Proliferating cell nuclear antigen binds DNA polymerase-β and mediates 1-methyl-4-phenylpyridinium-induced neuronal death

Targeting the "Undruggable": Small-Molecule Inhibitors of Proliferating Cell Nuclear Antigen (PCNA) in the Spotlight in Cancer Therapy

PCNA plays multiple roles in cancer development, including cell proliferation regulation, DNA repair, replication, and serving as a widely used biomarker and therapeutic target. Despite its significant role in oncology, PCNA has historically been considered "undruggable" due to the absence of known endogenous small molecule modulators and identifiable ligand binding sites. Unlike other protein-protein interfaces, PCNA lacks explicit binding grooves, featuring a relatively small and shallow surface pocket, which hinders the discovery of traditional small molecule targets. Recent breakthroughs have introduced promising PCNA-targeting candidates, with ATX-101 and AOH1996 entering phase I clinical trials for cancer therapy, garnering academic and industry interest. These achievements provide new evidence for PCNA as a drug target. This article provides insight and perspective on the application of small-molecule PCNA inhibitors in cancer treatment, covering PCNA function, its relationship with cancer, structural modification of small molecule inhibitors, and discovery strategies.

Alzheimer's Disease and Cancer: When Two Monsters Cannot Be Together

Alzheimer's disease (AD) and cancer are among the leading causes of human death around the world. While neurodegeneration is the main feature of AD, the most important characteristic of malignant tumors is cell proliferation, placing these two diseases in opposite sides of cell division spectrum. Interestingly, AD and cancer's pathologies consist of a remarkable common feature and that is the presence of active cell cycle in both conditions. In an in vitro model of primary adult neuronal culture, we previously showed that treating cell with beta amyloid forced neurons to start a cell cycle. Instead of cell division, however, neuronal cell cycle was aborted and a massive neurodegeneration was left behind as the consequence. A high level of cell cycle entry, which is a requirement for cancer pathogenesis, was reported in clinically diagnosed cases of AD, leading to neurodegeneration. The diverse clinical manifestation of a similar etiology, have puzzled researchers for many years. In fact, the evidence showed an inverse association between AD and cancer prevalence, suggesting that switching pathogenesis toward AD protects patients against cancer and vice versa. In this mini review, we discussed the possibility of involvement of cell proliferation and survival dysregulation as the underlying mechanism of neurodegeneration in AD, and the leading event to develop both disorders' pathology. As examples, the role of phosphoinositide 3 kinase/Akt/ mammalian target of rapamycin (PI3K/Akt/mTOR) signaling pathway in cell cycle re-entry and blocking autophagy are discussed as potential common intracellular components between AD and cancer pathogenesis, with diverse clinical diagnosis.

Impaired Memory in OT-II Transgenic Mice Is Associated with Decreased Adult Hippocampal Neurogenesis Possibly Induced by Alteration in Th2 Cytokine Levels

Recently, an increasing number of studies have focused on the effects of CD4+ T cell on cognitive function. However, the changes of Th2 cytokines in restricted CD4+ T cell receptor (TCR) repertoire model and their effects on the adult hippocampal neurogenesis and memory are not fully understood. Here, we investigated whether and how the mice with restricted CD4+ repertoire TCR exhibit learning and memory impairment by using OT-II mice. OT-II mice showed decreased adult neurogenesis in hippocampus and short- and long- term memory impairment. Moreover, Th2 cytokines in OT-II mice are significantly increased in peripheral organs and IL-4 is significantly increased in brain. Finally, IL-4 treatment significantly inhibited the proliferation of cultured adult rat hippocampal neural stem cells. Taken together, abnormal level of Th2 cytokines can lead memory dysfunction via impaired adult neurogenesis in OT-II transgenic.

Identification of 5-Methoxyflavone as a Novel DNA Polymerase-Beta Inhibitor and Neuroprotective Agent against Beta-Amyloid Toxicity

Cell-cycle reactivation is a core feature of degenerating neurons in Alzheimer's disease (AD) and Parkinson's disease (PD). A variety of stressors, including β-amyloid (Aβ) in the case of AD, can force neurons to leave quiescence and to initiate an ectopic DNA replication process, leading to neuronal death rather than division. As the primary polymerase (pol) involved in neuronal DNA replication, DNA pol-β contributes to neuronal death, and DNA pol-β inhibitors may prove to be effective neuroprotective agents. Currently, specific and highly active DNA pol-β inhibitors are lacking. Nine putative DNA pol-β inhibitors were identified in silico by querying the ZINC database, containing more than 35 million purchasable compounds. Following pharmacological evaluation, only 5-methoxyflavone (1) was validated as an inhibitor of DNA pol-β activity. Cultured primary neurons are a useful model to investigate the neuroprotective effects of potential DNA pol-β inhibitors, since these neurons undergo DNA replication and death when treated with Aβ. Consistent with the inhibition of DNA pol-β, 5-methoxyflavone (1) reduced the number of S-phase neurons and the ensuing apoptotic death triggered by Aβ. 5-Methoxyflavone (1) is the first flavonoid compound able to halt neurodegeneration via a definite molecular mechanism rather than through general antioxidant and anti-inflammatory properties.

The S-nitrosylation status of PCNA localized in cytosol impacts the apoptotic pathway in a Parkinson's disease paradigm

It is generally accepted that nitric oxide (NO) or its derivatives, reactive nitrogen species (RNS), are involved in the development of Parkinson's disease (PD). Recently, emerging evidence in the study of PD has indicated that protein S-nitrosylation triggers the signaling changes in neurons. In this study, SH-SY5Y cells treated with rotenone were used as a model of neuronal death in PD. The treated cells underwent significant apoptosis, which was accompanied by an increase in intracellular NO in a rotenone dose-dependent manner. The CyDye switch approach was employed to screen for changes in S-nitrosylated (SNO) proteins in response to the rotenone treatment. Seven proteins with increased S-nitrosylation were identified in the treated SH-SY5Y cells, which included proliferating cell nuclear antigen (PCNA). Although PCNA is generally located in the nucleus and participates in DNA replication and repair, significant PCNA was identified in the SH-SY5Y cytosol. Using immunoprecipitation and pull-down approaches, PCNA was found to interact with caspase-9; using mass spectrometry, the two cysteine residues PCNA-Cys81 and -Cys162 were identified as candidate S-nitrosylated residues. In addition, the evidence obtained from in vitro and the cell model studies indicated that the S-nitrosylation of PCNA-Cys81 affected the interaction between PCNA and caspase-9. Furthermore, the interaction of PCNA and caspase-9 partially blocked caspase-9 activation, indicating that the S-nitrosylation of cytosolic PCNA may be a mediator of the apoptotic pathway.