Effect of tBHQ on ATRA-induced differentiation and calcium homeostasis in ATRA-sensitive and -resistant NB4 variants.

(A) NB4-007/6 and NB4-306 cells were treated for 3 days with vehicle, 50 nM ATRA, 1 μM ATRA, 8 μM tBHQ, and 50 nM ATRA in combination with 8 μM tBHQ (as indicated in the figure); and NBT-reducing activity was examined. Mean ± SEM (n = 3). (B) Ast4 and HL-60_RES cells were treated for 3 days with vehicle (■) or with 3 μM tBHQ plus 50 nM ATRA in combination (▪), and cell differentiation was detected by NBT reduction. Mean ± SEM (n = 3). (C) NB4, NB4-007/6, and NB4-306 cells were treated for 3 days with vehicle, 50 nM ATRA, 1 μM ATRA, 8 μM tBHQ, and 50 nM ATRA in combination with 8 μM tBHQ (as indicated in the figure); and, after addition of 0.5 mM EGTA, the increase of cytosolic Ca²⁺concentration was recorded immediately after addition of 20 μM tBHQ, as described in “Materials and methods.” The increased intensities of peaks were measured for each treatment, and the results are expressed as percentage of cytosolic Ca²⁺ concentration increase compared with the basal cytosolic Ca²⁺concentration before addition of tBHQ. Mean ± SEM (n = 3). **P ≤ .01 and *P ≤ .05 according to the Duncan test for multiple comparisons.