Compromising the Unfolded Protein Response Induces Autophagy-Mediated Cell Death in Multiple Myeloma Cells

OBJECTIVE: To determine whether the Unfolded Protein Response (UPR) sensors (PERK, ATF6 and IRE-1) can be targeted to promote death of Multiple Myeloma (MM) cells. METHODS: We have knocked-down separately each UPR stress sensor in human MM cell lines using RNA interference and followed MM cell death by monitoring the membrane, mitochondrial and nuclear alterations. Involvement of caspases in MM cell death consecutive to UPR sensor knock-down was analyzed by western blotting, measurement of their enzymatic activity using fluorigenic substrates and susceptibility to a pan-caspase inhibitor. Activation of the autophagic process was measured directly by detection of autophagosomes (electronic microscopy), monodansylcadaverine staining, production of the cleaved form of the microtubule-associated protein 1A/1B light chain 3 (LC3) and indirectly by analyzing the impact of pharmacological inhibitors of autophagy such as 3MA and bafilomycin A1. RESULTS: We show that extinction of a single UPR stress sensor (PERK) induces a non-apoptotic form of cell death in MM cells that requires autophagy for its execution. We also show that this cytotoxic autophagic process represses the apoptosis program by reducing the cytosolic release of the apoptogenic factors Smac/DIABLO and cytochrome c. INTERPRETATION: Altogether our findings suggest that autophagy can contribute to execution of death in mammalian cells that are exposed to mild ER stress. They also suggest that the autophagic process can regulate the intrinsic apoptotic pathway by inhibiting production of death effectors by the mitochondria, thus preventing formation of a functional apoptosome. Altogether these findings give credit to the idea that UPR sensors can be envisaged as therapeutic targets for the treatment of MM

The cell death promoted by the PERK siRNA is morphologically distinct from apoptosis.

<p>NCI-H929 cells were either transfected with the non-targeting (C) or with the PERK (D–H) siRNA, treated with staurosporine (B) or left untreated (A) for 24 h and then examined by electron microscopy. The two stages or the death process induced by PERK silencing are illustrated in panels D and E. F–H Visualization at higher magnification of the cytosolic electron-dense structures identified by the arrowheads in D. In F, a pre-autophagosomal-like structure, in G and H, autophagosomes with a double membrane sequestering cellular material.</p

Silencing UPR sensors induces death of human MM cell lines.

<p>A. NCI-H929 cells were transfected with the PERK, ATF6, IRE1 or non-targeting siRNAs. The levels of expression of the targeted transcripts were determined by real-time RT-PCR 24 h after transfection. B. Percentages of transcript extinction induced by the targeting siRNAs in U266 and NCI-H929 cells (mean ± SD values of three independent experiments). C. Whole cell lysates were prepared from NCI-H929 cells transfected with the targeting or non-targeting siRNAs and blotted with anti-PERK, anti-ATF6, anti IRE1 or anti-ß actin mAbs. Representative of two independent experiments. D. NCI-H929 cells were transfected with the PERK (siPERK) or control siRNA (siCtrl). Percentages of cells with Δm loss (TMRE^lo) or with membrane alterations (Annexin V+ or PI+) were evaluated by TMRE and annexin V or PI stainings, respectively. Results represent the mean+SD values of 2 experiments.</p

Extinction of the UPR sensors induces the autophagic cell death of MM cells.

<p>A. NCI-H929 cells were serum-starved or transfected with the non-targeting or PERK siRNAs for 24 h then stained with MDC and examined by fluorescence microscopy. B. NCI-H929 cells cultured as in A, in the presence or absence of 3-MA, were stained with MDC and analyzed by flow cytometry. C and D, NCI-H929 cells were serum-starved (C) or transfected either with the non-targeting or the PERK siRNAs (D). All cultures were conducted with or without bafilomycin A1. Accumulation of autophagosomes was visualized by the conversion of endogenous LC3-I (18 kDa) to LC3-II (16 kDa). E. Average proportion of MDC⁺ cells in NCI-H929 cells 24 h after transfection with the targeting or non-targeting siRNAs or 24 h after culture in serum-starved conditions. All cultures were conducted with or without 3-MA. Results are expressed as the mean ± SD percentages of positive cells as calculated from duplicate determinations. The data shown are representative of two independent experiments. F. NCI-H929 cells were cultured as in E. Membrane alterations were estimated by Annexin V staining after 24 h of culture. Results are expressed as the mean ± SD percentages of positive cells as calculated from duplicate determinations and are representative of three independent experiments. (* <i>p</i><0.05; ** <i>p</i><0.01; ***<i>p</i><0.005; <i>ns</i> = non significant).</p

PERK silencing represses apoptosis of MM cells.

<p>A. NCI-H929 cells were transfected with the non-targeting or PERK siRNA and treated or not with STS 8 h later. Control cultures in which untransfected cells were treated with STS were also conducted. DNA fragmentation (A) and PS exposure (B) were assessed 24 h after transfection. Results are expressed as the mean ± SD percentages of positive cells as calculated from duplicate determinations and are representative of two independent experiments. C. NCI-H929 cells were cultured as in B and C in the presence or absence of 3-MA. DNA fragmentation was assessed by the TUNEL assay. Mean ± SD values of two independent experiments are shown. (* <i>p</i><0.05).</p

Silencing UPR sensors fails to promote DNA fragmentation and caspase activation in MM cells.

<p>A–B. NCI-H929 cells were transfected with the targeting or non-targeting siRNAs and cultured for 24 h with or without zVAD-fmk (100 µM). Cells were also treated with staurosporine (STS) as a positive control for caspase-dependent cell death. Oligonucleosomal DNA fragmentation was estimated both by the TUNEL (A) and the Apostain (B) assays. Results are expressed as the mean ± SD percentages of cells with fragmented DNA as calculated from duplicate determinations. The data shown are representative of three independent experiments. C–D. Whole cell extracts were prepared from NCI-H929 cells transfected either with the targeting or the non targeting siRNA or treated with tunicamycin (TN) or STS for 24 h. Activation of caspase-4 (C) and caspase-3 (D) was assessed by immunoblot analysis. The anti-ß actin mAb was used as a loading control. The proenzymatic forms of caspase-4 and caspase-3 were visualized as a 46 kD and a 32 kD band, respectively. Their cleaved active fragments were identified as a 30 kD band (caspase-4) and as a 16–18 kD doublet (caspase-3) (representative of three independent experiments). E. NCI-H929 cells were transfected with the non-targeting or PERK siRNAs or treated with tunicamycin (TN) for 24 h and examined for membrane, mitochondrial and nuclear alterations. Results are expressed as the mean ± SD percentages of positive cells (for annexin V, PI and TUNEL) or TMRE^low cells as calculated from duplicate determinations.</p

Extinction of the UPR sensors induces transient activation of the UPR in MM cells.

<p>The NCI-H929 MM cell line was transfected with the PERK (A, B), ATF6 (C), IRE1 (D) or non-targeting (ctrl) siRNAs. Whole cell lysates were prepared 12, 24 or 48 h after transfection and analyzed by Western blot for the expression of CHOP, unspliced XBP-1 (u), spliced XBP-1 (s), total and phosphorylated EIF2α and β actin. The data are representative of three different experiments.</p