Polo-like kinase: a novel marker of proliferation: Correlation with estrogen-receptor expression in human breast cancer

Previous data have shown that the mRNA-expression of the serin/threonine-ki nase polo-like kinase (PLK) is closely correlated with the survival of pati ents suffering from a subset of malignant tumors. PLK-mRNR and protein-expr ession are restricted to cells in the cell cycle. PLK-mRNA-transcripts are highly abundant in proliferating cells; no gene expression is found in G0-p hase cells. Here we investigated the mRNA- and protein-expression of PLK- a nd estrogen-receptor (ER) in human breast-carcinoma by northern-blotting, R T-PCR and immunohistochemistry. The expression of MIB-I was determined on s erial sections. Analysis of the immunohistochemical data revealed a close c orrelation between the ER and PLK-expression (r = 0.677; p = 0.001, n = 30) . No relationship between the mRNA-expression of ER and PLK was found. Furt hermore, no correlation for the protein expression of PLK and MIB-I exists. The influence of estrogen(ES) is known to have proliferative potential. Th e expression of ER correlates with the ES-plasma-level. In addition, the ho rmone cycle of premenopausal women undergoes rapid vacillations with varyin g effects on the proliferating tumor cells, e.g., growth induction. Our res ults therefore show that ER-expression is not only of therapeutic value for the clinician, but it may also be a tool for determining the tumor prolife ration index more precisely by integrating the hormone-mediated proliferati on stimulus.