Epigenetic Machinery Regulates Alternative Splicing of Androgen Receptor (AR) Gene in Castration-Resistant Prostate Cancer (CRPC)
Abstract
Androgen deprivation therapy (ADT) is the primary treatment for metastatic prostate cancer (PCa) since PCa depends on androgen for growth. Although initially responsive, most tumors progress into androgen-independent/castration-resistant PCa (CRPC). No curative therapy is available. The pro-growth function of androgen is mediated through androgen receptor (AR) in the cell where androgen binds to the ligand binding domain (LDB) of AR. Upon binding to androgen, AR enters into the nucleus and turns on gene expression required for cell growth. Newer and more powerful anti-androgen drugs such as enzalutamide and abiraterone are now used in clinics. Although they can significantly improve the survival rate of CRPC patients, disease progression and resistance to these agents will eventually develop. One of the reasons for the resistance to ADT and newer anti-androgen drugs is the constitutively active AR variants (AR-Vs) that are induced under ADT conditions. These AR-V molecules lack the LBD and thus bypass the need for androgen. For CRPC patients whose tumor is driven by AR-Vs, the current drug therapy will not be effective since they are designed to target LBD or androgen synthesis by tumor cells. Clearly, there is an urgent need to develop therapeutics to target ARVs. Among the AR variants, AR-V7 is by far the most characterized variant in terms of its expression, function, and clinical correlation with CRPC progression. AR is a high molecular weight protein encoded by a large gene that contains many exons split by introns. Exons are then joined together by the spliceosome protein complex at the pre-mRNA level to give rise mature mRNA where the protein is translated from. Introns are normally considered "junk" and are spliced out. However, it turns out that the "junk" sequence in the AR gene contains many cryptic exons (CEs) that can be alternative spliced into mRNA sequences in cancer cells because alternative splicing machinery goes berserk. For AR-V7, CE3 was used as an alternatively spliced exon. Because CE3 contains a stop codon, a truncated AR protein such as AR-V7 that lacks the C-terminal LBD for androgen binding is made. Our labs study mechanisms underlying PCa progression with the long-term goal of developing chemical inhibitors that block PCa progression. We focused on histone lysine demethylase KDM4B and studied how KDM4B modifies chromatin structure that modulates gene transcription. We found that elevated KDM4B in metastatic PCa was able to promote prostate tumor growth. We also identified several novel inhibitors of KDM4B that are very effective in blocking the tumor growth in mice. Most recently, we found that KDM4B is a part of the spliceosome complex and regulates AR-V7 expression in several PCa cell lines including CRPC cell lines. This is a novel discovery. Although KDM4B and AR-V7 are implicated in prostate tumorigenesis, the link between KDM4B and AR-V7 has not been made until now. We hypothesize that KDM4B promotes AR-V7 alternative splicing, leading to CPRC. In this proposal, we will test this hypothesis in a variety of CRPC cell lines including those that are resistant to anti-androgen since not all AR-V7-bearing tumors are the same. From this study, we expect to unveil the regulatory mechanism of AR-V7 expression by KDM4B in CRPC cells and other factors that are also involved in this regulation. The outcome will provide us insight to design targeted therapeutics for intervention. Finally, we will test the efficacy of our newly identified KDM4B inhibitor(s) as a monotherapy or combined with approved anti-androgen agents in AR-V7-expressing CRPC using preclinical models. Our research is highly translational, and the success of the proposal will have potent therapeutic impact.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jan 31, 2017
- Source ID
- W81XWH1610532
Entities
People
- Jer-Tsong Hsieh
Organizations
- United States Army
- University of Texas Southwestern Medical Center