Search

Summary: Coordinated temporal control of gene expression is essential for physiological homeostasis, especially during metabolic transitions. However, the interplay between chromatin architectural proteins and metabolism in regulating transcription is less understood. Here, we demonstrate a conserved bidirectional interplay between CTCF (CCCTC-binding factor) expression/function and metabolic inputs during feed-fast cycles. Our results indicate that its loci-specific functional diversity is associated with physiological plasticity in mouse hepatocytes. CTCF differential expression and long non-coding RNA-Jpx mediated changes in chromatin occupancy, unraveled its paradoxical yet tuneable functions, which are governed by metabolic inputs. We illustrate the key role of CTCF in controlling temporal cascade of transcriptional response, with effects on hepatic mitochondrial energetics and lipidome. Underscoring the evolutionary conservation of CTCF-dependent metabolic homeostasis, CTCF knockdown in flies abrogated starvation resistance. In summary, we demonstrate the interplay between CTCF and metabolic inputs that highlights the coupled plasticity of physiological responses and chromatin function.

Background: Accumulation of free fatty acids (FFAs) in hepatocytes is a hallmark of liver dysfunction and non-alcoholic fatty liver disease (NAFLD). Excessive deposition of FFAs alters lipid metabolism pathways increasing the oxidative stress and mitochondrial dysfunction. Attenuating hepatic lipid accumulation, oxidative stress, and improving mitochondrial function could provide potential targets in preventing progression of non-alcoholic fatty liver (NAFL) to non-alcoholic steatohepatitis (NASH). Earlier studies with Picrorhiza kurroa extract have shown reduction in hepatic damage and fatty acid infiltration in several experimental models and also clinically in viral hepatitis. Thus, the effect of P. kurroa's phytoactive, picroside II, needed mechanistic investigation in appropriate in vitro liver cell model. Objective(s): To study the effect of picroside II on FFAs accumulation, oxidative stress and mitochondrial function with silibinin as a positive control in in vitro NAFLD model. Materials and methods: HepG2 cells were incubated with FFAs-1000μM in presence and absence of Picroside II-10 μM for 20 hours. Results: HepG2 cells incubated with FFAs-1000μM lead to increased lipid accumulation. Picroside II-10μM attenuated FFAs-induced lipid accumulation (33%), loss of mitochondrial membrane potential (ΔΨm), ATP depletion, and production of reactive oxygen species (ROS). A concomitant increase in cytochrome C at transcription and protein levels was observed. An increase in expression of MnSOD, catalase, and higher levels of tGSH and GSH:GSSG ratios underlie the ROS salvaging activity of picroside II. Conclusion: Picroside II significantly attenuated FFAs-induced-lipotoxicity. The reduction in ROS, increased antioxidant enzymes, and improvement in mitochondrial function underlie the mechanisms of action of picroside II. These findings suggest a need to develop an investigational drug profile of picroside II for NAFLD as a therapeutic strategy. This could be evaluated through the fast-track path of reverse pharmacology.

The conserved NAD+-dependent deacylase SIRT1 plays pivotal, sometimes contrasting, roles in diverse physiological and pathophysiological conditions. In this study, we uncover a tissue-restricted isoform of SIRT1 (SIRT1-ΔE2) that lacks exon 2 (E2). Candidate-based screening of SIRT1 substrates demonstrated that the domain encoded by this exon plays a key role in specifying SIRT1 protein-protein interactions. The E2 domain of SIRT1 was both necessary and sufficient for PGC1α binding, enhanced interaction with p53, and thus downstream functions. Since SIRT1-FL and SIRT1-ΔE2 were found to have similar intrinsic catalytic activities, we propose that the E2 domain tethers specific substrate proteins. Given the absence of SIRT1-ΔE2 in liver, our findings provide insight into the role of the E2 domain in specifying “metabolic functions” of SIRT1-FL. Identification of SIRT1-ΔE2 and the conserved specificity domain will enhance our understanding of SIRT1 and guide the development of therapeutic interventions.

Summary: Inability to mediate fed-fast transitions in the liver is known to cause metabolic dysfunctions and diseases. Intuitively, a failure to inhibit futile translation of state-specific transcripts during fed-fast cycles would abrogate dynamic physiological transitions. Here, we have discovered hepatic fed microRNAs that target fasting-induced genes and are essential for a refed transition. Our findings highlight the role of these fed microRNAs in orchestrating system-level control over liver physiology and whole-body energetics. By targeting SIRT1, PGC1α, and their downstream genes, fed microRNAs regulate metabolic and mitochondrial pathways. MicroRNA expression, processing, and RISC loading oscillate during these cycles and possibly constitute an anticipatory mechanism. Fed-microRNA oscillations are deregulated during aging. Scavenging of hepatic fed microRNAs causes uncontrolled gluconeogenesis and failure in the catabolic-to-anabolic switching upon feeding, which are hallmarks of metabolic diseases. Besides identifying mechanisms that enable efficient physiological toggling, our study highlights fed microRNAs as candidate therapeutic targets. : Maniyadath et al. have discovered oscillatory hepatic miRNAs that exert network-level control on metabolism and mitochondrial functions to mediate fed-fast-refed transitions. Fed miRNAs possibly act as anticipatory mechanisms for physiological state switching and are deregulated in aging. Abrogating fed-miRNA fluctuations causes liver dysfunctions, abolishes catabolic-anabolic shift, and affects organismal homeostasis. Keywords: microRNA, fed, fast, liver, PGC1α, SIRT1, gluconeogenesis, fatty acid oxidation, mitochondria, energetics

Sir2, an evolutionarily conserved NAD+-dependent deacetylase, has been implicated as a key factor in mediating organismal life span. However, recent contradictory findings have brought into question the role of Sir2 and its orthologs in regulating organismal longevity. In this study, we report that Drosophila Sir2 (dSir2) in the adult fat body regulates longevity in a diet-dependent manner. We used inducible Gal4 drivers to knock down and overexpress dSir2 in a tissue-specific manner. A diet-dependent life span phenotype of dSir2 perturbations (both knockdown and overexpression) in the fat body, but not muscles, negates the effects of background genetic mutations. In addition to providing clarity to the field, our study contrasts the ability of dSir2 in two metabolic tissues to affect longevity. We also show that dSir2 knockdown abrogates fat-body dFOXO-dependent life span extension. This report highlights the importance of the interplay between genetic factors and dietary inputs in determining organismal life spans.