Phosphorylation of Tau Protein by CDK2/cyclin A and GSK3ß Recombinant Kinases: Analysis of Phosphorylation Patterns by Nuclear Magnetic Resonance Spectroscopy.

Posttranslational modifications (PTMs) of proteins can be investigated by Nuclear Magnetic Resonance (NMR) spectroscopy as a powerful analytical tool to define modification sites, their relative stoichiometry, and crosstalk between modifications. As a Structural Biology method, NMR provides important additional information on changes in protein conformation and dynamics upon modification as well as a mapping of binding sites upon biomolecular interactions. Indeed, PTMs not only mediate functional modulation in protein-protein interactions, but can also induce diverse structural responses with different biological outcomes. Here we present protocols that have been developed for the production and phosphorylation of the neuronal tau protein. Under its aggregated form, tau is a hallmark of Alzheimer's disease and other neurodegenerative diseases named tauopathies involving tau dysfunction and/or mutations. As a common feature shared by various tauopathies, tau aggregates are found into a form displaying an increased, abnormal phosphorylation, also referred to hyperphosphorylation. We have used NMR to investigate the phosphorylation patterns of tau induced by several kinases or cell extracts, how phosphorylation affects the local and overall conformation of tau, its interactions with partners (proteins, DNA, small-molecules, etc.) including tubulin and microtubules, and its capacity to form insoluble fibrillar aggregates. We present here detailed protocols for in vitro phosphorylation of tau by the recombinant kinases CDK2/cyclin A and GSK3ß, the production of the recombinant kinases thereof, as well as the analytical characterization of phosphorylated tau by NMR spectroscopy.

Skp2-Cyclin A Interaction Is Necessary for Mitotic Entry and Maintenance of Diploidy.

Skp2, the substrate recognition component of the SCFSkp2 ubiquitin ligase, has been implicated in the targeted destruction of a number of key cell cycle regulators and the promotion of S-phase. One of its critical targets is the Cyclin dependent kinase (Cdk) inhibitor p27, and indeed the overexpression of Skp2 in a number of cancers is directly correlated with the premature degradation of p27. Skp2 was first identified as a protein that interacts with Cyclin A in transformed cells, but its role in this complex has remained unclear. In this paper, we demonstrate that Skp2 interacts with Cyclin A in Drosophila and is required to maintain Cyclin A levels and permit mitotic entry. Failure of mitotic entry in Skp2 mutant cells results in polyploidy. If these cells enter mitosis again they are unable to properly segregate their chromosomes, leading to checkpoint dependent cell cycle arrest or apoptosis. Thus, Skp2 is required for mitosis and for maintaining diploidy and genome stability.

Enhanced Anti-tumor Effect of Flavopiridol in Combination With Gemcitabine in Pancreatic Cancer.

BACKGROUND/AIM: The efficacy of current chemotherapies for pancreatic ductal adenocarcinoma (PDAC) is still unsatisfactory. Flavopiridol inhibits multiple cyclin-dependent kinases, causing cell cycle arrest and inducing cancer cell apoptosis. This study aimed to evaluate the anti-tumor effect of flavopiridol and gemcitabine in PDAC in vitro and in vivo. MATERIALS AND METHODS: PANC-1 and MIA PaCa-2 cell lines were treated with gemcitabine and flavopiridol alone, in combination, and sequentially, and cell proliferation, apoptosis, and the cell cycle were evaluated. Proteins related to cell cycle progression (cyclin A, CDK2, E2F-1, and p53) were quantified using western blotting. A xenograft mouse model was generated, and the effects of gemcitabine and flavopiridol, administered alone or in combination, were evaluated by measuring tumor volume and apoptosis degree using the TUNEL assay. RESULTS: Sequential administration of gemcitabine and flavopiridol suppressed PDAC cell proliferation and induced apoptosis. Flavopiridol treatment led to an increase in the number of cells in the S and a decrease in those in the G0/G1 phases. Gemcitabine increased and decreased the number of S- and G2/M-phase cells, respectively. Furthermore, flavopiridol treatment decreased cyclin A and CDK2 expression and increased E2F-1 expression. In a xenograft mouse model, the combined administration of gemcitabine and flavopiridol demonstrated the most significant reduction in tumor volume and induction of apoptosis. CONCLUSION: Flavopiridol potentiates the anti-tumor activity of gemcitabine by inducing cell cycle arrest and apoptosis. Its synergistic inhibition of PDAC cell proliferation, when combined with gemcitabine, positions flavopiridol as a promising candidate for cancer treatment.

Microbiota derived D-malate inhibits skeletal muscle growth and angiogenesis during aging via acetylation of Cyclin A.

Metabolites derived from the intestinal microbiota play an important role in maintaining skeletal muscle growth, function, and metabolism. Here, we found that D-malate (DMA) is produced by mouse intestinal microorganisms and its levels increase during aging. Moreover, we observed that dietary supplementation of 2% DMA inhibits metabolism in mice, resulting in reduced muscle mass, strength, and the number of blood vessels, as well as the skeletal muscle fiber type I/IIb ratio. In vitro assays demonstrate that DMA decreases the proliferation of vascular endothelial cells and suppresses the formation of blood vessels. In vivo, we further demonstrated that boosting angiogenesis by muscular VEGFB injection rescues the inhibitory effects of D-malate on muscle mass and fiber area. By transcriptomics analysis, we identified that the mechanism underlying the effects of DMA depends on the elevated intracellular acetyl-CoA content and increased Cyclin A acetylation rather than redox balance. This study reveals a novel mechanism by which gut microbes impair muscle angiogenesis and may provide a therapeutic target for skeletal muscle dysfunction in cancer or aging.

Leukocyte-poor platelet-rich plasma and leukocyte-rich platelet-rich plasma promote myoblast proliferation through the upregulation of cyclin A, cdk1, and cdk2.

Muscle injuries are common among athletes and often treated with platelet-rich plasma (PRP). However, whether the leukocyte concentration affects the efficacy of PRP in treating muscle injuries remains unclear. This study investigated the effects of leukocyte-poor platelet-rich plasma (LP-PRP) and leukocyte-rich platelet-rich plasma (LR-PRP) on myoblast proliferation and the molecular mechanisms underlying these effects. Myoblasts were treated with 0.5% LP-PRP, 0.5% LR-PRP, 1% LP-PRP, or 1% LR-PRP for 24 h. The gene expression of the LP-PRP- and LR-PRP-treated myoblasts was determined using RNA sequencing analysis. Cell proliferation was evaluated using an bromodeoxyuridine (BrdU) assay, and cell cycle progression was assessed through flow cytometry. The expression of cyclin A, cyclin-dependent kinase 1 (cdk1), and cdk2 was examined using Western blotting. The expression of myoblast determination protein 1 (MyoD1) was examined through Western blotting and immunofluorescence staining. The LP-PRP and LR-PRP both promoted the proliferation of myoblasts and increased differential gene expression of myoblasts. Moreover, the LP-PRP and LR-PRP substantially upregulated the expression of cyclin A, cdk1, and cdk2. MyoD1 expression was induced in the LP-PRP and LR-PRP-treated myoblasts. Our results corroborate the finding that LP-PRP and LR-PRP have similar positive effects on myoblast proliferation and MyoD1 expression.

The no-observed-adverse-effect level of phthalates promotes proliferation and cell cycle progression in normal human breast cells.

OBJECTIVE: The data on the association between phthalates and breast cancer risk remains inconsistent. This study aimed to explore the possible mechanism of low-dose exposures of phthalates, including Butyl benzyl phthalate (BBP), di(n-butyl) phthalate (DBP), and di(20ethylhexyl) phthalate (DEHP), on breast tumorigenesis. METHODS AND METHODS: MCF-10A normal breast cells were treated with phthalates (10 and 100 nM) and 17ß-estradiol (E2, 10 nM), which were co-cultured with fibroblasts from normal mammary tissue. Cell viability, cycle, and apoptosis were detected by MTT assay, flow cytometry, and TUNEL assay respectively. The expression levels of related proteins were determined by Western blot. RESULTS: Like E2, both 10 nM and 100 nM phthalates exerted significantly higher cell viability, lower apoptosis, and increased cell numbers in the S and G2/M phases with up-regulation of cyclin D/CDK4, cyclin E/CDK2, cyclin A/CDK2, cyclin A/CDK1, and cyclin B/CDK1, compared with the control group. Significant increase in PDK1, P13K, p-AKT, p-mTOR, and BCL-2 expression and a decrease in Bax protein, cytochrome C, caspase 8, and caspase 3 levels were noted in cells treated with 10 nM and 100 nM phthalates and E2, compared with the control group and MCF-10A cells co-cultured with fibroblasts. The effects of the three phthalates were noted to be dose-dependent. CONCLUSIONS: The results indicate that phthalates at a level below its no-observed-adverse-effect concentration, as defined by the current standards, still induce cell cycle progression and proliferation as well as inhibit apoptosis of normal breast cells. Thus, the possibility of breast tumorigenesis through chronic phthalate exposure should be considered.

Cryo-EM structure of SKP1-SKP2-CKS1 in complex with CDK2-cyclin A-p27KIP1.

p27KIP1 (cyclin-dependent kinase inhibitor 1B, p27) is a member of the CIP/KIP family of CDK (cyclin dependent kinase) regulators that inhibit cell cycle CDKs. p27 phosphorylation by CDK1/2, signals its recruitment to the SCFSKP2 (S-phase kinase associated protein 1 (SKP1)-cullin-SKP2) E3 ubiquitin ligase complex for proteasomal degradation. The nature of p27 binding to SKP2 and CKS1 was revealed by the SKP1-SKP2-CKS1-p27 phosphopeptide crystal structure. Subsequently, a model for the hexameric CDK2-cyclin A-CKS1-p27-SKP1-SKP2 complex was proposed by overlaying an independently determined CDK2-cyclin A-p27 structure. Here we describe the experimentally determined structure of the isolated CDK2-cyclin A-CKS1-p27-SKP1-SKP2 complex at 3.4 Å global resolution using cryogenic electron microscopy. This structure supports previous analysis in which p27 was found to be structurally dynamic, transitioning from disordered to nascent secondary structure on target binding. We employed 3D variability analysis to further explore the conformational space of the hexameric complex and uncovered a previously unidentified hinge motion centred on CKS1. This flexibility gives rise to open and closed conformations of the hexameric complex that we propose may contribute to p27 regulation by facilitating recognition with SCFSKP2. This 3D variability analysis further informed particle subtraction and local refinement approaches to enhance the local resolution of the complex.

Regulation of the cell cycle and P13K/AKT/mTOR signaling pathway by phthalates in normal human breast cells.

OBJECTIVE: To investigate the impact of phthalates, including Butyl benzyl phthalate (BBP), di(n-butyl) phthalate (DBP), and di(2-ethylhexyl) phthalate (DEHP), in breast carcinogenesis. MATERIALS AND METHODS: MCF-10A normal breast cells were treated with phthalates (100 nM) and 17ß-estradiol (E2, 10 nM), which were co-cultured with fibroblasts from normal mammary tissue adjacent to estrogen receptor positive primary breast cancers. Cell viability was determined using a 3-(4,5-Dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay. Cell cycles were analyzed using flow cytometry. The proteins involving cell cycles and P13K/AKT/mTOR signaling pathway were then evaluated by Western blot analysis. RESULTS: MCF-10A co-cultured cells treated with E2, BBP, DBP, and DEHP exhibited a significant increase in cell viability using MTT assay. The expressions of P13K, p-AKT, and p-mTOR, as well as PDK1 expression, were significantly higher in MCF-10A cells treated with E2 and phthalates. E2, BBP, DBP, and DEHP significantly increased cell percentages in the S and G2/M phases. The significantly higher expression of cyclin D/CDK4, cyclin E/CDK2, cyclin A/CDK2, cyclin A/CDK1, and cyclin B/CDK1 in MCF-10A co-cultured cells were induced by E2 and these three phthalates. CONCLUSION: These results provide consistent data regarding the potential role of phthalates exposure in the stimulating proliferation of normal breast cells, enhancing cell viability, and driving P13K/AKT/mTOR signaling pathway and cell cycle progression. These findings strongly support the hypothesis that phthalates may play a crucial role in breast tumorigenesis.

Cyclin A participates in the TSO1-MYB3R1 regulatory module to maintain shoot meristem size and fertility in Arabidopsis.

The stem cell pools at the shoot apex and root tip give rise to all the above- and below-ground tissues of a plant. Previous studies in Arabidopsis identified a TSO1-MYB3R1 transcriptional module that controls the number and size of the stem cell pools at the shoot apex and root tip. As TSO1 and MYB3R1 are homologous to components of an animal cell cycle regulatory complex, DREAM, Arabidopsis mutants of TSO1 and MYB3R1 provide valuable tools for investigations into the link between cell cycle regulation and stem cell maintenance in plants. In this study, an Arabidopsis cyclin A gene, CYCA3;4, was identified as a member of the TSO1-MYB3R1 regulatory module and cyca3;4 mutations suppressed the tso1-1 mutant phenotype specifically in the shoot. The work reveals how the TSO1-MYB3R1 module is integrated with the cell cycle machinery to control cell division at the shoot meristem.

Cyclin A and Cks1 promote kinase consensus switching to non-proline-directed CDK1 phosphorylation.

Ordered protein phosphorylation by CDKs is a key mechanism for regulating the cell cycle. How temporal order is enforced in mammalian cells remains unclear. Using a fixed cell kinase assay and phosphoproteomics, we show how CDK1 activity and non-catalytic CDK1 subunits contribute to the choice of substrate and site of phosphorylation. Increases in CDK1 activity alter substrate choice, with intermediate- and low-sensitivity CDK1 substrates enriched in DNA replication and mitotic functions, respectively. This activity dependence is shared between Cyclin A- and Cyclin B-CDK1. Cks1 has a proteome-wide role as an enhancer of multisite CDK1 phosphorylation. Contrary to the model of CDK1 as an exclusively proline-directed kinase, we show that Cyclin A and Cks1 enhance non-proline-directed phosphorylation, preferably on sites with a +3 lysine residue. Indeed, 70% of cell-cycle-regulated phosphorylations, where the kinase carrying out this modification has not been identified, are non-proline-directed CDK1 sites.

Cyclin A-CDK1 suppresses the expression of the CDK1 activator CDC25A to safeguard timely mitotic entry.

Cyclin A and CDC25A are both activators of cyclin-dependent kinases (CDKs): cyclin A acts as an activating subunit of CDKs and CDC25A a phosphatase of the inhibitory phosphorylation sites of the CDKs. In this study, we uncovered an inverse relationship between the two CDK activators. As cyclin A is an essential gene, we generated a conditional silencing cell line using a combination of CRISPR-Cas9 and degron-tagged cyclin A. Destruction of cyclin A promoted an acute accumulation of CDC25A. The increase of CDC25A after cyclin A depletion occurred throughout the cell cycle and was independent on cell cycle delay caused by cyclin A deficiency. Moreover, we determined that the inverse relationship with cyclin A was specific for CDC25A and not for other CDC25 family members or kinases that regulate the same sites in CDKs. Unexpectedly, the upregulation of CDC25A was mainly caused by an increase in transcriptional activity instead of a change in the stability of the protein. Reversing the accumulation of CDC25A severely delayed G2-M in cyclin A-depleted cells. Taken together, these data provide evidence of a compensatory mechanism involving CDC25A that ensures timely mitotic entry at different levels of cyclin A.

Camptothecin Effectively Regulates Germline Differentiation through Bam-Cyclin A Axis in Drosophila melanogaster.

Camptothecin (CPT), first isolated from Chinese tree Camptotheca acuminate, produces rapid and prolonged inhibition of DNA synthesis and induction of DNA damage by targeting topoisomerase I (top1), which is highly activated in cancer cells. CPT thus exhibits remarkable anticancer activities in various cancer types, and is a promising therapeutic agent for the treatment of cancers. However, it remains to be uncovered underlying its cytotoxicity toward germ cells. In this study we found that CPT, a cell cycle-specific anticancer agent, reduced fecundity and exhibited significant cytotoxicity toward GSCs and two-cell cysts. We showed that CPT induced GSC loss and retarded two-cell cysts differentiation in a niche- or apoptosis-independent manner. Instead, CPT induced ectopic expression of a differentiation factor, bag of marbles (Bam), and regulated the expression of cyclin A, which contributed to GSC loss. In addition, CPT compromised two-cell cysts differentiation by decreasing the expression of Bam and inducing cell arrest at G1/S phase via cyclin A, eventually resulting in two-cell accumulation. Collectively, this study demonstrates, for the first time in vivo, that the Bam-cyclin A axis is involved in CPT-mediated germline stem cell loss and two-cell cysts differentiation defects via inducing cell cycle arrest, which could provide information underlying toxicological effects of CPT in the productive system, and feature its potential to develop as a pharmacology-based germline stem cell regulation agent.

Molecular mechanisms of bilirubin induced G1 cell cycle arrest and apoptosis in human breast cancer cell lines: involvement of the intrinsic pathway.

BACKGROUND: Bilirubin, as an essential constituent of cellular signaling pathways, may have a role in cell growth and apoptosis in breast cancer, although the biochemical relevance is still unclear. The purpose of the present study is to recognize the mechanism underlying bilirubin-induced apoptosis in breast cancer cell lines. METHODS AND RESULTS: To detect the cell viability, MTT assay was carried out. Apoptosis was assessed by flow cytometry analysis and caspase activities were determined by colorimetric method. The expression of AhR, cyclin D1, cyclin A, p53, p27, Bcl-2, and Bax were examined using real-time PCR. The cell viability has been reduced by bilirubin in a dose-dependent manner and an intrinsic apoptotic response has been occurred that was evidenced by the elevation of caspase-3 and - 9 activities. Bilirubin induced cell arrest in cell-cycle progression, which was associated with the induction of AhR expression, down-regulation of cyclin D1, cyclin A, and upregulation of p53 and p27 expression. Following bilirubin treatment, Bcl-2 was decreased and Bax protein was increased in both cell lines. CONCLUSIONS: To discuss, bilirubin, as a naturally occurring antiproliferative molecule, mediates growth inhibition by induction of cell cycle arrest and apoptosis in MCF-7 and MDA-MB-468 breast cancer cells. It is associated with the suppression of cyclin A, D1, and Bcl-2; induction of p53, p27, and Bax together with the activation of caspase-3 and - 9.

Cyclin-dependent kinase-mediated phosphorylation and the negative regulatory domain of transcription factor B-Myb modulate its DNA binding.

B-Myb is a highly conserved member of the vertebrate Myb family of transcription factors that plays a critical role in cell-cycle progression and proliferation. Myb proteins activate Myb-dependent promoters by interacting specifically with Myb-binding site (MBS) sequences using their DNA-binding domain (DBD). Transactivation of MBS promoters by B-Myb is repressed by its negative regulatory domain (NRD), and phosphorylation of the NRD by Cdk2-CyclinA relieves the repression to activate B-Myb-dependent promoters. However, the structural mechanisms underlying autoinhibition and activation of B-Myb-mediated transcription have been poorly characterized. Here, we determined that a region in the B-Myb NRD (residues 510-600) directly associates with the DBD and inhibits binding of the DBD to the MBS DNA sequence. We demonstrate using biophysical assays that phosphorylation of the NRD at T515, T518, and T520 is sufficient to disrupt the interaction between the NRD and the DBD, which results in increased affinity for MBS DNA and increased B-Myb-dependent promoter activation in cell assays. Our biochemical characterization of B-Myb autoregulation and the activating effects of phosphorylation provide insight into how B-Myb functions as a site-specific transcription factor.

Lidocaine Inhibited Tendon Cell Proliferation and Extracellular Matrix Production by Down Regulation of Cyclin A, CDK2, Type I and Type III Collagen Expression.

Lidocaine injection is a common treatment for tendon injuries. However, the evidence suggests that lidocaine is toxic to tendon cells. This study investigated the effects of lidocaine on cultured tendon cells, focusing on the molecular mechanisms underlying cell proliferation and extracellular matrix (ECM) production. Tendon cells cultured from rat Achilles tendons were treated with 0.5, 1.0, or 1.5 mg/mL lidocaine for 24 h. Cell proliferation was evaluated by Cell Counting Kit 8 (CCK-8) assay and bromodeoxyuridine (BrdU) assay. Cell apoptosis was assessed by Annexin V and propidium iodide (PI) stain. Cell cycle progression and cell mitosis were assessed through flow cytometry and immunofluorescence staining, respectively. The expression of cyclin E, cyclin A, cyclin-dependent kinase 2 (CDK2), p21, p27, p53, matrix metalloproteinases-2 (MMP-2), matrix metalloproteinases-9 (MMP-9), type I collagen, and type III collagen were examined through Western blotting, and the enzymatic activity of MMP-9 was determined through gelatin zymography. Lidocaine reduced cell proliferation and reduced G1/S transition and cell mitosis. Lidocaine did not have a significant negative effect on cell apoptosis. Lidocaine significantly inhibited cyclin A and CDK2 expression but promoted p21, p27, and p53 expression. Furthermore, the expression of MMP-2 and MMP-9 increased, whereas that of type I and type III collagen decreased. Lidocaine also increased the enzymatic activity of MMP-9. Our findings support the premise that lidocaine inhibits tendon cell proliferation by changing the expression of cell-cycle-related proteins and reduces ECM production by altering levels of MMPs and collagens.

Strongylopus grayii tadpole blastema extract exerts cytotoxic effects on embryonal rhabdomyosarcoma cells.

Amphibians have regenerative capacity and are resistant to developing cancer. This suggests that the developing blastema, located at the tissue regeneration site, may secrete anti-cancer factors. Here, we investigate the anti-cancer potential of tadpole tail blastema extracts (TAD) from the stream frog, Strongylopus grayii, in embryonal rhabdomyosarcoma (ERMS) cells. ERMS originates in skeletal muscle tissue and is a common pediatric soft tissue sarcoma. We show using MTT assays that TAD inhibited ERMS cell viability in a concentration-dependent manner, and phase contrast/fluorescent microscopy revealed that it induced morphological markers of senescence and apoptosis. Western blotting showed that this was associated with DNA damage (γH2AX) and activation of the p38/MAPK stress signaling pathway as well as molecular markers of senescence (p16INK4a), apoptosis (cleaved PARP), and inhibition of cell cycle promoters (cyclin A, CDK2, and cyclin B1). Furthermore, proteomics followed by gene ontology analyses showed that TAD treatment inhibited known tumor promoters and proteins required for cancer cell survival. Lastly, using the LINCS drug perturbation library, we show that there is an overlap between the proteomics signature induced by TAD and common anti-cancer drugs. Taken together, this study provides novel evidence that TAD exhibits cytotoxicity in ERMS cells.

P62/SQSTM1 mediates the autophagy-lysosome degradation of CDK2 protein undergoing PI3Kα/AKT T308 inhibition.

CDK2 forms a complex with cyclin A and cyclin E to promote the progress of cell cycle, but when cyclin A and cyclin E are dissociated from the complex and degraded by the ubiquitin proteasome pathway, the fate of the inactive CDK2 is unclear. In this study, we found that the inactive CDK2 protein was degraded by autophagy-lysosome pathway. In the classic model of G0/G1 phase arrest induced by serum starvation, we found that the mRNA level in CDK2 did not change but the protein level decreased. Subsequently, using PI3K and AKT inhibitors and gene knockout methods, it was found that CDK2 degradation was mediated by the inhibition of PI3Kα/AKTT308. In addition, P62/SQSTM1 was found to bind to the inactivated CDK2 protein to help it enter autophagy-lysosome degradation in a CTSB-dependent manner. Taken together, these results confirm that the PI3Kα/AKTT308 inhibition leads to degradation of CDK2 protein in the autophagy-lysosome pathway. These data reveal a new molecular mechanism of CDK2 protein degradation and provide a new strategy and method for regulating CDK2 protein.

Cortical Cyclin A controls spindle orientation during asymmetric cell divisions in Drosophila.

The coordination between cell proliferation and cell polarity is crucial to orient the asymmetric cell divisions to generate cell diversity in epithelia. In many instances, the Frizzled/Dishevelled planar cell polarity pathway is involved in mitotic spindle orientation, but how this is spatially and temporally coordinated with cell cycle progression has remained elusive. Using Drosophila sensory organ precursor cells as a model system, we show that Cyclin A, the main Cyclin driving the transition to M-phase of the cell cycle, is recruited to the apical-posterior cortex in prophase by the Frizzled/Dishevelled complex. This cortically localized Cyclin A then regulates the orientation of the division by recruiting Mud, a homologue of NuMA, the well-known spindle-associated protein. The observed non-canonical subcellular localization of Cyclin A reveals this mitotic factor as a direct link between cell proliferation, cell polarity and spindle orientation.

Inhibition of the CDK2 and Cyclin A complex leads to autophagic degradation of CDK2 in cancer cells.

Cyclin-dependent kinase 2 (CDK2) complex is significantly over-activated in many cancers. While it makes CDK2 an attractive target for cancer therapy, most inhibitors against CDK2 are ATP competitors that are either nonspecific or highly toxic, and typically fail clinical trials. One alternative approach is to develop non-ATP competitive inhibitors; they disrupt interactions between CDK2 and either its partners or substrates, resulting in specific inhibition of CDK2 activities. In this report, we identify two potential druggable pockets located in the protein-protein interaction interface (PPI) between CDK2 and Cyclin A. To target the potential druggable pockets, we perform a LIVS in silico screening of a library containing 1925 FDA approved drugs. Using this approach, homoharringtonine (HHT) shows high affinity to the PPI and strongly disrupts the interaction between CDK2 and cyclins. Further, we demonstrate that HHT induces autophagic degradation of the CDK2 protein via tripartite motif 21 (Trim21) in cancer cells, which is confirmed in a leukemia mouse model and in human primary leukemia cells. These results thus identify an autophagic degradation mechanism of CDK2 protein and provide a potential avenue towards treating CDK2-dependent cancers.

Daidzein and Chicory Extract Arrest the Cell Cycle via Inhibition of Cyclin D/CDK4 and Cyclin A/CDK2 Gene Expression in Hepatocellular Carcinoma.

BACKGROUND: Hepatocellular carcinoma (HCC) is one of the most common cancers, associated with a high rate of mortality. A disturbance between cell proliferation and cell death is one of the cancer hallmarks including HCC. Cell proliferation is mainly controlled by the cell cycle. The arrest of the cell cycle is one of the important targets of anticancer agents. OBJECTIVES: The present study tries to clarify the exact role of some natural products such as daidzein (DAZ) and alcoholic chicory leaf extract (CE), as possible regulators of cell cycle and apoptosis. METHODS: HCC in rats was induced using diethylnitrosamine (DENA). Ninety rats were allocated and divided equally into nine groups, treated with CE, DAZ, a combination of both, and sorafenib with non-treated control groups. RESULTS: Treatment with CE, DAZ, and their combination significantly downregulated hepatic tissue expression of cyclin D1/CDK4 axis as well as cyclin A/CDK2 axis. The suggested therapeutic protocol inhibited the proliferation and dampened Bcl-2 expression. Furthermore, the efficiency of combining CE and DAZ demonstrated a potency comparable to sorafenib in terms of cyclin D/CDK4 axis expression, as well as; this combination protocol was more potent in revealing a potentiated inhibitory effect on cyclin A and Ki-67 expression. CONCLUSION: Treatment with DAZ or CE alone, or in combination, could possess an inhibitory effect on hepatocarcinogenesis via cell cycle arrest, inhibition of proliferation through suppression of Ki-67 expression, and apoptosis induction, mediated by downregulation of Bcl-2.