Cytoplasmic retention of protein tyrosine kinase 6 promotes growth of prostate tumor cells

Protein tyrosine kinase 6 (PT K6) is an intracellular tyrosine kinase that is nuclear in epithelial cells of the normal prostate, but cytoplasmic in prostate tumors and in the PC3 prostate tumor cell line. The impact of altered PT K6 intracellular localization in prostate tumor cells has not been extensively explored. Knockdown of endogenous cytoplasmic PT K6 resulted in decreased PC3 cell proliferation and colony formation, suggesting that cytoplasmic PT K6 stimulates oncogenic pathways. In contrast, reintroduction of PT K6 into nuclei of PC3 cells had a negative effect on growth. Enhanced tyrosine phosphorylation of the PT K6 substrate Sam68 was detected in cells expressing nuclear-targeted PT K6. We found that mechanisms regulating nuclear localization of PT K6 are intact in PC3 cells. Transiently overexpressed PT K6 readily enters the nucleus. Ectopic expression of ALT-PT K6, a catalytically inactive splice variant of PT K6, did not affect localization of endogenous PT K6 in PC3 cells. Using leptomycin B, we confirmed that cytoplasmic localization of endogenous PT K6 is not due to Crm-1/exportin-1 mediated nuclear export. In addition, overexpression of the PT K6 nuclear substrate Sam68 is not sufficient to bring PT K6 into the nucleus. While exogenous PT K6 was readily detected in the nucleus when transiently expressed at high levels, low-level expression of inducible wild type PT K6 in stable cell lines resulted in its cytoplasmic retention. Our results suggest that retention of PT K6 in the cytoplasm of prostate cancer cells disrupts its ability to regulate nuclear substrates and leads to aberrant growth. In prostate cancer, restoring PT K6 nuclear localization may have therapeutic advantages.