The cell cycle and apoptosis

PAK2 activated by Cdc42 and caspase 3 mediates different cellular responses to oxidative stress-induced apoptosis

p21-activated protein kinase (PAK2) is a unique member of the PAK family kinases that plays important roles in stress signaling. It can be activated by binding to the small GTPase, Cdc42 and Rac1, or by caspase 3 cleavage. Cdc42-activated PAK2 mediates cytostasis, whereas caspase 3-cleaved PAK2 contributes to apoptosis. However, the relationship between these two states of PAK2 activation remains elusive. In this study, through protein biochemical analyses and various cell-based assays, we demonstrated that full-length PAK2 activated by Cdc42 was resistant to the cleavage by caspase 3 in vitro and within cells. When mammalian cells were treated by oxidative stress using hydrogen peroxide, PAK2 was highly activated through caspase 3 cleavage that led to apoptosis. However, when PAK2 was pre-activated by Cdc42 or by mild stress such as serum deprivation, it was no longer able to be cleaved by caspase 3 upon hydrogen peroxide treatment, and the subsequent apoptosis was also largely inhibited. Furthermore, cells expressing active mutants of full-length PAK2 became more resistant to hydrogen peroxide-induced apoptosis than inactive mutants. Taken together, this study identified two states of PAK2 activation, wherein Cdc42- and autophosphorylation-dependent activation inhibited the constitutive activation of PAK2 by caspase cleavage. The regulation between these two states of PAK2 activation provides a new molecular mechanism to support PAK2 as a molecular switch for controlling cytostasis and apoptosis in response to different types and levels of stress with broad physiological and pathological relevance.

Adeno-associated virus type 2 infection activates caspase dependent and independent apoptosis in multiple breast cancer lines but not in normal mammary epithelial cells

Background

In normal cells proliferation and apoptosis are tightly regulated, whereas in tumor cells the balance is shifted in favor of increased proliferation and reduced apoptosis. Anticancer agents mediate tumor cell death via targeting multiple pathways of programmed cell death. We have reported that the non-pathogenic, tumor suppressive Adeno-Associated Virus Type 2 (AAV2) induces apoptosis in Human Papillomavirus (HPV) positive cervical cancer cells, but not in normal keratinocytes. In the current study, we examined the potential of AAV2 to inhibit proliferation of MCF-7 and MDA-MB-468 (both weakly invasive), as well as MDA-MB-231 (highly invasive) human breast cancer derived cell lines. As controls, we used normal human mammary epithelial cells (nHMECs) isolated from tissue biopsies of patients undergoing breast reduction surgery.

Results

AAV2 infected MCF-7 line underwent caspase-independent, and MDA-MB-468 and MDA-MB-231 cell lines underwent caspase-dependent apoptosis. Death of MDA-MB-468 cells was marked by caspase-9 activation, whereas death of MDA-MB-231 cells was marked by activation of both caspase-8 and caspase-9, and resembled a mixture of apoptotic and necrotic cell death. Cellular demise was correlated with the ability of AAV2 to productively infect and differentially express AAV2 non-structural proteins: Rep78, Rep68 and Rep40, dependent on the cell line. Cell death in the MCF-7 and MDA-MB-231 lines coincided with increased S phase entry, whereas the MDA-MB-468 cells increasingly entered into G2. AAV2 infection led to decreased cell viability which correlated with increased expression of proliferation markers c-Myc and Ki-67. In contrast, nHMECs that were infected with AAV2 failed to establish productive infection or undergo apoptosis.

Conclusion

AAV2 regulated enrichment of cell cycle check-point functions in G1/S, S and G2 phases could create a favorable environment for Rep protein expression. Inherent Rep associated endonuclease activity and AAV2 genomic hair-pin ends have the potential to induce a cellular DNA damage response, which could act in tandem with c-Myc regulated/sensitized apoptosis induction. In contrast, failure of AAV2 to productively infect nHMECs could be clinically advantageous. Identifying the molecular mechanisms of AAV2 targeted cell cycle regulation of death inducing signals could be harnessed for developing novel therapeutics for weakly invasive as well as aggressive breast cancer types.

Adeno-associated virus type 2 induces apoptosis in human papillomavirus-infected cell lines but not in normal keratinocytes

The results of seroepidemiological studies suggest that infection with adeno-associated virus type 2 (AAV2) is negatively correlated with the incidence of human papillomavirus (HPV)-associated cervical cancer. We studied the potential of AAV2 oncosuppression of HPV and showed that HPV/AAV2 coinfection of cells culminated in apoptotic death, as determined by DNA laddering and caspase-3 cleavage. The induction of apoptosis coincided with AAV2 Rep protein expression; increased S-phase progression; upregulated pRb displaying both hyper- and hypophosphorylated forms; increased levels of p21(WAF1), p16(INK4), and p27(KIP1) proteins; and diminished levels of E7 oncoprotein. In contrast, normal keratinocytes that were infected with AAV2 or transfected with the cloned full-length AAV2 genome failed to express Rep proteins or undergo apoptosis. The failure of AAV2 to productively infect normal keratinocytes could be clinically advantageous. The delineation of the molecular mechanisms underlying the HPV/AAV2 interaction could be harnessed for developing novel AAV2-derived therapeutics for cervical cancer.

Taxanes, microtubules and chemoresistant breast cancer

The taxanes, paclitaxel and docetaxel are microtubule-stabilizing agents that function primarily by interfering with spindle microtubule dynamics causing cell cycle arrest and apoptosis. However, the mechanisms underlying their action have yet to be fully elucidated. These agents have become widely recognized as active chemotherapeutic agents in the treatment of metastatic breast cancer and early-stage breast cancer with benefits gained in terms of overall survival (OS) and disease-free survival (DFS). However, even with response to taxane treatment the time to progression (TTP) is relatively short, prolonging life for a matter of months, with studies showing that patients treated with taxanes eventually relapse. This review focuses on chemoresistance to taxane treatment particularly in relation to the spindle assembly checkpoint (SAC) and dysfunctional regulation of apoptotic signaling. Since spindle microtubules are the primary drug targets for taxanes, important SAC proteins such as MAD2, BUBR1, Synuclein-gamma and Aurora A have emerged as potentially important predictive markers of taxane resistance, as have specific checkpoint proteins such as BRCA1. Moreover, overexpression of the drug efflux pump MDR-1/P-gp, altered expression of microtubule-associated proteins (MAPs) including tau, stathmin and MAP4 may help to identify those patients who are most at risk of recurrence and those patients most likely to benefit from taxane treatment.

2-Methoxyestradiol induces mitochondria dependent apoptotic signaling in pancreatic cancer cells

The antiproliferative action of 2-methoxyestradiol (2-ME), an endogenous estrogen metabolite is specific for cancer cells and is mediated by the induction of programmed cell death or apoptosis. But the identity of the downstream effectors of apoptotic signaling induced by 2-ME is not known. In the present study, we explored the effect of 2-ME on apoptosis in a panel of human pancreatic cancer cell lines. We have identified two categories of pancreatic cancer cell lines, which are either sensitive to 2-ME such as MIA PaCa-2, CFPAC-1, PANC-1, or non-sensitive to 2-ME such as Hs 766T. The results presented here indicated that the cell lines responsive to 2-ME could undergo apoptosis either by G2-M arrest (PANC-1) with Bcl-x(L) phosphorylation or by the accumulation of tetraploid cells in G1-S region (MIA PaCa-2) without Bcl-2/ Bcl-x(L) phosphorylation. Furthermore, 2-ME induced apoptosis in pancreatic cancer cells is mitochondria dependent as evident by the release of cytochrome c into the cytosol. 2-ME exposed cells exhibit Bid cleavage that is accompanied by the translocation of Bax into the mitochondria. Also 2-ME could induce phosphorylation of Bcl-x(L) in G2-M arrested cells, thus indicating the involvement of various anti- and pro-apoptotic regulators in the signaling cascade. The dissection of differential response of pancreatic cancer cell lines holds promise for future therapeutic intervention.

Topoisomerase I inhibitor (camptothecin)-induced apoptosis in human gastric cancer cells and the role of wild-type p53 in the enhancement of its cytotoxicity

Camptothecin (CPT), a human topoisomerase I inhibitor, blocks DNA replication in human cancer cells. It represents a promising new class of chemotherapeutic agents with broad anti-tumor activity. However, its effect on gastric cancer cells remains unknown. We examined cell growth, apoptosis and cell cycle phase distribution in gastric cancer cells by exposing these cells to CPT for up to 72 h. Cell viability was determined by the Trypan blue exclusion assay. Cell cycle phase distribution and apoptosis were measured using flow cytometry, fluorescence microscopy and DNA ladder assay. Exposure of exponentially growing gastric AGS cancer cells to CPT induced time-dependent apoptosis and growth inhibition. Serum starvation-synchronized AGS cells (about 60% cells in G0/G1 phase) showed similar cellular responses. Analysis of cell cycle phase distribution of AGS cells treated with CPT for up to 72 h showed no obvious differences compared to untreated control cells. Although the induction of apoptosis was noticed in gastric cancer cell lines both with and without p53, cells lacking p53 showed less apoptosis compared to those cell lines possessing p53. Our data show that CPT is capable of inducing gastric cancer cell growth inhibition and apoptosis. Wild-type p53 may enhance the cytotoxicity of CPT against gastric carcinoma.

Adeno-associated virus type 2 Rep78 induces apoptosis through caspase activation independently of p53

Adeno-associated virus (AAV) type 2 Rep78 is a multifunctional protein required for AAV DNA replication, integration, and gene regulation. The biochemical activities of Rep78 have been described, but the effects of Rep proteins on the cell have not been characterized. We have analyzed Rep-mediated cytotoxicity. We demonstrated that Rep78 expression is sufficient to induce cell death and disruption of the cell cycle. Cell death was found to be mediated by apoptosis. Rep78 expression resulted in the activation of caspase-3, a terminal caspase directly involved in the execution of cell death. A peptidic inhibitor of caspase-3, Z-Asp-Glu-Val-Asp-fluoromethylketone (Z-DEVD-FMK), abrogated Rep78-induced apoptosis, indicating that Rep78-mediated apoptosis is caspase-3 dependent. Rep78 induced apoptosis in wild-type p53-containing human embryonal carcinoma NT-2 cells and in p53-null promyelocytic human HL-60 cells, indicating that at least one pathway of Rep78-induced apoptosis is p53 independent. Apoptosis was shown to occur during the G(1) and early S phases of the cell cycle. By analyzing the effects of Rep78 mutations on cell viability, the cause of cell death was attributed in part to two biochemical activities of Rep78, DNA binding and ATPase/helicase activity. The endonuclease activity of Rep78 did not contribute to apoptosis induction.

p53 accumulation due to down-regulation of ubiquitin: relevance for neuronal apoptosis

The p53 tumor suppressor protein is a major regulator of cell growth arrest and apoptosis in response to DNA damage. Both p53 function and stability are tightly controlled by Mdm2, which binds to the p53 N-terminus and targets p53 for ubiquitin-mediated proteolysis. Previous studies suggest that adrenalectomy-induced neuronal apoptosis is p53-dependent. Here we demonstrate both nuclear accumulation and functional activation of p53 protein in apoptotic hippocampal neurons from adrenalectomized rats. Increased p53 expression occurred despite the accumulation of its negative regulator, Mdm2, and the formation of p53-Mdm2 complexes. The persistence of p53 expression was explained by a striking decrease in free ubiquitin in p53-positive neurons. The addition of exogenous ubiquitin to p53-Mdm2 complexes from apoptotic neurons restored p53 degradation. These findings demonstrate a novel mechanism of p53 stabilization mediated by decreased ubiquitin levels. Regulation of free ubiquitin may therefore be an effective way to modulate p53-dependent apoptosis in certain cell types.