The role of XBP1 in UPR-mediated Mcl-1 expression. (A-F) RT-PCR analysis of XBP1 splicing in different MM cell lines. The MM cell lines OPM2, MMS1, U266, 5T33vt, RPMI-8226, and LP1 were treated with bortezomib and tunicamycin for 24 hours at varying doses. RNA isolation and RT-PCR were performed as described in “Methods.” In the human MM cell lines OPM2, MMS1, U266, RPMI-8226, and LP1, the 210- and 184-bp DNA fragments correspond to unspliced and spliced human XBP1 mRNAs, respectively. In 5T33vt cells, the 343- and 327-bp DNA fragments correspond to unspliced and spliced mouse Xbp1 mRNAs, respectively.