Absence of heme at the catalytic site of heme oxygenase-2 triggers its lysosomal degradation
Heme oxygenase-2 (HO2) and -1 (HO1) catalyze heme degradation to biliverdin, CO, and iron, forming an essential link in the heme metabolism network. Tight regulation of the cellular levels and catalytic activities of HO1 and HO2 is important for maintaining heme homeostasis. While transcriptional control of HO1 expression has been well-studied, how the cellular levels and activity of HO2 are regulated remains unclear. Here, the mechanism of post-translational regulation of cellular HO2 level by heme is elucidated. Under heme deficient conditions, HO2 is destabilized and targeted for degradation. In HO2, three heme binding sites are potential targets of heme-dependent regulation: one at its catalytic site; the others at its two heme regulatory motifs (HRMs). We report that, in contrast to other HRM-containing proteins, the cellular protein level and degradation rate of HO2 are independent of heme binding to the HRMs. Rather, under heme deficiency, loss of heme binding to the catalytic site destabilizes HO2. Consistently, a HO2 catalytic site variant that is unable to bind heme exhibits a constant low protein level and an enhanced protein degradation rate compared to the wild-type HO2. However, cellular heme overload does not affect HO2 stability. Finally, HO2 is degraded by the lysosome through chaperone-mediated autophagy, distinct from other HRM-containing proteins and HO1, which are degraded by the proteasome. These results reveal a novel aspect of HO2 regulation and deepen our understanding of HO2s role in maintaining heme homeostasis, paving the way for future investigation into HO2s pathophysiological role in heme deficiency response.