Androgen-independent Induction of Prostate-specific Antigen Gene Expression via Cross-talk between the Androgen Receptor and Protein Kinase A Signal Transduction Pathways

Marianne D Sadar

doi:10.1074/jbc.274.12.7777

Abstract

Transcription of the prostate-specific antigen (PSA) gene escapes regulation by androgens in advanced prostate cancer. To determine the molecular mechanism(s) of androgen-independent regulation of the PSA gene, the possibility that the androgen receptor (AR) is activated in the absence of androgen by stimulation of protein kinase A (PKA) was investigated. Activation of PKA by forskolin resulted in elevated expression of the PSA gene in androgen-depleted LNCaP cells, an effect that was blocked by the antiandrogen, bicalutamide. Further evidence that induction of PSA gene expression was dependent on AR was obtained from experiments using PC3 cells devoid of AR. Neither PSA, PB, nor ARR3 androgen-responsive reporters could be induced by activation of PKA in the absence of transfected AR. In addition, when nuclear AR from forskolin-treated LNCaP cells was incubated with oligonucleotides encoding an androgen response element of the PSA promoter and examined by electromobility shift assay, an increase in AR-androgen response element complex formation was observed. Lastly, cotransfection of an expression vector for a chimeric protein encoding the amino-terminal domain of the human AR linked to Gal4 and a 5xGal4UAS reporter gene construct resulted in activation of the amino-terminal domain of the AR by stimulation of PKA activity. These results demonstrate androgen-independent induction of PSA gene expression in prostate cancer cells by an AR-dependent pathway.

Highlights

The androgen receptor (AR)1 belongs to the superfamily of nuclear receptors that mediates the actions of lipophilic ligands, including steroids, retinoids, vitamin D3, and thyroid hormones (1)
Concentrations of forskolin in excess of 10 ␮M had little to no effect on prostate-specific antigen (PSA) mRNA levels when compared with those in controls, which is in agreement with other reports (18, 19)
Recent evidence shows that the human estrogen and the chicken, rat, and rabbit progesterone receptors can mediate extracellular signals in the absence of cognate ligand by dopamine, epidermal growth factor, heregulin, gonadotropin-releasing hormone, tumor growth factor ␣, insulin and insulin-like growth factor I, cAMP, okadaic acid, and vanadate (22–26)

Summary

Introduction

The androgen receptor (AR) belongs to the superfamily of nuclear receptors that mediates the actions of lipophilic ligands, including steroids, retinoids, vitamin D3, and thyroid hormones (1). It has been suggested that the AR can be transformed in the absence of androgen by elevation of cAMP levels and by growth factors (3–5) The mechanism of such ligand-independent activation of AR has not been clarified but may involve the bypassing of one of the above-mentioned processes associated with ligand-dependent transformation. When the tumor becomes androgenindependent, PSA mRNA is constitutively up-regulated through an unknown mechanism that presumably involves the promoter and enhancer regions of the PSA gene These regions have been sequenced as far as Ϫ5824 from the start site of transcription (8, 9), and the following DNA response elements have been characterized: 1) TATA box, Ϫ28 to Ϫ23 (10); 2) androgen response elements (AREs), Ϫ170 to Ϫ156 (10) and Ϫ4148 to Ϫ4134 (9); and 3) androgen response region (ARR), Ϫ395 to Ϫ376 (11). The experiments confirmed that PSA gene expression was induced by activation of PKA and demonstrated, for the first time, activation of the amino terminus of the AR by stimulation of PKA activity

Methods

Results

Conclusion