Radiosensitization of HL-60 human leukaemia cells by bryostatin-1 in the absence of increased DNA fragmentation or apoptotic cell death

Bryostatin-1 causes radiosensitization of BMG-1 malignant glioma cells through differential activation of protein kinase-Cδ not evident in the non-malignant AA8 fibroblasts

Bryostatin-1 (bryo-1), a non-phorbol ester, is known to sensitize mammalian cells against certain chemotherapeutic drugs. We assessed its ability to modify radiation response of mammalian cells using Chinese hamster fibroblasts AA8 cells and human malignant glioma BMG-1 cells. In the malignant glioma BMG-1 cell line, bryo-1 pre-treatment significantly enhanced radiation-induced growth inhibition and cytogenetic damage, and further reduced the clonogenic cell survival as compared to cells irradiated at the clinically relevant dose of 2 Gy. PKCδ expression increased significantly when bryo-1 pre-treated BMG-1 glioma cells were irradiated at 2 Gy and induced prolonged ERK-1/2 activation associated with p21 overexpression. Silencing PKCδ resulted in inhibition of bryo-1-induced radiosensitization. In contrast, bryo-1 failed to alter radiosensitivity (cell survival; growth inhibition; cytogenetic damage) or activate ERK1/2 pathway in the AA8 fibroblasts despite PKCδ phosphorylation at its regulatory (Y155) domain, indicating alternate mechanisms in these non-malignant cells as compared to the glioma cells. This study suggests that bryo-1 may effectively enhance the radiosensitivity of malignant cells and warrants further in-depth investigations to evaluate its radiosensitizing potential in various cell types.

Differential role of hydrogen peroxide and staurosporine in induction of cell death in glioblastoma cells lacking DNA-dependent protein kinase

Various DNA double-strand break repair mechanisms, in which DNA-dependent protein kinase (DNA-PK) has a major role, are involved both in the development and treatment of glioblastoma. The aim of the present study was to investigate how glioblastoma cells responded to hydrogen peroxide and staurosporine (STS) and how such a response is related to DNA-PK. Two human glioblastoma cell lines, M059J cells that lack DNA-PK activity, and M059K cells that express a normal level of DNA-PK, were exposed to hydrogen peroxide or STS. The response of the cells to hydrogen peroxide or STS was recorded by measuring cell death, which was detected by three different methods-MTT, annexin-V and propidium iodide staining, and JC-1 mitochondrial probe. The result showed that both hydrogen peroxide and STS were able to induce cell death of the glioblastoma cells and that the former was mainly associated with necrosis and the latter with apoptosis. Glioblastoma cells lacking DNA-PK were less sensitive to STS treatment than those containing DNA-PK. However, DNA-PK had no significant influence on hydrogen peroxide treatment. We further found that catalase, an antioxidant enzyme, could prevent cell death induced by hydrogen peroxide but not by STS, suggesting that the pathways leading to cell death by hydrogen peroxide and STS are different. We conclude that hydrogen peroxide and STS have differential effects on cell death of glioblastoma cells lacking DNA-dependent protein kinase. Such differential roles in the induction of glioblastoma cell death can be of significant value in selecting and/or optimizing the treatment for this malignant brain tumor.

The clinical development of the bryostatins

The bryostatins are a group of novel macrocyclic lactones derived from the marine bryozoan, Bugula neritina. In vitro evidence indicates that their main mechanism of action is modulation of protein kinase C (PKC) activity. Phase I studies suggested significant antineoplastic activity against several tumor types and defined the main dose-limiting toxicity as myalgia. Bryostatin-1 has subsequently been investigated extensively in phase II clinical trials as a single agent, although trial design has been hampered by lack of human pharmacokinetic data. Results have been generally disappointing but in vitro and animal data suggests an important role for bryostatin-1 in combination with cytotoxic agents. Preliminary results of phase I studies support these observations but further work needs to be done to define the future role of the bryostatins in the clinic.

Growth arrest and cell death in the breast tumor cell in response to ionizing radiation and chemotherapeutic agents which induce DNA damage

Breast tumor cells are relatively refractory to apoptosis in response to modalities which induce DNA damage such as ionizing radiation and the topoisomerase II inhibitor, adriamycin. Various factors which may modulate the apoptotic response to DNA damage include the p53 status of the cell, levels and activity of the Bax and Bcl-2 families of proteins, activation of NF-kappa B, relative levels of insulin like growth factor and insulin-like growth factor binding proteins, activation of MAP kinases and PI3/Akt kinases, (the absence of) ceramide generation and the CD95 (APO1/Fas) signaling pathway. Prolonged growth arrest associated with replicative senescence may represent an alternative and reciprocal response to DNA-damage induced apoptosis that is p53 and/or p21waf1/cip1 dependent while delayed apoptosis may occur in p53 mutant breast tumor cells which fail to maintain the growth-arrested state. Clearly, the absence of an immediate apoptotic response to DNA damage does not eliminate other avenues leading to cell death and loss of self-renewal capacity in the breast tumor cell. Nevertheless, prolonged growth arrest (even if ultimately succeeded by apoptotic or necrotic cell death) could provide an opportunity for subpopulations of breast tumor cells to recover proliferative capacity and to develop resistance to subsequent clinical intervention.

Pharmacology and clinical experience with bryostatin 1: a novel anticancer drug

Bryostatin 1 (bryo 1) is an example of a novel class of anticancer drug which modulates protein kinase C (PKC) activity. It has varied biological effects mediated largely by the initial activation of PKC, followed by its rapid downregulation. Bryo 1 stimulates in vitro and in vivo haematopoietic progenitor cell growth in a concentration-dependent and lineage-specific fashion. Granulocytes, lymphocytes, monocytes and platelets are all functionally stimulated by bryo 1. Stimulation of cytotoxic T-cell activity by bryo 1 has led to research utilising bryo 1 as an immunotherapeutic agent in mouse tumour xenograft models. The clinical development of bryo 1 followed the demonstration of direct in vitro activity against various tumour cell lines. Multiple Phase I trials have shown muscle pain and flu-like symptoms are the most common toxicities associated with administration of bryo 1. There is particular interest in the role of bryo 1 in haematologic malignancies because of its capacity to induce leukaemic cell differentiation. There is ample in vitro data demonstrating that bryo 1 can sensitise tumour cells to cytotoxic agents. Recent clinical work has focused on combining bryo 1 with traditional chemotherapeutic agents for both haematologic and non-haematologic cancers.

Comparative analysis of apoptosis in HL60 detected by annexin-V and fluorescein-diacetate

BACKGROUND

Our aim was to compare and evaluate apoptosis formation as detected by propidium-iodide (PI)/annexin-V or PI/fluorescein-diacetate (FDA) as dose-response parameters in a human promyelocytic leukemia cell line, HL60.

METHODS

In exponentially growing HL60 cells, apoptosis was induced by ionizing radiation, hyperthermia, topotecan, and cytosine beta-D-arabinofuranoside. At 4 consecutive days following induction, apoptosis was detected by double-labelling, either with PI/annexin-V or PI/FDA. Forward and side scatter, red (PI), and green (FDA or annexin-V) fluorescence were measured by flow cytometry.

RESULTS

While light scatter discriminated between morphologically damaged and undamaged cells, fluorescence differentiated vital, apoptotic, and dead cells. Equal proportions of these three subpopulations were detected by both staining techniques. Occasionally, early and mature apoptoses were identified as distinct clusters. During the 4-day observation period, no pronounced maxima of the apoptotic fractions were obtained with either treatment modality. The gradual increases usually showed a delay of 1-2 days.

CONCLUSIONS

FDA and annexin-V are equally suitable for detecting apoptosis. Separation improves with time after induction, indicating that, with respect to test specificity, mature apoptoses are superior to early stages. However, the sensitivity towards low rates of apoptosis after weak induction appears limited with both staining procedures.

The role of protein kinase C activity in the killing of Chinese hamster ovary cells by ionizing radiation and photodynamic treatment

In several recent studies it has been shown that protein kinase C (PKC) activity may either potentiate or antagonize cell killing by different cytotoxic agents. These apparently conflicting observations suggest that the effects of PKC activity on cell survival may depend on the different properties of different cell types but do not exclude the possibility that the effects may also depend on the nature of the cytotoxic agent. In this context the effects of PKC activation and PKC inhibition or down-regulation on Chinese hamster ovary (CHO) cell survival after photodynamic treatment and ionizing radiation were studied. It appeared that PKC activation by short-term incubation with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) protected CHO cells against ionizing radiation but, in contrast, sensitized the cells to photodynamic treatment. Conversely, inhibition of PKC by H7 and down-regulation of PKC activity by prolonged incubation with TPA sensitized CHO cells to ionizing radiation but protected the cells against photodynamic treatment. These results demonstrate that in one particular cell type PKC activity may have opposite effects on cell survival following cellular damage, depending on the nature of the cytotoxic agent.