p27 small interfering RNA induces cell death through elevating cell cycle activity in cultured cortical neurons: a proof-of-concept study

Recent research has demonstrated that cell cycle-associated molecules are activated in multiple forms of cell death in mature neurons, and raised a hypothesis that unscheduled cell cycle activity leads to neuronal cell death. But there is little evidence that changes in endogenous level of these molecules are causally associated with neuronal cell death. Here we transfected small interfering RNA (siRNA) targeting cyclin-dependent kinase (CDK) inhibitor p27, which plays an important role in cell cycle arrest at G1-S phase, into cultured cortical neurons. Transfection of p27 siRNA reduced neuronal viability in a time-dependent manner. p27 siRNA induced phosphorylation of retinoblastoma protein (Rb), a marker of cell cycle progression at late G1 phase. Moreover, phosphorylation of Rb and neuronal cell death provoked by p27 siRNA were abrogated by pharmacological CDK inhibitors, olomoucine and purvalanol A. Our data demonstrate that a decrease in endogenous p27 induces neuronal cell death through elevating cell cycle activity.     

Progesterone inhibits human endothelial cell proliferation through a p53-dependent pathway

Previous studies have shown that progesterone inhibits endothelial cell proliferation through a nuclear receptor-mediated mechanism. Here, we further demonstrate that progesterone at physiologic levels (5 – 500 nM) dose- and time-dependently inhibited DNA synthesis of cultured human umbilical vein endothelial cells (HUVEC). The mRNA and protein levels of p21, p27, and p53 in HUVEC were increased by progesterone. The formation of CDK2-p21 and CDK2-p27 were increased and the CDK2 activity was decreased in the progesterone-treated HUVEC. The progesterone-inhibited [3H]thymidine incorporation was completely blocked when the expressions of p21 and p27 were knocked-down together. Transfection of HUVEC with dominant negative p53 cDNA prevented the progesterone-induced increases in p21 and p27 promoter activity and protein level, decreases in thymidine incorporation, and capillary-like tube formation. Matrigel angiogenesis assay in mice demonstrated the antiangiogenic effect of progesterone in vivo. These findings demonstrate for the first time that progesterone inhibited endothelial cell proliferation through a p53-dependent pathway. Received 28 July 2008; received after revision 25 September 2008; accepted 26 September 2008     

Unscheduled CDK1 activity in G1 phase of the cell cycle triggers apoptosis in X-irradiated lymphocytic leukemia cells

Cyclin-dependent kinase 1 (CDK1) is a major component of the cell cycle progression engine. Recently, several investigations provided evidence demonstrating that unscheduled CDK1 activation may also be involved in apoptosis in cancerous cells. In this article, we demonstrate that X-ray irradiation induced G1 arrest in MOLT-4 lymphocytic leukemia cells, the arrest being accompanied by reduction in the activity of CDK2, but increased CDK1 activity and cell apoptosis in the G1 phase. Interestingly, this increase in CDK1 and apoptosis by ionizing radiation was prevented by pretreatment with the CDK1 inhibitor, roscovitine, suggesting that CDK1 kinase activity is required for radiation-induced apoptotic cell death in this model system. Furthermore, cyclin B1 and CDK1 were detected co-localizing and associating in G1 phase MOLT-4 cells, with the cellular lysates from these cells revealing a genotoxic stress-induced increase in CDK1 phosphorylation (Thr-161) and dephosphorylation (Tyr-15), as analyzed by postsorting immunoprecipitation and immunoblotting. Finally, X-irradiation was found to increase Bcl-2 phosphorylation in G1 phase cells. Taken together, these novel findings suggest that CDK1 is activated by unscheduled accumulation of cyclin B1 in G1 phase cells exposed to X-ray, and that CDK1 activation, at the wrong time and in the wrong phase, may directly or indirectly trigger a Bcl-2-dependent signaling pathway leading to apoptotic cell death in MOLT-4 cells. Received 30 March 2006; received after revision 23 June 2006; accepted 24 August 2006 J. Wu and Y. Feng contributed equally to this work.     

Selenocystine induces apoptosis of A375 human melanoma cells by activating ROS-mediated mitochondrial pathway and p53 phosphorylation

Selenocystine (SeC), a naturally occurring selenoamino acid, has been shown to be a novel compound with broad-spectrum anticancer activity. In this study, we showed that SeC triggered time- and dose-dependent apoptosis in A375 human melanoma cells by activating the mitochondria-mediated and death receptor-mediated apoptosis pathways. Pretreatment of cells with a general caspase inhibitor z-VAD-fmk significantly prevented SeC-induced apoptosis. A375 cells exposed to SeC showed an increase in levels of total p53 and phosphorylated p53 (serine-15). Silencing of p53 expression with RNA interference significantly suppressed SeC-induced p53 phosphorylation, caspase activation and apoptotic cell death. Moreover, generation of reactive oxygen species and subsequent induction of DNA strand breaks were found to be upstream mediators of p53 activation induced by SeC. In a nude mice xenograft experiment, SeC significantly inhibited the tumor growth of A375 cells via induction of apoptosis. Taken together, these results suggest the potential applications of SeC in cancer chemoprevention.     

Cyclin A1 is a p53-induced gene that mediates apoptosis, G2/ M arrest, and mitotic catastrophe in renal, ovarian, and lung carcinoma cells

We were the first to identify cyclin A1 as a p53-induced gene by cDNA expression profiling of p53-sensitive and -resistant tumor cells [Maxwell S. A. and Davis G. E. (2000) Proc. Natl. Acad. Sci. USA 97, 13009–13014]. We show here that cyclin A1 can induce G2 cell cycle arrest, polyploidy, apoptosis, and mitotic catastrophe in H1299 non-small cell lung, TOV-21G ovarian, or 786-0 renal carcinoma cells. More cdk1 protein and kinase activities were observed in cyclin A1-induced cells than in GFP control-induced cells. Thus, cyclin A1 might mediate apoptosis and mitotic catastrophe through an unscheduled or inappropriate activation of cdk1. Two primary renal cell carcinomas expressing mutated p53 exhibited reduced or absent expression of cyclin A1 relative to the corresponding normal tissue. Moreover, renal carcinoma-derived mutant p53s were deficient in inducing cyclin A1 expression in p53-null cells. Cyclin A1 but not cyclin A2 was upregulated in etoposide-treated tumor cells undergoing p53-dependent apoptosis and mitotic catastrophe. Forced upregulation of cyclin A2 did not induce apoptosis. The data implicate cyclin A1 as a downstream player in p53-dependent apoptosis and G2 arrest. Received 1 November 2005; received after revision 17 February 2006; accepted 13 April 2006     

α-1,3-Fucosyltransferase-VII stimulates the growth of hepatocarcinoma cells via the cyclin-dependent kinase inhibitor p27Kip1

After the transfection of -1,3-fucosyltransferase (FucT)-VII cDNA into H7721 human hepatocarcinoma cells, the protein expression of some cyclins, cyclin-dependent kinases (CDKs) and cyclin-dependent kinase inhibitors (CDIs) p16^INK4 and p21^waf1/Cip1 were unchanged. However, CDI p27^Kip1 protein, both the total amount and the amount that bound to CDK2, but not its mRNA, was significantly reduced. The de-inhibited CDK2 stimulated the phosphorylation of retinoblastoma (Rb) protein and facilitated the G1/S transition and growth rate of the cells. The decrease of p27^Kip1 protein, the increase of CDK2 activity and Rb phosphorylation, as well as the cell growth and percentage of S phase cells were correlated to the increased amount of cell surface sialyl Lewis X (SLe^x) antigen in cells with different -1,3-FucT-VII expression. The reduction in p27^Kip1 and the difference in its expression among different transfected cells were blocked by the SLe^x antibody KM93 in a dose-dependent manner, indicating that p27^Kip1 expression was influenced by -1,3-FucT-VII and its product SLe^x. The MEK/MAPK signaling pathway was more important than the PI-3K pathway in the regulation of p27^Kip1 expression.Received 5 August 2004; received after revision 25 October 2004; accepted 11 November 2005     

Calcium-mediated activation of PI3K and p53 leads to apoptosis in thyroid carcinoma cells

The molecular mechanism responsible for cadmium-induced cell death in thyroid cancer cells (FRO) is unknown. We demonstrated that apoptosis of FRO cells induced by cadmium was concentration and time dependent. Cadmium caused the rapid elevation of intracellular calcium and induced phosphorylation of Akt, p53, JNK, ERK and p38. Inhibition of PI3K/Akt attenuated the cadmium-induced apoptosis, but the inhibition of JNK inhibitor, ERK or p38 aggravated it, indicating that activation of PI3K/Akt was a pro-apoptosis signal in response to cadmium treatment, whereas the activation of stress-activated protein kinase JNK, ERK and p38 functioned as survival signals to counteract the cadmium-induced apoptosis. Buffering of the calcium response attenuated mitochondrial impairment, recovered the cadmium-activated Akt, p53, JNK, ERK and p38, and subsequently blocked the apoptosis. These results suggested that apoptosis induced by cadmium in FRO cells was initiated by the rapid elevation of intracellular calcium, followed by calcium-mediated activation of PI3K/Akt and mitochondrial impairment. Received 28 February 2007; received after revision 2 April 2007; accepted 23 April 2007     

Regulation of the cell cycle following DNA damage in normal and Ataxia telangiectasia cells

A proportion of the population is exposed to acute doses of ionizing radiation through medical treatment or occupational accidents, with little knowledge of the immedate effects. At the cellular level, ionizing radiation leads to the activation of a genetic program which enables the cell to increase its chances of survival and to minimize detrimental manifestations of radiation damage. Cytotoxic stress due to ionizing radiation causes genetic instability, alterations in the cell cycle, apoptosis, or necrosis. Alterations in the G1, S and G2 phases of the cell cycle coincide with improved survival and genome stability. The main cellular factors which are activated by DNA damage and interfere with the cell cycle controls are: p53, delaying the transition through the G1-S boundary; p21^WAF1/CIPI, preventing the entrance into S-phase; proliferating cell nuclear antigen (PCNA) and replication protein A (RPA), blocking DNA replication; and the p53 variant protein p53as together with the retinoblastoma protein (Rb), with less defined functions during the G2 phase of the cell cycle. By comparing a variety of radioresistant cell lines derived from radiosensitive ataxia talangiectasia cells with the parental cells, some essential mechanisms that allow cells to gain radioresistance have been identified. The results so far emphasise the importance of an adequate delay in the transition from G2 to M and the inhibition of DNA replication in the regulation of the cell cycle after exposure to ionizing radiation.     

Post-translational regulation of the tumor suppressor p27KIP1

Mitogenic signals stimulate cell division by activating cyclin/cyclin-dependent kinase (CDK) complexes. Their timely regulation ensures proper cell cycle progression. It is therefore not surprising that cyclin/CDK complexes are integrators of multiple signals from both the extracellular environment and intracellular cues. Important regulators of cyclin/CDKs are the CDK inhibitors that have attracted attention due to their association with disease. p27^KIP1 is a CDK inhibitor that controls CDK activity throughout the cell cycle. As a CDK inhibitor, p27^KIP1 has tumor suppressor activity. Besides CDKs, p27^KIP1 regulates additional cellular processes, including cell motility, some of which seem to mediate oncogenic activities of p27^KIP1. These activities of p27^KIP1 are regulated through multiple phosphorylation sites, targeted by several signal transduction pathways. Understanding functions and regulation of p27^KIP1 will be important to determine which isoform of p27^KIP1 has anti- or pro-tumorigenic activities. Such knowledge might be of prognostic value and may offer novel therapeutic windows. Received 26 May 2008; accepted 17 June 2008     

Protein kinase D inhibitor CRT0066101 suppresses bladder cancer growth in vitro and xenografts via blockade of the cell cycle at G2/M

The protein kinase D (PKD) family of proteins are important regulators of tumor growth, development, and progression. CRT0066101, an inhibitor of PKD, has antitumor activity in multiple types of carcinomas. However, the effect and mechanism of CRT0066101 in bladder cancer are not understood. In the present study, we show that CRT0066101 suppressed the proliferation and migration of four bladder cancer cell lines in vitro. We also demonstrate that CRT0066101 blocked tumor growth in a mouse flank xenograft model of bladder cancer. To further assess the role of PKD in bladder carcinoma, we examined the three PKD isoforms and found that PKD2 was highly expressed in eight bladder cancer cell lines and in urothelial carcinoma tissues from the TCGA database, and that short hairpin RNA (shRNA)-mediated knockdown of PKD2 dramatically reduced bladder cancer growth and invasion in vitro and in vivo, suggesting that the effect of the compound in bladder cancer is mediated through inhibition of PKD2. This notion was corroborated by demonstrating that the levels of phospho-PKD2 were markedly decreased in CRT0066101-treated bladder tumor explants. Furthermore, our cell cycle analysis by flow cytometry revealed that CRT0066101 treatment or PKD2 silencing arrested bladder cancer cells at the G2/M phase, the arrest being accompanied by decreases in the levels of cyclin B1, CDK1 and phospho-CDK1 (Thr161) and increases in the levels of p27^Kip1 and phospho-CDK1 (Thr14/Tyr15). Moreover, CRT0066101 downregulated the expression of Cdc25C, which dephosphorylates/activates CDK1, but enhanced the activity of the checkpoint kinase Chk1, which inhibits CDK1 by phosphorylating/inactivating Cdc25C. Finally, CRT0066101 was found to elevate the levels of Myt1, Wee1, phospho-Cdc25C (Ser216), Gadd45α, and 14-3-3 proteins, all of which reduce the CDK1-cyclin B1 complex activity. These novel findings suggest that CRT0066101 suppresses bladder cancer growth by inhibiting PKD2 through induction of G2/M cell cycle arrest, leading to the blockade of cell cycle progression.     

Phosphorylation and activation of the atypical kinase p53-related protein kinase (PRPK) by Akt/PKB

p53-related protein kinase (PRPK), the human homologue of yeast Bud32, belonging to a small subfamily of atypical protein kinases, is inactive unless it is previously incubated with cell lysates. Here we show that such an activation of PRPK is mediated by another kinase, Akt/PKB, which phosphorylates PRPK at Ser250. We show that recombinant PRPK is phosphorylated in vitro by Akt and its phospho-form is recognized by a Ser250-phospho-specific antibody; that cell co-transfection with Akt along with wild-type PRPK, but not with its Ser250Ala mutant, results in increased PRPK phosphorylation; and that the phosphorylation of p53 at Ser15, the only known substrate of PRPK, is markedly increased by co-transfection of Akt with wild-type PRPK, but not PRPK dead mutant, and is abrogated by cell treatment with the Akt pathway inhibitor LY294002. Our data disclose an unanticipated mechanism by which PRPK can be activated and provide a functional link between this enigmatic kinase and the Akt signaling pathway.     

Introduction: p53 – the first twenty years

The p53 protein was discovered 20 years ago, as a cellular protein tightly bound to the large T oncoprotein of the SV40 DNA tumour virus. Since then, research on p53 has developed in many exciting and sometimes unexpected directions. p53 is now known to be the product of a major tumour suppressor gene that is the most common target for genetic alterations in human cancer. The nonmutated wild-type p53 protein (wtp53) is often found within cells in a latent state and is activated in response to various intracellular and extracellular signals. Activation involves an increase in overall p53 protein levels, as well as qualitative changes in the protein. Upon activation, wtp53 can induce a variety of cellular responses, most notable among which are cell cycle arrest and apoptosis. To a great extent, these effects are mediated by the ability of p53 to activate specific target genes. In addition, the p53 protein itself possesses biochemical functions which may facilitate DNA repair as well as apoptosis. The role of p53 in normal development and particularly in carcinogenesis has been elucidated in depth through the use of mouse model systems. The insights provided by p53 research over the years are now beginning to be utilized towards better diagnosis, prognosis and treatment of cancer.     

Covalent and noncovalent modifiers of the p53 protein

Despite the massive attention it has received, there is still much to learn about the p53 tumour suppressor protein. Given that it plays complex and multiple roles in cells, it is not surprising that p53 is subjected to an intricate array of regulatory processes. p53 receives signals from cells in multiple ways, leading to its stabilization and activation. The functions of the protein are altered by phosphorylation and other covalent modifications. However, a number of proteins can regulate p53 function dramatically by noncovalent means. p53 is thus subjected to numerous signalling and regulatory pathways which we have only begun to decipher.