PP2A(Cdc55/B55), a possible therapeutic target in cyclin D1-dependent cancers.

Protein phosphatase 2A (PP2A) comprises a large family of heterotrimeric complexes required for a variety of cellular processes. PP2A often functions to oppose the activity of oncogenic kinases and negatively regulates cell cycle progression in human cells. Therefore, PP2A is thought to contribute to tumor suppression, and PP2A-activating agents appear to reduce tumor burden. However, conversely, PP2A inhibition may enhance cancer chemotherapy by DNA damaging agents. Survival of cancer cells in response to DNA damage depends on checkpoint-dependent cell cycle arrest. Therefore, PP2A inhibition, which promotes cell cycle progression and abrogates cell cycle arrest, may effectively induce mitotic catastrophes and subsequent cell death, indicating that modulating PP2A activity may hold good promise as cancer therapy. Thus, understanding how PP2A controls cell cycle progression is important.

In a recent report, McCourt et al. demonstrated an interesting link between PP2A and G1 cyclins, whose overexpression is frequently associated with human cancers. In humans, cyclin D1, one of the G1 cyclins that promote G1/S transitions, is regulated by proteasomal-dependent proteolysis. Degradation of cyclin D1 is dependent on glycogen synthase kinase 3β (GSK3β)-dependent phosphorylation and subsequent recognition by the SCFFbx4/aB-crystallin ubiquitin ligase. In budding yeast, G1 cyclins, Cln1 and Cln2, are known to undergo CDK1-dependent phosphorylation followed by ubiquitination via the SCFGrr1 ubiquitin ligase. Thus, phosphorylation-dependent degradation of G1 cyclins is conserved across evolution. However, phosphatases involved in dephosphorylation of G1 cyclins were not well-documented. Considering that phosphorylation status of G1 cyclins plays an important role in their stability, and that G1 cyclins are often deregulated in human cancers,,, identifying phosphatases involved in G1 cyclin stability plays a significant role in the improvement of cancer therapy.

In the course of understanding how PP2A regulates cellular processes, McCourt et al. identified an allele of grr1 as a synthetic lethal mutation with the loss of Cdc55 (B55 in humans), one of two regulatory B subunits of budding yeast PP2A. This genetic interaction was specific to PP2ACdc55, because the grr1 mutation was not synthetically lethal with the loss of Rts1, the second regulatory B subunit for PP2A. Grr1 is an F-box protein, which is a variable component of SCF ubiquitin ligases and responsible for substrate recognition., Further mutational analyses of grr1 revealed that mutations in domains required for substrate recognition are also synthetically lethal with cdc55 deletion, suggesting that accumulation of SCFGrr1 substrates is toxic in the absence of Cdc55. Indeed, Cln2, one of the SCFGrr1 substrates, was highly accumulated in grr1 mutant, and Cln2 overexpression was toxic in cdc55-deleted cells.

Interestingly, Cln2 was highly unstable in the absence of Cdc55. Cln2 degradation in cdc55-deleted cells was associated with the CDK1-dependent phosphorylation of Cln2, because the unphosphorylatable form of Cln2 was highly stable even in the absence of Cdc55. Furthermore, a temperature-sensitive mutation in Cdc53 (an SCFGrr1 component) stabilized Cln2 in cdc55 cells, indicating that Cln2 is a better SCFGrr1 substrate in the absence of Cdc55. Considering that SCFGrr1 targets phosphorylated Cln2, these results suggest that PP2ACdc55 regulates Cln2 stability through modulating its phosphorylation status. Consistent with this suggestion, the authors showed that PP2A physically associates with Cln2, indicating the role of PP2A in dephosphorylating Cln2. It would be interesting in the future to investigate whether PP2ACdc55 indeed directly dephosphorylates Cln2.

The authors took a further step and genetically demonstrated that PP2ACdc55 and SCFGrr1 act antagonistically to regulate G1 cyclin-dependent cell cycle events. Cellular amounts of human G1 cyclins, such as cyclin D1, must be tightly regulated to prevent uncontrolled growth and genomic instability associated with a variety of cancers. Indeed, several F-box proteins or associated factors, which are known to regulate cyclin D1 levels, are mutated in cancers, suggesting the importance of fine-tuning cyclin D1 levels in preventing cancer development. Therefore, targeting cyclin D1 is proposed to be an effective strategy in cancer therapy, and some compounds are reported to induce cyclin D1 degradation. Therefore, the modulation of phosphorylation status by inhibiting PP2A may constitute a new way of regulating cyclin D1 levels in cancer. Further research into the role of PP2A in G1 cyclin stability in human cancer cells would answer these questions. (Fig. 1)

Figure 1. Phosphorylation-dependent degradation of G1 cyclins in yeast and humans. In yeast, CDK1-dependent phosphorylation of Cln2 leads to Cln2 degradation via SCFGrr1. This degradation can be inhibited by Cln2 dephosphorylation by the PP2ACdc55 phosphatase. In humans, GSK3β phosphorylates Cyclin D1, resulting in Cyclin D1 ubiquitination by SCFFbx4/aB-crystallin.