Polo-like kinases (PLKs) are serine/threonine kinases, and function as regulators of cell cycle progression, centriole duplication, mitosis, cytokinesis and the DNA damage response (Zitouni et al., 2014). In humans, five structurally related PLKs have been identified (PLK1, 2, 3, 4, and 5). All PLKs share similar architecture with an amino-terminal (Ser/Thr) catalytic kinase domain and a carboxy-terminal region containing two or more polo boxes, which are organized in different domains (Zitouni et al., 2014). PLK1, 2 and 3 have very similar catalytic domains, whereas that of PLK4 has a divergent primary sequence, and PLK5 includes a pseudokinase domain. As a consequence, the consensus phosphorylation target sites and sensitivities to small-molecule competitive inhibitors of ATP are similar for PLK1, 2 and 3, whereas PLK4 has different properties. Phosphorylation of residues within the catalytic domain is required for PLK activation. The polo boxes of human PLKs regulate the catalytic activity of PLKs, and their localization and substrate binding (Zitouni et al., 2014).

PLK1 is mostly expressed in rapidly proliferating tissues, particularly in cancer cells with an unstable genome. Therefore, PLK1 has been considered as a potential target for cancer therapy (Liu et al., 2017), and PLK1 inhibitors have been evaluated in clinical phase I or II trials in patients with lymphoma(Vose et al., 2013), leukemia (Kobayashi et al., 2015), renal cancer (Stadler et al., 2014), pancreas cancer (Mross et al., 2012), or advanced solid malignancies (Olmos et al., 2011). However, the detailed molecular mechanism of PLK1 inhibitor-induced cell death is not fully understood.

Proteasome is a multienzyme complex that maintains protein homeostasis and important cellular functions by degrading misfolded, redundant, and damaged proteins via ubiquitin proteasome system (UPS)(Njomen and Tepe, 2019). To ensure appropriate destruction of damaged or misfolded proteins, UPS acts in a highly coordinated manner through several steps, including polyubiquitination, de-ubiquitination, and degradation of the target protein (Manasanch and Orlowski, 2017). Interestingly, ubiquitylation serves both pro- and anti-death functions (Bergmann, 2010). Proteasome inhibitors have been used for clinical cancer therapy (Manasanch and Orlowski, 2017). Meanwhile, the activation of proteasomes can elicit cell death via different mechanisms (Akabane et al., 2016; Sanchez et al., 2016; Valenti et al., 2008).

In this study, we found that PLK1 was preferentially expressed in several cancer cells and some normal cells. In PLK1 preferentially expressed cells, the inhibition of PLK1 induces cell death in vitro, and decreases tumor progress in nude mice model. Meanwhile, we found that PLK1 inhibitor-induced cell death is p53-, caspase-, PARP-and calpain-independent. Surprisingly, pharmacological inhibition of proteasome reversed PLK1 inhibitor-induced cell death in all cells preferentially expressed PLK1, suggesting that proteasome is critical for cell death induced by PLK1 inhibition.