2,2′-Dipyridyl diselenide (Py2Se2) induces G1 arrest and apoptosis in human lung carcinoma (A549) cells through ROS scavenging and reductive stress

Abstract

Organo-diselenides are well documented for pro-oxidant effects in tumor cells. However, the present study demonstrated that 2,2′-dipyridyl diselenide (Py₂Se₂) induced cytotoxicity in human non-small cell lung carcinoma (A549) cells through reductive stress marked by a significant decrease in the basal level of reactive oxygen species and a concurrent decrease in the ratio of oxidised (GSSG) and reduced (GSH) glutathione. The IC₅₀ (concentration inducing 50% cytotoxicity) of Py₂Se₂ in A549 and human normal lung fibroblast (WI38) cells was ∼8.5 μM and ∼5.5 μM, respectively, indicating that Py₂Se₂ did not exhibit selective toxicity towards cancer cells. Cell free studies indicated that Py₂Se₂ acted as a substrate of thioredoxin reductase (TrxR) and accordingly it was proposed that TrxR mediated reduction of Py₂Se₂ within cells might be generating intermediates leading to a reductive environment. Despite generating a reducing environment, Py₂Se₂ caused significant DNA damage, G1 phase arrest and apoptosis. The mechanistic investigation revealed that Py₂Se₂ induced G1 arrest was mediated through up-regulation of p21 transcript in a p53 independent manner. Further, the apoptotic effect of Py₂Se₂ was associated with the increase in the levels of unfolded protein response markers like BiP and CHOP, mitochondrial permeability (JC1) and apoptotic markers such as cleaved caspase-3 and poly (ADP-ribose) polymerase. Finally, pre-treatment with N-acetylcysteine (a stimulator of GSH biosynthesis) or L-buthionine sulfoximine (an inhibitor of GSH biosynthesis) increased and decreased the Py₂Se₂ mediated apoptosis, respectively. This confirmed that the cytotoxicity of Py₂Se₂ in A549 cells was triggered through reductive stress.