Abstract A55: Differential regulation of survivin by reactive oxygen species in breast cancer cells

Abstract

Survivin is an anti‐apoptotic protein that is highly expressed in most malignant tumors but minimally expressed in normal tissues. Here, we investigate how induced oxidative stress in the form of hydrogen peroxide (H₂O₂) affects survivin expression levels in the MCF‐7 breast cancer cell line. Numerous studies have demonstrated that intracellular H₂O₂ is a key regulator of signal transduction however its putative role in oncogenesis remains controversial. Conflicting reports have suggested that altering intracellular H₂O₂ levels can both inhibit and promote apoptosis in cancer cells. Recent studies have shown that compounds that raise cancer cells' intracellular H₂O₂ levels may make them more susceptible to chemotherapy, while studies at the molecular level show that an increase in intracellular H₂O₂ activates key oncogenes that block regulated apoptosis and promote an environment optimal for malignant transformation. Based on these reports, we investigated the effects of H₂O₂ treatment on survivin expression in MCF‐7 cells under two different conditions 1) normal proliferation in complete media, 2) cell‐cycle arrest by 16‐hour serum starvation. Our results show that H₂O₂ treatment in quiescent cells induced survivin expression in a dose-dependent manner. By contrast, H₂O₂ treatment in proliferating cells led to a dose‐dependent decrease in survivin expression. This result was unexpected as reports indicate that survivin expression is highly induced by growth factors present in serum‐complemented media. Further investigation into potential upstream signaling mediators revealed that activation of extracellular signal related kinases 1 and 2 (ERK1/2) may play an important role in H₂O₂‐induced survivin expression. We observed an increase in ERK1/2 phosphorylation that directly corresponded to an increase in survivin expression in serum‐deprived cells. However, phospho‐ERK1/2 levels remained relatively constant in proliferating MCF‐7 cells despite a dose‐dependent decrease in survivin expression. One possible explanation for this difference is a dramatic reduction in endogenous H₂O₂ levels following serum‐deprivation. Compared to normal cells, cancer cells have a significantly higher level of intracellular H₂O₂ potentially making them more susceptible to apoptosis following increased oxidative stress. This mechanism may account for the dose‐dependent decrease in survivin expression observed in MCF‐7 cells treated with H₂O₂ in the presence of complete media. However, growth factor withdrawal decreases intracellular H₂O₂ levels. Therefore, under conditions of serum deprivation, H₂O₂ may activate mitogen‐activated protein (MAP) kinase signal transduction cascades that promote growth and proliferation via oxidative stress signals. Together, our results support the hypothesis that endogenous levels of H₂O₂ may play a critical role in how cancer cells respond to treatment regulated by the MAP kinase pathway. Based on this model, agents that increase H₂O₂ levels in rapidly dividing cancer cells should be explored as an adjunct to chemotherapy, representing a novel approach to induce selective apoptosis by exploiting the oxidative environment tumor cells manifest in order to sustain their excessive metabolic demands.

Citation Information: Cancer Res 2009;69(23 Suppl):A55.