Function And The Underlying Mechanisms Of Nuclear Receptors RXR? And Nur77 In Nonresolving Inflammation And Cancer

Members of the nuclear receptor superfamily are critical regulators of cell growth,proliferation,differentiation,metabolism,inflammation,immune response and apoptosis.Abnormal expression and function of nuclear receptors can lead to various diseases and cancer.The current studies aim to investigate the function and the underlying molecular mechanism of nuclear receptors Nur77 and Retinoid X receptor-alpha(RXR?)in inflammation and cancer.The first part of the thesis relates to the function and mechanism of orphan nuclear receptor Nur77 in breast cancer.As an immediate-early response gene,Nur77 plays a critical role in regulating diverse biological processes including inflammation in response to external and internal stimuli.It is overexpressed in cancer cells,and its expression can be potently induced by a variety of inflammatory cytokines.Mechanistically,Nur77 like other nuclear receptors acts as a transcriptional factor.However,it could also act in the cytoplasm including its mitochondrial targeting and apoptosis induction.We recently reported that celastrol binding to Nur77 could promote its translocation to mitochondria where it interacts with TRAF2 and p62 to induce autophagy of dysfunctional mitochondria,thereby alleviating inflammation.Here we showed that celastrol is very effective in inhibiting the growth of triple-negative breast cancer cells.By using in vitro and PyMT transgenic mammary tumor animal models,we found that Nur77 was required for the inhibitory effect of celastrol.Mechanistically,we found that Nur77 was required for the anti-inflammatory effect of celastrol likely through its interaction with TRAF2 and p62 that we reported.Importantly,we found that Nur77 could mediate the inhibitory effect of celastrol on the activation of mTOR,which is critical for the growth and progression of breast cancer cells.Such an effect of Nur77 could be also attributed to its interaction with p62 that is required for mTOR activation.Together,these results provide important insight into the mechanism by which Nur77 modulates inflammation and mTOR activation and new strategy to develop celastrol-based therapeutics for treating triple-negative breast cancer.The second part relates to the function and mechanism of RXRa in nonalcoholic fatty liver disease(NAFLD).So far,few therapies exist for NAFLD and dietary therapy commonly prescribed remains problematic.Recent accelerated approval of Ocaliva(an agonist of nuclear receptor FXR)by FDA for treating primary biliary cirrhosis and fast track designation of another FXR agonist for treating non-alcoholic steatohepatitis(NASH)with liver fibrosis have stimulated excitement about targeting nuclear receptors for treating NAFLD.RXRa is highly expressed in the liver and it plays a critical role in modulating inflammatory responses in the liver.We recently reported our identification of a new RXRa ligand,K-80003,which binds RXRa through a very unique mechanism that may impact RXRa activity in NAFLD.The current study showed that compared with control group,long-term feeding of mice with MCD diet could cause abnormal RXRa expression in the liver,accompanied with macrophage infiltration and adipose accumulation.Interestingly,the cytoplasmic localization of RXRa was correlated with NF-?B activation,the secretion of several inflammatory factors secretion,and enhanced lipid drop accumulation in liver cells.The adverse effects of MCD diet were reversed when mice were administered with K-80003.Mechanistically,we found that that upon Toll-like receptor(TLR)4 activation,RXRa interacted with TRAF6,resulting in TRAF6 ubiquitination and NF-?B activation.Treatment of cells with K-80003 could suppress RXRa-dependent NF-?B activation by inhibiting RXRa interaction with TRAF6.Together,these data provide exciting new mechanistic insights into the role of RXRa in the onset and progression of NAFLD and suggest that K-80003 or analogs may represent promising leads for developing new therapeutics for treating NAFLD.