Colloquium of the Faculty of Biology and Pharmacy

The role of polo-like kinase 1 (Plk1) for the cell cycle of cancer and primary cells

PD Dr. Birgit Spänkuch

Center for Molecular Biomedicine, FSU Jena

Human polo-like kinase 1 (Plk1), a key component of mitotic progression, is over-expressed in all human cancer types and serves as negative prognostic factor for cancer patients. Thus, Plk1 represents a promising target to study mechanisms underlying cell cycle regulation in cancer versus primary cells and to develop new selective and specific therapeutic strategies. In first studies we applied nucleic acid-based agents (antisense oligonucleotides, siRNAs) to inhibit Plk1 kinase activity in cancer and primary cells. We observed reduced kinase activity, a G2/M arrest, reduced cell proliferation, apoptosis induction, naked centrosomes and impaired mitotic figures, and inhibition of tumor growth in a mouse model. In contrast, primary cells did not show such disturbed mitotic figures or any other signs of cell death. The following studies dealt with kinase inhibitors, which are divided in type I inhibitors, targeting the active conformation, and type II inhibitors, targeting the inactive conformation. Type II inhibitors occupy a hydrophobic pocket with less conserved amino acids leading to higher selectivity. We did in a collaboration a virtual screening using a homology-based structural pharmacophore model of human Plk1 to identify inhibitors against the inactive conformation. We identified one potent inhibitor which inhibits Plk1 activity with an IC50 of 200 pM, leading to decreased cell proliferation, G2/M arrest and apoptosis and abnormal mitotic figures in cancer cells. Notably, SBE13 did not influence activity of other kinases confirming its high selectivity towards the family members Plk2 and Plk3, which are known as tumor suppressor genes. In primary cells SBE13 induced a transient G0/G1 arrest, followed by completely normal cell cycle kinetics and normal cell proliferation, without any signs of cell death, although Plk1 kinase activity was reduced in these primary cells, which underlines the importance of selectivity.

The Plk1 inhibitors have been analyzed in combination with various other anti-cancer drugs: Earlier studies could show the enhanced sensitivity of cancer cells, but not of primary cells towards treatment with anti-neoplastic drugs like Paclitaxel or Herceptin after Plk1 inhibition using siRNAs or ASOs. Additionally, Plk1-specific siRNA is able to enhance the effects of irradiation and revealed the function of Plk1 as a predictive marker. The latest studies identified Plk1 as an enhancer of sensitivity of cancer cells towards treatment with PKCβ inhibitors (Enzastaurin) dependent on their p53 status. Ongoing studies aim at the mechanistic analysis of the interplay between Plk1 and Chk1 inhibition and of the interplay between Plk1 and HDAC inhibition.

In addition, we developed in a collaboration nanoparticle formulations for the targeted delivery of our Plk1 inhibitors. We used Herceptin to target the Her2 receptor and observed reduced Plk1 mRNA and protein, reduced cell proliferation and an induction of apoptosis after application of the Herceptin-conjugated HSA nanoparticles containing ASOs or expression plasmids for shRNAs targeting Plk1. Ongoing studies aim at the development of a nanoparticle formulation for the targeted delivery of SBE13 to cancer cells.

All these studies suggest and confirm the role of Plk1 as a target for anti-cancer therapeutics due to its over-expression in cancer cells, to its important role for cell cycle regulation and due to the fact that Plk1 inhibition using our Plk1-specific siRNA or our Plk1 kinase inhibitor SBE13 induces apoptosis in cancer cells, but leaves the cell cycle and thus cell proliferation of primary cells unimpaired. Taken together, especially our Plk1 inhibitor SBE13 could be confirmed as valuable tool to study the role of Plk1 in cell cycle regulation of cancer cells and primary cells and to develop selective and specific cancer therapeutics.