hBVR (human biliverdin reductase) is an enzyme that reduces biliverdin (the product of haem oxygenases HO-1 and HO-2 activity) to the antioxidant bilirubin. It also functions as a kinase and as a transcription factor in the MAPK (mitogen-activated protein kinase) signalling cascade. Fluorescence correlation spectroscopy was used to investigate the mobility of hBVR in living cells and its function in the nuclear transport of haematin for induction of HO-1. In transiently transfected HeLa cells only kinase-competent hBVR translocates to the nucleus. A reduced mobility in the nucleus of haematin-treated cells suggests formation of an hBVR–haematin complex and its further association with large nuclear components. The binding of haematin is specific, with the formation of a 1:1 molar complex, and the C-terminal 7-residue fragment KYCCSRK296 of hBVR contributes to the binding. The following data suggest formation of dynamic complexes of hBVR–haematin with chromatin: (i) the reduction of hBVR mobility in the presence of haematin is greater in heterochromatic regions than in euchromatic domains and (ii) hBVR mobility is not retarded by haematin in nuclear lysates that contain only soluble factors. Moreover, hBVR kinase activity is stimulated in the presence of double-stranded DNA fragments corresponding to HO-1 antioxidant and HREs (hypoxia response elements), as well as by haematin. Experiments with nuclear localization, export signal mutants and si-hBVR [siRNA (small interfering RNA) specific to hBVR] indicate that nuclear localization of hBVR is required for induction of HO-1 by haematin. Because gene regulation is energy-dependent and haematin regulates gene expression, our data suggest that hBVR functions as an essential component of the regulatory mechanisms for haem-responsive transcriptional activation.

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