Biological impact of freezing Plk1 in its inactive conformation in cancer cells

Human polo-like kinase 1 (Plk1), a key regulator of mitosis, is overexpressed in various human tumors. It is a negative prognostic factor for cancer patients and a measure for the aggressiveness of a tumor. Thus, targeting Plk1 might be a promising approach for cancer therapy. Kinase inhibitors are divided in type I inhibitors, targeting the highly conserved active conformation, and the more selective type II inhibitors, targeting the inactive conformation of kinases. We analyzed our previously identified type II Plk1 inhibitor SBE13 which is able to inhibit Plk1 activity. To determine its ability to induce cell death in cancer cells, we applied kinase assays, western blot analyses, FACScan analyses, Caspase assays and immunofluorescence studies. We detected decreased cell proliferation, delayed progression through the cell cycle in lower SBE13 concentrations, a G(2)/M arrest using higher SBE13 concentrations followed by apoptosis, and abnormal mitotic figures. Notably, SBE13 did not influence activity of other kinases (Plk2, Plk3, Aurora A), indicating the selectivity of this type II Plk1 inhibitor. This study suggests that Plk1 kinase inhibitors targeting the inactive conformation of Plk1 may be considered for the development of cancer therapeutics.