Statins Synergize Dexamethasone-induced Macrophage Ap2Expression By A Glucocorticoid Receptor-dependent Mechanism

Atherosclerosis is one of major causes of mortablity in developedcountries and is a complex pathophysiological process. The pathogenesisof atherosclerosis is initiated by the formation of cholesterol-rich lesionin the arterial wall. The formation of atherosclerotic lesion is a complexprocess involving the regulation of endothelial injury、moncyte andmacrophage accumulation、smooth muscle cells proliferation and avariety of cytokines. Monocyte-derived macrophages play a crucial rolein this process because they accumulate large amounts of lipid and covertto foam cells, the formation of foam cells promotes atheroscleroticplaques. Thus, the transformation of macrophage into foam cells is acritical component of atherogenic process.Cytoplasmic fatty acid-binding proteins (FABPs) consist of a familyof14to15kD proteins capable of binding hydrophobic ligands with highaffinity. Adipocyte fatty acid binding protein, aP2(also known as FABP4), is a member of the FABP family. Putative functions of aP2include shuttling of fatty acid ligands in the cytoplasm to variousintracellular compartments, modulation of intracellular lipid metabolismand regulation of gene expression. aP2is also expressed by macrophages.Genetic deletion of aP2expression prevents the high fat diet-inducedatherosclerosis while a small molecular inhibitor of aP2functions as aneffective therapeutic agent against the severe atherosclerosis and type2diabetes. Thus, aP2has an important relationship with the pathogenesisof atherosclerosis.Synthetic glucocorticoids (GCs), such as prednisolone anddexamethasone are widely used drugs in treatment of inflammatory andautoimmune diseases. Dexamathesone is also a major component ofadipocyte differentiating agents in vitro. The pontential inhibitors of3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase, statins, arerate-limiting enzyme for cholesterol biosynthesis. Statins are used clinically to lower plasma cholesterol in hypercholesterolemia patients.In contrast, statins have been demonstrated to the inhibition of adipocytedifferentiation and aP2expression. However, the interplay betweendexamethasone and statins on macrophage aP2expression is unknown. Inthis study, we investigated if statins are able to antagonize the action ofdexamethasone on aP2expression. We observed that althoughdexamethasone increased while statins reduced macrophage aP2proteinexpression moderately, unexpectedly, statins synergizeddexamethasone-induced macrophage aP2protein expression. Thesynergistic effects were statin-concentration and time dependent.Macrophage aP2mRNA levels were also elevated by dexamethasone andstatins in a synergistic manner indicating the regulation is transcriptional.In addition, the synergistic induction was observed with peritonealmacrophages indicating the physiological relevance. Cholesterol andmevalonate inhibited the synergistic induction, but the regulation of farnesylation and geranylgeranylation did not influence thedexamethasone-induced aP2expression. Promoter analysis disclosed thatinduction of macrophage aP2expression by dexamethasone wasglucocorticoid receptor dependent. Thus, the high-expressingglucocorticoid receptor increased macrophage aP2protein expression. Inaddition, there is a putative negative glucocorticoid regulatory element(nGRE) in aP2gene. Pitavastatin had little effect on expression ofglucocorticoid receptor but enhanced the dexamethasone-mediatednuclear translocation and inhibited the binding of glucocorticoid receptorto the nGRE that resulted in a synergistic binding of glucocorticoidreceptor to the positive glucocorticoid regulatory element (pGRE) of aP2gene and a synergistic transcription and expression of macrophage aP2.Furthermore, we investigated if the synergistic induction of aP2canoccur in vivo. Wild type mice were fed a normal chow or chowcontaining dexamethasone alone or pitavastatin alone or their combination for two weeks. After treatment, peritoneal macrophages、adipose tissuses and liver tissues were individually collected and used todetermine aP2protein expression. It was consistent with the in vitrostudy that administration of dexamethasone alone to mice induced aP2protein expression while pitavastatin alone inihibited it in macrophagesand adipocytes. However, the co-administration resulted in significantlyincrease compared to the dexamethasone alone suggesting pitavastatinsynergized dexamethasone-induced macrophage aP2expression in vivo.We also isolated human primary monocytes from a healthy donor’s blood.All results were consistent with the results in cell culture. The results inthis study demonstrate that statins enhance dexamethasone-inducedmacrophages aP2expression in vivo. To disclose if the synergisticinduction of aP2expression by statins plus dexamethasone would impacton the development of atherosclerosis, apoE knockout mice were fed aWestern diet or Western diet containing dexamethasone alone or pitavastatin alone or dexamethasone plus pitavastatin for12weeks, weobserved that dexamethasone plus pitavastatin aggervate atherosclerosisplaque induced by western diet in apoE knockout mice.Taken together our study, for the first time, discloses pitavastatinsynergize dexamethasone-induced macrophage aP2expression. Thecombination of statins and dexamethasone treatment possiblely causesside effects which aggravate atherosclerosis. Also, our results may haveimplication for the analysis of therapeutic agents inanti-inflammation/immuosuppression and hypercholesterolemia.