1316

KLF6

BCD1|CBA1|COPEB|CPBP|GBF|PAC1|ST12|ZF9;Kruppel-like factor 6;KLF6;Kruppel-like factor 6

B-cell-derived protein 1|GC-rich binding factor|GC-rich sites-binding factor GBF|Krueppel-like factor 6|Kruppel-like zinc finger protein Zf9|core promoter element binding protein|core promoter element-binding protein|proto-oncogene BCD1|protooncogene B-ce

10p15

protein-coding

Kruppel-like factor 6 (KLF6) is a zinc finger transcription factor of unknown function. Here, we show that the KLF6 gene is mutated in a subset of human prostate cancer. Loss-of-heterozygosity analysis revealed that one KLF6 allele is deleted in 77% (17 of 22) of primary prostate tumors. Sequence analysis of the retained KLF6 allele revealed mutations in 71% of these tumors. Functional studies confirm that whereas wild-type KLF6 up-regulates p21 (WAF1/CIP1) in a p53-independent manner and significantly reduces cell proliferation, tumor-derived KLF6 mutants do not. Our data suggest that KLF6 is a tumor suppressor gene involved in human prostate cancer.

Gliomas are the most common tumors of the central nervous system and have a grave prognosis. Deletion of chromosome 10p15 is one of the most common chromosomal alterations in gliomas. Recently, a candidate tumor suppressor gene, KLF6, which is mapped to chromosome 10p, was found to be frequently mutated in prostate cancer. KLF6 is a zinc finger transcription factor and transactivates p21/WAF1/CIP expression. To elucidate the role of genetic alterations of KLF6 in gliomas, we analyzed the 4 exons of the gene by direct DNA sequencing in 155 gliomas. Of these, mutations of KLF6 were found in 9 of 76 (11.8%) glioblastomas multiforme, 2 of 28 (7.1%) anaplastic astrocytomas, 2 of 36 (5.5%) low-grade diffuse astrocytomas and in none of the 15 oligodendrogliomas. All 13 mutations were located in the transactivation domain and most of them affected either serine residues or codons next to serine residues. Of the 13 cases with KLF6 mutation, loss of heterozygosity (LOH) at the KLF6 locus was inferred from the LOH displayed by the flanking microsatellite markers in 11 cases. We conclude that mutations of the KLF6 gene play a role in the pathogenesis of astrocytic gliomas.

BACKGROUND & AIMS: Kruppel-like factor 6 (KLF6) is a ubiquitous zinc finger tumor suppressor that is often mutated in prostate cancer. Our aims were to establish the frequency of KLF6 inactivation in sporadic and inflammatory bowel disease (IBD)-associated colorectal cancers (CRC); to correlate these abnormalities with mutation and/or loss of TP53, APC, and K-RAS; and to characterize the behavior of mutant KLF6 in colon-derived cell lines. METHODS: We analyzed DNA isolated from 50 microdissected CRC cases, including 35 sporadic and 15 IBD-associated tumors. Microsatellite analysis and direct sequencing were used to establish the incidence of microsatellite instability, KLF6 and TP53 allelic imbalance, and KLF6, K-RAS, TP53, and APC mutation. Loss of growth suppressive function of the CRC-derived KLF6 mutants was characterized by in vitro thymidine incorporation assays and Western blotting. RESULTS: KLF6 was inactivated by loss and/or mutation in most sporadic and IBD-related CRCs. The KLF6 locus was deleted in at least 55% of tumors, and mutations were identified in 44%. Rates of KLF6 loss and mutation were similar to those of TP53 and K-RAS in the same samples. KLF6 mutations were present in tumors with either microsatellite or chromosomal instability and were more common, particularly in the IBD-related cancers, in the presence of wild-type APC. Unlike wild-type KLF6, cancer-derived KLF6 mutants neither suppressed growth nor induced p21 following transfection into cultured cells. CONCLUSIONS: Deregulation of KLF6 by a combination of allelic imbalance and mutation may play a role in the development of CRC.

The IGF system plays an important role in prostate cancer initiation and progression. Most of the biological actions of IGF-I and IGF-II are mediated by activation of the IGF-I receptor (IGF-IR). Evidence accumulated in recent years indicates that acquisition of the malignant phenotype is initially IGF-IR dependent, but progression toward metastatic stages is usually associated with a decrease in IGF-IR levels. The Kruppel-like factor 6 (KLF6) is a zinc finger-containing transcription factor that was shown to be mutated in a significant portion of prostate and other types of cancer. To examine the potential regulation of IGF-IR gene expression by KLF6, we measured KLF6 levels in prostate-derived cell lines displaying different levels of IGF-IR. The results of Western analysis showed that KLF6 levels were higher in nontumorigenic P69 cells expressing high IGF-IR levels than in metastatic M12 cells containing reduced IGF-IR levels. Transient coexpression of wild-type, but not mutated, KLF6 together with an IGF-IR promoter-luciferase reporter plasmid resulted in an approximately 3.4-fold stimulation of IGF-IR promoter activity. Furthermore, KLF6 expression induced a significant increment in endogenous IGF-IR levels. Deletion analysis of the IGF-IR promoter revealed that a cluster of four GC boxes located between nucleotides -399 and -331 mediates a significant portion of the transactivating effect of KLF6. KLF6, although unable to stimulate IGF-IR promoter activity in Sp1-null Drosophila-derived Schneider cells, significantly enhanced the effect of Sp1. To assess the potential interactions between KLF6 and p53 in the regulation of IGF-IR gene expression, transfections were performed in the colorectal cancer cell line HCT116(+/+), which expresses p53, and its HCT116(-/-) derivative, which lacks p53. KLF6 exhibited an enhanced activity in p53-containing, compared with p53-null, cells. In addition, we were able to detect a physical interaction between KLF6 and p53. In summary, we have identified the IGF-IR gene as a novel downstream target for transcription factor KLF6. The regulation of IGF-IR gene expression by KLF6 may have significant implications in terms of cancer initiation and/or progression.

Kruppel-like factor 6 (KLF6) is a tumor suppressor gene inactivated in prostate and colon cancers, as well as in astrocytic gliomas. Here, we establish that KLF6 mediates growth inhibition through an interaction with cyclin D1, leading to reduced phosphorylation of the retinoblastoma protein (Rb) at Ser(795). Furthermore, introduction of KLF6 disrupts cyclin D1-cyclin-dependent kinase (cdk) 4 complexes and forces the redistribution of p21(Cip/Kip) onto cdk2, which promotes G(1) cell cycle arrest. Our data suggest that KLF6 converges with the Rb pathway to inhibit cyclin D1/cdk4 activity, resulting in growth suppression.

Kruppel-like transcription factors (KLFs) represent one of the most diverse set of regulators in vertebrate organisms. KLF family members are involved in cell proliferation and differentiation control in normal as well as in pathological situations. Here, we demonstrate that KLF6 behaves as a functional antagonist of the c-Jun proto-oncoprotein. Thus, KLF6 overexpression downregulated c-Jun-dependent transcription and a physical interaction between c-Jun and KLF6 was detected. Moreover, cell proliferation induced by c-Jun was significantly decreased by KLF6. The inhibition of c-Jun functions correlates directly with c-Jun protein degradation induced by KLF6. We also show that all KLF6 effects on c-Jun were largely dependent on phorbol ester (TPA/ionomycin) extracellular stimulation, which enhanced KLF6 nuclear translocation and transcriptional activity and modified its phosphorylation status. Our data are consistent with a novel mechanism of KLF6s role as an inhibitor of cell proliferation by counteracting the function of the c-Jun proto-oncoprotein involving enhanced c-Jun degradation by the proteasome-dependent pathway, and further reinforces KLF6 as a potential tumor suppressor gene product.

Hepatocellular carcinoma (HCC) is a leading cause of cancer death worldwide, reflecting incomplete characterization of underlying mechanisms and lack of early detection. Kruppel-like factor 6 (KLF6) is a ubiquitously expressed zinc finger transcription factor that is deregulated in multiple cancers through loss of heterozygosity (LOH) and/or inactivating somatic mutation. We analyzed the potential role of the KLF6 tumor suppressor gene in 41 patients who had HCC associated with hepatitis C virus (16 patients), hepatitis B virus (12 patients, one of whom was coinfected with hepatitis C virus), and other etiologies (14 patients) by determining the presence of LOH and mutations. Overall, LOH and/or mutations were present in 20 (49%) of 41 tumors. LOH of the KLF6 gene locus was present in 39% of primary HCCs, and the mutational frequency was 15%. LOH and/or mutations were distributed across all etiologies of HCC evaluated, including patients who did not have cirrhosis. Functionally, wild-type KLF6 decreased cellular proliferation of HepG2 cells, while patient-derived mutants did not. In conclusion, we propose that KLF6 is deregulated by loss and/or mutation in HCC, and its inactivation may contribute to pathogenesis in a significant number of these tumors.

Prostate cancer is a leading and increasingly prevalent cause of cancer death in men. Whereas family history of disease is one of the strongest prostate cancer risk factors and suggests a hereditary component, the predisposing genetic factors remain unknown. We first showed that KLF6 is a tumor suppressor somatically inactivated in prostate cancer and since then, its functional loss has been further established in prostate cancer cell lines and other human cancers. Wild-type KLF6, but not patient-derived mutants, suppresses cell growth through p53-independent transactivation of p21. Here we show that a germline KLF6 single nucleotide polymorphism, confirmed in a tri-institutional study of 3,411 men, is significantly associated with an increased relative risk of prostate cancer in men, regardless of family history of disease. This prostate cancer-associated allele generates a novel functional SRp40 DNA binding site and increases transcription of three alternatively spliced KLF6 isoforms. The KLF6 variant proteins KLF6-SV1 and KLF6-SV2 are mislocalized to the cytoplasm, antagonize wtKLF6 function, leading to decreased p21 expression and increased cell growth, and are up-regulated in tumor versus normal prostatic tissue. Thus, these results are the first to identify a novel mechanism of self-encoded tumor suppressor gene inactivation and link a relatively common single nucleotide polymorphism to both regulation of alternative splicing and an increased risk in a major human cancer.

The KLF6 gene encodes the Kruppel-like factor 6, a transcription factor that has been individualized as a tumor-suppressor gene involved in the regulation of cell proliferation and differentiation. Recently, high frequency (42%) of KLF6 mutations have been reported in colorectal cancers (CRC) as in prostate cancers, astrocytic gliomas and hepatocellular carcinomas. The aims of the study was to confirm the frequency of KLF6 mutations in a larger series of CRC than that previously published by using DNA extracted from frozen tissue samples, which have been proved to generate less mutational artefact than that extracted from formalin-fixed paraffin-embedded tissue samples, in order to compare KLF6 mutation frequency with that of other common genetic alterations and to determine genotype-phenotype correlations. Amplification and direct sequencing of KLF6 exon 2 of 76 CRC and matched normal frozen tissues was performed. Polymorphisms were observed in 14 cases, among which two (T35T and S116S) had not already been reported. No KLF6 somatic mutation was observed. Our data suggest a minor role of KLF6 mutation in colorectal carcinogenesis and underline the fact that the validity of sequence informations obtained from DNA extracted from formalin-fixed tissues may be limited.

The tumor suppressor KLF6 is a member of the Kruppel-like family of transcription factors, which has been implicated in the pathogenesis of several human carcinomas. Uncovering the transcriptional targets relevant for its tumorigenic properties, including cellular proliferation and invasion, will be essential to understanding possible mechanisms by which KLF6 and its antagonistic splice form, KLF6-SV1, regulate this development. To begin defining possible metastatic-related pathways, we analysed the effect of KLF6 dysregulation on a recognized suppressor of cellular invasion, E-cadherin. Targeted KLF6 reduction in an ovarian cancer cell line, SKOV-3, resulted in a 50% reduction of E-cadherin expression (P<0.01) and conversely, KLF6-SV1 silencing upregulated E-cadherin approximately fivefold (P<0.0001). These changes resulted from KLF6 directly transactivating the E-cadherin promoter as demonstrated by luciferase promoter assay and chromatin immunoprecipitation (ChIP). KLF6-mediated changes in E-cadherin levels were accompanied by downstream changes in both the subcellular localization of beta-catenin and c-myc expression levels. Moreover, and consistent with these experimental findings, patient-derived epithelial ovarian tumors with low KLF6 and high KLF6-SV1 expression ratios had significantly decreased E-cadherin expression (P<0.0001). These combined findings highlight the E-cadherin pathway as a novel and functionally important mediator by which changes in KLF6 and KLF6-SV1 can directly alter ovarian tumor invasion and metastasis.

To investigate whether the KLF6 gene plays an important role in the development and/or progression of colorectal cancers, we searched for mutations and allelic loss of the KLF6 gene in 123 colorectal adenocarcinomas by performing PCR-SSCP sequencing. We found five somatic missense mutations: S155N, G163S, G163D, P183L and G195S. Three of them affected the activation domain of KLF6 and four mutations were predicted to disrupt the putative phosphorylation sites. On LOH analysis, 63 cases were heterozygous for at least one marker and 27 cases (42.9%) showed allelic loss at these markers. These data further support that the KLF6 gene may be one of the candidate tumor suppressor genes in colorectal cancers and that genetic alteration of the KLF6 gene might play a role in the development of colorectal carcinomas.

BACKGROUND AND AIM: The Kruppel-like transcription factor KLF6 is a novel tumor-suppressor gene. It was inactivated in human prostate cancer and other tumors tissue, as the result of frequent mutation and loss of heterozygosity (LOH). However, there is no data reporting the levels of KLF6 both mRNA and protein in hepatocellular carcinomas (HCCs). We therefore detected mutations and expression of KLF6 in HCC tissues and further observed the effect of it on cell growth in HCC cell lines. METHODS: We analyzed the exon-2 of KLF6 gene by direct DNA sequencing, and detected the expression of KLF6 by RT-PCR and Western blot in 23 HCC tissues and corresponding nontumorous tissues. Loss of growth suppressive effect of the HCC-derived KLF6 mutant was characterized by in vitro growth curves plotted, flow cytometry and Western blotting. RESULTS: KLF6 mutations were found in 2 of 23 HCC tissues and one of mutations was missense. expression of KLF6 mRNA or protein was down-regulated in 8 (34.7%) or 9 (39.1%) of 23 HCC tissues. Wild-type KLF6 (wtKLF6) inhibited cellular proliferation and prolonged G1-S transition by inducing the expression of p21WAF1 following stable transfection into cultured HepG2 cells, but tumor-derived KLF6 mutant (mKLF6) had no effects. CONCLUSION: Our findings suggest that KLF6 may be involved in pathogenesis of HCC.

Kruppel-like factor 6 (KLF6) is a cancer gene (). Here, we demonstrate that KLF6 protein is rapidly degraded when apoptosis is induced via the intrinsic pathway by cisplatin, adriamycin, or UVB irradiation in multiple cell lines (HCT116, SW40, HepG2, PC3-M, Skov3, NIH-3T3, 293T, GM09706, and MEF, IMR-90). KLF6 degradation occurred in the presence or absence of p53, was associated with ubiquitination, mediated by the proteasome (half-life 16min, unstimulated), and independent of caspases and calpain. KLF6 was unchanged by apoptosis via the extrinsic/death-receptor pathway. Deregulation of KLF6 stability may alter its tumor suppressor function and/or the response of tumors to chemotherapeutics.

Kruppel-like factor (KLF) 6 is a tumor-suppressor gene functionally inactivated by loss of heterozygosity, somatic mutation and/or alternative splicing that generates a dominant-negative splice form, KLF6-SV1. Wild-type KLF6 (wtKLF6) expression is decreased in many human malignancies, which correlates with reduced patient survival. Additionally, loss of the KLF6 locus in the absence of somatic mutation in the remaining allele occurs in a number of human cancers, raising the possibility that haploinsufficiency of the KLF6 gene alone contributes to cellular growth dysregulation and tumorigenesis. Our earlier studies identified the cyclin-dependent kinase inhibitor p21 as a transcriptional target of the KLF6 gene in cultured cells, but not in vivo. To address this issue, we have generated two genetic mouse models to define the in vivo role of KLF6 in regulating cell proliferation and p21 expression. Transgenic overexpression of KLF6 in the liver resulted in a runted phenotype with decreased body and liver size, with evidence of decreased hepatocyte proliferation, increased p21 and reduced proliferating cell nuclear antigen expression. In contrast, mice with targeted deletion of one KLF6 allele (KLF6+/-) display increased liver mass with reduced p21 expression, compared to wild type littermates. Moreover, in primary hepatocellular carcinoma samples, there is a significant correlation between wtKLF6 and p21 mRNA expression. Combined, these data suggest that haploinsufficiency of the KLF6 gene may regulate cellular proliferation in vivo through decreased transcriptional activation of the cyclin-dependent kinase inhibitor p21.

Glioblastoma multiforme (GBM) is the most common and aggressive primary brain tumor and possesses a high incidence of 10p loss. The KLF6 (Kruppel-like transcription factor) tumor suppressor gene on 10p15 is inactivated by loss of heterozygosity (LOH) and/or somatic mutation in a number of human cancers and forced expression of KLF6 in GBM lines inhibits their growth and transformation. In addition, increased expression of its alternatively spliced, cytoplasmic isoform KLF6-SV1 has now been shown to play a role in cancer pathogenesis. On the basis of these findings we examined the role of KLF6 and KLF6-SV1 in the development and progression of GBM. LOH analysis of 17 primary GBM patient samples using KLF6-specific microsatellite markers revealed that 88.2% (15/17) had LOH of the KLF6 locus. Interestingly, no KLF6 somatic mutations were identified. RNA analysis revealed concomitant decreases in all primary GBM tumors (n = 11) by approximately 80% in KLF6 expression (p < 0.001) coupled with increased KLF6-SV1 expression (p < 0.001) when compared to normal astrocytes. To determine the biological relevance of these findings, we examined the effect of KLF6 expression and KLF6-SV1 knockdown in A235 and CRL2020 cell lines. Reconstitution of KLF6 decreased cell proliferation by almost 50%, whereas targeted KLF6 reduction increased cell proliferation 2.5-4.5 fold. Conversely, targeted KLF6-SV1 reduction decreased cell proliferation by 50%. Taken together, our findings demonstrate that KLF6 allelic imbalance and decreased KLF6 and increased KLF6-SV1 expression are common findings in primary GBM tumors, and these changes have antagonistic effects on the regulation of cellular proliferation in GBM cell lines.

BACKGROUND & AIMS: Hepatocellular carcinoma (HCC) is the fifth most prevalent cancer worldwide and the third most lethal. Dysregulation of alternative splicing underlies a number of human diseases, yet its contribution to liver cancer has not been explored fully. The Kruppel-like factor 6 (KLF6) gene is a zinc finger transcription factor that inhibits cellular growth in part by transcriptional activation of p21. KLF6 function is abrogated in human cancers owing to increased alternative splicing that yields a dominant-negative isoform, KLF6 splice variant 1 (SV1), which antagonizes full-length KLF6-mediated growth suppression. The molecular basis for stimulation of KLF6 splicing is unknown. METHODS: In human HCC samples and cell lines, we functionally link oncogenic Ras signaling to increased alternative splicing of KLF6 through signaling by phosphatidylinositol-3 kinase and Akt, mediated by the splice regulatory protein ASF/SF2. RESULTS: In 67 human HCCs, there is a significant correlation between activated Ras signaling and increased KLF6 alternative splicing. In cultured cells, Ras signaling increases the expression of KLF6 SV1, relative to full-length KLF6, thereby enhancing proliferation. Abrogation of oncogenic Ras signaling by small interfering RNA (siRNA) or a farnesyl-transferase inhibitor decreases KLF6 SV1 and suppresses growth. Growth inhibition by farnesyl-transferase inhibitor in transformed cell lines is overcome by ectopic expression of KLF6 SV1. Down-regulation of the splice factor ASF/SF2 by siRNA increases KLF6 SV1 messenger RNA levels. KLF6 alternative splicing is not coupled to its transcriptional regulation. CONCLUSIONS: Our findings expand the role of Ras in human HCC by identifying a novel mechanism of tumor-suppressor inactivation through increased alternative splicing mediated by an oncogenic signaling cascade.

Phorbol 12-myristate 13-acetate (PMA) modulates cell proliferation and survival by activating several intracellular signaling pathways. Protein kinase C (PKC) plays a key role in PMA-induced growth arrest of non-small cell lung cancer (NSCLC) cells. Kruppel-like transcription factor 6 (KLF6), which is associated with negative control of cell proliferation, is downregulated in many cancers, including NSCLC. In this study, we found that KLF6 is downregulated in 17 lung cancer cell lines and in cells representing early stages of lung cancer development. Moreover, PMA induced cell growth arrest through KLF6 induction in H358 NSCLC cells. The increase in KLF6 by PMA was associated with upregulation of the cyclin-dependent kinase inhibitors (CDKIs) p21(WAF1/CIP1) and p27(KIP1). In addition, inhibition of PKC or JNK activation decreased PMA-induced KLF6 induction and activation of PKC alone by Bryostatin-1 and Thymeleatoxin increased KLF6 levels. Moreover, siRNA-mediated knockdown of KLF6 reduced PMA-induced cell growth inhibition concomitantly with decreased expression of both p21(WAF1/CIP1) and p27(KIP1), and in accordance, overexpression of KLF6 alone upregulated both CDKIs protein levels. Our results demonstrate the induction of the tumor suppressor KLF6 following PKC activation and its importance for PMA-mediated cancer cell growth arrest.

The tumor suppressor Kruppel-like factor 6 (KLF6) is frequently inactivated in hepatocellular carcinoma (HCC). To unearth downstream transcriptional targets of KLF6, cDNA microarray analysis of whole liver was compared between KLF6+/+ and KLF6+/- mice. Pituitary tumor transforming gene 1 (PTTG1), an oncogene, was the most up-regulated transcript in KLF6+/- liver. In human HCCs, KLF6 mRNA was significantly decreased, associated with increased PTTG1. In HepG2, KLF6 transcriptionally repressed PTTG1 by direct promoter interaction. Whereas KLF6 downregulation by siRNA increased HepG2 proliferation, siRNA to PTTG1 was anti-proliferative. PTTG1 downregulation represents a novel tumor suppressor pathway of KLF6.

BACKGROUND: Kruppel-like factor 6 (KLF6) is an evolutionarily conserved and ubiquitously expressed protein that belongs to the mammalian Sp1/KLF family of transcriptional regulators. Though KLF6 is a transcription factor and harbors a nuclear localization signal it is not systematically located in the nucleus but it was detected in the cytoplasm of several tissues and cell lines. Hence, it is still not fully settled whether the tumor suppressor function of KLF6 is directly associated with its ability to regulate target genes. METHODOLOGY/PRINCIPAL FINDINGS: In this study we analyzed KLF6 expression and sub-cellular distribution by immunohistochemistry in several normal and tumor tissues in a microarray format representing fifteen human organs. Results indicate that while both nuclear and cytoplasmic distribution of KLF6 is detected in normal breast tissues, breast carcinomas express KLF6 mainly detected in the cytoplasm. expression of KLF6 was further analyzed in breast cancer tissues overexpressing ERBB2 oncoprotein, which is associated with poor disease prognosis and patients survival. The analysis of 48 ductal carcinomas revealed a significant population expressing KLF6 predominantly in the nuclear compartment (X(2)p = 0.005; Fisher p = 0.003). Moreover, this expression pattern correlates directly with early stage and small ductal breast tumors and linked to metastatic events in lymph nodes. CONCLUSIONS/SIGNIFICANCE: Data are consistent with a preferential localization of KLF6 in the nuclear compartment of early stage and small HER2-ERBB2 overexpressing ductal breast tumor cells, also presenting lymph node metastatic events. Thus, KLF6 tumor suppressor could represent a new molecular marker candidate for tumor prognosis and/or a potential target for therapy strategies.

BACKGROUND: A putative tumor suppressor gene at chromosome 10p15, which contains KLF6 and other genes, is predicted to be lost during melanoma development, and its identity is unknown. In this study, we investigated the biological roles and identity of this tumor suppressor gene. METHODS: The human UACC 903 melanoma cell line containing introduced DNA fragments from the 10p15 region with (10E6/3, 10E6/11, and 10E6/18) and without (10ER4S.2/1) the tumor suppressor gene was used. Xenograft tumors were generated in a total of 40 mice with melanoma cell lines, and tumor size was measured. Cells were cultured on plastic or a gel of type I collagen. Viability, proliferation, and apoptosis were assessed. expression of KLF6 protein was assessed by immunohistochemistry and immunoblot analysis. expression of phosphorylated Erk1/2 and cyclin D1 was assessed by immunoblot analysis. Protein expression of KLF6 was inhibited with small interfering RNA (siRNA). KLF6 protein expression was assessed in 17 human nevi and human melanoma specimens from 29 patients. Statistical analyses were adjusted for multiple comparisons by use of Dunnett method. All statistical tests were two-sided. RESULTS: Melanoma cells containing KLF6 generated smaller subcutaneous xenograft tumors with fewer proliferating cells than control cells. When grown on collagen 1, viability of cells with ectopic KLF6 expression (72%) was lower than that of control cells (100%) (group difference = -28%, 95% confidence interval = -31.3% to -25.2%, P < .001). Viability of melanoma cells with or without the KLF6 tumor suppressor gene on plastic dishes was similar. When KLF6 expression was inhibited with KLF6 siRNA, viability of cells with the tumor suppressor gene on collagen I gel increased compared with that of control cells carrying scrambled siRNA. KLF6 protein was detected in all nevi examined but not in human metastatic melanoma tissue examined. Ectopic expression of KLF6 protein in melanoma cells grown on collagen I decreased levels of phosphorylated Erk1/2 and cyclin D1 in the mitogen-activated protein kinase signaling pathway. CONCLUSIONS: In melanoma cells, the tumor suppressor gene at 10p15 appears to be KLF6. Signaling from the collagen I-rich extracellular matrix appears to be involved in the tumor suppressive activity of KLF6 protein.

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.

Although energy restriction has been recognized as an important target for cancer prevention, the mechanism by which energy restriction-mimetic agents (ERMAs) mediate apoptosis remains unclear. By using a novel thiazolidinedione-derived ERMA, CG-12 (Wei, S., Kulp, S. K., and Chen, C. S. (2010) J. Biol. Chem. 285, 9780-9791), vis-a-vis 2-deoxyglucose and glucose deprivation, we obtain evidence that epigenetic activation of the tumor suppressor gene Kruppel-like factor 6 (KLF6) plays a role in ERMA-induced apoptosis in LNCaP prostate cancer cells. KLF6 regulates the expression of many proapoptotic genes, and shRNA-mediated KLF6 knockdown abrogated the ability of ERMAs to induce apoptosis. Chromatin immunoprecipitation analysis indicates that this KLF6 transcriptional activation was associated with increased histone H3 acetylation and histone H3 lysine 4 trimethylation occupancy at the promoter region. Several lines of evidence demonstrate that the enhancing effect of ERMAs on these active histone marks was mediated through transcriptional repression of histone deacetylases and H3 lysine 4 demethylases by down-regulating Sp1 expression. First, putative Sp1-binding elements are present in the promoters of the affected histone-modifying enzymes, and luciferase reporter assays indicate that site-directed mutagenesis of these Sp1 binding sites significantly diminished the promoter activities. Second, shRNA-mediated knockdown of Sp1 mimicked the repressive effect of energy restriction on these histone-modifying enzymes. Third, ectopic Sp1 expression protected cells from the repressive effect of CG-12 on these histone-modifying enzymes, thereby abolishing the activation of KLF6 expression. Together, these findings underscore the intricate relationship between energy restriction and epigenetic regulation of tumor suppressor gene expression, which has therapeutic relevance to foster novel strategies for prostate cancer therapy.

BACKGROUND/AIMS: Colorectal cancer is one of the common cancers of the gastrointestinal tract in Egypt. It is characterized by a relatively earlier onset compared to that in the western world. Studying genetic alterations involved in colorectal cancer progression may help in identifying molecular biomarkers that can be used for early detection. METHODOLOGY: We analyzed DNA isolated from 83 cases including 38 colorectal carcinomas, 23 polyps (16 of which were adenomatous) and 22 cases with inflammatory bowel disease (IBD). mutations at KLF6 tumor suppressor gene (exon 1-4) were examined by PCR-SSCP silver staining technique followed by direct sequencing. 10p15 LOH was analyzed using KLF6 M1, KLF6 M2 and KLF6 M4 markers by microsatellite assay. RESULTS: KLF6 mutations were found in 45%, 27% and 26% of colorectal carcinoma, ulcerative colitis and polyp cases, respectively. Most of the mutations detected were located at exon 2. The majority of mutations found in KLF6 were missense mutation and their type and locations were different from those previously described in the western population. The frequencies of LOH at the three markers examined were 29%, 36%, and 52% for colorectal carcinomas, IBD, and polyp cases, respectively. LOH was detected in mutant KLF6 as well as wild type. No significant association was found between genetic alterations examined with different clinicopathological factors. CONCLUSIONS: Our data highlights for the first time an association of KLF6 gene in colorectal cancer in an Egyptian population. Detecting mutational sites different from those in western population is a characteristic feature in our study which may be related to environmental and/or genetic factors that have to be further identified.

microRNAs have emerged as crucial regulators of tumorigenesis. However, it remains unknown whether miR-181a is involved in the pathogenesis of gastric cancer. In this study, we found that miR-181a is overexpressed in human gastric cancer tissues. Ectopic expression of miR-181a mimic promoted the proliferation, colony formation, migration, and invasion and inhibited the apoptosis of SGC-7901 gastric cancer cells, whereas ectopic expression of miR-181a inhibitor inhibited the malignant phenotypes of SGC-7901 cells. Site-directed mutagenesis and luciferase reporter assay demonstrated that miR-181a repressed KLF6 expression by targeting its 3-UTR. Western blot analysis further showed that KLF6 protein was significantly decreased or increased when miR-181a mimic or inhibitor was transfected into SGC-7901 cells, respectively. In summary, these data suggest that KLF6 gene is a direct target of miR-181a and miR-181a functions as an oncomir in gastric cancer by repressing the expression of tumor suppressor KLF6.

KLF6, a ubiquitously expressed Kruppel-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 3beta (GSK3beta) phosphorylation consensus site, we investigated the role of GSK3beta in the regulation of KLF6 function. Based on transient transfection, GSK3beta augments the transactivation of a p21 promoter luciferase by KLF6. Reciprocal co-immunoprecipitation of hemagglutinin (HA)-GSK3beta and Flag-KLF6 validated the interaction between these two proteins. KLF6 phosphorylation is augmented in the presence of GSK3beta 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. GSK3beta augmented the transactivation by wild-type KLF6, but not KLF6-4A, towards the p21 promoter, and increased p21 protein. Functionally, GSK3beta enhanced KLF6-mediated growth suppression, which was abrogated by the KLF6-4A phosphomutant. These data establish that GSK3beta 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.