Androgen deprivation therapies in prostate cancer patients are effective in reducing systemic levels of androgens. However, studies demonstrate that intratumoral androgens do not change significantly, and those levels are sufficient to activate AR growth signaling pathways [
1‐
4], suggesting that, in a castrate environment, tumors have the capability to make local androgens. Subsequent studies showed that in CRPC, tumor tissues exhibit enhanced levels of androgen biosynthetic enzyme expression [
5,
6]. Production of tumoral androgens in the castrate environment can be enhanced from adrenal precursors such as dehydroepiandrosterone (DHEA) and androstenediol, and directly from cholesterol. Several studies suggest that, downstream of the CYP17 gene in the androgen pathway, there could be a viable alternate target for intratumoral androgen production [
7‐
10]. How expression of these androgen biosynthetic enzymes is regulated in CRPC remains to be investigated. We showed for the first time that ERG transcription factor regulates androgen biosynthesis and subsequent AR activation in prostate cancer cells [
11]. ERG transcription factor is expressed in PC tumors from TMPRSS2-ERG fusion gene and the presence of this fusion is highly prevalent in PC patients, including CRPC. The presence of TMPRSS2-ERG fusions is associated with high grade disease [
12], and different subsets of rearrangements including 2 + Edel, T2-E4, and the presence of a 72 bp insert in the ERG gene are associated with aggressive disease characteristics [
13‐
15]. ERG has been shown to bind AR responsive gene promoters throughout the genome and regulate AR signaling, interfere with prodifferentiation functions, and mediate oncogenic signaling [
16]. Also cooperation between AR and ERG drives invasive adenocarcinoma [
17] even in the castrate environment [
18]. However, mechanisms underlying the cooperation of AR and ERG with each other in a low androgen environment remain unknown. In a low androgen CRPC state, AR reprograms gene expression for enhanced cell cycle progression [
19], which is distinct from the prodifferentiation function. ERG fusion status is also related to abiraterone acetate therapy responsiveness, where fusion positive patients responded well to therapy, suggesting a role of the fusion gene in androgen production [
20]. Our previous study supports this notion and defined a feed forward loop consisting of TMPRSS2-ERG fusions, androgen biosynthetic enzymes and AR activation operating in prostate cancer cells which drives intracellular DHT production. In this study, we tested the biological significance of ERG induced androgen production on bone tumor growth and targeting AR signaling with enzalutamide.
Bone metastases are highly prevalent occurrences in both hormone-naïve and CRPC patients. Androgen receptor expression is prevalent in bone metastases where both full length [
21] and shorter AR variants [
22,
23] are expressed in metastasized prostate tumor cells. Nuclear AR staining in bone metastases associate with poor outcome in patients, suggesting AR signaling in promoting bone tumor growth [
21]. At a cellular level, AR expression and signaling has been active in mature osteoblasts, osteocytes, and stromal cells [
24‐
26] that maintain normal bone function. In support of this argument, a clinical trial with abiraterone acetate showed that treated patients have lower testosterone levels in bone marrow tumor biopsies [
27], suggesting the prevalence of androgen production in bone metastatic sites.
Our previous study demonstrates that AR/ERG/Androgen biosynthetic enzymes form a feed forward loop in TMPRSS2-ERG fusion positive cells for androgen production and growth [
11]. This study is focused on determining the impact of breaking this feedforward loop on bone tumor growth. We utilized ERG knockdown approach in TMPRSS2-ERG positive cells and targeting the AR function with enzalutamide for bone tumor growth. We show that ERG positive cells/tumors are highly responsive to enzaluatimde treatment. AR responsive genes in CRPC state are inhibited by enzaluatamide treatment in ERG positive cells. Androgen biosynthetic enzyme gene expression is inhibited in ERG knockdown tumors andenzalutamide significantly inhibited androgen biosynthetic enzyme expression in ERG positive tumors likely through abrogating the feedback back mechanism involving AR/ERG/androgen biosynthetic enzyme expression [
11]. This data supports the notion that TMPRSS2-ERG fusion positive tumors are better responders to enzalutamide therapy.