Activation of KEAP1/NRF2 stress signaling involved in the molecular basis of hemin-induced cytotoxicity in human pro-erythroid K562 cells.

During hemolysis, free heme released from damaged RBCs impairs adjacent cells. As a response, heme induces its metabolic degradation via heme oxygenase-1 (HO-1), activated by NF-E2-related factor 2 (NRF2), the master stress response transcription factor. Heme is well considered a signaling molecule, but how heme does activate NRF2 is not well understood. K562, human pro-erythroid cells responding to hemin (ferric chloride heme), were employed to uncover the major role of Kelch-like ECH-associated protein 1 (KEAP1)/NRF2 stress response signaling, embedded in hemin-induced cytotoxicity (HIC), at ≥50 μM. The intracellular pools of hemin were found to determine the progression from the reversible cell growth inhibition to non-apoptotic cell death. Hemin-induced accumulation of both reactive oxygen species (ROS) and ubiquitinated proteins provoked disturbed cellular proteostasis. Immediate accumulation and nuclear translocation of NRF2 were recorded as defensive adaptation. The NRF2-driven genes encoding glutamate-cysteine ligase (GCLC) and cystine/glutamate antiporter (xCT) were substantially activated. Hemin orchestrated a defensive pathway involving the management of cellular non-protein thiols, via an increase in GSH levels and secretion of cysteine. Mechanistically, hemin stabilized NRF2 protein levels selectively by inhibiting the KEAP1-driven ubiquitination of NRF2, while allowing KEAP1 ubiquitination. High-molecular-weight ubiquitinated KEAP1 variants formed in hemin-treated cells degraded in proteasomes, while a portion of them translocated into the nucleus. The KEAP1/NRF2 system can be revealed as a basic homeostatic mechanism, activated in cells encountering free heme, both in healthy and diseased state. Its activation provides a multi-target cytoprotective platform to develop agents preventing heme toxicity.