KLF6 degradation after apoptotic DNA damage

The kruppel-like factor (KLF) family, diseases, and physiological events

The Kruppel-Like Factor family of regulatory proteins, which has 18 members, is transcription factors. This family contains zinc finger proteins, regulates the activation and suppression of transcription, and binds to DNA, RNA, and proteins. Klfs related to the immune system are Klf1, Klf2, Klf3, Klf4, Klf6, and Klf14. Klfs related to adipose tissue development and/or glucose metabolism are Klf3, Klf7, Klf9, Klf10, Klf11, Klf14, Klf15, and Klf16. Klfs related to cancer are Klf3, Klf4, Klf5, Klf6, Klf7, Klf8, Klf9, Klf10, Klf11, Klf12, Klf13, Klf14, Klf16, and Klf17. Klfs related to the cardiovascular system are Klf4, Klf5, Klf10, Klf13, Klf14, and Klf15. Klfs related to the nervous system are Klf4, Klf7, Klf8, and Klf9. Klfs are associated with diseases such as carcinogenesis, oxidative stress, diabetes, liver fibrosis, thalassemia, and the metabolic syndrome. The aim of this review is to provide information about the relationship of Klfs with some diseases and physiological events and to guide future studies.

Krüppel-like factor 6-mediated loss of BCAA catabolism contributes to kidney injury in mice and humans

Altered cellular metabolism in kidney proximal tubule (PT) cells plays a critical role in acute kidney injury (AKI). The transcription factor Krüppel-like factor 6 (KLF6) is rapidly and robustly induced early in the PT after AKI. We found that PT-specific Klf6 knockdown (Klf6 ^PTKD) is protective against AKI and kidney fibrosis in mice. Combined RNA and chromatin immunoprecipitation sequencing analysis demonstrated that expression of genes encoding branched-chain amino acid (BCAA) catabolic enzymes was preserved in Klf6 ^PTKD mice, with KLF6 occupying the promoter region of these genes. Conversely, inducible KLF6 overexpression suppressed expression of BCAA genes and exacerbated kidney injury and fibrosis in mice. In vitro, injured cells overexpressing KLF6 had similar decreases in BCAA catabolic gene expression and were less able to utilize BCAA. Furthermore, knockdown of BCKDHB, which encodes one subunit of the rate-limiting enzyme in BCAA catabolism, resulted in reduced ATP production, while treatment with BCAA catabolism enhancer BT2 increased metabolism. Analysis of kidney function, KLF6, and BCAA gene expression in human chronic kidney disease patients showed significant inverse correlations between KLF6 and both kidney function and BCAA expression. Thus, targeting KLF6-mediated suppression of BCAA catabolism may serve as a key therapeutic target in AKI and kidney fibrosis.

Two Sides of the Same Coin: The Roles of KLF6 in Physiology and Pathophysiology

The Krüppel-like factors (KLFs) family of proteins control several key biological processes that include proliferation, differentiation, metabolism, apoptosis and inflammation. Dysregulation of KLF functions have been shown to disrupt cellular homeostasis and contribute to disease development. KLF6 is a relevant example; a range of functional and expression assays suggested that the dysregulation of KLF6 contributes to the onset of cancer, inflammation-associated diseases as well as cardiovascular diseases. KLF6 expression is either suppressed or elevated depending on the disease, and this is largely due to alternative splicing events producing KLF6 isoforms with specialised functions. Hence, the aim of this review is to discuss the known aspects of KLF6 biology that covers the gene and protein architecture, gene regulation, post-translational modifications and functions of KLF6 in health and diseases. We put special emphasis on the equivocal roles of its full-length and spliced variants. We also deliberate on the therapeutic strategies of KLF6 and its associated signalling pathways. Finally, we provide compelling basic and clinical questions to enhance the knowledge and research on elucidating the roles of KLF6 in physiological and pathophysiological processes.

KLF6 Acetylation Promotes Sublytic C5b-9-Induced Production of MCP-1 and RANTES in Experimental Mesangial Proliferative Glomerulonephritis

Rat Thy-1 nephritis (Thy-1N) is an experimental mesangial proliferative glomerulonephritis (MsPGN) for studying human MsPGN. Although sublytic C5b-9 complex formation on glomerular mesangial cells (GMCs) and renal MCP-1 and RANTES production in rats with Thy-1N have been proved, the role and mechanism of MCP-1 or RANTES synthesis in GMCs induced by sublytic C5b-9 are poorly elucidated. In this study, we first found the expression of transcription factor (KLF6), co-activator (KAT7) and chemokines (MCP-1 and RANTES) was all up-regulated both in renal tissue of Thy-1N rats (in vivo) and in sublytic C5b-9-induced GMCs (in vitro). Further in vitro experiments revealed that KLF6 bound to MCP-1 promoter (-297 to -123 nt) and RANTES promoter (-343 to -191 nt), leading to MCP-1 and RANTES gene transcription. Meanwhile, KAT7 also bound to the same region of MCP-1 and RANTES promoter in a KLF6-dependent manner, and KLF6 was acetylated by KAT7 at lysine residue 100, which finally promoted MCP-1 and RANTES expression. Moreover, our in vivo experiments discovered that knockdown of renal KAT7 or KLF6 gene obviously reduced MCP-1 and RANTES production, GMCs proliferation, ECM accumulation, and proteinuria secretion in Thy-1N rats. Collectively, our study indicates that sublytic C5b-9-induced MCP-1 and RANTES synthesis is associated with KAT7-mediated KLF6 acetylation and elevated KLF6 transcriptional activity, which might provide a new insight into the pathogenesis of rat Thy-1N and human MsPGN.

KLF6 Induces Apoptosis in Human Lens Epithelial Cells Through the ATF4-ATF3-CHOP Axis

Background

Many studies have confirmed that high myopia is related to the high prevalence of cataracts, which results from apoptosis of lens epithelial cells (LECs) due to endoplasmic reticulum stress. Krüppel-like factor 6 (KLF6) is a tumor suppressor that is involved in the regulation of cell proliferation and apoptosis.

Purpose

In this study, our purpose was to find the relationship between KLF6-induced apoptosis in LECs and ATF4 (activating transcription factor 4)-ATF3 (activating transcription factor 3)-CHOP (C/EBP homologous protein) signaling pathway.

Methods

KLF6, ATF4, ATF3, and CHOP were ectopically expressed using cDNAs subcloned into the pCDNA3.1+ vector. ATF4, ATF3, and CHOP knockdown were performed by small interfering RNA (siRNA). Expression of relative gene was tested using QT-PCR and western-blot. Then, accompanied by UVB stimulation, cell viability was measured by CCK-8 assay; The cell damage was examined by live & dead staining; The apoptotic markers Bax and Bcl-2 were detected by immunoblotting; Quantitative apoptotic levels were measured with the Apoptosis Detection Kit; The expression level of reactive oxygen-free radical (ROS) was analyzed by DCFH-DA` probe.

Results

Ectopically expressed ATF4, ATF3, and CHOP-induced apoptosis in cells, whereas ATF4, ATF3, and CHOP knockdown by small interfering RNA (siRNA) blocked KLF6-induced apoptosis. In addition, we determined that ATF4 regulates ATF3 and CHOP expression and that ATF3 silencing reduces CHOP upregulation without changing ATF4 levels; however, ATF4 and ATF3 expression was unaffected by blockade of CHOP, suggesting that KLF6 triggers endoplasmic reticulum stress in LECs by mediating the ATF4-ATF3/CHOP axis. Besides, KLF6 overexpression significantly induced LEC apoptosis under UV radiation, as demonstrated by the elevated Bax/Bcl-2 ratio.

Conclusion

The ATF4-ATF3-CHOP pathway plays an important role in KLF6-induced apoptosis in HLECs. Our results increase our understanding of the mechanisms that regulate LEC apoptosis and contribute to the development of a new preventative strategy for cataract.

The Klf6-related super enhancer regulates Klf6-SV2 expression mediated proliferation in human hepatoma (HepG2) cells

Background

The Klf6 gene, which belongs to Krüppel-like family of C2H2 zinc finger transcription factors, is greatly related to tumorigenesis via a high rate of somatic mutation in the carcinomas of prostate, liver, colon, stomach, lung, neck, pituitary, and nervous system: Furthermore, the pathways regulating the expressions of Klf6 splice variants termed Klf6-SV1, -SV2, and -SV3 remain obscure although their functional outcomes have been clear. In this study, the functional roles of Klf6 variants in the inhibition of cell proliferation induced by the disruption of Klf6-related super enhancer in human hepatoma (HepG2) cells were evaluated.

Results

As a result, the disruption of Klf6-related super enhancer not only induced the upregulation of Klf6-SV2 but also led to a significant reduction of proliferation in HepG2 cells. In addition, the disruption of Klf6-related super enhancer led to the induction of p21 and Bax genes mediated by the upregulation of Klf6-SV2.

Conclusion

In conclusion, it was demonstrated that Klf6-related super enhancer modulates cell proliferation via the regulation of Klf6-SV2 expression in human hepatoma (HepG2) cells. The results provide the functional significance of Klf6-related super enhancer in understanding the transcriptional regulation mechanism of Klf6.

Krüppel-Like Factor 6 Is Required for Oxidative and Oncogene-Induced Cellular Senescence

Krüppel-like factor 6 (KLF6) is a transcription factor involved in the regulation of several cellular processes. Regarding its role in tumorigenesis, KLF6 is considered a tumor suppressor. Numerous reports demonstrate its frequent genomic loss or down-regulation, implying a functional inactivation in a broad range of human cancers. Previous work from our laboratory showed that the down-regulation of KLF6 expression in normal fibroblasts leads to cellular transformation, while its ectopic expression interferes with the oncogenic transformation triggered by activated Ras through a cell cycle arrest. We hypothesize that the growth suppressor activity of KLF6 may involve the induction of cellular senescence thereby helping to prevent the proliferation of cells at risk of neoplastic transformation. Here, we explored the association of KLF6 up-regulation in two different cellular senescence scenarios. We found that KLF6 silencing bypasses both oxidative and oncogene-induced senescence. In this context, KLF6 expression per se was capable to trigger cellular senescence in both normal and tumoral contexts. As such, the findings presented in this report provide insights into a potential mechanism by which KLF6 may play a suppressing role of uncontrolled or damaged cell proliferation.

Effects of Ferroportin-Mediated Iron Depletion in Cells Representative of Different Histological Subtypes of Prostate Cancer

AIMS

Ferroportin (FPN) is an iron exporter that plays an important role in cellular and systemic iron metabolism. Our previous work has demonstrated that FPN is decreased in prostate tumors. We sought to identify the molecular pathways regulated by FPN in prostate cancer cells.

RESULTS

We show that overexpression of FPN induces profound effects in cells representative of multiple histological subtypes of prostate cancer by activating different but converging pathways. Induction of FPN induces autophagy and activates the transcription factors tumor protein 53 (p53) and Kruppel-like factor 6 (KLF6) and their common downstream target, cyclin-dependent kinase inhibitor 1A (p21). FPN also induces cell cycle arrest and stress-induced DNA-damage genes. Effects of FPN are attributable to its effects on intracellular iron and can be reproduced with iron chelators. Importantly, expression of FPN not only inhibits proliferation of all prostate cancer cells studied but also reduces growth of tumors derived from castrate-resistant adenocarcinoma C4-2 cells in vivo.

INNOVATION

We use a novel model of FPN expression to interrogate molecular pathways triggered by iron depletion in prostate cancer cells. Since prostate cancer encompasses different subtypes with a highly variable clinical course, we further explore how histopathological subtype influences the response to iron depletion. We demonstrate that prostate cancer cells that derive from different histopathological subtypes activate converging pathways in response to FPN-mediated iron depletion. Activation of these pathways is sufficient to significantly reduce the growth of treatment-refractory C4-2 prostate tumors in vivo.

CONCLUSIONS

Our results may explain why FPN is dramatically suppressed in cancer cells, and they suggest that FPN agonists may be beneficial in the treatment of prostate cancer.

Podocyte-Specific Loss of Krüppel-Like Factor 6 Increases Mitochondrial Injury in Diabetic Kidney Disease

Mitochondrial injury is uniformly observed in several murine models as well as in individuals with diabetic kidney disease (DKD). Although emerging evidence has highlighted the role of key transcriptional regulators in mitochondrial biogenesis, little is known about the regulation of mitochondrial cytochrome c oxidase assembly in the podocyte under diabetic conditions. We recently reported a critical role of the zinc finger Krüppel-like factor 6 (KLF6) in maintaining mitochondrial function and preventing apoptosis in a proteinuric murine model. In this study, we report that podocyte-specific knockdown of Klf6 increased the susceptibility to streptozotocin-induced DKD in the resistant C57BL/6 mouse strain. We observed that the loss of KLF6 in podocytes reduced the expression of synthesis of cytochrome c oxidase 2 with resultant increased mitochondrial injury, leading to activation of the intrinsic apoptotic pathway under diabetic conditions. Conversely, mitochondrial injury and apoptosis were significantly attenuated with overexpression of KLF6 in cultured human podocytes under hyperglycemic conditions. Finally, we observed a significant reduction in glomerular and podocyte-specific expression of KLF6 in human kidney biopsies with progression of DKD. Collectively, these data suggest that podocyte-specific KLF6 is critical to preventing mitochondrial injury and apoptosis under diabetic conditions.

Krüppel-like factors: Crippling and un-crippling metabolic pathways

Krüppel-like factors (KLFs) are DNA-binding transcriptional factors that regulate various pathways that control metabolism and other cellular mechanisms. Various KLF isoforms have been associated with cellular, organ or systemic metabolism. Altered expression or activation of KLFs has been linked to metabolic abnormalities, such as obesity and diabetes, as well as with heart failure. In this review article we summarize the metabolic functions of KLFs, as well as the networks of different KLF isoforms that jointly regulate metabolism in health and disease.

Krüppel-like factors in mammalian stem cells and development

Krüppel-like factors (KLFs) are a family of zinc-finger transcription factors that are found in many species. Recent studies have shown that KLFs play a fundamental role in regulating diverse biological processes such as cell proliferation, differentiation, development and regeneration. Of note, several KLFs are also crucial for maintaining pluripotency and, hence, have been linked to reprogramming and regenerative medicine approaches. Here, we review the crucial functions of KLFs in mammalian embryogenesis, stem cell biology and regeneration, as revealed by studies of animal models. We also highlight how KLFs have been implicated in human diseases and outline potential avenues for future research.

The activation of peroxisome proliferator-activated receptor γ is regulated by Krüppel-like transcription factors 6 & 9 under steatotic conditions

Liver steatosis is characterised by lipid droplet deposition in hepatocytes that can leads to an inflammatory and fibrotic phenotype. Peroxisome proliferator-activated receptors (PPARs) play key roles in energetic homeostasis by regulating lipid metabolism in hepatic tissue. In adipose tissue PPARγ regulates the adipocyte differentiation by promoting the expression of lipid-associated genes. Within the liver PPARγ is up-regulated under steatotic conditions; however, which transcription factors participate in its expression is not completely understood. Krüppel-like transcription factors (KLFs) regulate various cellular mechanisms, such as cell proliferation and differentiation. KLFs are key components of adipogenesis by regulating the expression of PPARγ and other proteins such as the C-terminal enhancer binding protein (C/EBP). Here, we demonstrate that the transcript levels of Klf6, Klf9 and Pparγ are increased in response to a steatotic insult in vitro. Chromatin immunoprecipitation (ChIp) experiments showed that klf6 and klf9 are actively recruited to the Pparγ promoter region under these conditions. Accordingly, the loss-of-function experiments reduced cytoplasmic triglyceride accumulation. Here, we demonstrated that KLF6 and KLF9 proteins directly regulate PPARγ expression under steatotic conditions.

Krüppel-like factor 6 rendered rat Schwann cell more sensitive to apoptosis via upregulating FAS expression

Krüppel-like factor 6 (KLF6) is a tumor suppressor gene and play a role in the regulation of cell proliferation and apoptosis. After the peripheral nerve injury (PNI), the microenvironment created by surrounding Schwann cells (SCs) is a critical determinant of its regenerative potential. In this study, we examined the effects of KLF6 on SCs responses during PNI. Both KLF6 mRNA and protein expression levels were upregulated in the injured sciatic nerve, and immunofluorescence results showed that many KLF6-positive cells simultaneously expressed the SC markers S-100 and p75NTR. The apoptosis inducers TNFα and cisplatin upregulated KLF6 expression in primary cultured SCs and the SC line RSC96. Although KLF6 overexpression exacerbated cisplatin- and TNFα-induced apoptosis, expression levels of the apoptosis regulators Bcl2 and Bax were not significantly affected in either KLF6-overexpressing or KLF6-depleted RSC96 cells. Realtime PCR arrays and qRT-PCR demonstrated that KLF6 overexpression upregulated four pro-apoptotic genes, FAS, TNF, TNFSF12, and PYCARD, and inhibited expression of the anti-apoptotic IL10 gene expression. Further analysis revealed that FAS protein expression was positively correlated with KLF6 expression in SCs. These data suggest that KLF6 upregulation may render SCs more vulnerable to apoptosis after injury via upregulating FAS expression.

GSK3β phosphorylation of the KLF6 tumor suppressor promotes its transactivation of p21

KLF6, a ubiquitously expressed Krüppel-like transcription factor, is frequently inactivated in human cancer and has significant roles in cellular proliferation, apoptosis, differentiation and development. A key mechanism of KLF6-mediated growth suppression is through p53-independent transactivation of p21. Several cancer-derived KLF6 mutants lead to the loss of p21-mediated growth suppression through an unknown mechanism. Because several colorectal cancer and hepatocellular carcinoma-derived KLF6 mutations affect a glycogen synthase kinase 3β (GSK3β) phosphorylation consensus site, we investigated the role of GSK3β in the regulation of KLF6 function. Based on transient transfection, GSK3β augments the transactivation of a p21 promoter luciferase by KLF6. Reciprocal co-immunoprecipitation of hemagglutinin (HA)-GSK3β and Flag-KLF6 validated the interaction between these two proteins. KLF6 phosphorylation is augmented in the presence of GSK3β based on in vitro and in vivo (32)P incorporation assays. Site-directed mutagenesis of the candidate phosphorylation sites to alanines ('KLF6-4A' phosphomutant) eliminated a higher molecular weight phosphorylated isoform of KLF6 based on western blot. GSK3β augmented the transactivation by wild-type KLF6, but not KLF6-4A, towards the p21 promoter, and increased p21 protein. Functionally, GSK3β enhanced KLF6-mediated growth suppression, which was abrogated by the KLF6-4A phosphomutant. These data establish that GSK3β directly phosphorylates KLF6, which augments its induction of p21 and resultant growth suppression. This interaction may account for the growth-promoting effects of cancer-derived KLF6 mutants that lack tumor suppressor activity.

Reduced hepatic stellate cell expression of Kruppel-like factor 6 tumor suppressor isoforms amplifies fibrosis during acute and chronic rodent liver injury

Kruppel-like factor 6 (KLF6), a zinc finger transcription factor and tumor suppressor, is induced as an immediate-early gene during hepatic stellate cell (HSC) activation. The paradoxical induction of a tumor suppressor in HSCs during proliferation led us to explore the biology of wildtype KLF6 (KLF6(WT) ) and its antagonistic, alternatively spliced isoform KLF6(SV1) in cultured HSCs and animal models. The animal models generated include a global heterozygous KLF6 mouse (Klf6+/-), and transgenic mice expressing either hKLF6(WT) or hKLF6(SV1) under the control of the Collagen α2 (I) promoter to drive HSC-specific gene expression following injury. The rat Klf6 transcript has multiple splice forms that are homologous to those of the human KLF6 gene. Following a transient increase, all rat Klf6 isoforms decreased in response to acute carbon tetrachloride (CCl(4)) liver injury and culture-induced activation. After acute CCl(4), Klf6+/- mice developed significantly increased fibrosis and enhanced fibrogenic messenger RNA (mRNA) and protein expression. In contrast, HSC-specific transgenic mice overexpressing KLF6(WT) or KLF6(SV1) developed significantly diminished fibrosis with reduced expression of fibrogenic genes. Chromatin IP and quantitative reverse-transcription polymerase chain reaction in mouse HSCs overexpressing KLF6(WT) demonstrated KLF6(WT) binding to GC boxes in promoters of Colα1 (I), Colα2 (I), and beta-platelet-derived growth factor receptor (β-Pdgfr) with reduced gene expression, consistent with transcriptional repression by KLF6. Stellate cells overexpressing either KLF6(WT) or KLF6(SV1) were more susceptible to apoptotic stress based on poly (ADP-ribose) polymerase (PARP) cleavage.

CONCLUSION

KLF6 reduces fibrogenic activity of HSCs by way of two distinct mechanisms, direct transcriptional repression of target fibrogenic genes and increased apoptosis of activated HSCs. These results suggest that following its initial induction, sustained down-regulation of KLF6 in liver injury may allow de-repression of fibrogenic genes and decreased stellate cell clearance by inhibiting apoptosis.

GSK3β phosphorylation of the KLF6 tumor suppressor promotes its transactivation of p21

KLF6, a ubiquitously expressed Krüppel-like transcription factor, is frequently inactivated in human cancer and has significant roles in cellular proliferation, apoptosis, differentiation and development. A key mechanism of KLF6-mediated growth suppression is through p53-independent transactivation of p21. Several cancer-derived KLF6 mutants lead to the loss of p21-mediated growth suppression through an unknown mechanism. Because several colorectal cancer and hepatocellular carcinoma-derived KLF6 mutations affect a glycogen synthase kinase 3β (GSK3β) phosphorylation consensus site, we investigated the role of GSK3β in the regulation of KLF6 function. Based on transient transfection, GSK3β augments the transactivation of a p21 promoter luciferase by KLF6. Reciprocal co-immunoprecipitation of hemagglutinin (HA)-GSK3β and Flag-KLF6 validated the interaction between these two proteins. KLF6 phosphorylation is augmented in the presence of GSK3β based on in vitro and in vivo (32)P incorporation assays. Site-directed mutagenesis of the candidate phosphorylation sites to alanines ('KLF6-4A' phosphomutant) eliminated a higher molecular weight phosphorylated isoform of KLF6 based on western blot. GSK3β augmented the transactivation by wild-type KLF6, but not KLF6-4A, towards the p21 promoter, and increased p21 protein. Functionally, GSK3β enhanced KLF6-mediated growth suppression, which was abrogated by the KLF6-4A phosphomutant. These data establish that GSK3β directly phosphorylates KLF6, which augments its induction of p21 and resultant growth suppression. This interaction may account for the growth-promoting effects of cancer-derived KLF6 mutants that lack tumor suppressor activity.

Fbw7γ-mediated degradation of KLF13 prevents RANTES expression in resting human but not murine T lymphocytes

RANTES (CCL5) is a chemokine implicated in many human diseases. We previously showed that the transcription factor Kruppel-like factor 13 (KLF13) controls the late (3-5 days after activation) expression of RANTES in T lymphocytes and that KLF13 itself is translationally regulated through the 5'-untranslated region of its mRNA. Here, we show that KLF13 levels are further regulated by ubiquitination and degradation. KLF13 protein is undetectable in resting human T lymphocytes, but treatment with either proteosomal or lysosomal inhibitors increases KLF13 protein levels. Glycogen synthase kinase 3β (GSK3β)-mediated phosphorylation of KLF13 triggers the ubiquitination of KLF13 by the E3 ligase Fbw7γ, resulting in KLF13 protein degradation. Knockdown of either Fbw7γ or GSK3β by small interfering RNA increases KLF13 expression in resting human T lymphocytes. In contrast, in murine T lymphocytes, KLF13 protein is abundant because of the absence of Fbw7γ. Treatment of unactivated human lymphocytes with lysosomal inhibitors stabilizes KLF13 protein, resulting in an increase of RANTES mRNA and protein. Taken together, these studies found that tightly regulated control of both synthesis and degradation allows rapid changes in the level of KLF13 in human T lymphocytes.

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.

Biology of Krüppel-like factor 6 transcriptional regulator in cell life and death

An essential role for the Krüppel-like transcription factor family has been determined in the regulation of remarkable processes including cell proliferation, differentiation, signal transduction, oncogenesis, and cell death. A member of this group, Krüppel-like factor 6 (KLF6), identified on the basis of its ability to regulate a group of genes belonging to the carcinoembryonic antigen gene family, has been involved in human carcinogenesis. Early studies proposed a tumor suppressor function for KLF6 because of its ability to reduce cell proliferation through several biochemical mechanisms including regulation of cell cycle components, oncogene products, and apoptosis. Mutations within the klf6 gene, decreased expression and/or loss-of-heterozygosity were associated with the development of different human malignancies, and, hence, further supporting the tumor suppressor function of KLF6. This view has been challenged by other studies in distinct types of human cancers describing infrequent genetic alterations of klf6 gene or even enhanced expression in some tumors. The scenario about KLF6 function became still more complex as the description of oncogenic KLF6 splice variant 1 (SV1) with dominant negative activity against the wild type KLF6 (wtKLF6) protein. Additionally, increased evidence is suggesting that KLF6 is a bonafide target of several signaling cascades, which ultimate regulatory effect on this protein could drive decisions of cell life and death, facing the dilemma about how wtKLF6 could be involved in both processes. These apparently conflicting situations, emerged by apparently opposite effects mediated by wtKLF6, may be related, at least in part, to the biological cross-talk with the c-Jun oncoprotein. Depending on the stimulus received by the cell, wtKLF6 interaction with c-Jun determines different cell outcomes such as proliferation control or apoptosis. Thus, KLF6 responsiveness represents a kind of cell warning signal on receiving different stimuli, including oncogenic activation and microbial infections, orchestrating the implementation of proliferation and apoptotic programs.

The tumor suppressor protein menin inhibits AKT activation by regulating its cellular localization

Multiple endocrine neoplasia type 1 (MEN1) is an autosomal dominant disorder associated mainly with tumors of multiple endocrine organs. Mutations in the MEN1 gene that encodes for the menin protein are the predominant cause for hereditary MEN1 syndrome. Though menin is a tumor suppressor, its molecular mechanism of action has not been defined. Here, we report that menin interacts with AKT1 in vitro and in vivo. Menin downregulates the level of active AKT and its kinase activity. Through interaction with AKT1, menin suppresses both AKT1-induced proliferation and antiapoptosis in nonendocrine and endocrine cells. Confocal microscopy analysis revealed that menin regulates AKT1 in part by reducing the translocation of AKT1 from the cytoplasm to the plasma membrane during growth factor stimulation. Our findings may be generalizable to other cancers, insofar as we found that loss of menin expression was also associated with AKT activation in a mouse model of pancreatic islet adenoma. Together, our results suggest menin as an important novel negative regulator of AKT kinase activity.

Mammalian Krüppel-like factors in health and diseases

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.

Nucleo-cytoplasmic localization domains regulate Krüppel-like factor 6 (KLF6) protein stability and tumor suppressor function

Background

The tumor suppressor KLF6 and its oncogenic cytoplasmic splice variant KLF6-SV1 represent a paradigm in cancer biology in that their antagonistic cancer functions are encoded within the same gene. As a consequence of splicing, KLF6-SV1 loses both the C-terminus C₂H₂ three zinc finger (ZF) domain, which characterizes all KLF proteins, as well as the adjacent 5′ basic region (5BR), a putative nuclear localization signal (NLS). It has been hypothesized that this NLS is a functional domain critical to direct the distinct subcellular localization of the tumor suppressor and its splice variant.

Methodology/Principal Findings

In this study, we demonstrate using EGFP fusion constructs that KLF6/KLF6-SV1 nucleo-cytoplasmic transport is not regulated by the 5′ basic region but activated by a novel NLS encoded within the ZF domain, and a nuclear export signal (NES) located in the first 16 amino acids of the shared N-terminus sequence. We demonstrate KLF6 nuclear export to be Crm1-dependent. The dysregulation of nucleo-cytoplasmic transport when disrupting the KLF6 NLS using site-directed mutagenesis showed that its integrity is necessary for appropriate protein stability. Moreover, these mutations impaired transcriptional induction of two KLF6 well-characterized target genes, E-cadherin and p21, as shown by RT-PCR and luciferase promoter assays. The addition of the ZF domain to KLF6-SV1 results in its nuclear localization and a markedly decreased half-life similar to wild type KLF6.

Conclusions/Significance

We describe the domains that control KLF6 nucleo-cytoplasmic shuttling and how these domains play a role in KLF6 protein half-life and tumor suppressor function. The results begin to mechanistically explain, at least in part, the opposing functions of KLF6 and KLF6-SV1 in cancer.

KLF6-SV1 is a novel antiapoptotic protein that targets the BH3-only protein NOXA for degradation and whose inhibition extends survival in an ovarian cancer model

Defects in apoptosis are not only a hallmark of cancer initiation and progression but can also underlie the development of chemoresistance. How the tightly regulated cascade of protein-protein interactions between members of three competing protein families regulating the apoptotic cascade is subverted in tumor cells is incompletely understood. Here, we show that KLF6-SV1, whose overexpression is associated with poor survival in several different cancers and is an alternatively spliced isoform of the Krüppel-like tumor suppressor KLF6, is a critical prosurvival/antiapoptotic protein. KLF6-SV1 binds the proapoptotic BH3-only protein NOXA, which results in their mutual HDM2-dependent degradation. In turn, this increases the intracellular concentration of the prosurvival binding partner of NOXA, Mcl-1, and effectively blocks apoptosis. In an ovarian cancer model, systemically delivered small interfering RNA against KLF6-SV1 induces spontaneous apoptosis of tumor cells, decreases tumor burden, and restores cisplatin sensitivity in vivo. Moreover, i.p. delivery of siKLF6-SV1 RNA halts ovarian tumor progression and improves median and overall survival (progression-free for >15 months; P < 0.0002) in mice in a dose-dependent manner. Thus, KLF6-SV1 represents a novel regulator of protein interactions in the apoptotic cascade and a therapeutically targetable control point.

Tumor suppressor and hepatocellular carcinoma

A few signaling pathways are driving the growth of hepatocellular carcinoma. Each of these pathways possesses negative regulators. These enzymes, which normally suppress unchecked cell proliferation, are circumvented in the oncogenic process, either the over-activity of oncogenes is sufficient to annihilate the activity of tumor suppressors or tumor suppressors have been rendered ineffective. The loss of several key tumor suppressors has been described in hepatocellular carcinoma. Here, we systematically review the evidence implicating tumor suppressors in the development of hepatocellular carcinoma.