Computer analysis of
protein phosphorylation site sequences revealed that transcriptional factors and viral
oncoproteins are prime targets for regulation of
proline-directed
protein phosphorylation, suggesting an association of the
proline-directed protein kinase (PDPK) family with neoplastic transformation and
tumorigenesis. In this report, an immunoprecipitate activity assay of
proline-directed protein kinase F(A)/
glycogen synthase kinase-3alpha (PDPK F(A)/GSK-3alpha) has been optimized to demonstrate significantly increased (p < 0.01) activity in poorly differentiated human prostate
carcinoma PC-3 cells (55.5+/-3.8 units/mg) when compared to well-differentiated LNCaP cells (28.1+/-2.3 units/mg). Immunoblotting analysis revealed that increased activity of this PDPK in PC-3 cells is due not to overexpression of the
protein, but to enhanced
tyrosine phosphorylation of the
kinase. When treated with
genistein (a
protein tyrosine kinase PTK inhibitor), the enhanced
tyrosine phosphorylation/activation of the
kinase in PC-3 cells can be blocked. Conversely, when treated with
vanadate (a
protein tyrosine phosphatase PTP inhibitor), the
phosphotyrosine content of PDPK F(A)/GSK-3alpha in LNCaP cells can be promoted to the level of PC-3 cells. In sharp contrast, the PTK inhibitor has little effect on the
tyrosine phosphorylation level of the
kinase in LNCaP cells, whereas the PTP inhibitor has little effect on the
tyrosine phosphorylation level of the
kinase in PC-3 cells. Taken together, the results provide initial evidence that the
tyrosine phosphorylation/activation levels of this oncogenic PDPK can be differentially regulated in well- and poorly differentiated prostate
carcinoma cells.