Deficiency of XBP1 or IRE1α compromises MEF and murine BMSC support of MM cell growth. (A) Western blot demonstrating the effect of XBP1 deficiency on baseline and TNFα (10 ng/mL for 24 hours)–induced VCAM-1 protein expression in MEF cells. The basal level VCAM-1 protein expression in XBP1^+/+ MEF cells was normalized to β-actin and defined as 1. (B) ELISA assay shows that IL-6 secretion was decreased significantly in XBP1^−/− MEF cells compared with XBP1^+/+ MEF cells (n = 3; mean ± SD; *P < .05). (C) Effect of XBP1 deficiency on the capacity of MEF cells to support MM cell growth (n = 3, mean ± SD, *P < .05). (D) The effects of TNFα (10 ng/mL for 24 hours) on baseline and TNFα-induced VCAM-1 protein expression in IRE1α^+/+ and IRE1α^−/− MEF cells, determined by Western blotting. The basal level VCAM-1 protein expression in IRE1α^+/+ MEF cells was normalized to β-actin and defined as 1. (E) ELISA assay shows that IL-6 secretion was significantly decreased in IRE1α^−/− MEF cells compared with that in IRE1α^+/+ MEF cells (n = 3; mean ± SD; *P < .05). (F) Effect of IRE1α deficiency on the capacity of MEF cells to support 5TGM1 cell growth (n = 3, mean ± SD, *P < .05). (G) Effects of murine XBP1 knock-down on the baseline and TNFα (10 ng/mL for 24 hours)–induced VCAM-1 protein expression in mBMSCs. The basal level VCAM-1 protein expression in mBMSCs with control shRNA was normalized to β-actin and defined as 1. (H) Effects of murine XBP1 knockdown on IL-6 secretion in mBMSCs (n = 3; mean ± SD; *P < .05). (I) XBP1 knockdown reduced mBMSC support of 5TGM1 cell growth (n = 3, mean ± SD, *P < .05).