Loss of Selenoprotein F in Prostate Cancer

Abstract

ObjectiveSelenium‐containing proteins, or selenoproteins, are likely to play a role in prostate cancer progression and mortality. Selenoprotein F (SELENOF) is a selenium‐containing protein implicated in prostate cancer etiology due to its dramatic reduction in prostate cancer tissue when compared to adjacent benign regions. The reduction in SELENOF levels in immortalized prostate epithelial cells results in phenotypic features associated with cellular transformation, indicating that SELENOF functions as a tumor suppressor in the prostate. The mechanism behind the reduction of SELENOF levels in prostate cancer remains unknown, although it is suspected that it involves events at the level of protein synthesis, based on work done in tissue culture cells.SELENOF synthesis is controlled by sequences within the 3′ untranslated sequences of SELENOF mRNA known as selenocysteine insertion sequences (SECIS) that are required for the recognition of in‐frame UGA codons as the selenocysteine amino acid. Elongation initiation factor 4a3 (eIF4a3) binds SECIS during times of low selenium availability and suppresses the synthesis of several selenoproteins. Moreover, higher levels of eIF4a3 are associated with increased grade of prostate cancer. The objective of this study is to determine if elevated eIF4a3 levels suppress SELENOF synthesis and contribute to prostate cancer progression.MethodologyIn order to establish a model for SELENOF loss in prostate‐derived cells, SELENOF levels were determined by western blot and mRNA levels by RT‐PCR in PC3 prostate cancer cells and non‐tumorigenic RWPE‐1 cells. Reporter constructs in which the production of light‐emitting luciferase requires SECIS function to recognize a UGA placed in the open reading frame of the luciferase gene were generated by in vitro mutagenesis. An eIF4a3 overexpression construct was used to overexpress that protein in transfected cells to directly determine its impact on SELENOF synthesis.ResultsSELENOF levels were 18‐fold lower in PC3 cells when compared to RWPE‐1 cells, although mRNA levels were similar between the two cell lines. Using the luciferase reporter constructs described above, it was determined that the readthrough of the UGA in the luciferase gene and supported by the SELENOF SECIS element was more efficient in the RWPE‐1 cells as compared to PC3 cells. In addition, over‐expression of eIF4a3 in RWPE‐1 cells was achieved and the consequences on SELENOF translation and the transformed phenotype are currently under investigation.ConclusionThe loss of SELENOF in prostate cancer is likely to occur at the level of translation and mediated through molecular events involving the SECIS element. A likely candidate for how this occurs has been identified as eIF4a3, an RNA helicase whose levels are elevated in prostate cancer. Enhancing SELENOF levels and/or diminishing eIF4a3 are potentially novel approaches to treat prostate cancers that are no longer responsive to existing treatments.Support or Funding InformationThis work was supported by grant PC170236 from the Department of Defense Prostate Cancer Research Program