PORIMIN: The key to (+)-Usnic acid-induced liver toxicity and oncotic cell death in normal human L02 liver cells

ETHNOPHARMACOLOGICAL RELEVANCE

Usnic acid (UA) is one of the well-known lichen metabolites that induces liver injury. It is mainly extracted from Usnea longissima and U. diffracta in China or from other lichens in other countries. U. longissima has been used as traditional Chinese medicine for treatment of cough, pain, indigestion, wound healing and infection. More than 20 incidences with hepatitis and liver failure have been reported by the US Food and Drug Administration since 2000. UA is an uncoupler of oxidative phosphorylation causing glutathione and ATP depletion. Previous histological studies observed extensive cell and organelle swellings accompanied with hydrotropic vacuolization of hepatocytes.

AIM OF THE STUDY

This study was to investigate the mechanism of UA-induced liver toxicity in normal human L02 liver cells and ICR mice using various techniques, such as immunoblotting and siRNA transfection.

MATERIALS AND METHODS

Assays were performed to evaluate the oxidative stress and levels of GSH, MDA and SOD. Double flouresencence staining was used for the detection of apoptotic cell death. The protein expressions, such as glutathione S transferase, glutathione reductase, glutathione peroxidase 4, catalase, c-Jun N-terminal protein kinase, caspases, gastamin-D and porimin were detected by Western blotting. Comparisons between transfected and non-transfected cells were applied for the elucidation of the role of porimin in UA-induced hepatotoxicity. Histopathological examination of mice liver tissue, serum total bilirubin and hepatic enzymes of alanine aminotransferase and aspatate aminotransferase were also studied.

RESULTS

The protein expressions of glutathione reductase, glutathione S transferase and glutathione peroxidase-4 were increased significantly in normal human L02 liver cells. Catalase expression was diminished in dose-dependent manner. Moreover, (+)-UA did not induce the activation of caspase-3, caspase-1 or gasdermin-D. No evidence showed the occurrence of pyroptosis. However, the porimin expressions were increased significantly. In addition, (+)-UA caused no cytotoxicity in the porimin silencing L02 cells.

CONCLUSIONS

In conclusion, (+)-UA induces oncotic L02 cell death via increasing protein porimin and the formation of irreversible membrane pores. This may be the potential research area for future investigation in different aspects especially bioactivity and toxicology.

Review: Usnic acid-induced hepatotoxicity and cell death

Increasing prevalence of herbal and dietary supplement-induced hepatotoxicity has been reported worldwide. Usnic acid (UA) is a well-known hepatotoxin derived from lichens. Since 2000, more than 20 incident reports have been received by the US Food and Drug Administration after intake of UA containing dietary supplement resulting in severe complications. Scientists and clinicians have been studying the cause, prevention and treatment of UA-induced hepatotoxicity. It is now known that UA decouples oxidative phosphorylation, induces adenosine triphosphate (ATP) depletion, decreases glutathione (GSH), and induces oxidative stress markedly leading to lipid peroxidation and organelle stress. In addition, experimental rat liver tissues have shown massive vacuolization associated with cellular swellings. Additionally, various signaling pathways, such as c-JNK N-terminal kinase (JNK), store-operated calcium entry, nuclear erythroid 2-related factor 2 (Nrf2), and protein kinase B/mammalian target of rapamycin (Akt/mTOR) pathways are stimulated by UA causing beneficial or harmful effects. Nevertheless, there are controversial issues, such as UA-induced inflammatory or anti-inflammatory responses, cytochrome P450 detoxifying UA into non-toxic or transforming UA into reactive metabolites, and unknown mechanism of the formation of vacuolization and membrane pore. This article focused on the previous and latest comprehensive putative mechanistic findings of UA-induced hepatotoxicity and cell death. New insights on controversial issues and future perspectives are also discussed and summarized.