LDs as buffers of DHT induced ROS and DHT promotes ROS homeostasis. A, Confocal micrograph showing HCS LipidTox stained LDs in LNCaP cells cultured in CSS containing medium for 3 days and treated with DMSO (vehicle) and 1 nmol/L DHT alone or combined with 2.5 mmol/L NAC for 3 days. B, LD quantification of LNCaP cells from A. Data represent LDs per cell and error bars represent SEM. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001. C, Quantification of ROS, detected with CM-H2DCFDA in LNCaP treated as described above in A. Data are presented as mean ± SEM from at least three independent determinations. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001. D, Illustration of concept that DHT initially induces ROS to trigger proliferation (1 day of DHT), and subsequently decreases intracellular ROS levels. LD accumulation is observed as DHT decreases ROS levels. E, Quantification of ROS in control (#1) or Sigma1 shRNA (#5) transduced LNCaP cells treated as described above in A. Each datapoint represents mean CM-H2DCFDA signal per cell from three fields in three independent wells. F, Redox balance. Total GSH levels and ratio of GSH-to-GSSG measured in nonspecific control shRNA (ns) and in Sigma1 shRNA (#5) transduced LNCaP cells treated as described above in A.
ARTICLE ABSTRACT
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.
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.
