Regulatory functional territory of PLK-1 and their substrates beyond mitosis

Abstract

Polo-like kinase 1 (PLK-1) is a well-known (Ser/Thr) mitotic protein kinase and is considered as a proto-oncogene. As hyper-activation of PLK-1 is broadly associated with poor prognosis and cancer progression, it is one of the most extensively studied mitotic kinases. During mitosis, PLK-1 regulates various cell cycle events, such as spindle pole maturation, chromosome segregation and cytokinesis. However, studies have demonstrated that the role of PLK-1 is not only restricted to mitosis, but PLK-1 can also regulate other vital events beyond mitosis, including transcription, translation, ciliogenesis, checkpoint adaptation and recovery, apoptosis, chromosomes dynamics etc. Recent reviews have tried to define the regulatory role of PLK-1 during mitosis progression and tumorigenesis, but its' functional role beyond mitosis is still largely unexplored. PLK-1 can regulate the activity of many proteins that work outside of its conventional territory. The dysregulation of these proteins can cause diseases such as Alzheimer's disease, tumorigenesis etc. and may also lead to drug resistance. Thus, in this review, we discussed the versatile role of PLK-1 and tried to collect data to validate its' functional role in cell cycle regulation apart from mitosis.

Keywords: DNA damage response; checkpoint adaptation and recovery; ciliogenesis; polo-like kinase-1; transcription and translation.

Figure 1. A substrate priming model for PLK-1-targeted substrates during cell cycle - The substrate priming mechanism of PLK-1-targeted substrates mediated either by CDK-1 (non-self-priming) or PLK-1 (self-priming)

In non-self-priming, CDK-1 phosphorylates the PLK-1-targeted substrate to facilitate the binding of its PBD to the phosphorylated substrate. PBD-substrate binding is required for partial activation of PLK-1 through physical dissociation of the KD from the PBD. In self-priming, inactivated PLK-1 interacts with a phosphorylated substrate and facilitates its kinase domain dissociation from the PBD. PLK-1 activation is achieved by Aurora-A-Bora to catalyze the phosphorylation substrates. Activated PLK-1 catalyzes phosphorylation of either the same substrate bound to the PBD (processive phosphorylation) or another substrate (binds to the PBD bound substrate, distributive phosphorylation). PLK-1 may also facilitate both models for phosphorylating the targeted substrate at same time.

Figure 2. A schematic role of PLK-1-mediated phosphorylation of p150glued in neuronal cell death in a β-amyloid-dependent manner in AD - Overexpressed γ-secretase cleaves mutated β-amyloid to generate a binary complex (NFT) with a hyper-phosphorylated Tau protein

The NFT binds to the microtubules of cortical neurons in a phosphorylated p150^glued and dynactin-dynein-dependent manner. In the hippocampus of AD patients, overexpression of PLK-1 phosphorylates p150^glued in the dynactin and dynein complex to induce neuronal cell death in a NFT-dependent manner. Subsequently, the hippocampal region is shrunken in AD patients because of neuronal cell death.

Figure 3. A model illustrating the dynamic cycle of primary cilia disassembly in the presence and absence of PLK-1 a. In the presence of PLK-1, non-canonical Wnt5a triggers the CK-1ε-mediated phosphorylation of Dvl2 to recruit PLK-1, which generates the CK-1ε-PLK-1 complex

This complex interacts with Smad-3 and stabilizes the HEF1 for interacting with Aur-A. Because of the unavailability of Smad-3 for generating a HEF1 destruction complex (HEF1-Smad-3-APC complex), stabilized HEF1 interacts with Aur-A to achieve the disassembly of the primary cilia just prior to mitosis. b. In the absence of PLK-1, Smad-3 is available for generating the HEF1 destruction complex, HEF1-Smad-3-APC. HEF1 is ubiquitinated and degraded by the proteasome in APC-dependent manner. Consequently, HEF1 is not available to interact with Aur-A to facilitate the disassembly of the primary cilia in the G0/G1 phase.

Figure 4. A schematic model of PLK-1-mediated regulation of p53 in TOPORS and GTSE1-dependent manner - During prolonged cell cycle arrest, a. activated PLK-1 phosphorylates TOPORS to facilitate the ubiquitination and proteasomal degradation of p53, which promotes checkpoint recovery

During normal cell cycle progression, TOPORS sumoylates and stabilizes p53 for inducing its pro-apoptotic functions. b. PLK-1 phosphorylates and translocate GTSE1 into the nucleus. Nucleated GTSE1 binds to the C-terminus of p53 and catalyzes the shuttling of p53 from the nucleus to the cytoplasm. GTSE1-dependent shuttling of p53 results in inactivation of p53, which leads to checkpoint recovery.

Figure 5. The role of PLK-1 in the DNA damage response- During the DNA damage response, ATM or ATR kinases delay cell cycle progression to provide adequate time to repair the damage in a Chk-1- or Chk-2-dependent manner, respectively

The ATR-Chk-2 complex inhibits PLK-1 and CDC25 to block the G2/M transition. Moreover, ATR-Chk-2 catalyzes p53 phosphorylation to inhibit CDK-1, which facilitates the G1/S transition. Simultaneously, ATM-Chk-1 triggers p53 to activate its’ downstream targets p21 and 14-3-3. p21 and 14-3-3 both inhibit the CDK-1-cyclin B1 binary complex, which is required for the G2/M transition and, arrests cells in the G2/M phase for short periods of time. Inhibition of the G2/M transition provides time for damage repair and recovery from DNA damage.

Figure 6. The role of PLK-1 in checkpoint adaptation and recovery - a. During prolonged cell cycle arrest, PLK-1 is upregulated in cells that are stressed for a long period of time

Up-regulated PLK-1 inhibits the damage repair mechanism and releases the cells from G2/M arrest with damaged DNA. PLK-1 inhibits/degrades the various proteins that are associated either with the damage repair mechanism or with G2/M arrest, including Chk-2, p53, MYT1, WEE1, and claspin. PLK-1 also phosphorylates CDC25 to activate its’ phosphatase activity. Phosphorylated CDC25 activates the CDK-1-cyclin B1 complex to release the cell cycle from G2/M arrest with damaged DNA. b. In the HR repair pathway, PLK-1 phosphorylates RAD51 to activate BRCA1/2 for activating the repair pathways in a CK-II-dependent manner. Phosphorylated RAD51 interacts with NBS, and the RAD51-NBS complex is recruited to the damage site to repair the damage. Moreover, PLK-1 and CK-II-dependent phosphorylation blocks the G1/S transition and provides time for damage repair.