Pparα-agonist Combined With All-trans Retinoic Acid Induces "browning" Of White Adipose Cells Through P38mapk Signaling Pathway

BackgroudsThere are two diffrernt kinds of adipose cells in mammals including whiteadipocytes and brown adipocytes.In contrast to the primarily fat-storing function ofwhite adipose tissue (WAT), brown adipose tissue(BAT) has non-shiveringthermogenic properties owing to expression of uncoupling protein-1(UCP1) andincreased mitochondrial content[1]. Recent findings that inducing white adiposetissue to brown adipose tissue (referred as "browning" of white adipose tissue) canreduce the weight of the animal and improve glucose homeostasis.In addition, higherBAT levels are associated with resistance to metabolic diseases[2].A muscle-derived hormone,irisin,has been identified that promotes induction ofbrown adipocytes in white fat recently. Investigating at the molecular level find thatone possible mechanism might be increased expression of PPARα.Whilepharmacological inhibition with a PPARα-selective antagonist limited the inductionof the browning programme by irisin[6].However,a PPARα-agonist (WY14643)failed to regulate the expression of the UCP1gene of white adipocytes unlesscombined with retinoic acid[8-9]. Retinoic acid could reduce body weight, increases body temperature and adiposity in rodent models and stimulates UCP1expression inbrown adipose tissue and skeletal muscle. Due to the effect of RA impacting on lipidmetabolism involves the modulation of the activity of several important proteinkinases,for example, p38mitogen-activated protein kinase (p38MAPK).Tthemechanisms of white adipocytes transforming to brown adipocytes and PPARα-agonists WY14643combined with t-RA on the“browning” of mice white adipocytesare currently unclear.ObjectiveThe ability of mammals to resist body fat accumulation is linked to their abilityto expand the number of “brown adipocytes” within white fat depots.All-transretinoic acid (t-RA) and Peroxisome proliferator-activated receptorsα(PPARα) hasbeen implicated in “browning-like” or “browning” programme respectively.However,a PPARα-agonist (WY14,643) failed to regulate the expression of the UCP1geneunless combined with retinoic acid.This study investigated the effects of PPARα-agonists WY14643combined with t-RA on the“browning”of mice white adipocytesmediated by uncoupling protein1(UCP1) and the molecular mechanism.MethodsWe compared different results of5umol/L WY14643,WY14643combined with(0、10-5、10-4、10-3、10-2、10-1mmol/L)t-RA or10umol/L inhibitor-P38MAPKSB203580treated with white adipocytes after24h by which UCP1examined byRT-PCR and Western blot. We also determined the mechanism by which p38MAPKand phospho-p38MAPK examined by Western blot.ResultsRT-PCR and Western blot showed that all different concentration of WY14643can’t induce UCP1mRNA、protein expression or phosphorylate p38MAPK(P<0.05)；WY14643combined with t-RA can induce UCP1mRNA、protein expression or phosphorylate p38MAPK(P<0.05);SB203580combined with WY14643and t-RAsuppress UCP1mRNA、 protein expression and p38MAPK phosphorylation(P<0.05).Since mice white adipocytes have no expression of UCP1, we examined whetherWY14643combined with t-RA could induce expression of UCP1. In theseexperiments, we used differentiated mice multipotent adipose-derived3T3-L1preadipocytes.Treatment with WY14643over a range of concentrations from2.5–10uM, was unable to significantly induce the transcripts for UCP1; but WY14643combined with t-RA could. Western blot showed that the ability of WY14643combined with t-RA to triggers expression was synchronous to the effect of theyinduce phosphorylation of p38MAPK. However,pretreatment with SB203580, whichis a p38MAPK inhibitor, completely blocked the effect of WY14643combined witht-RA， suggesting that p38MAPK activity is important for“browning”of whiteadipocytes.ConclusionWY14643combined with tRA can induce white adipocytes transform to brownadipocytes through activating P38MAPK signaling pathway.