Oxidized LDL upregulates macrophage DPP4 expression via TLR4/TRIF/CD36 pathways

Rao, Xiaoquan; Zhao, Shi; Braunstein, Zachary; Mao, Hong; Razavi, Michael; Duan, Lihua; Wei, Yingying; Toomey, Amelia C.; Rajagopalan, Sanjay; Zhong, Jixin

In a large cross-sectional study in China, plasma DPP4 activity was shown to correlate with carotid intima media thickness. DPP4 activity in this study associated positively with the degree of insulin resistance (HOMA-IR), oxidized LDL and other measures of oxidative stress including nitrotyrosine and 8-isoPGF2a [24] . Interestingly, our recent study indicates there is no correlation between plasma DPP4 activity and atherosclerosis, suggesting that catalytic activity of circulating/soluble DPP4 alone may not be sufficient to serve as a mediator of atherosclerosis [14] . Additionally, one could conclude that monocytes may not be an important source of circulating DPP4. Indeed, adipocytes, endothelial cells and bone marrow derived cells (other than monocytes) have been confirmed as important sources of circulating DPP4 [10, 25] . We have previously shown that monocyte DPP4 expression is positively associated with non-HDL cholesterol and triglycerides, but not with fasting blood glucose or insulin levels, suggesting that the increase of monocyte DPP4 in obese patients might be related to dysregulated lipid metabolism in insulin resistance [14] . Since both LDL and its oxidatively modified forms are highly prevalent in atherosclerosis and may link diabetes with accelerated atherosclerosis [18, [26] [27] [28] [29] [30] [31] [32] , we examined the effect of both native LDL and oxLDL on DPP4 expression. Interestingly, only oxLDL, but not native LDL and free form of oxidized cholesterol (27OH-Ch), enhanced the expression of DPP4 on macrophages. In addition to DPP4 upregulation, IL-1β expression was also upregulated following oxLDL treatment, accompanied by a trend of increase in inflammasome components NLRP3 and caspase-1. This result is consistent with previous reports [33, 34] . Recent studies have demonstrated that CD36 may coordinate with TLRs to enhance oxLDL-induced NLRP3 inflammasome activation and atherosclerosis [35] [36] [37] . However, the causal relationship between DPP4 upregulation and IL-1β production was not examined in this study and requires further investigation. Activation of TLRs and downstream inflammatory signaling could be activated by excessive free fatty acids, very low density lipoprotein, apolipoprotein CIII, and lipoprotein oxidation [18, [38] [39] [40] [41] [42] [43] . In addition, the expression and activation of TLRs were also reported to increase in patients with metabolic syndrome [18, 38, [43] [44] [45] . In line with previous findings, we found that TLR4 knockdown or deficiency of downstream molecule TRIF abolished oxLDL-mediated up-regulation of DPP4 on macrophages. This suggests that oxLDL-induced TLR4/TRIF signaling may be responsible for elevated monocyte DPP4 in obesity. MyD88 and TRIF are the two major downstream adaptor proteins for TLRs. In the present study, we found TRIF rather than MyD88 is required for oxLDL-induced DPP4 up-regulation. TRIF is responsible for mediating the activation of NF-κB and IFN-β production mediated by TLR3 and TLR4 [46, 47] . In our study, oxLDL-induced DPP4 upregulation was even slightly enhanced in MyD88 deficient macrophages. This could be a result of the compensatory enhancement of TRIF signaling in MyD88 knockout macrophages [48] . The role of TRIF signaling in Fig. 6 . CD36 is associated with DPP4 expression on macrophages. A&B, Peritoneal macrophages were co-stained with CD36 and DPP4 and then examined by imaging flow cytometry. CD36 + and CD36 − macrophages were gat