Background: Xanthine oxidoreductase (XOR) is involved in oxidative metabolism of purines and is a source of reactive oxygen species (ROS). As such, XOR has been implicated in oxidant-mediated injury in multiple cardiopulmonary diseases. XOR enzyme activity is regulated, in part, via a phosphorylation-dependent, post-translational mechanism, although the kinase(s) responsible for such hyperactivation are unknown.
Methods and results: Using an in silico approach, we identified a cyclin-dependent kinase 5 (CDK5) consensus motif adjacent to the XOR flavin adenine dinucleotide (FAD) binding domain. CDK5 is a proline-directed serine/threonine kinase historically linked to neural development and injury. We tested the hypothesis that CDK5 and its activators are mediators of hypoxia-induced hyperactivation of XOR in pulmonary microvascular endothelial cells (EC) and the intact murine lung. Using complementary molecular and pharmacologic approaches, we demonstrated that hypoxia significantly increased CDK5 activity in EC. This was coincident with increased expression of the CDK5 activators, cyclin-dependent kinase 5 activator 1 (CDK5r1 or p35/p25), and decreased expression of the CDK5 inhibitory peptide, p10. Expression of p35/p25 was necessary for XOR hyperactivation. Further, CDK5 physically associated with XOR and was necessary and sufficient for XOR phosphorylation and hyperactivation both in vitro and in vivo. XOR hyperactivation required the target threonine (T222) within the CDK5-consensus motif.
Conclusions and significance: These results indicate that p35/CDK5-mediated phosphorylation of T222 is required for hypoxia-induced XOR hyperactivation in the lung. Recognizing the contribution of XOR to oxidative injury in cardiopulmonary disease, these observations identify p35/CDK5 as novel regulators of XOR and potential modifiers of ROS-mediated injury.