The ER inducer tunicamycin increases Mcl-1 expression and triggers UPR activation in MM cells. (A) Tunicamycin treatment induces Mcl-1 accumulation and cleavage in OPM2 cells. OPM2 cells were treated with different doses of tunicamycin for 24 hours. Cell extracts were probed with Mcl-1, ATF4, GRP-78, CHOP, and β-actin pAbs. Left panel shows the representative blots of 3 independent experiments; right panel shows densitometric analysis. (B-D) qRT-PCR analysis of the expression of Mcl-1 and the UPR-related components in response to tunicamycin treatment in RPMI-8226 (B), OPM2 (C), and U266 (D) cells. RPMI-8226, OPM2, and U266 cells were treated with 2.5 μg/mL of tunicamycin for 24 hours. RNA isolation, cDNA synthesis, and qRT-PCR were performed as described in “Methods.” *P < .05 and **P < .01 versus vehicle-treated samples. Data represent the means ± SD of 3 separate experiments, each performed in triplicate.