PS-341 inhibits spontaneous proliferation of Tax transgenic mouse tumors in culture. (A) A total of 3 × 10⁶ splenocytes or tumor cells from 5 Tax transgenic and 5 nontransgenic mouse splenocytes were cultured for 4 hours following drug or mock treatment. Nontransgenic splenocytes were also stimulated with 200 U/mL IL-2 plus 10 μg/mL PHA. Then, 1 μCi/well (1.1 MBq/well) [³H] thymidine was added to cultures, and thymidine incorporation was measured after a 16-hour incubation. Percent proliferation is reported ± SEM considering the [³H] thymidine incorporation of untreated samples as 100%. The statistical significance of the inhibition was evaluated by a paired t test: P ≤ .01 for untreated compared to sodium salicylate treatments and P ≤ .05 for untreated compared to PS-341 treatments. (B) Cell viability was measured by trypan blue exclusion after 20 hours of culture with sodium salicylate or PS-341 and values from spleens or tumors from 5 Tax transgenic and 5 nontransgenic mouse spleens are reported ± SEM.