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.
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Suryaprakash Raichur, Patrick Lau, Bart Staels, and George E O Muscat
Francesco Paolo Zummo, Alexandre Berthier, Céline Gheeraert, Manjula Vinod, Marie Bobowski-Gérard, Olivier Molendi-Coste, Laurent Pineau, Matthieu Jung, Loic Guille, Julie Dubois-Chevalier, David Dombrowicz, Bart Staels, Jérôme Eeckhoute, and Philippe Lefebvre
The functional versatility of the liver is paramount for organismal homeostasis. Adult liver functions are controlled by a tightly regulated transcription factor network including nuclear receptors (NRs), which orchestrate many aspects of hepatic physiology. NRs are transcription factors sensitive to extracellular cues such as hormones, lipids, xenobiotics, etc. and are modulated by intracellular signaling pathways. While liver functional zonation and adaptability to fluctuating conditions rely on a sophisticated cellular architecture, a comprehensive knowledge of NR functions within liver cell populations is still lacking. As a step toward the accurate mapping of NR functions in the liver, we characterized their levels of expression in the whole liver from C57Bl6/J male mice as a function of time and diet. Nr1d1 (Rev-erba), Nr1d2 (Rev-erbb), Nr1c2 (Pparb/d), and Nr1f3 (Rorg) exhibited a robust cyclical expression in ad libitum-fed mice which was, like most cyclically expressed NRs, reinforced upon time-restricted feeding. In a few instances, cyclical expression was lost or gained as a function of the feeding regimen. NR isoform expression was explored in purified hepatocytes, cholangiocytes, Kupffer cells, hepatic stellate cells, and liver sinusoidal cells. The expression of some NR isoforms, such as Nr1h4 (Fxra) and Nr1b1 (Rara) isoforms, was markedly restricted to a few cell types. Leveraging liver single-cell RNAseq studies yielded a zonation pattern of NRs in hepatocytes, liver sinusoidal cells, and stellate cells, establishing a link between NR subtissular localization and liver functional specialization. In summary, we provide here an up-to-date compendium of NR expression in mouse liver in space and time.
