Cytoprotective role of nitric oxide in HepG2 cell apoptosis induced by hypocrellin B photodynamic treatment

- HB/light treatment displayed potent cytotoxic activity against HepG2 cells with an IC50 of 3.10 μM.
- HB/light treatment activated of caspase-3, -9 and induced cell apoptosis.
- iNOS activity was upregulated significantly by HB/light treatment with NO generation.
- The pretreatment of NOS inhibitor l-NMMA and NO scavenger cPTIO enhanced HB/light-induced apoptosis.

Hypocrellin B (HB), a natural perylenequinone pigment, has been successfully employed in the photodynamic therapy (PDT) in a variety of human cancer cells due to its high singlet oxygen yield. To investigate the generation of nitric oxide (NO) and its role on cancer cell death induced by PDT, we used human hepatocellular carcinoma (HepG2) cells and HB as a photosensitizer. HB/light treatment decreased the growth of HepG2 cells in a dose-dependent manner with an IC50 of 3.10 μM, activated caspase-3, -9 and induced apoptosis in HepG2 cells. It was found that exposure of the cells to HB/light resulted in inducible nitric oxide synthase (iNOS) activation and followed by significant increase in NO generation. Incubating cells with a NOS inhibitor Nω-monomethyl-l-arginine (l-NMMA) and an NO scavenger 2-(4-carboxyphenyl)-4, 4, 5, 5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) enhanced HB/light-induced caspase-3, -9 activation and apoptosis significantly while decreasing DAF fluorescence-assessed NO generation substantially. Cells could be rescued from HB/light-induced apoptosis by an exogenous NO donor, sodium nitroprusside (SNP). Our findings suggested that induced NO was acting cytoprotectively and PDT efficacy of HB could be improved by using pharmacological modulators of NO or NOS.

Graphical Abstract