Hemopexin-dependent heme uptake via endocytosis regulates the Bach1 transcription repressor and heme oxygenase gene activation

•An efficient purification method of recombinant hemopexin (rHx) was established.•rHx showed chemical and biological properties similar to those of native Hx.•rHx-bound heme induced the expression of heme oxygenase-1 (HO-1) by promoting Bach1 degradation.•The induction of HO-1 by rHx-bound heme was dependent on endocytosis.

BackgroundIntracellular heme plays versatile roles in a variety of physiological processes including mitochondrial respiration. Heme also induces the expression of genes such as heme oxygenase-1 (HO-1) by inactivating the transcription repressor Bach1 through direct binding. However, the source of heme for the regulation of the Bach1–HO-1 axis has been unclear. Considering that extracellular heme exists as a complex with hemopexin (Hx) in serum under the physiological conditions, heme–Hx complex may deliver heme for the gene regulation.MethodsUsing a mammalian expression system, high secretory recombinant Hx (rHx) was developed. We examined the effects of rHx-bound heme on HO-1 expression and Bach1 in Hepa-1c1c7 liver cells and THP-1 macrophage cells. We investigated the uptake pathway of rHx-bound heme by treating cells with chlorpromazine (CPZ).ResultsrHx-bound heme induced the expression of HO-1 and decreased the level of Bach1 protein. CPZ inhibited the induction of the HO-1 expression by rHx-bound heme.ConclusionrHx-bound heme was internalized into the cells via endocytosis, resulting in HO-1 expression and inactivation of Bach1.General significanceThe Bach1-dependent repression of the HO-1 expression is under the control of the Hx-dependent uptake of extracellular heme. Heme may regulate Bach1 as an extracellular signaling molecule.