Integrative Survival Response Evoked by Heme Oxygenase-1 and Heme Metabolites

Heme oxygenase (HO) catalyzes the rate-limiting step in heme degradation to produce carbon monoxide (CO), iron, and biliverdin. Biliverdin is subsequently converted to bilirubin by its reductase, and iron is recycled for heme synthesis. The inducible HO isoform, HO-1, is involved in the protection of multiple tissues and organs. The mechanism of protective actions of HO-1 has not been completely elucidated, but recent evidence suggests that one or more of heme metabolites can mediate the protective effects of HO-1. Particularly, CO mimics the antioxidant, anti-inflammatory, anti-apoptotic and antiproliferative actions of HO-1. Many of these effects of CO depend on the production of cyclic guanosine monophosphate (cGMP), and the modulation of mitogen-activated protein kinase (MAPK) pathways. The transcription factors, including nuclear factor E2-related factor-2 (Nrf2), and their upstream kinases, including MAPK pathway, play an important regulatory role in HO-1 expression by dietary antioxidants and drugs. This review attempts to concisely summarize the molecular and biochemical characteristics of HO-1, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by dietary antioxidants and drugs. In addition, the cytoprotective roles of HO-1 shall be discussed from the perspective of each of the metabolic by-products.