Is induction of type 2 programmed death in cancer cells from solid tumors directly related to mitochondrial mass?

Mitochondria and energetic depression in cell pathophysiology

Mitochondrial dysfunction is a hallmark of almost all diseases. Acquired or inherited mutations of the mitochondrial genome DNA may give rise to mitochondrial diseases. Another class of disorders, in which mitochondrial impairments are initiated by extramitochondrial factors, includes neurodegenerative diseases and syndromes resulting from typical pathological processes, such as hypoxia/ischemia, inflammation, intoxications, and carcinogenesis. Both classes of diseases lead to cellular energetic depression (CED), which is characterized by decreased cytosolic phosphorylation potential that suppresses the cell's ability to do work and control the intracellular Ca(2+) homeostasis and its redox state. If progressing, CED leads to cell death, whose type is linked to the functional status of the mitochondria. In the case of limited deterioration, when some amounts of ATP can still be generated due to oxidative phosphorylation (OXPHOS), mitochondria launch the apoptotic cell death program by release of cytochrome c. Following pronounced CED, cytoplasmic ATP levels fall below the thresholds required for processing the ATP-dependent apoptotic cascade and the cell dies from necrosis. Both types of death can be grouped together as a mitochondrial cell death (MCD). However, there exist multiple adaptive reactions aimed at protecting cells against CED. In this context, a metabolic shift characterized by suppression of OXPHOS combined with activation of aerobic glycolysis as the main pathway for ATP synthesis (Warburg effect) is of central importance. Whereas this type of adaptation is sufficiently effective to avoid CED and to control the cellular redox state, thereby ensuring the cell survival, it also favors the avoidance of apoptotic cell death. This scenario may underlie uncontrolled cellular proliferation and growth, eventually resulting in carcinogenesis.

Mitochondria: A novel target for the chemoprevention of cancer

The mitochondria have emerged as a novel target for anticancer chemotherapy. This tenet is based on the observations that several conventional and experimental chemotherapeutic agents promote the permeabilization of mitochondrial membranes in cancerous cells to initiate the release of apoptogenic mitochondrial proteins. This ability to engage mitochondrial-mediated apoptosis directly using chemotherapy may be responsible for overcoming aberrant apoptosis regulatory mechanisms commonly encountered in cancerous cells. Interestingly, several putative cancer chemopreventive agents also possess the ability to trigger apoptosis in transformed, premalignant, or malignant cells in vitro via mitochondrial membrane permeabilization. This process may occur through the regulation of Bcl-2 family members, or by the induction of the mitochondrial permeability transition. Thus, by exploiting endogenous mitochondrial-mediated apoptosis-inducing mechanisms, certain chemopreventive agents may be able to block the progression of premalignant cells to malignant cells or the dissemination of malignant cells to distant organ sites as means of modulating carcinogenesis in vivo. This review will examine cancer chemoprevention with respect to apoptosis, carcinogenesis, and the proapoptotic activity of various chemopreventive agents observed in vitro. In doing so, I will construct a paradigm supporting the notion that the mitochondria are a novel target for the chemoprevention of cancer.

Caspase-dependent and -independent panc-1 cell death due to actinomycin D and MK 886 are additive but increase clonogenic survival

In human panc-1 pancreatic cancer cells, actinomycin D (act D) induces a type 1 (apoptotic, extrinsic, death domain, receptor-dependent, and caspase-positive) form of programmed cell death (PCD) and MK 886, a 5-lipoxygenase inhibitor serving among other functions as a surrogate for increasing oxidative stress, a type 2 form, defined as an intrinsic, mitochondria-dependent, autophagic form of cellular suicide. Using both agents simultaneously should allow for examination of their interaction in cells able to express either form of PCD. Activation of both forms might result in synergistic, additive, null, or inhibitory effects on the reduction in proliferation, PCD, and clonogenicity of surviving cells. Co-culture of panc-1 cells with act D and MK 886, which both inhibit their proliferation, had an additive effect on increasing the development of these forms of PCD, as determined by morphology, a nucleosome assay, and flow cytometry. Initially, laddering on agarose detected with propidium iodide, present in act D, and act D plus MK 886-treated cells was partially obscured by randomly degraded DNA. With the use of the more sensitive SYBR green dye and reduced exposure of detached cells to 37 degrees C, a limited laddering of DNA from MK 886-treated cells was also detected. Caspase activity was present in act-D-cultured cells but was absent in cells cultured with MK 886. Combined culture reduced caspase activity in act D-treated cells, consistent with interference from type 2 of type 1 PCD. Removal after 48 hr of act D or MK 886 allowed regrowth of residual cells, the latter agent to a greater extent than the former. In combination, the number of clones was increased compared with act D alone. These features distinguish two forms of PCD. In therapeutic settings in which the modes of cell death have not been identified, unintentional activation of several cellular suicide pathways with "crosstalk" between them occurs. Their intentional simultaneous activation and responses, as modulated by the history of cells in or out of cycle, could reduce the intended therapeutic outcome with survival of additional clonogenic cells due to various forms of mutual interference.