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Abstract
Tumor cell survival and proliferation is attributable in part to suppression of apoptotic pathways, yet the mechanisms by which cancer cells resist apoptosis are not fully understood. Many cancer cells constitutively express heme oxygenase-1 (HO-1), which catabolizes heme to generate biliverdin, Fe(2+), and carbon monoxide (CO). These breakdown products may play a role in the ability of cancer cells to suppress apoptotic signals. K(+) channels also play a crucial role in apoptosis, permitting K(+) efflux which is required to initiate caspase activation. Here, we demonstrate that HO-1 is constitutively expressed in human medulloblastoma tissue, and can be induced in the medulloblastoma cell line DAOY either chemically or by hypoxia. Induction of HO-1 markedly increases the resistance of DAOY cells to oxidant-induced apoptosis. This effect was mimicked by exogenous application of the heme degradation product CO. Furthermore we demonstrate the presence of the pro-apoptotic K(+) channel, Kv2.1, in both human medulloblastoma tissue and DAOY cells. CO inhibited the voltage-gated K(+) currents in DAOY cells, and largely reversed the oxidant-induced increase in K(+) channel activity. p38 MAPK inhibition prevented the oxidant-induced increase of K(+) channel activity in DAOY cells, and enhanced their resistance to apoptosis. Our findings suggest that CO-mediated inhibition of K(+) channels represents an important mechanism by which HO-1 can increase the resistance to apoptosis of medulloblastoma cells, and support the idea that HO-1 inhibition may enhance the effectiveness of current chemo- and radiotherapies.
Highlights
Heme oxygenase-1 (HO-1) is constitutively expressed in many cancers which are highly resistant to apoptosis
This enzyme could be induced by both hemin and cobalt protoporphyrin IX (CoPPIX), known inducers of HO-1 expression in other tissues, in a concentration-dependent manner (Fig. 2, A and B; note, that at the highest concentration (200 M) CoPPIX was toxic to cells)
One possible reason to account for the lack of constitutive expression of HO-1 in DAOY cells, yet clear expression in medulloblastoma tissue (Fig. 1), is the fact that hypoxia is a key feature of the tumor microenvironment, yet DAOY cells were cultured at high levels of ambient O2 levels (20%) typically encountered in tissue culture
Summary
Heme oxygenase-1 (HO-1) is constitutively expressed in many cancers which are highly resistant to apoptosis. Many cancer cells constitutively express heme oxygenase-1 (HO-1), which catabolizes heme to generate biliverdin, Fe2؉, and carbon monoxide (CO) These breakdown products may play a role in the ability of cancer cells to suppress apoptotic signals. Induction of HO-1 markedly increases the resistance of DAOY cells to oxidant-induced apoptosis This effect was mimicked by exogenous application of the heme degradation product CO. P38 MAPK inhibition prevented the oxidant-induced increase of K؉ channel activity in DAOY cells, and enhanced their resistance to apoptosis. Our findings suggest that CO-mediated inhibition of K؉ channels represents an important mechanism by which HO-1 can increase the resistance to apoptosis of medulloblastoma cells, and support the idea that HO-1 inhibition may enhance the effectiveness of current chemo- and radiotherapies.
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