Reactive oxygen species (ROS) resulting from chronic inflammation cause liver injury leading to transformation of regenerating hepatocytes. Metallothioneins (MT), induced at high levels by oxidative stress, are potent scavengers of ROS. Here, we report that the levels of MT-1 and MT-2A are drastically reduced in primary human hepatocellular carcinomas (HCCs) and in diethylnitrosamine-induced liver tumors in mice, which is primarily due to transcriptional repression. Expression of the transcription factor, MTF-1, essential for MT expression, and its target gene Zn-T1 that encodes the zinc transporter-1 was not significantly altered in HCCs. Inhibitors of both phosphatidylinositol 3-kinase (PI3K) and its downstream target AKT increased expression of MT genes in HCC cells but not in liver epithelial cells. Suppression of MT-1 and MT-2A by ectopic expression of the constitutively active PI3K or AKT and their up-regulation by dominant-negative PI3K or AKT mutant confirmed negative regulation of MT expression by PI3K/AKT signaling pathway. Further, treatment of cells with a specific inhibitor of glycogen synthase kinase-3 (GSK-3), a downstream effector of PI3K/AKT, inhibited MT expression specifically in HCC cells. Short interfering RNAmediated depletion of CCAAT/enhancer binding protein A (C/EBPA), a target of GSK-3, impeded MT expression, which could not be reversed by PI3K inhibitors. DNA binding activity of C/EBPA and its phosphorylation at T222 and T226 by GSK-3 are required for MT expression. MTF-1 and C/EBPA act in concert to increase MT-2A expression, which probably explains the high level of MT expression in the liver. This study shows the role of PI3K/AKT signaling pathway and C/EBPA in regulation of MT expression in hepatocarcinogenesis.

Adipogenesis is the process by which mature, insulin-responsive adipocytes are generated from undifferentiated preadipocytes and mesenchymal progenitor cells (1). This process is crucial to the normal development of adipose tissue and its expansion in response to excess dietary energy intake. Alternatively, most lipodystrophic syndromes are characterized by a suppression of adipogenesis and an increase in adipocyte death.Cells destined to the adipose lineage arise late in development from multipotential stem cells of mesodermal origin (1-4). The commitment of the multipotent stem cells to the adipocyte lineage is a poorly understood process. However, once committed to the adipocyte lineage, nonproliferating preadipocytes become responsive to external stimuli that induce their differentiation to mature adipocytes. These stimuli include insulin-like growth factor-1 or insulin (which appears to work through the insulin-like growth factor-1 receptor), glucocorticoids, and agents that elevate intracellular cAMP levels (1).Exposure of these cells to adipogenic inducers initiates a temporally orchestrated cascade of gene expression events that characterize adipogenic differentiation. These agents initially induce a period of mitotic expansion during which expression of CCAAT/enhancer binding proteins (C/EBPs) 2 ␤ and ␦ is increased, whereas expression of factors like Pref-1, necdin, and Wnt10b are diminished. Following mitotic expansion, differentiation begins, during which peroxisome-proliferator-activated receptor ␥ (PPAR␥) and C/EBP ␣ are up-regulated. These transcription factors regulate the expression of many of the factors that characterize the mature adipocyte phenotype like GLUT4 (5), adiponectin, aP2 (6), and perilipin (7).We previously reported that the activity of the transcription factor CREB was stimulated by cAMP mimetics and insulin in both preadipocytes (8), suggesting that CREB might play a role in adipogenic conversion. Subsequent experiments demonstrated that ectopic expression of constitutively active forms of CREB could induce adipogenesis of 3T3-L1 cells (9, 10) and prevent their apoptotic death in response to insulin and/or serum deprivation and tumor necrosis factor ␣ (11). Alternately, ectopic expression of dominant negative forms of CREB blocked adipogenic conversion and stimulated apoptosis of mature adipocytes. Recent studies using these techniques as well as CREB-specific antisense and siRNA confirm these results and indicate that CREB may promote adipogenesis by

The transcription factor CCAAT/enhancer-binding protein ␣ (C/EBP␣) is required during adipogenesis for development of insulin-stimulated glucose uptake. Modes for regulating this function of C/EBP␣ have yet to be determined. Phosphorylation of C/EBP␣ on Ser-21 has been implicated in the regulation of granulopoiesis and hepatic gene expression. To explore the role of Ser-21 phosphorylation on C/EBP␣ function during adipogenesis, we developed constructs in which Ser-21 was mutated to alanine (S21A) to model dephosphorylation. In two cell culture models deficient in endogenous C/EBP␣, enforced expression of S21A-C/EBP␣ resulted in normal lipid accumulation and expression of many adipogenic markers. However, S21A-C/ EBP␣ had impaired ability to activate the Glut4 promoter specifically, and S21A-C/EBP␣ expression resulted in diminished GLUT4 and adiponectin expression, as well as reduced insulinstimulated glucose uptake. No defects in insulin signaling or GLUT4 vesicle trafficking were identified with S21A-C/EBP␣ expression, and when exogenous GLUT4 expression was enforced to normalize expression in S21A-C/EBP␣ cells, insulin-responsive glucose transport was reconstituted, suggesting that the primary defect was a deficit in GLUT4 levels. Mice in which endogenous C/EBP␣ was replaced with S21A-C/EBP␣ displayed reduced GLUT4 and adiponectin protein expression in epididymal adipose tissue and increased blood glucose compared with wild-type littermates. These results suggest that phosphorylation of C/EBP␣ on Ser-21 may regulate adipocyte gene expression and whole body glucose homeostasis.Although adipocytes were initially thought to be passive storage vessels for caloric excess, it is now known that adipose tissue acts as an important metabolic and endocrine organ (1-4). One of the major regulatory hormones for fat cell function is insulin, which regulates whole body energy balance by increasing glucose uptake into muscle and adipose tissue via translocation of the glucose transporter GLUT4 to the cell surface and by inhibiting hepatic gluconeogenesis (4 -7). Several studies have identified C/EBP␣ 2 as a critical factor for development of insulin-sensitive glucose uptake in developing adipocytes (8 -11).Adipogenesis is a coordinated transcriptional cascade of gene expression regulated by many transcription factors, including PPAR␥ and members of the C/EBP and KLF families of transcription factors (12-17). Mouse models suggest that C/EBP␣ and PPAR␥ are each required for complete development of adipose tissue in vivo (18 -20). In vitro cell culture studies determined that although constitutive expression of either PPAR␥ or C/EBP␣ is sufficient to convert fibroblasts into fatladen, adipocyte-like cells (21), C/EBP␣ is required for the establishment of insulin-stimulated glucose uptake in adipocytes (10, 11). In both C/EBP␣ Ϫ/Ϫ mouse embryonic fibroblasts (MEFs) and NIH-3T3 fibroblasts, which are also deficient in C/EBP␣, overexpression of PPAR␥ is sufficient to induce lipid accumulation, but these "adipocytes" do not transpo...

No abstract

No abstract