We previously showed that targeted expression of non-
receptor tyrosine kinase Etk/BMX in mouse prostate induces prostate intraepithelial
neoplasia, implying a possible causal role of Etk in
prostate cancer development and progression. Here, we report that Etk is upregulated in both human and mouse prostates in response to
androgen ablation. Etk expression seems to be differentially regulated by
androgen and
interleukin 6 (IL-6), which is possibly mediated by the
androgen receptor (AR) in
prostate cancer cells. Our immunohistochemical analysis of tissue microarrays containing 112 human prostate
tumor samples revealed that Etk expression is elevated in
hormone-resistant
prostate cancer and positively correlated with
tyrosine phosphorylation of AR (Pearson correlation coefficient rho = 0.71, P < 0.0001). AR
tyrosine phosphorylation is increased in Etk-overexpressing cells, suggesting that Etk may be another
tyrosine kinase, in addition to Src and Ack-1, which can phosphorylate AR. We also showed that Etk can directly interact with AR through its Src homology 2 domain, and such interaction may prevent the association of AR with Mdm2, leading to stabilization of AR under
androgen-depleted conditions. Overexpression of Etk in
androgen-sensitive LNCaP cells promotes
tumor growth while knocking down Etk expression in
hormone-insensitive
prostate cancer cells by a specific
shRNA that inhibits
tumor growth under
androgen-depleted conditions. Taken together, our data suggest that Etk may be a component of the adaptive compensatory mechanism activated by
androgen ablation in prostate and may play a role in
hormone resistance, at least in part, through direct modulation of the AR signaling pathway.