The main findings of this study lie in the effects of sitagliptin in reducing the formation of atherosclerotic lesion area in the aortic root and abdominal aorta, and changing the histological composition of atherosclerotic plaques by reducing the content of collagen fiber and tending to reduce smooth muscle cells and macrophages in the aorta, although with no statistical significance. These results are consistent with previous researches on the anti-atherosclerotic effects of DPP-4 inhibitors in animals [6–11, 30–34]. It has been shown that sitagliptin is cardioprotective in the diabetic Akita mouse even at low doses . A pooled analysis of 25 randomised clinical trials indicate that sitagliptin does not increase cardiovascular risk in patients with T2DM . And chronic treatment with sitagliptin may have cardioprotective effects in diabetes patients presenting with acute coronary syndrome .
Furthermore, the anti-atherosclerotic effects of sitagliptin in this study occurred with no differences in food intake, body weight or blood glucose levels between sitagliptin-fed and control animals. Our results are roughly similar to those reported by Junichi Matsubara et al., who reported that sitagliptin reduced atherosclerotic lesion formation in ApoE-deficient mice independent of fasting glucose and lipid profiles . In addition, we unexpected discovered that sitagliptin significantly increase the HDL cholesterol levels and tends to increase the LDL cholestrerol levels in ApoE-/- mice. HDL cholesterol has been demonstrated to reduce the risk of atherosclerosis by multiple pathophysiologic mechanisms . And LDL has been regarded as a positive risk for atherosclerosis when they invade the endothelium and become oxidized. Previous study suggests that sitagliptin is a sound agent for use in the comprehensive treatment of patients with T2DM because it improves not only glycemic control, but also blood pressure and lipid profiles . A systematic review and meta-analysis reported that DPP-4 inhibitors appear to have a beneficial effect on total cholesterol and triglyceride levels, whereas the effect on the other lipids concluded HDL and LDL cholesterol has not been confirmed yet . Whether DPP-4 inhibitor has influence on HDL and LDL cholesterol in mice remains contentious. It has been reported that anagliptin significantly reduced total cholesterol level especially VLDL and LDL without affecting triglyceride level and vildagliptin analogue decreased plasma levels of LDL by 27% in Apoe-/- mice [9, 30]. However, some reports showed that sitagliptin has no effect on TG, TC, LDL or HDL levels induced by HFD in Apoe-/- mice [6, 10]. Therefore, the definitive effects of DPP-4 inhibitor include sitagliptin on the blood lipid profiles in mice remains contentious.
After observing the anti-atherosclerotic effect of sitagliptin, we then explored its relative mechanisms in further. It has previously been reported that exendin-4, a GLP-1R agonist, attenuates atherosclerosis through PKA–PI3K/Akt–eNOS–p38 MAPK–JNK- dependent pathways via a GLP-1R-dependent mechanism, without affecting metabolic parameters [5, 16, 18, 38–40]. Although studies have shown that sitagliptin can inhibit the formation of atherosclerosis in ApoE-/- mice, the mechanisms through which it attenuates the progress of atherosclerosis are complex and not completely understood.
Recent evidences have shown a promising role for AMPK in the attenuation of atherosclerosis involving vascular dysfunction and endothelial inflammation by upregulating the Akt/eNOS signaling pathway [25, 41] and suppressing the activation of ERK1/2 in vascular tissues [24, 26]. And it has been demonstrated that mitogen-activated protein kinase (MAPK) may play a role in anti-atherosclerosis [24–29], and moreover alogliptin can inhibit the ERK-mediated expression of matrix metalloproteinases, which are involved in atherosclerosis [17, 30]. These informations provided important clues for our study.
Another major finding of this study is that it shows, for the first time, that sitagliptin can activate the AMPK pathway and inhibit MAPK signaling by increasing the phosphorylation of AMPK and its downstream molecule Akt, while reducing the phosphorylation of p38 and ERK1/2 MAPK in the aorta. As a result, sitagliptin reduces serum soluble VCAM-1 and P-selectin levels, which play an important role in regulating the binding of leukocytes to endothelial cells as a key initial step in the formation of atherosclerosis, and also reduces the expression of inflammation factors such as MCP-1 and IL-6. As is well know, inflammation in the vascular endothelium and subsequent leukocyte recruitment are initiating events in the progression of atherosclerosis.
A limitation of the current study is that we have only demonstrated the mechanisms described in aortic tissues, and were unable to provide direct evidence of sitagliptin activating the AMPK and MAPK signaling pathways. Another limitation is that we were unable to measure the plasma levels of active GLP-1 after ingestion. The definitive mechanisms of action for sitagliptin still need further investigation in vitro by culturing primary aortic endothelial cells.
In summary, our study confirms that sitagliptin can attenuate the development of atherosclerosis and alter the composition of the atherosclerotic plaques induced by HFD in ApoE-/- mice. In addition, the beneficial effects of sitagliptin also contain increased HDL cholesterol and decreased adhesion molecules as well as inflammatory cytokines. Our present observations indicate that sitagliptin has protective actions against atherosclerosis via anti-inflammation potentially through AMPK and MAPK-dependent mechanisms. Given that the prevention and treatment of diabetic vascular complications remains important and challenging, sitagliptin, as an effective medicine for diabetes, may open a new therapeutic window for the treatment of atherosclerosis and related diseases.