hTERT Overexpression Alleviates Intracellular ROS Production, Improves Mitochondrial Function, and Inhibits ROS-Mediated Apoptosis in Cancer Cells

Abstract

The human telomerase reverse transcriptase (hTERT) is the catalytic subunit of the telomerase holoenzyme. Evidence is accumulating to link hTERT to activities other than telomere maintenance and immortalization. Here, we show that hTERT overexpression not only reduces the basal cellular reactive oxygen species (ROS) levels but also inhibits endogenous ROS production in response to stimuli that induce intracellular ROS generation. Conversely, siRNA-mediated gene silencing of hTERT potentiated the increase in cellular ROS levels following exposure to oxidative stress. This antioxidant effect of hTERT is mediated via a significant increase in the ratio of reduced to oxidized glutathione (GSH:GSSG) as well as efficient recovery of the oxidized peroxiredoxin to its nonoxidized form. Our data also provide evidence for mitochondrial localization of hTERT, and a significantly higher activity of cytochrome C oxidase, the rate-limiting enzyme in the mitochondrial electron transport chain, in hTERT overexpressing cells. To ascertain whether the improved mitochondrial function and antioxidant effect of hTERT could provide cancer cells with a survival advantage, the effect of oxidative stress on mitochondrial apoptosis was evaluated. Indeed, hTERT overexpressing cells inhibited cytosolic acidification, translocation of Bax, the drop in mitochondrial transmembrane potential, the release of cytochrome C to the cytosol, and cell death. Taken together, these data demonstrate a hitherto undefined role of hTERT in alleviating cellular ROS levels by way of potentiating the cellular antioxidant defense systems, and in doing so endowing cancer cells with the ability to evade death stimuli. Cancer Res; 71(1); 266-76. Ó2010 AACR.

Extra-Telomeric Effects of Telomerase (hTERT) in Cell Death

“…This investigation also showed that the antioxidant function of TERT may be linked to an increase in the ratio of reduced glutathione to oxidized glutathione in addition to an improved recovery of the peroxiredoxin in its reduced state. As a substantiation of the previous results we mentioned earlier, the authors of this study were able to show that TERT induces an increase in complex IV activity (cytochrome c oxidase) (Indran et al, 2011). In the meantime it was also confirmed that these cells overexpressing TERT display a higher resistance to H2O2-induced apoptosis.…”

“…Another recent report also confirmed the role of TERT as a modulator of ROS production (Indran et al, 2010(Indran et al, , 2011. Indeed it was observed that TERT overexpression induces reduction of basal levels of ROS and inhibits the ROS production induced by oxidative stress (Figure 2).…”

“…Additional studies demonstrated the ability of TERT to antagonize apoptosis induced by topoisomerase inhibitors in PC12 cell line or by oxidative stress in lymphocytes CD4+ model (Lu et al, 2001;Luiten et al, 2003). Such results illustrating the protective role of TERT against ROS were confirmed later in other models (Ahmed et al, 2008;Haendeler et al, 2009;Indran et al, 2011). Although most of these early investigations did not explore the involvement of the reverse transcriptase activity of TERT in this anti-apoptotic mechanism, these results had already outlined a potential function of TERT unrelated to its enzymatic activity and ability to lengthen telomeres (Sung et al, 2005).…”

“…This mechanism is regulated by a family of proteins called the Bcl-2 family of proteins, which are responsible for the regulation of the apoptotic mitochondrial pathway through the activation of caspases (Antonsson, 2004). On the other hand it is also of critical importance to understand that mitochondria are a major producer of ROS which activate many downstream pathways involved in the modulation of mechanisms such as cell death or cell survival, cell proliferation, senescence and ageing (Indran et al, 2011;Saretzki, 2009). …”

Anticancer activity of 23,24-dihydrocucurbitacin B against the HeLa human cervical cell line is due to apoptosis and G2/M cell cycle arrest

“…Thus, the results of the present study indicate that 23,24-dihydrocucurbitacin B may induce apoptosis by increasing intracellular ROS and decreasing the ΔΨm. Several anticancer drugs act against cancer by producing ROS (21,22); for example, afferent A disrupts the ΔΨm and induces oxidative stress, ultimately inducing apoptosis in osteosarcoma cells (23).…”