Abstract 957: TMPRSS2:ERG overexpression limits vitamin D receptor action in prostate cancer cells
Authors: Kim Jung-Sun , Wang Jianghua , Cai Yi , Shao Long-Jiang , Ittmann Michael , Weigel Nancy 
Published: August 2012

Treatment with the active metabolite of vitamin D (1,25D), or the less calcemic analog of 1,25D (EB1089) decreases growth of many prostate cancer (PCa) cell lines. The actions of 1,25D are mediated by the vitamin D receptor (VDR), a hormone activated transcription factor. A recent study showed that the prostates of mice fed with a low vitamin D diet produced levels of metabolites similar to those found in humans with low normal levels, exhibited a significant increase in proliferation, and decrease in apoptosis compared to mice fed a diet supplemented to achieve higher levels of vitamin D metabolites. This suggests that 1,25D plays a role in limiting proliferation of normal prostate cells. 1,25D is inactivated by the enzyme CYP24. 1,25D dependent induction of CYP24 provides a negative feedback mechanism to limit VDR signaling. VDR dependent induction of CYP24 requires an ETS transcription factor. The majority of PCa contain a TMPRSS2:ETS factor fusion gene linking the promoter of the androgen regulated TMPRSS2 gene with the coding regions of ETS transcription factors. The most common fusion gene is TMPRSS2:ERG (T/E). This fusion promotes growth and invasiveness. We speculated that T/E might also cause hyperinduction of CYP24 limiting the beneficial anti-proliferative actions of 1,25D. To test this, we used siRNA targeting ERG to reduce expression of T/E in VCaP cells. Depletion of ERG reduced 1,25D induction of CYP24 by 95%. We have generated an LNCaP cell line that expresses a T/E isoform in response to doxycycline. Induction of ERG caused an increase in both potency and efficacy of 1,25D dependent CYP24 induction. In the presence of increased ERG expression, 1 nM 1,25D caused an induction of CYP24 equivalent to 100 nM 1,25D in control cells. This increase in CYP24 in ERG over-expressing cells suggests that physiological levels of 1,25D (typically &lt;1 nM) would be rapidly metabolized in T/E expressing cells limiting the beneficial differentiating effects of 1,25D. We had previously shown that 1,25D induces TMPRSS2 and found that both 1,25D and EB1089 induced T/E expression in VCaP PCa cells in vitro. Although both 1,25D and the synthetic androgen, R1881, induced expression of T/E, the combination yielded little additional increase. EB1089 inhibited growth of VCaP cells in vitro. To test the effects of elevated VDR agonist in vivo, VCaP cells stably expressing luciferase were injected into SCID mouse prostate, and mice were treated with vehicle or EB1089 for five weeks. EB1089 increased tumor T/E expression 2-fold, and also induced T/E target gene expression. However, EB1089 did not alter overall tumor growth in vivo. Our previous studies showed that EB1089 inhibits growth of T/E negative LNCaP xenografts. These new findings suggest that induction of T/E may counteract other beneficial actions of EB1089. Thus, 1,25D or EB1089 may be more beneficial for T/E negative patients than for patients with T/E positive tumors.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 957. doi:1538-7445.AM2012-957