Physiological concentrations of insulin promote binding of nuclear proteins to the insulin-like growth factor I gene

Amelioration of exosome and mesenchymal stem cells in rats infected with diabetic nephropathy by attenuating early markers and aquaporin-1 expression

Diabetic nephropathy (DN) is a prevalent diabetic microvascular condition. It is the leading cause of kidney disease in the advanced stages. There is no currently effective treatment available. This research aimed to investigate the curative potentials of exosomes isolated from mesenchymal stem cells affecting DN. This study was performed on 70 male adult albino rats. Adult rats were randomized into seven groups: Group I: Negative control group, Group II: DN group, Group III: Balanites treated group, Group IV: MSCs treated group, Group V: Exosome treated group, Group VI: Balanites + MSCs treated group and Group VII: Balanites + exosome treated group. Following the trial period, blood and renal tissues were subjected to biochemical, gene expression analyses, and histopathological examinations. Results showed that MDA was substantially increased, whereas TAC was significantly decreased in the kidney in the DN group compared to normal health rats. Undesired elevated values of MDA levels and a decrease in TAC were substantially ameliorated in groups co-administered Balanites aegyptiacae with MSCs or exosomes compared to the DN group. A substantial elevation in TNF-α and substantially diminished concentration of IGF-1 were noticed in DN rats compared to normal health rats. Compared to the DN group, the co-administration of Balanites aegyptiacae with MSCs or exosomes substantially improved the undesirable elevated values of TNF-α and IGF-1. Furthermore, in the DN group, the mRNA expression of Vanin-1, Nephrin, and collagen IV was significantly higher than in normal healthy rats. Compared with DN rats, Vanin-1, Nephrin, and collagen IV Upregulation were substantially reduced in groups co-administered Balanites aegyptiacae with MSCs or exosomes. In DN rats, AQP1 expression was significantly lower than in normal healthy rats. Furthermore, the groups co-administered Balanites aegyptiacae with MSCs or exosomes demonstrated a substantial increase in AQP1 mRNA expression compared to DN rats.

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.

The combined hyperlipidemia caused by impaired Wnt-LRP6 signaling is reversed by Wnt3a rescue

The underlying molecular genetic basis of combined hyperlipidemia, the most common atherogenic lipid disorder, is poorly characterized. Rare, nonconservative mutations in the Wnt coreceptor, LRP6, underlie autosomal dominant atherosclerosis, combined hyperlipidemia, and fatty liver disease. Mice with LRP6(R611C) mutation similarly developed elevated plasma LDL and TG levels and fatty liver. Further investigation showed that LRP6(R611C) mutation triggers hepatic de novo lipogenesis, lipid and cholesterol biosynthesis, and apoB secretion by an Sp1-dependent activation of IGF1, AKT, and both mTORC1 and mTORC2. These pathways were normalized after in vitro treatment of primary hepatocytes from LRP6(R611C) mice with either the IGF1R antagonist PPP, rapamycin, or rmWnt3a. Strikingly, in vivo administration of rmWnt3a to LRP6(R611C) mice normalized the altered expression of enzymes of DNL and cholesterol biosynthesis, and restored plasma TG and LDL levels to normal. These findings identify Wnt signaling as a regulator of plasma lipids and a target for treatment of hyperlipidemia.

Regulation of insulin-response element binding protein-1 in obesity and diabetes: potential role in impaired insulin-induced gene transcription

One of the major mechanisms by which insulin modulates glucose homeostasis is through regulation of gene expression. Therefore, reduced expression of transcription factors that are required for insulin-regulated gene expression may contribute to insulin resistance. We recently identified insulin response element-binding protein-1 (IRE-BP1) as a transcription factor that binds and transactivates multiple insulin-responsive genes, but the regulation of IRE-BP1 in vivo is largely unknown. In this study, we show that IRE-BP1 interacts with the insulin response sequence of the IGF-I, IGFBP-1, and IGFBP-3 genes using chromatin immunoprecipitation assay. Furthermore, activation by IRE-BP1 is sequence specific and mimics that of the insulin effect on gene transcription. Tissue expression of IRE-BP1 is 50- to 200-fold higher in classical insulin target compared with nontarget tissues in lean animals, with a significantly reduced level of expression in the skeletal muscle and adipose tissue in obese and diabetic animals. In the liver, IRE-BP1 is localized to the nucleus in lean rats but is sequestered to the cytoplasm in obese and diabetic animals. Cytoplasmic sequestration appears to be related to inhibition of insulin-mediated phosphatidylinositol-3 kinase signaling. Therefore, in diabetes and obesity, the mechanisms involved in reducing the transactivation of the insulin response sequence by IRE-BP1 include decreased gene transcription and nuclear exclusion to prevent DNA binding. Our study supports the notion that IRE-BP1 may be relevant to the action of insulin in vivo and may play a role in the development of insulin resistance and diabetes.

Alteration of the gene and protein levels of insulin-like growth factors in streptozotocin-induced diabetic male rats

Insulin-like growth factor (IGFs: IGF-I and IGF-II) systems have been reported to be associated with the onset of diabetic mellitus. Therefore, we investigated the effect of diabetes on regulation of the IGF system in the liver, kidneys and heart, which are important organs in the pathogenesis of diabetes. The experimental groups were subdivided into three groups: 1) controls, 2) streptozotocin (STZ)-induced untreated diabetic group, and 3) an insulin-treated group (plus diabetic rats). In the present study, starting on the second day after STZ treatment, the diabetic group exhibited hyperglycemia, polyuria, and polydipsia, which are characteristic of diabetes melittus. Serum levels of IGF-I were decreased, but those of IGF-II were increased in the diabetic group compared with the controls. The expression levels of IGF-I and IGF-II protein in the livers of the diabetic group had a similar pattern to the serum. In addition, the expression levels of liver IGF-I mRNA and IGF-II mRNA were decreased in the diabetic groups. In the heart, IGF-I levels were decreased, but IGF-II levels were increased in the untreated diabetic groups, which was consistent with the expression levels of their mRNA. However, both the IGF-I and IGF-II levels in the kidneys were increased in the untreated diabetic groups, but the mRNA levels were decreased. Insulin treatment ameliorated the changes of IGF system in the serum, liver, kidneys, and heart. In conclusion, diabetes induced alteration of the IGF system tissue-specifically, and this was blocked by insulin treatment.

Activation of the nuclear transcription factor SP-1 by insulin rapidly increases the expression of protein kinase C delta in skeletal muscle

SP-1, a ubiquitous transcription factor involved in regulation of target genes participating in specific signaling pathways, is utilized by insulin for induction of gene transcription. Transcriptional activation generally occurs only after several (14-24) hours. A major element rapidly activated by insulin in skeletal muscle is PKCdelta, which plays a positive regulatory role in insulin signaling. We recently reported that insulin stimulation of skeletal muscle increases PKCdelta RNA expression and PKCdelta protein levels within 5 min. These effects were blocked by inhibitors of either translation or transcription. In this study, we investigated the possibility that SP-1 may participate in this unusually rapid effect. Studies were performed on myoblasts and myotubes of the L6 skeletal muscle cell line. Insulin rapidly increased SP-1 levels and stimulated SP-1 phosphorylation in the nuclear fraction of L6 myotubes. The increase in nuclear SP-1 was blocked by inhibition of nuclear import. Inhibition of SP-1, either pharmacologically or by suppression of SP-1 by RNAi, nearly completely abrogated insulin-induced increase in PKCdelta promoter activity. Insulin induced a rapid association of SP-1 with the PKCalpha promoter. In addition, SP-1 inhibition blocked insulin-induced increases in both PKCdelta RNA expression and PKCdelta protein levels. We conclude that insulin rapidly stimulates SP-1, which mediates the ability of this hormone to induce the rapid transcription of a major target gene utilized in the insulin signaling cascade.

An insulin-response element-binding protein that ameliorates hyperglycemia in diabetes

Insulin modulates glucose homeostasis, but the role of insulin-responsive transcription factors in such actions is not well understood. Recently, we have identified the insulin-response element-binding protein-1 (IRE-BP1) as a transcription factor that appears to mediate insulin action on multiple target genes. To examine the possibility that IRE-BP1 is an insulin-responsive glucoregulatory factor involved in the metabolic actions of insulin, we investigated the effect of adenoviral overexpression of hepatic IRE-BP1 on the glycemic control of insulin-resistant diabetic rats. Adenoviral IRE-BP1 lowered both fasting and postprandial glucose levels, and microarray of hepatic RNA revealed modulation of the expression of genes involved in gluconeogenesis, lipogenesis, and fatty acid oxidation. The insulin mimetic effects of IRE-BP1 were also confirmed in L6 myocytes; stable constitutive expressions of IRE-BP1 enhanced glucose transporter expression, glucose uptake, and glycogen accumulation in these cells. These findings showed physiologic sufficiency of IRE-BP1 as the transcriptional mediator of the metabolic action of insulin. Understanding IRE-BP1 action should constitute a useful probe into the mechanisms of metabolic regulation and an important target to develop therapeutic agents that mimic or enhance insulin action.

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.

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

The diabetes-induced decrease in insulin-like growth factor-I transcription appears to be mediated by footprint region V in exon 1. Since region V contains both an Sp1 site and an AT-rich element that recognizes an insulin-responsive binding protein (IRBP), we tested the hypothesis that Sp1 interactions are facilitated by an IRBP. Binding of nuclear extracts to region V probes was reduced by mutational or chemical interference with the AT-rich element. Blocking the AT site also reduced interactions of Sp1 with region V in vitro and blunted transactivation of region V reporter constructs by Sp1 in vivo. Sp1 binding was enhanced by small quantities of hepatic nuclear extracts, but enhancement was reduced by the AT mutation and abolished by a 5-base pair insertion between the AT-rich and GC-rich sites, and transactivation by Sp1 in vivo was diminished by inserting bases between the AT-rich and GC-rich elements. However, treating cells with insulin increased the ability of nuclear extracts to enhance Sp1 binding. These findings indicate that the presence of the AT-rich element is essential for the actions of Sp1 in vitro and in vivo, and the combination of both spacing requirements and insulin responsiveness suggests that IRBP may interact directly with Sp1.