Associations of saposin C, Src, and androgen receptor upregulate the expression and function of androgen receptor in human prostate cancer cells.

We previously demonstrated that ectopic expression of neurotrophic peptide (NP) derived from saposin C promotes androgen receptor (AR) expression and transactivation in human prostate cancer cells. This prompted us to investigate how NP or saposin C can function in cells. We constructed plasmids expressing saposin C or a chimeric peptide of a viral TAT transduction domain and saposin C (TAT-saposin C) with His-tag. Intracellular localization of saposin C and NP was predominantly shown in transfected cells, while TAT-saposin C was detected around membrane and in cytosol by immunofluorescence staining. Furthermore, induction of the AR expression and activation of the AR transcriptional function were observed in cells transfected with saposin C or TAT-saposin C, compared to control cells transfected with an empty plasmid. The effects of saposin C and TAT-saposin C on AR activity were examined in the presence of inhibitors of GPCR, MAPK1/2, and PI3K/Akt. Interestingly, we found that these inhibitors only affect AR activities in cells with TAT-saposin C expression but not with saposin C expression. Immunostaining images showed that co-localization of saposin C, Src, and the AR occurred in transfected cells. Physical interactions of saposin C/NP, Src, and the AR were then demonstrated by co-immunoprecipitation assays. Blockage of Src activity by specific inhibitor led to a decrease in the saposin C-mediated enhancement of AR transactivity, suggesting that intracellular expression of saposin C caused stimulation of AR expression and activity by associations with Src in LNCaP cells. This effect may not be mediated by GPCR.