Mitochondrial respiratory uncoupling promotes keratinocyte differentiation and blocks skin carcinogenesis.

Decreased mitochondrial oxidative metabolism is a hallmark bioenergetic characteristic of malignancy that may have an adaptive role in carcinogenesis. By stimulating proton leak, mitochondrial uncoupling proteins (UCP1-3) increase mitochondrial respiration and may thereby oppose cancer development. To test this idea, we generated a mouse model that expresses an epidermal-targeted keratin-5-UCP3 (K5-UCP3) transgene and exhibits significantly increased cutaneous mitochondrial respiration compared with wild type (FVB/N). Remarkably, we observed that mitochondrial uncoupling drove keratinocyte/epidermal differentiation both in vitro and in vivo. This increase in epidermal differentiation corresponded to the loss of markers of the quiescent bulge stem cell population, and an increase in epidermal turnover measured using a bromodeoxyuridine (BrdU)-based transit assay. Interestingly, these changes in K5-UCP3 skin were associated with a nearly complete resistance to chemically-mediated multistage skin carcinogenesis. These data suggest that targeting mitochondrial respiration is a promising novel avenue for cancer prevention and treatment.

Cell compartmentalization in redox signaling.

From a growing body of evidence on the role of Reactive Oxygen Species as intracellular signaling molecules, the concept starts to emerge that cell responses to redox changes are function of the intracellular site where oxidants are produced and/or meet their molecular targets. In particular, a major distinction between oxidative events in the cytosolic versus the mitochondrial compartment appears to exist in terms of physiological stimuli, signaling mechanisms and functional consequences. Experimental data supporting this view are reviewed here, and the potential implications of this new perspective in redox signaling are discussed.

Deregulated manganese superoxide dismutase expression and resistance to oxidative injury in p53-deficient cells.

Loss of function of the tumor suppressor protein p53 represents a very frequent event in human carcinogenesis, but the molecular mechanisms linking impaired p53 activity to increased cell malignancy are still incompletely understood. p53 is normally involved in both cell cycle control and the induction of cell death and is involved in the latter mainly through the transcriptional regulation of pro- and antiapoptotic proteins. Reactive oxygen species are known to be powerful inducers of p53 activity; moreover, they play a role in the execution of p53-dependent apoptosis. Here we show that transformed mouse fibroblasts lacking p53 are significantly more resistant than wild-type (wt) controls to the cytotoxic effect of a number of pro-oxidant treatments. Interestingly, these cells also exhibit deregulated expression of the antioxidant enzyme manganese superoxide dismutase (MnSOD), a protein known to protect cancer cells from the oxidative injury inflicted by antitumoral cytokines and anticancer drugs. MnSOD activity was also increased in liver tissue from p53-deficient mice in comparison with wt tissue. Transient transfection of wt p53 in HeLa cells led to a significant reduction in steady-state MnSOD mRNA levels and enzymatic activity, confirming that the expression of this antioxidant enzyme is negatively regulated by p53. Forced expression of MnSOD rendered HeLa cells resistant to p53-dependent cytotoxic treatments and, in cotransfection experiments, counteracted the growth-inhibitory effect of p53. Taken together, these data identify MnSOD as a potential target for tumor suppressor protein p53 and underscore the relevance of MnSOD modulation in the context of normal p53 functions because it is consistent with many reports of abnormally increased MnSOD expression in human cancers.

A redox signaling mechanism for density-dependent inhibition of cell growth.

Reactive oxygen species (ROS) have recently drawn significant attention as putative mitogenic mediators downstream of activated growth factor receptors and oncogenic Ras; however, the possibility that a redox-related mechanism also operates in the negative control of cell proliferation by inhibitory signals has not been investigated thus far. Here we show that the arrest of growth induced by cell confluence ("contact inhibition") is due, at least in part, to a decrease in the steady-state levels of intracellular ROS and the consequent impairment of mitogenic redox signaling. In confluent fibroblast cultures, the decrease in the concentration of oxygen species was associated with diminished activity of the small GTPase Rac-1, a signal transducer directly involved in the ligand-dependent generation of oxygen-derived molecules, and was effectively mimicked by exposure of sparse cultures to dithiothreitol (DTT) and inhibitors of enzymes (phospholipase A2 and lipoxygenase) acting in the arachidonic acid cascade downstream of growth factor receptors and Rac-1. Sparse fibroblasts treated with nontoxic amounts of DTT underwent growth arrest, whereas a low concentration of hydrogen peroxide significantly increased thymidine incorporation in confluent cultures, demonstrating a causal link between redox changes and growth control by cell density. Removal of oxygen species from sparse cultures was accompanied by a drastic decrease of protein tyrosine phosphorylation after epidermal growth factor stimulation, which, at a biochemical level, reproduced the signaling hallmarks of contact inhibition. Moreover, the cytosolic tyrosine phosphatase SHP-2 was identified as a putative target for redox signaling by cell density because the enzyme itself and the associated substrates appear markedly dephosphorylated in both confluent and reductant-treated cells after exposure to epidermal growth factor, and SHP-2 enzymatic activity is strongly activated by DTT in vitro. Taken together, these data support a model in which impaired generation of ROS and increased protein tyrosine phosphatase activity impede mitogenic signaling in contact-inhibited cells.

Redox regulation of lymphocyte signaling.

Compelling evidence exists that reactive oxygen species can deliver intracellular signals in mammalian cells, and elicit a broad array of physiological responses according to the cell type, the oxidative burden and the cellular compartment where radicals are generated. When applied to immune cells, these concepts gain a particular relevance, in relation to the plasticity of immune functions and the biological complexity of lymphocyte response to antigens. Here we review some recent and somehow conflicting observations on the involvement of oxygen radicals and redox balance in lymphocyte activation, and propose models for how radical species could contribute to normal and pathological immunity.

Endogenous oxygen radicals modulate protein tyrosine phosphorylation and JNK-1 activation in lectin-stimulated thymocytes.

Molecular events mediating the T-lymphocyte response to lectins are still incompletely understood, although much evidence suggests that both the mitogenic and the death-promoting effects of these agents involve the biochemical cascade initiated by the CD3/T-cell antigen receptor (TCR) complex. Reactive oxygen species (ROS) and in particular H(2)O(2) have been shown to have a role in cell response to cytokines and growth factors. Here we report that the proliferation of mouse thymocytes in response to the mitogenic lectin concanavalin A (ConA) is strongly and selectively inhibited by the intracellular ROS scavenger N-acetylcysteine (NAC) and by diphenyleneiodonium (DPI), a potent inhibitor of NADPH-dependent membrane oxidases activated by surface receptors. A rapid 'burst' of intracellular oxygen radicals was observed in mouse thymocytes stimulated by ConA, with kinetics that paralleled the appearance of tyrosine-phosphorylated proteins. This burst was abrogated by the pretreatment of cells with NAC or DPI. Only a modest increase in intracellular oxygen species was found in thymocytes stimulated by strong cross-linking of TCR together with CD4 or CD28. Pharmacological interference with ROS production in ConA-stimulated thymocytes resulted in a decreased tyrosine phosphorylation of multiple protein species, including a 38 kDa band able to recruit the adapter protein Grb2 and corresponding to the recently identified transducer LAT (linker for activation of T-cells), a molecule involved in linking activated TCR to the production of interleukin 2 and the proliferation of T-cells. Furthermore, ROS inhibition markedly attenuated the activation of stress-activated protein kinase/JNK-1 (c-Jun N-terminal kinase 1) in response to lectins. Taken together, these results identify ROS as important modulators of the signalling cascade initiated by mitogenic lectins in thymocytes and, by extension, as a novel class of mediators downstream of antigen receptors.

Increased growth capacity of cervical-carcinoma cells over-expressing manganous superoxide dismutase.

Increases in the expression of manganese-dependent superoxide dismutase (MnSOD) have been detected in several classes of human and experimental tumors and appear to correlate with poorer prognosis in human neuro-epithelial, ovarian and cervical tumors. To delineate the relevance of MnSOD expression to tumor-cell growth and survival, a human MnSOD cDNA was over-expressed in the HeLa cervical-carcinoma cell line. MnSOD over-expression had marginal effects on the growth of HeLa cells in standard medium, but markedly protected the cells from growth suppression and cell death in conditions of serum deprivation. Serum starvation did not affect expression of endogenous MnSOD in wild-type HeLa cells, but was associated with increases in cell death and in the generation of intracellular oxygen radicals. By contrast, in HT29 colon-carcinoma cells, which are relatively resistant to growth-factor withdrawal, serum deprivation was associated with increases in MnSOD expression and activity. Together these observations suggest that MnSOD provides a mechanism for counteracting the intracellular oxidative processes that impair cell growth and viability in the context of growth-factor withdrawal and, in this context, may promote tumor-cell survival in vivo in conditions normally unfavorable to cell growth.