Nitric oxide (NO), synthesized from L-arginine by NO synthases, is a small, lipophilic, diffusible, highly reactive molecule with dichotomous regulatory roles in many biological events under physiological and pathological conditions. NO can promote apoptosis (pro-apoptosis) in some cells, whereas it inhibits apoptosis (anti-apoptosis) in other cells. This complexity is a consequence of the rate of NO production and the interaction with biological molecules such as metal ion, thiol, protein tyrosine, and reactive oxygen species. Long-lasting overproduction of NO acts as a pro-apoptotic modulator, activating caspase family proteases through the release of mitochondrial cytochrome c into cytosol, up-regulation of the p53 expression, and alterations in the expression of apoptosis-associated proteins, including the Bcl-2 family. However, low or physiological concentrations of NO prevent cells from apoptosis that is induced by the trophic factor withdrawal, Fas, TNFalpha/ActD, and LPS. The anti-apoptotic mechanism is understood on the basis of gene transcription of protective proteins. These include: heat shock protein, hemeoxygenase, or cyclooxygenase-2 and direct inhibition of the apoptotic executive effectors caspase family protease by S-nitrosylation of the cysteine thiol group in their catalytic site in a cell specific way. Our current understanding of the mechanisms by which NO exerts both pro- and anti-apototic action is discussed in this review article.

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Apigenin is a member of the flavone subclass of flavonoids present in fruits and vegetables. Apigenin has long been considered to have various biological activities, such as antioxidant, anti-inflammatory, and antitumorigenic properties, in various cell types. Cisplatin was known to exhibit cytotoxic effect to renal cells by inducing apoptosis through activation of p53. The present study investigated the antiapoptotic effects of apigenin on the cisplatin-treated human renal proximal tubular epithelial (HK-2) cells. HK-2 cells were pretreated with apigenin (5, 10, 20 μM) for 1 h and then treated with 40 μM cisplatin for various times. Apigenin inhibited the cisplatin-induced apoptosis of HK-2 cells. Interestingly, apigenin itself exerted cytostatic activity because of its ability to induce cell cycle arrest. Apigenin inhibited caspase-3 activity and PARP cleavage in cisplatin-treated cells. Apigenin reduced cisplatin-induced phosphorylation and expression of p53, with no significant influence on production of ROS that is known to induce p53 activation. Furthermore, apigenin promoted cisplatin-induced Akt phosphorylation, suggesting that enhanced Akt activation may be involved in cytoprotection. Taken together, these results suggest that apigenin ameliorates cisplatin-induced apoptosis through reduction of p53 activation and promotion of PI3K/Akt pathway in HK-2 cells.

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