Retinoid-related orphan receptor γ (RORγ) is an orphan nuclear hormone receptor (NR) that is preferentially expressed in skeletal muscle and several other tissues, including pancreas, thymus, prostate, liver and testis. Surprisingly, the specific role of RORγ in skeletal muscle, a peripheral tissue, has not been examined. Muscle is one of the most energy demanding tissues which accounts for ~40% of the total body mass and energy expenditure, >75% of glucose disposal and relies heavily on β-oxidation of fatty acids. We hypothesize that RORγ regulates metabolism in this major mass lean tissue. This hypothesis was examined by gain and loss of function studies in an in vitro mouse skeletal muscle cell culture model. We show that RORγ mRNA and protein are dramatically induced during skeletal muscle cell differentiation. We utilize stable ectopic over-expression of VP16-RORγ (gain of function), native RORγ and RORγΔH12 (loss of function) vectors to modulate RORγ mRNA expression and function. Ectopic VP16 (herpes simplex virus transcriptional activator)-RORγ and native RORγ expression increases RORα mRNA expression. Candidate-driven expression profiling of lines that ectopically express the native and variant forms of RORγ suggested that this orphan NR has a function in regulating the expression of genes that control lipid homeostasis (fatty acid-binding protein 4, CD36 (fatty acid translocase), lipoprotein lipase and uncoupling protein 3), carbohydrate metabolism (GLUT5 (fructose transporter), adiponectin receptor 2 and interleukin 15 (IL-15)) and muscle mass (including myostatin and IL-15). Surprisingly, the investigation revealed a function for RORγ in the pathway that regulates production of reactive oxygen species.
Suryaprakash Raichur, Patrick Lau, Bart Staels and George E O Muscat
K D Senali Abayratna Wansa and George E O Muscat
The NR4A1–3 (Nur77, NURR1 and NOR-1) subfamily of nuclear hormone receptors (NRs) has been implicated in Parkinson’s disease, schizophrenia, manic depression, atherogenesis, Alzheimer’s disease, rheumatoid arthritis, cancer and apoptosis. This has driven investigations into the mechanism of action, and the identification of small molecule regulators, that may provide the platform for pharmaceutical and therapeutic exploitation. Recently, we found that the purine antimetabolite 6-Mercaptopurine (6-MP), which is widely used as an anti-neoplastic and anti-inflammatory drug, modulated the NR4A1–3 subfamily. Interestingly, the agonist-mediated activation did not involve modulation of primary coactivators’ (e.g. p300 and SRC-2/GRIP-1) activity and/or recruitment. However, the role of the subsequently recruited coactivators, for example CARM-1 and TRAP220, in 6-MP-mediated activation of the NR4A1–3 subfamily remains obscure. In this study we demonstrate that 6-MP modulates the activity of the coactivator TRAP220 in a dose-dependent manner. Moreover, we demonstrate that TRAP220 potentiates NOR-1-mediated transactivation, and interacts with the NR4A1–3 subgroup in an AF-1-dependent manner in a cellular context. The region of TRAP220 that mediated 6-MP activation and NR4A interaction was delimited to amino acids 1–800, and operates independently of the critical PKC and PKA phosphorylation sites. Interestingly, TRAP220 expression does not increase the relative induction by 6-MP, however the absolute level of NOR-1-mediated trans-activation is increased. This study demonstrates that 6-MP modulates the activity of the NR4A subgroup, and the coactivator TRAP220.
Shu-Ching M Wang, Dennis H Dowhan and George E O Muscat
Breast cancer is a heterogeneous disease, and the complexity of breast carcinogenesis is associated with epigenetic modification. There are several major classes of epigenetic enzymes that regulate chromatin activity. This review will focus on the nine mammalian protein arginine methyltransferases (PRMTs) and the dysregulation of PRMT expression and function in breast cancer. This class of enzymes catalyse the mono- and (symmetric and asymmetric) di-methylation of arginine residues on histone and non-histone target proteins. PRMT signalling (and R methylation) drives cellular proliferation, cell invasion and metastasis, targeting (i) nuclear hormone receptor signalling, (ii) tumour suppressors, (iii) TGF-β and EMT signalling and (iv) alternative splicing and DNA/chromatin stability, influencing the clinical and survival outcomes in breast cancer. Emerging reports suggest that PRMTs are also implicated in the development of drug/endocrine resistance providing another prospective avenue for the treatment of hormone resistance and associated metastasis. The complexity of PRMT signalling is further underscored by the degree of alternative splicing and the scope of variant isoforms (with distinct properties) within each PRMT family member. The evolution of PRMT inhibitors, and the ongoing clinical trials of PRMT inhibitors against a subgroup of solid cancers, coupled to the track record of lysine methyltransferases inhibitors in phase I/II clinical trials against cancer underscores the potential therapeutic utility of targeting PRMT epigenetic enzymes to improve survival outcomes in aggressive and metastatic breast cancer.