Systematic and functional characterization of novel androgen receptor variants arising from alternative splicing in the ligand-binding domain.

The presence of intact ligand-binding domain (LBD) ensures the strict androgen-dependent regulation of androgen receptor (AR): binding of androgen induces structural reorganization of LBD resulting in release of AR from HSP90, suppression of nuclear export which otherwise dominates over import and nuclear translocation of AR as a transcription factor. Thus, loss or defects of the LBD abolish constraint from un-liganded LBD as exemplified by constitutively active AR variants (AR-Vs), which are associated with emerging resistance mechanism to anti-AR therapy in castration-resistant prostate cancer (mCRPC). Recent analysis of the AR splicing landscapes revealed mCRPC harboring multiple AR-Vs with diverse patterns of inclusion/exclusion of exons (exons 4-8) corresponding to LBD to produce namely exon-skipping variants. In silico construction for these AR-Vs revealed four novel AR-Vs having unique features: Exclusion of specified exons introduces a frameshift in variants v5es, v6es and v7es. ARv56es maintains the reading frame resulting in the inclusion of the C-terminal half of the LBD. We systematically characterized these AR-Vs regarding their subcellular localization, affinity for HSP90 and transactivation capability. Notably, ARv5es was free from HSP90, exclusively nuclear, and constitutively active similarly as previously reported for v567es. In contrast, v6es and v7es were similar in that they are cytoplasmic, transcriptionally inactive and bind HSP90, ARv56es was present in both nucleus and cytoplasm, does not bind HSP90 and is transcriptionally inactive. Converting these transcriptionally inactive AR-Vs into active forms, we identified the two separate elements that allosterically suppress otherwise constitutively active AR-Vs; one in exon 5 for v6es and v7es and the other in exon 8 for v56es. Our findings identify a novel constitutively active AR-V, ARv5es and establish a method to predict potential activities of AR-Vs carrying impaired LBD.

Extracellular influences on tumour angiogenesis in the aged host.

Whether tumours are epithelial or non-epithelial in origin, it is generally accepted that once they reach a certain size all solid tumours are dependent upon a vascular supply to provide nutrients. Accordingly, there is great interest in how the extracellular environment enhances or inhibits vascular growth. In this minireview, we will examine key extracellular components, their changes with ageing, and discuss how these alterations may influence the subsequent development of tumour vasculature in the aged host. Because of the tight correlation between advanced age and development of prostate cancer, we will use prostate cancer as the model throughout this review.

Regulation of proliferation of prostate epithelial cells by 1,25-dihydroxyvitamin D3 is accompanied by an increase in insulin-like growth factor binding protein-3.

The biologically active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) has been shown to regulate the proliferation of human prostate epithelial cell lines. Since the insulin-like growth factor (IGF) system is involved in the transformation process of epithelial cells, the following study was undertaken to determine if the IGF system, in particular IGF binding protein-3 (IGFBP-3), is altered by 1,25-(OH)2D3 in normal prostate epithelial cells as part of a mechanism for inhibition of transformation. Two cell systems were used in this study: (1) primary cultures of benign human prostate epithelial cells (PECs) and (2) an SV40-T immortalized prostate epithelial cell line (P153) that is non-tumorigenic. 1,25-(OH)2D3 was added to parallel sets of PECs and P153 cells in addition to the presence or absence of IGF-I or des(1-3)IGF-I. Treatment with 1,25-(OH)2D3 resulted in significant growth inhibition of both PECs and P153 cells. Furthermore, 1,25-(OH)2D3 inhibited IGF-induced proliferation, but this was partially reversed by high concentrations of IGF-I. Western ligand blots of condition media demonstrated a significant increase in IGFBP-3; likewise Northern blots demonstrated an increase in mRNA for IGFBP-3. Proliferation assays using an antibody designed to block the IGF-independent effects of IGFBP-3 failed to reverse the inhibitory effect of 1,25-(OH)2D3. Thus, IGFBP-3 acts in an IGF-dependent manner to inhibit cell growth of benign prostate epithelial cells.

Transcriptional regulation of insulin-like growth factor-I receptor gene expression in prostate cancer cells.

A marked decrease in the type 1 insulin-like growth factor (IGF) receptor (IGF-IR) occurs in prostate epithelial cells during transformation from the benign to the metastatic state. One of the principal regulators of IGF-IR gene expression, the WT1 tumor suppressor, is expressed in prostate cancer and in prostate cancer cell lines. The purpose of this study was to determine whether the decrease in IGF-IR expression was transcriptionally regulated, and whether WT1 action may be involved in the repression of the IGF-IR gene in prostate cancer cells. The P69 cell line was derived by immortalization of human primary prostate epithelial cells with simian virus-40 T antigen and is rarely tumorigenic. The M12 line was derived from the P69 line by selection for tumor formation in nude mice and is tumorigeneic and metastatic. P69 cells express 20,000 IGF-IR/cell, whereas M12 cells express 3,500 IGF-IR/cell. These differences in receptor number are reflected in proportional differences in IGF-IR mRNA levels. To assess IGF-IR promoter activity in these cell lines, each was transiently transfected with luciferase reporter vectors containing the IGF-IR gene transcription start site and 476 bp of 5'-flanking sequence, 640 bp of 5'-untranslated region sequence, or both regions. The promoter activity of the full-length construct was 50% lower (P < 0.01) in M12 cells compared with P69 cells, the activity of the 5'-flanking region construct was 53% lower (P < 0.0001), and that of the 5'-untranslated region construct was 36% lower (P = 0.01). P69 clones stably transfected with a WT1 expression vector exhibited decreased expression of the endogenous IGF-IR gene and decreased promoter activity in transient transfection assays with IGF-IR promoter constructs containing multiple WT1 binding sites. The observed reduction in endogenous IGF-IR expression was sufficient to inhibit IGF-I-stimulated cell proliferation. These data suggest that most of the decreased expression of the IGF-IR seen in malignant prostate epithelium is the result of transcriptional repression of the IGF-IR gene, and that this repression may be due in part to the increased expression of the WT1 tumor suppressor in metastatic prostate cancer.

Insulin-like growth factor binding protein-related protein 1 (IGFBP-rP1) is a potential tumor suppressor protein for prostate cancer.

Insulin-like growth factor binding protein-related protein-1 (IGFBP-rP1) has been shown to have decreased expression in the progression from benign to malignant prostate epithelial cells (V. Hwa et al., J. Clin Endocrinol. Metab., 83: 4355-4362, 1998). The present study was undertaken to determine the effects of the re-expression of IGFBP-rP1 in a cell line from a model of human prostate cancer, M12, in which IGFBP-rP1 expression had been demonstrated to decrease from the parent epithelial cell, P69, to the malignant subline, M12. An IGFBP-rP1 cDNA encoding the protein was transfected into M12 cells in a plasmid that resulted in constitutive-expression of IGFBP-rP1. Clones of transfected M12 cells were selected for low (L) and high (H) levels of expression, and the plasmid vector alone was transfected into M12 as a control. After transfection, there was a marked alteration in the morphology of the M12 cells such that the H clones had an elongated appearance when compared with the M12 control cells. The M12 clones overexpressing IGFBP-rP1 had a dose-related increase in population doubling time, decreased colony formation in soft agar, an increased propensity to undergo apoptosis in response to 6-hydroxyurea, and decreased tumor formation in male athymic, nude mice. These data suggest that IGFBP-rP1 may have a suppressive effect on prostate cancer development.