The Krüppel-like factor (KLF) family of transcription factors regulates diverse biological processes that include proliferation, differentiation, growth, development, survival, and responses to external stress. Seventeen mammalian KLFs have been identified, and numerous studies have been published that describe their basic biology and contribution to human diseases. KLF proteins have received much attention because of their involvement in the development and homeostasis of numerous organ systems. KLFs are critical regulators of physiological systems that include the cardiovascular, digestive, respiratory, hematological, and immune systems and are involved in disorders such as obesity, cardiovascular disease, cancer, and inflammatory conditions. Furthermore, KLFs play an important role in reprogramming somatic cells into induced pluripotent stem (iPS) cells and maintaining the pluripotent state of embryonic stem cells. As research on KLF proteins progresses, additional KLF functions and associations with disease are likely to be discovered. Here, we review the current knowledge of KLF proteins and describe common attributes of their biochemical and physiological functions and their pathophysiological roles.

A cDNA clone, named gut-enriched Krü ppel-like factor (GKLF), was isolated from an NIH 3T3 library using a probe encoding the zinc finger region of the immediate-early transcription factor zif/268. The deduced GKLF amino acid sequence contains three tandem zinc fingers that are related to members of the Krü ppel family of transcription factors. By indirect immunofluorescence, GKLF is localized to the cell nucleus. In cultured fibroblasts, GKLF mRNA is found in high levels in growth-arrested cells and is nearly undetectable in cells that are in the exponential phase of proliferation. The growth-arresting nature of GKLF is demonstrated by an inhibition of DNA synthesis in cells transfected with a GKLF-expressing plasmid construct. In the mouse, GKLF mRNA is present in select tissues and is most abundant in the colon, followed by the testis, lung, and small intestine. In situ hybridization experiments indicate that GKLF mRNA is enriched in epithelial cells located in the middle to upper crypt region of the colonic mucosa. Taken together, these results suggest that GKLF is potentially a negative regulator of cell growth in tissues such as the gut mucosa, where cell proliferation is intimately coupled to growth arrest and differentiation.

Previous studies have shown that differentiation of 3T3-L1 preadipocytes leads to the transcriptional activation of a group of adipose-specific genes. As an approach to defining the mechanism responsible for activating the expression of these genes, we investigated the binding of nuclear factors to the promoters of two differentiation-induced genes, the 422(aP2) and stearoyl-CoA desaturase 1 (SCDl) genes. DNase I footprinting and gel retardation analysis identified two binding regions within the promoters of each gene that interact with nuclear factors present in differentiated 3T3-L1 adipocytes. One differentiation-induced nuclear factor interacts specifically with a single binding site in the promoter of each gene. Competition experiments showed that the interaction of this nuclear factor with the SCDl promoter was prevented specifically by a synthetic oligonucleotide corresponding to the site footprinted in the 422(aP2) promoter. Several lines of evidence indicate that the differentiation-induced nuclear factor is CCAAT/enhancer binding protein (C/EBP), a DNA-binding protein first isolated from rat liver. Bacterially expressed recombinant C/EBP binds to the same site at which the differentiation-specific nuclear factor interacts within the promoter of each gene. Northern analysis with RNA from 3T3-L1 cells shows that C/EBP mRNA abundance increases markedly during differentiation. Transient cotransfection studies using a C/EBP expression vector demonstrate that C/EBP can function as a trans-activator of both the 422(aP2) and SCDl gene promoters.

Background & Aims-Liver transplantation is the primary treatment for various end-stage hepatic diseases but is hindered by the lack of donor organs and by complications associated with rejection and immunosuppression. There is increasing evidence to suggest the bone marrow is a transplantable source of hepatic progenitors. We previously reported that multipotent bone marrow-derived mesenchymal stem cells differentiate into functional hepatocyte-like cells with almost 100% induction frequency under defined conditions, suggesting the potential for clinical applications. The aim of this study was to critically analyze the various parameters governing the success of bone marrow-derived mesenchymal stem cell-based therapy for treatment of liver diseases.

An important mechanism by which the tumor suppressor p53 maintains genomic stability is to induce cell cycle arrest through activation of the cyclin-dependent kinase inhibitor p21 WAF1/Cip1 gene. We show that the gene encoding the gut-enriched Krü ppel-like factor (GKLF, KLF4) is concurrently induced with p21 WAF1/Cip1during serum deprivation and DNA damage elicited by methyl methanesulfonate. The increases in expression of both Gklf and p21 WAF1/Cip1 due to DNA damage are dependent on p53. Moreover, during the first 30 min of methyl methanesulfonate treatment, the rise in Gklf mRNA level precedes that in p21, suggesting that GKLF may be involved in the induction of p21 Indeed, GKLF activates p21WAF1/Cip1 through a specific Sp1-like cis-element in the p21 WAF1/Cip1 proximal promoter. The same element is also required by p53 to activate the p21 WAF1/Cip1 promoter, although p53 does not bind to it. Potential mechanisms by which p53 activates the p21 WAF1/Cip1 promoter include a physical interaction between p53 and GKLF and the transcriptional induction of Gklf by p53. Consequently, the two transactivators cause a synergistic induction of the p21 WAF1/Cip1 promoter activity. The physiological relevance of GKLF in mediating p53-dependent induction of p21 WAF1/Cip1 is demonstrated by the ability of antisense Gklf oligonucleotides to block the production of p21 WAF1/Cip1 in response to p53 activation. These findings suggest that GKLF is an essential mediator of p53 in the transcriptional induction of p21 WAF1/Cip1 and may be part of a novel pathway by which cellular responses to stress are modulated.

Recent advances in molecular cloning have led to the identification of a large number of mammalian zinc finger-containing transcription factors that exhibit homology to the Drosophila melanogaster protein, Krüppel. Although the amino acid sequences in the zinc finger domains of these Krüppel-like factors (KLFs) are closely related to one another, the regions outside the zinc fingers of the proteins are usually unique. KLFs display seemingly different and broad biological properties with each functioning as an activator of transcription, a repressor or both. This review article provides a current phylogenetic classification of the identified KLFs to date. More importantly, the currently known biological activities of the KLFs in regulating transcription, cell proliferation, differentiation and development are summarized and compared. Further characterization of this interesting protein family should provide additional insights into the their respective regulatory role in various important biological processes.

Krüppel-like factor 4 (KLF4) is an evolutionarily conserved zinc finger-containing transcription factor that regulates diverse cellular processes such as cell growth, proliferation, and differentiation. Since its discovery in 1996, KLF4 has been gaining a lot of attention, after it was shown in 2006 as one of four factors required for the induction of pluripotent stem cells (iPSCs). Here we review the current knowledge on the different functions and roles of KLF4 in various tissue and organ systems.

Purpose: Near-IR fluorescence imaging has great potential for noninvasive in vivo imaging of tumors. In this study, we show the preferential uptake and retention of two hepatamethine cyanine dyes, IR-783 and MHI-148, in tumor cells and tissues.Experimental Design: IR-783 and MHI-148 were investigated for their ability to accumulate in human cancer cells, tumor xenografts, and spontaneous mouse tumors in transgenic animals. Time-and concentration-dependent dye uptake and retention in normal and cancer cells and tissues were compared, and subcellular localization of the dyes and mechanisms of the dye uptake and retention in tumor cells were evaluated using organelle-specific tracking dyes and bromosulfophthalein, a competitive inhibitor of organic anion transporting peptides. These dyes were used to detect human cancer metastases in a mouse model and differentiate cancer cells from normal cells in blood.Results: These near-IR hepatamethine cyanine dyes were retained in cancer cells but not normal cells, in tumor xenografts, and in spontaneous tumors in transgenic mice. They can be used to detect cancer metastasis and cancer cells in blood with a high degree of sensitivity. The dyes were found to concentrate in the mitochondria and lysosomes of cancer cells, probably through organic anion transporting peptides, because the dye uptake and retention in cancer cells can be blocked completely by bromosulfophthalein. These dyes, when injected to mice, did not cause systemic toxicity.Conclusions: These two heptamethine cyanine dyes are promising imaging agents for human cancers and can be further exploited to improve cancer detection, prognosis, and treatment.