Abstract
Heme oxygenase (HO) isoforms catalyze the conversion of heme to carbon monoxide (CO) and biliverdin with a concurrent release of iron, which can drive the synthesis of ferritin for iron sequestration. Most of the studies so far were directed at evaluating the protective effect of these enzymes because of their ability to generate antioxidant and antiapoptotic molecules such as CO and bilirubin. Recent evidences are suggesting that HO may possess other important physiological functions, which are not related to its enzymatic activity and for which we would like to introduce for the first time the term “non canonical functions”. Recent evidence suggest that both HO isoforms may form protein-protein interactions (i.e. cytochrome P450, adiponectin, CD91) thus serving as chaperone-like protein. In addition, truncated HO-1 isoform was localized in the nuclear compartment under certain experimental conditions (i.e. excitotoxicity, hypoxia) regulating the activity of important nuclear transcription factors (i.e. Nrf2) and DNA repair. In the present review, we discuss three potential signaling mechanisms that we refer to as the non-canonical functions of the HO isoforms: protein-protein interaction, intracellular compartmentalization, and extracellular secretion. The aim of the present review is to describe each of this mechanism and all the aspects warranting additional studies in order to unravel all the functions of the HO system.
Highlights
Heme oxygenases catalyze the degradation of heme into biliverdin, carbon monoxide (CO) and ferric iron [1,2,3,4,5] (Figure 1A)
The above-mentioned studies suggest that the Heme oxygenase (HO) system may possess important biological functions beyond its enzymatic activity
We reported what we called the non-canonical function of the heme oxygenases
Summary
Heme oxygenases catalyze the degradation of heme into biliverdin, carbon monoxide (CO) and ferric iron [1,2,3,4,5] (Figure 1A). Heme functions as the prosthetic group in hemoproteins, e.g., nitric oxide synthase, cyclooxygenases, soluble guanylate cyclase, cytochrome P450, peroxidase, and catalase and since HO is the sole physiological pathway of heme degradation. It plays a critical role in the regulation of cellular heme-dependent enzyme levels [6,7,8,9,10] (Figure 1B). HO-2 is responsible for the most HO constitutive activity [48-
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