Effects of signal transduction inhibitors on histamine levels and HDC expression in Ba/F3 cells and CML cells. (A) Ton.B210-X cells were cultured in the absence (Control) or in the presence of doxycycline, 1 μg/mL (to express BCR/ABL) for 24 hours. BCR/ABL-expressing cells were incubated in control medium (BCR/ABL) or in the presence of inhibitors (PD98059, 50 μM; LY294002, 20 μM; rapamycin, 20 nM; imatinib, 1 μM) for another 24 hours. Thereafter, cells were harvested, and the levels of histamine were determined by RIA. Results represent the mean ± SD from 3 independent experiments. The asterisk indicates significant inhibition compared with control = BCR/ABL (P < .05). (B) KU812 cells were incubated with control medium (Control) or various signal transduction inhibitors (same type and dose as in panel A) for 14 hours. Thereafter, total RNA was isolated and subjected to Northern blot analysis. HDC mRNA expression was analyzed using an HDC-specific cDNA probe. β-actin served as a loading control. (C-D) Histamine levels (determined by RIA) in KU812 cells (C) and in PBMCs obtained from patients with CML (chronic phase, n = 5; accelerated phase, n = 4) (D). KU812 cells and isolated primary PBMCs were incubated with control medium (Control), PD98059 (50 μM), LY294002 (20 μM), rapamycin (20 nM), or imatinib (STI571; 1 μM) for 24 hours. Then cells were lysed in distilled water and subjected to RIA. Results represent the mean ± SD from 3 (C) or 9 (D) independent experiments, respectively. ^*P < .05 compared with controls.