FIG 4.

6-TG and 6-TGo activate the UPR. (A) A549 cells were treated with 6-thioguanine (6-TG), 6-thioguanosine (6-TGo), 6-mercaptopurine (6-MP), 5-fluorouracil (5-FU), or ribavirin at the indicated concentrations for 6 h prior to harvesting lysates for immunoblotting for the indicated cellular proteins. Tunicamycin (TM) at 5 μg/ml served as a positive control for UPR activation. ATF6* indicates a lower-molecular-weight species that is not cleaved to its active form. (B) cDNA was generated from total RNA that was isolated from treated cells. XBP1 mRNA splicing was determined by the semiquantitative RT-PCR splicing assay. XBP1u₁ and XBP1u₂ indicate the cleaved products from digesting the unspliced XBP1 cDNA with PstI.