Reply to “Paeonol Suppresses Vasculogenesis Through Regulating Vascular Smooth Muscle Phenotypic Switching”

We read the recent publication of Xu et al ¹ which investigated the effect of paeonol on vascular smooth muscle (VSMCs) phenotypic switching. Vascular smooth muscles can undergo phenotypic switching from a differentiated phenotype to a dedifferentiated state in several vascular pathological conditions including atherosclerosis or aneurysm. ² Dedifferentiated VSMCs are characterized by enhanced proliferation, migration, and decreased extracellular matrix deposition. ²

In this study, the investigators aimed to determine the role of paeonol using 3 models: (1) common carotid artery ligation-induced vascular injury to investigate its effect on vascular neointima formation, (2) in vitro experiments using rat aortic VSMCs to determine the effect of paeonol on phenotypic switching, and (3) murine hind limb ischemia model to explore if paeonol can suppress vasculogenesis.

Using hematoxylin and eosin (H&E) staining, the authors found that paeonol attenuated vascular injury-induced neointima formation in the carotid artery ligation-induced vascular injury model. It would have been of interest to analyze cell death on the sections as paeonol might induce apoptosis, which could partly explain the decreased neointima formation. In vitro experiments on rat VSMCs showed that treatment with paeonol increased the expression of smooth muscle-specific marker genes and decreased the expression of cell growth regulated genes, suggesting that paeonol suppressed phenotyping switching. However, only gene expressions were analyzed using quantitative polymerase chain reaction and these results should be confirmed based on protein expression. It has previously been described that subsets of dedifferentiated VSMCs can contribute to neointimal formation in mouse injury and atherosclerosis models. ³ It would be of interest to confirm whether paeonol changes VSMC phenotype in vivo and could partly explain the decreased neointima formation observed in the carotid artery ligation-induced vascular injury model. In vitro experiments further showed that paeonol suppressed the migration and proliferation of VSMCs. This was confirmed on the hind limb ischemia injury model, where mice pretreated with paeonol had a decreased neoangiogenesis, as shown by a decreased thickness and VSMCs number in the major arteries. Note that it would have been of interest to perform a functional test in mice after the ligation of the proximal femoral artery to validate the model of limb ischemia. It would also have been relevant to discuss the current knowledge on the pharmacodynamics of paeonol and what was the rationale to perform a pretreatment of 7 days.

While the individual results are of interest, the conclusion should be tempered. Indeed, while the authors observed a decreased phenotyping switching as a well as a decreased angiogenesis, it is extremely difficult to conclude to a causal relationship between the first and the latter. Finally, inflammation plays a key role in vascular remodeling ⁴ and it would be of interest to investigate the systemic and local role of paeonol on immune cells. Further studies should also be oriented toward investigating the role of paeonol on vascular immune cells and cross-talking with VSMCs.