Inhibition of TLR4 Signaling Affects Mitochondrial Fitness Overcoming Bortezomib Resistance in Myeloma Plasma Cells

BACKGROUND Multiple myeloma (MM) is a B-cell malignancy critically dependent for survival and proliferation on signals coming from its inflammatory microenvironment in which toll-like receptors (TLR) may be potential linking elements between inflammation and cancer. It has been recently demonstrated that TLR4 pathway provides a protective effect against bortezomib (BTZ)-induced endoplasmic reticulum (ER) stress and pre-treatment of MM cells with LPS significantly reduces BTZ-induced apoptosis. AIM Since the acquisition of BTZ resistance is accompanied by an increased reliance on mitochondrial respiration, we investigated the role of TLR4 as stress-responsive mechanism that protect mitochondria during BTZ-induced ER stress as potential mechanism of drug resistance. RESULTS The activation of TLR4 signaling by LPS increased mitochondrial mass in human MM cell lines (HMCL: U266, MM1.S, OPM2, NCI-H929) and induced up-regulation of mitochondrial biogenesis markers (PGC1a, PRC and TFAM). After treatment with BTZ for 24h, all HMCL over-expressed TLR4 and its signaling was functional as suggested by up-regulation of MyD88 and MAPK activation. Compared to BTZ-sensitive cells (U266-S), U266-R showed higher levels of TLR4, p-p38 and p-ERK proteins and higher mitochondrial mass. Using a selective TLR4 inhibitor (TAK-242), we next treated U266-R cells with either 15nM BTZ, 20 μM TAK-242 or their combination. Combinatorial treatment significantly induced cell apoptosis (about 52%; p<0.001) that appeared to result from the deleterious effects of oxidative stress. Indeed, BTZ-induced intracellular ROS returned to normal levels after 3h and cells were able to up-regulate two anti-oxidant enzymes (GPX1 and GSTP1). On the contrary, TAK-242/BTZ activated a strong pro-oxidant status incresing ROS and RNS (reactive nitrogen species) levels, decreasing GSH ones and down-regulating GPX1 and GSTP1. Analyzing the effect of each treatment on mitochondrial polarization status, we observed about 6,7% of depolarized mitochondria after BTZ treatment, while TAK-242/BTZ combination induced a mitochondrial depolarization of about 69,3% (p<0.001). Moreover, cells treated with BTZ alone showed a compensatory up-regulation of the OXPHOS- (NDUFA-6 and MT-ND4) and mitochondrial fusion-related genes (mitofusin and OPA1) and TFAM. On the contrary, all these genes were down-regulated by TAK-242/BTZ combination. Also a dramatic drop in mitochondrial respiration was observed with a marked decrease in ATP production, consequent accumulation of AMP and a decreased NAD+/NADH and NADP+/NADPH ratio. Since high levels of oxidative stress and mitochondrial impairment activate mitophagy sensitizing cells to apoptosis, we evaluated co-localization of mitochondria (stained with MitoTracker) with the autophagosome marker LC3 using confocal microscopy. BTZ and TAK-242/BTZ increased Mitotracker/LC3 co-localization respectively of about 4,5 and 50 fold compared with control (BTZ vs combination: p<0.001). To evaluate whether TLR4 inhibition resensitizes resistant primary cells, CD138+ cells derived from 5 refractory/relapsed MM patients were treated with 5nM BTZ, 10mM TAK-242 or their combination. Compared to BTZ alone, combination treatment induced higher mitochondrial depolarization after 24h and significantly decreased viability of CD138+ cells after 48h. TLR4 inhibitor alone or in combination did never show cytotoxicity toward CD138- cells. CONCLUSION Taken together, these findings indicate thatTLR4 signaling is involved in the acquisition of bortezomib resistance protecting mitochondria during BTZ exposure and sustaining mitochondrial dynamics in BTZ-resitant cells. Inhibition of TLR4 may overcome bortezomib resistance in patients with relapsed/refractory MM. Disclosures Conticello: Celgene: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding. Palumbo:Celgene: Honoraria; Amgen: Honoraria; Hospira: Honoraria; Teva: Honoraria; Novartis: Honoraria; Janssen: Honoraria. Di Raimondo:Takeda: Consultancy; Amgen: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding.