Wee1 and Cdc25: Tools, pathways, mechanisms, questions

From dusk till dawn: cell cycle progression in the red seaweed Gracilariopsis chorda (Rhodophyta)

The conserved eukaryotic functions of cell cycle genes have primarily been studied using animal/plant models and unicellular algae. Cell cycle progression and its regulatory components in red (Rhodophyta) seaweeds are poorly understood. We analyzed diurnal gene expression data to investigate the cell cycle in the red seaweed Gracilariopsis chorda. We identified cell cycle progression and transitions in G. chorda which are induced by interactions of key regulators such as E2F/DP, RBR, cyclin-dependent kinases, and cyclins from dusk to dawn. However, several typical CDK inhibitor proteins are absent in red seaweeds. Interestingly, the G1-S transition in G. chorda is controlled by delayed transcription of GINS subunit 3. We propose that the delayed S phase entry in this seaweed may have evolved to minimize DNA damage (e.g., due to UV radiation) during replication. Our results provide important insights into cell cycle-associated physiology and its molecular mechanisms in red seaweeds.

Sequential drug treatment targeting cell cycle and cell fate regulatory programs blocks non-genetic cancer evolution in acute lymphoblastic leukemia

BACKGROUND

Targeted therapies exploiting vulnerabilities of cancer cells hold promise for improving patient outcome and reducing side-effects of chemotherapy. However, efficacy of precision therapies is limited in part because of tumor cell heterogeneity. A better mechanistic understanding of how drug effect is linked to cancer cell state diversity is crucial for identifying effective combination therapies that can prevent disease recurrence.

RESULTS

Here, we characterize the effect of G2/M checkpoint inhibition in acute lymphoblastic leukemia (ALL) and demonstrate that WEE1 targeted therapy impinges on cell fate decision regulatory circuits. We find the highest inhibition of recovery of proliferation in ALL cells with KMT2A-rearrangements. Single-cell RNA-seq and ATAC-seq of RS4;11 cells harboring KMT2A::AFF1, treated with the WEE1 inhibitor AZD1775, reveal diversification of cell states, with a fraction of cells exhibiting strong activation of p53-driven processes linked to apoptosis and senescence, and disruption of a core KMT2A-RUNX1-MYC regulatory network. In this cell state diversification induced by WEE1 inhibition, a subpopulation transitions to a drug tolerant cell state characterized by activation of transcription factors regulating pre-B cell fate, lipid metabolism, and pre-BCR signaling in a reversible manner. Sequential treatment with BCR-signaling inhibitors dasatinib, ibrutinib, or perturbing metabolism by fatostatin or AZD2014 effectively counteracts drug tolerance by inducing cell death and repressing stemness markers.

CONCLUSIONS

Collectively, our findings provide new insights into the tight connectivity of gene regulatory programs associated with cell cycle and cell fate regulation, and a rationale for sequential administration of WEE1 inhibitors with low toxicity inhibitors of pre-BCR signaling or metabolism.

Rigosertib and Cholangiocarcinoma: A Cell Cycle Affair

Rigosertib is multi-kinase inhibitor that could represent an interesting therapeutic option for non-resectable patients with cholangiocarcinoma, a very aggressive hepatic cancer with limited effective treatments. The Western blotting technique was used to evaluate alterations in the expression of proteins involved in the regulation of the cell cycle of cholangiocarcinoma EGI-1 cells. Our results show an increase in EMI1 and Cyclin B protein levels after Rigosertib treatment. Moreover, the phosphorylation of CDK1 is significantly reduced by Rigosertib, while PLK1 expression increased after 24 h of treatment and decreased after 48 h. Finally, we evaluated the role of p53. Its levels increase after Rig treatment, and, as shown in the cell viability experiment with the p53 inhibitor Pifithrin, its activity is necessary for the effects of Rigosertib against the cell viability of EGI-1 cells. In conclusion, we hypothesized the mechanism of the action of Rigosertib against cholangiocarcinoma EGI-1 cells, highlighting the importance of proteins involved in the regulation of cell cycles. The CDK1-Cyclin B complex and p53 play an important role, explaining the Block in the G2/M phase of the cell cycle and the effect on cell viability.

Crosstalk between the mTOR and DNA Damage Response Pathways in Fission Yeast

Cells have developed response systems to constantly monitor environmental changes and accordingly adjust growth, differentiation, and cellular stress programs. The evolutionarily conserved, nutrient-responsive, mechanistic target of rapamycin signaling (mTOR) pathway coordinates basic anabolic and catabolic cellular processes such as gene transcription, protein translation, autophagy, and metabolism, and is directly implicated in cellular and organismal aging as well as age-related diseases. mTOR mediates these processes in response to a broad range of inputs such as oxygen, amino acids, hormones, and energy levels, as well as stresses, including DNA damage. Here, we briefly summarize data relating to the interplays of the mTOR pathway with DNA damage response pathways in fission yeast, a favorite model in cell biology, and how these interactions shape cell decisions, growth, and cell-cycle progression. We, especially, comment on the roles of caffeine-mediated DNA-damage override. Understanding the biology of nutrient response, DNA damage and related pharmacological treatments can lead to the design of interventions towards improved cellular and organismal fitness, health, and survival.

Caffeine Stabilises Fission Yeast Wee1 in a Rad24-Dependent Manner but Attenuates Its Expression in Response to DNA Damage

The widely consumed neuroactive compound caffeine has generated much interest due to its ability to override the DNA damage and replication checkpoints. Previously Rad3 and its homologues was thought to be the target of caffeine’s inhibitory activity. Later findings indicate that the Target of Rapamycin Complex 1 (TORC1) is the preferred target of caffeine. Effective Cdc2 inhibition requires both the activation of the Wee1 kinase and inhibition of the Cdc25 phosphatase. The TORC1, DNA damage, and environmental stress response pathways all converge on Cdc25 and Wee1. We previously demonstrated that caffeine overrides DNA damage checkpoints by modulating Cdc25 stability. The effect of caffeine on cell cycle progression resembles that of TORC1 inhibition. Furthermore, caffeine activates the Sty1 regulated environmental stress response. Caffeine may thus modulate multiple signalling pathways that regulate Cdc25 and Wee1 levels, localisation and activity. Here we show that the activity of caffeine stabilises both Cdc25 and Wee1. The stabilising effect of caffeine and genotoxic agents on Wee1 was dependent on the Rad24 chaperone. Interestingly, caffeine inhibited the accumulation of Wee1 in response to DNA damage. Caffeine may modulate cell cycle progression through increased Cdc25 activity and Wee1 repression following DNA damage via TORC1 inhibition, as TORC1 inhibition increased DNA damage sensitivity.

Caffeine as a tool for investigating the integration of Cdc25 phosphorylation, activity and ubiquitin-dependent degradation in Schizosaccharomyces pombe

The evolutionarily conserved Cdc25 phosphatase is an essential protein that removes inhibitory phosphorylation moieties on the mitotic regulator Cdc2. Together with the Wee1 kinase, a negative regulator of Cdc2 activity, Cdc25 is thus a central regulator of cell cycle progression in Schizosaccharomyces pombe. The expression and activity of Cdc25 is dependent on the activity of the Target of Rapamycin Complex 1 (TORC1). TORC1 inhibition leads to the activation of Cdc25 and repression of Wee1, leading to advanced entry into mitosis. Withdrawal of nitrogen leads to rapid Cdc25 degradation via the ubiquitin- dependent degradation pathway by the Pub1 E3- ligase. Caffeine is believed to mediate the override of DNA damage checkpoint signalling, by inhibiting the activity of the ataxia telangiectasia mutated (ATM)/Rad3 homologues. This model remains controversial, as TORC1 appears to be the preferred target of caffeine in vivo. Recent studies suggest that caffeine induces DNA damage checkpoint override by inducing the nuclear accumulation of Cdc25 in S. pombe. Caffeine may thus modulate Cdc25 activity and stability via inhibition of TORC1. A clearer understanding of the mechanisms by which caffeine stabilises Cdc25, may provide novel insights into how TORC1 and DNA damage signalling is integrated.

To control or to be controlled? Dual roles of CDK2 in DNA damage and DNA damage response

CDK2 (cyclin-dependent kinase 2), a member of the CDK family, has been shown to play a role in many cellular activities including cell cycle progression, apoptosis and senescence. Recently, accumulating evidence indicates that CDK2 is involved in DNA damage and DNA repair response (DDR). When DNA is damaged by internal or external genotoxic stresses, CDK2 activity is required for proper DNA repair in vivo and in vitro, whereas inactivation of CDK2 by siRNA techniques or by inhibitors could result in DNA damage and stimulate DDR. Hence, CDK2 seems to play dual roles in DNA damage and DDR. On one aspect, it is activated and stimulates DDR to repair DNA damage when DNA damage occurs; on the other hand, its inactivation directly leads to DNA damage and evokes DDR. Here, we describe the roles of CDK2 in DNA damage and DDR, and discuss the potential application of CDK2 inhibitors as anti-cancer agents.