Heme oxygenase-1 comes back to endoplasmic reticulum

► Although multiple compartmentalization of HO-1 has been documented, the functional implication of this enzyme at these subcellular organelles is only partially elucidated. ► HO-1 expression at ER is induced by a diverse set of conditions that cause ER stressors. ► CO may induce HO-1 expression in human ECs by activating Nrf2 through PERK phosphorylation in a positive-feedback manner. ► ER-residing HO-1 and its cytoprotective activity against ER stress is discussed.