Bilirubin participates in protecting of heme oxygenase-1 induction by quercetin against ethanol hepatotoxicity in cultured rat hepatocytes

To attenuate alcohol liver disease (ALD) is extremely urgent since ALD has been emerged as a major liver disease. The aim of the present study is to investigate the hepatoprotective effect against ethanol-induced injury of bilirubin, a product of heme metabolism degradation via HO and biliverdin reductase catalysis. Ethanol-incubated primary rat hepatocytes (100 mmol/L) were treated by quercetin, bilirubin, inflammatory factors, and/or HO-1 inducer/inhibitor for 24 h, and the cellular damage was assayed. Quercetin lowered ethanol-induced glutathione depletion and superoxide dismutase inactivation, inhibited the overproduction of malondialdehyde and reactive oxygen species, and decreased the leakage of cellular aspartate aminotransferase and lactate dehydrogenase, accompanying the normalization of bilirubin level. The effect of quercetin was mimicked by exogenous bilirubin in a dose-dependent manner to some extent (within 25 μmol/L) and pharmacological HO-1 inducer hemin, but abolished by HO-1 inhibitor zinc protoporphyrin-IX. Inflammatory challenge of TNF-α plus IL-6 further aggravated ethanol-induced oxidative damage, which was also attenuated by bilirubin in part. These findings shed a light on the anti-oxidative and anti-inflammatory role of bilirubin released from quercetin/HO-1 and biliverdin reductase pathway against ethanol hepatotoxicity and highlight a prospective strategy of nutritional intervention for ALD by naturally occurring quercetin to induce HO-1 with the release of bioactive end-products.