Nuclear translocation of the membrane oxoeicosanoid/androgen receptor, OXER1: Possible mechanisms involved

Abstract

OXER1, the receptor for the arachidonic acid metabolite 5-οxo-eicosatetraenoic acid (5-oxo-ETE), has been reported to also bind and mediate the membrane-initiated actions of androgens. Indeed, androgens antagonize the 5-oxo-ETE effects through OXER1, affecting a number of signaling pathways and inhibiting cancer cell proliferation and migration. OXER1, being a GPCR, was classically described to be localized in the plasma membrane. However, for numerous GPCRs, there is now strong evidence that they can be also found in other cellular compartments, including the nucleus. The aim of the present work was to investigate OXER1's possible localization in the nucleus and identify the mechanism(s) involved. For this purpose, we verified OXER1's nuclear presence by immunofluorescence and western blot, in whole cells and nuclei of two different prostate cancer cell lines (DU-145 and LNCaP) and in CHO cells transfected with a GFP labelled OXER1, both in untreated and OXER1 ligands' treated cells. Mutated, OXER1-tGFP expressing, CHO cells were used to verify that OXER1 agonist (5-oxo-ETE) binding is necessary for OXER1 nuclear translocation. NLS sequences were in silico identified, and a specific inhibitor, as well as, specific importins' siRNAs were also utilized to explore the mechanism involved. Moreover, we examined the role of palmitoylation in OXER1 nuclear translocation by in silico identifying possible palmitoylation sites and using a palmitoylation inhibitor. Our results clearly show that OXER1 can be localized in the nucleus, in an agonist-dependent manner, that is inhibited by androgens. We also provide evidence for two possible mechanisms for its nuclear trafficking, that involve receptor palmitoylation and importin-mediated cytoplasmic-nuclear transport. In our knowledge, it is the first time that a membrane androgen receptor is identified into the nucleus, suggesting an alternative, more direct, mode of action, involving nuclear mechanisms. Therefore, our findings provide new insights on androgen-mediated actions and androgen-lipid interactions, and reveal new possible therapeutic targets, not only for cancer, but also for other pathological conditions in which OXER1 may have an important role.