Previous studies of diabetic HDL have been focusing on its pro-inflammatory role and dysfunctions such as reverse cholesterol transport (3). In diabetes mellitus, disorders of carbohydrate, fat and protein metabolism result in modifications of HDL protein and lipid components, which may lead to functional deficiency of plasma HDL . Consistent high levels of blood glucose may not only cause protein glycosylation, resulting in dysfunction of reverse cholesterol transport and anti-inflammatory effects, but may also cause disorder in lipid metabolism which possibly alters the function of HDL particle in T2DM. These modifications greatly affect clearance of HDL and apoA-I from circulation [24, 25]. On the other hand, the increased non-enzymatic glycosylation of the major apolipoprotein, including apoA-I secondary to hyperglycemia impairs cholesterol transport and its anti-inflammatory function [26, 27]. In fact, all changes should gradually occur because T2DM is a chronic disease. Therefore, the body of patient tries to compensate the dysfunctional changes at the beginning of abnormal metabolism while disease itself may be a result of decompensation of the body. During the compensated stage, patients usually exhibit few symptoms of diabetes without complications of vascular damage due to the body’s compensatory protective regulations. However, there is insufficient evidence showing whether there are some compensatory protective effects, including the effects of HDL in patients with T2DM. The relationship between the changes of level of HDL-associated S1P and the disease progression, like atherosclerosis, is still unclear due to the lack of longitudinal studies.
In this study, we have generated a body of data supporting the hypothesis that HDL and S1P play some compensatory protective effects in the diabetic status. Subjects of T2DM had higher levels of triglycerides, total cholesterol and LDL-c, but lower levels of HDL-c compared with that of controls. On the other hand, the levels of HDL-associated S1P suggesting that S1P may be one of compensatory mechanisms in the early stage of T2DM. This increased content of S1P associated with HDL is possibly due to alternated capacity of HDL loading more S1P from plasma, or increased concentration of plasma S1P, or both . Also, the levels of S1P associated with HDL from T2DM-As were decreased than early T2DM, suggesting that diabetic HDL may lose its compensatory effects as disease progresses. The levels of S1P associated with diabetic HDL3 were higher than that of native HDL3 implying that this difference of S1P might be because of the alteration of S1P in HDL3. It is now known that diabetes causes alteration in the sphingolipids metabolism . However, these studies have focused on sphingosine, but not S1P, a downstream product of the metabolism. S1P is generated from the phosphorylation of sphingosine catalyzed by two sphingosine kinase isoenzymes, SphK1 and SphK2 . Slight changes in the levels of sphingomyelin, ceramide or sphingosine are able to significantly increase the level of S1P. Studies of S1P biological functions suggest that S1P may play different roles in various conditions. For instance, the increased S1P levels have been observed in the animal models of type 1 diabetes, type 2 ob/ob mice and diabetic humans and implicated in pro-thrombotic and pro-inflammatory effects [24, 31]. S1P accounts for several antiatherogenic and anti-inflammatory effects of HDL. S1P also antagonizes endothelial dysfunction by preventing monocyte/endothelial interactions through activation S1PR1 in the type 1 NOD mouse model and has vascular protective properties, whereas S1PR1 activation promotes eNOS activation and nitric oxide production . Our data also showed that HDL with higher content of S1P upregulated COX-2 expression and PGI-2 release by human vascular endothelial cells, two important athero-protective factors .
S1P receptors belong to G-coupled receptors while human vascular endothelial mainly express two of S1P receptors, S1PR1 and S1PR3. To investigate whether the effects of HDL on HUVEC were mediated through S1P, we employed a common G-PRC inhibitor pertussis toxin and S1PR1 and 3 specific antagonist VPC 23019. It has been shown in a previous study in which the expression of S1PR1 was higher in diabetes than controls, suggesting that S1P and its receptors might play important role in diabetes . Our data showed that up to 80% HDL-mediated effects on COX-2 expression and PGI-2 release was blockaded by these two inhibitors, suggesting that the effects of HDL on HUVEC largely contribute the contents of S1P in HDL. G-PCR is large group of receptors which located on cell membrane. We used PTX as a general blocker of G-PCR. On the other hand, we used VPC23109, a selective blocker of S1PR1 and 3, to narrow down the range of these blockages inhibiting the specific effects of S1P and its receptors. Selective blockage of S1PR1 and 3 seemed weaker than that of general G-PCR inhibitor PTX, particularly in COX-2 expression. A potential explanation is that other G-PCRs except for S1PR1 and 3 may also have similar effects. Previous studies have shown that S1P was associated with apolipoprotein M (apoM) in HDL exerted its biological function . But the effects of S1P associated with apoM in metabolic disease are still unknown. Whether any changes of modification and function of apoM in T2DM needs further investigations.
Normally, with developing of disease, the metabolic process starts dynamically from balance to unbalance,from compensation to decompensation. Similar status may occur in S1P in HDL, while HDL might finally lose the compensatory effects. Some clinical trials have shown that “glycemic lowering therapies” and “intensive lipid lowering treatment” might not decrease mortality caused by macrovascular events in T2DM . Therefore, it is necessary to further understand the potential mechanism of the lipid metabolism in HDL in T2DM, particularly in the early stage of the disease.
We also evaluated other effects of diabetic HDL, especially leucocyte-endothelial interaction. Compared with N-HDL, diabetic HDL enhanced more THP-1 cells to adhere to HUVECs, which is possibly associated with increased expression of adhesion molecules, including ICAM-1 and VCAM-1 (Additional file 1: Figure S2). Previous studies have shown that advanced glycation end products (AGEs) play an important role in impairing endothelial progenitor cell (EPC) functions, which may contribute to the development of vascular diseases in diabetes .
In conclusion, our data suggests that diabetic HDL with higher content S1P may exert some protective effects on vascular endothelial system, as a kind of the body’s compensatory mechanism preventing or delaying complications from the disease. Future studies should extend its other protective effects and focus on therapeutic potential in this area.