The cancer cell’s “power plants” as promising therapeutic targets: An overview

This introductory article to the review series entitled “The Cancer Cell’s Power Plants as Promising Therapeutic Targets” is written while more than 20 million people suffer from cancer. It summarizes strategies to destroy or prevent cancers by targeting their energy production factories, i.e., “power plants.” All nucleated animal/human cells have two types of power plants, i.e., systems that make the “high energy” compound ATP from ADP and P _i . One type is “glycolysis,” the other the “mitochondria.” In contrast to most normal cells where the mitochondria are the major ATP producers (>90%) in fueling growth, human cancers detected via Positron Emission Tomography (PET) rely on both types of power plants. In such cancers, glycolysis may contribute nearly half the ATP even in the presence of oxygen (“Warburg effect”). Based solely on cell energetics, this presents a challenge to identify curative agents that destroy only cancer cells as they must destroy both of their power plants causing “necrotic cell death” and leave normal cells alone. One such agent, 3-bromopyruvate (3-BrPA), a lactic acid analog, has been shown to inhibit both glycolytic and mitochondrial ATP production in rapidly growing cancers (Ko et al., Cancer Letts., 173, 83–91, 2001), leave normal cells alone, and eradicate advanced cancers (19 of 19) in a rodent model (Ko et al., Biochem. Biophys. Res. Commun., 324, 269–275, 2004). A second approach is to induce only cancer cells to undergo “apoptotic cell death.” Here, mitochondria release cell death inducing factors (e.g., cytochrome c). In a third approach, cancer cells are induced to die by both apoptotic and necrotic events. In summary, much effort is being focused on identifying agents that induce “necrotic,” “apoptotic” or apoptotic plus necrotic cell death only in cancer cells. Regardless how death is inflicted, every cancer cell must die, be it fast or slow.     

Inhibition of deoxyribonuclease I by actin is to protect cells from premature cell death

Deoxyribonuclease I (Dnase1) is the major extracellular endonuclease. It is secreted by digestive glands into the alimentary tract and into the plasma, lacrimal fluid and urine by hepatocytes, lacrimal glands and renal proximal tubular cells, respectively. In many species the activity of Dnase1 is inhibited by monomeric actin. However, the biological significance of this high affinity interaction is unknown. We generated a Dnase1 mutant with extremely reduced actin binding capacity. EGFP-constructs of wild-type and mutant Dnase1 were transfected into MCF-7 breast cancer cells and apoptosis or necrosis was induced by staurosporine or oxidative stress. During apoptosis faster chromatin fragmentation occurred in cells transfected with mutant Dnase1. When wt (wild-type)- or mutated Dnase1 were added to cells after induction of necrosis, faster chromatin degradation occurred in the presence of mutant Dnase1. Inclusion of actin under these conditions inhibited chromatin degradation by wt- but not by mutated Dnase1. Thus, inhibition of Dnase1 by actin may serve as a self-protection mechanism against premature DNA degradation during cell damage.     

Sustained Calpain Inhibition Improves Locomotor Function and Tissue Sparing Following Contusive Spinal Cord Injury

Following contusive spinal cord injury (SCI), calpain activity is dramatically increased and remains elevated for days to weeks. Although calpain inhibition has previously been demonstrated to be neuroprotective following spinal cord injury, most studies administered the calpain inhibitor at a single time point. We hypothesized that sustained calpain inhibition would improve functional and pathological outcomes, as compared to the results obtained with a single postinjury administration of the calpain inhibitor. Contusion SCI was produced in female Long-Evans rats using the Infinite Horizon spinal cord injury impactor at the 200 kdyn force setting. Open-field locomotor function was evaluated until 6 weeks postinjury. Histological assessment of lesion volume and tissue sparing was performed at 6 weeks after SCI. Calpain inhibitor MDL28170 administered as a single postinjury i.v. bolus (20 mg/kg) or as a daily i.p. dose (1 mg/kg) improved locomotor function, but did not increase tissue sparing. Combined i.v. and daily i.p. MDL28170 administration resulted in significant improvement in both functional and pathological outcome measures, supporting the calpain theory of SCI proposed by Dr. Banik and colleagues. Special issue in honor of Naren Banik.     

1-Methylnicotinamide ameliorates lipotoxicity-induced oxidative stress and cell death in kidney proximal tubular cells

Free fatty acid-bound albumin (FFA-albumin)-related oxidative stress is involved in the pathogenesis of proximal tubular cell (PTC) damage and subsequent renal dysfunction in patients with refractory proteinuria. Nicotinamide adenine dinucleotide (NAD) metabolism has recently been focused on as a novel therapeutic target for several modern diseases, including diabetes. This study was designed to identify a novel molecule in NAD metabolism to protect PTCs from lipotoxicity-related oxidative stress. Among 19 candidate enzymes involved in mammalian NAD metabolism, the mRNA expression level of nicotinamide n-methyltransferase (NNMT) was significantly increased in both the kidneys of FFA-albumin-overloaded mice and cultured PTCs stimulated with palmitate-albumin. Knockdown of NNMT exacerbated palmitate-albumin-induced cell death in cultured PTCs, whereas overexpression of NNMT inhibited it. Intracellular concentration of 1-Methylnicotinamide (1-MNA), a metabolite of NNMT, increased and decreased in cultured NNMT-overexpressing and -knockdown PTCs, respectively. Treatment with 1-MNA inhibited palmitate-albumin-induced mitochondrial reactive oxygen species generation and cell death in cultured PTCs. Furthermore, oral administration of 1-MNA ameliorated oxidative stress, apoptosis, necrosis, inflammation, and fibrosis in the kidneys of FFA-albumin-overloaded mice. In conclusion, NNMT-derived 1-MNA can reduce lipotoxicity-mediated oxidative stress and cell damage in PTCs. Supplementation of 1-MNA may have potential as a new therapy in patients with refractory proteinuria.     

Retinal damage induced by commercial light emitting diodes (LEDs)

Spectra of “white LEDs” are characterized by an intense emission in the blue region of the visible spectrum, absent in daylight spectra. This blue component and the high intensity of emission are the main sources of concern about the health risks of LEDs with respect to their toxicity to the eye and the retina. The aim of our study was to elucidate the role of blue light from LEDs in retinal damage. Commercially available white LEDs and four different blue LEDs (507, 473, 467, and 449 nm) were used for exposure experiments on Wistar rats. Immunohistochemical stain, transmission electron microscopy, and Western blot were used to exam the retinas. We evaluated LED-induced retinal cell damage by studying oxidative stress, stress response pathways, and the identification of cell death pathways. LED light caused a state of suffering of the retina with oxidative damage and retinal injury. We observed a loss of photoreceptors and the activation of caspase-independent apoptosis, necroptosis, and necrosis. A wavelength dependence of the effects was observed. Phototoxicity of LEDs on the retina is characterized by a strong damage of photoreceptors and by the induction of necrosis.     

Suppression of PI3K/mTOR pathway rescues LLC cells from cell death induced by hypoxia

Cancer cells in solid tumors are challenged by various microenvironmental stresses, including hypoxia, and cancer cells in hypoxic regions are resistant to current cancer therapies. To investigate the mechanism of resistance to hypoxia in cancer cells, we examined mouse Lewis lung carcinoma (LLC) cells, which died due to necrosis at high density under hypoxic but not under normoxic conditions. Levels of mammalian target of rapamycin (mTOR), a central regulator of cellular energy, are reported to be suppressed in hypoxia. We found that phosphorylation of two molecules downstream to it, ribosomal p70 S6 kinase (S6K) and ribosomal protein S6, was markedly suppressed by hypoxia. Overexpression of the active form of S6K increased the sensitivity of LLC cells to hypoxia. On the other hand, inhibition of PI3K or mTOR dramatically reduced hypoxia-induced cell death under hypoxic conditions. Under hypoxic conditions, blockade of the PI3K or mTOR pathway increased levels of intracellular ATP and delayed decreases in pH and glucose level in culture medium, without affecting the cell cycle.     

ERK1/2-dependent regulation of U937 cell survival after exposure to peroxynitrite

A short-term growth of U937 cells in serum-free medium causes a prompt, mitochondrial permeability transition (MPT)-dependent necrotic response after exposure to an otherwise non-toxic concentration of peroxynitrite. This event is mediated by inhibition of extracellular signal-regulated kinase 1 and 2 (ERK1/2) phosphorylation, essential for the cytosolic phospholipase A(2)-dependent arachidonic acid (AA) release evoked by peroxynitrite. Reduced availability of the lipid messenger would therefore limit the efficiency of the AA-dependent survival signalling and cause an MPT-based necrosis. Since peroxynitrite further reduces the extent of ERK1/2 phosphorylation, regardless of whether cells had been grown in serum-free or -containing medium, it appears that basal ERK1/2 phosphorylation is a critical determinant for the survival response of U937 cells to a non-toxic, but nevertheless MPT-committing, concentration of peroxynitrite.     

In vivo cryochemotherapy of a human lung cancer model

Cryotherapy, an efficient technique to destroy tumour cells, is sometimes applied locally as a palliative treatment in lung cancers. It can be performed in combination with chemotherapy. Our aims were to determine in vivo: (1) the effects of cryochemotherapy in a human lung adenocarcinoma, (2) if it presents a benefit compared to the separate treatments and (3) if cryotherapy allows a tumour retention of the drug. Cells from the A549 cell line were xenografted into SCID mice. Tumours were treated by cryotherapy, chemotherapy (injection of Vinorelbine: Navelbine) or both and were studied morphologically at variable time points. Apoptosis was analysed by immunohistochemical staining of cleaved caspase-3 and by TUNEL. Intratumour Navelbine concentration was assessed by high performance liquid chromatography. Necrosis was important 2 h after cryochemotherapy (45% of the tumour surface) and at the later time points. Expression of cleaved caspase-3 was not significantly different from that of untreated tumours, except at the time point of 2 h where it was maximal (58%). Navelbine concentration was more important in tumours treated by chemotherapy than in tumours treated by cryochemotherapy, demonstrating that in our model, the benefit of the association observed 2h after treatment was not due to a concentration-dependent effect.