Mycoplasma infection suppresses p53, activates NF-kappaB and cooperates with oncogenic Ras in rodent fibroblast transformation.

Prokaryotes of the genus Mycoplasma are the smallest cellular organisms that persist as obligate extracellular parasites. Although mycoplasma infection is known to be associated with chromosomal instability and can promote malignant transformation, the mechanisms underlying these phenomena remain unknown. Since persistence of many cellular parasites requires suppression of apoptosis in host cells, we tested the effect of mycoplasma infection on the activity of the p53 and nuclear factor (NF)-kappaB pathways, major mechanisms controlling programmed cell death. To monitor the activity of p53 and NF-kappaB in mycoplasma-infected cells, we used a panel of reporter cell lines expressing the bacterial beta-galactosidase gene under the control of p53- or NF-kappaB-responsive promoters. Cells incubated with media conditioned with different species of mycoplasma showed constitutive activation of NF-kappaB and reduced activation of p53, common characteristics of the majority of human tumor cells, with M. arginini having the strongest effect among the species tested. Moreover, mycoplasma infection reduced the expression level and inducibility of an endogenous p53-responsive gene, p21(waf1), and inhibited apoptosis induced by genotoxic stress. Infection with M. arginini made rat and mouse embryo fibroblasts susceptible to transformation with oncogenic H-Ras, whereas mycoplasma-free cells underwent irreversible p53-dependent growth arrest. Mycoplasma infection was as effective as shRNA-mediated knockdown of p53 expression in making rodent fibroblasts permissive to Ras-induced transformation. These observations indicate that mycoplasma infection plays the role of a p53-suppressing oncogene that cooperates with Ras in cell transformation and suggest that the carcinogenic and mutagenic effects of mycoplasma might be due to inhibition of p53 tumor suppressor function by this common human parasite.

[Transfection with the E1A and E1B-19kDa oncogenes does not prevent rat embryo fibroblasts from cell cycle arrest after gamma-radiation]. / Perenos onkogenov E1A i E1B-19kDa v émbrional'nye fibroblasty krysy ne otmeniaet sposobnosti kletok ostanavlivat'sia v kletochnom tsikle posle gamma-oblucheniia.

Introduction of the E1A early region of the human adenovirus type 5 impairs the ability of mammalian cells to arrest the cell cycle at G1/S after damage. Two-parameter fluorescent-activated cell sorting (FACS) with iododeoxyuridine revealed the radiation-induced G1/S arrest in rat embryo fibroblasts transformed with the complementing E1A + E1B-19 kDa oncogenes. This was due to selective inhibition of CycIE/Cdk2-associated kinase activity, while activities of type 2 kinase and of CyclA/Cdk2 complexes remained unchanged. The inhibitor of G1-phase cyclin kinases, p21/Waf1, was accumulated and interacted with target kinases both in normal and in transformed cells after irradiation. As shown by immunoprecipitation, p21/Waf1 formed complexes with the E1A on coproducts in the transformants, which possibly accounted for its functional inactivation. Kinase modification in cyclin-kinase complexes was assumed to play a key role in regulation of cyclin-dependent kinases in the transformants with inactivated p21/Waf1.

[Anti-apoptotic and antiproliferative effect of bcl-2 gene transferred to E1A+cHa-ras-transformed cells]. / Antiapoptoticheskoe i antiproliferativnoe deistvie gena bcl-2 pri perenose ego v transformanty E1A+cHa-ras.

Transformed rat embryo fibroblasts E1A + cHa-ras known to possess high proapoptotic sensitivity and not to be arrested after DNA damage or upon serum starvation, were transfected with bcl-2 gene using calcium-phosphate precipitation method. Triple transformants E1A + cHa-ras + bcl-2 appeared to be protected from damage- and serum depletion-induced apoptosis and to restore cell cycle checkpoint control. Using the method of flow cytometry we have shown that these transformants are arrested in different phases of cell cycle in response to irradiation, adriamycin treatment and serum deprivation. Overexpression of bcl-2 in E1A + cHa-ras-transformed cells entirely suppresses adriamycin-induced apoptosis and significantly reduces the level of apoptosis triggered by irradiation and growth factor withdrawal, as we have revealed by the test of clonogenic survival and electrophoretic analysis of oligonucleosomal DNA fragmentation. Our results have demonstrated, for the first time, that the oncogenic Ras co-immunoprecipitates with transfected Bcl-2 in E1A + cHa-ras + bcl-2 transformed cells after irradiation but not after adriamycin treatment. Bcl-2-Ras complexes were also observed in transformants E1A + cHa-ras + bcl-2 after serum starvation. Taken together, these data suggest that Bcl-2 and Ras interaction might play a crucial role in the cell cycle checkpoints restoration and apoptotic events regulation in transformants E1A + cHa-ras + bcl-2 exposed to DNA-damaging factors or growth factor-deprived.

[Distribution of rat embryonal fibroblasts through cell cycle phases in the presence of inhibitors of active oxygen species formation and N-acetylcysteine]. / Raspredelenie émbrional'nykh fibroblastov krysy po fazam kletochnogo tsikla v prisutstvii ingibitorov obrazovaniia aktivnykh form kisloroda i N-atetiltsisteina.

Intracellular level of reactive oxygen species (ROS) and distribution of primary rat embryo fibroblast throughout the cell cycle have been studied. Serum-starved cells were activated by the addition of 10% serum to the culture medium in the presence on N-acetyl-cystein (NAC) and ROS-inhibitors, diphenileniodonium (DPI) and rothenone. It has been shown that serum starvation could block the cells at the G1/S boundary. Activation of serum-starved cells by the addition of serum reactivated the cell cycle and caused cell progression into S phase, which was paralleled with the increase in the intracellular level of ROS. Effects of NAC, PAI and rothenone, similar to that of serum starvation, blocked cell progression into S phase and decreased ROS formation due to the action of serum growth factors. The antiproliferative effect of NAC is discussed.

[Rat embryo fibroblasts transformed by complementation with oncogenes E1A+E1B-19 and E1A+cHa-ras differ in the ability to realize the G1/S block in serum free media]. / Transformanty, poluchennye iz émbrional'nykh fibroblastov krysy komplementatsiei onkogenov E1A+E1B-19 i E1A+cHa-ras, razlichaiutsia po sposobnosti realizovat' blok G1/S v usloviiakh syvorotochnogo golodaniia.

The capability of REF cells transformed by EA + E1B-19 kDa and EA + cHa-ras oncogenes to realize the G1/S cell cycle arrest upon serum starvation was studied. The amount of cyclin-kinase inhibitor protein p27/Kip was shown to increase in both normal and transformed cells. However, the p27/Kip-bound cyclin-kinase complexes of transformed cells were found to be active, implying the functional inactivation of p27/Kip inhibitor. Nevertheless, in contrast to E1A + cHa-ras transformants, E1A + E1B-19 kDa transformants undergo the G1 cell cycle arrest. The G1 cell cycle block correlates with the decrease in cyclinE-Cdk2 activity. Since cyclinE-Cdk2 complexes need Thr-160 phosphorylation of Cdk2 by CAK-kinase for full activity, we have analysed the Cdk-7 associated activity upon serum starvation using gst-Cdk2 as a substrate. Serum starvation did not affect CAK activity either in E1A + cHa-ras or in E1A + E1B-19 kDa transformants. Thus, selective suppression of cyclineE-Cdk2 activity in E1A + E1B-19 kDa transformants upon serum starvation does not arise from the action of cyclin-kinase inhibitors, or from change in CAK activity.

[Function of an inhibitor of cyclin-dependent kinase p27/Kip in cells transformed by E1A + E1B19 kDa + E1A + cHa-Ras, differing in their ability to realize a G1-block during serum starvation]. / Izuchenie funktsionirovania ingibitora tsiklinzavisimykh kinaz p27/Kip v kletkakh-transformantakh E1A + E1B19 kDa i E1A + cHa-Ras, razlichaiushchikhsia po sposobnosti realizovat' G1-blok pri syvorotochnom golodanii.

We studied the capability of E1A + cHa-ras and E1A + E1B19kDa transformants to undergo the G1/S arrest of the cell cycle following depletion of serum growth factors. It has been shown that serum starvation induced the G1/S arrest both in normal rat embryo fibroblasts (REF) and in E1A + E1B19kDa transformants, whereas E1A + cHa-ras transformed cells lost this feature. To analyse the mechanisms underlying these differences, we studied the expression of p27/KIP, its intracellular distribution and association with E1A oncoproducts. The content of the p27/KIP inhibitor of cyclin-dependent kinases was found to change a little upon transformation by two complementary oncogene pairs. However, serum starvation for 24 h led to a significant increase in the content of p27/KIP in E1A + E1B19kDa transformants, while E1A + cHa-ras cells accumulated p27/KIP less markedly. According to the immunofluorescence study, the p27/KIP inhibitor is located in the nucleus of both normal and transformed cells. Moreover, serum starvation did not lead to its inhibition due to redistribution to the cytoplasm in both cell lines. Also, we were unable to detect association of p27/KIP with E1A oncoproducts in immunoprecipitated complexes. The obtained data indicate that, in contrast to E1A + cHa-ras transformants, in E1A + E1B19kDa cells the p27/KIP inhibitor is functional and it is capable of inducing the G1/S block after serum starvation.

Deregulation of p53/p21Cip1/Waf1 pathway contributes to polyploidy and apoptosis of E1A+cHa-ras transformed cells after gamma-irradiation.

The p53/p21Cip1/Waf1-dependent checkpoint control of G1/S and G2/M phases of the cell cycle in response to DNA damage is an important mechanism of genome stability maintenance in normal cells. In many tumor cells, due to frequent point mutations and deletions of p53, the stringent control of the cell cycle and apoptosis is compromised. We have examined the cell cycle control and cell death of the rat embryo fibroblast cells (REF) transformed by E1A+cHa-ras oncogenes and expressing wild type p53. Gamma-irradiation at a dosage of 6 Gy has been used to analyse the p53-dependent trans-activation of the target p21cip1/waf1 gene and the levels of activity of cyclin-dependent kinases. Our results show that the cell cycle inhibitors p21Cip1/Waf1 and p27KIP accumulate in response to irradiation both in REF and E1A+cHa-ras cells. In contrast to normal REF cells, the accumulation of p21Cip1/Waf1 and p27KIP inhibitors, however, does not lead to inhibition of Cdk2 and cyclins E, A-associated kinase activities and to a G1/S block in E1A+cHa-ras cells. It is unlikely that the lack of inhibitory function of p21Cip1/Waf1 can be explained by its inability to bind Cdk2 and Cdk4 kinases or PCNA. Moreover, the p21Cip1/Waf1-associated kinase activity is increased upon gamma-irradiation of E1A+cHa-ras cells. We suggest that inactivation of p21Cip1/Waf1 may be accounted for by its interaction with E1A oncoproducts as the inhibitor is detected in immunoprecipitates using E1A-specific antibodies. During a temporary G2/M delay induced by gamma-irradiation, E1A+cHa-ras transformants continue DNA replication, which leads to accumulation of polyploid cells with lobulated nuclei and micronuclei. Thus, DNA damage of E1A+cHa-ras transformed cells, with a combination of functionally active wild type p53 and inactive p21Cip1/Waf1, contributes to formation of polyploid cells which then die due to apoptosis.

[Apoptotic cell death in rat embryo fibroblasts transformed by EiA + cHa-RAS oncogenes after gamma irradiation]. / Analiz kharaktera apopticheskoi gibeli émbrional'nykh fibroblastov krysy, transformirovannykh onkogenami E1A + cHa-RAS, posle gamma-oblucheniia.

A mechanism of apoptotic death of normal rat embryo fibroblasts and of those transformed by E1A + cHa-Ras oncogenes following gamma irradiation has been investigated. The E1A + cHa-Ras transformed cells were shown to express wild type p53 which was able to trans-activate a reporter pG13-luc Plasmid. As a result of trans-activation, an accumulation of universal inhibitor of cyclin-dependent kinases--p21/Waf1 protein and an increase in the proportion of p21/Waf1 expressing cells were observed, The accumulated p21/Waf1 was found to bind with PCNA. The association with PCNA, however, did not lead to suppression of DNA replication according to the data of iododeoxyuridine (IdUr) incorporation. A high proportion of S-phase cells, in combination with cell cycle blocking in G2-phase, promoted polyploidization of E1A + cHa-Ras transformed cells after gamma irradiation. The polyploidic cells with DNA content equal and higher than 8c die 48-72 h following irradiation due to apoptosis. A significant proportion of E1A + cHa-Ras cells with incorporated IdUr contains labeled micronuclei, the fact being a morphological evidence of apoptosis of cells in S-phase of the cell cycle.