Sigma1 Regulates Lipid Droplet-mediated Redox Homeostasis Required for Prostate Cancer Proliferation

Lipid droplets (LD) are dynamic organelles that serve as hubs of cellular metabolic processes. Emerging evidence shows that LDs also play a critical role in maintaining redox homeostasis and can mitigate lipid oxidative stress. In multiple cancers, including prostate cancer, LD accumulation is associated with cancer aggressiveness, therapy resistance, and poor clinical outcome. Prostate cancer arises as an androgen receptor (AR)-driven disease. Among its myriad roles, AR mediates the biosynthesis of LDs, induces autophagy, and modulates cellular oxidative stress in a tightly regulated cycle that promotes cell proliferation. The factors regulating the interplay of these metabolic processes downstream of AR remain unclear. Here, we show that Sigma1/SIGMAR1, a unique ligand-operated scaffolding protein, regulates LD metabolism in prostate cancer cells. Sigma1 inhibition triggers lipophagy, an LD selective form of autophagy, to prevent accumulation of LDs which normally act to sequester toxic levels of reactive oxygen species (ROS). This disrupts the interplay between LDs, autophagy, buffering of oxidative stress and redox homeostasis, and results in the suppression of cell proliferation in vitro and tumor growth in vivo. Consistent with these experimental results, SIGMAR1 transcripts are strongly associated with lipid metabolism and ROS pathways in prostate tumors. Altogether, these data reveal a novel, pharmacologically responsive role for Sigma1 in regulating the redox homeostasis required by oncogenic metabolic programs that drive prostate cancer proliferation.

SIGNIFICANCE

To proliferate, cancer cells must maintain productive metabolic and oxidative stress (eustress) while mitigating destructive, uncontrolled oxidative stress (distress). LDs are metabolic hubs that enable adaptive responses to promote eustress. Targeting the unique Sigma1 protein can trigger distress by disrupting the LD-mediated homeostasis required for proliferation.

Abstract 284: Sigma1 regulates lipid droplet mediated redox homeostasis required for prostate cancer proliferation

Lipid droplets (LDs) are dynamic organelles that serve as hubs of cellular metabolic processes. Emerging evidence shows that LDs also play a critical role in maintaining redox homeostasis and can mitigate lipid oxidative stress. In multiple cancers, including prostate cancer (PCa), LD accumulation is associated with cancer aggressiveness, therapy resistance, and poor clinical outcome. PCa arises as an androgen receptor (AR) driven disease. Among its myriad roles, AR mediates the biosynthesis of LDs, induces autophagy, and modulates cellular oxidative stress in a tightly regulated cycle that promotes cell proliferation. The factors regulating the interplay of these metabolic processes downstream of AR remain unclear. Here, we show that Sigma1/SIGMAR1, a unique ligand-operated scaffolding protein, regulates LD metabolism in PCa cells. Sigma1 inhibition triggers lipophagy, an LD selective form of autophagy, to prevent accumulation of LDs which normally act to sequester toxic levels of reactive oxygen species (ROS). This disrupts the interplay between LDs, autophagy, buffering of oxidative stress and redox homeostasis, and results in the suppression of cell proliferation in vitro and tumor growth in vivo. Consistent with these experimental results, SIGMAR1 transcripts are strongly associated with lipid metabolism and reactive oxygen species pathways in prostate tumors. Altogether, these data reveal a novel, pharmacologically responsive role for Sigma1 in regulating the redox homeostasis required by oncogenic metabolic programs that drive PCa proliferation.

Citation Format: Halley M. Oyer, Alexandra R. Steck, Charles Longen, Sanjana Venkat, Konuralp Bayrak, Eleanor Munger, Dan Fu, Christina Sanders, Justin Myers, Matthew Schiewer, Nan Chen, Elahe Mostaghel, Felix J. Kim. Sigma1 regulates lipid droplet mediated redox homeostasis required for prostate cancer proliferation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 284.

Small-Molecule Modulators of Sigma1 and Sigma2/TMEM97 in the Context of Cancer: Foundational Concepts and Emerging Themes

There are two known subtypes of the so-called sigma receptors, Sigma1 and Sigma2. Sigma1 (encoded by the SIGMAR1 gene and also known as Sigma-1 receptor, S1R) is a unique pharmacologically regulated integral membrane chaperone or scaffolding protein that allosterically modulates the activity of its associated proteins. Sigma2, recently identified as transmembrane protein 97 (TMEM97), is an integral membrane protein implicated in cellular cholesterol homeostasis. A number of publications over the past two decades have suggested a role for both sigma proteins in tumor biology. Although there is currently no clinically used anti-cancer drug that targets Sigma1 or Sigma2/TMEM97, a growing body of evidence supports the potential of small-molecule compounds with affinity for these proteins, putative sigma ligands, as therapeutic agents to treat cancer. In preclinical models, these compounds have been reported to inhibit cancer cell proliferation, survival, adhesion, and migration; furthermore, they have been demonstrated to suppress tumor growth, to alleviate cancer-associated pain, and to exert immunomodulatory properties. Here, we will address the known knowns and the known unknowns of Sigma1 and Sigma2/TMEM97 ligand actions in the context of cancer. This review will highlight key discoveries and published evidence in support of a role for sigma proteins in cancer and will discuss several fundamental questions regarding the physiological roles of sigma proteins in cancer and sigma ligand mechanism of action.

Small-Molecule Sigma1 Modulator Induces Autophagic Degradation of PD-L1

Emerging evidence suggests that Sigma1 (SIGMAR1, also known as sigma-1 receptor) is a unique ligand-regulated integral membrane scaffolding protein that contributes to cellular protein and lipid homeostasis. Previously, we demonstrated that some small-molecule modulators of Sigma1 alter endoplasmic reticulum (ER)-associated protein homeostasis pathways in cancer cells, including the unfolded protein response and autophagy. Programmed death-ligand 1 (PD-L1) is a type I integral membrane glycoprotein that is cotranslationally inserted into the ER and is processed and transported through the secretory pathway. Once at the surface of cancer cells, PD-L1 acts as a T-cell inhibitory checkpoint molecule and suppresses antitumor immunity. Here, we demonstrate that in Sigma1-expressing triple-negative breast and androgen-independent prostate cancer cells, PD-L1 protein levels were suppressed by RNAi knockdown of Sigma1 and by small-molecule inhibition of Sigma1. Sigma1-mediated action was confirmed by pharmacologic competition between Sigma1-selective inhibitor and activator ligands. When administered alone, the Sigma1 inhibitor decreased cell surface PD-L1 expression and suppressed functional interaction of PD-1 and PD-L1 in a coculture of T cells and cancer cells. Conversely, the Sigma1 activator increased PD-L1 cell surface expression, demonstrating the ability to positively and negatively modulate Sigma1 associated PD-L1 processing. We discovered that the Sigma1 inhibitor induced degradation of PD-L1 via autophagy, by a mechanism distinct from bulk macroautophagy or general ER stress-associated autophagy. Finally, the Sigma1 inhibitor suppressed IFNγ-induced PD-L1. Our data demonstrate that small-molecule Sigma1 modulators can be used to regulate PD-L1 in cancer cells and trigger its degradation by selective autophagy.Implications: Sigma1 modulators sequester and eliminate PD-L1 by autophagy, thus preventing functional PD-L1 expression at the cell surface. This posits Sigma1 modulators as novel therapeutic agents in PD-L1/PD-1 blockade strategies that regulate the tumor immune microenvironment.Visual Overview: http://mcr.aacrjournals.org/content/molcanres/16/2/243/F1.large.jpg Mol Cancer Res; 16(2); 243-55. ©2017 AACR.

Haloperidol and Prostate Cancer Prevention: More Epidemiologic Research Needed

CONTEXT

The antipsychotic drug haloperidol has antiproliferative and growth-inhibiting properties on prostate cancer cell lines in vitro by binding the sigma 1 protein. Evidence is needed regarding a possible preventive association in men.

OBJECTIVE

To examine whether our epidemiologic data support an inverse association of haloperidol use with risk of prostate cancer.

DESIGN

These case-control analyses used conditional logistic regression to estimate relative risk by odds ratios (ORs) adjusting for race/ethnicity and aspects of medical care related to detection of prostate cancer. We tested 3 other commonly used antipsychotic drugs, risperidone, quetiapine, and olanzapine, for sigma 1 protein binding and inhibition of clonogenic growth of prostate cancer cells. Use of any of these by men was considered use of a comparator drug.

MAIN OUTCOME MEASURES

1) association of haloperidol with prostate cancer; 2) sigma 1 binding and clonogenic growth.

RESULTS

Probably owing to small numbers of haloperidol recipients, evidence of a preventive association was inconsistent, depending on the definition of long-term use. If duration of use was greater than 1 year, the odds ratio (OR) was 0.38 (95% confidence interval (CI) = 0.14-1.01) for haloperidol and 0.80 (95% CI = 0.66-0.98) for the comparator drug; if the duration of use was greater than 2 years, the OR was 0.66 (95% CI = 0.24-1.76) for haloperidol and 0.84 (95% CI = 0.66-1.08) for the comparator drug. Unlike haloperidol, risperidone, quetiapine, and olanzapine did not bind sigma 1 or inhibit clonogenic growth.

CONCLUSION

Given the laboratory evidence, our ambiguous epidemiologic findings should encourage more epidemiologic evaluation of haloperidol use and risk of prostate cancer. Finding a negative association could be a scientific advance in prostate cancer prevention but would not be sufficient basis for recommending the prescription of haloperidol for that purpose.

Abstract 3202: Pharmacologic modulation of Sigma1 induces autophagic degradation of programmed death-ligand 1 in cancer cells

Emerging evidence suggests that Sigma1 (also known as sigma1 receptor) is a unique ligand-operated integral membrane chaperone or scaffolding protein that contributes to cellular protein homeostasis. Previously, we found that treatment of various cancer cell lines with some prototypic small molecule modulators of Sigma1 can engage endoplasmic reticulum (ER) associated protein homeostasis pathways including the unfolded protein response and autophagy. Programmed death-ligand 1 (PD-L1) is a type 1 integral membrane glycoprotein that is processed and transported through the ER and secretory pathway of tumor cells. PD-L1 expressed at the surface of tumor cells can act as a T-cell inhibitory checkpoint molecule that inactivates tumor infiltrating immune cells that express PD-1, its cognate receptor. Here, we show that Sigma1 physically associates with PD-L1. In triple negative breast and androgen-independent prostate cancer cells, PD-L1 protein levels are suppressed by both RNAi mediated knockdown of Sigma1 and pharmacological modulation of Sigma1. We observe decreased cell surface and intracellular levels of PD-L1 by flow cytometry and biochemical subcellular fractionation respectively, which corresponds with a dose-responsive decrease in functional PD-L1/PD-1 interaction in a co-culture of cancer cells and T-cells. Inhibitors of autophagy block this suppression of PD-L1 protein levels, suggesting PD-L1 is degraded away by autophagy after Sigma1 modulation. Through confocal microscopy, we show that Sigma1 modulation results in colocalization of PD-L1 and GFP-LC3, a marker of autophagosomes. From these conclusions, we hypothesize that autophagic degradation of nascent PD-L1 after Sigma1 modulation plays a key role in preventing the transport of functional PD-L1 to the plasma membrane. Together, these data demonstrate that Sigma1 modulators have the potential to act as novel therapeutic agents in PD-1/PD-L1 blockade strategies.

Citation Format: Christina M. Maher, Jeffrey D. Thomas, Charles G. Longen, Derick A. Haas, Halley M. Oyer, Jane Y. Tong, Felix J. Kim. Pharmacologic modulation of Sigma1 induces autophagic degradation of programmed death-ligand 1 in cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3202. doi:10.1158/1538-7445.AM2017-3202

Novel small molecule guanidine Sigma1 inhibitors for advanced prostate cancer

Prostate cancer is the most frequently diagnosed malignancy and the leading cause of cancer related death in men. First line therapy for disseminated disease relies on androgen deprivation, leveraging the addiction of these tumors on androgens for both growth and survival. Treatment typically involves antagonizing the androgen receptor (AR) or blocking the synthesis of androgens. Recurrence is common and within 2-3years patients develop castration resistant tumors that become unresponsive to AR-axis targeted therapies. In order to provide a more effective treatment, we are utilizing an approach that targets a key scaffolding protein, Sigma1 (also known as sigma-1 receptor), a unique 26-kilodalton integral membrane protein that is critical in stabilizing the AR. Herein we report on a new series of Sigma1 compounds for lead optimization derived from a hybrid pharmacophore approach.

Sigma1 Targeting to Suppress Aberrant Androgen Receptor Signaling in Prostate Cancer

Suppression of androgen receptor (AR) activity in prostate cancer by androgen depletion or direct AR antagonist treatment, although initially effective, leads to incurable castration-resistant prostate cancer (CRPC) via compensatory mechanisms including resurgence of AR and AR splice variant (ARV) signaling. Emerging evidence suggests that Sigma1 (also known as sigma-1 receptor) is a unique chaperone or scaffolding protein that contributes to cellular protein homeostasis. We reported previously that some Sigma1-selective small molecules can be used to pharmacologically modulate protein homeostasis pathways. We hypothesized that these Sigma1-mediated responses could be exploited to suppress AR protein levels and activity. Here we demonstrate that treatment with a small-molecule Sigma1 inhibitor prevented 5α- dihydrotestosterone-mediated nuclear translocation of AR and induced proteasomal degradation of AR and ARV, suppressing the transcriptional activity and protein levels of both full-length and splice-variant AR. Consistent with these data, RNAi knockdown of Sigma1 resulted in decreased AR levels and transcriptional activity. Furthermore, Sigma1 physically associated with ARV7 and ARv567es as well as full-length AR. Treatment of mice xenografted with ARV-driven CRPC tumors with a drug-like small-molecule Sigma1 inhibitor significantly inhibited tumor growth associated with elimination of AR and ARV7 in responsive tumors. Together, our data show that Sigma1 modulators can be used to suppress AR/ARV-driven prostate cancer cells via regulation of pharmacologically responsive Sigma1-AR/ARV interactions, both in vitro and in vivoCancer Res; 77(9); 2439-52. ©2017 AACR.