The aim of our study was to explore the antiproliferative and pro-apoptotic action of roscovitine (ROSC) on human breast cancer MCF-7 cells. We examined the effect of ROSC on cell proliferation, cell cycle progression, nucleolar morphology, posttranslational modifications of histones as well as on induction of apoptosis. The effects of ROSC on the argyrophilic nucleolar organizer regions (AgNORs) and nucleolar RNA of MCF-7 cells were marked: ROSC treatment changed the pattern of AgNORs in a time-dependent manner. The disintegration of nucleoli manifested by increasing number of nucleolar fragments already began at 6 hr posttreatment. This was accompanied by a redistribution of the nucleolin from the nucleolus beginning after 6 hr and preceded a decrease of histone acetylation and phosphorylation. Congenital and acquired resistance to chemotherapy and radiotherapy has been a major obstacle for successful cure of human malignancies. Therefore, alternative drugs intended to overcome this clinical hindrance are searched. 1 The inhibitors of cyclindependent kinases (CDKs) represent a well-defined group of biologically active compounds suitable for nonconventional treatments. 2,3 One potential adjunct of the CDK inhibitors to classical cytostatics is direct induction of programmed cell death (PCD). The CDK inhibitors stimulate cell death from any stage of the cell cycle, 4 whereas the DNA-damaging drugs kill more efficaciously S-phase cells. 1 Roscovitine (ROSC) belongs to chemical inhibitors of CDK; the mechanism turns out to be a competitive inhibition of ATP binding. 3,5,6 ROSC occupies the purine-binding pocket, located in the cleft between the small and large lobes of the kinase. The kinase specificity of ROSC was investigated with 25 highly purified kinases. 3 Most kinases were not significantly inhibited by ROSC. However, cdc2/cyclin B, cdk2/cyclin A, cdk2/cyclin E and cdk5/p35 were substantially inhibited by ROSC. 3 Considering this unique selectivity for some CDK, ROSC clearly provides a useful reagent not only for cell cycle studies 3 but also offers a potent drug for efficient anticancer therapy. 1 The effect of ROSC on the cell cycle progression is independent of the tumour-suppressor protein p53, a primary controller of the endogenous cell-cycle inhibitor p21 waf1/Cip1 , because an inhibition was observed irrespective of p53 status: normal vs. mutated/deleted. Recently, ROSC has been shown to drive the estrogen-negative MDA-MB-231 human breast cancer cells in apoptotic cell death. 7 Therefore, it appeared interesting to address the question whether ROSC's capability to initiate the apoptosis in breast cancer cells is dependent on the estrogen-receptor status. Furthermore, it also seemed attractive to examine whether wt p53 in human breast carcinoma cells can be activated by ROSC. The p53 protein, a product of a tumour suppressor gene, is known to exert in response to various types of stress not only antiproliferative effects including a transient cell cycle arrest or terminal cellular senescence, but also...

Triazoloacridone C-1305 is a novel inhibitor of DNA topoisomerase II, which exhibits potent antitumor activity toward solid tumors. In this study, antiproliferative action of C-1305 and its close analog C-1533 was investigated in nontransformed mouse fibroblasts and two mutant cell lines in which the PARP-1 gene was specifically disrupted. Unexpectedly, C-1305 very strongly affected proliferation of cells lacking poly(ADPribose) polymerase-1 (PARP-1), whereas the action of less active compound C-1533 toward normal and PARP-1-negative cells was comparable. The IC 50 concentration of C-1305 determined for PARP-1 knockout cells was ϳ150-fold lower than that determined for cells with functional PARP-1. Both studied triazoloacridones exhibited very low direct cytotoxicity as evidenced by accumulation of 7-amino-actinomycin D, and only low levels of apoptosis were observed after a 24-h exposure to studied drugs. Analysis of DNA damage induced by C-1305 by the Comet assay showed that this drug induced very low levels of DNA strand breaks. C-1305 strongly affected cell cycle progression in normal and PARP-1 mutant cells and arrested both cell types in G 2 -M phase. However, the G 2 -M arrest induced by C-1305 was greatly prolonged in PARP-1-deficient cells as compared with normal fibroblasts. Together, these results show that mouse cells lacking PARP-1 are extremely sensitive to C-1305, a new topoisomerase II inhibitor. This is in striking contrast with previous reports in which PARP-1-deficient cells were shown to be resistant to classical topoisomerase II inhibitors. Our data also suggest that the PARP-1 status might be essential for the maintenance of the G 2 arrest induced by C-1305.

The efficacy of distinct anti-cancer drugs used in the chemotherapy of human malignancies varies between tumor tissues and depends largely on the ability of the therapeutic agents to simultaneously inhibit cell proliferation and to eliminate malignant cells by apoptosis. Especially, detection of early apoptotic changes seems to be important because early stages of apoptosis differ from those of necrosis. Therefore, the development of a novel test allowing fast and concomitant screening of the anti-proliferative and pro-apoptotic action of a number of anti-cancer drugs is of great interest. For this purpose, we choose as an experimental model a well characterized anti-proliferative and pro-apoptotic effect of cisplatin (CP) on human cervical carcinoma HeLaS3 cells. As previously reported, exposure of HeLaS3 to CP resulted in a concomitant inhibition of cell proliferation and induction of apoptosis in a dose- and time-dependent manner. In the present study we performed two independent approaches. In the first approach, we examined the cell proliferation and activity of caspases-3/7 in two separate microtiter plates using the CellTiter-Glo Luminescent Cell Viability Assay and the Caspase-Glo 3/7 Assay, respectively. In the second approach, we determined the same parameters sequentially in one microtiter plate by a mutiplexing assay using CellTiter-Blue Cell Viability Assay and Caspase-Glo 3/7 Assay. The both approaches gave very similar results indicating that this new multiplexing assay offers an important advantage for simultaneous detection of cell number and activation of caspases-3/7. The new multiplexing assay offers a range of benefits over standard assays.

We reported recently that roscovitine (ROSC), a selective cyclin-dependent kinase (CDK) inhibitor, arrested human MCF-7 breast cancer cells in G2 phase of the cell cycle and concomitantly induced apoptosis. On the other hand, ROSC-induced G1 arrest observed by another group has not been accompanied by apoptosis. Therefore, we decided to prove to which extent components of tissue culture media could affect the primary action of ROSC. For this purpose we compared the efficacy of the ROSC treatment on MCF-7 cells cultivated in medium with and without phenol red. The kinetics of MCF-7 cell proliferation strongly depended on the presence of phenol red that has been recognized previously as a weak estrogen. Exposure of MCF-7 cells cultivated in phenol red-deprived medium to ROSC resulted in a strong G2 arrest and apoptosis. However, the anti-proliferative and pro-apoptotic action of ROSC was strongly diminished in cells maintained in medium containing phenol red. The ratio of the G2 cell population after 12 h ROSC was reduced by approximately 20% in the latter and correlated with the lack of CDK2 inactivation. Moreover, the kinetics of ROSC-induced apoptosis was delayed in the presence of phenol red. These results clearly evidence that the efficacy of the therapy of ER-positive breast cancers by CDK inhibitors is diminished in the presence of estrogen-mimicking compounds and indicate that phytoestrogens and xenoestrogens could interfere with the therapy. Therefore, the exposure of cancer patients to the estrogen mimics should be avoided at least during chemotherapy by CDK inhibitors.

We reported previously that treatment of rats with the hepatocarcinogen N-nitrosomorpholine (NNM) caused severe hepatotoxicity associated with apoptosis of hepatocytes beginning 12 h after administration of NNM. We observed that poly(ADP-ribose) polymerase 1 (PARP-1), one of the major nuclear targets for caspases, was proteolytically degraded generating primarily 64 and 54 kDa fragments. Interestingly, at 20, 30, and 40 h post-treatment a 85 kDa cleavage product of PARP-1 resembling that generated by caspase-3 appeared additionally in hepatocytes. More detailed analysis performed in the present study revealed that the 85 kDa fragment of PARP-1 was generated in the liver in 10 of 17 (60%) animals examined between 20 and 40 h after NNM administration. The caspase-3 generated 85 kDa fragment was detected solely in hepatocytes undergoing apoptosis as evidenced by immunostaining performed with the antibody recognizing exclusively PARP-1 cleaved at position 214/215. The appearance of the 85 kDa fragment of PARP-1 in the liver nuclei coincided temporally with an significant increase of caspase-3 activity in hepatocytes. In contrast, in testis samples obtained from the same animals, no changes characteristic for apoptosis such as induction of caspases activity or degradation of nuclear PARP-1 could be detected. Our results evidence unequivocally that PARP-1 in liver is not resistant to caspases and can be processed in vivo by activated caspase-3 producing the p85 kDa fragment. Moreover, the caspase-3 induced PARP-1 fragmentation coinciding with the increase of caspase-3 activity was detected solely in the target organ and exclusively in hepatocytes undergoing apoptosis. Considering the fact that the caspase-3 mediated PARP-1 cleavage occurred only in 60% of animals tested between 20 and 40 h, it becomes obvious that the cellular response in vivo to the same trigger(s) strongly varies and may depend on a variety of intrinsic factors. It remains to elucidate which additional factors may be involved in the modulation of cellular response to the strong insults thereby activating different pathways and generating distinct outcomes.

We examined the action of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on HeLa cells and compared it with that of cisplatin (CP). MNNG directly killed a substantial number of cells within 1 hour and resulted in strong DNA-damage as evidenced by Comet measurements. Despite appearance of DNA lesions, p53 protein was not activated. Analysis of HeLa cells treated with MNNG for 1h, 3h and 6h by flow cytometry and by Hoechst staining did not reveal any sub-G(1) cell population and chromatin condensation/fragmentation characteristic for apoptosis, respectively. Also, no biochemical changes typical for apoptosis such as activation of caspase-3 or release of cytochrome C from mitochondria were detected. Inactivation of PARP-1 reduced the direct cytotoxicity exerted by MNNG. Our results showing that despite appearance of severe DNA lesions after short exposure of HeLa cells to MNNG neither activation of p53 response nor induction of apoptosis occurred implicate that generation of strong DNA damage is not sufficient to stabilize p53 protein in HeLa cells. Our data unequivocally show that the conscientious determination of the type of cell death induced by genotoxic agents is necessary. The assessment of the changes based on at least a few independent criteria is required to discriminate between apoptosis and necrosis. Since the alkylating agents generate DNA strand breaks, the recruitment of methods based on determination of DNA cleavage such as DNA ladder or TUNEL assay for evaluation of apoptosis is not adequate.

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Inhibition of cyclin-dependent kinases (CDKs) is a novel strategy in the therapy of human malignancies. The pharmacological CDK inhibitors representing a few distinct classes of compounds exert different target specificity. Considering the fact that dividing and quiescent cells differ in their CDK activity and in the pattern of their expression, one might expect that anti-proliferative efficiency of the pharmacological CDK inhibitors would depend on the mitotic index of treated cells. The present article shows that olomoucine (OLO), a weak CDK2 inhibitor has new, unexpected activity. At concentrations up to 100 microM OLO did not inhibit proliferation of normal human cells, but arrested growth of human HL-60 leukemia cells. The anti-proliferative effect of OLO was clearly weaker than that of roscovitine (ROSC). Surprisingly, OLO at low doses strongly up-regulated a cellular protein with approximately 65 kDa in normal, but not in immortalized and cancer cells. By mass spectrometric analysis CLIMP-63, a cytoskeleton-linking membrane protein was identified as the major component of the up-regulated protein band. These results were subsequently confirmed by immunoblotting. Further experiments revealed that OLO, but not ROSC, strongly up-regulates CLIMP-63 in a dose- and time-dependent manner solely in senescent cells.