RNA Editing of Androgen Receptor Gene Transcripts in Prostate Cancer Cells

Abstract

Androgen-independent (AI)2 prostate cancer (CaP) typically develops from the selective outgrowth of tumor cells to castrate levels of testosterone in response to androgen-deprivation therapy (1). AI CaP cells have evolved different strategies for overriding the androgen-dependent (AD) growth and survival characteristics of early stage, organ-confined CaP or early stage metastatic CaP (2). Aberrant AR activation is the primary mechanism for the growth and survival of AI CaP in response to castrate levels of androgen (2). With the exception of prostatic small cell neuroendocrine carcinoma (3), the most parsimonious model of AI CaP arises from the inappropriate activation of AR-dependent cell growth and survival pathways. Unfortunately, this model is incomplete because examples of AR-independent CaP do exist. Most notably, the well established human CaP cell lines DU145 and PC3, which were derived from brain and bone metastases, respectively (4, 5), are supposedly devoid of AR mRNA and protein (6, 7) and thus represent bona fide models of AR-independent CaP. Interestingly, this classification was recently called into question as detectable levels of AR mRNA and protein were observed in both DU145 and PC3 cells (8). More importantly, several studies have demonstrated low AR activity in PC3 cells, suggesting these cells also utilize AR-dependent mechanisms for growth and survival similar to other established AD CaP cell lines (e.g. LNCaP, 22Rv1, and C4-2) (8 -11).AR displays variable immunoreactivity in individual tumor cells of AI CaP, with cells showing a strong, weak, or undetectable AR signal (12). These results illustrate the heterogeneity of AR expression in AI CaP and suggest that both AR-dependent and AR-independent mechanisms of growth and survival are present in AI CaP. Practically speaking, determining whether AI CaP utilizes AR-dependent or AR-independent mechanisms for growth and survival is a controversial and arduous endeavor that has profound clinical ramifications on how to treat advanced stage, AI CaP. Here we have investigated whether DU145 and PC3 cells are representative models of AR-independent CaP by testing if they respond to androgens or require AR expression for cell growth in vitro. We show that DU145 and PC3 cells are androgen-responsive and require AR for optimal growth in vitro, thus demonstrating each cell line is a bona fide model of androgen-responsive, AR-dependent CaP. We show AR gene transcripts in DU145 and PC3 cells harbor nucleotide transitions suggesting the AR pre-mRNA is the target of multiple RNA editing enzymes. We propose that RNA editing enzymes are modulators of AR activity through the introduction of loss-of-function or gain-of-function mutations into AR gene transcripts in advanced stage AI CaP.

A triad of telomerase, androgen receptor and early growth response 1 in prostate cancer cells

“…While LNCaP cells harbor DNA mutations, DU145 and PC3 cells acquire these mutations due to RNA editing. 21 Thus, the differential effects of AR on telomerase activity in ADPC and CRPC cells cannot be attributed solely to AR mutations. Instead, differential effects on the telomerase activity may signify a divergent transcriptional program of AR in ADPC and CRPC cells.…”

Enigmatic In Vivo iduronate-2-sulfatase (IDS) mutant transcript correction to wild-type in Hunter syndrome

“…Although rarely encountered, a number of different forms of mRNA editing have been reported in the human genome, the most common being cytidine to uridine and adenosine to inosine (dbRES; http://bioinfo.au.tsinghua.edu.cn/dbRES). U-to-C mRNA editing, which would be required to account for the appearance of wild-type IDS mRNA in pts #1 and #2, is highly unusual but not without precedent in the human genome: it has previously been reported to occur in the adenosine deaminase, RNA-specific, B1 (ADARB1) gene in both normal and systemic lupus erythematosus T cells [Laxminarayana et al, 2007] and in the androgen receptor (AR) gene in prostate cancer cells [Martinez et al, 2008]. In the same vein, a U deletional form of RNA editing would need to be hypothesised in the case of pt #3 (hemizygous for the insertion of a T nucleotide in the c.10 position) to account for the appearance of wild-type IDS mRNA in this individual.…”

ADAR1: a promising new biomarker for esophageal squamous cell carcinoma?

“…In pediatric glioblastoma multiforme, the overexpression of ADAR1 p110 may cause the formation of an inactive ADAR1/ADAR2 heterodimer or the sequestration of ADAR2 from editing substrates, leading to the altered ADAR2 editing activity and the subsequent decreased editing frequencies [28]. In prostate cancer (CaP), high expression of both enzymes ADAR1 and ADAR2 in androgen-independent CaP cells (DU145 and PC3) resulted in the higher number of RNA editing mutation when compared with androgen-dependent CaP cells (LNCaP and 22Rv1) [29]. Due to the interferonresponsive ADAR1 activity, ADAR1 p150 was upregulated in chronic myeloid leukemia patient samples, which was correlated with BCR/ABL (oncogenic gene fusion protein) amplification and the increased A-to-I editing level [30].…”