Krüppel-like Factor 4 Is Acetylated by p300 and Regulates Gene Transcription via Modulation of Histone Acetylation

Klf4 interacts directly with Oct4 and Sox2 to promote reprogramming

Somatic cells can be reprogrammed to induced pluripotent stem (iPS) cells by ectopic expression of specific sets of transcription factors. Oct4, Sox2, and Klf4, factors that share many target genes in embryonic stem (ES) cells, are critical components in various reprogramming protocols. Nevertheless, it remains unclear whether these factors function together or separately in reprogramming. Here we show that Klf4 interacts directly with Oct4 and Sox2 when expressed at levels sufficient to induce iPS cells. Endogenous Klf4 also interacts with Oct4 and Sox2 in iPS cells and in mouse ES cells. The Klf4 C terminus, which contains three tandem zinc fingers, is critical for this interaction and is required for activation of the target gene Nanog. In addition, Klf4 and Oct4 co-occupy the Nanog promoter. A dominant negative mutant of Klf4 can compete with wild-type Klf4 to form defective Oct4/Sox2/Klf4 complexes and strongly inhibit reprogramming. In the absence of Klf4 overexpression, interaction of endogenous Klf4 with Oct4/Sox2 is also required for reprogramming. This study supports the idea that direct interactions between Klf4, Oct4, and Sox2 are critical for somatic cell reprogramming.

Role of Krüppel-like factor 4 in phenotypic switching and proliferation of vascular smooth muscle cells

Phenotypic switching and proliferation of vascular smooth muscle cells (VSMCs) are critical components in the development of many vascular proliferation diseases such as atherosclerosis and restenosis after percutaneous coronary interventions. Krüppel-like factor 4 (KLF4) has been shown to play a key role in VSMC proliferation and differentiation. The focus of this review is to provide an overview for understanding the physiological and pathobiological roles of KLF4 in phenotypic switching and proliferation of VSMCs.

The emerging role of Krüppel-like factors in endocrine-responsive cancers of female reproductive tissues

Krüppel-like factors (KLFs), of which there are currently 17 known protein members, belong to the specificity protein (Sp) family of transcription factors and are characterized by the presence of Cys(2)/His(2) zinc finger motifs in their carboxy-terminal domains that confer preferential binding to GC/GT-rich sequences in gene promoter and enhancer regions. While previously regarded to simply function as silencers of Sp1 transactivity, many KLFs are now shown to be relevant to human cancers by their newly identified abilities to mediate crosstalk with signaling pathways involved in the control of cell proliferation, apoptosis, migration, and differentiation. Several KLFs act as tumor suppressors and/or oncogenes under distinct cellular contexts, underscoring their prognostic potential for cancer survival and outcome. Recent studies suggest that a number of KLFs can influence steroid hormone signaling through transcriptional networks involving steroid hormone receptors and members of the nuclear receptor family of transcription factors. Since inappropriate sensitivity or resistance to steroid hormone actions underlies endocrine-related malignancies, we consider the intriguing possibility that dysregulation of expression and/or activity of KLF members is linked to the pathogenesis of endometrial and breast cancers. In this review, we focus on recently described mechanisms of actions of several KLFs (KLF4, KLF5, KLF6, and KLF9) in cancers of the mammary gland and uterus. We suggest that understanding the mode of actions of KLFs and their functional networks may lead to the development of novel therapeutics to improve current prospects for cancer prevention and cure.

Inner cell mass localization of NANOG precedes OCT3/4 in rhesus monkey blastocysts

The mechanism by which the inner cell mass (ICM) and trophectoderm (TE) become specified is poorly understood. Considerable species variation is evident in the expression of lineage-specific and embryonic stem cell (ESC) regulatory markers. We sought to investigate localization patterns of these markers in rhesus macaque compact morulae and blastocysts. NANOG protein was restricted to the ICM of blastocysts. In contrast to a previous report, the expression of CDX2 was detected in the primate blastocyst, localized specifically to the TE. Unlike the mouse embryo, OCT4 protein was detected using two different antibodies in both the ICM and TE. The ubiquitous pattern of OCT4 expression is consistent with observations in human, cow, and pig embryos. Significantly, lack of restricted OCT4 protein, and ICM localization of NANOG in primate blastocysts, suggests that NANOG may determine inner cell mass fate more specifically during primate development or may be less susceptible to culture artifacts. These results contrast markedly with current mechanistic hypotheses, although other factors may lie upstream of NANOG to constitute a complex interactive network. This difference may also underlie observations that regulatory mechanisms in ESC differ between mice and primates.

KLF4 interacts with beta-catenin/TCF4 and blocks p300/CBP recruitment by beta-catenin

Wnt signaling is crucial in the organization and maintenance of the human intestinal epithelium, and somatic mutations that result in deregulated Wnt signaling are an early event in the development of colorectal cancer. The Wnt ligand ultimately results in the stabilization of cytoplasmic beta-catenin, which is then free to enter the nucleus and activate transcription through its interaction with the transcription factor TCF4. Our laboratory recently found that KLF4, a transcription factor highly expressed in the adult intestine and critical for intestinal differentiation, interacts with beta-catenin and inhibits Wnt signaling. In this study, we characterize the molecular mechanisms of KLF4-mediated inhibition of Wnt/beta-catenin signaling. We find that the KLF4 directly interacts with the C-terminal transactivation domain of beta-catenin and inhibits p300/CBP recruitment by beta-catenin. KLF4 inhibits p300/CBP-mediated beta-catenin acetylation as well as histone acetylation on Wnt target genes. In addition, we observe that KLF4 directly interacts with TCF4 independently of beta-catenin and that KLF4 and TCF4 are expressed in similar patterns within the large intestine, with greatest staining near the epithelial surface. These results provide a deeper understanding of the regulation of beta-catenin in the intestine and will have important implications in cancer and stem cell research.

Wip1 confers G2 checkpoint recovery competence by counteracting p53-dependent transcriptional repression

Activation of the DNA damage checkpoint causes a cell-cycle arrest through inhibition of cyclin-dependent kinases (cdks). To successfully recover from the arrest, a cell should somehow be maintained in its proper cell-cycle phase. This problem is particularly eminent when a cell arrests in G2, as cdk activity is important to establish a G2 state. Here, we identify the phosphatase Wip1 (PPM1D) as a factor that maintains a cell competent for cell-cycle re-entry during an ongoing DNA damage response in G2. We show that Wip1 function is required throughout the arrest, and that Wip1 acts by antagonizing p53-dependent repression of crucial mitotic inducers, such as Cyclin B and Plk1. Our data show that the primary function of Wip1 is to retain cellular competence to divide, rather than to silence the checkpoint to promote recovery. Our findings uncover Wip1 as a first in class recovery competence gene, and suggest that the principal function of Wip1 in cellular transformation is to retain proliferative capacity in the face of oncogene-induced stress.

Transforming growth factor-beta1 up-regulation of human alpha(1)(I) collagen is mediated by Sp1 and Smad2 transacting factors

Hepatic fibrosis results from excessive deposition of type I collagen. The roles of Smads in mediating the effect of transforming growth factor-beta1 (TGFbeta1) on activation of the alpha(1)(I) collagen promoter were determined. Smads bind in association with Sp1 to the CC(GG)-rich TGFbeta1 responsive element of the promoter that lacks the classical Smad recognition element, and enhance binding of Sp1. In transfection experiments, TGFbeta1 activated a proximal promoter, but not promoters mutated at sites that prevented Sp1 binding. Sp1 alone or the combination of Smad2 and Smad4 activated the promoter in transfected human LX-2 stellate cells. Sp1 or Smad2 knockdowns with siRNAs prevented the effect of TGFbeta1 in enhancing the promoter. In conclusion, this study shows that Smads bind in association with Sp1 to the CC(GG)-rich TGFbeta1 responsive element of the human alpha(1)(I) collagen promoter that lacks the classical Smad recognition element, thus enhancing the binding of Sp1 and in this manner activating the collagen promoter.

The molecular mechanism of induced pluripotency: a two-stage switch

Pluripotent stem cells are basic cells with an indefinite self-renewal capacity and the potential to generate all the cell types of the three germinal layers. So far, the major source for pluripotent stem cells is the inner cell mass of the blastocysts: embryonic stem (ES) cells. Potential clinical application of ES cells is faced with many practical and ethical concerns. So, a major breakthrough was achieved in 2006, when it was shown that pluripotent stem cells could be obtained by transducing mouse embryonic and adult fibroblasts with a limited set of defined transcription factors. These reprogrammed cells, named induced pluripotent stem (iPS) cells, resembled ES cells in many of their characteristics. Since this initial study, iPS cell research has taken an incredible flight, and to date iPS cells have been generated from cells from several species using different sets of reprogramming factors. Given the potential to generate patient-specific cell populations without the need for human embryonic cells, iPS cell technology has been received with great excitement by research and medical communities. However, many questions regarding the actual molecular process of induced reprogramming remain unanswered and need to be addressed before iPS cells can go to the clinic. In this review, we start by summarizing recent advances in iPS cell research and inventory the hurdles that still need to be taken before safe clinical application. Our major aim, however, is to review the available data on the molecular processes underlying pluripotency reprogramming and present a two-stage switch model.

All-trans retinoic acid increases KLF4 acetylation by inducing HDAC2 phosphorylation and its dissociation from KLF4 in vascular smooth muscle cells

The zinc finger transcription factor Krüppel-like factor 4 (KLF4) has been implicated in vascular smooth muscle cell differentiation induced by all-trans retinoic acid (ATRA). However, the molecular mechanism whereby ATRA regulates KLF4 activity is still poorly understood. Here, we show that ATRA-induced histone deacetylase 2 (HDAC2) phosphorylation at Ser424 in VSMCs and inhibited the interaction of HDAC2 with KLF4. Inhibiting JNK by JNK inhibitor SP600125 or knockdown of JNK by JNK siRNA abrogated ATRA-induced HDAC2 phosphorylation and reversed ATRA-induced suppression of the interaction of HDAC2 with KLF4. We further demonstrated that HDAC2 directly deacetylated KLF4, and that KLF4 acetylation and binding activity of KLF4 to the SM22alpha promoter were significantly increased in ATRA-treated VSMCs. Collectively, our results indicate that ATRA induces HDAC2 phosphorylation mediated by JNK signaling, and thus causes HDAC2 dissociation from KLF4, subsequently leading to the increase in KLF4 acetylation.

KLF4 positively regulates human ghrelin expression

Ghrelin, an endogenous ligand of the GH (growth hormone) secretagogue receptor, influences many metabolic processes including GH secretion, food intake, energy balance, insulin secretion and adipogenesis. Although ghrelin exhibits a variety of biological functions, the mechanism by which ghrelin expression is regulated is unknown. Ghrelin is expressed in the gastrointestinal tract, predominantly in the stomach, as is KLF4 (Krüppel-like factor 4). Therefore we investigated whether ghrelin expression is associated with KLF4, and found that the tissue distribution of ghrelin corresponded with that of KLF4. Furthermore, treatment with butyrate, an inducer of KLF4 expression, stimulated ghrelin expression, and fasting, which induces ghrelin expression, also increased KLF4 expression, suggesting that ghrelin expression is associated with KLF4. Then, we investigated the effects of KLF4 on the human ghrelin-promoter activity and identified a KLF4-responsive region in the promoter. KLF4 expression specifically stimulated human ghrelin-promoter activity in a dose-dependent manner in human gastric-cancer AGS cells. However, this effect was not seen in response to a mutant KLF4 construct. Transfection studies using mutant constructs containing 5'-deletions in the human ghrelin promoter revealed that the KLF4-responsive element is located between -1228 and -1105. Electrophoretic mobility shift assays using oligonucleotides containing -1165/-1146 revealed the binding of KLF4 to the human ghrelin promoter. Finally, deletion of the -1165/-1146 region abrogated KLF4-induced transactivation of the ghrelin promoter. Collectively, these results indicate that KLF4 positively regulates human ghrelin expression via binding to a KLF-responsive region in the promoter.

Epigallocatechin-3-gallate, a histone acetyltransferase inhibitor, inhibits EBV-induced B lymphocyte transformation via suppression of RelA acetylation

Because the p300/CBP-mediated hyperacetylation of RelA (p65) is critical for nuclear factor-kappaB (NF-kappaB) activation, the attenuation of p65 acetylation is a potential molecular target for the prevention of chronic inflammation. During our ongoing screening study to identify natural compounds with histone acetyltransferase inhibitor (HATi) activity, we identified epigallocatechin-3-gallate (EGCG) as a novel HATi with global specificity for the majority of HAT enzymes but with no activity toward epigenetic enzymes including HDAC, SIRT1, and HMTase. At a dose of 100 micromol/L, EGCG abrogates p300-induced p65 acetylation in vitro and in vivo, increases the level of cytosolic IkappaBalpha, and suppresses tumor necrosis factor alpha (TNFalpha)-induced NF-kappaB activation. We also showed that EGCG prevents TNFalpha-induced p65 translocation to the nucleus, confirming that hyperacetylation is critical for NF-kappaB translocation as well as activity. Furthermore, EGCG treatment inhibited the acetylation of p65 and the expression of NF-kappaB target genes in response to diverse stimuli. Finally, EGCG reduced the binding of p300 to the promoter region of interleukin-6 gene with an increased recruitment of HDAC3, which highlights the importance of the balance between HATs and histone deacetylases in the NF-kappaB-mediated inflammatory signaling pathway. Importantly, EGCG at 50 micromol/L dose completely blocks EBV infection-induced cytokine expression and subsequently the EBV-induced B lymphocyte transformation. These results show the crucial role of acetylation in the development of inflammatory-related diseases.

Stochasticity and the molecular mechanisms of induced pluripotency

The generation of induced pluripotent stem cells from adult somatic cells by ectopic expression of key transcription factors holds significant medical promise. However, current techniques for inducing pluripotency rely on viral infection and are therefore not, at present, viable within a clinical setting. Thus, there is now a need to better understand the molecular basis of stem cell pluripotency and lineage specification in order to investigate alternative methods to induce pluripotency for clinical application. However, the complexity of the underlying molecular circuitry makes this a conceptually difficult task. In order to address these issues, we considered a computational model of transcriptional control of cell fate specification. The model comprises two mutually interacting sub-circuits: a central pluripotency circuit consisting of interactions between stem-cell specific transcription factors OCT4, SOX2 and NANOG coupled to a differentiation circuit consisting of interactions between lineage-specifying master genes.The molecular switches which arise from feedback loops within these circuits give rise to a well-defined sequence of successive gene restrictions corresponding to a controlled differentiation cascade in response to environmental stimuli. Furthermore, we found that this differentiation cascade is strongly unidirectional: once silenced, core transcription factors cannot easily be reactivated. In the context of induced pluripotency, this indicates that differentiated cells are robustly resistant to reprogramming to a more primitive state. However, our model suggests that under certain circumstances, amplification of low-level fluctuations in transcriptional status (transcriptional "noise") may be sufficient to trigger reactivation of the core pluripotency switch and reprogramming to a pluripotent state. This interpretation offers an explanation of a number of experimental observations concerning the molecular mechanisms of cellular reprogramming by defined factors and suggests a role for stochasticity in reprogramming of somatic cells to pluripotency.

Kruppel-like factor 4 induces p27Kip1 expression in and suppresses the growth and metastasis of human pancreatic cancer cells

The zinc finger transcription factor Krüppel-like factor 4 (KLF4) has been implicated in both tumor suppression and progression. However, its function in pancreatic cancer has not been well characterized. Here, we show that pancreatic cancer cell lines expressed various levels of KLF4 RNA and protein. Ectopic expression of KLF4 by FG and BxPC-3 pancreatic cancer cells resulted in cell cycle arrest and marked inhibition of cell growth in vitro and attenuation of tumor growth and metastasis in an orthotopic mouse model. Overexpression of KLF4 also led to significant induction of p27(Kip1) expression, at both the RNA and protein levels, in a dose- and time-dependent manner, indicating that KLF4 transcriptionally regulates the expression of p27(Kip1). Chromatin immunoprecipitation assays consistently showed that KLF4 protein physically interacts with the p27(Kip1) promoter. Promoter deletion and point mutation analyses indicated that a region between nucleotides -435 and -60 of the p27(Kip1) promoter and intact of the three KLF4-binding sites within that region were required for the full induction of p27(Kip1) promoter activity by KLF4. Our findings suggest that KLF4 transactivates p27(Kip1) expression and inhibits the growth and metastasis of human pancreatic cancer.

Epigenetic regulation of stem cell fate

Stem cell-based regenerative medicine holds great promise for repair of diseased tissue. Modern directions in the field of epigenetic research aimed to decipher the epigenetic signals that give stem cells their unique ability to self-renew and differentiate into different cell types. However, this research is only the tip of the iceberg when it comes to writing an 'epigenetic instruction manual' for the ramification of molecular details of cell commitment and differentiation. In this review, we discuss the impact of the epigenetic research on our understanding of stem cell biology.

Roles of Krüpel-like factor 4 in normal homeostasis, cancer and stem cells

Krüpel-like factor 4 (KLF4) is a zinc finger-type transcription factor expressed in a variety of tissues, including the epithelium of the intestine and the skin, and it plays an important role in differentiation and cell cycle arrest. Depending on the gene targeted, KLF4 can both activate and repress transcription. Moreover, in certain cellular contexts, KLF4 can function as a tumor suppressor or an oncogene. Finally, KLF4 is important in reprogramming differentiated fibroblasts into inducible pluripotent stem cells, which highly resemble embryonic stem cells. This review summarizes what is known about the diverse functions of KLF4 as well as their molecular mechanisms.

Induction of pluripotent stem cells from adult human fibroblasts by defined factors

Successful reprogramming of differentiated human somatic cells into a pluripotent state would allow creation of patient- and disease-specific stem cells. We previously reported generation of induced pluripotent stem (iPS) cells, capable of germline transmission, from mouse somatic cells by transduction of four defined transcription factors. Here, we demonstrate the generation of iPS cells from adult human dermal fibroblasts with the same four factors: Oct3/4, Sox2, Klf4, and c-Myc. Human iPS cells were similar to human embryonic stem (ES) cells in morphology, proliferation, surface antigens, gene expression, epigenetic status of pluripotent cell-specific genes, and telomerase activity. Furthermore, these cells could differentiate into cell types of the three germ layers in vitro and in teratomas. These findings demonstrate that iPS cells can be generated from adult human fibroblasts.