The main finding of the present study is that propofol could protect HUVECs against 15 mM glucose-induced endothelial adhesion molecules expression and mononuclear-endothelial interaction. Our data also suggested that the protective effects of propofol might be achieved by attenuating O2.- accumulation, inhibiting PKCβ2 ser660 phosphorylation, PKC activity and NF-κB activation.
Hyperglycemia could up-regulate endothelial adhesion molecules expression, thus leading to the adhesion of monocytes to endothelial cells[3–5]. Many studies indicated that NF-κB signal pathway was involved in the hyperglycemia-mediated up-regulation of gene and protein expression of endothelial adhesion molecules, including VCAM-1, ICAM-1, and E-selectin[3–5, 13–16]. These findings were consistent with our study (Figures1,2,3,4). Further, we suggested that PKCβ2 was involved in this process and its activation laid up-stream of NF-κB activation (Figure5,7,8,9). This was consistent with a previous study, which demonstrated that PKCβ2 and NF-κB played a key role in high glucose-mediated up-regulation of VCAM-1 in vascular endothelial cells. However, other pathways were also claimed to be responsible. Kim S et al. reported that high glucose-induced endothelial adhesion molecules expression was mediated through the p38 mitogen-activated protein kinase (MAPK) signaling pathway. In our preliminary experiments, we examined the activity of p38 MAPK, but did not detect its activation in response to 15 mM glucose (data not shown). We noticed that endothelial cells were treated with 25 mM glucose for 24 h in their study, while we treated endothelial cells with 15 mM glucose for 4 h. One potential explanation for the discrepancy between their findings and ours is that different concentration or duration of glucose treatment may up-regulate the expression of endothelial adhesion molecules through different pathways.
The necessity to regulate plasma glucose concentration to normal levels in perioperative hyperglycemic patients is debatable, because it may cause severe hypoglycemia and other serious adverse events. As such, scientists are devoting to explore novel strategies, which may exert beneficial effects without tightly regulating plasma glucose levels. Recently, endothelial adhesion molecules have been widely considered to be potential targets for the effective treatment of high glucose-induced endothelial injury. It has been shown that high glucose-induced mononuclear-endothelial cell adhesion and endothelial injury can be attenuated by several compounds, such as fasudil and cannabidiol. Interestingly, these compounds have the property to reduce superoxide generation and decrease endothelial adhesion molecules expression[21, 22]. So, antioxidant may be a novel vascular protective strategy for hyperglycemic patients.
Propofol is an intravenous anesthetic agent which is widely used in clinical settings. Besides anesthetic properties, other characteristics of propofol have been widely studied in recent years. Propofol is chemically similar to endogenous antioxidant a-tocopheral (Vitamin E), and theoretically it should demonstrate similar properties. Chen J et al. reported that propofol could attenuate the adhesion of monocytes to oxidative stress-activated HUVECs. This was quite consistent with ours. In the present study, we found that propofol could significantly alleviate 15 mM glucose-induced O2.- accumulation (Figure6), endothelial adhesion molecules expression (Figure2) and mononuclear-endothelial cell interaction (Figure9). So we believe the antioxidative property of propofol is one of the main mechanisms for its protective effects on HUVECs.
The activation of NF-κB consists of several steps, starting with the degradation of IκB, a cytoplasmic inhibitor of NF-κB. Among IκB family, IκBα is the most widely studied. IκBα degradation leads to the translocation of NF-κB from the cytoplasm compartment to the nuclear, where it could recognize and bind to the promoter of target genes and regulate the expression of these genes. Studies have indicated that propofol could inhibit NF-κB activation in endothelial cells exposed to hydrogen peroxide and lipopolysaccharide. In the present study, we found that pre-treatment of cells with propofol attenuated 15 mM glucose-induced cytoplasmic IκBα degradation and NF-κB translocation (Figure3). In addition, we demonstrated that propofol could inhibit 15 mM glucose-induced endothelial adhesion molecules expression (Figure2) and mononuclear-endothelial cell interaction (Figure9). These results strongly suggested that the beneficial effects of propofol on 15 mM glucose-induced endothelial adhesion molecules expression may result from its inhibitory effects on NF-κB signal pathway.
It is of interest to determine the leading cause of PKC activation in response to high glucose. Gallo A et al., Gopalakrishna R et al. and Pricci F et al. reported that high glucose increased oxidative stress, which is responsible for the translocation and activation of PKC in vascular tissues[28–30]. It has also been reported that activation of PKCβ2 was associated with high glucose-induced vascular ROS generation and high glucose-mediated nitric oxide reduction. In the present study, we found propofol could attenuate 15 mM glucose-induced O2.- accumulation (Figure6). Consistently, our previous study showed that propofol could restore high glucose-mediated O2.- accumulation and nitric oxide reduction via re-coupling endothelial nitric oxide synthase. Accordingly, we suggested that the antioxidative effect of propofol is a potential mechanism responsible for the beneficial effect on high glucose-mediated PKC activation and mononuclear-endothelial adhesion.
In a recent study, vascular healing responses were described after drug-eluting stent implantation in experimental models, employing everolimus or paclitaxel. In these models, anesthesia was achieved with propofol. We may raise the assumption that the obtained favorable effects were at least partially due to propofol. Further studies in in vivo models are necessary to verify this hypothesis.
This study has some limitations. First, the study was carried out in HUVECs, which is an in vitro system. It differs from in vivo settings, especially when drug effectiveness and toxicity is considered. Secondly, we did not investigate the effect of propofol and high glucose on mononuclear. So further experiments are required to determine whether the effect of propofol on high glucose-induced mononuclear-endothelial interaction is mediated by endothelial cells, mononuclear cells, or both.