Spermidine Confers Liver Protection by Enhancing NRF2 Signaling Through a MAP1S-Mediated Noncanonical Mechanism

Spermidine (SPD), a naturally occurring polyamine, has been recognized as a caloric restriction mimetic that confers health benefits, presumably by inducing autophagy. Recent studies have reported that oral administration of SPD protects against liver fibrosis and hepatocarcinogenesis through activation of microtubule associated protein 1S (MAP1S)–mediated autophagy. Nuclear factor (erythroid-derived 2)-like 2 (NRF2) is a transcription factor that mediates cellular protection by maintaining the cell's redox, metabolic, and proteostatic balance. In this study, we demonstrate that SPD is a noncanonical NRF2 inducer, and that MAP1S is a component of this noncanonical pathway of NRF2 activation. Mechanistically, MAP1S induces NRF2 signaling through two parallel mechanisms, both resulting in NRF2 stabilization: (1) MAP1S competes with Kelch-like ECH-associated protein 1 (KEAP1) for NRF2 binding through an ETGE motif, and (2) MAP1S accelerates p62-dependent degradation of KEAP1 by the autophagy pathway. We further demonstrate that SPD confers liver protection by enhancing NRF2 signaling. The importance of both NRF2 and p62-dependent autophagy in SPD-mediated liver protection was confirmed using a carbon tetrachloride–induced liver fibrosis model in wild-type, Nrf2^-/-, p62^-/- and Nrf2^-/-;p62^-/- mice, as the protective effect of SPD was significantly reduced in NRF2 or p62 single knockout mice, and completely abolished in the double knockout mice. Conclusion: Our results demonstrate the pivotal role of NRF2 in mediating the health benefit of SPD, particularly in the context of liver pathologies.