Tion represses CD36 expression, remains to be investigated. Current reports recommend that FXR activation reduces CD36 expression within the murine liver and in macrophages [32,33]. Besides activating gene expression, FXR can also directly act as a transcriptional repressor. As an illustration, hepatic lipase and apoA-I, which are each relevant to HDL metabolism, are repressed by FXR [34,35]. When SR-BI levels have been strongly DAPK manufacturer lowered in HepG2 cells, there was nonetheless considerable residual HDL cell association apparent (examine Figs. four and six). Other receptors such as the low affinity binding site beneath the manage of F1-ATPase/P2Y13 at the same time as CD36 may possibly account for this residual activity. In line, SR-BI does not seem to become the significant factor figuring out hepatic HDL endocytosis [6,10]. In contrast, SR-BI will be the principal receptor mediating selective lipid uptake from HDL. Our results show that SR-BI expression is unaltered after FXR activation (Fig. six). Current studies report that FXR activates SR-BI expression [24,25,36]. Even so, it was also located that FXR activation represses SR-BI by a mechanism comparable to the repression for Cyp7a1 [26].The causes for these discrepancies remain unknown. Resulting from unaltered SR-BI expression immediately after CDCA or GW4064 treatment in our experiments, cholesteryl-ester uptake from HDL was unchanged. This resulted in an increase of calculated selective uptake, because HDL particle association was lowered (Fig. 6). Altogether, our data have implications for the connection amongst HDL endocytosis and selective uptake. When HDL uptake in HepG2 cells was reduced either by extracellular or transcriptional mechanisms, no concomitant reduction in cholesteryl-ester uptake was observed. In contrast, selective CE uptake seemed to become differentially regulated. HDL endocytic trafficking is accompanied by lipid exchange [5,37]. Moreover, pharmacological interference with HDL endocytosis resulted in induced flux of HDL P2Y6 Receptor drug cholesterol in the plasma for the liver and enhanced biliary cholesterol secretion [38]. Having said that, HDL endocytosis is no prerequisite for selective lipid uptake: liposomes containing purified SR-BI take up CE efficiently [39]. In addition, diverse experimental approaches to block HDL endocytosis usually do not influence selective uptake [40,41]. Regularly, our data presented right here suggest that HDL endocytosis and selective CE uptake usually are not necessarily linked with every single other. Certainly, in-vivo research suggest that bile acids improve selective lipid uptake, thereby enhancing the clearance of HDL cholesterol from the plasma. Bile acid feeding lowers HDL cholesterol in mice [42]. Consistently, GW4064 administration decreases HDL cholesterol in mice [36] as well as the synthetic FXR agonist PX 20606 decreased plasma HDL levels in cynomolgus monkeys [43]. In contrast, FXR knockout mice have increased HDL cholesterol levels [23]. Taken collectively, our results indicate that bile acids lower HDL endocytosis by transcriptional and non-transcriptional mechanisms. Having said that, lowered HDL endocytosis isn’t accompanied by reduced cholesteryl-ester transfer.AcknowledgmentsWe thank Dr. Michael Trauner and Dr. Monika Strobl for useful scientific discussion. We are grateful to Jelena Brankovic for great technical help.Author ContributionsConceived and made the experiments: CR HS. Performed the experiments: CR KE SF. Analyzed the information: CR KE SF HS. Wrote the paper: CR SF HS.
Int. J. Mol. Sci. 2013, 14, 21647-21659; doi:10.3390/ijmsOPEN ACCESSInternational Jo.
