Insulin-like growth factor-i regulates Kruppel-like factor-6 gene expression in a p53-dependent manner

Krüppel-like factor 6 is a transcriptional activator of autophagy in acute liver injury

Krüppel-like factor 6 (KLF6) is a transcription factor and tumor suppressor. We previously identified KLF6 as mediator of hepatocyte glucose and lipid homeostasis. The loss or reduction of KLF6 is linked to the progression of hepatocellular carcinoma, but its contribution to liver regeneration and repair in acute liver injury are lacking so far. Here we explore the role of KLF6 in acute liver injury models in mice, and in patients with acute liver failure (ALF). KLF6 was induced in hepatocytes in ALF, and in both acetaminophen (APAP)- and carbon tetrachloride (CCl₄)-treated mice. In mice with hepatocyte-specific Klf6 knockout (DeltaKlf6), cell proliferation following partial hepatectomy (PHx) was increased compared to controls. Interestingly, key autophagic markers and mediators LC3-II, Atg7 and Beclin1 were reduced in DeltaKlf6 mice livers. Using luciferase assay and ChIP, KLF6 was established as a direct transcriptional activator of ATG7 and BECLIN1, but was dependent on the presence of p53. Here we show, that KLF6 expression is induced in ALF and in the regenerating liver, where it activates autophagy by transcriptional induction of ATG7 and BECLIN1 in a p53-dependent manner. These findings couple the activity of an important growth inhibitor in liver to the induction of autophagy in hepatocytes.

Insulin Downregulates the Transcriptional Coregulator CITED2, an Inhibitor of Proangiogenic Function in Endothelial Cells

In patients with atherosclerotic complications of diabetes, impaired neovascularization of ischemic tissue in the myocardium and lower limb limits the ability of these tissues to compensate for poor perfusion. We identified 10 novel insulin-regulated genes, among them Adm, Cited2, and Ctgf, which were downregulated in endothelial cells by insulin through FoxO1. CBP/p300-interacting transactivator with ED-rich tail 2 (CITED2), which was downregulated by insulin by up to 54%, is an important negative regulator of hypoxia-inducible factor (HIF) and impaired HIF signaling is a key mechanism underlying the impairment of angiogenesis in diabetes. Consistent with impairment of vascular insulin action, CITED2 was increased in cardiac endothelial cells from mice with diet-induced obesity and from db/db mice and was 3.8-fold higher in arterial tissue from patients with type 2 diabetes than control subjects without diabetes. CITED2 knockdown promoted endothelial tube formation and endothelial cell proliferation, whereas CITED2 overexpression impaired HIF activity in vitro. After femoral artery ligation, induction of an endothelial-specific HIF target gene in hind limb muscle was markedly upregulated in mice with endothelial cell deletion of CITED2, suggesting that CITED2 can limit HIF activity in vivo. We conclude that vascular insulin resistance in type 2 diabetes contributes to the upregulation of CITED2, which impairs HIF signaling and endothelial proangiogenic function.

The age factor in axonal repair after spinal cord injury: A focus on neuron-intrinsic mechanisms

Age is an important consideration for recovery and repair after spinal cord injury. Spinal cord injury is increasingly affecting the middle-aged and aging populations. Despite rapid progress in research to promote axonal regeneration and repair, our understanding of how age can modulate this repair is rather limited. In this review, we discuss the literature supporting the notion of an age-dependent decline in axonal growth after central nervous system (CNS) injury. While both neuron-intrinsic and extrinsic factors are involved in the control of axon growth after injury, here we focus on possible intrinsic mechanisms for this age-dependent decline.

Insulin-like Growth Factor 1 Signaling Axis Meets p53 Genome Protection Pathways

Clinical, epidemiological, and experimental evidence indicate that the insulin-like growth factors (IGFs) are important mediators in the biochemical chain of events that lead from a phenotypically normal to a neoplastic cell. The IGF1 receptor (IGF1R), which mediates the biological actions of IGF1 and IGF2, exhibits potent pro-survival and antiapoptotic activities. The IGF1R is highly expressed in most types of cancer and is regarded as a promising therapeutic target in oncology. p53 is a transcription factor with tumor suppressor activity that is usually activated in response to DNA damage and other forms of cellular stress. On the basis of its protective activities, p53 is commonly regarded as the guardian of the genome. We provide evidence that the IGF signaling axis and p53 genome protection pathways are tightly interconnected. Wild-type, but not mutant, p53 suppresses IGF1R gene transcription, leading to abrogation of the IGF signaling network, with ensuing cell cycle arrest. Gain-of-function, or loss-of-function, mutations of p53 in tumor cells may disrupt its inhibitory activity, thus generating oncogenic molecules capable of transactivating the IGF1R gene. The interplay between the IGF1 and p53 pathways is also of major relevance in terms of metabolic regulation, including glucose transport and glycolysis. A better understanding of the complex physical and functional interactions between these important signaling pathways will have major basic and translational relevance.

The Krüppel-like factors in female reproductive system pathologies

Female reproductive tract pathologies arise largely from dysregulation of estrogen and progesterone receptor signaling, leading to aberrant cell proliferation, survival, and differentiation. The signaling pathways orchestrated by these nuclear receptors are complex, require the participation of many nuclear proteins serving as key binding partners or targets, and involve a range of paracrine and autocrine regulatory circuits. The members of the Krüppel-like factor (KLF) family of transcription factors are ubiquitously expressed in reproductive tissues and have been increasingly implicated as critical co-regulators and integrators of steroid hormone actions. Herein, we explore the involvement of KLF family members in uterine pathology, describe their currently known molecular mechanisms, and discuss their potential as targets for therapeutic intervention.

Transcriptional and epigenetic control of IGF1R gene expression: implications in metabolism and cancer

IGF1R plays an important role in protection from apoptosis, regulation of cell growth, differentiation and oncogenic transformation. IGF1R aberrations lead to intrauterine and postnatal growth failure, microcephaly, mental retardation and deafness. High levels of IGF1R are detected in a diversity of human tumors. IGF1R gene transcription is controlled by complex interactions involving DNA-binding and non DNA-binding transcription factors. This review highlights selected examples of a series of tumor suppressors, including the breast cancer gene-1 (BRCA1), p53, the Wilm's tumor protein-1 (WT1) and the von Hippel-Lindau gene (VHL), whose mechanisms of action involve regulation of IGF1R gene expression. IGF1R gene transcription is also dependent on the presence of stimulatory nuclear proteins, including zinc-finger protein Sp1, EWS-WT1, E2F1, Krüppel-like factor-6 (KLF6), high-mobility group A1 (HMGA1), and others. Loss-of-function of tumor suppressor genes, usually caused by mutations, may result in non-functional proteins unable to control IGF1R promoter activity. Impaired regulation of the IGF1R gene is linked to defective cell division, chromosomal instability and increased incidence of cancer.

Krüppel-like factor 6 expression changes during trophoblast syncytialization and transactivates ßhCG and PSG placental genes

Background

Krüppel-like factor-6 (KLF6) is a widely expressed member of the Sp1/KLF family of transcriptional regulators involved in differentiation, cell cycle control and proliferation in several cell systems. Even though the highest expression level of KLF6 has been detected in human and mice placenta, its function in trophoblast physiology is still unknown.

Methodology/Principal Findings

Herein, we explored KLF6 expression and sub-cellular distribution in human trophoblast cells differentiating into the syncytial pathway, and its role in the regulation of genes associated with placental development and pregnancy maintenance. Confocal immunofluorescence microscopy demonstrated that KLF6 is expressed throughout human cytotrophoblast differentiation showing no evident modifications in its nuclear and cytoplasmic localization pattern. KLF6 transcript and protein peaked early during the syncytialization process as determined by qRT-PCR and western blot assays. Overexpression of KLF6 in trophoblast-derived JEG-3 cells showed a preferential nuclear signal correlating with enhanced expression of human β-chorionic gonadotropin (βhCG) and pregnancy-specific glycoprotein (PSG) genes. Moreover, KLF6 transactivated βhCG5, PSG5 and PSG3 gene promoters. Deletion of KLF6 Zn-finger DNA binding domain or mutation of the consensus KLF6 binding site abolished transactivation of the PSG5 promoter.

Conclusions/Significance

Results are consistent with KLF6 playing a role as transcriptional regulator of relevant genes for placental differentiation and physiology such as βhCG and PSG, in agreement with an early and transient increase of KLF6 expression during trophoblast syncytialization.

Expression and Function of Kruppel Like-Factors (KLF) in Carcinogenesis

Krüppel-like factor (KLF) family members share a three C2H2 zinc finger DNA binding domain, and are involved in cell proliferation and differentiation control in normal as in pathological situations. Studies over the past several years support a significant role for this family of transcription factors in carcinogenesis. KLFs can both activate and repress genes that participate in cell-cycle regulation. Among them, many up-regulated genes are inhibitors of proliferation, whereas genes that promote cell proliferation are repressed. However, several studies do present KLFs as positive regulator of cell proliferation. KLFs can be deregulated in multiple cancers either by loss of heterozygosity (LOH), somatic mutation or transcriptional silencing by promoter hypermethylation. Accordingly, KLF expression was shown to mediate growth inhibition when ectopically expressed in multiple cancer-derived cell lines through the inhibition of a number of key oncogenic signaling pathways, and to revert the tumorogenic phenotype in vivo. Taken together, these observations suggest that KLFs act as tumor suppressor. However, in some occasion, KLFs could act as tumor promoters, depending on “cellular context”. Thus, this review will discuss the roles and the functions of KLF family members in carcinogenesis, with a special focus on cancers from epithelial origin.

Type 2 diabetes and cancer: what is the connection?

Epidemiological studies have demonstrated an association between type 2 diabetes and cancer. Type 2 diabetes is characterized by insulin resistance and hyperinsulinemia. Hyperinsulinemia may lead to cancer through insulin's effect on its cognate receptor and the insulin-like growth factor system. The effects of insulin and insulin-like growth factor I on cancer development and progression have been demonstrated in animal and human studies. Type 2 diabetes has been positively associated with cancers of the breast, colon, and pancreas. An inverse relationship has been observed between type 2 diabetes and prostate cancer, and this may be due to lower testosterone levels in men with type 2 diabetes. Medications used to treat type 2 diabetes may affect cancer cells directly or indirectly by affecting serum insulin levels. Hyperinsulinemia may be an important risk factor for cancer as well as a target for cancer therapy.

Preadipocytes of type 2 diabetes subjects display an intrinsic gene expression profile of decreased differentiation capacity

OBJECTIVE

Insulin resistance and type 2 diabetes mellitus (T2DM) are associated with increased adipocyte size, altered secretory pattern and decreased differentiation of preadipocytes. In this study, we identified the underlying molecular processes in preadipocytes of T2DM patients, a characteristic for the development of T2DM.

DESIGN AND PARTICIPANTS

Preadipocyte cell cultures were prepared from subcutaneous fat biopsies of seven T2DM patients (age 53 ± 12 years; body mass index (BMI) 34 ± 5  kg m(-2)) and nine control subjects (age 51 ± 12 years; BMI 30 ± 3 kg m(-2)). Microarray analysis was used to identify altered processes between the T2DM and control preadipocytes.

RESULTS

Gene expression profiling showed changed expression of transcription regulators involved in adipogenesis and in extracellular matrix remodeling, actin cytoskeleton and integrin signaling genes, which indicated decreased capacity to differentiate. Additionally, genes involved in insulin signaling and lipid metabolism were downregulated, and inflammation/apoptosis was upregulated in T2DM preadipocytes.

CONCLUSION

Decreased expression of genes involved in differentiation can provide a molecular basis for the reduced adipogenesis of preadipocytes of T2DM subjects, leading to reduced formation of adipocytes in subcutaneous fat depots, and ultimately leading to ectopic fat storage.

Increased apoptosis in bovine blastocysts exposed to high levels of IGF1 is not associated with downregulation of the IGF1 receptor

The hypothesis that high concentrations of IGF1 can impair embryo development was investigated in a bovine in vitro model to reflect conditions in polycystic ovary syndrome (PCOS) patients. Embryos were either cultured in the absence or presence of a physiological (100 ng/ml) or supraphysiological (1000 ng/ml) IGF1 concentration. Cell allocation, apoptosis, transcript and protein expression of selected genes involved in apoptosis, glucose metabolism and the IGF system were analysed. Supraphysiological IGF1 concentration did not improve blastocyst formation over controls, but induced higher levels of apoptosis, decreased TP53 protein expression in the trophectoderm and increased the number of cells in the inner cell mass (ICM). The increase in ICM cells corresponded with an increase in IGF1 receptor (IGF1R) protein in the ICM. A small, but significant, percentage of blastocysts displayed a hypertrophic ICM, not observed in controls and virtually absent in embryos treated with physiological concentrations of IGF1. Physiological IGF1 concentrations increased total IGF1R protein expression and upregulated IGFBP3 transcripts leading to an increase in blastocyst formation with no effects on cell number or apoptosis. In conclusion, the results support the hypothesis of detrimental effects of supraphysiological IGF1 concentrations on early pregnancy. However, our results do not support the premise that increased apoptosis associated with high levels of IGF1 is mediated via downregulation of the IGF1R as previously found in preimplantation mouse embryos. This in vitro system with the bovine preimplantation embryo reflects critical features of fertility in PCOS patients and could thus serve as a useful model for in-depth mechanistic studies.

Insulin-like growth factor-I activates extracellularly regulated kinase to regulate the p450 side-chain cleavage insulin-like response element in granulosa cells

IGF regulates steroidogenesis in granulosa cells through expression of the cytochrome P450 side-chain cleavage enzyme (P450scc) (CYP11A1), the rate-limiting enzyme in this biosynthetic process. We showed previously that the polypyrimidine tract-binding protein-associated splicing factor (PSF) acts as a repressor, whereas Sp1 is an activator, of P450 gene expression. The aim of the present study was to investigate IGF-stimulated ERK signaling regulating P450scc gene expression in the immortalized porcine granulosa cell line JC-410. We used a reporter gene under control of the IGF response element from the P450scc promoter. Inhibition of ERK phosphorylation with U0126 [1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene] blocked IGF-I induction of IGF response element reporter gene activity. Western blotting revealed that IGF-I treatment resulted in phosphorylation of ERK that was specifically inhibited by U0126. ERK activation led to phosphorylation of T739 (an ERK site) on Sp1 that was diminished by U0126 or overexpression of PSF. Coimmunoprecipitation and Western blotting of nuclear extracts showed that phosphorylated ERK (pERK) bound PSF under basal conditions. IGF-I caused dissociation of pERK from PSF. Finally, chromatin immunoprecipitation analysis showed that PSF and Sp1 constitutively occupy the P450scc promoter independent of IGF-I treatment. These events provide a potential molecular mechanism for release of PSF repression of P450scc expression by dissociation of pERK and subsequent pERK-mediated phosphorylation of Sp1 to drive transcriptional induction of the P450scc gene in the absence of altered binding of PSF or Sp1 to the promoter. Understanding IGF-I regulation of these critical ovarian signaling pathways is the first step to delineating ovarian hyperstimulation syndromes such as polycystic ovarian syndrome.

Identification of Insulin-Like Growth Factor-I Receptor (IGF-IR) Gene Promoter-Binding Proteins in Estrogen Receptor (ER)-Positive and ER-Depleted Breast Cancer Cells

The insulin-like growth factor I receptor (IGF-IR) has been implicated in the etiology of breast cancer. Overexpression of the IGF-IR gene is a typical feature of most primary breast cancers, whereas low IGF-IR levels are seen at advanced stages. Hence, evaluation of IGF-IR levels might be important for assessing prognosis. In the present study, we employed a proteomic approach based on DNA affinity chromatography followed either by mass spectroscopy (MS) or Western blot analysis to identify transcription factors that may associate with the IGF-IR promoter in estrogen receptor (ER)-positive and ER-depleted breast cancer cells. A biotinylated IGF-IR promoter fragment was bound to streptavidin magnetic beads and incubated with nuclear extracts of breast cancer cells. IGF-IR promoter-binding proteins were eluted with high salt and analyzed by MS and Western blots. Among the proteins that were found to bind to the IGF-IR promoter we identified zinc finger transcription factors Sp1 and KLF6, ER-α, p53, c-jun, and poly (ADP-ribosylation) polymerase. Furthermore, chromatin immune-precipitation (ChIP) analysis confirmed the direct in vivo binding of some of these transcription factors to IGF-IR promoter DNA. The functional relevance of binding data was assessed by cotransfection experiments with specific expression vectors along with an IGF-IR promoter reporter. In summary, we identified nuclear proteins that are potentially responsible for the differential expression of the IGF-IR gene in ER-positive and ER-depleted breast cancer cells.

Role of Kruppel-like factor 6 in transforming growth factor-beta1-induced epithelial-mesenchymal transition of proximal tubule cells

Krüppel-like factor 6 (KLF6) is a DNA-binding protein containing a triple zinc-fingered motif and plays a key role in the regulation of cell proliferation, differentiation, and development. More recently it has been implicated in hepatic fibrosis via its binding to the transforming growth factor (TGF)-beta control element. In the kidney, epithelial-mesenchymal transition (EMT) is a major contributor to the pathogenesis of renal fibrosis, with TGF-beta1 being a key mediator of EMT. The present study aimed to determine the role of KLF6 and TGF-beta1 in EMT in proximal tubule cells. To determine the relevance in clinical disease, KLF6 was measured in kidneys of streptozotocin-induced diabetic Ren-2 rats and in cells exposed to high (30 mM) glucose. TGF-beta1 was confirmed to induce EMT by morphological change, loss of E-cadherin, and gain in vimentin expression. KLF6 mRNA expression was concomitantly measured. To determine the role of KLF6 in EMT, the above markers of EMT were determined in KLF6-silenced (small interfering RNA) and KLF6-overexpressing proximal tubule cells. KLF6 overexpression significantly promoted a phenotype consistent with EMT. High glucose induced KLF6 in proximal tubule cells (P < 0.05). This increase in KLF6 in response to high glucose was TGF-beta1 mediated. In an in vivo model of diabetic nephropathy KLF6 increased at week 8 (P < 0.05). KLF6 plays a permissive role in TGF-beta1-induced EMT in proximal tubule cells. Its upregulation in in vivo models of diabetic nephropathy suggests it as a potential therapeutic target.