FXR and Anti-HIV Therapy Figure 5. FXR and PPARa ligands does not impact on liver expression of genes involved in lipid and cholesterol metabolism in ApoE2/2 mice administered the HIV-PI ritonavir. In each panel, data are mean6 SE of 6 animal per group. *P,0.05 versus wild type na¨ıve mice. doi:10.1371/journal.pone.0013238.g005 inhibitors included in the HAART increases the risk of myocardial infarction [28,29]. Beside the DAD study, smaller studies have provided evidence that long-term use of HAART regimens based on HIV PI lopinavir/ritonavir increases the levels of triglycerides and results in a pro-atherosclerotic lipoprotein profile [5–7]. These studies indicate that the hyperlipidemia produced by HIV PI is primarily attributable to an excessive FFA mobilization occurring because of insulin resistance in the adipose tissue, resulting in increased VLDL–triglyceride production and apoB synthesis. Likewise, t in fed mice, ritonavir had no effect on serum glucose and cholesterol, whereas triglyceride and fatty acids increased by 57% to 108% [30]. In fasted mice, ritonavir increased serum glucose by 29%, cholesterol by 40%, and triglyceride by 99%. The hyperlipidemia was attributed to elevated FFA and cholesterol synthesis by the liver and adipose tissue caused by activation of SREBP1c and 2, whereas studies in cultured liver cells support the notion that ritonavir might cause a decreased degradation of nascent apoB attributable to inhibition of the 20S proteasome. In the present study we have confirmed that administration of ritonavir to wild type mice increases triacylglycerols synthesis. A two-fold increase in FFA, triacylglycerols and LDL plasma levels was observed in mice administered 5 mg/kg/day ritonavir for 10 days, indicating that this model reproduces all major metabolic features observed in HIV infected persons administered liponavir/ ritonavir. A major observation of the present study was the demonstration that such lipid abnormalities are highly dependent on the activity of the nuclear receptor FXR. Thus, not only activation of FXR with CDCA protects against development of dyslipidemia caused by ritonavir, but feeding ritonavir to FXR2/2 mice resulted in an exacerbation of biochemical derangements caused by the HIV PI in wild type mice. Together, these observations provide a strong support to the notion that FXR might be a novel therapeutic target for preventing/treating lipid metabolism abnormalities caused by HIV PI. Feeding with FXR agonists attenuates dyslipidemia in a variety of rodent models [16–20]. A key regulatory pathway involved in these effects is the inhibition of SREBP1c. Administering rodents with natural and synthetic FXR ligands inhibits the expression of SREBP1c in the liver. Originally, this effect was thought to be mediated by SHP, an atypical nuclear receptor that lacks a DNA binding domain. Based on studies in SHP-deficient mice, it has been proposed that the induction of SHP mediates the downregulation of SREBP-1c expression in response to bile acids treatment [18,19,31]. Counter-intuitively, however, a decreased, and not increased, SREBP-1c mRNA is observed in ob/ob/SHP2/2 double mutants [32]. In addition to the observation that transgenic mice constitutively expressing SHP in the liver exhibited an enhanced expression of SREBP-1c mRNA levels and a concomitant accumulation of hepatic triglycerides [33], these data suggest that also a SHP-independent mechanism is activated by FXR ligands. In the present study we have shown that CDCA induced the liver expression of SHP and was as effective as the PPARa ligand gemfibrozil in correcting lipid abnormalities caused by ritonavir. PLoS ONE | www.plosone.org 7 October 2010 | Volume 5 | Issue 10 | e13238