Recent studies show that a remarkable percentage (>50%) of PCa contain chromosomal deletions or translocations that produce a TMPRSS2-ETS factor gene fusion. This links the androgen regulated TMPRSS2 promoter to the coding sequence of an ETS factor, most commonly the proto-oncogene ERG. Recent studies suggest that this protein stimulates migration and that some forms stimulate tumor growth. In contrast, there is good epidemiological evidence that high levels of vitamin D reduce the risk for PCa. The active metabolite of vitamin D, 1,25-dihydroxyvitamin D3 (1,25D), binds to and activates the vitamin D receptor (VDR), a hormone-activated transcription factor. 1,25D reduces growth and enhances differentiation of normal prostate cells and some prostate cancer cells, but cells bearing these fusions have not been studied. We have two reasons to suspect that cells expressing these fusions may respond differently to VDR agonists than other PCa cells. First, some of the ETS factors cooperate with VDR to induce CYP24, an enzyme that inactivates 1,25D. Thus, tumors with this rearrangement may metabolize 1,25D so quickly that its beneficial effects are lost. On the other hand, we have discovered that unexpectedly a synthetic 1,25D analog, EB1089, induces the fusion. Thus, activation of VDR may be harmful in these tumors. We hypothesize that VDR-dependent induction of TMPRSS2-ETS fusions limits the effectiveness of endogenous 1,25D as a growth-inhibitory and -differentiating agent in PCa both through reducing the levels of 1,25D available to carry out the beneficial actions of VDR and through the stimulation of cell growth, migration, and tumor growth caused by the fusions. However, it is our hope that the beneficial actions of 1,25D outweigh the negative effects of additional induction. Because patients are supplemented with vitamin D and vitamin D is necessary for good health, it is critical to determine whether beneficial actions of 1,25D are being lost in these tumors through metabolism or whether 1,25D is harmful in these tumors because of the induction of the fusion; that is the goal of this proposal. These findings potentially have immediate implications for patients. It is relatively easy to screen for the fusions. If the induction by 1,25D is harmful and not counteracted by the other beneficial actions of 1,25D, then means to prevent the induction of the fusion must be found. On the other hand, if the net actions of 1,25D are beneficial in these tumors, but the local levels of 1,25D are being reduced by the induction of CYP24, then a combination of a CYP24 inhibitor and vitamin D supplementation would be particularly useful for this group of patients. This is an important problem with the potential to affect more than 50% of prostate cancer patients at all stages.