Inhibition and stimulation of c-myc gene transcription by insulin in rat hepatoma cells. Insulin alters the intragenic pausing of c-myc transcription

The role of metabolic states in development and disease

During development, cells adopt distinct metabolic strategies to support growth, produce energy, and meet the demands of a mature tissue. Some of these metabolic states maintain a constrained program of nutrient utilization, while others providing metabolic flexibility as a means to couple developmental progression with nutrient availability. Here we discuss our understanding of metabolic programs, and how they support specific aspects of animal development. During phases of rapid proliferation a subset of metabolic programs provide the building blocks to support growth. During differentiation, metabolic programs shift to support the unique demands of each tissue. Finally, we discuss how a model system, such as Drosophila egg development, can provide a versatile platform to discover novel mechanisms controlling programmed shift in metabolism.

Insulin attenuates TNFα-induced hemopexin mRNA: An anti-inflammatory action of insulin in rat H4IIE hepatoma cells

Proinflammatory cytokines, including TNF-α and IL-6, can contribute to insulin resistance. Conversely, insulin has some actions that can be considered anti-inflammatory. Hemopexin is a Class 2 acute phase reactant and control of its transcription is predominantly regulated by IL-6, with TNF-α and IL-1β also inducing hemopexin gene expression. Thus, we asked whether insulin could inhibit the ability of TNF-α to stimulate hemopexin mRNA expression. In cultured rat hepatoma (H4IIE) cells, TNF-α significantly increased hemopexin mRNA accumulation. The TNF-α-induced increase of hemopexin mRNA was dramatically attenuated by insulin, even though TNF-α reduced peak insulin activation of ERK. Thus, even though TNF-α can contribute to insulin resistance, the residual insulin response was still able to counteract TNF-α actions.

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The TNF-α-induced increase of hemopexin mRNA was dramatically attenuated by insulin.

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This occurred even though TNF-α significantly decreased insulin activation of ERK.

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This suggests an additional mechanism for the anti-inflammatory action of insulin.

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Cytokine-induced insulin resistance does not abolish insulin’s anti-inflammatory effect.

Prenylation differentially inhibits insulin-dependent immediate early gene mRNA expression

Increased activity of prenyl transferases is observed in pathological states of insulin resistance, diabetes, and obesity. Thus, functional inhibitors of farnesyl transferase (FTase) and geranylgeranyl transferase (GGTase) may be promising therapeutic treatments. We previously identified insulin responsive genes from a rat H4IIE hepatoma cell cDNA library, including β-actin, EGR1, Pip92, c-fos, and Hsp60. In the present study, we investigated whether acute treatment with FTase and GGTase inhibitors would alter insulin responsive gene initiation and/or elongation rates. We observed differential regulation of insulin responsive gene expression, suggesting a differential sensitivity of these genes to one or both of the specific protein prenylation inhibitors.

Insulin upregulates the expression of zinc finger and BTB domain-containing 7A in HepG2 cells

Zinc finger and BTB domain-containing 7A (Zbtb7A) is a proto-oncogene overexpressed in numerous cancers. In this study, we explored the mechanism of insulin-induced Zbtb7A expression. Real-time PCR and western blotting were used to detect Zbtb7A expression. Zbtb7A promoter activity was monitored by Luciferase reporter assay. It was shown that insulin elevates the mRNA and protein levels of the Zbtb7A gene in HepG2 cells. Using chemical inhibitors of insulin downstream pathways, we demonstrated that the insulin-induced Zbtb7A gene expression was completely blocked by LY294002, a PI3K/AKT inhibitor, and partially attenuated by the MAPK inhibitor PD98059. Transfection of HepG2 cells with a 1 kb Zbtb7A promoter-luciferase reporter construct revealed a dose-dependent activation of the Zbtb7A promoter by insulin, while mutation of the Sp1 binding site within the Zbtb7A promoter resulted in the failure of insulin-induced promoter activation, suggesting that insulin increases Zbtb7A expression through transcriptional regulation mediated by Sp1 in HepG2 cells.

Insulin alters heterogeneous nuclear ribonucleoprotein K protein binding to DNA and RNA

The interaction of the multimodular heterogeneous nuclear ribonucleoprotein (hnRNP) K protein with many of its protein and nucleic acid partners is regulated by extracellular signals. Acting as a docking platform, K protein could link signal-transduction pathways to DNA- and RNA-directed processes such as transcription, mRNA processing, transport, and translation. Treatment of hepatocyte culture with insulin increased K protein tyrosine phosphorylation. Insulin altered K protein interaction with RNA and DNA in vitro. Administration of insulin into mice had similar effects on K protein in liver. Coimmunoprecipitations of RNA with K protein revealed preferential in vivo K protein binding of a subset of transcripts, including the insulin-inducible c-fos mRNA. These results suggest a class of insulin pathways that signal nucleic acid-directed processes that involve K protein.

Insulin induction of pip 92, CL-6, and novel mRNAs in rat hepatoma cells

Insulin directly affects many aspects of cellular metabolism. An additional, poorly studied effect of insulin is the regulation of multiple genes whose products are important in many cellular functions. Using differential screening techniques, we identified insulin-regulated genes induced in insulin-treated rat H4IIE (H4) hepatoma cells. Two of the mRNAs identified were homologous to the previously characterized mouse pip 92 and rat CL-6 immediate-early genes. The pip 92 clone was initially isolated from mitogen-stimulated mouse Balb/c 3T3 fibroblasts, whereas the CL-6 clone was first isolated from regenerating rat liver. In this article, we demonstrated that in rat H4 cells, the transcription rates of both pip 92 and CL-6 are induced by insulin alone. Additionally, we showed that the transcription rates of two other genes, whose sequences are not homologous to any other sequences in gene bank ("novel genes"), were rapidly and transiently induced by insulin. These results demonstrate that insulin regulates the expression of several novel genes with a time-course similar to members of the immediate-early response gene family.

Initial expression of phosphoenolpyruvate carboxykinase gene in fetal hepatocytes: role of transcription factors

BACKGROUND/AIMS

Hepatic phosphoenolpyruvate carboxykinase (PEPCK; EC 4.1.1.32) gene is absent in fetal liver. However, this gene can be initially expressed in 20-day-old fetal hepatocyte primary cultures under specific hormonal stimulation. The role of transcriptional factors involved is also studied.

METHODS

Primary 20-day-old fetal hepatocytes have been cultured and Northern-blot and nuclear run-on transcription assays have been performed.

RESULTS

Fetal hepatocytes in culture initially expressed PEPCK gene by dibutyryl cAMP, in the presence of dexamethasone. Dibutyryl cAMP increased by 8-fold the rate of transcription of PEPCK gene at 30 min, and produced a 50-fold increase in its mRNA content at 3 h. This induction of PEPCK expression by cAMP occurred in the presence of sustained levels of CCAAT/enhancer binding protein (C/EBP) alpha-delta mRNAs, and was accompanied by an increase in the rate of transcription and mRNA content of C/EBP beta gene, and a decrease in the expression of c-myc, in the absence of c-fos expression. In addition, insulin or phorbol esters decreased by 50% the PEPCK rate of transcription and its mRNA accumulation induced by dibutyryl cAMP. This inhibitory effect of insulin or phorbol esters on PEPCK gene expression was accompanied by an increase in the rate of transcription and mRNA content of nuclear factors such as c-fos and c-myc, the expression of C/EBPs remaining essentially unmodified.

Regulation of gamma-actin gene expression by insulin

Insulin exerts rapid effects on cellular metabolism and can cause morphological changes by inducing rearrangements of cytoskeletal components. The regulation of specific cytoskeletal genes by insulin, however, has not been studied extensively. In the present work insulin was found to rapidly, but transiently, increase transcription of the cytoskeletal gamma-actin gene in rat H4IIE (H4) hepatoma cells. Insulin-induced transcription of the gamma-actin gene was evident within 5 min and was maximal by 15 min at 10-fold above control levels. The stimulation of transcription was transient, with a return towards basal levels by 120 min. Transcription of gamma-actin was increased at insulin concentrations as low as 1 x 10(-11) M and was maximal at 1 x 10(-9) to 1 x 10(-8) M. Transcription of several control genes (skeletal and cardiac alpha-actin and beta-tubulin) were unaltered by insulin administration. Messenger RNA (mRNA) levels for the gamma-actin gene increased, but to a lesser degree than transcription. Since the gamma-actin message is an abundant and stable mRNA, its levels would not be expected to change dramatically from a transient induction of transcription. Like insulin, phorbol esters transiently increased transcription of the gamma-actin gene. In addition, pretreatment of cells with phorbol esters for 24 h reduced the ability of insulin to induce gamma-actin transcription. These data support our hypothesis that insulin and phorbol esters share intracellular signalling pathways in the control of transcription of specific genes.

Post-transcriptional regulation of insulin-like growth factor binding protein-2 mRNA in diabetic rat liver

IGFBP-1 and IGFBP-2 mRNAs are increased in the livers of streptozotocin-diabetic rats. A corresponding increase is observed in transcription of the IGFBP-1 but not the IGFBP-2 gene, indicating that the increase in steady-state levels of IGFBP-2 mRNA is a post-transcriptional effect. IGFBP-1 and IGFBP-2 mRNAs also differ in the rapidity of their response to insulin treatment: hepatic IGFBP-1 mRNA is normalized within 1 h, IGFBP-2 mRNA decreases more slowly. These differences suggest that IGFBP-2 may provide more chronic adaptation to metabolic change than IGFBP-1.

Rapid regulation of albumin transcription by insulin and phorbol esters in rat hepatoma cells

The short-term effects of insulin and phorbol esters on the regulation of the albumin gene in rat H4IIE (H4) hepatoma cells were investigated and compared to the expression of a gene known to be inhibited by these agents, phosphoenol pyruvate carboxykinase (PEPCK). Both insulin and phorbol esters inhibited transcription of the albumin gene in a rapid, dose-dependent manner. Within 15 min, albumin transcription was reduced by approx. 80%. The inhibitory effects of insulin were evident at concentrations of insulin as low as 5.10(-11)M, suggesting that these effects were mediated through insulin-specific pathways. The ability of both phorbol esters and insulin to inhibit albumin transcription suggests that the negative control of this gene is a stable feature in H4 cells. The effect of phorbol esters to mimic insulin action on the albumin gene, and on several other genes in this cell line, implies that a common pathway may be shared by both insulin and phorbol esters.

Short- and long-term effects of insulin on tyrosine aminotransferase gene expression

In the present study the relationship between changes in tyrosine aminotransferase (TAT) enzyme activity, cytoplasmic mRNA levels, and gene transcription in response to both short- and long-term exposure to insulin was investigated. Insulin acutely inhibited transcription of the TAT gene by 50% in serum-deprived rat H4 hepatoma cells. Following this initial 50% decrease in transcription, there was a 2.5-fold induction in TAT activity that could not be accounted for by a concomitant increase in TAT mRNA levels. Insulin had no effect on the half-life of TAT mRNA. Insulin exposure for short periods of time also inhibited the glucocorticoid- and cAMP-induced transcription of the TAT gene. Like insulin, protein synthesis inhibitors acutely inhibited basal and glucocorticoid-induced TAT transcription. TAT activity gradually returned toward basal levels after 8 h of insulin treatment. A second insulin-induced increase in TAT activity (3.5-fold above basal levels) was observed by 24 h of insulin treatment. This second phase of insulin-induced TAT activity was associated with elevated levels of TAT transcription and TAT mRNA levels, and therefore, unlike the earlier stimulation, could be accounted for by changes in gene expression. Thus, the insulin-mediated regulation of the TAT gene in H4 cells is complex. Different transcriptional and post-transcriptional mechanisms are likely to be involved in the biphasic responses to insulin.

Protein synthesis and insulin regulation of p33 and PEPCK gene expression

Insulin stimulates transcription and cytoplasmic accumulation of a specific mRNA (termed p33), while inhibiting transcription and accumulation of phosphoenolpyruvate carboxykinase (PEPCK) mRNA in rat H4IIE (H4) hepatoma cells. The present work examines the role of protein synthesis in regulation of these genes by insulin and dexamethasone. Like insulin, cycloheximide and anisomycin, two protein synthesis inhibitors, induced p33 transcription and reduced PEPCK transcription. The combination of either protein synthesis inhibitor and insulin did not induce p33 transcription or inhibit PEPCK transcription beyond that observed with either protein synthesis inhibitor alone. Dexamethasone induced both p33 and PEPCK transcription. The combination of insulin and dexamethasone, or protein synthesis inhibitors and dexamethasone, abolished dexamethasone-induced PEPCK transcription. Thus, protein synthesis inhibitors regulate transcription of the p33 and the PEPCK genes in an insulin-like manner.