Sequential Cytoprotective Responses to Sigma1 Ligand–Induced Endoplasmic Reticulum Stress

Abstract

The Sigma1 receptor (Sigma1) is an endoplasmic reticulum (ER) integral membrane protein that is highly expressed in a number of cancer cell lines. Small molecule compounds targeting Sigma1 (Sigma1 ligands) inhibit cancer cell proliferation and induce apoptotic cell death in vitro and inhibit tumor growth in xenograft experiments. However, the cellular pathways activated by Sigma1 protein-ligand interaction are not well defined. Here, we find that treatment with some Sigma1 ligands induces ER stress and activates the unfolded protein response (UPR) in a dose-and time-responsive manner in a range of adenocarcinoma cell lines. Autophagy is engaged after extended treatment with Sigma1 ligands, which suggests that protracted UPR results in autophagy as a secondary response. Inhibition of UPR by RNAi-mediated knockdown of inositol-requiring enzyme 1a and activating transcription factor 4 abrogates autophagosome formation, as does knockdown of essential autophagy gene products Beclin1 and autophagy protein 5. Knockdown of Sigma1 also suppresses IPAG [1-(4-iodophenyl)-3-(2-adamantyl) guanidine] induced UPR marker and autophagosome levels, indicating that this response is indeed Sigma1-mediated. We find that UPR activation precedes autophagosome formation and autophagy precedes apoptosis in Sigma1 ligandtreated cells. These processes are reversible, and washout of IPAG before cell death results in a return of autophagosomes and UPR markers toward basal levels. However, inhibition of Sigma1 ligand-induced UPR or autophagy accelerates apoptotic cell death. Together, these data suggest that UPR and autophagy are engaged as primary and secondary cytoprotective responses, respectively, to Sigma1 ligand-induced disruption of cancer cell protein homeostasis.

Sigma1 Targeting to Suppress Aberrant Androgen Receptor Signaling in Prostate Cancer

“…Cell lysis, protein extraction, SDS-PAGE, and immunoblotting were performed as described previously (13), with a few modifications. The Luminata Western HRP Substrate chemiluminescence kit (Millipore) was used to reveal immunoblotted proteins.…”

“…However, emerging evidence from our laboratory and others (9, 11) suggest that Sigma1 may in fact function as a novel molecular chaperone or a scaffolding protein in cancer cells. We have found that Sigma1 is involved in aspects of cancer cell protein homeostasis including protein synthesis, folding, transport, and degradation (12, 13). …”

Novel small molecule guanidine Sigma1 inhibitors for advanced prostate cancer

“…In vitro radio-ligand binding assays using membranes from MDA-MB-468 cancer cells, which express Sigma1 11,28,29 provided confirmation of Sigma1 binding affinity for our new hybrid analogs compared to IPAG and haloperidol (Table 1). The hybrids 1 , 4 , 5 , 6 all demonstrated reasonable affinity to Sigma1 with about a 5–10-fold decrease in binding affinity compared to IPAG.…”

“…Sigma1 is highly expressed in prostate cancer cells and RNAi-mediated knockdown of Sigma1 results in tumor growth inhibition. 11 Upon profiling known Sigma1 inhibitors for their ability to affect Sigma1 mediated translational repression 10 and ER homeostasis pathways, 11 we identified IPAG (1-adamantan-1-yl)-3-(4-iodophenyl)guanidine) and the anti-psychotic drug, haloperidol, as potent inhibitors of Sigma1-mediated AR signaling, protein homostasis, as well as inhibitors of cancer cell proliferation and growth. 10,11 In an effort to create new composition of matter for intellectual property protection, and to provide a new series of Sigma1 compounds for lead optimization, we utilized a hybrid pharmacophore approach to identify novel Sigma1 compounds.…”

Peeking into Sigma-1 Receptor Functions Through the Retina

“…While autophagy is found to be an active process in the RPE in vivo [72], evidence from AMD donors indicates a decline of autophagic flux in the RPE [73]. Although not yet specifically investigated in RPE cells in vivo, S1R has been reported to influence autophagy in vitro [31, 32, 74]. A possible protective role of the S1R via autophagic regulations in RPE cells needs to be further determined experimentally.…”