Xylarianaphthol-1, a novel dinaphthofuran derivative,activates p21 promoter in a p53-independent manner

Xylarianaphthol-1, a novel dinaphthofuran derivative, was isolated from a marine sponge-derived fungus of order Xylariales on the guidance of a bioassay using the transfected human osteosarcoma MG63 cells (MG63^luc+). The chemical structure of xylarianaphthol-1 was determined from the ¹H and ¹³C NMR analysis and was further confirmed by the total synthesis. Xylarianaphthol-1 activated p21 promoter stably transfected in MG63 cells dose-dependently. Expression of p21 protein in the wild-type MG63 cells was also increased by xylarianaphthol-1 treatment.     

CtIP promotes G2/M arrest in etoposide-treated HCT116 cells in a p53-independent manner

Acquired resistance to cytotoxic antineoplastic agents is a major clinical challenge in tumor therapy; however, the mechanisms involved are still poorly understood. In this study, we show that knockdown of CtIP, a corepressor of CtBP, promotes cell proliferation and alleviates G2/M phase arrest in etoposide (Eto)-treated HCT116 cells. Although the expression of p21 and growth arrest and DNA damage inducible α (GADD45a), which are important targets of p53, was downregulated in CtIP-deficient HCT116 cells, p53 deletion did not affect G2/M arrest after Eto treatment. In addition, the phosphorylation levels of Ser317 and Ser345 in Chk1 and of Ser216 in CDC25C were lower in CtIP-deficient HCT116 cells than in control cells after Eto treatment. Our results indicate that CtIP may enhance cell sensitivity to Eto by promoting G2/M phase arrest, mainly through the ATR-Chk1-CDC25C pathway rather than the p53-p21/GADD45a pathway. The expression of CtIP may be a useful biomarker for predicting the drug sensitivity of colorectal cancer cells.     

p53-, SIRT1-, and PARP-1-independent downregulation of p21WAF1 expression in nicotinamide-treated cells

Nicotinamide at mM concentration is a potent inhibitor of certain key molecules involved in cell survival, such as SIRT1 and PARP-1, and affects cell survival in various conditions in vivo and in vitro. However, the effect of an acute treatment of nicotinamide on gene expression has rarely been closely examined. In our study, the treatment of 10 mM nicotinamide downregulated p21WAF1 expression in various human cells including p53-negative or SIRT1-knockdown cells indicating gene regulation not mediated by p53 or SIRT1. Meanwhile, in the nicotinamide-treated cells, Sp1 activity and protein level was substantially reduced due to increased proteasome-mediated degradation. Our results indicate that nicotinamide treatment attenuates p21WAF1 expression through Sp1 downregulation, and suggest a possible involvement of nicotinamide metabolism in cellular gene expression.     

Mechanism of p53 downstream effectors p21 and Gadd45 in DNA damage surveillance

Both p21 (WAF1/CIP1) and Gadd45 were activated in a p53-dependent manner in MCF-7 cells after being exposed to ionizing radiation. In order to investigate their roles in DNA damage surveillance, p21~(as)/MCF-7 cells stably transfected by p21 antisense expression plasmid pC-WAF1-AS and Gadd45~(as)/MCF-7 stably transfected by Gadd45 antisense expression plasmid pCMVas45 were established. It was observed that G_1 arrest induced by radiation was significantly reduced in Gadd45~(as)/MCF-7 cells as well as in p21~(as)/MCF-7 cells. Repair of radiation damaged report gene greatly reduced in Gadd45~(as)/MCF-7 and p21~(as)/MCF-7 cells. Apoptosis significantly increased in p21~(as)/MCF-7 after exposure to radiation. These results suggest that both p21 and Gadd45 support cellular survival by taking roles in G_1 arrest and DNA repair, furthermore, p21 protects cells from death by inhibiting apoptosis after exposure to ionizing radiation.     

A dominant negative form of p63 inhibits apoptosis in a p53-independent manner

Stem cells are a source of differentiated cells in multiple tissues. If genetic alterations occur in stem cells, the problem persists and malignant cancers may arise. DeltaNp63alpha-a homologue of the tumor suppressor p53-is exclusively expressed in proliferating undifferentiated epithelial cells and cancer cells of epidermal origin. Here, we show that DeltaNp63alpha antagonizes DNA damage-induced apoptosis in a p53-independent manner. We found that upon cellular injury, DeltaNp63alpha must be downregulated before apoptotic program can be activated. The 5637 cell line has abundant levels of DeltaNp63alpha and mutant p53, and it is resistant to DNA damage-induced apoptosis. The knockdown of DeltaNp63alpha by RNA interference sensitized these cells to apoptosis upon genotoxic insult. This suggests that DeltaNp63alpha plays an anti-apoptotic role regardless of the p53 status. Considering the frequent mutations of p53 in tumor cells, our results provide important implications for the treatment of cancers in which p63 is amplified.     

Cordycepin causes p21WAF1-mediated G2/M cell-cycle arrest by regulating c-Jun N-terminal kinase activation in human bladder cancer cells

Cordycepin (3′-deoxyadenosine), a bioactive compound of Cordycepsmilitaris, has many pharmacological activities. The present study reveals novel molecular mechanisms for the anti-tumor effects of cordycepin in two different bladder cancer cell lines, 5637 and T-24 cells. Cordycepin treatment, at a dose of 200 μM (IC₅₀) during cell-cycle progression resulted in significant and dose-dependent growth inhibition, which was largely due to G2/M-phase arrest, and resulted in an up-regulation of p21WAF1 expression, independent of the p53 pathway. Moreover, treatment with cordycepin-induced phosphorylation of JNK (c-Jun N-terminal kinases). Blockade of JNK function using SP6001259 (JNK-specific inhibitor) and small interfering RNA (si-JNK1) rescued cordycepin-dependent p21WAF1 expression, inhibited cell growth, and decreased cell cycle proteins. These results suggest that cordycepin could be an effective treatment for bladder cancer.     

Resveratrol- induced apoptosis is mediated by p53-dependent pathway in Hep G2 cells

Resveratrol, a phytoalexin found in many plants, has been reported to possess a wide range of pharmacological properties and is one of the promising chemopreventive agents for cancer. Here, we examined the antiproliferation effect of resveratrol in two human liver cancer cell lines, Hep G2 and Hep 3B. Our results showed that resveratrol inhibited cell growth in p53-positive Hep G2 cells only. This anticancer effect was a result of cellular apoptotic death induced by resveratrol via the p53-dependent pathway. Here we demonstrated that the resveratrol-treated cells were arrested in G1 phase and were associated with the increase of p21 expression. In addition, we also illustrated that the resveratrol-treated cells had enhanced Bax expression but they were not involved in Fas/APO-1 apoptotic signal pathway. In contrast, the p53-negative Hep 3B cells treated with resveratrol did not show the antiproliferation effect neither did they show significant changes in p21 nor Fas/APO-1 levels. In summary, our study demonstrated that the resveratrol effectively inhibited cell growth and induced programmed cell death in Hepatoma cells on a molecular basis. Furthermore, these results implied that resveratrol might also be a new potent chemopreventive drug candidate for liver cancer as it played an important role to trigger p53-mediated molecules involved in the mechanism of p53-dependent apoptotic signal pathway.     

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.     

DNAJB1 stabilizes MDM2 and contributes to cancer cell proliferation in a p53-dependent manner

Both MDM2 and MDMX regulate p53, but these proteins play different roles in this process. To clarify the difference, we performed a yeast 2 hybrid (Y2H) screen using the MDM2 acidic domain as bait. DNAJB1 was found to specifically bind to MDM2, but not MDMX, in vitro and in vivo. Further investigation revealed that DNAJB1 stabilizes MDM2 at the post-translational level. The C-terminus of DNAJB1 is essential for its interaction with MDM2 and for MDM2 accumulation. MDM2 was degraded faster by a ubiquitin-mediated pathway when DNAJB1 was depleted. DNAJB1 inhibited the MDM2-mediated ubiquitination and degradation of p53 and contributed to p53 activation in cancer cells. Depletion of DNAJB1 in cancer cells inhibited activity of the p53 pathway, enhanced the activity of the Rb/E2F pathway, and promoted cancer cell growth in vitro and in vivo. This function was p53 dependent, and either human papillomavirus (HPV) E6 protein or siRNA against p53 was able to block the contribution caused by DNAJB1 depletion. In this study, we discovered a new MDM2 interacting protein, DNAJB1, and provided evidence to support its p53-dependent tumor suppressor function.     

Loss of p21 expression in senescent cells after DNA damage accompanied with increase of miR-93 expression and reduced p53 interaction with p21 gene promoter

To answer what is a critical event for higher incidence of tumor development in old than young individuals, primary culture of human diploid fibroblasts were employed and DNA damage was induced by doxorubicin or X-ray irradiation. Response to the damage was different between young and old cells; loss of p21^sdi1 expression in spite of p53^S15 activation in old cells along with [³H]thymidine and BrdU incorporation, but not in young cells. The phenomenon was confirmed by other tissue fibroblasts obtained from different donor ages. Induction of miR-93 expression and reduced p53 binding to p21 gene promoter account for loss of p21^sdi1 expression in senescent cells after DNA damage, suggesting a mechanism of in vivo carcinogenesis in aged tissue without repair arrest.