Insulin-responsive nuclear proteins facilitate Sp1 interactions with the insulin-like growth factor-I gene

Sp1 and the 'hallmarks of cancer'

For many years, transcription factor Sp1 was viewed as a basal transcription factor and relegated to a role in the regulation of so-called housekeeping genes. Identification of Sp1's role in recruiting the general transcription machinery in the absence of a TATA box increased its importance in gene regulation, particularly in light of recent estimates that the majority of mammalian genes lack a TATA box. In this review, we briefly consider the history of Sp1, the founding member of the Sp family of transcription factors. We review the evidence suggesting that Sp1 is highly regulated by post-translational modifications that positively and negatively affect the activity of Sp1 on a wide array of genes. Sp1 is over-expressed in many cancers and is associated with poor prognosis. Targeting Sp1 in cancer treatment has been suggested; however, our review of the literature on the role of Sp1 in the regulation of genes that contribute to the 'hallmarks of cancer' illustrates the extreme complexity of Sp1 functions. Sp1 both activates and suppresses the expression of a number of essential oncogenes and tumor suppressors, as well as genes involved in essential cellular functions, including proliferation, differentiation, the DNA damage response, apoptosis, senescence and angiogenesis. Sp1 is also implicated in inflammation and genomic instability, as well as epigenetic silencing. Given the apparently opposing effects of Sp1, a more complete understanding of the function of Sp1 in cancer is required to validate its potential as a therapeutic target.

Models to predict changes in serum IGF1 and body composition in response to GH replacement therapy in GH-deficient adults

OBJECTIVE

Clinical response to GH therapy in GH-deficient (GHD) adults varies widely. Good predictors of treatment response are lacking. The aim of the study was to develop mathematical models to predict changes in serum IGF1 and body composition (BC) in response to GH therapy in GHD adults.

DESIGN AND METHODS

One hundred and sixty-seven GHD patients (103 men, median age 50 years) were studied before and after 12 months of GH treatment. GH dose was tailored according to serum IGF1 concentrations. Good responders (GR) and poor responders (PR) to GH therapy were defined as patients with a response >60th and <40th percentile respectively, for changes in serum IGF1 levels (adjusted for GH cumulative dose) and in BC (lean body mass (LBM) and body fat determined using dual-energy X-ray absorptiometry). A logistic regression model was used to predict the probability of being a GR or PR.

RESULTS

In the IGF1 prediction model, men (odds ratio (OR) 5.62: 95% confidence interval 2.59-12.18) and patients with higher insulin levels (OR 1.06: 1.00-1.12) were more likely to be GR. The accuracy of the prediction model was 70%. In the BC model, men (OR 10.72: 1.36-84.18) and GHD patients with lower LBM (OR 0.82: 0.73-0.92) and greater height (OR 1.23: 1.08-1.40) at baseline were more likely to be GR. The accuracy of the prediction model was 80%.

CONCLUSION

Accurate mathematical models to predict GH responsiveness in GHD adults were developed using gender, body height, baseline LBM, and serum insulin levels as the major clinical predictors.

Insulin regulates cytokines and intercellular adhesion molecule-1 gene expression through nuclear factor-kappaB activation in LPS-induced acute lung injury in rats

Diabetic patients have increased susceptibility to infection, which may be related to impaired inflammatory response observed in experimental models of diabetes, and restored by insulin treatment. The goal of this study was to investigate whether insulin regulates transcription of cytokines and intercellular adhesion molecule 1 (ICAM-1) via nuclear factor-kappaB (NF-kappaB) signaling pathway in Escherichia coli LPS-induced lung inflammation. Diabetic male Wistar rats (alloxan, 42 mg/kg, i.v., 10 days) and controls were instilled intratracheally with saline containing LPS (750 microg/0.4 mL) or saline only. Some diabetic rats were given neutral protamine Hagedorn insulin (4 IU, s.c.) 2 h before LPS. Analyses performed 6 h after LPS included: (a) lung and mesenteric lymph node IL-1 beta, TNF-alpha, IL-10, and ICAM-1 messenger RNA (mRNA) were quantified by real-time reverse transcriptase-polymerase chain reaction; (b) number of neutrophils in the bronchoalveolar lavage (BAL) fluid, and concentrations of IL-1 beta, TNF-alpha, and IL-10 in the BAL were determined by the enzyme-linked immunosorbent assay; and (c) activation of NF-kappaB p65 subunit and phosphorylation of I-kappaB alpha were quantified by Western blot analysis. Relative to controls, diabetic rats exhibited a reduction in lung and mesenteric lymph node IL-1 beta (40%), TNF-alpha (approximately 30%), and IL-10 (approximately 40%) mRNA levels and reduced concentrations of IL-1 beta (52%), TNF-alpha (62%), IL-10 (43%), and neutrophil counts (72%) in the BAL. Activation of NF-kappaB p65 subunit and phosphorylation of I-kappaB alpha were almost suppressed in diabetic rats. Treatment of diabetic rats with insulin completely restored mRNA and protein levels of these cytokines and potentiated lung ICAM-1 mRNA levels (30%) and number of neutrophils (72%) in the BAL. Activation of NF-kappaB p65 subunit and phosphorylation of I-kappaB alpha were partially restored by insulin treatment. In conclusion, data presented suggest that insulin regulates transcription of proinflammatory (IL-1 beta, TNF-alpha) and anti-inflammatory (IL-10) cytokines, and expression of ICAM-1 via the NF-kappaB signaling pathway.

Resistin gene variation is associated with systemic inflammation but not plasma adipokine levels, metabolic syndrome or coronary atherosclerosis in nondiabetic Caucasians

OBJECTIVE

Resistin causes insulin resistance and diabetes in mice whereas in humans it is linked to inflammation and atherosclerosis. Few human genetic studies of resistin in inflammation and atherosclerosis have been performed. We hypothesized that the -420C>G putative gain-of-function resistin variant would be associated with inflammatory markers and atherosclerosis but not with metabolic syndrome or adipokines in humans.

DESIGN AND METHODS

We examined the association of three resistin polymorphisms, -852A>G, -420C>G and +157C>T, and related haplotypes with plasma resistin, cytokines, C-reactive protein (CRP), adipokines, plasma lipoproteins, metabolic syndrome and coronary artery calcification (CAC) in nondiabetic Caucasians (n = 851).

RESULTS

Resistin levels were higher, dose-dependently, with the -420G allele (CC 5.9 +/- 2.7 ng/ml, GC 6.5 +/- 4.0 ng/ml and GG 7.2 +/- 4.8 ng/ml, trend P = 0.04) after age and gender adjustment [fold higher for GC + GG vs. CC; 1.07 (1.00-1.15), P < 0.05)]. The -852A>G single nucleotide polymorphism (SNP) was associated with higher soluble tumour necrosis factor-receptor 2 (sol-TNFR2) levels in fully adjusted models [1.06 (95% CI 1.01-1.11), P = 0.01)]. The estimated resistin haplotype (GGT) was associated with sol-TNFR2 (P = 0.04) and the AGT haplotype was related to CRP (P = 0.04) in the fully adjusted models. Resistin SNPs and haplotypes were not associated with body mass index (BMI), fasting glucose, insulin resistance, metabolic syndrome, adipokines or CAC scores.

CONCLUSIONS

Despite modest associations with plasma resistin and inflammatory biomarkers, resistin 5' variants were not associated with metabolic parameters or coronary calcification. This suggests that resistin is an inflammatory cytokine in humans but has little influence on adiposity, metabolic syndrome or atherosclerosis.

Effect of the -420C/G variant of the resistin gene promoter on metabolic syndrome, obesity, myocardial infarction and kidney dysfunction

OBJECTIVE

Resistin is an adipokine that has been suggested to be correlated with markers of inflammation and to be predictive of coronary atherosclerosis and type II diabetes in humans. A common single nucleotide polymorphism (SNP) (-420C/G) in the promoter of resistin is associated with increased resistin plasma levels and susceptibility to type II diabetes. The aim of this study was to investigate the association of the -420C/G polymorphism with metabolic syndrome, obesity, myocardial infarction and kidney disease.

DESIGN AND RESULTS

First we studied 1542 subjects from the PLIC study (a population based cohort). GG carriers showed an higher prevalence of obesity and metabolic syndrome as well as increased plasma triglycerides levels, BMI, systolic and diastolic blood pressure and cardiovascular risk according to Framingham algorithm (P < 0.05 for all). Next we investigated the presence of the -420C/G resistin polymorphism in a case-control study that included 300 subject with myocardial infarction and 300 age and sex matched controls and then we studied the role of the -420C/G SNP in 88 patients with mild to moderate renal dysfunction. No statistically significant differences in allele frequencies between the PLIC study, the myocardial infarction (MI) cases and the subjects with renal dysfunction were observed. Pro-inflammatory gene expression profiling of peripheral blood mononuclear cells failed to detect any difference between wild type subjects and carriers of the rare allele.

CONCLUSION

Our data suggest that the presence of the -420C/G SNP of the resistin gene is associated with increased obesity and metabolic syndrome, although it is not different in subjects at high cardiovascular risk such as patients with myocardial infarction or patients with renal dysfunction compared with controls.

Transcriptional regulation by insulin: from the receptor to the gene

Insulin, after binding to its receptor, regulates many cellular processes and the expression of several genes. For a subset of genes, insulin exerts a negative effect on transcription; for others, the effect is positive. Insulin controls gene transcription by modifying the binding of transcription factors on insulin-response elements or by regulating their transcriptional activities. Different insulin-signaling cascades have been characterized as mediating the insulin effect on gene transcription. In this review, we analyze recent data on the molecular mechanisms, mostly in the liver, through which insulin exerts its effect. We first focus on the key transcription factors (viz. Foxo, sterol-response-element-binding protein family (SREBP), and Sp1) involved in the regulation of gene transcription by insulin. We then present current information on the way insulin downregulates and upregulates gene transcription, using as examples of downregulation phosphoenolpyruvate carboxykinase (PEPCK) and insulin-like growth factor binding protein 1 (IGFBP-1) genes and of upregulation the fatty acid synthase and malic enzyme genes. The last part of the paper focuses on the signaling cascades activated by insulin in the liver, leading to the modulation of gene transcription.

Value of insulin-like growth factor system markers in the assessment of growth hormone status

Insulin-like growth factor-I (IGF-I) has been measured extensively in a variety of clinical settings. Total IGF-I frequently is used to assess the clinical impact of disorders of GH secretion and to monitor patients' response to therapy. It does not have sufficient precision to be used as a stand-alone test in the diagnosis of GH deficiency. Free IGF-I, IGF binding protein-3, or acid-labile subunit may provide useful information regarding GH secretion in specific conditions but are not superior to IGF-I for making the diagnosis of GH deficiency or acromegaly.

Molecular characterization of the insulin-like growth factor-I (IGF-I) gene in channel catfish (Ictalurus punctatus)

The insulin-like growth factor I (IGF-I) gene was characterized in channel catfish. Partial cDNA sequence, missing exon 1 and part of exon 2, was obtained in 5'- and 3'-RACE experiments. Direct sequencing of two bacterial artificial chromosome clones revealed gene structure and provided sequence from 640 bp upstream of the initiator methionine to 136 bp beyond the polyadenylation site. Genomic sequence contained a putative TATA box 506 bp upstream of the initiator methionine. The 477-bp reading frame within five exons encoded a 159-amino acid (aa) pre-propeptide highly similar to IGF-I in higher vertebrates. The sequence encoding the signal peptide was unique in catfish and contained 70% G+C content with the potential for a stable stem-loop structure. Full-length cDNA was only maintained in recombination-deficient (DH10B) strain E. coli. Levels of IGF-I mRNA were highest in liver, followed by brain and muscle, then heart and kidney (P<0.05). A CT/GA dinucleotide microsatellite in intron 1 was highly polymorphic in commercial channel catfish, and permitted placement of the IGF-I gene on the catfish genetic map. However, specific IGF-I alleles were not correlated with differences in growth rate from 100 to 130 days post-hatch in USDA103 line catfish.

Insulin increases the abundance of the growth hormone receptor in liver and adipose tissue of periparturient dairy cows

After parturition, increased growth hormone (GH) secretion is important to preserve the metabolic homeostasis of energy-deficient dairy cows. Elevated plasma GH promotes lipid mobilization from adipose tissue, but paradoxically, is associated with depressed concentration of insulin-like growth factor-I (IGF-I), a growth factor produced in a GH-dependent fashion in liver. Primary factors regulating GH responses of liver and adipose tissue are poorly understood in periparturient dairy cows. Consistent with insulin being such a factor, its plasma concentration declined concomitantly with net energy balance (EB) and with plasma IGF-I in a group of 9 periparturient dairy cows. To test the role of insulin in regulating cellular determinants of GH responsiveness, hyperinsulinemic-euglycemic clamps were performed on 6 dairy cows in late pregnancy (28 d prepartum) before the reductions in EB, insulin, and IGF-I were initiated, and when they were completed in early lactation (10 d postpartum). Infusion of insulin nearly doubled the plasma concentration of IGF-I (P < 0.001) and hepatic levels of IGF-I mRNA during both states (P < 0.05). In liver, these responses were associated with increased abundance of the GH receptor protein (GHR; P < 0.05), whereas the abundance of intracellular mediators of GH actions (JAK2, STAT5, or STAT3) remained unaffected. Insulin also doubled GHR abundance in adipose tissue (P < 0.01), indicating that this effect is not liver specific. These results raise the possibility that insulin regulates the efficiency of GH signaling in liver and adipose tissue of dairy cows by acting as a rheostat of GHR synthesis.

Using DNA microarray to identify Sp1 as a transcriptional regulatory element of insulin-like growth factor 1 in cardiac muscle cells

High throughput gene expression profiling with DNA microarray provides an opportunity to analyze transcriptional regulation of hundreds or thousands of similarly regulated genes. Transcriptional regulation of gene expression plays an important role in myocardial remodeling. We have studied cardiac muscle gene expression with DNA microarray and used a computational strategy to identify common promoter motifs that respond to insulin-like growth factor 1 (IGF-1) stimulation in cardiac muscle cells. The analysis showed that the Sp1 binding site is a likely target of IGF-1 action. Further experiments with gel shift assay indicated that IGF-1 regulated the Sp1 site in cardiomyocytes, by increasing the abundance of Sp1 and Sp3 proteins. Using firefly luciferase as reporter gene, additional experiments showed that IGF-1 activated the promoter of cyclin D3 and Glut1. Both promoters contain one Sp1 site. The effect of IGF-1 on these two promoters was abolished with siRNA for Sp1. Thus, the transcriptional activation of these two promoters by IGF-1 requires the induction of Sp1 protein. These experiments suggest that the global transcriptional regulatory actions of IGF-1 involve activation of the Sp1 site in cardiac muscle. The computational model we have developed is a prototypical method that may be further developed to identify unique cis- and trans-acting elements in response to hormonal stimulation during cardiac muscle growth, repair, and remodeling in normal and abnormal cardiac muscle.

Interaction of p38 and Sp1 in a mechanical force-induced, beta 1 integrin-mediated transcriptional circuit that regulates the actin-binding protein filamin-A

Connective tissue cells in mechanically active environments survive applied physical forces by modifying actin cytoskeletal structures that stabilize cell membranes. In fibroblasts, tensile forces induce the expression of filamin-A, a mechanoprotective actin-binding protein, but the mechanisms and protein interactions by which force activates filamin-A transcription are not defined. We found that in fibroblasts, application of tensile forces through collagen-coated magnetite beads to cell surface beta(1) integrins induced filamin-A expression. This induction required actin filaments and selective activation of the p38 mitogen-activated protein kinase. Force promoted the redistribution of p38 to the integrin/bead locus and the nucleus as well as enhanced binding of the transcription factor Sp1 to proximal, regulatory domains of the filamin-A promoter. Force application increased association of Sp1 with p38 and phosphorylation of Sp1. Transcriptional activation of filamin-A in force-treated fibroblasts was subsequently mediated by Sp1-binding sites on the filamin-A promoter. These results provide evidence for a mechanically coupled transcriptional circuit that originates at the magnetite bead/integrin locus, activates p38, tethers p38 to actin filaments, promotes binding of p38 to Sp1 in the nucleus, and induces filamin-A expression.