Figure 4

MIRA-1 induced an ER stress response in MM cells. (A) MM.1S and LP1 cells were treated with MIRA-1 for 8 h, total proteins were harvested and analysed by Western blot for the expression of the indicated proteins. (B) XBP1 mRNA splicing by MIRA-1. RT–PCR analysis with XBP1 or GAPDH-specific primers of total RNA extracted from MM.1S or 8226 cells that were incubated with 10 or 20 μmol l⁻¹ MIRA-1, respectively, for 8 h as indicated. XBP1U = unspliced XBP1; XBP1S = spliced XBP1. Thapsigargin (TS) (100 nmol l⁻¹) was used as a positive control ER stress-inducing agent. (C) Proposed model for the p53-independent apoptotic function of MIRA-1 in MM cells. MIRA-1 may directly interact with mitochondria function inhibiting Bcl2 family members Mcl-1 and/or Myc through upregulation of Puma and Bax. Alternatively, inhibition of Mcl-1 by MIRA-1 may be indirectly mediated by activation of ER stress signalling.