ACE2 has been the first congenic compound of an angiotensin-converting enzyme discovered in recent years[13]. It is an important member of the renin-angiotensin system, which plays an important pathological role in cardiovascular and cerebrovascular diseases[14, 15][16]. Mainly expressed in the cardiovascular system, it can convert Angiotensin II to Angiotensin (1–7)[17, 18]; reduce macrophage infiltration; reduce monocyte MCP-1, IL-6, TNF-α, nuclear factor-kappaB (NF-κB), VCAM-1 and ROS levels; inhibit apoptosis, and increase NO release. It protects the endothelial cells and prevents the development of atherosclerotic plaques in vivo[19] and can regulate vascular function by regulating the release of nitric oxide and oxidative stress[20, 21]. Several studies have demonstrated the role of ACE2 in vascular injury. However, the potential role of ACE2 in KD has not yet been reported; therefore, the main purpose of this study was to detect the expression of ACE2 in children with KD and investigate the relationship between serum ACE2 levels and coronary artery injury in patients with KD. Our results showed that serum ACE2 levels were significantly higher in patients with KD than in the controls. Serum ACE2 levels in patients with KD were also positively correlated with CK-MB.
ACE2 is mainly present on the vascular endothelium, and in children with KD, it is also shed from the vascular endothelium into the serum, which in turn causes an increase in ACE2 in the serum. This finding is similar to that of ACE2 in coronary atherosclerosis and hypertensive heart disease [5, 22] [23] can induce endothelial cell apoptosis [19]and activate NF-κB through myeloid differentiation gene response 88- dependent and -independent pathways by upregulating the expression of Toll-like receptor 4 (TLR4) on the surface of dendritic cells, initiating inflammatory cytokine transcription, mediating inflammatory mediator secretion, apoptosis, and producing oxygen-free radicals production [24]. The mRNA expression of TLR4 and the levels of its related factors in children in the acute phase of KD are significantly higher than those in normal children [25]. In this study, the content of ACE2 in the serum of children with KD was significantly increased, which indicated that the up-regulation of serum ACE2 may be the mechanism of vascular injury and inflammation in the acute phase KD. In addition, the level of ACE2 in KD-CAL and KD-CAA was higher than that in the control group; however, this difference was not statistically significant, which may have been related to the small number of samples or the expression of ACE2 on a variety of cells. One recent study [26] has demonstrated that the expression of ACE2 is mainly in the endothelial tissues of arteries, arterioles, heart, and kidney, but is also found on the vascular smooth muscle of tubular epithelial cells, intrarenal arteries, and coronary vessels. This indicates that ACE2 may be insensitive and specific for the formation of CAL in KD patients; however, ACE2 may lead to the formation of CAL, and our study found that the level of ACE2 in KD-CAA was also increased. Its increasing trend was more obvious than that in CAL group, which may be because the vascular injury and aneurysm formation in KD is caused by inflammatory factors and oxidative stress. ACE2 has been reported to have antioxidant effects in most vascular injuries [20, 27]; hence, we speculate that the negative results of this study may have been related to the small sample size.
In this study, correlation analysis showed a negative correlation between serum ACE2 and platelet levels in patients with KD. In general, the platelet count of children with KD gradually increased from the first week of onset, peaked in the 2nd–3rd week of onset, and then gradually decreased. Platelet activation level was enhanced and closely related to cardiovascular injury and mortality. The increase in platelet activation level may be involved in coronary artery injury in children with KD, and 1% – 2% of children with KD had decreased platelets after onset, suggesting that those with thrombocytopenia were prone to CAA[28]. In children with KD, ACE2 on the vascular endothelium sheds into the serum and loses the effect of antagonizing platelets, further leading to platelet aggregation on the vascular endothelium. This in turn induces the formation of thrombosis on the vascular endothelium, which leads to vasodilatation and vascular endothelial injury, indicating that ACE2 may be related to vascular endothelial injury and the formation of hemangioma.
In addition, our study found that the CK-MB levels in the KD-CAL group were significantly higher than those in the KD-NCAL group. CK-MB is a marker for clinical judgment of myocarditis and myocardial injury; its increase is associated with the formation of coronary artery aneurysms [29, 30]. However, the level of ACE2 was also significantly increased in the KD-CAL group, suggesting that ACE2 may lead to the formation of CAL, leading to vascular endothelial injury. However, our study did not find a specific link between ACE2 and CK-MB in the KD-CAL group, which needs to be confirmed in further studies.
In conclusion, this study provides the first evidence that serum ACE2 levels are significantly increased and are correlated with platelet count in patients with KD. These results suggest that high ACE2 levels may play a major role in the inflammatory response and vascular injury in the acute phase of KD. However, whether and how ACE2 can cause CAL, and the association of ACE2-related genes with vascular endothelial injury needs to be further studied.