Fig. 7: Model of heme-triggered DNA damage and cytotoxicity in HCEC versus CRC cells and role of HO-1.

a Heme iron is taken up into HCEC, where it catalyzes the formation of reactive oxygen species (ROS) and induces oxidative DNA lesions as well as DNA strand breaks (indicated by red asterisks). This finally results in cytotoxicity, which is more prominent in HCEC than in CRC cell lines. In contrast, internalized inorganic iron causes little ROS production and DNA damage, and only slightly impairs cell viability. Heme-dependent ROS formation activates the transcription factor Nrf2, which shuttles from the cytoplasm to the nucleus, where it drives the transcription of its target genes such as heme oxygenase-1 (HO-1). This enzyme degrades heme to Fe²⁺, carbon monoxide (CO) and biliverdin (BV). Concomitant to HO-1 induction, ferritin heavy chain gene (FtH) is upregulated by hemin. Genetic abrogation of HO-1 by siRNA or its pharmacological inhibition by ZnPP potentiated heme-induced ROS, DNA damage and cell death, strongly suggesting that heme iron, and not its breakdown product Fe²⁺, initiates ROS formation and thus DNA damage induction. b Internalization of heme iron and inorganic iron in CRC cells. Similar to HCEC, inorganic iron causes little ROS production and DNA damage, and only slightly impairs viability in CRC cells. Hemin is taken up differentially into CRC cells (Caco-2 > HCT116), causing less ROS formation and oxidative DNA damage as in HCEC. Furthermore, CRC cells are in general more resistant against hemin-triggered cytotoxicity and HO-1 abrogation moderately affects cell survival in the presence of high heme concentrations. This figure was created using Servier Medical Art templates, which are licensed under a Creative Commons Attribution 3.0 Unported License; https://smart.servier.com.