Induction of peripheral-type benzodiazepine receptors during differentiation of mouse erythroleukemia cells. A possible involvement of these receptors in heme biosynthesis.

To search for a possible role for peripheral-type benzodiazepine receptors (PBR) during erythroid differentiation, we cloned the PBR isoquinoline carboxamide-binding protein (PBR/IBP), an 18-kDa protein on PBR, from a mouse erythroleukemia (MEL) cell cDNA library. Sequence analysis revealed that PBR/IBP comprises 169 amino acid residues (M(r) 18,828), and has a high homology with PBR/IBP from other sources. The cDNA allows for the expression of active PBR/IBP, exhibiting a high affinity for isoquinoline carboxamide, [3H]PK11195, with Kd of 0.80 and 1.56 nM. RNA blot analysis revealed that treatment of MEL cells with dimethyl sulfoxide led to an increase in PBR/IBP mRNA (delta 1.0 kilobases) for up to 72 h, with a concomitant induction of mRNAs for heme biosynthetic enzymes, coproporphyrinogen oxidase and ferrochelatase. The induction of PBR/IBP mRNA was also observed in MEL cells induced with diazepam. The binding activity of [3H]PK11195 in MEL cells showed a high affinity with Kd of 0.69-2.13 nM, and increased during erythroid differentiation. The order of potency of different ligands to compete against [3H]PK11195 binding in induced MEL cells was PK11195 > protoporphyrin IX > diazepam > coproporphyrinogen III > coproporphyrin III > estazolam. In contrast to the induction of PBR/IBP in induced MEL cells, the voltage-dependent anion channel (mitochondrial porin) associated with PBR remained unchanged. These results suggest that PBR/IBP on PBR may be involved in porphyrin transport and may even be a critical factor in erythroid-specific induction of heme biosynthesis.