HIF1α-KLF4 signaling dictates extra-erythrocyte expression of hemoglobin conferring sorafenib resistance in hepatocellular carcinoma.

Cancer cells must deal with excessive reactive oxygen species (ROS) to survive severe hypoxia and anticancer therapy; however, anti-ROS mechanisms other than the well-known NFE2L2/NRF2 signaling pathway are poorly recognized. Here, we report a ROS scavenging mechanism mediated by HIF1α-KLF4-induced hemoglobin extraerythrocytically expressed in hepatocellular carcinoma (HCC). We found that the ROS pathway was aberrantly activated in HCC and was associated with hemoglobin upregulation, which independently predicts poor outcomes. Network analysis further identified heme binding as the top ROS-associated functional module, suggesting a previously unrecognized role of hemoglobin in maintaining redox homeostasis in HCC. Hemoglobin expression in cancer cells is transcriptionally controlled by HIF1α via KLF4 but not HIF2α or the recently identified KDM5A-KLF1 signaling. The upregulated hemoglobin counteracts the detrimental effects of oxidative stress by scavenging ROS, promoting sorafenib resistance, which could be effectively reversed by interfering with hemoglobin expression, leading to tumor suppression. Both in vitro and in vivo experiments consistently supported the functional importance of hemoglobin in regulating oxidative stress adaptation and therapeutic response. Overall, a previously unrecognized mechanism was identified for cancer cell survival under oxidative stress, where HIF1α-KLF4 signaling induces hemoglobin to scavenge ROS produced during hypoxia and anticancer therapy, providing a promising target of synthetic lethality for cancer therapeutics.