Bcl-2 inhibits early apoptotic events and reveals post-mitotic multinucleation without affecting cell cycle arrest in human epithelial tumor cells exposed to etoposide

Defective apoptotic mechanisms are considered to play a role in both the development of malignancy and resistance to chemotherapeutic drugs. The Bcl-2 family of proteins regulate the cellular commitment to survive or die when challenged with various apoptotic stimuli.

PURPOSE

The purpose of this study was to identify the point at which Bcl-2 interrupts the apoptotic cascade initiated following exposure of human tumor cells to etoposide.

METHODS

A stable Bcl-2-expressing HeLa-transfected clonal cell line, along with its control-vector-transfected counterpart, were utilized in this study. Following etoposide exposure, cells were examined for cell cycle arrest, formation of hyperdiploid cells, apoptotic DNA degradation, loss of plasma membrane integrity, levels of expression of members of the Bcl-2 protein family, caspase activation, degradation of poly(ADP-ribose) polymerase and movement of Bax from cytosol to cellular membrane fractions.

RESULTS

Caspase activation, poly(ADP-ribose) polymerase degradation and Bax membrane insertion were initiated rapidly following etoposide removal, concomitantly with cell cycle arrest. Whereas Bcl-2 had no effect on etoposide-induced cell arrest, it interrupted all aspects of apoptosis, including activation of caspases, poly(ADP-ribose) polymerase degradation, DNA fragmentation and loss of plasma membrane integrity. Surprisingly, Bcl-2 also blocked Bax membrane insertion. In addition, Bcl-2 also prevented the increase in cellular levels of Bak, Bax and Bcl-xL, along with degradation of actin and Bax. However, inhibition of etoposide-induced apoptosis by Bcl-2 resulted in the accumulation of giant, multinucleated cells that eventually lost the ability to exclude trypan blue without apoptotic morphology or DNA degradation.

CONCLUSIONS

These results indicate that biochemical apoptotic processes are initiated concomitant with etoposide-induced cell cycle arrest and are interrupted by Bcl-2 overexpression. However, the aberrant mitotic events induced by etoposide are sufficient to kill these cells even in the absence of apoptosis.

Expression of Bcl-2 in human epithelial tumor (HeLa) cells enhances clonogenic survival following exposure to 5-fluoro-2'-deoxyuridine or staurosporine, but not following exposure to etoposide or doxorubicin

A reduced capacity for apoptosis induction is considered to play a significant role both in the development of malignancy and in tumor cell resistance to chemotherapeutic drugs. The Bcl-2 oncoprotein inhibits apoptosis induced by antitumor agents at a point downstream of drug-target interactions. Stable expression of Bcl-2 in the human epithelial tumor (HeLa) cell line results in inhibition of apoptosis following exposure to the topoisomerase II poison, etoposide. However, Bcl-2 is unable to enhance clonogenic survival as a result of alternate pathways to reproductive death induced by the drug.

PURPOSE

The purpose of this study was to further investigate the role of Bcl-2 in human epithelial tumor cell drug resistance using 5-fluoro-2'-deoxyuridine, staurosporine, and doxorubicin, in addition to etoposide.

METHODS

The ability of Bcl-2 to enhance clonogenic cell survival was studied by colony-forming assays, while delay of cell death induction was assessed by trypan blue viability measurements. The proportion of apoptotic cells was measured by morphological criteria, as well as detection of apoptotic DNA fragmentation using the terminal deoxynucleotidyl transferase assay.

RESULTS

Despite profound inhibition to loss of plasma membrane integrity for all agents tested, Bcl-2 was only able to significantly increase clonogenic survival following exposure to 5-fluoro-2'-deoxyuridine and staurosporine, but not following exposure to etoposide or doxorubicin. Furthermore, the time-course of apoptosis induction following exposure of HeLa cells to equitoxic concentrations of staurosporine and etoposide was profoundly different.

CONCLUSIONS

These results indicate that Bcl-2 enhances clonogenic survival of human epithelial tumor cells in an agent-specific fashion, which may be determined by the initial cytotoxic lesion induced by a particular drug.

Potentiation of etoposide-induced apoptosis by staurosporine in human tumor cells is associated with events downstream of DNA-protein complex formation

Protein kinase inhibitors have demonstrated potential for use in the therapy of human cancers, in particular leukemia. Staurosporine, a protein kinase inhibitor with broad specificity, enhances the cytotoxic effects of various antitumor agents with different modes of action. The topoisomerase II inhibitor, etoposide, has shown clinical activity against a wide range of tumor types.

PURPOSE

The purpose of this study was to assess the effects of staurosporine on etoposide-induced cell death processes in a human tumor of epithelial origin.

METHODS

Modulation of etoposide-induced apoptosis by staurosporine in HeLa cells was assessed by cell morphology, extraction of low molecular weight DNA, quantitation of DNA-protein complexes, and measurements of rates of DNA synthesis. The effects on cellular genes implicated in apoptosis were determined by Northern and Western blotting, along with assays of cyclin-dependent kinase activities.

RESULTS

Staurosporine exhibited a two- to three-fold potentiation of apoptosis caused by etoposide in HeLa cells when applied concurrently, or immediately following etoposide removal, but did not alter the quantity of DNA-protein complexes produced by etoposide. Etoposide-induced apoptosis, and its potentiation by staurosporine, were associated with reduced c-myc expression, and a moderate increase in p21WAF1/CIP1 mRNA and protein levels. Inhibitors of cyclic AMP-dependent protein kinase and protein kinase C, which exhibit greater specificity than staurosporine, were without effect on apoptosis caused by etoposide, whereas use of the tyrosine phosphatase inhibitor, vanadate, resulted in its abrogation. The potentiation of etoposide-induced apoptosis by staurosprine was associated with a significant increase in cyclin A-dependent kinase activity. In addition, etoposide caused substantial inhibition of DNA synthesis.

CONCLUSION

These results indicate that staurosporine potentiates apoptosis through events which occur downstream of DNA damage, and implicate unscheduled activation of cyclin A-dependent kinase during inhibition of DNA synthesis as a possible cause.

Dual modes of death induced by etoposide in human epithelial tumor cells allow Bcl-2 to inhibit apoptosis without affecting clonogenic survival

The Bcl-2 oncoprotein, which is expressed in a variety of human malignancies, blocks apoptosis induced by chemotherapeutic drugs, including the topoisomerase II inhibitor, etoposide. To determine the significance of Bcl-2 in etoposide-induced death of human epithelial tumor cells, HeLa S3 cells were transfected with human bcl-2 cDNA in the pSFFV expression vector, and stable Bcl-2-expressing clones established. In agreement with previous studies, Bcl-2 inhibited loss of cell viability (by trypan blue exclusion), the appearance of morphologically apoptotic cells, and the amount of low molecular weight DNA extracted after etoposide exposure (25 microns, 4 h). The degree of inhibition, compared to wild-type and vector control-transfected clones, differed according to the level of Bcl-2 protein expressed in the two clones studied. However, when cell survival was assessed by colony-forming assays, no significant differences were detected at any of the etoposide concentrations used. Although Bcl-2 inhibited etoposide-induced apoptosis, it had no effect on the formation of giant, multinucleated cells characteristic of mitotic catastrophe. Consequently, the ability of Bcl-2 to prevent apoptosis caused by chemotherapeutic drugs may not necessarily translate into increased survival of cells that express Bcl-2.

Differential ability of 2,4-dinitrophenol to modulate etoposide cytotoxicity in mammalian tumor cell lines associated with inhibition of macromolecular synthesis

The inhibitor of oxidative phosphorylation, 2,4-dinitrophenol (DNP), abrogates etoposide cytotoxicity in murine leukemia L1210 cells without affecting the quantity of drug-induced DNA lesions. Cell cycle arrest and events associated with cell death were followed in etoposide treated L1210 cells under conditions in which DNP reduced cytotoxicity greater than 100-fold. Micronucleation, associated with mitotic catastrophe, and apoptotic internucleosomal degradation of DNA, were both inhibited by DNP co-treatment to an extent consistent with clonogenic survival. However, the ability of etoposide to cause cell cycle arrest was minimally affected by DNP. For the same proportion of cells arresting in G(2), DNP co-treatment profoundly reduced etoposide cytotoxicity, suggesting a separation between etoposide-induced G(2) arrest and cell death. At the same concentration used to treat L1210 cells, DNP was unable to abrogate etoposide cytotoxicity in HeLa, Chinese hamster ovary or HL60 cells. The relationship between ongoing macromolecular synthesis during etoposide treatment and clonogenic survival was further studied in L1210 and HeLa cells. In general, L1210 cells were more sensitive than HeLa cells to inhibition of macromolecular synthesis by DNP. A higher DNP concentration did partially block etoposide cytotoxicity in HeLa cells, in association with increased inhibition of macromolecular synthesis. It was not possible to attribute the reduced cytotoxicity of etoposide in HeLa cells to inhibition of DNA or RNA synthesis alone, because inhibitors with greater specificity (aphidicolin and DRB) had no effect on clonogenic survival. However, aphidicolin partially abrogated etoposide cytotoxicity in L1210 cells, although to a lesser extent than DNP. These data indicate that inhibition of DNA or RNA synthesis alone during etoposide exposure is insufficient to abrogate killing of HeLa cells, that inhibition of etoposide cytotoxicity in HeLa cells may require the additional inhibition of protein synthesis, and that the modulating effects of ongoing DNA synthesis on etoposide cytotoxicity are cell line dependent.