Characterization of heme oxygenase and biliverdin reductase gene expression in zebrafish (Danio rerio): Basal expression and response to pro-oxidant exposures

- hmox1a, hmox2a, hmox2b and bvrb are sexually dimorphic in expression.
- hmox paralogs were induced in adult tissues by cadmium exposure.
- hmox1a, hmox1b and bvrb were induced by multiple pro-oxidants zebrafish embryos.
- Differential expression of zebrafish hmox paralogs suggest partitioning of function.
- Nrf2a mediates the induction of hmox1b and bvrb by cadmium in zebrafish embryos.

While heme is an important cofactor for numerous proteins, it is highly toxic in its unbound form and can perpetuate the formation of reactive oxygen species. Heme oxygenase enzymes (HMOX1 and HMOX2) degrade heme into biliverdin and carbon monoxide, with biliverdin subsequently being converted to bilirubin by biliverdin reductase (BVRa or BVRb). As a result of the teleost-specific genome duplication event, zebrafish have paralogs of hmox1 (hmox1a and hmox1b) and hmox2 (hmox2a and hmox2b). Expression of all four hmox paralogs and two bvr isoforms were measured in adult tissues (gill, brain and liver) and sexually dimorphic differences were observed, most notably in the basal expression of hmox1a, hmox2a, hmox2b and bvrb in liver samples. hmox1a, hmox2a and hmox2b were significantly induced in male liver tissues in response to 96 h cadmium exposure (20 μM). hmox2a and hmox2b were significantly induced in male brain samples, but only hmox2a was significantly reduced in male gill samples in response to the 96 h cadmium exposure. hmox paralogs displayed significantly different levels of basal expression in most adult tissues, as well as during zebrafish development (24 to 120 hpf). Furthermore, hmox1a, hmox1b and bvrb were significantly induced in zebrafish eleutheroembryos in response to multiple pro-oxidants (cadmium, hemin and tert-butylhydroquinone). Knockdown of Nrf2a, a transcriptional regulator of hmox1a, was demonstrated to inhibit the Cd-mediated induction of hmox1b and bvrb. These results demonstrate distinct mechanisms of hmox and bvr transcriptional regulation in zebrafish, providing initial evidence of the partitioning of function of the hmox paralogs.