UV-induced apoptosis in keratinocytes is a highly complex process in which various molecular pathways are involved. These include the extrinsic pathway via triggering of death receptors and the intrinsic pathway via DNA damage and reactive oxygen species (ROS) formation. In this study we investigated the effect of catalase and CuZn-superoxide dismutase (SOD) overexpression on apoptosis induced by UVB exposure at room temperature or 4 degrees C on normal human keratinocytes. Irradiation at low temperature reduced UV-induced apoptosis by 40% in normal keratinocytes independently of any change in p53 and with a decrease in caspase-8 activation. Catalase overexpression decreased apoptosis by 40% with a reduction of caspase-9 activation accompanied by a decrease in p53. Keeping cells at low temperature and catalase overexpression had additive effects. CuZn-SOD overexpression had no significant effect on UVB-induced apoptosis. UVB induced an increase in ROS levels at two distinct stages: immediately following irradiation and around 3 h after irradiation. Catalase overexpression inhibited only the late increase in ROS levels. We conclude that catalase overexpression has a protective role against UVB irradiation by preventing DNA damage mediated by the late ROS increase.

Hypoxia-inducible factor-1 (HIF-1) is a major transcription factor sensitive to oxygen levels, which responds to stress factors under both hypoxic and nonhypoxic conditions. UV irradiation being a common stressor of skin, we looked at the effect of UVB on HIF-1alpha expression in keratinocytes. We found that UVB induces a biphasic HIF-1alpha variation through reactive oxygen species (ROS) generation. Whereas rapid production of cytoplasmic ROS down-regulates HIF-1alpha expression, delayed mitochondrial ROS generation results in its up-regulation. Indeed, activation of p38 MAPK and JNK1 mediated by mitochondrial ROS were required for HIF-1alpha phosphorylation and accumulation after UVB irradiation. Our experiments also revealed a key role of HIF-1alpha in mediating UVB-induced apoptosis. We conclude that the broad impact of the HIF-1 transcription factor on gene expression could make it a key regulator of UV-responsive genes and photocarcinogenesis.

Chemotherapies alter cellular redox balance and reactive oxygen species (ROS) content. Recent studies have reported that chemoresistant cells have an increased oxidative state in hematologic malignancies. In this study, we demonstrated that chemoresistant acute myeloid leukemia (AML) cells had a lower level of mitochondrial and cytosolic ROS in response to cytarabine (AraC) and overexpressed myeloperoxidase (MPO), a heme protein that converts hydrogen peroxide to hypochlorous acid (HOCl), compared with sensitive AML cells. High MPO-expressing AML cells were less sensitive to AraC in vitro and in vivo. They also produced higher levels of HOCl and exhibited an increased rate of mitochondrial oxygen consumption when compared with low MPO-expressing AML cells. Targeting MPO expression or enzyme activity sensitized AML cells to AraC treatment by triggering oxidative damage and sustaining oxidative stress, particularly in high MPO-expressing AML cells. This sensitization stemmed from mitochondrial superoxide accumulation, which impaired oxidative phosphorylation and cellular energetic balance, driving apoptotic death and selective eradication of chemoresistant AML cells in vitro and in vivo. Altogether, this study uncovers a noncanonical function of MPO enzyme in maintaining redox balance and mitochondrial energetic metabolism, therefore affecting downstream pathways involved in AML chemoresistance. Significance: These findings demonstrate the role of myeloperoxidase in the regulation of ROS levels and sensitivity of AML cells to cytarabine, an essential chemotherapeutic backbone in the therapy of AML.