Understanding the role of AP-1 in prostate cancer
Abstract
Activator protein 1 (AP-1) is a group of dimeric transcription factors composed of proteins including activating transcription factor 2 (ATF2), c-Jun and c-Fos. Increased cytoplasmic phosphorylated ATF2 was observed in prostate cancer compared to normal prostate suggesting that altered localization of ATF2 may contribute to the pathogenesis of the disease. On the other hand, the c-Jun downregulation was also observed in a fraction of advanced prostate cancer patients and in castration-resistant prostate cancer patients. The objective of the study was to elucidate the mechanisms regulating ATF2 nuclear-cytoplasmic shuttling and to characterize the role of AP-1 in prostate cancer. In the first study, a novel chromosome region maintenance 1 (CRM1)-dependent nuclear export signal (NES) was identified in the most N-terminus of ATF2 (termed N-NES). Mutation of the hydrophobic residues in N-NES abrogated CRM1/N-NES interaction and increased nuclear localization of ATF2. Concomitantly, the transcriptional activity of ATF2 was enhanced following N-NES mutation. Therefore, the N-NES negatively regulates ATF2 nuclear localization and transcriptional activity. Mutation of both N-NES and a previously identified NES leaded to predominant nuclear localization, suggesting that both NESs function in cooperation to export ATF2 out of the nucleus. The function of N-NES was validated in various cell lines including prostate cancer cells. N-NES is unique in that it is only present in higher organisms and that it is also missed in certain splicing variants of human ATF2. In a second study, I demonstrated that c-Jun functions as a potent inhibitor for androgen receptor (AR) in prostate cancer cells. Overexpression of c-Jun inhibits the activity of various androgen-responsive promoters. By generating LNCaP stable cell lines with inducible c-Jun expression, I showed that c-Jun downregulates transcripts of multiple AR target genes. Interestingly, long-term c-Jun overexpression also downregulated AR at both protein and mRNA levels, suggesting c-Jun suppresses AR signaling through a dual mechanism. Overexpression of c-Jun not only inhibits AR signaling but also cell proliferation of both hormone-naïve and castration-resistant prostate cancer cells. Molecular analysis reveals that c-Jun inhibits AR transactivation potential via an unknown target gene. These results suggest a novel mechanism by which c-Jun antagonizes the AR signaling. Future identification of the c-Jun target gene that mediates the AR inhibition will provide a new opportunity to develop novel therapeutics.
Degree
Ph.D.
Advisors
Hu, Purdue University.
Subject Area
Cellular biology|Biochemistry|Oncology
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