranolol promoted dysregulated adipogenesis and inhibited the HemSCs from becoming functional adipocytes, ultimately resulting in cell death. Understanding this mechanism behind propranolol's effectiveness will be a vital factor in producing more effective therapies in the future.