BACKGROUND: Disruption of iron homeostasis induced by cigarette smoke (CS) exposure is a major risk factor for smoking-related osteoporosis. However, the specific mechanisms remain unclear.
METHODS: For in vivo experiments, wild-type (WT) and Nuclear factor E2-related factor 2 (Nrf2)-knockout mice received daily intraperitoneal injections of fresh CS extract for 60 days (0.2 mL/20 g/d), after which bone tissues were harvested for analysis of bone mineral density, histomorphometry, and molecular markers. In vitro experiments were performed using bone marrow-derived mesenchymal stem cells (BMSCs) exposed to CS, with or without treatment of the Nrf2 agonist CDDO. The changes of Nrf2/HECT and RLD domain containing E3 ubiquitin protein ligase 2 (HERC2) pathway in both in vivo and in vitro models were evaluated.
RESULTS: CS triggered ferroptosis in murine bone and BMSCs by stimulating nuclear receptor coactivator 4 (NCOA4)-mediated ferritinophagy. This process shifted BMSCs differentiation towards adipogenesis via enhancing peroxisome proliferator-activated receptor-γ, while suppressing osteogenesis by inhibiting β-catenin. Mechanistically, CS upregulated Kelch-like ECH-associated protein 1, inhibiting Nrf2 nuclear translocation. Consequently, suppressed Nrf2 reduced transcription of downstream HERC2, impairing its role in degrading NCOA4 and ultimately activating ferroautophagy. Nrf2-knockout mice exhibited exacerbated CS-induced reduction in bone mineral density loss and decreased BMSCs markers. Similarly, Nrf2 or HERC2 knockdown in BMSCs amplified CS-induced ferroptosis and adipogenic commitment. Conversely, Nrf2 activation via agonist bardoxolone restored HERC2 expression, thereby attenuating ferroautophagy and rebalancing osteogenic-adipogenic differentiation.
CONCLUSION: CS induces bone loss by dysregulating the Nrf2/HERC2 axis, leading to ferroautophagy-driven ferroptosis and impaired osteogenesis. Pharmaceutical Nrf2 activation mitigates these effects, highlighting a novel therapeutic target for smoking-related osteoporosis.
