The high prevalence of TMPRSS2-ERG rearrangements (B60%) in prostate cancer (CaP) leads to androgenic induction of the ETS-related gene (ERG ) expression. However, the biological functions of ERG overexpression in CaP remain to be understood. ERG knockdown in TMPRSS2-ERG expressing CaP cells induced striking morphological changes and inhibited cell growth both in cell culture and SCID mice. Evaluation of the transcriptome and specific gene promoters in ERG siRNA-treated cells and investigation of gene expression signatures of human prostate tumors revealed ERG-mediated activation of C-MYC oncogene and the repression of prostate epithelial differentiation genes (PSA and SLC45A3/ Prostein). Taken together, these data combining cell culture and animal models and human prostate tumors reveal that ERG overexpression in prostate tumor cells may contribute to the neoplastic process by activating C-MYC and by abrogating prostate epithelial differentiation as indicated by prostate epithelial specific markers.

Protein kinase signaling pathways, such as Janus kinase 2-Signal transducer and activator of transcription 5A/B (JAK2-STAT5A/B), are of significant interest in the search for new therapeutic strategies in both breast and prostate cancers. In prostate cancer, the components of the JAK2-STAT5A/B signaling pathway provide molecular targets for small-molecule inhibition of survival and growth signals of the cells. At the same time, new evidence suggests that the STAT5A/B signaling pathway is involved in the transition of organ-confined prostate cancer to hormonerefractory disease. This implies that the active JAK2-STAT5A/B signaling pathway potentially provides the means for pharmacological intervention of clinical prostate cancer progression. In addition, active STAT5A/B may serve as a prognostic marker for identification of those primary prostate cancers that are likely to progress to aggressive disease. In breast cancer, the role of STAT5A/B is more complex. STAT5A/B may have a dual role in the regulation of malignant mammary epithelium. Data accumulated from mouse models of breast cancer suggest that in early stages of breast cancer STAT5A/B may promote malignant transformation and enhance growth of the tumor. This is in contrast to established breast cancer, where STAT5A/B may mediate the critical cues for maintaining the differentiation of mammary epithelium. In addition, present data suggest that activation of STAT5A/B in breast cancer predicts favorable clinical outcome. The dual nature of STAT5A/B action in breast cancer makes the therapeutic use of STAT5 A/B more complex.

The oncogenic activation of the ETS related gene (ERG) due to gene fusions is present in over half of prostate cancer (CaP) in Western countries. Due to its high incidence and oncogenic role, ERG and components of ERG network have emerged as potential drug targets for CaP. Utilizing gene expression datasets, from matched normal and prostate tumor epithelial cells, an association of NOTCH transcription factors with ERG expression status was identified; confirming that NOTCH factors are direct transcriptional targets of ERG. Inhibition of ERG in TMPRSS2-ERG positive VCaP cells led to decreased levels of NOTCH-1 and -2 proteins and downstream transcriptional targets and partially recapitulated the phenotypes associated with ERG inhibition. Regulation of NOTCH-1 and -2 genes by ERG were also noted with ectopic ERG expression in LNCaP (ERG-negative CaP) and RWPE-1 (benign prostate derived immortalized) cells. Furthermore, inhibition of NOTCH by the small molecule gamma-Secretase inhibitor 1, GSI-1, conferred an increased sensitivity to androgen receptor (AR) inhibitors (Bicalutamide, Enzalutamide,) or the androgen biosynthesis inhibitor (Abiraterone) in VCaP cells. Combined treatment with Bicalutamide and GSI-1 showed strongest inhibition of AR, ERG, NOTCH1, NOTCH2, and PSA protein levels along with decreased cell growth, cell survival and enhanced apoptosis. Intriguingly, this effect was not observed in ERG-negative prostate cancer cells or immortalized benign/normal prostate epithelial cells. These data underscore the synergy of AR and NOTCH inhibitors in reducing the growth of ERG-positive CaP cells. Implications Combinational targeting of NOTCH and AR signaling has therapeutic potential in advanced ERG-driven prostate cancers.