LPA5-Dependent Signaling Regulates Regeneration of the Intestinal Epithelium Following Irradiation

Lysophosphatidic acid (LPA) is a bioactive lipid molecule that regulates a wide array of cellular functions, including proliferation, differentiation, and survival, via activation of cognate receptors. The LPA5 receptor is highly expressed in the intestinal epithelium, but its function in restoring intestinal epithelial integrity following injury has not been examined. Here, we use a radiation-induced injury model to study the role of LPA5 in regulating intestinal epithelial regeneration. Control mice (Lpar5f/f) and mice with an inducible, epithelial cell-specific deletion of Lpar5 in the small intestine (Lpar5IECKO) were subjected to 10 Gy total body X-ray irradiation and analyzed during recovery. Repair of the intestinal mucosa was delayed in Lpar5IECKO mice, with reduced epithelial proliferation and increased crypt cell apoptosis. These effects were accompanied by reduced numbers of OLFM4+ intestinal stem cells (ISCs). The effects of LPA5 on ISCs were corroborated by studies using organoids derived from Lgr5-lineage tracking reporter mice with deletion of Lpar5 in Lgr5+-stem cells (Lgr5Cont or Lgr5ΔLpar5). Irradiation of organoids resulted in fewer numbers of Lgr5ΔLpar5 organoids retaining Lgr5+-derived progenitor cells compared to Lgr5Cont organoids. Finally, we observed that impaired regeneration in Lpar5IECKO mice was associated with reduced numbers of Paneth cells and decreased expression of YAP, a critical factor for intestinal epithelial repair. Our study highlights a novel role for LPA5 in regeneration of the intestinal epithelium following irradiation and its effect on the maintenance of Paneth cells that support the stem cell niche.

Inducible intestine-specific deletion of Krüppel-like factor 5 is characterized by a regenerative response in adult mouse colon

Krüppel-like factor 5 (KLF5) is a pro-proliferative transcriptional regulator primarily expressed in the intestinal crypt epithelial cells. Constitutive intestine-specific deletion of Klf5 is neonatal lethal suggesting a crucial role for KLF5 in intestinal development and homeostasis. We have previously shown Klf5 to play an active role regulating intestinal tumorigenesis. Here we examine the effect of inducible intestine-specific deletion of Klf5 in adult mice. Klf5 is lost from the intestine beginning at day 3 after the start of a 5-day treatment with the inducer tamoxifen. Although the mice have no significant weight loss or lethality, the colonic tissue shows signs of epithelial distress starting at day 3 following induction. Accompanying the morphological changes is a significant loss of proliferative crypt epithelial cells as revealed by BrdU or Ki67 staining at days 3 and 5 after start of tamoxifen. We also observed a loss of goblet cells from the colon and Paneth cells from the small intestine upon induced deletion of Klf5. In addition, loss of Klf5 from the colonic epithelium is accompanied by a regenerative response that coincides with an expansion in the zone of Sox9 expression along the crypt axis. At day 11, both proliferation and Sox9 expression return to baseline levels. Microarray and quantitative PCR analyses reveal an up-regulation of several regeneration-associated genes (Reg1A, Reg3G and Reg3B) and down-regulation of many Klf5 targets (Ki-67, cyclin B, Cdc2 and cyclin D1). Sox9 and Reg1A protein levels are also increased upon Klf5 loss. Lentiviral-mediated knockdown of KLF5 and exogenous expression of KLF5 in colorectal cancer cell lines confirm that Sox9 expression is negatively regulated by KLF5. Furthermore, ChIP assays reveal a direct association of KLF5 with both the Sox9 and Reg1A promoters. We have shown that disruption of epithelial homeostasis due to Klf5 loss from the adult colon is followed by a regenerative response led by Sox9 and the Reg family of proteins. Our study demonstrates that adult mouse colonic tissue undergoes acute physiological changes to accommodate the loss of Klf5 withstanding epithelial damage further signifying importance of Klf5 in colonic homeostasis.

Notch1 regulates the effects of matrix metalloproteinase-9 on colitis-associated cancer in mice

BACKGROUND & AIMS

Inflammatory bowel disease increases the risks of colon cancer and colitis-associated cancer (CAC). Epithelial cell-derived matrix metalloproteinase (MMP)-9 mediates inflammation during acute colitis and the cleavage and activation of the transcription factor Notch1, which prevents differentiation of progenitor cells into goblet cells. However, MMP-9 also protects against the development of CAC and acts as a tumor suppressor. We investigated the mechanisms by which MMP-9 protects against CAC in mice.

METHODS

C57/B6 wild-type mice were given a single dose of azoxymethane and 2 cycles of dextran sulfate sodium (DSS). Mice were also given the γ-secretase inhibitor difluorophenacetyl-l-alanyl-S-phenylglycine t-butyl ester (DAPT) or dimethyl sulfoxide (control) during each DSS cycle; they were killed on day 56. We analyzed embryonic fibroblasts isolated from wild-type and MMP-9-/- mice and HCT116 cells that were stably transfected with MMP-9.

RESULTS

Wild-type mice were more susceptible to CAC following inhibition of Notch1 by DAPT, shown by increased numbers of tumors and level of dysplasia compared with controls. Inhibition of Notch1 signaling significantly reduced protein levels of active Notch1, p53, p21WAF1/Cip1, Bax-1, active caspase-3, as well as apoptosis, compared with controls. Similar results were observed in transgenic HCT116 cells and embryonic fibroblasts from MMP-9-/- mice on γ-radiation-induced damage of DNA.

CONCLUSIONS

MMP-9 mediates Notch1 signaling via p53 to regulate apoptosis, cell cycle arrest, and inflammation. By these mechanisms, it might prevent CAC.

Krüppel-like factor 5 is important for maintenance of crypt architecture and barrier function in mouse intestine

BACKGROUND & AIMS

Krüppel-like factor 5 (KLF5) is transcription factor that is expressed by dividing epithelial cells of the intestinal epithelium. KLF5 promotes proliferation in vitro and in vivo and is induced by mitogens and various stress stimuli. To study the role of KLF5 in intestinal epithelial homeostasis, we examined the phenotype of mice with conditional deletion of Klf5 in the gut.

METHODS

Mice were generated with intestinal-specific deletion of Klf5 (Vil-Cre;Klf5fl/fl). Morphologic changes in the small intestine and colon were examined by immunohistochemistry, immunoblotting, and real-time polymerase chain reaction.

RESULTS

Klf5 mutant mice were born at a normal Mendelian ratio but had high mortality compared with controls. Complete deletion of Klf5 from the intestinal mucosa resulted in neonatal lethality that corresponded with an absence of epithelial proliferation. Variegated intestinal-specific deletion of Klf5 in adult mice resulted in morphologic changes that included a regenerative phenotype, impaired barrier function, and inflammation. Adult mutant mice exhibited defects in epithelial differentiation and migration. These changes were associated with reduced expression of Caudal type homeobox (Cdx) 1, Cdx2, and Eph and ephrin signaling proteins. Concomitantly, Wnt signaling to β-catenin was reduced. Proliferation in regenerative crypts was associated with increased expression of the progenitor cell marker Sox9.

CONCLUSIONS

Deletion of Klf5 in the gut epithelium of mice demonstrated that KLF5 maintains epithelial proliferation, differentiation, and cell positioning along the crypt radial axis. Morphologic changes that occur with deletion of Klf5 are associated with disruption of canonical Wnt signaling and increased expression of Sox9.

Krüppel-like factor 5 protects against dextran sulfate sodium-induced colonic injury in mice by promoting epithelial repair

BACKGROUND & AIMS

Krüppel-like factor 5 (KLF5) is a transcription factor that promotes proliferation, is highly expressed in dividing crypt cells of the gastrointestinal epithelium, and is induced by various stress stimuli. We sought to determine the role of KLF5 in colonic inflammation and recovery by studying mice with dextran sulfate sodium (DSS)-induced colitis.

METHODS

Wild-type (WT) and Klf5(+/-) mice were given DSS in the drinking water to induce colitis. For recovery experiments, mice were given normal drinking water for 5 days after DSS administration. The extent of colitis was determined using established clinical and histological scoring systems. Immunohistochemical and immunoblotting analyses were used to examine proliferation, migration, and expression of the epidermal growth factor receptor.

RESULTS

Klf5 expression was increased in colonic tissues of WT mice given DSS; induction of Klf5 was downstream of mitogen-activated protein kinase signaling. In DSS-induced colitis, Klf5(+/-) mice exhibited greater sensitivity to DSS than WT mice, with significantly higher clinical and histological colitis scores. In recovery experiments, Klf5(+/-) mice showed poor recovery, with continued weight loss and higher mortality than WT mice. Klf5(+/-) mice from the recovery period had reduced epithelial proliferation and cell migration at sites of ulceration compared to WT mice; these reductions correlated with reduced expression of epidermal growth factor receptor.

CONCLUSIONS

Epithelial repair is an important aspect of recovery from DSS-induced colitis. The transcription factor KLF5 regulates mucosal healing through its effects on epithelial proliferation and migration.

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.

Altered intestinal epithelial homeostasis in mice with intestine-specific deletion of the Krüppel-like factor 4 gene

The zinc finger transcription factor, Krüppel-like factor 4 (KLF4), is expressed in the post-mitotic, differentiated epithelial cells lining the intestinal tract and exhibits a tumor suppressive effect on intestinal tumorigenesis. Here we report a role for KLF4 in maintaining homeostasis of intestinal epithelial cells. Mice with conditional ablation of the Klf4 gene from the intestinal epithelium were viable. However, both the rates of proliferation and migration of epithelial cells were increased in the small intestine of mutant mice. In addition, the brush-border alkaline phosphatase was reduced as was expression of ephrine-B1 in the small intestine, resulting in mispositioning of Paneth cells to the upper crypt region. In the colon of mutant mice, there was a reduction of the differentiation marker, carbonic anhydrase-1, and failure of differentiation of goblet cells. Mechanistically, deletion of Klf4 from the intestine resulted in activation of genes in the Wnt pathway and reduction in expression of genes encoding regulators of differentiation. Taken together, these data provide new insights into the function of KLF4 in regulating postnatal proliferation, migration, differentiation, and positioning of intestinal epithelial cells and demonstrate an essential role for KLF4 in maintaining normal intestinal epithelial homeostasis in vivo.

A small ubiquitin-related modifier-interacting motif functions as the transcriptional activation domain of Krüppel-like factor 4

The zinc finger transcription factor, Krüppel-like factor 4 (KLF4), regulates numerous biological processes, including proliferation, differentiation, and embryonic stem cell self-renewal. Although the DNA sequence to which KLF4 binds is established, the mechanism by which KLF4 controls transcription is not well defined. Small ubiquitin-related modifier (SUMO) is an important regulator of transcription. Here we show that KLF4 is both SUMOylated at a single lysine residue and physically interacts with SUMO-1 in a region that matches an acidic and hydrophobic residue-rich SUMO-interacting motif (SIM) consensus. The SIM in KLF4 is required for transactivation of target promoters in a SUMO-1-dependent manner. Mutation of either the acidic or hydrophobic residues in the SIM significantly impairs the ability of KLF4 to interact with SUMO-1, activate transcription, and inhibit cell proliferation. Our study provides direct evidence that SIM in KLF4 functions as a transcriptional activation domain. A survey of transcription factor sequences reveals that established transactivation domains of many transcription factors contain sequences highly related to SIM. These results, therefore, illustrate a novel mechanism by which SUMO interaction modulates the activity of transcription factors.

Krüppel-like factor 5 is a crucial mediator of intestinal tumorigenesis in mice harboring combined ApcMin and KRASV12 mutations

Background

Both mutational inactivation of the adenomatous polyposis coli (APC) tumor suppressor gene and activation of the KRAS oncogene are implicated in the pathogenesis of colorectal cancer. Mice harboring a germline Apc^Min mutation or intestine-specific expression of the KRAS^V12 gene have been developed. Both mouse strains develop spontaneous intestinal tumors, including adenoma and carcinoma, though at a different age. The zinc finger transcription factor Krüppel-like factor 5 (KLF5) has previously been shown to promote proliferation of intestinal epithelial cells and modulate intestinal tumorigenesis. Here we investigated the in vivo effect of Klf5 heterozygosity on the propensity of Apc^Min/KRAS^V12 double transgenic mice to develop intestinal tumors.

Results

At 12 weeks of age, Apc^Min/KRAS^V12 mice had three times as many intestinal tumors as Apc^Min mice. This increase in tumor number was reduced by 92% in triple transgenic Apc^Min/KRAS^V12/Klf5^+/- mice. The reduction in tumor number in Apc^Min/KRAS^V12/Klf5^+/- mice was also statistically significant compared to Apc^Min mice alone, with a 75% decrease. Compared with Apc^Min/KRAS^V12, tumors from both Apc^Min/KRAS^V12/Klf5^+/- and Apc^Min mice were smaller. In addition, tumors from Apc^Min mice were more distally distributed in the intestine as contrasted by the more proximal distribution in Apc^Min/KRAS^V12 and Apc^Min/KRAS^V12/Klf5^+/- mice. Klf5 levels in the normal-appearing intestinal mucosa were higher in both Apc^Min and Apc^Min/KRAS^V12 mice but were attenuated in Apc^Min/KRAS^V12/Klf5^+/- mice. The levels of β-catenin, cyclin D1 and Ki-67 were also reduced in the normal-appearing intestinal mucosa of Apc^Min/KRAS^V12/Klf5^+/- mice when compared to Apc^Min/KRAS^V12 mice. Levels of pMek and pErk1/2 were elevated in the normal-appearing mucosa of Apc^Min/KRAS^V12 mice and modestly reduced in Apc^Min/KRAS^V12/Klf5^+/- mice. Tumor tissues displayed higher levels of both Klf5 and β-catenin, irrespective of the mouse genotype from which tumors were derived.

Conclusions

Results of the current study confirm the cumulative effect of Apc loss and oncogenic KRAS activation on intestinal tumorigenesis. The drastic reduction in tumor number and size due to Klf5 heterozygosity in Apc^Min/KRAS^V12 mice indicate a critical function of KLF5 in modulating intestinal tumor initiation and progression.

Haploinsufficiency of Krüppel-like factor 5 rescues the tumor-initiating effect of the Apc(Min) mutation in the intestine

Inactivation of the tumor suppressor adenomatous polyposis coli, with the resultant activation of beta-catenin, is the initiating event in the development of a majority of colorectal cancers. Krüppel-like factor 5 (KLF5), a proproliferative transcription factor, is highly expressed in the proliferating intestinal crypt epithelial cells. To determine whether KLF5 contributes to intestinal adenoma formation, we examined tumor burdens in Apc(Min/+) mice and Apc(Min/+)/Klf5(+/-) mice. Compared with Apc(Min/+) mice, Apc(Min/+)/Klf5(+/-) mice had a 96% reduction in the number of intestinal adenomas. Reduced tumorigenicity in the Apc(Min/+)/Klf5(+/-) mice correlated with reduced levels and nuclear localization of beta-catenin as well as reduced expression of two beta-catenin targets, cyclin D1 and c-Myc. In vitro studies revealed a physical interaction between KLF5 and beta-catenin that enhanced the nuclear localization and transcriptional activity of beta-catenin. Thus, KLF5 is necessary for the tumor-initiating activity of beta-catenin during intestinal adenoma formation in Apc(Min/+) mice, and reduced expression of KLF5 offsets the tumor-initiating activity of the Apc(Min) mutation by reducing the nuclear localization and activity of beta-catenin.

The Role of Inflammation in the Pathogenesis of Colorectal Cancer

Chronic inflammatory disorders are often associated with an increased risk of developing cancer. A classic example of the connection between inflammation and cancer is the increased risk of colorectal cancer in patients with inflammatory bowel disease (IBD). In this review, we discuss aspects of IBD that promote colorectal cancer and highlight key molecular mediators that contribute to cancer risk. Additionally, we report on progress in identifying molecular targets that may prove efficacious in blocking the progression of IBD-related inflammation to cancer.

SUMOylation regulates nuclear localization of Krüppel-like factor 5

SUMOylation is a form of post-translational modification shown to control nuclear transport. Krüppel-like factor 5 (KLF5) is an important mediator of cell proliferation and is primarily localized to the nucleus. Here we show that mouse KLF5 is SUMOylated at lysine residues 151 and 202. Mutation of these two lysines or two conserved nearby glutamates results in the loss of SUMOylation and increased cytoplasmic distribution of KLF5, suggesting that SUMOylation enhances nuclear localization of KLF5. Lysine 151 is adjacent to a nuclear export signal (NES) that resembles a consensus NES. The NES in KLF5 directs a fused green fluorescence protein to the cytoplasm, binds the nuclear export receptor CRM1, and is inhibited by leptomycin and site-directed mutagenesis. SUMOylation facilitates nuclear localization of KLF5 by inhibiting this NES activity, and enhances the ability of KLF5 to stimulate anchorage-independent growth of HCT116 colon cancer cells. A survey of proteins whose nuclear localization is regulated by SUMOylation reveals that SUMOylation sites are frequently located in close proximity to NESs. A relatively common mechanism for SUMOylation to regulate nucleocytoplasmic transport may lie in the interplay between neighboring NES and SUMOylation motifs.

Krüppel-like factor 5 mediates transmissible murine colonic hyperplasia caused by Citrobacter rodentium infection

BACKGROUND & AIMS

Krüppel-like factor 5 (KLF5) is a transcription factor that is highly expressed in proliferating crypt cells of the intestinal epithelium. KLF5 has a pro-proliferative effect in vitro and is induced by mitogenic and stress stimuli. To determine whether KLF5 is involved in mediating proliferative responses to intestinal stressors in vivo, we examined its function in a mouse model of transmissible murine colonic hyperplasia triggered by colonization of the mouse colon by the bacteria Citrobacter rodentium.

METHODS

Heterozygous Klf5 knockout (Klf5(+/-)) mice were generated from embryonic stem cells carrying an insertional disruption of the Klf5 gene. Klf5(+/-) mice or wild-type (WT) littermates were infected with C rodentium by oral gavage. At various time points postinfection, mice were killed and distal colons were harvested. Colonic crypt heights were determined morphometrically from sections stained with H&E. Frozen tissues were stained by immunofluorescence using antibodies against Klf5 and the proliferation marker, Ki67, to determine Klf5 expression and numbers of proliferating cells per crypt.

RESULTS

Infection of WT mice with C rodentium resulted in a 2-fold increase in colonic crypt heights at 14 days postinfection and was accompanied by a 1.7-fold increase in Klf5 expression. Infection of Klf5(+/-) mice showed an attenuated induction of Klf5 expression, and hyperproliferative responses to C rodentium were reduced in the Klf5(+/-) animals as compared with WT littermates.

CONCLUSION

Our study shows that Klf5 is a key mediator of crypt cell proliferation in the colon in response to pathogenic bacterial infection.

Krüppel-like factor 5 mediates cellular transformation during oncogenic KRAS-induced intestinal tumorigenesis

BACKGROUND & AIMS

Krüppel-like factor 5 (KLF5) is a zinc finger-transcription factor that regulates cell proliferation. Oncogenic KRAS mutations are commonly found in colorectal cancers. We aimed to determine whether KLF5 mediates KRAS functions during intestinal tumorigenesis.

METHODS

The effects of KLF5 on proliferation and transformation were examined in IEC-6 intestinal epithelial cells stably transfected with inducible KRAS(V12G). KLF5 expression was examined in intestinal tumors derived from transgenic mice expressing KRAS(V12G) under villin promoter and in human colorectal cancers with mutated KRAS.

RESULTS

Induction of KRAS(V12G) in IEC-6 cells resulted in increased expression of KLF5, accompanied by increased rates of proliferation and anchorage-independent growth. Inhibition of KLF5 expression by mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK) inhibitors or KLF5-specific small interfering RNA reduced proliferation and anchorage-independent growth despite KRAS(V12G) induction. Human colorectal cancer cell lines with mutated KRAS contained high levels of KLF5 and reduction of KLF5 by MEK inhibitors or KLF5 small interfering RNA also led to reduced proliferation and transformation. In vivo, both intestinal tumors derived from mice transgenic for villin-KRAS(V12G) and human primary colorectal cancers with mutated KRAS contained high levels of KLF5 and increased staining of the proliferative marker Ki67.

CONCLUSIONS

Elevated levels of KLF5 protein are strongly correlated with activating KRAS mutations in intestinal tumors in vitro and in vivo. Inhibition of KLF5 expression in tumor cells resulted in significantly reduced rates of proliferation and transforming activities. We conclude that KLF5 is an important mediator of oncogenic KRAS transforming functions during intestinal tumorigenesis.

Haploinsufficiency of Krüppel-like factor 4 promotes adenomatous polyposis coli dependent intestinal tumorigenesis

The zinc finger transcription factor Krüppel-like factor 4 (KLF4) is frequently down-regulated in colorectal cancer. Previous studies showed that the expression of KLF4 was activated by the colorectal cancer tumor suppressor adenomatous polyposis coli (APC) and that KLF4 repressed the Wnt/beta-catenin pathway. Here, we examined whether KLF4 plays a role in modulating intestinal tumorigenesis by comparing the tumor burdens in mice heterozygous for the Apc(Min) allele (Apc(Min/+)) and those heterozygous for both the Apc(Min) and Klf4 alleles (Klf4(+/-)/Apc(Min/+)). Between 10 and 20 weeks of age, Klf4(+/-)/Apc(Min/+) mice developed, on average, 59% more intestinal adenomas than Apc(Min/+) mice (P < 0.0001). Immunohistochemical staining showed that Klf4 protein levels were lower in the normal-appearing intestinal tissues of Klf4(+/-)/Apc(Min/+) mice compared with wild-type, Klf4(+/-), or Apc(Min/+) mice. In contrast, the levels of beta-catenin and cyclin D1 were higher in the normal-appearing intestinal tissues of Klf4(+/-)/Apc(Min/+) mice compared with the other three genotypes. Klf4 levels were further decreased in adenomas from both Apc(Min/+) and Klf4(+/-)/Apc(Min/+) mice compared with their corresponding normal-appearing tissues. Reverse transcription-PCR showed an inverse correlation between adenoma size and Klf4 mRNA levels in both Klf4(+/-)/Apc(Min/+) and Apc(Min/+) mice. There was also a progressive loss of heterozygosity of the wild-type Apc allele in adenomas with increasing size from Klf4(+/-)/Apc(Min/+) and Apc(Min/+) mice. Results from this study show that KLF4 plays an important role in promoting the development of intestinal adenomas in the presence of Apc(Min) mutation.

The diverse functions of Krüppel-like factors 4 and 5 in epithelial biology and pathobiology

The Krüppel-like factors (KLFs) comprise a family of evolutionarily conserved zinc finger transcription factors that regulate numerous biological processes including proliferation, differentiation, development and apoptosis. KLF4 and KLF5 are two closely related members of this family and are both highly expressed in epithelial tissues. In the intestinal epithelium, KLF4 is expressed in terminally differentiated epithelial cells at the villus borders of the mucosa and inhibits cell growth, while KLF5 is expressed in proliferating epithelial cells at the base of the intestinal crypts and promotes cell growth. KLF4 and KLF5 respond to a myriad of external stress stimuli and are likely involved in restoring cellular homeostasis following exposure to stressors. Confirming their importance in maintaining tissue integrity, KLF4 and KLF5 are both dysregulated in various types of cancer. Here we review the recent advances in defining the physiological and pathobiological roles of KLF4 and KLF5, focusing on their functions in the intestinal epithelium.

Kruppel-like factor 5 is an important mediator for lipopolysaccharide-induced proinflammatory response in intestinal epithelial cells

Lipopolysaccharide (LPS) is a bacterially-derived endotoxin that elicits a strong proinflammatory response in intestinal epithelial cells. It is well established that LPS activates this response through NF-κB. In addition, LPS signals through the mitogen-activated protein kinase (MAPK) pathway. We previously demonstrated that the Krüppel-like factor 5 [KLF5; also known as intestine-enriched Krüppel-like factor (IKLF)] is activated by the MAPK. In the current study, we examined whether KLF5 mediates the signaling cascade elicited by LPS. Treatment of the intestinal epithelial cell line, IEC6, with LPS resulted in a dose- and time-dependent increase in KLF5 messenger RNA (mRNA) and protein levels. Concurrently, mRNA levels of the p50 and p65 subunits of NF-κB were increased by LPS treatment. Pretreatment with the MAPK inhibitor, U0126, or the LPS antagonist, polymyxin B, resulted in an attenuation of KLF5, p50 and p65 NF-κB subunit mRNA levels from LPS treatment. Importantly, suppression of KLF5 by small interfering RNA (siRNA) resulted in a reduction in p50 and p65 subunit mRNA levels and NF-κB DNA binding activity in response to LPS. LPS treatment also led to an increase in secretion of TNF-α and IL-6 from IEC6, both of which were reduced by siRNA inhibition of KLF5. In addition, intercellular adhesion molecule-1 (ICAM-1) levels were increased in LPS-treated IEC6 cells and this increase was associated with increased adhesion of Jurkat lymphocytes to IEC6. The induction of ICAM-1 expression and T cell adhesion to IEC6 by LPS were both abrogated by siRNA inhibition of KLF5. These results indicate that KLF5 is an important mediator for the proinflammatory response elicited by LPS in intestinal epithelial cells.

TMS1/ASC: the cancer connection

TMS1/ASC is a bipartite protein comprising two protein-protein interaction domains, a pyrin domain (PYD) and a caspase recruitment domain (CARD). Proteins containing these domains play pivotal roles in regulating apoptosis and immune response pathways, and mutations in a number of PYD- and CARD-containing proteins have been linked to autoinflammatory diseases and cancer. Indeed, one of the ways in which TMS1/ASC was identified was as a target of methylation-mediated silencing in breast cancer cells. This review discusses the mounting evidence supporting a correlation between the silencing of TMS1/ASC expression and cancer. In addition, it addresses the reported functions of TMS1/ASC that include apoptosis, activation of inflammatory caspases and regulation of NF-kappa B, and discusses the potential ways in which loss of TMS1/ASC contributes to carcinogenesis.

TMS1/ASC: the cancer connection

TMS1/ASC is a bipartite protein comprising two protein-protein interaction domains, a pyrin domain (PYD) and a caspase recruitment domain (CARD). Proteins containing these domains play pivotal roles in regulating apoptosis and immune response pathways, and mutations in a number of PYD- and CARD-containing proteins have been linked to autoinflammatory diseases and cancer. Indeed, one of the ways in which TMS1/ASC was identified was as a target of methylation-mediated silencing in breast cancer cells. This review discusses the mounting evidence supporting a correlation between the silencing of TMS1/ASC expression and cancer. In addition, it addresses the reported functions of TMS1/ASC that include apoptosis, activation of inflammatory caspases and regulation of NF-kappa B, and discusses the potential ways in which loss of TMS1/ASC contributes to carcinogenesis.

Activation of a caspase-9-mediated apoptotic pathway by subcellular redistribution of the novel caspase recruitment domain protein TMS1

Genetic and epigenetic alterations affecting proteins involved in apoptosis can contribute to the establishment and progression of cancer. Recently, our laboratory has isolated a novel gene, TMS1, that is aberrantly methylated and silenced in a significant proportion of human breast cancers. TMS1 contains a caspase recruitment domain (CARD), suggesting a role in caspase-mediated cell death. In the present study, we characterize the participation of TMS1 in apoptosis and examine the subcellular localization of the protein. Inducible expression of TMS1 inhibited cellular proliferation and induced DNA fragmentation in a time-dependent manner. These apoptotic events were blocked by the general caspase inhibitor, Z-VAD-fmk. The ability of TMS1 to trigger apoptosis was also suppressed by a dominant negative form of caspase-9 but not by a dominant negative form of caspase-8, indicating that TMS1 functions through activation of caspase-9. Unlike a number of other CARD-containing proteins, TMS1 did not activate nuclear factor kappaB-dependent transcription, consistent with a proapoptotic role for TMS1 in death signaling pathways. Timed localization studies revealed that TMS1-induced apoptosis was accompanied by the redistribution of TMS1 from the cytoplasm to perinuclear spherical structures. Whereas the apoptotic activity of TMS1 was blocked by caspase inhibition, the formation of TMS1-containing subcellular structures was not, suggesting that the redistribution of TMS1 precedes caspase activation. Both the proapoptotic activity of TMS1 and aggregate formation were dependent on the CARD. In summary, the data indicate that TMS1-induced apoptosis proceeds through a CARD-dependent aggregation step followed by activation of a caspase-9-mediated pathway.

TMS1, a novel proapoptotic caspase recruitment domain protein, is a target of methylation-induced gene silencing in human breast cancers

Gene silencing associated with aberrant methylation of promoter region CpG islands is an acquired epigenetic alteration that serves as an alternative to genetic defects in the inactivation of tumor suppressor and other genes in human cancers. The hypothesis that aberrant methylation plays a direct causal role in carcinogenesis hinges on the question of whether aberrant methylation is sufficient to drive gene silencing. To identify downstream targets of methylation-induced gene silencing, we used a human cell model in which aberrant CpG island methylation is induced by ectopic expression of DNA methyltransferase. Here we report the isolation and characterization of TMS1 (target of methylation-induced silencing), a novel CpG island-associated gene that becomes hypermethylated and silenced in cells overexpressing DNA cytosine-5-methyltransferase-1. We also show that TMS1 is aberrantly methylated and silenced in human breast cancer cells. Forty percent (11 of 27) of primary breast tumors exhibited aberrant methylation of TMS1. TMS1 is localized to chromosome 16p11.2-12.1 and encodes a 22-kDa predicted protein containing a COOH-terminal caspase recruitment domain, a recently described protein interaction motif found in apoptotic signaling molecules. Ectopic expression of TMS1 induced apoptosis in 293 cells and inhibited the survival of human breast cancer cells. The data suggest that methylation-mediated silencing of TMS1 confers a survival advantage by allowing cells to escape from apoptosis, supporting a new role for aberrant methylation in breast tumorigenesis.

Induced expression of p16(INK4a) inhibits both CDK4- and CDK2-associated kinase activity by reassortment of cyclin-CDK-inhibitor complexes

To investigate the mode of action of the p16(INK4a) tumor suppressor protein, we have established U2-OS cells in which the expression of p16(INK4a) can be regulated by addition or removal of isopropyl-beta-D-thiogalactopyranoside. As expected, induction of p16(INK4a) results in a G1 cell cycle arrest by inhibiting phosphorylation of the retinoblastoma protein (pRb) by the cyclin-dependent kinases CDK4 and CDK6. However, induction of p16(INK4a) also causes marked inhibition of CDK2 activity. In the case of cyclin E-CDK2, this is brought about by reassortment of cyclin, CDK, and CDK-inhibitor complexes, particularly those involving p27(KIP1). Size fractionation of the cellular lysates reveals that a substantial proportion of CDK4 participates in active kinase complexes of around 200 kDa. Upon induction of p16(INK4a), this complex is partly dissociated, and the majority of CDK4 is found in lower-molecular-weight fractions consistent with the formation of a binary complex with p16(INK4a). Sequestration of CDK4 by p16(INK4a) allows cyclin D1 to associate increasingly with CDK2, without affecting its interactions with the CIP/KIP inhibitors. Thus, upon the induction of p16(INK4a), p27(KIP1) appears to switch its allegiance from CDK4 to CDK2, and the accompanying reassortment of components leads to the inhibition of cyclin E-CDK2 by p27(KIP1) and p21(CIP1). Significantly, p16(INK4a) itself does not appear to form higher-order complexes, and the overwhelming majority remains either free or forms binary associations with CDK4 and CDK6.

The alternative product from the human CDKN2A locus, p14(ARF), participates in a regulatory feedback loop with p53 and MDM2

The two distinct proteins encoded by the CDKN2A locus are specified by translating the common second exon in alternative reading frames. The product of the alpha transcript, p16(INK4a), is a recognized tumour suppressor that induces a G1 cell cycle arrest by inhibiting the phosphorylation of the retinoblastoma protein by the cyclin-dependent kinases, CDK4 and CDK6. In contrast, the product of the human CDKN2A beta transcript, p14(ARF), activates a p53 response manifest in elevated levels of MDM2 and p21(CIP1) and cell cycle arrest in both G1 and G2/M. As a consequence, p14(ARF)-induced cell cycle arrest is p53 dependent and can be abrogated by the co-expression of human papilloma virus E6 protein. p14(ARF) acts by binding directly to MDM2, resulting in the stabilization of both p53 and MDM2. Conversely, p53 negatively regulates p14(ARF) expression and there is an inverse correlation between p14(ARF) expression and p53 function in human tumour cell lines. However, p14(ARF) expression is not involved in the response to DNA damage. These results place p14(ARF) in an independent pathway upstream of p53 and imply that CDKN2A encodes two proteins that are involved in tumour suppression.

Inhibitors of cyclin-dependent kinases induce features of replicative senescence in early passage human diploid fibroblasts

After a limited number of population doublings (PDs), cultures of normal mammalian diploid cells undergo an irreversible growth arrest known as replicative senescence [1]. As well as contributing to cellular ageing, senescence is viewed as an important mechanism of tumour suppression by preventing the emergence of immortal cell clones [2-4]. Senescent cells have a number of characteristics that distinguish them from cycling or quiescent cells including elevated levels of two cyclin-dependent kinase (Cdk) inhibitors, p16INK4a and p21CIP1 [5-11]. Here, we demonstrate that both of these Cdk inhibitors, as well as other members of their protein families (the INK4 and CIP/KIP families, respectively [12]), induce several facets of the senescent phenotype when ectopically expressed in young human diploid fibroblasts. These include a reduced proliferative capacity, an altered size and shape, the presence of underphosphorylated retinoblastoma protein (pRb), increased expression of plasminogen activator inhibitor (PAI-1) and the appearance of senescence-associated beta-galactosidase (SA-beta-gal) activity [2,3,13-15]. A 20 amino acid peptide from p16INK4a that inhibits Cdks active in the G1 phase of the cell cycle [16] produces similar effects in a dose-dependent manner suggesting that, in primary fibroblasts, inhibition of G1-specific Cdk activity is sufficient to induce phenotypic changes that normally occur at the end of their finite lifespan.

Two new mutations in the human E1 beta subunit of branched chain alpha-ketoacid dehydrogenase associated with maple syrup urine disease

Maple syrup urine disease (MSUD) is an autosomal recessive disorder caused by defective function of the mitochondrial branched chain alpha-ketoacid dehydrogenase (BCKD) complex. Mutations in both alleles of any of three genes for component proteins result in the clinical phenotype. Two discrete mutant alleles for the E1 beta subunit of the decarboxylase component in a proband with MSUD are defined and parental origin of each allele identified. The maternal mutation, an A to T transversion at nucleotide 526 in the coding sequence, potentiates an asparagine to tyrosine change at position 126 (N126Y). The paternal mutant allele contains a C to T transition at nucleotide 970 introducing a stop codon (R274 ). Western blot analysis revealed a 75% reduction in the E1 beta-N126Y protein and an absence of the R274* truncated protein in proband cells. Both mutant proteins could be synthesized, imported into mitochondria, and processed in vitro. Functional analysis of the mutant proteins provided new information on the role of E1 beta in the activity of BCKD. In vivo the E1 beta-N126Y protein associated into macromolecular complexes indistinguishable from those formed with the wild type E1 beta protein. However, catalytic activity of these complexes in proband cells was < 1% of wild type activity. Alignment comparisons with other thiamin pyrophosphate-requiring enzymes suggests the N126Y substitution could interfere with interactions of the protein with the cofactor causing inactivity. The truncated E1 beta-R274* protein is unstable and not found in mitochondria from the patient derived cells.

Influence of subunit transcript and protein levels on formation of a mitochondrial multienzyme complex

Constitutive expression of nuclear genes encoding mitochondrial proteins raises the question of whether these proteins are present in similar amounts in mitochondria of different tissues. We report that amounts of a single multienzyme complex can vary on a per mitochondrion basis depending on the number of mitochondria per cell. Human branched-chain alpha-keto acid dehydrogenase (BCKD) expression is used as a paradigm in these studies. Expression is compared and contrasted in HepG2 and DG75 cells in which mitochondrial content is twofold higher in the hepatocarcinoma line than in the lymphoblastoid line. Per cell, BCKD activity is equal in the two cells types, but BCKD protein concentration per mitochondrion is twofold higher in DG75 cells. Steady-state mRNA levels do not appear to be directly related to amounts of protein in the two cell lines. To test whether one subunit is limiting in formation of complex, overexpression of each BCKD subunit was elicited by plasmid transfection of the DG75 cells. Only overexpression of the beta-subunit of the decarboxylase component induced more BCKD activity without apparent increase in mRNA for the other endogenously expressed subunits. This implies that free BCKD subunits exist in a cell and can be recruited into an active complex when the limiting subunit becomes available.