Androgen receptor expression and activity: Roles of inflammation, age-induced oxidative stress, and epigenetic modifications.

摘要

The androgen receptor (AR) plays a central role in the initiation, development and progression of prostate cancer. Increased AR expression is a key determinant in prostate cancer progression to androgen-independence and therapy resistance. Evidence suggests that various cancers including prostate cancer are linked to chronic inflammation and oxidative stress. Consistent with the importance of inflammation and redox stress in tumorigenesis, the pro-inflammatory cytokine tumor necrosis factor-alpha (TNFalpha) and oxidative stress, which is a hallmark of aging and many age-associated diseases, are known to exert autocrine and paracrine influences on normal and malignant prostate cells. My dissertation focused on the molecular mechanisms that direct AR expression and activity by stress conditions. I also examined the epigenetic regulation of prostate cancer cell proliferation by microRNAs.;I have demonstrated that (1) TNFalpha inhibits AR expression in androgen-dependent LNCaP human prostate cancer cells through transcription repression; (2) Aging and oxidative stress lead to down-regulation of AR expression in the rat liver and in rat hepatoma cells; (3) The histone methyltransferase Set9 regulates AR activity positively or negatively in a cell type-dependent manner; (4) The microRNA 454 (miR-454) enhances prostate cancer cell proliferation and I have identified the target mRNA for miR-454 in prostate cancer cells.;In part 1, I show that TNFalpha mediated AR repression requires interdependent activity of NF-kappaB and the B-Myb proto-oncogene at a conserved composite sequence in the AR gene 5' untranslated region. Recruitment of B-Myb and NF-kappaB-p65, p50 to the AR chromatin paralleled TNFalpha-induced enrichment of the same negatively responsive region for corepressors SMRT and mSin3A, and histone deacetylase HDAC1. Negative regulation of AR was absent in androgen-independent C4-2 prostate cancer cells, although TNFalpha activation of NF-kappaB and B-Myb were maintained. These results, linking androgen dependence to the cross talk of NF-kappaB and AR signaling, allude to potential therapeutic targets in prostate cancer. In Part 2, I show that inhibition of AR expression in the aging rat liver and in oxidatively stressed cells involves exchange of a PARP-1-associated, p/CAF-containing coactivator assembly for a p53-interacting, mSin3A- and Groucho/TLE1-included corepressor complex at an age- and oxidant-responsive element in the AR gene. AR expression is regulated positively by PARP-1, hnRNPK, B-Myb and c-Myb and negatively by p53, revealed from siRNA-mediated silencing of each protein. I conclude that reciprocal chromatin dynamics of p53 and PARP-1 at the ADF element, mediated by the Myb transcription factors, is integral to the regulatory network that orchestrates loss of AR in aging and in response to oxidative stress. In Part 3, I show that Set9 methylates the lysine 630 of AR both in vitro and in vivo. I further show that Set9 can repress AR transactivation function, since androgen-induced rat probasin promoter function was inhibited when Set9 and AR were cotransfected into the 293A kidney cells. Lysine to alanine substitution of AR at the Set9-targeted residues showed augmented functional interaction with coactivators compared to the wild-type counterpart, as evident from increased coactivation of the mutant AR activity by p160 family of coactivators and by the histone/non-histone acetylases p300 and p/CAF. However, in the Set9-silenced LNCaP cells endogenous mRNAs for prostate specific antigen (PSA) and ornithine decarboxylase 1 (ODC1) were reduced, indicating a Set9 context-dependent positive or negative regulatory role of in the regulation of AR activity. In Part 4, I show that miR-454 enhances prostate cancer cell proliferation, is present at higher abundance in prostate cancer cells that are androgen-independent and it targets 3'UTR mRNA of the Slain family protein Slain1 for translation suppression. Future studies involve assessment of the role of Slain1 in prostate tumor growth in vivo, and in localized versus metastatic prostate cancer.