Heme oxygenase (HO) activity is exhibited by inducible (HO-1) and constitutive (HO-2) proteins. HO-1 protects against ischemic and nephrotoxic acute kidney injury (AKI). We have previously demonstrated that HO-2 protects against heme protein-induced AKI. The present study examined whether HO-2 is protective in ischemic AKI. Renal ischemia was imposed on young and aged HO-2+/+ and HO-2-/- mice. On days 1 and 2 after renal ischemia, there were no significant differences in renal function between young male HO-2+/+ and HO-2-/- mice, between young female HO-2+/+ and HO-2-/- mice, or between aged female HO-2+/+ and HO-2-/- mice. However, in aged male mice, HO-2 deficiency worsened renal function on days 1 and 2 after ischemic AKI, and, on day 2 after ischemia, such deficiency augmented upregulation of injury-related genes and worsened histological injury. Renal HO activity was markedly decreased in unstressed aged male HO-2-/- mice and remained so after ischemia, despite exaggerated HO-1 induction in HO-2-/- mice after ischemia. Such exacerbation of deficiency of HO-2 protein and HO activity may reflect phosphorylated STAT3, as activation of this proinflammatory transcription factor was accentuated early after ischemia in aged male HO-2-/- mice. This exacerbation may not reflect impaired induction of nephroprotectant genes, since the induction of HO-1, sirtuin 1, and β-catenin was accentuated in aged male HO-2-/- mice after ischemia. We conclude that aged male mice are hypersensitive to ischemic AKI and that HO-2 mitigates such sensitivity. We speculate that this protective effect of HO-2 may be mediated, at least in part, by suppression of phosphorylated STAT3-dependent signaling.
Keywords: acute kidney injury; age; biology of sex differences; heme oxygenase-2; phospho-STAT3.