Enhanced transcription of rRNA genes by purified androgen receptor complexes in vitro

Co-staining of microRNAs and their target proteins by miRNA in situ hybridization and immunohistofluorescence on prostate cancer tissue microarrays

The co-expression of miRNAs and their target proteins was studied on tissue microarrays from different prostate cancer (PCa) patients. PCa of primary Gleason pattern 4 (GP4), lymph node metastases of GP4, distant metastases, and normal tissue from the transitional and peripheral zones were co-stained by fluorescent miRNA in situ hybridization (miRisH) and protein immunohistofluorescence (IHF). The miRNAs and corresponding target proteins include the pairs miR-145/ERG, miR-143/uPAR, and miR-375/SEC23A. The fluorescence-stained and scanned tissue microarrays (TMAs) were evaluated by experienced uropathologists. The pair miR-145/ERG showed an exclusive staining for miR-145 in the nuclei of stromal cells, both in tumor and normal tissue, and for ERG in the cytoplasm with/without co-expression in the nucleus of tumor cells. The pair miR-143/uPAR revealed a clear distinction between miR-143 in the nuclei of stromal cells and uPAR staining in the cytoplasm of tumor cells. Metastases (lymph node and distant) however, showed tumor cells with cytoplasmic staining for miR-143/uPAR. In normal tissues, beside the nuclei of the stroma cells, gland cells could also express miR-143 and uPAR in the cytoplasm. miR-375 showed particular staining in the nucleoli of GP4 and metastatic samples, suggesting that nucleoli play a special role in sequestering proteins and miRNAs. Combined miRisH/IHF allows for the study of miRNA expression patterns and their target proteins at the single-cell level.

Androgen-regulated metabolism and biosynthesis in prostate cancer

Metabolic changes are a well-described hallmark of cancer and are responses to changes in the activity of diverse oncogenes and tumour suppressors. For example, steroid hormone biosynthesis is intimately associated with changes in lipid metabolism and represents a therapeutic intervention point in the treatment of prostate cancer (PCa). Both prostate gland development and tumorigenesis rely on the activity of a steroid hormone receptor family member, the androgen receptor (AR). Recent studies have sought to define the biological effect of the AR on PCa by defining the whole-genome binding sites and gene networks that are regulated by the AR. These studies have provided the first systematic evidence that the AR influences metabolism and biosynthesis at key regulatory steps within pathways that have also been defined as points of influence for other oncogenes, including c-Myc, p53 and hypoxia-inducible factor 1α, in other cancers. The success of interfering with these pathways in a therapeutic setting will, however, hinge on our ability to manage the concomitant stress and survival responses induced by such treatments and to define appropriate therapeutic windows.

Angiogenin mediates androgen-stimulated prostate cancer growth and enables castration resistance

The androgen receptor (AR) is a critical effector of prostate cancer development and progression. Androgen-dependent prostate cancer is reliant on the function of AR for growth and progression. Most castration-resistant prostate cancer (CRPC) remains dependent on AR signaling for survival and growth. Ribosomal RNA (rRNA) is essential for both androgen-dependent and castration-resistant growth of prostate cancer cells. During androgen-dependent growth of prostate cells, androgen-AR signaling leads to the accumulation of rRNA. However, the mechanism by which AR regulates rRNA transcription is unknown. Here, investigation revealed that angiogenin (ANG), a member of the secreted ribonuclease superfamily, is upregulated in prostate cancer and mediates androgen-stimulated rRNA transcription in prostate cancer cells. Upon androgen stimulation, ANG undergoes nuclear translocation in androgen-dependent prostate cancer cells, where it binds to the rDNA promoter and stimulates rRNA transcription. ANG antagonists inhibit androgen-induced rRNA transcription and cell proliferation in androgen-dependent prostate cancer cells. Interestingly, ANG also mediates androgen-independent rRNA transcription through a mechanism that involves its constitutive nuclear translocation in androgen-insensitive prostate cancer cells, resulting in a constant rRNA overproduction and thereby stimulating cell proliferation. Critically, ANG overexpression in androgen-dependent prostate cancer cells enables castration-resistant growth of otherwise androgen-dependent cells. Thus, ANG-stimulated rRNA transcription is not only an essential component for androgen-dependent growth of prostate cancer but also contributes to the transition of prostate cancer from androgen-dependent to castration-resistant growth status.

IMPLICATIONS

The ability of angiogenin to regulate rRNA transcription and prostate cancer growth makes it a viable target for therapy.

Protein B23/nucleophosmin/numatrin nuclear dynamics in relation to protein kinase CK2 and apoptotic activity in prostate cells

Protein B23/nucleophosmin/numatrin (B23) is a key nucleolar/nuclear matrix-associated protein required for cell growth-related functions, such as rRNA synthesis. Protein kinase CK2 (CK2) (formerly casein kinase 2, a protein Ser/Thr kinase signal that is involved in cell growth and cell death) mediates phosphorylation of B23, thereby influencing its functional activity. Here we have delineated the dynamics of B23 and its link to CK2 status in response to altered growth stimuli and induction of apoptosis in cultured prostate cells and in rat prostate cells in vivo. Our studies employing PC-3 and ALVA-41 prostate cancer cells demonstrated colocalization of CK2 and B23 in the nucleus. Further, CK2 and B23 underwent coordinate modulation in the nucleus related to their nucleocytoplasmic shuttling in response to induction of apoptotic activity in cells caused by downregulation of CK2 or by treatment with other apoptosis-inducing agents. These alterations in nuclear association of B23 occurred in the absence of a significant change in the level of cytoplasmic B23. Similar studies in the in vivo model of rat prostate epithelial cells subjected to androgen deprivation (that resulted in loss of nuclear CK2 and induction of apoptosis) demonstrated dynamic modulation of nuclear matrix-associated B23 without a significant change in its cytoplasmic level. These changes were reversed by androgen-mediated growth response in the prostate. Our results suggest that CK2-mediated phosphorylation of B23 is essential for its retention in the nucleus and that coordinated nuclear localization of B23 and CK2 is dynamically regulated in response to altered growth status in the cell.

Angiogenin is translocated to the nucleus of HeLa cells and is involved in ribosomal RNA transcription and cell proliferation

Angiogenin is an angiogenic protein known to play a role in rRNA transcription in endothelial cells. Nuclear translocation of angiogenin in endothelial cells decreases as cell density increases and ceases when cells are confluent. Here we report that angiogenin is constantly translocated to the nucleus of HeLa cells in a cell density-independent manner. Down-regulation of angiogenin expression by antisense and RNA interference results in a decrease in rRNA transcription, ribosome biogenesis, proliferation, and tumorigenesis both in vitro and in vivo. Exogenous angiogenin rescues the cells from antisense and RNA interference inhibition. The results showed that angiogenin is constitutively translocated into the nucleus of HeLa cells where it stimulates rRNA transcription. Thus, besides its angiogenic activity, angiogenin also plays a role in cancer cell proliferation.

Androgen regulation of the largest subunit of RNA polymerase II in the rat ventral prostate

One of the dramatic changes in the prostate during androgen manipulation is the alteration in cellular content of total RNA - the amount of total RNA in each cell. The abundance of cellular total RNA correlates with the RNA polymerase (RNAP) activity in the prostate. One possible mechanism of androgen regulation of RNAP activity involves the regulation of RNAP expression. Western blot analysis showed that the largest subunit of the RNAP II, an essential component of the transcriptional machinery for mRNA, is indeed regulated by androgens. Castration down-regulates the protein level of RNAP II, whereas androgen replacement up-regulates the protein. However, androgen manipulation does not have consistent effects on the phosphorylation of the C-terminal domain (CTD) of the RNAP II. Androgen regulation of the RNAP II protein expression was also observed in the seminal vesicles but not in the thymus and liver, indicating that androgen regulation of RNAP II protein expression appears to be limited to the male sex accessory organs. These observations suggest that RNAP II plays an essential role in androgen action in male sex accessory organs.

Androgen regulation of ribosomal RNA synthesis in LNCaP cells and rat prostate

Androgen-dependent growth of prostate tissue has been well documented. An additional prerequisite for cellular growth is the accumulation of ribosomes. It is thus reasonable to hypothesize that ribosomal DNA (rDNA) transcription in prostate tissue must be stimulated by androgen either directly or indirectly. This hypothesis was tested using both LNCaP cells, an androgen-dependent tissue culture line and in a rat animal model. Nuclear run-on assays confirmed that the administration of DHT to LNCaP cells resulted in a two- to three-fold increase in the rate of rRNA synthesis when compared to cells maintained in the absence of androgen. Enzymatic analysis and Western blots were carried out to measure the amount (activity and mass) of RNA polymerase I in DHT treated LNCaP cells. These assays demonstrated that neither the catalytic activity of RNA polymerase I nor the amount of the enzyme varied in response to DHT. However, Western blots revealed that the amount of the auxiliary RNA polymerase I transcription factor UBF, was significantly increased (two- to three-fold) in cells grown in the presence of DHT. Similar experiments were carried out with prostatic tissue obtained from orchiectomized rats maintained on either placebo or testosterone pellets. In this model, both the catalytic activity as well as the amount of RNA polymerase I protein decreased. However, in agreement with the tissue culture model, UBF protein decreased in prostates from orchiectomized rats and was maintained in animals supplemented with testosterone. These lines of evidence are consistent with the hypothesis that androgens stimulate rRNA synthesis by increasing the quantities of the components of the rDNA transcription system.

Role of protein phosphorylation in post-translational regulation of protein B23 during programmed cell death in the prostate gland

Protein B23 is a nucleolar and nuclear matrix-associated phosphoprotein that is involved in ribosome synthesis. Its expression and phosphorylation in rat ventral prostate, an androgen target organ, are profoundly influenced by androgens. Induction of programmed cell death (apoptosis) in the prostatic epithelium by androgen deprivation in the animal induces an early decline in protein B23 in the absence of a corresponding loss of protein B23 mRNA. We have now demonstrated that prostatic nuclei retain the ability to transcribe the B23 mRNA and that a significant amount of this mRNA persists even after 7 days of androgen deprivation when > 80% of the prostatic epithelial cells have undergone apoptosis. The B23 mRNA from these nuclei is also translatable in vitro. However, the majority of the B23 mRNA is associated with free and short-stretch polysomes, which may account for the castration-induced decline in synthesis of protein B23 in vivo. In addition, the mechanism of down-regulation of protein B23 in apoptotic prostatic cells appears to relate to two coordinate signals, which include loss of phosphorylation of the protein as well as the expression of a protease active toward dephosphorylated protein B23, under these conditions.

Present concept of the relevance of steroid receptors for prostatic cancer

Suitable assays for nuclear androgen-receptors in human prostatic cancer tissue have been presented. Correlations between NAR-content and response to hormonal therapy are demonstrated. The methods are time consuming and require expensive equipment as well as experience. Individual laboratory standards seem to determine response rates. The earlier methodological difficulties are replaced by a new set of problems for which as yet no single solution is available. As there is a tendency to use aspiration cytology more and more for diagnosis as well as follow-up of prostatic cancer, a cytochemical method would be extremely valuable. Research in that direction should be pursued.