Recent Advances and New Strategies in Targeting Plk1 for Anticancer Therapy

Abstract

Polo-like kinase 1 (Plk1) plays key roles in regulating mitotic processes that are crucial for cellular proliferation. Overexpression of Plk1 is tightly associated with the development of particular cancers in humans, and a large body of evidence suggests that Plk1 is an attractive target for anticancer therapeutic development. Drugs targeting Plk1 can potentially be directed at two distinct sites: the N-terminal catalytic kinase domain (KD), which phosphorylates substrates, and the C-terminal polo-box domain (PBD) which is essential for protein-protein interactions. In this review we summarize recent advances and new challenges in the development of Plk1 inhibitors targeting these two domains. We also discuss novel strategies for designing and developing next-generation inhibitors to effectively treat Plk1-associated human disorders.

Figure 1

A schematic diagram illustrating the structures of the human Plk family and subcellular localization of polo-like kinase 1 (Plk1) during the cell cycle. (A) A Phylogenetic tree is shown at left. Amino acid residues and sequence identities of Plk2–5 compared to Plk1 are indicated. PB1, polo-box motif 1; PB2, polo-box motif 2; CPB, cryptic polo-box; PB3, polo-box motif 3. (B) Subcellular localization pattern of Plk1 in HeLa cells is shown with a centromere/kinetochore-associating protein, PBIP1 [151]. DNA is stained with DAPI. Arrowheads, centrosomes; arrows, midzone and midbody.

Figure 2

A schematic diagram illustrating cancer cell–selective killing by the reversal of addictions to oncogenes and non-oncogenes. (A) Cancer cells arise as a consequence of mutations in oncogenes or deregulation of non-oncogenes. Reversal of this addiction can induce apoptotic cell death in cancer cells. (B) In a hypothetical cancer cell, where p53 is downregulated and Ras is mutationally activated, Plk1 is overexpressed. Because of cancer cells’ addiction to Plk1, depletion or inhibition of Plk1 can induce selective killing of cancer cells, but not normal cells.

Figure 3

A schematic diagram showing two strategies that target either the catalytic activity or the PBD of Plk1. (A) The catalytic activity of Plk1 is mostly targeted by ATP-competitive inhibitors, while the PBD is inhibited by PBD-binding antagonists. K.D., kinase domain; PBD, polo-box domain. (B) Catalytic inhibitors are unselective because they can abolish the catalytic activity of Plk1 in both normal and cancer cells. (C) PBD inhibitors can be designed in such a way that they can selectively inhibit cancer cell–enriched PBD-dependent interactions. The normal and cancer-enriched targets are drawn in different sizes in (B) and (C) to indicate altered levels of PBD-dependent interactions between normal and cancer cells.

Figure 4

The mode of small-molecule inhibitors binding to the ATP-binding site of Plk1–3. (A) The Plk1 catalytic domain with BI 2536 (brown; PDB 2RKU) [53] and volasertib (green; PDB 3FC2) [94]; (B) the Plk2 catalytic domain with 1C8 (PDB 4I6H), [152]; and (C) the Plk3 catalytic domain with 9ZP (PDB 4B6L) are shown. (Left) The active sites of Plks are displayed in stick models in the left panel. Residues establishing interactions with the ligands are displayed in gray stick models. Residues at and around the DFG motif are shown in yellow stick models. (Right) The binding modes of specific ligands are shown in electrostatic surface representations. Residues specific to each Plk are indicated in red, while residues present in two of Plk1–3 are shown in green. The catalytic domain of Plk4 is excluded because of a low level of similarity to Plk1–3. (D) Unique residues in each Plk isoform are indicated in red, while residues overlapped in two of Plk1–3 are denoted in green.

Figure 5

A structural comparison of Plk1–3 PBDs and the binding mode of Plk1 PBD with its ligands. (A) The apo structures of three Plk1–3 PBDs are represented in stick models: Plk1 PBD (PDB 1UMW), Plk2 PBD (PDB 4RS6), and Plk3 PBD (a model generated using the Robetta server [153]). The PBDs are arbitrarily divided into three regions based on the elements required for 4j binding to Plk1 PBD: a Pro pocket (cyan), a phospho-binding pocket (pink), and a hydrophobic channel (gray). The residues specific to each Plk are shown in red, and the overlapped residues in two of Plk1–3 are indicated in green. (B) Unique residues in each Plk are indicated in red, while residues overlapped in two of Plk1–3 are denoted in green. (C) 4j (green) bound to Plk1 PBD (PDB 3RQ7; [120]) is displayed in an electrostatic surface representation, with overlaid thymoquinone (gray) bound to Plk1 PBD (PDB 4H71; [130]). (D) A schematic diagram showing three elements that are critical for Plk1 PBD binding. The Pro pocket and the hydrophobic channel have been shown to increase the Plk1 PBD-binding affinity ~40-fold and ~500-fold, respectively [82, 120]. The critical role of the SpT anchor has been demonstrated previously [47, 82]. The spinal backbone of 4j is indicated in a dotted rectangular box.