OB-R gene and protein expression in PR9 cells, NB4 cells, and primary APL cells. (A) OB-R long- and short-isoform mRNA levels before and after the induction of PML-RARα (200 μM ZnSO₄ treatment) in U937/PR9 cells, in NB4 cells, and in primary APL cells (TaqMan RT-PCR). OB-R mRNA expression of each sample was normalized to β₂-microglobulin. PR9 cells without ZnSO₄ were used as calibrators (OB-R mRNA level in PR9 cells without ZnSO₄ = 1). Percentages of blasts and promyelocytes in APL samples: no. 1, PB, 82%; no. 2, BM, 95%, no. 3, PB, 75%; no. 4, PB, 91% (PB indicates peripheral blood). Graphs represent the mean ± SD of results from 3 different experiments. (B) Amplification plot of OB-R long-isoform cDNA expression in PR9 cells after ZnSO₄ treatment in NB4 and primary APL cells. Blue lines indicate PR9; magenta lines, NB4; and red lines, primary APL cells. (C) Representative immunocytochemistry results for OB-R expression in the cytosol of PR9 cells: (i) before ZnSO₄ treatment and (ii) after 24 hours of ZnSO₄ (200 μM) treatment. (iii) Immunoreaction of OB-R in cytosol of NB4 cells (original magnification, × 200).