Effects of ionizing radiation and caffeine treatment on cyclin dependent kinase complexes in V79 hamster cells

The long form of CDK2 arises via alternative splicing and forms an active protein kinase with cyclins A and E

We have reinvestigated the long form of cyclin-dependent kinase (CDK)2 that is expressed in many rodent cells. We show that the mRNA encoding CDK2L arises by alternative splicing and that the encoded protein can bind to, and be activated by, cyclins A and E. The complex of CDK2L with cyclin A has about half the specific activity of the equivalent CDK2-cyclin A complex. Also, CDK2L--cyclin A is inhibited to the same extent and by the same concentrations of p21(CIP1) as CDK2--cyclin A. The nucleotide sequences of intron V in the human and murine CDK2 genes, where the sequences encoding the 48-residue insert in CDK2L are located, show very high conservation in the position of the alternatively spliced exon and its surroundings. Despite this, we were not able to detect significant expression of CDK2L in human cell lines, although a low level is expressed in COS-1 cells from monkeys.

Mitogen-activated protein kinase kinase 2 activation is essential for progression through the G2/M checkpoint arrest in cells exposed to ionizing radiation

An increasing body of evidence suggests that mitogen-induced activation of the RAF/ERK signaling pathway is functionally separate from the stress-induced activation of the SEK/JNK/p38 signaling pathway. In general, stress stimuli strongly activate the p38s and the JNKs while only weakly activating ERK1 and ERK2. However, a number of independent groups have now shown that the RAF/ERK signaling pathway is strongly activated by ionizing radiation. In this work, we examine this paradox. We show that both mitogen-activated protein (MAP) kinase kinase 1 (MEK1) and MAP kinase kinase 2 (MEK2) are activated by ionizing radiation. Blockage of this activation through the use of dominant negative MEK2 increases sensitivity of the cell to ionizing radiation and decreases the ability of a cell to recover from the G2/M cell cycle checkpoint arrest. Blocking MEK2 activation does not affect double-strand DNA break repair, however. Although MEK1 is activated to a lesser extent by ionizing radiation, expression of a dominant negative MEK1 does not affect radiation sensitivity of the cell, the G2/M checkpoint of the cell, or double-strand break repair. Because ionizing radiation leads to a different cell cycle arrest (G2/M arrest) than that typically seen with other stress stimuli, and because we have shown that MEK2 can affect G2/M checkpoint kinetics, these results provide an explanation for the observation that the MEKs can be strongly activated by ionizing radiation and only weakly activated by other stressful stimuli.

Caffeine and staurosporine enhance the cytotoxicity of cisplatin and camptothecin in human brain tumor cell lines

Caffeine and staurosporine have been shown to attenuate G2 delay produced by DNA-damaging agents and to augment the cytotoxicity of these agents in a number of cell lines in vitro. Studies in rodent brain tumor cell lines suggest that modulation of the G2/M transition may not contribute to the enhanced cytotoxicity produced by caffeine in brain tumor cells. To evaluate the impact of agents that decrease G2 delay on the cytotoxicity of chemotherapy in human brain tumor cells, we examined the ability of caffeine and staurosporine to modulate the G2 delay and cytotoxicity produced by cisplatin (CDDP) and camptothecin (CPT) in U251 glioma and DAOY medulloblastoma cells. Synchronized U251 were incubated with 20 microM CDDP in the presence or absence of 2 mM caffeine. DAOY cells were incubated with 100 nM CPT in the presence or absence of 2 nM staurosporine. Caffeine and staurosporine attenuated G2 delay produced by CDDP and CPT, respectively. Clonogenic assays indicated that continuous exposure to 2 mM caffeine substantially lowered the ID50 and ID90 of CDDP in U251 cells without significantly altering plating efficiency. Twenty-four-hour exposure to 2 nM staurosporine lowered the ID50 and ID90 of CPT in DAOY cells without significantly altering plating efficiency. Evaluation of programmed cell death using terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling assay indicated that one mechanism for synergistic cytotoxicty of caffeine with CDDP and staurosporine with CPT in U251 and DAOY cells, respectively, is to promote apoptosis. These results underscore the importance of understanding regulation of G2/M transition in brain tumor cells. Such an understanding may lead to novel therapies that target G2 check points to augment the efficacy of currently available treatments for brain tumors.

Effects of ionizing- and UV B-radiation on proteins controlling cell cycle progression in human cells: comparison of the MCF-7 adenocarcinoma and the SCL-2 squamous cell carcinoma cell line

MCF-7 and SCL-2 cells were irradiated with UV B-radiation or with 137Cs gamma-radiation, in order to investigate cell cycle checkpoint control mechanisms. Effects of both qualities of radiation were investigated for the two cell lines in regard to p53 protein levels, and alterations in Cdk1 (cyclin dependent kinase 1) and Cdk2 phosphorylation were monitored. SCL-2 cells constitutively overexpressed a form of p53 protein whose abundance remained unchanged after irradiation, whereas MCF-7 cells expressed wild type p53 whose abundance increased after irradiation. Accordingly, MCF-7 cells showed a strong G1 phase arrest, whereas SCL-2 cells were only delayed in S phase (after UV B-irradiation) and arrested in G2 phase (after gamma-irradiation and UV B-irradiation), as monitored by flow cytometry. In MCF-7 cells increased p53 levels were observed for up to 30 h after gamma-irradiation and up to 20 h after UV B-irradiation. Only in SCL-2 cells was there a significant radiation induced inactivation of Cdk1 by hyperphosphorylation. This effect was prevented by culturing cells in the presence of caffeine after irradiation. After UV B-irradiation the inactivation of Cdk1 was less pronounced and only partially diminished in the presence of caffeine. No alteration in Cdk2 phosphorylation was observed after irradiation in either cell line.

UV-B-induced cell cycle perturbations, micronucleus induction, and modulation by caffeine in human keratinocytes

UV-B-induced perturbations of cell cycle progression in asynchronous human keratinocytes were analysed during two cell cycles with respect to their cell cycle stage at the time of irradiation using BrdUrd/Hoechst flow cytometry. Exponentially growing SCL-2-keratinocytes exposed to UV-B radiation showed a short delay in G1-phase exit and were blocked in the S and G2/M phases of the first cell cycle. UV-A wavelengths did not show any detectable effect on cell cycle progression. In contrast, 137Cs-irradiation of these cells induced a temporary G2 block only. Micronucleus frequency increased in gamma-irradiated cells as soon as the cells started to divide and reached a plateau when most of the cells had divided. Continuous treatment with caffeine starting immediately after 137Cs gamma-irradiation prevented accumulation of cells in G2 phase, but did not influence the frequency of micronuclei. In UV-B-irradiated keratinocytes, however, the damage-induced cell cycle perturbations were merely reduced by caffeine, but not eliminated. Compared with gamma-irradiation a moderate induction of micronuclei was observed in UV-B-irradiated cells. Caffeine, however, potentiated the induction of micronuclei by UV-B. These different effects on cell cycle kinetics and micronucleus induction indicate different mechanisms of DNA damage caused by UV-B- and gamma-irradiation that may be repaired through different pathways.