Molecular cloning and characterization of the human p27Kip1 gene promoter

Up-regulation of cofilin-1 in cell senescence associates with morphological change and p27kip1 -mediated growth delay

Morphological change is an explicit characteristic of cell senescence, but the underlying mechanisms remains to be addressed. Here, we demonstrated, after a survey of various actin-binding proteins, that the post-translational up-regulation of cofilin-1 was essential for the reduced rate of actin depolymerization morphological enlargement in senescent cells. Additionally, up-regulated cofilin-1 mainly existed in the serine-3 phosphorylated form, according to the 2D gel immunoblotting assay. The up-regulation of cofilin-1 was also detected in aged mammalian tissues. The over-expression of wild-type cofilin-1 and constitutively phosphorylated cofilin-1 promoted cell senescence with an increased cell size. Additionally, senescent phenotypes were also reduced by knockdown of total cofilin-1, which led to a decrease in phosphorylated cofilin-1. The senescence induced by the over-expression of cofilin-1 was dependent on p27^Kip1 , but not on the p53 and p16^INK4 expressions. The knockdown of p27^Kip1 alleviated cell senescence induced by oxidative stress or replicative stress. We also found that the over-expression of cofilin-1 induced the expression of p27^Kip1 through transcriptional suppression of the transcriptional enhancer factors domain 1 (TEAD1) transcription factor. The TEAD1 transcription factor played a transrepressive role in the p27^Kip1  gene promoter, as determined by the promoter deletion reporter gene assay. Interestingly, the down-regulation of TEAD1 was accompanied by the up-regulation of cofilin-1 in senescence. The knockdown and restoration of TEAD1 in young cells and old cells could induce and inhibit p27^Kip1 and senescent phenotypes, respectively. Taken together, the current data suggest that cofilin-1/TEAD1/p27^Kip1 signaling is involved in senescence-related morphological change and growth arrest.

Transcriptional and post-transcriptional upregulation of p27 mediates growth inhibition of isorhapontigenin (ISO) on human bladder cancer cells

There are few approved drugs available for the treatment of muscle-invasive bladder cancer (MIBC). Recently, we have demonstrated that isorhapontigenin (ISO), a new derivative isolated from the Chinese herb Gnetum cleistostachyum, effectively induces cell-cycle arrest at the G0/G1 phase and inhibits anchorage-independent cell growth through the miR-137/Sp1/cyclin D1 axis in human MIBC cells. Herein, we found that treatment of bladder cancer (BC) cells with ISO resulted in a significant upregulation of p27, which was also observed in ISO-treated mouse BCs that were induced by N-butyl-N-(4-hydroxybutyl) nitrosamine (BBN). Importantly, knockdown of p27 caused a decline in the ISO-induced G0-G1 growth arrest and reversed ISO suppression of anchorage-independent growth in BC cells. Mechanistic studies revealed that ISO promoted p27 expression at mRNA transcription level through increasing direct binding of forkhead box class O1 (FOXO1) to its promoter, while knockdown of FOXO1 attenuated ISO inhibition of BC cell growth. On the other hand, ISO upregulated the 3'-untranslated region (3'-UTR) activity of p27, which was accompanied by a reduction of miR-182 expression. In line with these observations, ectopic expression of miR-182 significantly blocked p27 3'-UTR activity, whereas mutation of the miR-182-binding site at p27 mRNA 3'-UTR effectively reversed this inhibition. Accordingly, ectopic expression of miR-182 also attenuated ISO upregulation of p27 expression and impaired ISO inhibition of BC cell growth. Our results not only provide novel insight into understanding of the underlying mechanism related to regulation of MIBC cell growth but also identify new roles and mechanisms underlying ISO inhibition of BC cell growth.

High-throughput screening reveals alsterpaullone, 2-cyanoethyl as a potent p27Kip1 transcriptional inhibitor

p27^Kip1 is a cell cycle inhibitor that prevents cyclin dependent kinase (CDK)/cyclin complexes from phosphorylating their targets. p27^Kip1 is a known tumor suppressor, as the germline loss of p27^Kip1 results in sporadic pituitary formation in aged rodents, and its presence in human cancers is indicative of a poor prognosis. In addition to its role in cancer, loss of p27^Kip1 results in regenerative phenotypes in some tissues and maintenance of stem cell pluripotency, suggesting that p27^Kip1 inhibitors could be beneficial for tissue regeneration. Because p27^Kip1 is an intrinsically disordered protein, identifying direct inhibitors of the p27^Kip1 protein is difficult. Therefore, we pursued a high-throughput screening strategy to identify novel p27^Kip1 transcriptional inhibitors. We utilized a luciferase reporter plasmid driven by the p27^Kip1 promoter to transiently transfect HeLa cells and used cyclohexamide as a positive control for non-specific inhibition. We screened a “bioactive” library consisting of 8,904 (4,359 unique) compounds, of which 830 are Food and Drug Administration (FDA) approved. From this screen, we successfully identified 111 primary hits with inhibitory effect against the promoter of p27^Kip1. These hits were further refined using a battery of secondary screens. Here we report four novel p27^Kip1 transcriptional inhibitors, and further demonstrate that our most potent hit compound (IC₅₀ = 200 nM) Alsterpaullone 2-cyanoethyl, inhibits p27^Kip1 transcription by preventing FoxO3a from binding to the p27^Kip1 promoter. This screen represents one of the first attempts to identify inhibitors of p27^Kip1 and may prove useful for future tissue regeneration studies.

Integrity of the LXXLL motif in Stat6 is required for the inhibition of breast cancer cell growth and enhancement of differentiation in the context of progesterone

Background

Progesterone is essential for the proliferation and differentiation of mammary gland epithelium. Studies of breast cancer cells have demonstrated a biphasic progesterone response consisting of an initial proliferative burst followed by sustained growth arrest. However, the transcriptional factors acting with the progesterone receptor (PR) to mediate the effects of progesterone on mammary cell growth and differentiation remain to be determined. Recently, it was demonstrated that signal transducer and activator of transcription 6 (Stat6) is a cell growth suppressor. Similar to progesterone-bound PR, Stat6 acts by inducing the expression of the G1 cyclin-dependent kinase inhibitors p21 and p27. The possible interaction between Stat6 and progesterone pathways in mammary cells was therefore investigated in the present study.

Methods

ChIP and luciferase were assayed to determine whether Stat6 induces p21 and p27 expression by recruitment at the proximal Sp1-binding sites of the gene promoters. Immunoprecipitation and Western blotting were performed to investigate the interaction between Stat6 and PR-B. The cellular DNA content and cell cycle distribution in breast cancer cells were analyzed by FACS.

Results

We found that Stat6 interacts with progesterone-activated PR in T47D cells. Stat6 synergizes with progesterone-bound PR to transactivate the p21 and p27 gene promoters at the proximal Sp1-binding sites. Moreover, Stat6 overexpression and knockdown, respectively, increased or prevented the induction of p21 and p27 gene expression by progesterone. Stat6 knockdown also abolished the inhibitory effects of progesterone on pRB phosphorylation, G1/S cell cycle progression, and cell proliferation. In addition, knockdown of Stat6 expression prevented the induction of breast cell differentiation markers, previously identified as progesterone target genes. Finally, Stat6 gene expression levels increased following progesterone treatment, indicating a positive auto-regulatory loop between PR and Stat6.

Conclusions

Taken together, these data identify Stat6 as a coactivator of PR mediating the growth-inhibitory and differentiation effects of progesterone on breast cancer cells.

TGF-β Signaling Regulates Pancreatic β-Cell Proliferation through Control of Cell Cycle Regulator p27 Expression

Proliferation of pancreatic β-cells is an important mechanism underlying β-cell mass adaptation to metabolic demands. Increasing β-cell mass by regeneration may ameliorate or correct both type 1 and type 2 diabetes, which both result from inadequate production of insulin by β-cells of the pancreatic islet. Transforming growth factor β (TGF-β) signaling is essential for fetal development and growth of pancreatic islets. In this study, we exposed HIT-T15, a clonal pancreatic β-cell line, to TGF-β signaling. We found that inhibition of TGF-β signaling promotes proliferation of the cells significantly, while TGF-β signaling stimulation inhibits proliferation of the cells remarkably. We confirmed that this proliferative regulation by TGF-β signaling is due to the changed expression of the cell cycle regulator p27. Furthermore, we demonstrated that there is no observed effect on transcriptional activity of p27 by TGF-β signaling. Our data show that TGF-β signaling mediates the cell-cycle progression of pancreatic β-cells by regulating the nuclear localization of CDK inhibitor, p27. Inhibition of TGF-β signaling reduces the nuclear accumulation of p27, and as a result this inhibition promotes proliferation of β-cells.

Undifferentiated embryonic cell transcription factor-1 (UTF1) inhibits the growth of cervical cancer cells by transactivating p27Kip1

Undifferentiated embryonic cell transcription factor-1 (UTF1) is an important transcription factor during development, which plays critical roles in cell fate determination. However, its expression and function in somatic tissues remain unclear. Here, we investigated the expression pattern of the UTF1 in the human normal and cancerous lesions of cervix and found that UTF1 was downregulated in cervical carcinogenesis, which was related to the hypermethylation of UTF1 promoter. Exogenous expression of UTF1 resulted in the significant inhibition of cell proliferation in vitro and tumorigenesis in vivo through attenuating cell cycle arrest via increasing the level of p27 (Kip1) . Luciferase reporter assay indicated that the region containing an intact activating transcription factor site between nucleotides -517 and -388 of the p27 (Kip1) promoter was indispensable for its activation by UTF1. Chromatin immunoprecipitation analysis confirmed the physical interaction between UTF1 and the p27 (Kip1) promoter. Taken together, our findings reveal that UTF1 attenuates cell proliferation and is inactivated in cervical carcinogenesis through epigenetic modification, which strongly supports that UTF1 is a potential tumor suppressor.

Melatonin MT1 receptor-induced transcriptional up-regulation of p27(Kip1) in prostate cancer antiproliferation is mediated via inhibition of constitutively active nuclear factor kappa B (NF-κB): potential implications on prostate cancer chemoprevention and therapy

Our laboratory has recently demonstrated a melatonin MT1 receptor-mediated antiproliferative signaling mechanism in androgen receptor (AR)-positive prostate epithelial cells which involves up-regulation of p27(Kip1) through dual activation of Gα(s)/protein kinase A (PKA) and Gα(q)/protein kinase C (PKC) in parallel, and down-regulation of activated AR signaling via PKC stimulation. The aim of the present investigation was to identify the transcription factor that mediates melatonin's up-regulatory effect on p27(Kip1) in LNCaP and 22Rv1 prostate cancer cells. Deletion mapping and reporter assays of the p27(Kip1) promoter revealed that the putative melatonin-responsive transcription factor binds to a 116 base-pair region of the promoter sequence, which contains a potential nuclear factor kappa B (NF-κB) binding site. When the NF-κB binding site was abolished by site-directed mutagenesis, the stimulatory effect of melatonin on p27(Kip1) promoter activity was mitigated. Notably, melatonin inhibited the DNA binding of activated NF-κB via MT1 receptor-induced PKA and PKC stimulation. Furthermore, melatonin's up-regulatory effect on p27(Kip1) transcription and consequent cell antiproliferation were abrogated by NF-κB activator but mimicked by NF-κB inhibitor. The results indicate that inhibition of constitutively active NF-κB via melatonin MT1 receptor-induced dual activation of (Gα(s)) PKA and (Gα(q)) PKC can de-repress the p27(Kip1) promoter leading to transcriptional up-regulation of p27(Kip1). MT1 receptor-mediated inhibition of activated NF-κB signaling provides a novel mechanism supporting the use of melatonin in prostate cancer chemoprevention and therapy.

Stat6 cooperates with Sp1 in controlling breast cancer cell proliferation by modulating the expression of p21(Cip1/WAF1) and p27 (Kip1)

BACKGROUND

The signal transducer and activator of transcription 6 (Stat6), a member of the family of DNA-binding proteins, has been identified as a critical cell differentiation modulator in breast cancer cells. As of yet, the mechanisms underlying this function have remained largely unknown. To further elucidate the role of Stat6 in breast cancer development, we investigated the consequences of exogenous Stat6 expression.

METHODS

Proliferation assays and flow cytometry assays were conducted to evaluate the putative role of Stat6 on cell proliferation. To this end, we produced synchronized cells after a double thymidine block, as confirmed by FACS analysis. mRNA levels of Stat6 were measured by RNase protection analysis. To confirm the interaction among proteins, we employed GST pull-down assays and immunoprecipitation assays. Luciferase assays and ChIP assays were used to assess the transcriptional activity.

RESULTS

Compared to control breast cancer cells, we found that exogenous Stat6 expression plays a critical role in controlling cell proliferation. Also in different breast tumor cell lines, endogenous Stat6 expression was found to be positively related to a lower proliferation rate. Interestingly, in human breast cancer cells Stat6 functions in G1/S cell cycle progression, and the growth-inhibitory effect of Stat6 was shown to be mediated by induction of the G1 cyclin-dependent kinase inhibitors p21(Cip1/WAF1) (p21) and p27(Kip1) (p27). Simultaneously, G1-related cyclin/cyclin-dependent kinase activities and pRB phosphorylation were markedly reduced, and cell cycle progression was blocked in the G1 phase. Stat6 knockdown resulted in enhanced cell proliferation and a decrease in p21 and p27 mRNA levels in the steroid-responsive and non-responsive T-47D and MDA-MB-231 cell lines, respectively. In addition, the stimulatory effect of Stat6 on p21 and p27 gene transcription was found to be associated with interaction of Stat6 with the transcription factor Sp1 at the proximal Sp1-binding sites in their respective promoters.

CONCLUSIONS

Together, these results identify Stat6 as an important cell differentiation regulatory protein functioning, at least in part, by interacting with Sp1 to activate the p21 and p27 gene promoters in breast cancer cells.

Regulation of p27Kip1 by Sox2 maintains quiescence of inner pillar cells in the murine auditory sensory epithelium

Sox2 plays critical roles in cell fate specification during development and in stem cell formation; however, its role in postmitotic cells is largely unknown. Sox2 is highly expressed in supporting cells (SCs) of the postnatal mammalian auditory sensory epithelium, which unlike non-mammalian vertebrates remains quiescent even after sensory hair cell damage. Here, we induced the ablation of Sox2, specifically in SCs at three different postnatal ages (neonatal, juvenile and adult) in mice. In neonatal mice, Sox2-null inner pillar cells (IPCs, a subtype of SCs) proliferated and generated daughter cells, while other SC subtypes remained quiescent. Furthermore, p27(Kip1), a cell cycle inhibitor, was absent in Sox2-null IPCs. Similarly, upon direct deletion of p27(Kip1), p27(Kip1)-null IPCs also proliferated but retained Sox2 expression. Interestingly, cell cycle control of IPCs by Sox2-mediated expression of p27(Kip1) gradually declined with age. In addition, deletion of Sox2 or p27(Kip1) did not cause a cell fate change. Finally, chromatin immunoprecipitation with Sox2 antibodies and luciferase reporter assays with the p27(Kip1) promoter support that Sox2 directly activates p27(Kip1) transcription in postmitotic IPCs. Hence, in contrast to the well known activity of Sox2 in promoting proliferation and cell fate determination, our data demonstrate that Sox2 plays a novel role as a key upstream regulator of p27(Kip1) to maintain the quiescent state of postmitotic IPCs. Our studies suggest that manipulating Sox2 or p27(Kip1) expression is an effective approach to inducing proliferation of neonatal auditory IPCs, an initial but necessary step toward restoring hearing in mammals.

Functional characterization of a rare germline mutation in the gene encoding the cyclin-dependent kinase inhibitor p27Kip1 (CDKN1B) in a Spanish patient with multiple endocrine neoplasia-like phenotype

OBJECTIVE

The aim of this study was to investigate the presence of germline mutations in the CDKN1B gene that encodes the cyclin-dependent kinase (Cdk) inhibitor p27 in multiple endocrine neoplasia 1 (MEN1)-like Spanish index patients. The CDKN1B gene has recently been identified as a tumor susceptibility gene for MEN4, with six germline mutations reported so far in patients with a MEN-like phenotype but negative for MEN1 mutations.

DESIGN AND METHODS

Fifteen Spanish index cases with MEN-like symptoms were screened for mutations in the CDKN1B gene and the mutant variant was studied functionally by transcription/translation assays in vitro and in transiently transfected HeLa cells.

RESULTS

We report the identification of a heterozygous GAGA deletion in the 5'-UTR of CDKN1B, NM_004064.3:c.-32_-29del, in a patient affected by gastric carcinoid tumor and hyperparathyroidism. This deletion falls inside the region that is responsible for CDKN1B transcription and is predicted to destroy a secondary stem and loop structure that includes the GAGAGA element responsible for ribosome recruitment. Accordingly, in vitro studies of coupled transcription/translation assays and transient transfection in HeLa cells showed that the GAGA deletion in the CDKN1B 5'-UTR significantly impairs the transcription of downstream reporter luciferase (of ∼40-60%) and, possibly, the translation of the corresponding mRNA. This mutation was associated with a significant reduction in the amount of CDKN1B mRNA in peripheral blood leukocytes from the patient, as demonstrated by quantitative real-time PCR.

CONCLUSIONS

Our results confirm that germline CDKN1B mutations may predispose to a human MEN4 condition and add novel evidence that alteration in the transcription/translation rate of CDKN1B mRNA might be the mechanism implicated in tumor susceptibility.

Krüppel-like factor 4 functions as a tumor suppressor in cervical carcinoma

BACKGROUND

The Krüppel-like factor 4 (KLF4) protein, a zinc finger transcription factor that is highly expressed in epithelial tissues such as the gut and skin, has been implicated in both tumor suppression and progression. However, the role of KLF4 in human cervical carcinoma is still unclear.

METHODS

The expression of KLF4 in cervical carcinoma tissues and cervical cancer cell lines was examined with immunohistochemistry and Western blot assay. The effects of KLF4 silencing and overexpression on the cell proliferation, cell viability, and tumor formation of cervical cancer cells were investigated.

RESULT

KLF4 protein expression showed a pattern of gradual decrease from normal cervix to cervical carcinoma in situ and then to invasive cervical carcinomas (P < .05). Overexpression of KLF4 in SiHa and C33A cells resulted in significantly inhibited cell growth and significantly attenuated tumor formation. Consistently, KLF4 silencing in HeLa cells significantly promoted cell growth and tumor formation. Furthermore, KLF4 overexpression caused cell cycle arrest at the G1/S transition, along with the up-regulated expression of p27(Kip1) protein. Promoter analysis revealed that KLF4 transactivated the expression of p27(Kip1) through the specific motif that is between the nucleotides of -435 and -60 in its promoter. The results from chromatin immunoprecipitation assays demonstrated the physical interaction between KLF4 protein and this specific motif in p27(Kip1) promoter.

CONCLUSIONS

KLF4 may function as a tumor suppressor in cervical carcinoma by inhibiting cell growth and tumor formation.

Upstream molecular signaling pathways of p27(Kip1) expression in human breast cancer cells in vitro: differential effects of 4-hydroxytamoxifen and deficiency of either D-(+)-glucose or L-leucine

BACKGROUND

The objective of this study was to investigate whether the levels of glucose or certain amino acids could regulate the expression of a cell cycle repressor protein p27(Kip1), thereby dictating the risk of cancer in either obesity or caloric/dietary restriction. Previously, we identified and reported four different upstream molecular signaling pathways of p27 expression in human breast cancer cells. We called these four pathways as pathway #1, #2, #3 and #4. We found that 4-hydroxytamoxifen - but not tamoxifen - up-regulated the expression of p27 using pathway #1 which consisted mainly of receptor tyrosine kinases and mTORC1. We now investigate, using 4-hydroxytamoxifen as a reference anti-cancer agents, whether (a) the moderate increase in the concentration of D-(+)-glucose could down-regulate and, conversely, (b) the deficiency of D-(+)-glucose or certain L-amino acids could up-regulate the expression of p27 in these cells using pathway #2 which consists mainly of AMPK and mTORC1.

RESULTS

Using human MDA-MB-231 breast cancer cells in vitro, these hypotheses were tested experimentally by performing p27-luciferase reporter transfection assays and western immunoblot analyses. The results obtained are consistent with these hypotheses. Furthermore, the results indicated that, although 4-hydroxytamoxifen used primarily pathway #1 to down-regulate the phosphorylation of 4E-BP1 and up-regulate the expression of p27, it also secondarily down-regulated the phosphorylation of S6K1. In contrast, the deficiency of D-(+)-glucose or L-leucine used primarily pathway #2 to down-regulate the phosphorylation of S6K1, but they also secondarily down-regulated the phosphorylation of 4E-BP1 and up-regulated the expression of p27. Finally, deficiency of D-(+)-glucose or L-leucine - but not 4-hydroxytamoxifen - up-regulated the expression of mitochondrial ATP5A and SIRT3.

CONCLUSIONS

(a) 4-Hydroxitamoxifen used primarily pathway #1 to up-regulate the expression of p27. (b) Moderate increase in the concentration of D-(+)-glucose used primarily pathway #2 to down-regulate the expression of p27. (c) Deficiency of D-(+)-glucose or L-leucine also used primarily pathway #2 to up-regulate the expression of p27. (d) Deficiency of D-(+)-glucose or L-leucine - but not 4-hydroxytamoxifen - up-regulated the expression of mitochondrial ATP5A in the Complex V of respiratory oxidation-phosphorylation chain and mitochondrial SIRT3. The SIRT3 is one of the seven mammalian anti-aging as well as anti-metabolic sirtuins.

Upstream molecular signaling pathways of p27(Kip1) expression: effects of 4-hydroxytamoxifen, dexamethasone, and retinoic acids

Background

p27(Kip1) is a cyclin-dependent kinase inhibitor that inhibits G1-to-S phase transition of the cell cycle. It is known that a relatively large number of nutritional and chemopreventive anti-cancer agents specifically up-regulate expression of p27 without directly affecting the expression of other G1-to-S phase cell cycle regulatory proteins including p21(Cip1Waf1). However, the upstream molecular signaling pathways of how these agents up-regulate the expression of p27 have not been well characterized. The objective of this study was to identify such pathways in human breast cancer cells in vitro using 4-hydroxytamoxifen, dexamethasone, and various retinoic acids as examples of such anti-cancer agents.

Results

Experimental evidence presented in the first half of this report was obtained by transfecting human breast cancer cells in vitro with proximal upstream region of p27 gene-luciferase reporter plasmids. 1) The evidence indicated that 4-hydroxytamoxifen, dexamethasone, and various retinoic acids up-regulated expression of p27 in both estrogen receptor-positive and negative human breast cancer cells in vitro. 2) The degree of up-regulation of p27 expression by these anti-cancer agents in human breast cancer cells in vitro linearly correlated with the degree of inhibition of methylnitrosourea (MNU)-induced rat mammary adenocarcinoma in vivo. 3) Lastly, up-regulation of the expression of p27 was likely due to the activation of translation initiation rather than transcription of p27 gene. The experimental evidence presented in the second half of this report was obtained by a combination of Western immunoblot analysis and transfection analysis. It indicated that 4-hydroxytamoxifen and dexamethasone up-regulated expression of p27 by down-regulating phosphorylation of eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1) at Ser65 and this phosphorylation was likely to be mediated by upstream receptor tyrosine kinases/phosphoinositide-3-kinase/Akt/5'-AMP-activated protein kinase/mammalian target of rapamycin (RTKs/PI3K/Akt/AMPK/mTOR) protein kinase signaling pathways. Retinoic acids up-regulated expression of p27 without using either 4E-BP1 or RTKs/PI3K/Akt/AMPK/mTOR protein kinase signaling pathways.

Conclusions

4-Hydroxytamoxifen and dexamethasone up-regulated translation initiation of p27 by down-regulating 4E-BP1 phosphorylated at Ser65 and this down-regulation seemed to be mediated by upstream RTKs/PI3K/Akt/AMPK/mTOR protein kinase signaling pathways. Retinoic acids also up-regulated translation initiation of p27, but without using any of these pathways.

Mitogenic regulation of p27(Kip1) gene is mediated by AP-1 transcription factors

The abundance of cyclin-dependent kinase inhibitor p27(Kip1) during the cell cycle determines whether cells will proliferate or become quiescent. Although the post-translational regulation of p27(Kip1) is well established, its transcriptional regulation is poorly understood. Here, we report that mitogenic stimulation of quiescent HEK293 and Huh7 cells showed a rapid decline in the levels of p27(Kip1) transcript by 2.4 +/- 0.1-fold. Inhibition of the p27(Kip1) gene in response to mitogens involved transcriptional down-regulation and required newly synthesized protein(s). Mutation of the AP-1 element at position -469 in the human p27(Kip1) promoter abrogated the effect of mitogens. The recruitment of the AP-1 complex to the p27(Kip1) promoter was confirmed by in vitro DNA binding and chromatin immunoprecipitation studies. Reporter gene analysis combined with enforced expression of Jun/Fos proteins suggested the involvement of Jun/Fos heterodimer in the transrepression process. Both MAPK and phosphatidylinositol 3-kinase signaling pathways appeared to mediate p27(Kip1) transcription. Furthermore, hepatitis B virus X protein-mediated down-regulation of p27(Kip1) in a transgenic environment correlated with an increase in c-Fos levels, reiterating the physiological relevance of AP-1 in the transcriptional regulation of p27(Kip1). Collectively, our studies present the first evidence demonstrating the role of the AP-1 complex in transcriptional down-regulation of the p27(Kip1) gene following mitogenic stimulation.

The PI3K/Akt/FOXO3a/p27Kip1 signaling contributes to anti-inflammatory drug-suppressed proliferation of human osteoblasts

Akt has been reported to suppress p27(Kip1) promoter activity through Forkhead box O (FOXO) in different kinds of cells. Previous studies indicated that anti-inflammatory drugs up-regulated p27(Kip1), and this effect might play an important role in anti-inflammatory drug-induced cell cycle arrest of human osteoblasts (hOBs). In this study, we hypothesized that these drugs might increase p27(Kip1) expression in hOBs by altering the Akt/FOXO signaling. We tested this hypothesis by examining the influences of three anti-inflammatory drugs on the levels and/or activities of Akt, FOXO and p27(Kip1) as well as the relationship between these factors and proliferation of hOBs. We tested the effects of indomethacin (10(-5) and 10(-4)M), celecoxib (10(-6) and 10(-5)M), and dexamethasone (10(-7) and 10(-6)M) using PI3K inhibitor, LY294002 (10(-5)M) as the basis of comparison. The three drugs suppressed the canonical level of phosphorylated Akt in hOBs. This was accompanied by elevated FOXO3a level and increased promoter activity, mRNA expression and protein level of p27(Kip1). Furthermore, the anti-inflammatory drugs suppressed the EGF-induced increases in proliferation, phosphorylation, and nucleus translocation of Akt. Simultaneously, they suppressed EGF-induced decreases of FOXO3a nucleus accumulation and p27(Kip1) mRNA expression. On the other hand, FOXO silencing significantly attenuated the drug-induced up-regulation of p27(Kip1) and suppression of proliferation in hOBs. To the best of our knowledge, this study represents the first to demonstrate that Akt/FOXO3a/p27(Kip1) pathway contributes to suppression of hOB proliferation by anti-inflammatory drugs. We suggest that anti-inflammatory drugs suppress hOB proliferation, at least partly, through inactivating Akt, activating FOXO3a, and eventually up-regulating p27(Kip1) expression.

Bcl-2 blocks 2-methoxyestradiol induced leukemia cell apoptosis by a p27(Kip1)-dependent G1/S cell cycle arrest in conjunction with NF-kappaB activation

2-Methoxyestradiol (2-ME2) induces leukemia cells to undergo apoptosis in association with Bcl-2 inactivation but the mechanisms whereby Bcl-2 contributes to protection against programmed cell death in this context remain unclear. Here we showed that 2-ME2 inhibited the proliferation of Jurkat leukemia cells by markedly suppressing the levels of cyclins D3 and E, E2F1 and p21(Cip1/Waf1) and up-regulating p16(INK4A). Further, 2-ME2 induced apoptosis of Jurkat cells in association with down-regulation and phosphorylation of Bcl-2 (as mediated by JNK), up-regulation of Bak, activation of caspases-9 and -3 and PARP-1 cleavage. To determine the importance and mechanistic role of Bcl-2 in this process, we enforced its expression in Jurkat cells by retroviral transduction. Enforcing Bcl-2 expression in Jurkat cells abolished 2-ME2-induced apoptosis and instead produced a G1/S phase cell cycle arrest in association with markedly increased levels of p27(Kip1). Bcl-2 and p27(Kip1) were localized mainly in the nucleus in these apoptotic resistant cells. Interestingly, NF-kappaB activity and p50 levels were increased by 2-ME2 and suppression of NF-kappaB signaling reduced p27(Kip1) expression and sensitized cells to 2-ME2-induced apoptosis. Importantly, knocking-down p27(Kip1) in Jurkat Bcl-2 cells sensitized them to spontaneous and 2-ME2-induced apoptosis. Thus, Bcl-2 prevented the 2-ME2-induced apoptotic response by orchestrating a p27(Kip1)-dependent G1/S phase arrest in conjunction with activating NF-kappaB. Thus, we achieved a much better understanding of the penetrance and mechanistic complexity of Bcl-2 dependent anti-apoptotic pathways in cancer cells and why Bcl-2 inactivation is so critical for the efficacy of apoptosis and anti-proliferative inducing drugs like 2-ME2.

miR-181a regulates cap-dependent translation of p27(kip1) mRNA in myeloid cells

p27(kip1) (p27) is a cell cycle inhibitor and tumor suppressor whose expression is tightly regulated in the cell. Translational control of p27 mRNA has emerged as a prominent mechanism to regulate p27 expression during differentiation, quiescence, and cancer progression. The microRNAs miR-221 and miR-222 repress p27 expression in various cancer cells, and this repression promotes tumor cell proliferation. In addition, the presence of an internal ribosome entry site in the 5' untranslated region (UTR) of p27 mRNA has been reported. Here, we show that p27 mRNA is translated via a cap-dependent mechanism in HeLa and HL60 cells and that the previously reported IRES activity can be attributed to cryptic promoters in the sequence corresponding to the p27 5' UTR. Furthermore, cap-dependent translation of p27 mRNA is repressed by miR-181a in undifferentiated HL60 cells. Repression by miR-181a is relieved during differentiation of HL60 into monocyte-like cells, allowing the accumulation of p27, which is necessary to fully block cell cycle progression and reach terminal differentiation. These results identify miR-181a as a regulator of p27 mRNA translation during myeloid cell differentiation.

Structure and activity of the internal ribosome entry site within the human p27 Kip1 5'-untranslated region

The cyclin dependent kinase inhibitor p27(Kip1) is a key cell cycle regulatory protein that is often downregulated in cancer cells. The cellular levels of p27(Kip1) are regulated, in part, through translational control mechanisms. The 5'-UTR of the p27(Kip1) mRNA is known to harbor an IRES that may facilitate expression of p27(Kip1) under conditions of stress such as loss of cell adhesion or growth factor and nutrient deprivation. The results presented here further characterize the p27(Kip1) 5'-UTR and its IRES activity. We confirm that the major transcription start site of the p27(Kip1) gene produces an mRNA with a 5'-UTR of approximately 472 nucleotides. Other minor transcripts are also observed but the 472 nucleotide 5'-UTR displays the highest IRES activity. A structural model for the 472 nucleotide 5'-UTR was derived from nuclease digestion patterns coupled with MFOLD secondary structural prediction software. These results indicate that the 5'-UTR has significant secondary structure but also contains a large single-stranded loop that extends from nucleotides -31 to -66 relative to the start codon. Mapping of the ribosome entry window indicates that the ribosome is recruited to this single-stranded loop. The single-stranded loop also includes a U-rich sequence that has previously been shown to bind several proteins, including HuR. This is significant because HuR has previously been shown to inhibit p27(Kip1) IRES activity and cause downregulation of endogenous p27(Kip1) protein levels. Thus HuR may inhibit IRES activity by blocking the ribosome entry site.

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

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

Activation of protein kinase G Increases the expression of p21CIP1, p27KIP1, and histidine triad protein 1 through Sp1

The anticancer role of cyclic guanosine 3',5'-monophosphate (cGMP)-dependent protein kinase G (PKG) has become of considerable interest, but the underlying mechanisms are not fully established. In this study, we examined the effects of activation of PKG on the expression of three tumor suppressor proteins in human SW480 colon cancer cells. Our results revealed that treatment with cell permeable cGMP derivatives, or the cGMP phosphodiesterase inhibitor sulindac sulfone (exisulind, aptosyn, hereafter called exisulind) led to increased expression of the tumor suppressor proteins p21(CIP1), p27(KIP1), and Histidine triad protein 1 (HINT1), and their corresponding mRNAs. Overexpression of PKG Ibeta also caused increased expression of the p21(CIP1), p27(KIP1), and HINT1 proteins. Both the p21(CIP1) and p27(KIP1) promoters contain Sp1 binding sites and they were activated by PKG in luciferase reporter assays. Specific Sp1 sites in the p21 and p27 promoters were sufficient to mediate PKG-induced luciferase reporter activity, suggesting an interaction between Sp1 and PKG. Indeed, we found that PKG can phosphorylate Sp1 on serine residue(s) and this resulted in transcriptional activation of Sp1. Knockdown of Sp1 expression with siRNA inhibited the increased expression of p21(CIP1), p27(KIP1), and HINT1 induced by the cGMP derivative 8-pCPT-cGMP in SW480 cells. These novel effects of PKG activation on the expression of three tumor suppressor genes may explain, at least in part, the anticancer effects of activation of PKG. They also provide a rationale for further developing activators of PKG for the prevention and treatment of cancer.

FoxO3a preferentially induces p27Kip1 expression while impairing muscle precursor cell-cycle progression

Previous work has demonstrated that forkhead transcription factors, which include the FoxO subfamily, play a critical role in muscle atrophy by inducing expression of the atrophy-related ubiquitin ligases. The proliferation of muscle precursor cells (MPC) is also essential for skeletal muscle mass. The hypothesis was tested that the FoxO forkhead transcription factor FoxO3a hinders MPC proliferation. The present studies were designed to determine the effects of overexpression of FoxO3a on in vitro proliferation of MPCs. MPCs infected with an adenovirus for wild-type FoxO3a had decreased DNA synthesis as detected by the incorporation of 5-bromo-2' deoxyuridine. In general, cyclin-dependent kinase inhibitors, including p27(Kip1)and p21(Waf/Cip1), inhibit cell proliferation. Associated with the impaired MPC proliferation, we found an increase in the promoter activity and protein levels of the cyclin-dependent kinase inhibitor p27(Kip1), whereas there was no effect and a decrease in the promoter activity and protein levels of p21(Waf/Cip1). FoxO3a overexpression had no effect on either the phosphorylation of retinoblastoma protein (ser780) or cyclin D1 protein levels, suggesting that FoxO3a does not effect the early phase of the G(1)-S transition. In addition to its ability to induce muscle atrophy, these studies identify FoxO3a as a negative regulator of MPC proliferation. Our findings suggest that attenuating increased FoxO3a may restore MPC proliferation to prevent atrophy and improve the regenerative capacity of skeletal muscle.

L-Maf regulates p27kip1 expression during chick lens fiber differentiation

Organ formation requires spatio-temporal proliferation and differentiation of precursor cells. During lens development, placodal cells in the posterior lens vesicle exit from the cell cycle and enter into the process of differentiation. Cyclin-dependent kinase inhibitors play critical roles in cell cycle exit and promote differentiation in several tissues. We have found that p27kip1 is expressed in the posterior lens cells that undergo differentiation to form the differentiated fiber cells. The transcription factor L-Maf is expressed in these cells earlier than p27kip1. From in ovo gain- or loss-of-function experiments, we have found that L-Maf can, respectively, induce or inhibit the expression of p27kip1 in lens cells. Promoter assays using the 5' upstream sequences of the human p27kip1 gene indicate that L-Maf can activate p27kip1 transcription through the basal regulatory region. We suggest that L-Maf regulates cell cycle exit of the posterior lens cells by activating p27kip1, and thus directs fiber cell differentiation during lens formation in chick.

Rac1-dependent transcriptional up-regulation of p27Kip1 by homophilic cell-cell contact in vascular endothelial cells

The mechanism for the transcriptional up-regulation of p27Kip1 due to the formation of the cell-cell contact was investigated in vascular endothelial cells. The induction of the cell-cell contact by adding an extra number of endothelial cells activated Rac1, up-regulated p27Kip1 mRNA and protein, and also facilitated the cell cycle arrest. Transduction of the Rac1 inhibitor protein using the cell-penetrating peptide or treatment with a Rac1 inhibitor NSC23766 inhibited the p27Kip1 up-regulation and delayed the cell cycle arrest. Rac1 was therefore suggested to mediate the contact-induced transcriptional up-regulation of p27Kip1. The role of Rac1 in the regulation of the p27Kip1 promoter activity was next examined with a luciferase reporter assay. The promoter activity was increased by inducing the cell-cell contact, which was significantly inhibited by the Rac1 inhibitory protein and NSC23766. The evaluation of various truncated promoter regions determined region -620 to -573 nucleotides from the initiation codon to be responsible for the contact-induced, Rac1-dependent activation of the p27Kip1 promoter. The present study thus demonstrated for the first time that the activation of Rac1 due to the cell-cell contact plays a critical role in the transcriptional up-regulation of p27Kip1 in vascular endothelial cells.

Transcription factor AP-2alpha represses both the mucin MUC4 expression and pancreatic cancer cell proliferation

MUC4 is a transmembrane mucin expressed in pancreatic ductal adenocarcinoma (DAC) in contrast to normal pancreas, and is an independent predictor of poor prognosis in patients with invasive DAC. Our aim was therefore to investigate the mechanisms that control MUC4 expression in pancreatic cancer cells. We focused our study on activator protein (AP)-2alpha transcription factor that acts as a tumour suppressor gene in several cancers. In a series of 18 human DAC, using immunohistochemistry, we confirmed that MUC4 was exclusively expressed in cancerous or preneoplastic lesions in 83% of the samples. On the contrary, AP-2 was mainly expressed by non-tumoural ductal cells (61%) or endocrine cells (67%). Moreover, MUC4 and AP-2 were never found co-expressed suggesting an inhibitory role of AP-2alpha in normal ductal cells. In CAPAN-1 and CAPAN-2 cells, transient AP-2alpha over-expression decreased both MUC4 mRNA and apomucin levels by 20-40% by a mechanism involving inhibition of MUC4 promoter. By chromatin immunoprecipitation and gel-shift assays, we demonstrated that this inhibition involved two AP-2 cis-elements located in the -475/-238 region of the promoter. CAPAN-1 clones, which stably over-expressed AP-2alpha, displayed a strong MUC4 down-regulation (-38 to -100%), a significant decrease of both cell proliferation and invasion concomitant to the up-regulation of p27 cyclin-dependent kinase inhibitor. In conclusion, our data provide evidence that AP-2alpha is an important in vivo negative regulator of MUC4 expression in human pancreatic tissue and that AP-2alpha may play a tumour-suppressive role in pancreatic DAC.

Vitamin K2 inhibits the proliferation of HepG2 cells by up-regulating the transcription of p21 gene

AIM

Vitamin K2 has been reported to inhibit the growth of human hepatocellular carcinoma (HCC) in vitro and suppress hepatocarcinogenesis in vivo. However, its inhibitory mechanism has not yet been clarified.

METHODS

Different concentrations of vitamin K2 (30, 10, 1, 0.1 and 0.01 muM) were added to the HCC cell line HepG2 to assess effects on cell growth. The effect of vitamin K2 on cell cycle progression was determined by flow-cytometric analysis. The expression of cell cycle regulatory proteins p21 and p27 was then examined by Western blot. Whether vitamin K2 regulates the gene expression through action on the p21 promoter region was investigated by luciferase assay.

RESULTS

Vitamin K2 inhibited the growth of HepG2 cells dose-dependently, and its inhibitory rate reached approximately 50% at the dose of 30 muM after 96 h treatment. After treatment with vitamin K2, the proportion of cells in G0-G1 phase increased, and in S phase decreased. Apoptotic cells were not detected. The expression of cell cycle regulatory protein p21 was induced by vitamin K2 treatment, but p27 was not. By the luciferase assay, vitamin K2 significantly activated the promoter of p21. Knock-down of p21 by siRNA reversed the growth inhibition of HepG2 cells by vitamin K2.

CONCLUSIONS

The findings suggest that vitamin K2 suppresses the proliferation of HCC cells by blocking the cell cycle G1/S progression through the transcriptional induction of p21.

Id3 is a novel regulator of p27kip1 mRNA in early G1 phase and is required for cell-cycle progression

P27kip is a key inhibitory protein of the cell-cycle progression, which is rapidly downregulated in early G1 phase by a post-translational mechanism involving the proteosomal degradation. In this study, using a wounding model that induces cell-cycle entry of human dermal fibroblasts, we demonstrate that p27mRNA is downregulated when cells progress into the G1 phase, and then it returns to its basal level when cells approach the S phase. By using a quantitative polymerase chain reaction screening we identified inhibitors of differentiation (Id3), a bHLH transcriptional repressor, as a candidate mediator accounting for p27 mRNA decrease. Id3 silencing, using an small interfering RNA approach, reversed the injury mediated p27 downregulation demonstrating that Id3 is involved in the transcriptional repression of p27. Reporter gene experiments and a chromatin immunoprecipitation assay showed that Id3 likely exerts its repressive action through ELK1 inhibition. By inhibiting early p27 downregulation, Id3 depletion blocked (i) the G1-phase progression as assessed by the inhibition of pRb phosphorylation and p130 degradation and (ii) the G1/S transition as observed by the inhibition of cyclin A induction, demonstrating that p27 mRNA decrease is required for cell proliferation. Apart from its effect on the early p27 diminution, Id3 appears also involved in the control of the steady-state level of p27 at the G1/S boundary. In conclusion, this study identifies a novel mechanism of p27 regulation which besides p27 protein degradation also implicates a transcriptional mechanism mediated by Id3.

Functional role of VDR in the activation of p27Kip1 by the VDR/Sp1 complex

Our previous study demonstrate that vitamin D3 induces the binding of vitamin D3 receptor (VDR) to Sp1 transcription factor and stimulates p27Kip1 expression via the Sp1 consensus sequences in the promoter. Both VDR and Sp1 are transcriptional activators, it is unclear which protein functions as the transcription component of the VDR/Sp1 complex. To address this issue, we constructed the AF-2 deletion mutant of VDR and tested the effect of vitamin D3 on p27Kip1 expression. In consistent with our previous results, we found that expression of wild-type VDR in SW620 colon cancer cells, which expressed very low level of endogenous VDR, increased vitamin D3-stimulated p27Kip1 promoter activity and protein expression. On the contrary, expression of AF-2 deletion mutant had little effect. DNA affinity precipitation assay (DAPA) showed that both wild-type and deletion mutant of VDR bound to the DNA probe corresponding to the Sp1 binding site in the p27Kip1 promoter in a vitamin D3-dependent manner indicating deletion of AF-2 domain does not affect the interaction between VDR and Sp1. Chromatin immunoprecipitation (CHIP) assay also confirmed that VDR and its AF-2 deletion mutant bound to p27Kip1 promoter in vivo. We found that deletion of AF-2 domain abolished the interaction of coactivators SRC-1 and DRIP205 with VDR. Taken together, our results suggest that VDR functions as the transactivation component of the VDR/Sp1 complex to trigger gene expression.

Bone morphogenetic protein signaling regulates the size of hair follicles and modulates the expression of cell cycle-associated genes

Bone morphogenetic protein (BMP) signaling is involved in the regulation of a large variety of developmental programs, including those controlling organ sizes. Here, we show that transgenic (TG) mice overexpressing the BMP antagonist noggin (promoter, K5) are characterized by a marked increase in size of anagen hair follicles (HFs) and by the replacement of zig-zag and auchen hairs by awl-like hairs, compared with the age-matched WT controls. Markedly enlarged anagen HFs of TG mice show increased proliferation in the matrix and an increased number of hair cortex and medulla cells compared with WT HFs. Microarray and real-time PCR analyses of the laser-captured hair matrix cells show a strong decrease in expression of Cdk inhibitor p27(Kip1) and increased expression of selected cyclins in TG vs. WT mice. Similar to TG mice, p27(Kip1) knockout mice also show an increased size of anagen HFs associated with increased cell proliferation in the hair bulb. Primary epidermal keratinocytes (KC) from TG mice exhibit significantly increased proliferation and decreased p27(Kip1) expression, compared with WT KC. Alternatively, activation of BMP signaling in HaCaT KC induces growth arrest, stimulates p27(Kip1) expression, and positively regulates p27(Kip1) promoter activity, thus further supporting a role of p27(Kip1) in mediating the effects of BMP signaling on HF size. These data suggest that BMP signaling plays an important role in regulating cell proliferation and controls the size of anagen HFs by modulating the expression of cell-cycle-associated genes in hair matrix KC.

Molecular mechanism of adaphostin-mediated G1 arrest in prostate cancer (PC-3) cells: signaling events mediated by hepatocyte growth factor receptor, c-Met, and p38 MAPK pathways

Adaphostin (NSC680410), a small molecule congener of tyrphostin AG957, has been demonstrated previously to have significant anti-proliferative effects in several leukemia models. However, this effect of adaphostin in adherent cells/solid tumor models has not been examined. In this study, we investigated the anti-proliferative effects of adaphostin in the human prostate cancer cell line PC-3. Specifically, we explored the potential molecular mechanism(s) by which adaphostin elicits its anti-proliferative effect(s). We demonstrate that adaphostin inhibits the proliferation of PC-3 cells by inducing a G(1) phase cell cycle arrest. This adaphostin-induced G(1) arrest was associated with an increase in the expression of p21 and p27 and a decrease in the expression of G(1)-specific cyclins (cyclin A, D1, and D3) and cyclin-dependent kinases 4 and 6. Consequently, a dramatic decrease in the phosphorylation of retinoblastoma protein was also observed. Additionally, we found that adaphostin treatment induced a decrease in the phosphorylation of nucleophosmin, a major nuclear phosphoprotein, and that this decreased phosphorylation was a result of the p21- and p27-mediated inactivation of cyclin E-cyclin-dependent kinase 2 complex kinase activity. Furthermore, we have determined that the adaphostin-mediated cell cycle arrest of PC-3 cells is dependent upon activation of the p38 MAPK. We also demonstrate that the hepatocyte growth factor receptor-c-Met is involved in the adaphostin-mediated signaling events that regulate p38 MAPK. Taken together, these results identify for the first time a signaling cascade of adaphostin-mediated G(1) phase-specific cell cycle arrest in PC-3 cells. These findings suggest that the tyrphostin member has a broader spectrum of activity than originally predicted.

"Nutritional and chemopreventive anti-cancer agents up-regulate expression of p27Kip1, a cyclin-dependent kinase inhibitor, in mouse JB6 epidermal and human MCF7, MDA-MB-321 and AU565 breast cancer cells"

Background

p27(Kip1) is a cyclin-dependent kinase inhibitor. When up-regulated, p27 inhibits G1-to-S phase transition of the cell cycle. This report addresses the question of whether various nutritional and chemopreventive anti-cancer agents up-regulate the expression of p27 in preneoplastic and neoplastic cells.

Results

Experimental evidence presented in the first half of this report shows that these agents fairly faithfully up-regulate expression of p27 in mouse epidermal (JB6) and human breast cancer (MCF7, MDA-MB-321, and AU565) cells. Up-regulation appears to be specific to p27 because expression of cyclin D1, E, and A, and p21Cip1/Waf1 was not modulated by these agents. Up-regulation of the expression of p27 is likely due to the activation of translation rather than transcription of p27 because (a) up-regulation is mediated by the 5'-untranslated region (-575) of the p27 gene and (b) the antibiotic actinomycin D, an inhibitor of transcription, did not attenuate the up-regulation of p27. This latter finding is likely to preclude the existence of cryptic transcription factor binding site(s) in the 5'-untranslated region of p27 gene. The experimental evidence, presented in the second half of this report, was obtained using the 5'-untranslated region (-575) of p27 gene. The evidence suggests that cancer preventive agents up-regulate expression of p27 by at least four different molecular signaling pathways: (a) Caloric restriction is likely to up-regulate p27 expression via 5'-AMP-activated protein kinase (AMPK; a metabolic energy sensor or cellular fuel gauge), tuberous sclerosis complex (TSC), and mammalian target of rapamycin (mTOR). Amino acid deficiencies also up-regulate the expression of p27 using some components of this pathway. (b) 4-Hydroxytamoxifen (but not tamoxifen), genistein (but not genistin), daidzein, and probably other nutritional and chemopreventive anti-cancer agents could up-regulate expression of p27 via receptor protein tyrosine kinases (RPTKs), phosphoinositide 3-kinase (PI3K), phosphoinosite-dependent kinase (PDK), Akt/PKB and mTOR. (c) Expression of p27 could also be up-regulated via RPTKs followed by MAPKs – MEK, ERK and p38MAPK – and probably MNK. Finally, (d) global hypomethylation of 5'-m⁷G cap of mRNAs could also up-regulate expression of p27.

Conclusion

Based on these findings, we conclude that various nutritional and chemopreventive anti-cancer agents up-regulate expression of p27 in (pre)neoplastic cells.

A novel function of OLIG2 to suppress human glial tumor cell growth via p27Kip1 transactivation

The basic helix-loop-helix transcription factor OLIG2 is specifically expressed in cells of the oligodendrocyte lineage. It is also expressed in various tumors originating from glial cells; however, the expression of OLIG2 is rare or weak in glioblastomas, the most malignant gliomas. The role of OLIG2 in glioma remains unclear. To investigate the function of OLIG2 in glial tumor cells, we have established a glioblastoma cell line, U12-1, in which the expression of OLIG2 is induced by the Tet-off system. Induction of OLIG2 resulted in suppression of both the proliferation and anchorage-independent growth of U12-1. It also resulted in an increase in the expression of p27Kip1. A luciferase assay revealed that the CTF site of the p27Kip1 gene promoter was essential for OLIG2-dependent activation of p27Kip1 gene transcription. Electrophoretic mobility shift assays confirmed that a nuclear extract of OLIG2-expressing U12-1 cells contained a protein complex that binds to the CTF site of the p27Kip1 gene promoter. Furthermore, siRNA against p27Kip1 rescued the OLIG2-mediated growth and DNA synthesis inhibition of U12-1 cells. These results indicate that OLIG2 suppresses the proliferation of U12-1 and that this effect is mediated by transactivation of the p27Kip1 gene, and low expression of OLIG2 may be related to the malignant behavior of human glioblastoma.

Ciglitazone-induced p27 gene transcriptional activity is mediated through Sp1 and is negatively regulated by the MAPK signaling pathway

We have previously demonstrated that the PPARgamma ligand, ciglitazone, increases p27kip1 protein levels in HT-29 colon cancer cells through both inhibition of proteasome associated degradation and activation of transcriptional activity. [F. Chen, L.E. Harrison, Cell Signal. 17 (2005) 809] The purpose of this investigation was to further elucidate the mechanism of ciglitazone-induced activation of p27 gene transcription. We observed that the region -774/-462 of the p27 promoter plays a key role in ciglitazone-induced gene transcriptional activity and this region contains two Sp1 binding sites. When the p27PF-luc reporter was co-transfected with Sp1 expression plasmids, ciglitazone-induced p27PF-luc activity significantly increased, while mithramycin A, a Sp1 inhibitor, was able to abrogate its effects. Ciglitazone exposure increased both Sp1 protein expression and Sp1-DNA binding, which was also associated with a decrease of Erk1/2 phosphorylation. A similar increase of Sp1-DNA binding was observed when phosphorylation of Erk1/2 was inhibited by pretreatment with the MAP kinase inhibitor, U0126. In addition, a significant increase of p27PF-luc reporter luciferase activity was noted after MAP kinase inhibition, which could be abolished with co-treatment with mithramycin A. Based on these data, we postulate that ciglitazone induces p27 gene transcription through increased Sp1 binding to its promoter region, which in turn is mediated through increased Sp1 protein levels and decreased inhibitory regulation by the MAP kinase pathway.

The MLL fusion gene, MLL-AF4, regulates cyclin-dependent kinase inhibitor CDKN1B (p27kip1) expression

MLL, involved in many chromosomal translocations associated with acute myeloid and lymphoid leukemia, has >50 known partner genes with which it is able to form in-frame fusions. Characterizing important downstream target genes of MLL and of MLL fusion proteins may provide rational therapeutic strategies for the treatment of MLL-associated leukemia. We explored downstream target genes of the most prevalent MLL fusion protein, MLL-AF4. To this end, we developed inducible MLL-AF4 fusion cell lines in different backgrounds. Overexpression of MLL-AF4 does not lead to increased proliferation in either cell line, but rather, cell growth was slowed compared with similar cell lines inducibly expressing truncated MLL. We found that in the MLL-AF4-induced cell lines, the expression of the cyclin-dependent kinase inhibitor gene CDKN1B was dramatically changed at both the RNA and protein (p27kip1) levels. In contrast, the expression levels of CDKN1A (p21) and CDKN2A (p16) were unchanged. To explore whether CDKN1B might be a direct target of MLL and of MLL-AF4, we used chromatin immunoprecipitation (ChIP) assays and luciferase reporter gene assays. MLL-AF4 binds to the CDKN1B promoter in vivo and regulates CDKN1B promoter activity. Further, we confirmed CDKN1B promoter binding by ChIP in MLL-AF4 as well as in MLL-AF9 leukemia cell lines. Our results suggest that CDKN1B is a downstream target of MLL and of MLL-AF4, and that, depending on the background cell type, MLL-AF4 inhibits or activates CDKN1B expression. This finding may have implications in terms of leukemia stem cell resistance to chemotherapy in MLL-AF4 leukemias.

Galectin-1 interacts with the {alpha}5{beta}1 fibronectin receptor to restrict carcinoma cell growth via induction of p21 and p27

Surface binding of galectin family members has the potential to link distinct glycan structures to growth regulation. Therefore, we addressed the antiproliferative potential of galectin-1 (Gal-1) in a panel of carcinoma cell lines. We discovered growth inhibition by Gal-1 in epithelial tumor cell lines from different origins and provide evidence that this effect requires functional interaction with the alpha5beta1 integrin. Antiproliferative effects result from inhibition of the Ras-MEK-ERK pathway and consecutive transcriptional induction of p27. We have further identified two Sp1-binding sites in the p27 promoter as crucial for Gal-1 responsiveness. Inhibition of the Ras-MEK-ERK cascade by Gal-1 increased Sp1 transactivation and DNA binding due to reduced threonine phosphorylation of Sp1. Furthermore, Gal-1 induced p21 transcription and selectively increased p27 protein stability. Gal-1-mediated accumulation of p27 and p21 inhibited cyclin-dependent kinase 2 activity and ultimately resulted in G(1) cell cycle arrest and growth inhibition. These data define a novel mechanism whereby Gal-1 regulates epithelial tumor cell homeostasis via carbohydrate-dependent interaction with the alpha5beta1 integrin.

Transcription factor KLF7 is important for neuronal morphogenesis in selected regions of the nervous system

The Krüppel-like transcription factors (KLFs) are important regulators of cell proliferation and differentiation in several different organ systems. The mouse Klf7 gene is strongly active in postmitotic neuroblasts of the developing nervous system, and the corresponding protein stimulates transcription of the cyclin-dependent kinase inhibitor p21waf/cip gene. Here we report that loss of KLF7 activity in mice leads to neonatal lethality and a complex phenotype which is associated with deficits in neurite outgrowth and axonal misprojection at selected anatomical locations of the nervous system. Affected axon pathways include those of the olfactory and visual systems, the cerebral cortex, and the hippocampus. In situ hybridizations and immunoblots correlated loss of KLF7 activity in the olfactory epithelium with significant downregulation of the p21waf/cip and p27kip1 genes. Cotransfection experiments extended the last finding by documenting KLF7's ability to transactivate a reporter gene construct driven by the proximal promoter of p27kip1. Consistent with emerging evidence for a role of Cip/Kip proteins in cytoskeletal dynamics, we also documented p21waf/cip and p27kip1 accumulation in the cytoplasm of differentiating olfactory sensory neurons. KLF7 activity might therefore control neuronal morphogenesis in part by optimizing the levels of molecules that promote axon outgrowth.

Hes1 directly controls cell proliferation through the transcriptional repression of p27Kip1

A transcriptional regulator, Hes1, plays crucial roles in the control of differentiation and proliferation of neuronal, endocrine, and T-lymphocyte progenitors during development. Mechanisms for the regulation of cell proliferation by Hes1, however, remain to be verified. In embryonic carcinoma cells, endogenous Hes1 expression was repressed by retinoic acid in concord with enhanced p27(Kip1) expression and cell cycle arrest. Conversely, conditional expression of a moderate but not maximal level of Hes1 in HeLa cells by a tetracycline-inducible system resulted in reduced p27(Kip1) expression, which was attributed to decreased basal transcript rather than enhanced proteasomal degradation, with concomitant increases in the growth rate and saturation density. Hes1 induction repressed the promoter activity of a 5' flanking basal enhancer region of p27(Kip1) gene in a manner dependent on Hes1 expression levels, and this was mediated by its binding to class C sites in the promoter region. Finally, hypoplastic fetal thymi, as well as livers and brains of Hes1-deficient mice, showed significantly increased p27(Kip1) transcripts compared with those of control littermates. These results have suggested that Hes1 directly contributes to the promotion of progenitor cell proliferation through transcriptional repression of a cyclin-dependent kinase inhibitor, p27(Kip1).

Role of Sp proteins in regulation of vascular endothelial growth factor expression and proliferation of pancreatic cancer cells

Sp proteins play an important role in angiogenesis and growth of cancer cells, and specificity protein 1 (Sp1) has been linked to vascular endothelial growth factor (VEGF) expression in pancreatic cancer cells. RNA interference was used to investigate the role of Sp family proteins on regulation of VEGF expression and proliferation of Panc-1 pancreatic cancer cells. Using a series of constructs containing VEGF promoter inserts, it was initially shown that Sp1 and Sp3 were required for transactivation, and this was primarily dependent on proximal GC-rich motifs. We also showed that Sp4 was expressed in Panc-1 cells, and RNA interference assays suggested that Sp4 cooperatively interacted with Sp1 and Sp3 to activate VEGF promoter constructs in these cells. However, the relative contributions of Sp proteins to VEGF expression were variable among different pancreatic cancer cell lines. Small inhibitory RNAs for Sp3, but not Sp1 or Sp4, inhibited phosphorylation of retinoblastoma protein, blocked G0/G1-->S-phase progression, and up-regulated p27 protein/promoter activity of Panc-1 cells; similar results were observed in other pancreatic cancer cells, suggesting that Sp3-dependent growth of pancreatic cancer cells is caused by inhibition of p27 expression.

The prognostic and therapeutic relevance of p27kip1 in Ewing's family tumors

PURPOSE

Ewing's family tumors (EFTs) display the characteristic fusion gene EWS-Fli1. We have reported EWS-Fli1 may promote the cell cycle progression accompanied by the suppression of the expression of cyclin-dependent kinase inhibitor p27(kip1) in EFT cells. Here, we describe the prognostic and therapeutic relevance of p27 in EFTs.

EXPERIMENTAL DESIGN

We examined tumor samples taken from 21 patients with primary EFTs for the expression of p27 protein immunohistochemically and evaluated its correlation with clinical outcome. We also investigated the usefulness of p27 as a therapeutic strategy in vitro and in vivo using p27 expression adenovirus. Finally, we examined the process of EWS-Fli1-mediated reduction of p27 expression.

RESULTS

Immunohistochemical analysis showed that a low expression level of p27 protein was related to poor event-free survival in an univariate analysis and that the expression level of p27 correlated more significantly with patient survival than several clinical factors in a multivariate survival analysis. Overexpression of p27 with the adenoviral vector remarkably inhibited the cell growth in all EFT cells tested and further induced apoptosis in the wild-type p53 EFT cells. In vivo studies demonstrated a reduction in tumor growth of EFT xenograft in nude mice treated with the intratumoral injection of p27-expressing adenovirus. EWS-Fli1 did not significantly affect the p27 promoter activity and p27 mRNA levels. However, the challenge of the proteasome inhibitor caused accumulation of p27 protein in EFT cells. These data strongly suggest EWS-Fli1 might attenuate p27 protein level via activation of the proteasome-mediated degradation pathway.

CONCLUSIONS

Our findings provide the first evidence of the prognostic relevance of p27 expression in EFTs. We propose p27 as a novel and powerful therapeutic factor for the molecular target therapy of EFTs.

Vitamin D3 receptor/Sp1 complex is required for the induction of p27Kip1 expression by vitamin D3

1alpha,25-dihydroxyvitamin D3 (vitamin D3) has been shown to upregulate p27Kip1 expression via Sp1 and NF-Y binding sites in the p27Kip1 promoter. However, whether vitamin D3 receptor (VDR) involves in this process is unclear. In this study, we demonstrated that expression of VDR in SW620 cells, which exhibited low level of endogenous VDR, increased vitamin D3-stimulated p27Kip1 promoter activity. On the contrary, suppression of Sp1 expression by small interference RNA reduced the stimulation of p27Kip1 promoter activity by vitamin D3 in LNCaP cells. DNA affinity precipitation assay and chromatin immunoprecipitation assay showed that VDR bound to the p27Kip1 promoter in vitro and in vivo. In addition, we also demonstrated that VDR interacted with Sp1 in vitro and in cells. Collectively, our results suggest that VDR is involved in the induction of p27Kip1 by vitamin D3 and may interact with Sp1 to modulate the expression of target genes that lack VDR response element (VDRE) in their promoters.

Beta-hydroxybutyrate-induced growth inhibition and collagen production in HK-2 cells are dependent on TGF-beta and Smad3

BACKGROUND

Ketonuria is common in diabetes. The major form of ketone body is beta-hydroxybutyrate (beta-HB), which is metabolized by the proximal tubule. Transforming growth factor beta (TGF-beta) and tubulopathy are important in diabetic nephropathy. Thus, the role of TGF-beta and the downstream Smad3 in beta-HB-induced effects in the human proximal tubule (HK-2 cell) was studied.

METHODS

Effects of beta-HB (0.1 to 10 mmol/L) on HK-2 cells were determined for: proliferation, cell cycle distribution, collagen production, tubular transdifferentiation [expression of alpha-smooth muscle actin (alpha-SMA) protein], TGF-beta, Smad2/3, p21WAF1, and p27kip1.

RESULTS

Beta-HB (0.1 to 10 mmol/L) dose dependently decreased proliferation, arrested the cells in G0/G1 phase of the cell cycle, and increased p21WAF1/p27kip1 protein expression at 48 hours (without affecting p21WAF1/p27kip1 mRNA and transcription). beta-HB (1 mmol/L) increased p21WAF1/p27kip1 protein half-lives. Beta-HB (1 mmol/L) increased TGF-beta transcription at 24 hours and TGF-beta1 mRNA/bioactivity at 48 hours. Beta-HB (1 mmol/L) increased nuclear Smad2/3 protein expression and increased collagen production (without affecting tubular transdifferentiation), which were reversed by Smad7, dominant-negative Smad3, and N-acetylcysteine. Dominant-negative Smad3 reversed beta-HB-induced TGF-beta transcription at 24 hours, and reversed TGF-beta1 bioactivity at 48 hours. Dominant-negative Smad3 reversed beta-HB-induced p21WAF1/p27kip1 protein expression at 48 hours. Finally, N-acetylcysteine, TGF-beta antibody, Smad7, and dominant-negative Smad3 reversed beta-HB (1 mmol/L)-induced growth inhibition at 48 hours.

CONCLUSION

Beta-HB activated Smad 2/3 by oxidative stress. TGF-beta and Smad3 mediate beta-HB-induced cell cycle-dependent growth inhibition while Smad3 mediate beta-HB-induced collagen production and p21WAF1/p27kip1 protein expression in HK-2 cells. Moreover, beta-HB increased p21WAF1/p27kip1 protein expression by increasing p21WAF1/p27kip1 protein stability.

Trichostatin A activates the osteopontin gene promoter through AP1 site

In this study, we investigated osteoblastic differentiation by trichostatin A (TSA), a histone deacetylase inhibitor in mouse undifferentiated mesenchymal cell line. TSA increased the osteopontin (OPN) mRNA level and OPN protein. Deletion analysis of the promoter region revealed TSA-induced luciferase response was regulated by -75 to -65 of the OPN promoter. There was an AP1-binding sequence at the site of the OPN promoter. In an electrophoretic mobility shift assay, bands of the complexes were supershifted by addition of antibody to c-fos and phosphorylated c-jun. These data suggested that AP1 plays a crucial role in the TSA-induced OPN expression.

Forkhead transcription factor FoxO1 transduces insulin-like growth factor's signal to p27Kip1 in primary skeletal muscle satellite cells

The insulin-like growth factor I (IGF-I) stimulates muscle satellite cell proliferation. Chakravarthy et al., (2000, J Biol Chem 275:35942-35952.) previously found that IGF-I-stimulated proliferation of primary satellite cells was associated with the activation of phosphatidylinositol 3'-kinase (PI3K)/Akt and the downregulation of a cell-cycle inhibitor p27Kip1. To understand mechanisms by which IGF-I signals the downregulation of p27Kip1 in rat skeletal satellite cells, the role of Forkhead transcription factor FoxO1 in transcriptional activity of p27Kip1 was examined. When primary rat satellite cells were transfected with a p27Kip1 promoter-reporter gene construct, IGF-I (100 ng/ml) inhibited specific p27Kip1 promoter activity. Addition of LY294002, an inhibitor of PI3K, reversed the IGF-I-mediated downregulation of p27Kip1 promoter activity. Co-transfection of wild type (WT) FoxO1 into satellite cells increased p27Kip1 promoter activity in the absence of IGF-I supplementation. Addition of IGF-I reversed the induction of p27Kip1 promoter activity by WT FoxO1. When a mutated FoxO1 (without Thr24, Ser256, and Ser316 Akt phosphorylation sites) was used, IGF-I was no longer able to reverse the FoxO1 induced stimulation of p27Kip1 promoter activity that had been seen when WT FoxO1 was present. When the satellite cells were treated with IGF-I, phosphorylation of Akt-Ser473 and FoxO1-Ser256 was increased. In addition, when the cells were pre-incubated with LY294002 before IGF-I stimulation, the phosphorylation of Akt-Ser473 and FoxO1-Ser256 was inhibited, implying that phosphorylation of Akt and FoxO1 was downstream of IGF-I-induced PI3K signaling. However, IGF-I did not induce phosphorylation of FoxO1 on residues Thr24 and Ser316. These results suggested that IGF-I induced the phosphorylation of Ser256 and inactivated FoxO1 thereby downregulating the activation of the p27Kip1 promoter. Thus, inactivation of FoxO1 by IGF-I plays a critical role in rat skeletal satellite cell proliferation through regulation of p27Kip1 expression.

Involvement of PKA and Sp1 in the induction of p27(Kip1) by tamoxifen

We have previously shown that tamoxifen (Tam) inhibits proliferation of estrogen receptor-negative human non-small cell lung cancer cells and this inhibition is associated with induction of p27(Kip1). In this study, we investigated the mechanism by which Tam increases p27(Kip1) expression. Because intracellular p27(Kip1) protein level is mainly controlled via posttranslational regulation, we first tested whether Tam might affect protein stability of p27(Kip1). Metabolic labeling and pulse chase assays showed that Tam did not affect the half-life of this protein. We next examined whether Tam enhanced p27(Kip1) expression through transcriptional activation. Our results demonstrated that Tam directly stimulated the p27(Kip1) promoter in lung cancer cells. Deletion and mutation analysis revealed that two Sp1 consensus sites located between -545 and -532bp from the transcription start site were crucial for the induction of p27(Kip1) by Tam. Conversely, mutation in a CTF site (-525/-520) nearby these two Sp1 sites had little effect. Electromobility shift assays showed that Sp1 transcription factor bound to these consensus sites and the DNA binding activity of Sp1 was enhanced by Tam. Our data also demonstrated that induction of p27(Kip1) by Tam was inhibited by protein kinase A inhibitor H89, but not by protein kinase C inhibitor calphostin C and mitogen-activated kinase kinase inhibitor PD98059. Taken together, our results suggest that Tam transcriptionally activates p27(Kip1) expression via the Sp1 consensus sites in the p27(Kip1) promoter and PKA is involved in this process.

Induction of G1 phase arrest in MCF human breast cancer cells by pentagalloylglucose through the down-regulation of CDK4 and CDK2 activities and up-regulation of the CDK inhibitors p27(Kip) and p21(Cip)

Pentagalloylglucose (5GG) is a potent and specific inhibitor of NADPH dehydrogenase or xanthine oxidase. In our previous study, we showed that 5GG was able to induce apoptosis in HL-60 cells in a time- and concentration-dependent manner via the activation of caspase-3. Recently, we found that 5GG was capable of perturbing the cell cycle of the human breast cancer cell line MCF-7. DNA flow cytometric analysis showed that 5GG exhibited the ability of blocking MCF-7 cell cycle progression at the G1 phase. The level of several G1 phase-related cyclins and cyclin-dependent kinases did not change in these cells during a 24-hr exposure to 5GG. However, the activity of cyclin E/CDK2 was decreased in a concentration- and time-dependent manner and the activity of cyclin D/CDK4 was inhibited when serum-starved synchronized cells were released from synchronization. p27(Kip) and p21(Cip), inhibitors of cyclin/CDK complexes in G1-phase, were gradually increased after 5GG treatment in a time-dependent manner and the induction of p21(Cip) was correlated with an increase in p53 levels. These results suggest that the suppression of cell-cycle progression in the G1 phase by 5GG was mediated in MCF-7 cells, at least in part, by either the inhibition of cyclin D/CDK4 and cyclin E/CDK2 activity or the induction of the CDK inhibitors p27(Kip) and p21(Cip).

ELAV/Hu proteins inhibit p27 translation via an IRES element in the p27 5'UTR

p27Kip1 restrains cell proliferation by binding to and inhibiting cyclin-dependent kinases. To investigate the mechanisms of p27 translational regulation, we isolated a complete p27 cDNA and identified an internal ribosomal entry site (IRES) located in its 5'UTR. The IRES allows for efficient p27 translation under conditions where cap-dependent translation is reduced. Searching for possible regulators of IRES activity we have identified the neuronal ELAV protein HuD as a specific binding factor of the p27 5'UTR. Increased expression of HuD or the ubiquitously expressed HuR protein specifically inhibits p27 translation and p27 IRES activity. Consistent with an inhibitory role of Hu proteins in p27 translation, siRNA mediated knockdown of HuR induced endogenous p27 protein levels as well as IRES-mediated reporter translation and leads to cell cycle arrest in G1.

BRCA1 transactivates the cyclin-dependent kinase inhibitor p27(Kip1)

The p27(Kip1) is a member of the universal cyclin-dependent kinase inhibitor family. Previously, immunochemical analysis of a series of breast cancer cell lines demonstrated a correlation between the expression of p27(Kip1) and the breast cancer susceptibility gene BRCA1. BRCA1 has a number of activities including DNA repair, growth inhibition and as a transcription factor. Here we demonstrate that BRCA1 transactivates expression of p27(Kip1). This transactivation is dependent on the presence of a functional C-terminal transactivation domain. Promoter-deletion analysis identified the presence of a putative BRCA1-responsive element located at position -615 to -511 of the p27(Kip1) promoter. These results suggest that the transcriptional regulation of p27(Kip1) by BRCA1 may be a mechanism for BRCA1- induced growth inhibition.

Deregulated expression of the homeobox gene Cux-1 in transgenic mice results in downregulation of p27(kip1) expression during nephrogenesis, glomerular abnormalities, and multiorgan hyperplasia

Cux-1 is a murine homeobox gene that is highly expressed in the developing kidney with expression restricted to the nephrogenic zone. Cux-1 is highly expressed in cyst epithelium of polycystic kidneys from C57BL/6J-cpk/cpk mice, but not in kidneys isolated from age-matched phenotypically normal littermates. To further elucidate the role of Cux-1 in renal development, we generated transgenic mice expressing Cux-1 under the control of the CMV immediate early gene promoter. Mice constitutively expressing Cux-1 developed multiorgan hyperplasia and organomegaly, but not an overall increase in body size. Transgenic kidneys were enlarged 50% by 6 weeks of age, with the increased growth primarily restricted to the cortex. Proliferating cells were found in proximal and distal tubule epithelium throughout the cortex, and the squamous epithelium that normally lines Bowman's capsule was replaced with proximal tubule epithelium. However, the total number of nephrons was not increased. In the developing kidneys of transgenic mice, Cux-1 was ectopically expressed in more highly differentiated tubules and glomeruli, and this was associated with reduced expression of the cyclin kinase inhibitor, p27. Transient transfection experiments revealed that Cux-1 is an inhibitor of p27 promoter activity. These results suggest that Cux-1 regulates cell proliferation during early nephrogenesis by inhibiting expression of p27.

Transcriptional up-regulation of p27(Kip1) during contact-induced growth arrest in vascular endothelial cells

By plating porcine aortic endothelial cells at two different densities and thereby inducing two different time courses of contact-induced growth arrest, the temporal correlation between p27(Kip1) expression and cell cycle progression was investigated. When the quiescent cells were replated, they synchronously entered S phase with a peak at 20 h in both cases, while the cells plated at 25 and 80% of confluent densities exited the cell cycle by 96 and 48 h, respectively. Nuclear p27(Kip1) disappeared when the cells reentered the cell cycle and then recovered when the cells exited the cell cycle. The change in p27(Kip1) was associated with a concomitant change in Kip1 mRNA. The p27(Kip1) degradation activity did not increase in the cells reentering the cell cycle, nor did it decrease in the cells exiting the cell cycle. The Kip1 mRNA stability decreased in the growing cells and increased in the cells exiting the cell cycle and at confluence. A nuclear run-on assay revealed a close correlation between the Kip1 transcriptional activity and the level of Kip1 mRNA. We conclude that the cell-cell contact up-regulated the Kip1 gene transcription and increased the Kip1 mRNA stability, which was related to the recovery of p27(Kip1) protein during contact-induced growth arrest in endothelial cells.