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- _3 value " View this table: [in this window] [in a new window] TABLE 1 Summary of the various functions of HO isoforms, their tissue distribution, and gene regulation Maines et al. (1986) were the first to report the identification of a second form of HO from rat liver microsomes, designated HO-2. Only recently, a third isoform, HO-3, was discovered (McCoubrey et al., 1997). HO-1 and HO-2 share little similarity in amino acid sequence (40%), whereas the HO-2 and HO-3 isoform are far more homologous (90%). All HO isoforms are highly conserved among species in evolution. HO is expressed in virtually all life forms; in prokaryotic bacteria as well as in fungi, plants, and humans, regulating a wide spectrum of cellular processes (Terry et al., 2002). The homology between rat, mouse, and human is for HO-1 and HO-2 proteins higher than 80% and 90%, respectively. Under normal physiological conditions, most cells express low or undetectable levels of HO-1 protein, whereas HO-2 proteins are constitutively expressed. HO-3 protein expression awaits further characterization. HO-2 transcription is only up-regulated by few agents, such as opiates and adrenal glucocorticoids (Li and David Clark, 2000; Liu et al., 2000). HO-1 gene expression is highly inducible by more diverse stimuli than any other enzyme described to date (Maines, 1997) and involves a multitude of signaling pathways (Table 2) (Immenschuh and Ramadori, 2000). HO-1 expression is mainly regulated at the transcriptional level. View this table: [in this window] [in a new window] TABLE 2 Selection of different inducers of HO-1 gene expression " provenance.