MUC1 expression is associated with ST3GAL2 and negatively correlated with the androgen receptor in castration-resistant prostate cancer.

Stage-specific embryonic antigen-4 (SSEA-4) is a developmentally regulated antigen, while expression level of SSEA-4 and / or its synthase ST3GAL2 is associated with prognosis in various malignancies. We have reported a prominent increase of SSEA-4 in castration-resistant prostate cancer (CRPC) and its negative correlation with the androgen receptor (AR). Meanwhile, loss of AR has increased to approximately 30% with the growing use of androgen receptor signaling inhibitor for metastatic CRPC (mCRPC). However, monitoring the progression status of AR-negative prostate cancer is a challenge because it does not produce prostate-specific antigen. Based on the negative relationship of expression between AR and SSEA-4, we hypothesized that a soluble molecule synchronized with SSEA-4 in expression could be a serum marker candidate for AR-negative prostate cancer. Thus, we investigated the molecular background of SSEA-4 expression by ST3GAL2-knockout in DU145 cells. Here we show that MUC1 is identified as a molecule associated with ST3GAL2 and expressed in AR-negative prostate cancer. A negative correlation of expression between AR and MUC1 was observed in prostate cancer cell lines and CRPC tissues. The average rate of MUC1 expression was nearly 60% in AR-negative prostate cancer cells in CRPC tissues. Level of serum CA15-3 (MUC1) was the highest in mCRPC among various stages and its higher level was associated with faster progression of mCRPC. Our results demonstrate that MUC1 is identified as a ST3GAL2-associated molecule and expressed in AR-negative CRPC cells. Furthermore, level of serum CA15-3 may reflect the progression status of mCRPC.