Antitumoral activity of allosteric inhibitors of protein kinase CK2

INTRODUCTION

Due to its physiological role into promoting cell survival and its dysregulation in most cancer cells, protein kinase CK2 is a relevant physiopathological target for development of chemical inhibitors. We report the discovery of azonaphthalene derivatives, as a new family of highly specific CK2 inhibitors. First, we demonstrated that CK2 inhibition (IC50= 0.4 µM) was highly specific, reversible and non ATP-competitive. Small Angle X-ray Scattering experiments showed that this inhibition was due to large conformational change of CK2α upon binding of these inhibitors. We showed that several compounds of the family were cell-potent CK2 inhibitors promoting cell cycle arrest of human glioblastoma U373 cells. Finally, in vitro and in vivo assays showed that these compounds could decrease U373 cell tumor mass by 83 % emphasizing their efficacy against these apoptosis-resistant tumors. In contrast, Azonaphthalene derivatives inactive on CK2 activity showed no effect in colony formation and tumor regression assays. These findings illustrate the emergence of nonclassical CK2 inhibitors and provide exciting opportunities for the development of novel allosteric CK2 inhibitors.

BACKGROUND

CK2 is an emerging therapeutic target and ATP-competitive inhibitors have been identified. CK2 is endowed with specific structural features providing alternative strategies for inhibition.

RESULTS

Azonaphthalene compounds are allosteric CK2 inhibitors showing antitumor activity.

CONCLUSION

CK2 may be targeted allosterically.

SIGNIFICANCE

These inhibitors provide a foundation for a new paradigm for specific CK2 inhibition.

A complex small RNA repertoire is generated by a plant/fungal-like machinery and effected by a metazoan-like Argonaute in the single-cell human parasite Toxoplasma gondii

In RNA silencing, small RNAs produced by the RNase-III Dicer guide Argonaute-like proteins as part of RNA-induced silencing complexes (RISC) to regulate gene expression transcriptionally or post-transcriptionally. Here, we have characterized the RNA silencing machinery and exhaustive small RNAome of Toxoplasma gondii, member of the Apicomplexa, a phylum of animal- and human-infecting parasites that cause extensive health and economic damages to human populations worldwide. Remarkably, the small RNA-generating machinery of Toxoplasma is phylogenetically and functionally related to that of plants and fungi, and accounts for an exceptionally diverse array of small RNAs. This array includes conspicuous populations of repeat-associated small interfering RNA (siRNA), which, as in plants, likely generate and maintain heterochromatin at DNA repeats and satellites. Toxoplasma small RNAs also include many microRNAs with clear metazoan-like features whose accumulation is sometimes extremely high and dynamic, an unexpected finding given that Toxoplasma is a unicellular protist. Both plant-like heterochromatic small RNAs and metazoan-like microRNAs bind to a single Argonaute protein, Tg-AGO. Toxoplasma miRNAs co-sediment with polyribosomes, and thus, are likely to act as translational regulators, consistent with the lack of catalytic residues in Tg-AGO. Mass spectrometric analyses of the Tg-AGO protein complex revealed a common set of virtually all known RISC components so far characterized in human and Drosophila, as well as novel proteins involved in RNA metabolism. In agreement with its loading with heterochromatic small RNAs, Tg-AGO also associates substoichiometrically with components of known chromatin-repressing complexes. Thus, a puzzling patchwork of silencing processor and effector proteins from plant, fungal and metazoan origin accounts for the production and action of an unsuspected variety of small RNAs in the single-cell parasite Toxoplasma and possibly in other apicomplexans. This study establishes Toxoplasma as a unique model system for studying the evolution and molecular mechanisms of RNA silencing among eukaryotes.

Toxoplasma gondii is an important human parasite that causes life-threatening diseases in developing fetuses and in immunocompromised individuals, especially AIDS and transplant patients. Curiously, the Toxoplasma genome is deprived of most of the basic transcription factors that regulate gene expression in other eukaryotic cells. Therefore, alternative strategies must exist to modulate the many phases of the Toxoplasma complex life cycle that includes invasion of several hosts. Here, we investigate one of these strategies, by studying the repertoire of Toxoplasma silencing small RNAs (sRNAs). In eukaryotes, most of these regulatory molecules, 20–30nt-long, are produced by members of the Dicer RNase-III family, and exert their various functions through ubiquitous proteins called Argonaute (Ago). The surprising diversity of the Toxoplasma sRNAome uncovered in our study is consistent with those molecules exerting key functions during the parasite's life cycle, including, possibly, during virulent infection. The study also unravels an unsuspected level of complexity in the origin and mechanisms of action of the factors that generate and affect Toxoplasma sRNA, prompting a re-evaluation of our current views on RNA silencing in eukaryotes.

In vitro and in vivo assays of protein kinase CK2 activity

Protein kinase CK2 (formerly casein kinase 2) is recognized as a central component in the control of the cellular homeostasis; however, much remains unknown regarding its regulation and its implication in cellular transformation and carcinogenesis. Moreover, study of CK2 function and regulation in a cellular context is complicated by the dynamic multisubunit architecture of this protein kinase. Although a number of robust techniques are available to assay CK2 activity in vitro, there is a demand for sensitive and specific assays to evaluate its activity in living cells. We hereby provide a detailed description of several assays for monitoring the CK2 activity and its subunit interaction in living cells. The guidelines presented herein should enable researchers in the field to establish strategies for cellular screenings of CK2 inhibitors.

Structure-based discovery of small molecules targeting different surfaces of protein-kinase CK2

Protein kinase CK2 is an unfavorable pronostic marker in several cancers and has consequently emerged as a relevant therapeutic target. Several classes of ATP-competitive inhibitors have been identified, showing variable effectiveness. The molecular architecture of this multisubunit enzyme could offer alternative strategies to develop small molecule inhibitors targeting different surfaces of the kinase. Polyoxometalates were identified as original CK2 inhibitors targeting key structural elements located outside the active site. In addition, the CK2 subunit interface represents an exosite distinct from the catalytic cavity that can be targeted by peptides or small molecules to achieve functional effects.

Toxoplasma gondii gene expression is under the control of regulatory pathways acting through chromatin structure

The activity state of a gene is determined by a complex regulatory network of co-acting factors affecting the structure of the chromatin into which the gene is embedded. While significant changes of the transcriptome occur during cell differentiation in apicomplexan parasites, basic mechanisms controlling gene expression are still unknown. Recent studies support and expand the concept of the chromatin environment being key factor for the control of transcriptional activity in these lower eukaryotes organisms. Here, we review recent advances in the field of epigenetic gene regulation in Toxoplasma gondii, the model apicomplexan.

The histone methylase KMTox interacts with the redox-sensor peroxiredoxin-1 and targets genes involved in Toxoplasma gondii antioxidant defences

The ability of living cells to alter their gene expression patterns in response to environmental changes is essential for viability. Oxidative stress represents a common threat for all aerobic life. In normally growing cells, in which hydrogen peroxide generation is transient or pulsed, the antioxidant systems efficiently control its concentration. Intracellular parasites must also protect themselves against the oxidative burst imposed by the host. In this work, we have investigated the role of KMTox, a new histone lysine methyltransferase, in the obligate intracellular parasite Toxoplasma gondii. KMTox is a nuclear protein that holds a High Mobility Group domain, which is thought to recognize bent DNA. The enzyme methylates both histones H4 and H2A in vitro with a great preference for the substrate in reduced conditions. Importantly, KMTox interacts specifically with the typical 2-cys peroxiredoxin-1 and the binding is to some extent enhanced upon oxidation. It appears that the cellular functions that are primarily regulated by the KMTox are antioxidant defences and maintenance of cellular homeostasis. KMTox may regulate gene expression in T. gondii by providing the rapid re-arrangement of chromatin domains and by interacting with the redox-sensor TgPrx1 contribute to establish the antioxidant 'firewall' in T. gondii.

SET8-mediated methylations of histone H4 lysine 20 mark silent heterochromatic domains in apicomplexan genomes

Posttranslational histone modifications modulate chromatin-templated processes in various biological systems. H4K20 methylation is considered to have an evolutionarily ancient role in DNA repair and genome integrity, while its function in heterochromatin function and gene expression is thought to have arisen later during evolution. Here, we identify and characterize H4K20 methylases of the Set8 family in Plasmodium and Toxoplasma, two medically important members of the protozoan phylum Apicomplexa. Remarkably, parasite Set8-related proteins display H4K20 mono-, di-, and trimethylase activities, in striking contrast to the monomethylase-restricted human Set8. Structurally, few residues forming the substrate-specific channel dictate enzyme methylation multiplicity. These enzymes are cell cycle regulated and focally enriched at pericentric and telomeric heterochromatin in both parasites. Collectively, our findings provide new insights into the evolution of Set8-mediated biochemical pathways, suggesting that the heterochromatic function of the marker is not restricted to metazoans. Thus, these lower eukaryotes have developed a diverse panel of biological stages through their high capacity to differentiate, and epigenetics only begins to emerge as a strong determinant of their biology.