Epigenetic control of a VDR-governed feed-forward loop that regulates p21(waf1/cip1) expression and function in non-malignant prostate cells

In non-malignant RWPE-1 prostate epithelial cells signaling by the nuclear receptor Vitamin D Receptor (VDR, NR1I1) induces cell cycle arrest through targets including CDKN1A (encodes p21((waf1/cip1))). VDR dynamically induced individual histone modification patterns at three VDR binding sites (R1, 2, 3) on the CDKN1A promoter. The magnitude of these modifications was specific to each phase of the cell cycle. For example, H3K9ac enrichment occurred rapidly only at R2, whereas parallel accumulation of H3K27me3 occurred at R1; these events were significantly enriched in G(1) and S phase cells, respectively. The epigenetic events appeared to allow VDR actions to combine with p53 to enhance p21((waf1/cip1)) activation further. In parallel, VDR binding to the MCM7 gene induced H3K9ac enrichment associated with rapid mRNA up-regulation to generate miR-106b and consequently regulate p21((waf1/cip1)) expression. We conclude that VDR binding site- and promoter-specific patterns of histone modifications combine with miRNA co-regulation to form a VDR-regulated feed-forward loop to control p21((waf1/cip1)) expression and cell cycle arrest. Dissection of this feed-forward loop in a non-malignant prostate cell system illuminates mechanisms of sensitivity and therefore possible resistance in prostate and other VDR responsive cancers.     

G1阻滞中p21-E2F间的相互作用

前列腺素A2(PGA2)具有强的体内、外抗增殖活性,引起细胞周期阻滞,同时,可诱导cdk抑制物p21蛋白的表达,后者亦可介导多种细胞的G1阻滞.提示p21^waf1/cip1在PGA2诱导的细胞周期阻滞中具有重要作用.主要介绍了近两年来有关p21^waf1/cip1与转录因子E2F间的相互作用的研究,阐述p21^waf1/cip1在PGA2介导的细胞周期阻滞中的作用机制.     

A Multistep Model for Paclitaxel-Induced Apoptosis in Human Breast Cancer Cell Lines

Despite extensive previous investigation, the events occurring between paclitaxel-induced mitotic arrest and the subsequent onset of apoptosis remain incompletely understood. In the present study, the sequential morphological and biochemical changes that occur after paclitaxel treatment were examined in MDA-MB-468 (p53 mutant) and MCF-7 (p53 wild-type) breast cancer cells. Flow cytometry indicated that paclitaxel induces tetraploidy that persists until the onset of apoptosis in both cell lines. Light and electron microscopy indicated that the cells transiently arrest in mitosis and then enter a multinucleated interphase state characterized by the absence of punctate staining for CENP-F, a G(2) marker, but the presence of cyclin E, a G(1) cyclin, and p21(waf1/cip1), a cyclin-dependent kinase inhibitor. Despite high p21(waf1/cip1) levels, paclitaxel-treated cells incorporated thymidine into DNA. Aphidicolin inhibited this DNA synthesis but not the subsequent onset of apoptosis. Conversely, the broad-spectrum caspase inhibitor benzyloxycarbonyl-val-ala-asp(OMe)-fluoromethylketone inhibited apoptosis and enhanced the number of multinucleated cells but did not facilitate generation of octaploid cells. These results are consistent with a multistep model in which breast cancer cells exposed to paclitaxel undergo an aberrant mitotic exit; proceed through a tetraploid, multinucleated G(1) state; initiate an aphidicolin-suppressible process of DNA repair; and subsequently undergo apoptosis.     

The antiproliferative activity of aloe-emodin is through p53-dependent and p21-dependent apoptotic pathway in human hepatoma cell lines

The aim of this study is to investigate the anticancer effect of aloe-emodin in two human liver cancer cell lines, Hep G2 and Hep 3B. We observed that aloe-emodin inhibited cell proliferation and induced apoptosis in both examined cell lines, but with different the antiproliferative mechanisms. In Hep G2 cells, aloe-emodin induced p53 expression and was accompanied by induction of p21 expression that was associated with a cell cycle arrest in G1 phase. In addition, aloe-emodin had a marked increase in Fas/APO1 receptor and Bax expression. In contrast, with p53-deficient Hep 3B cells, the inhibition of cell proliferation of aloe-emodin was mediated through a p21-dependent manner that did not cause cell cycle arrest or increase the level of Fas/APO1 receptor, but rather promoted aloe-emodin induced apoptosis by enhancing expression of Bax. These findings suggest that aloe-emodin may be useful in liver cancer prevention.     

Apoptosis and cell recovery in response to oxidative stress in p53-deficient prostate carcinoma cells

We have studied the effects of different concentrations of H(2)O(2) on the proliferation of PC-3 prostate carcinoma cells. Since this cell line lacks functional p53, we sought to characterize whether apoptotic response to the oxidative insult was altered such that, unlike in cells containing functional p53 apoptosis may be reduced and replaced by other mechanisms of cellular arrest and death. We did not observe necrosis in PC-3 cells treated with H(2)O(2) concentrations of up to 500 microM. In the presence of 50 microM H(2)O(2), arrest was observed in the G2-phase of the cell cycle, along with p53-independent apoptosis. In the presence of 500 microM H(2)O(2), addition of l-buthionine sulfoximine increased the percentage of apoptotic cell death. Senescence-associated cell arrest was never observed. Moreover, some of the treated cells seemed to be resistant to oxidative damage. These cells re-entered the cell cycle and proliferated normally. Analysis of the expression of p21(waf1) and of p21 protein levels, as well as the activity of caspase-3 and caspase-8, allowed us to characterize some aspects of the arrest of PC-3 cells in G2 and the apoptotic response to oxidative stress in the absence of functional p53.     

Bacterial cyclomodulin Cif blocks the host cell cycle by stabilizing the cyclin-dependent kinase inhibitors p21 and p27

The cycle inhibiting factor (Cif) is a cyclomodulin produced by enteropathogenic and enterohemorrhagic Escherichia coli. Upon injection into the host cell by the bacterial type III secretion system, Cif inhibits the G2/M transition via sustained inhibition of the mitosis inducer CDK1 independently of the DNA damage response. In this study, we show that Cif induces not only G2, but also G1 cell cycle arrest depending on the stage of cells in the cell cycle during the infection. In various cell lines including differentiated and untransformed enterocytes, the cell cycle arrests are correlated with the accumulation of the cyclin-dependent kinase inhibitors p21(waf1/cip1) and p27(kip1). Cif-induced cyclin-dependent kinase inhibitor accumulation is independent of the p53 pathway but occurs through inhibition of their proteasome-mediated degradation. Our results provide a direct link between the mode of action of Cif and the host cell cycle control.     

Theaflavins induce G2/M arrest by modulating expression of p21waf1/cip1, cdc25C and cyclin B in human prostate carcinoma PC-3 cells

Cancer of the prostate gland (PCA) is the most common invasive malignancy and is the second leading cause of cancer-related death in males. The polyphenolic constituents of black tea have gained considerable attention as chemopreventive agents. Many studies have shown that black tea reduces the risk of several cancer types. In the present study, we studied the effect of a black tea polyphenol, theaflavin (TF), on cellular proliferation and cell death in the human prostate cancer cell line, PC-3. We showed that TF inhibits cell proliferation in a dose- and time-dependent manner. Studies on cell cycle progression have shown that the anti-proliferative effect of TF is associated with an increase in the G2/M phase of PC-3 cells. Western blot results showed that TF-induced G2/M phase arrest was mediated through the inhibition of cyclin-regulated signaling pathways. TF induces cyclin kinase inhibitor p21(waf1/cip1) expression and inhibits cdc25C and cyclin B expression. Increased exposure time to TF caused apoptosis of PC-3 cells, which was associated with up-regulation of the pro-apoptotic proteins Bax, caspase-3 and caspase-9 and down-regulation of anti-apoptotic protein Bcl-2. The role of caspase-induced apoptosis was further confirmed by a reduction in mitochondria membrane potential and the appearance of a DNA laddering pattern. Thus, it can be concluded that TF acts as an effective anti-proliferative agent by modulating cell growth regulators in prostate cancer cells.     

Linkage of Curcumin-Induced Cell Cycle Arrest and Apoptosis by Cyclin-Dependent Kinase Inhibitor p21/WAF1/CIP1

We have recently shown that curcumin induces apoptosis in prostate cancer cells through Bax translocation to mitochondria and caspase activation, and enhances the therapeutic potential of TRAIL. However, the molecular mechanisms by which it causes growth arrest are not well-understood. We studied the molecular mechanism of curcumin-induced cell cycle arrest in prostate cancer androgen-sensitive LNCaP and androgen-insensitive PC-3 cells. Treatment of both cell lines with curcumin resulted in cell cycle arrest at G1/S phase and that this cell cycle arrest is followed by the induction of apoptosis. Curcumin induced the expression of cyclin-dependent kinase (CDK) inhibitors p16/INK4a, p21/WAF1/CIP1 and p27/KIP1, and inhibited the expression of cyclin E and cyclin D1, and hyperphosphorylation of retinoblastoma (Rb) protein. Lactacystin, an inhibitor of 26 proteasome, blocks curcumin-induced down-regulation of cyclin D1 and cyclin E proteins, suggesting their regulation at level of posttranslation. The suppression of cyclin D1 and cyclin E by curcumin may inhibit CDK-mediated phosphorylation of pRb protein. The inhibition of p21/WAF1/CIP1 by siRNA blocks curcumin-induced apoptosis, thus establishing a link between cell cycle and apoptosis. These effects of curcumin result in the proliferation arrest and disruption of cell cycle control leading to apoptosis. Our study suggests that curcumin can be developed as a chemopreventive agent for human prostate cancer.     

Perturbation of the p53 response by human papillomavirus type 16 E7.

The p53 tumor suppressor protein can induce both cell cycle arrest and apoptosis in DNA-damaged cells. In human carcinoma cell lines expressing wild-type p53, expression of E7 allowed the continuation of full cell cycle progression following DNA damage, indicating that E7 can overcome both G1 and G2 blocks imposed by p53. E7 does not interfere with the initial steps of the p53 response, however, and E7 expressing cells showed enhanced expression of p21(waf1/cip1) and reductions in cyclin E- and A-associated kinase activities following DNA damage. One function of cyclin-dependent kinases is to phosphorylate pRB and activate E2F, thus allowing entry into DNA synthesis. Although E7 may substitute for this activity during cell division by directly targeting pRB, continued cell cycle progression in E7-expressing cells was associated with phosphorylation of pRB, suggesting that E7 permits the retention of some cyclin-dependent kinase activity. One source of this activity may be the E7-associated kinase, which was not inhibited following DNA damage. Despite allowing cell cycle progression, E7 was unable to protect cells from p53-induced apoptosis, and the elevated apoptotic response seen in these cells correlated with the reduction of cyclin A-associated kinase activity. It is possible that inefficient cyclin A-dependent inactivation of E2F at the end of DNA synthesis contributes to the enhanced apoptosis displayed by E7-expressing cells.     

CDKs和CKIs在胃癌细胞周期调控中的相关性

研究 CDKs和 CKIs在调节胃癌细胞周期进程中的作用表明 ,全反式视黄酸 ( ATRA)通过诱导细胞滞留在 G1/G0 期而抑制胃癌细胞生长 .Western blot分析显示 ,ATRA可上调 p2 1 waf1/ cip1的表达 ,而抑制 p1 6ink4 的表达 .免疫沉淀及活性测定表明 ,CDK2 激酶活性可被 ATRA抑制 ,而CDK4 活性先被诱导上升 ,2 4 h后逐渐下降 .另外 ,ATRA可以调节 Rb蛋白的磷酸化和 c- myc蛋白的表达 .由此证实 ,ATRA诱导胃癌细胞滞留于 G1/G0 期与其上调 p2 1 waf1/ cip1的表达和抑制CDK2 和 CDK4 激酶活性 ,进而抑制 Rb蛋白的磷酸化和 c- myc的表达有关 . Rb蛋白是 ATRA抑制胃癌细胞生长的下游调节因子 .另外 ,p1 6ink4 的功能在胃癌细胞中可能丧失 .     

MDM2 promotes p21waf1/cip1 proteasomal turnover independently of ubiquitylation

The CDK inhibitor p21waf1/cip1 is degraded by a ubiquitin-independent proteolytic pathway. Here, we show that MDM2 mediates this degradation process. Overexpression of wild-type or ring finger-deleted, but not nuclear localization signal (NLS)-deleted, MDM2 decreased p21waf1/cip1 levels without ubiquitylating this protein and affecting its mRNA level in p53(-/-) cells. This decrease was reversed by the proteasome inhibitors MG132 and lactacystin, by p19(arf), and by small interfering RNA (siRNA) against MDM2. p21waf1/cip1 bound to MDM2 in vitro and in cells. The p21waf1/cip1-binding-defective mutant of MDM2 was unable to degrade p21waf1/cip1. MDM2 shortened the half-life of both exogenous and endogenous p21waf1/cip1 by 50% and led to the degradation of its lysine-free mutant. Consequently, MDM2 suppressed p21waf1/cip1-induced cell growth arrest of human p53(-/-) and p53(-/-)/Rb(-/-)cells. These results demonstrate that MDM2 directly inhibits p21waf1/cip1 function by reducing p21waf1/cip1 stability in a ubiquitin-independent fashion.     

Notch1 activation reduces proliferation in the multipotent hematopoietic progenitor cell line FDCP-mix through a p53-dependent pathway but Notch1 effects on myeloid and erythroid differentiation are independent of p53

Signaling mediated by activation of the transmembrane receptor Notch influences cell-fate decisions, differentiation, proliferation, and cell survival. Activated Notch reduces proliferation by altering cell-cycle kinetics and promotes differentiation in hematopoietic progenitor cells. Here, we investigated if the G(1) arrest and differentiation induced by activated mNotch1 are dependent on tumor suppressor p53, a critical mediator of cellular growth arrest. Multipotent wild-type p53-expressing (p53(wt)) and p53-deficient (p53(null)) hematopoietic progenitor cell lines (FDCP-mix) carrying an inducible mNotch1 system were used to investigate the effects of proliferation and differentiation upon mNotch1 signaling. While activated Notch reduced proliferation of p53(wt)-cells, no change was observed in p53(null)-cells. Activated Notch upregulated the p53 target p21(cip/waf) in p53(wt)-cells, but not in p53(null)-cells. Induction of the p21(cip/waf) gene by activated Notch was mediated by increased binding of p53 to p53-binding sites in the p21(cip/waf) promoter and was independent of the canonical RBP-J binding site. Re-expression of p53(wt) in p53(null) cells restored the inhibition of proliferation by activated Notch. Thus, activated Notch inhibits proliferation of multipotent hematopoietic progenitor cells via a p53-dependent pathway. In contrast, myeloid and erythroid differentiation was similarly induced in p53(wt) and p53(null) cells. These data suggest that Notch signaling triggers two distinct pathways, a p53-dependent one leading to a block in proliferation and a p53-independent one promoting differentiation.     

Histone deacetylase inhibitors differentially stabilize acetylated p53 and induce cell cycle arrest or apoptosis in prostate cancer cells

In LNCaP prostate cancer cells CG-1521, a new inhibitor of histone deacetylases, alters the acetylation of p53 in a site-specific manner. While p53 is constitutively acetylated at Lys320 in LNCaP cells, treatment with CG-1521, stabilizes the acetylation of p53 at Lys373, elevating p21 (and inducing cell cycle arrest). Treatment with CG-1521 also promotes Bax translocation to the mitochondria and cleavage, and apoptosis. TSA stabilizes the acetylation of p53 at Lys382, elevating p21 levels and inducing cell cycle arrest, but does not induce Bax translocation or apoptosis. In LNCaP cells CG-1521, but not TSA, promotes the rapid degradation of HDAC2. These data suggest that the acetylation of p53 at Lys373 is required for the p53-mediated induction of cell cycle arrest and apoptosis, while acetylation of p53 at Lys382 induces only cell cycle arrest. In p53(-/-) PC3 cells both compounds induce p21 and cell cycle arrest, but not Bax translocation or apoptosis, suggesting that both compounds can also induce p21 through a p53-independent mechanism.     

JNK supports survival in melanoma cells by controlling cell cycle arrest and apoptosis

JNK1/2 proteins belong to the family of stress-activated protein kinases. They play a complex role in growth regulation, inducing either cell death or growth support. In this report, we provide evidence that, in human melanoma cells, JNK inhibition with the small molecule inhibitor SP600125 induces either predominantly a G2/M arrest or apoptosis depending on the cell line. In 1205Lu cells, JNK inhibition induced cell cycle arrest through p53-dependent induction of p21 Cip1/Waf1 expression, while in WM983B cells, induction of apoptosis by JNK inhibition was accompanied by p53, Bad and Bax induction, not p21 Cip1/Waf1. JNK inhibition with the small molecule inhibitor SP600125 slowed growth of all cell lines, although the effect was markedly greater in cells exhibiting high phospho- (P-)JNK1 levels. Specific gene knockdown of JNK1 by means of siRNA oligonucleotides inhibited cell growth only in melanoma cell lines exhibiting high P-JNK1 levels. siRNAs directed against JNK2 did not reduce cell growth in any of the cell lines tested. Together, our findings demonstrate that JNK, and in particular the JNK1 isoform, support the growth of melanoma cells, by controlling either cell cycle progression or apoptosis depending on the cellular context.     

视黄酸对胃癌细胞周期的调控

视黄酸(RA)能够抑制许多类型癌细胞生长、诱导细胞凋亡和调节细胞周期。本文研究了全反式视黄酸(ATRA)对人胃癌细胞的作用机理。结果表明,ATRA通过诱导细胞滞留在G_0/G_1期而显著抑制胃癌细胞生长,但ATRA不能诱导胃癌细胞凋亡;ATRA调控细胞周期与c-myc、磷酸化Rb水平的下调和p21~(WAF1/CIP1)、p53水平的上调有关,而cyclinD_1和CDK_4水平没有明显变化。在RA抗性细胞中,ATRA不能调节这些基因表达。结果证实,ATRA对胃癌细胞生长抑制与其诱导细胞滞留在G_0/G_1期有关,而与细胞凋亡的诱导无关,许多重要的、与周期相关的分子,包括cmyc、p21~(WAF1/CIP1、p53和Rb等参与细胞周期的调控。     

Serine palmitoyltransferase inhibitor myriocin induces growth inhibition of B16F10 melanoma cells through G(2) /M phase arrest

Objectives: Melanoma is the most aggressive form of skin cancer, and it resists chemotherapy. Candidate drugs for effective anti‐cancer treatment have been sought from natural resources. Here, we have investigated anti‐proliferative activity of myriocin, serine palmitoyltransferase inhibitor, in the de novo sphingolipid pathway, and its mechanism in B16F10 melanoma cells. Material and methods: We assessed cell population growth by measuring cell numbers, DNA synthesis, cell cycle progression, and expression of cell cycle regulatory proteins. Ceramide, sphingomyelin, sphingosine and sphingosine‐1‐phosphate levels were analysed by HPLC. Results: Myriocin inhibited proliferation of melanoma cells and induced cell cycle arrest in the G₂/M phase. Expressions of cdc25C, cyclin B1 and cdc2 were decreased in the cells after exposure to myriocin, while expression of p53 and p21^waf1/cip1 was increased. Levels of ceramide, sphingomyelin, sphingosine and sphingosine‐1‐phosphate in myriocin‐treated cells after 24 h were reduced by approximately 86%, 57%, 75% and 38%, respectively, compared to levels in control cells. Conclusions: Our results suggest that inhibition of sphingolipid synthesis by myriocin in melanoma cells may inhibit expression of cdc25C or activate expression of p53 and p21^waf1/cip1, followed by inhibition of cyclin B1 and cdc2, resulting in G₂/M arrest of the cell cycle and cell population growth inhibition. Thus, modulation of sphingolipid metabolism by myriocin may be a potential target of mechanism‐based therapy for this type of skin cancer.     

Decreased translation of p21waf1 mRNA causes attenuated p53 signaling in some p53 wild-type tumors

DNA damage induces cell cycle arrest through both Chk1 and the p53 tumor suppressor protein, the latter arresting cells through induction of p21^waf1 protein. Arrest permits cells to repair the damage and recover. The frequent loss of p53 in tumor cells makes them more dependent on Chk1 for arrest and survival. However, some p53 wild type tumor cell lines, such as HCT116 and U2OS, are also sensitive to inhibition of Chk1 due to attenuated p21^waf1 induction upon DNA damage. The purpose of this study is to determine the cause of this attenuated p21^waf1 protein induction. We find that neither the induction of p21^waf1 mRNA nor protein half-life is sufficient to explain the low p21^waf1 protein levels in HCT116 and U2OS cells. The induced mRNA associates with polysomes but little protein is made suggesting these two cell lines have a reduced rate of p21^waf1 mRNA translation. This represents a novel mechanism for disruption of the p53-p21^waf1 pathway as currently known mechanisms involve either mutation of p53 or reduction of p53 protein levels. As a consequence, this attenuated p21^waf1 expression may render some p53 wild type tumors sensitive to a combination of DNA damage plus checkpoint inhibition.