Transcriptional regulation of energy metabolism in the liver

Regulation of hepatocyte cell cycle re-entry by RNA polymerase II-associated Gdown1

Liver is the central organ responsible for whole-body metabolism, and its constituent hepatocytes are the major players that carry out liver functions. Although they are highly differentiated and rarely divide, hepatocytes re-enter the cell cycle following hepatic loss due to liver damage or injury. However, the exact molecular mechanisms underlying cell cycle re-entry remain undefined. Gdown1 is an RNA polymerase II (Pol II)-associated protein that has been linked to the function of the Mediator transcriptional coactivator complex. We recently found that Gdown1 ablation in mouse liver leads to down-regulation of highly expressed liver-specific genes and a concomitant cell cycle re-entry associated with the induction of cell cycle-related genes. Unexpectedly, in view of a previously documented inhibitory effect on transcription initiation by Pol II in vitro, we found that Gdown1 is associated with elongating Pol II on the highly expressed genes and that its ablation leads to a reduced Pol II occupancy that correlates with the reduced expression of these genes. Based on these observations, we discuss the in vitro and in vivo functions of Gdown1 and consider mechanisms by which the dysregulated Pol II recruitment associated with Gdown1 loss might induce quiescent cell re-entry into the cell cycle.

Transcriptional down-regulation of metabolic genes by Gdown1 ablation induces quiescent cell re-entry into the cell cycle

Here, Jishage et al. show that hepatocyte-specific ablation of RNA polymerase II (Pol II)-associated Gdown1 leads to down-regulation of highly expressed genes involved in plasma protein synthesis and metabolism, a concomitant cell cycle re-entry associated with induction of cell cycle-related genes (including cyclin D1). Their findings establish an important physiological function for a Pol II regulatory factor (Gdown1) in the maintenance of normal liver cell transcription through constraints on cell cycle re-entry of quiescent hepatocytes.

Liver regeneration and metabolism are highly interconnected. Here, we show that hepatocyte-specific ablation of RNA polymerase II (Pol II)-associated Gdown1 leads to down-regulation of highly expressed genes involved in plasma protein synthesis and metabolism, a concomitant cell cycle re-entry associated with induction of cell cycle-related genes (including cyclin D1), and up-regulation of p21 through activation of p53 signaling. In the absence of p53, Gdown1-deficient hepatocytes show a severe dysregulation of cell cycle progression, with incomplete mitoses, and a premalignant-like transformation. Mechanistically, Gdown1 is associated with elongating Pol II on the highly expressed genes and its ablation leads to reduced Pol II recruitment to these genes, suggesting that Pol II redistribution may facilitate hepatocyte re-entry into the cell cycle. These results establish an important physiological function for a Pol II regulatory factor (Gdown1) in the maintenance of normal liver cell transcription through constraints on cell cycle re-entry of quiescent hepatocytes.

Evaluation of apoptosis and hypoxia-related factors in gingival tissues of smoker and non-smoker periodontitis patients

OBJECTIVE

Smoking causes pathological changes in all tissues, including gingiva and alveolar bone. The aim of present study was to evaluate apoptotic tissue alterations and tissue destruction in smoker and non-smoker periodontitis patients and healthy individuals.

METHODS

Gingival biopsy samples from 15 systemically and orally healthy individuals (Group 1), 15 systemically healthy periodontitis patients (Group 2), 15 systemically and orally healthy smokers (Group 3), and 15 systemically healthy smoker periodontitis patients (Group 4) were enrolled in the present study. Clinical periodontal measurements as plaque index (PI), gingival index (GI), and clinical attachment levels (CAL) were recorded, and gingival biopsies were obtained. Biopsy samples were fixed in formalin solution and embedded in paraffin. Fibroblast and inflammatory cell counts were determined via histomorphometrically. Hypoxia-inducible factor alpha (HIF-1α), vascular endothelial growth factor(VEGF), tissue inhibitor of matrix metalloproteinase-1(TIMP-1), matrix metalloproteinases-8(MMP-8) expressions, Bax, Bcl-2, and caspase-3 expressions were evaluated via immunohistochemistry.

RESULTS

Demographic data of the study groups were similar. Smoking levels of the smokers were also similar. The highest fibroblast cell counts were observed in healthy controls and the counts were similar in other groups. The highest inflammatory cell counts were found in smoker periodontitis group, and the lowest counts were found in healthy control groups. The differences were statistically significant. HIF-1α and Bax expressions were elevated and Bcl-2 decreased in smoker periodontitis patients compared with healthy individuals. However, there were no differences in VEGF, MMP-8, and TIMP-1 expressions.

CONCLUSION

Within limits of present study, it can be suggested that both smoking and periodontitis caused similar decrease in fibroblast counts while causing a dramatic increase in inflammatory cell counts. Increased apoptosis and hypoxia also accompanied to the increased inflammation.

Contrasting Patterns in the Evolution of Vertebrate MLX Interacting Protein (MLXIP) and MLX Interacting Protein-Like (MLXIPL) Genes

ChREBP and MondoA are glucose-sensitive transcription factors that regulate aspects of energy metabolism. Here we performed a phylogenomic analysis of Mlxip (encoding MondoA) and Mlxipl (encoding ChREBP) genes across vertebrates. Analysis of extant Mlxip and Mlxipl genes suggests that the most recent common ancestor of these genes was composed of 17 coding exons. Single copy genes encoding both ChREBP and MondoA, along with their interacting partner Mlx, were found in diverse vertebrate genomes, including fish that have experienced a genome duplication. This observation suggests that a single Mlx gene has been retained to maintain coordinate regulation of ChREBP and MondoA. The ChREBP-β isoform, the more potent and constitutively active isoform, appeared with the evolution of tetrapods and is absent from the Mlxipl genes of fish. Evaluation of the conservation of ChREBP and MondoA sequences demonstrate that MondoA is better conserved and potentially mediates more ancient function in glucose metabolism.

Myostatin inhibits proliferation and insulin-stimulated glucose uptake in mouse liver cells

Although myostatin functions primarily as a negative regulator of skeletal muscle growth and development, accumulating biological and epidemiological evidence indicates an important contributing role in liver disease. In this study, we demonstrate that myostatin suppresses the proliferation of mouse Hepa-1c1c7 murine-derived liver cells (50%; p < 0.001) in part by reducing the expression of the cyclins and cyclin-dependent kinases that elicit G1-S phase transition of the cell cycle (p < 0.001). Furthermore, real-time PCR-based quantification of the long noncoding RNA metastasis associated lung adenocarcinoma transcript 1 (Malat1), recently identified as a myostatin-responsive transcript in skeletal muscle, revealed a significant downregulation (25% and 50%, respectively; p < 0.05) in the livers of myostatin-treated mice and liver cells. The importance of Malat1 in liver cell proliferation was confirmed via arrested liver cell proliferation (p < 0.05) in response to partial Malat1 siRNA-mediated knockdown. Myostatin also significantly blunted insulin-stimulated glucose uptake and Akt phosphorylation in liver cells while increasing the phosphorylation of myristoylated alanine-rich C-kinase substrate (MARCKS), a protein that is essential for cancer cell proliferation and insulin-stimulated glucose transport. Together, these findings reveal a plausible mechanism by which circulating myostatin contributes to the diminished regenerative capacity of the liver and diseases characterized by liver insulin resistance.

Peripheral signalling involved in energy homeostasis control

The alarming prevalence of obesity has led to a better understanding of the molecular mechanisms controlling energy homeostasis. Regulation of energy intake and expenditure is more complex than previously thought, being influenced by signals from many peripheral tissues. In this sense, a wide variety of peripheral signals derived from different organs contributes to the regulation of body weight and energy expenditure. Besides the well-known role of insulin and adipokines, such as leptin and adiponectin, in the regulation of energy homeostasis, signals from other tissues not previously thought to play a role in body weight regulation have emerged in recent years. The role of fibroblast growth factor 21 (FGF21), insulin-like growth factor 1 (IGF-I), and sex hormone-binding globulin (SHBG) produced by the liver in the regulation of body weight and insulin sensitivity has been recently described. Moreover, molecules expressed by skeletal muscle such as myostatin have also been involved in adipose tissue regulation. Better known is the involvement of ghrelin, cholecystokinin, glucagon-like peptide 1 (GLP-1) and PYY(3-36), produced by the gut, in energy homeostasis. Even the kidney, through the production of renin, appears to regulate body weight, with mice lacking this hormone exhibiting resistance to diet-induced obesity. In addition, the skeleton has recently emerged as an endocrine organ, with effects on body weight control and glucose homeostasis through the actions of bone-derived factors such as osteocalcin and osteopontin. The comprehension of these signals will help in a better understanding of the aetiopathology of obesity, contributing to the potential development of new therapeutic targets aimed at tackling excess body fat accumulation.