Cyclin-dependent kinase inhibitors: novel anticancer agents

Radiotherapy and radio-sensitization in H3K27M -mutated diffuse midline gliomas

BACKGROUND

H3^K27M mutated diffuse midline gliomas (DMGs) are extremely aggressive and the leading cause of cancer-related deaths in pediatric brain tumors with 5-year survival <1%. Radiotherapy is the only established adjuvant treatment of H3^K27M DMGs; however, the radio-resistance is commonly observed.

METHODS

We summarized current understandings of the molecular responses of H3^K27M DMGs to radiotherapy and provide crucial insights into current advances in radiosensitivity enhancement.

RESULTS

Ionizing radiation (IR) can mainly inhibit tumor cell growth by inducing DNA damage regulated by the cell cycle checkpoints and DNA damage repair (DDR) system. In H3K27M DMGs, the aberrant genetic and epigenetic changes, stemness genotype, and epithelial-mesenchymal transition (EMT) disrupt the cell cycle checkpoints and DDR system by altering the associated regulatory signaling pathways, which leads to the development of radio-resistance.

CONCLUSIONS

The advances in mechanisms of radio-resistance in H3^K27M  DMGs promote the potential targets to enhance the sensitivity to radiotherapy.

Synthesis and Evaluation of Some New 4H-Pyran Derivatives as Antioxidant, Antibacterial and Anti-HCT-116 Cells of CRC, with Molecular Docking, Antiproliferative, Apoptotic and ADME Investigations

Colorectal cancer oncogenesis is linked to dysbiosis, oxidative stress and overexpression of CDK2. The 4H-pyran scaffold is considered an antitumoral, antibacterial and antioxidant lead as well as a CDK2 inhibitor. Herein, certain 4H-pyran derivatives were evaluated as antibacterial, antioxidant and cytotoxic agents against HCT-116 cells. Derivatives 4g and 4j inhibited all the tested Gram-positive isolates, except for B. cereus (ATCC 14579), with lower IC₅₀ values (µM) than ampicillin. In addition, 4g and 4j demonstrated the strongest DPPH scavenging and reducing potencies, with 4j being more efficient than BHT. In cell viability assays, 4d and 4k suppressed the proliferation of HCT-116 cells, with the lowest IC₅₀ values being 75.1 and 85.88 µM, respectively. The results of molecular docking simulations of 4d and 4k, inhibitory kinase assays against CDK2, along with determination of CDK2 protein concentration and the expression level of CDK2 gene in the lysates of HCT-116 treated cells, suggested that these analogues blocked the proliferation of HCT-116 cells by inhibiting kinase activity and downregulating expression levels of CDK2 protein and gene. Moreover, 4d and 4k were found to induce apoptosis in HCT-116 cells via activation of the caspase-3 gene. Lastly, compounds 4g, 4j, 4d and 4k were predicted to comply with Lipinski’s rule of five, and they are expected to possess excellent physiochemical and pharmacokinetic properties suitable for in vivo bioavailability, as predicted by the SwissADME web tool.

Antitumor Effects of Scorpion Peptide Smp43 through Mitochondrial Dysfunction and Membrane Disruption on Hepatocellular Carcinoma

Smp43, a cationic antimicrobial peptide identified from the venom gland of the Egyptian scorpion Scorpio maurus palmatus, shows cytotoxicity toward hepatoma cell line HepG2 by membrane disruption. However, its underlying detailed mechanisms still remain to be further clarified. In the present study, we evaluated the cellular internalization of Smp43 and explored its effects on cell viability, cell cycle, apoptosis, autophagy, necrosis, and factor expression related to these cellular processes in human HepG2. Smp43 was found to suppress the growth of HepG2, Huh7, and human primary hepatocellular carcinoma cells while showing low toxicity to normal LO2 cells. Furthermore, Smp43 could interact with the cell membrane and be internalized into HepG2 cells via endocytosis and pore formation, which caused a ROS production increase, mitochondrial membrane potential decline, cytoskeleton disorganization, dysregulation of cyclin expression, mitochondrial apoptotic pathway activation, and alteration of MAPK as well as PI3K/Akt/mTOR signaling pathways. Finally, Smp43 showed effective antitumor protection in the HepG2 xenograft mice model. Overall, these findings indicate that Smp43 significantly exerts antitumor effects via induction of apoptosis, autophagy, necrosis, and cell cycle arrest due to its induction of mitochondrial dysfunction and membrane disruption. This discovery will extend the antitumor mechanisms of antimicrobial peptides and contribute to the development of antitumor agents against hepatocellular carcinoma.

Fraxetin exerts anticancer effect in glioma by suppressing MiR-21-3p

Fraxetin (FXT) exerts anticancer function in multiple cancers, but its function on glioma was ill-defined. This article expounded the mechanism by which FXT exerts an anticancer effect in glioma. The effect of gradient concentration of FXT on the viability of glioma cell lines was determined by cell counting kit 8. Effects of FXT on proliferation, apoptosis, and cell cycle in glioma cell lines were determined by colony formation assay, flow cytometry, and Hoechst 33342 staining. Expressions of apoptosis-related gene, cycle-related gene, and glioma-related miRNAs after FXT (25 and 50 μmol/L) treatment were determined by quantitative reverse transcription polymerase chain reaction and western blot as needed. After miR-21-3p overexpression, cell viability and apoptosis of glioma cell lines treated with FXT (50 μmol/L) were tested again. Although 1 μmol/L FXT had no significant effect on cell viability, 5, 10, 25, and 50 μmol/L FXT suppressed cell viability in a concentration-dependent manner. FXT inhibited proliferation, promoted apoptosis, and induced cell cycle arrest in G0/G1 phase in glioma cell lines. These effects may be achieved by elevated expressions of Bax and cleaved caspase-3 and diminished expressions of Bcl-2, Bcl-XL, cyclin E1, cyclin D1, and cyclin-dependent kinase-6. FXT attenuated the contents of miR-21-3p and miR-455-3p, and escalated the contents of miR-124-3p and miR-7-5p. The regulation of FXT on cell viability, proliferation and apoptosis was reversed by miR-21-3p overexpression. FXT suppressed the development of glioma cells by downregulating miR-21-3p.

Screening of differentially expressed miRNAs in tensile strain‑treated HepG2 cells by miRNA microarray analysis

Cirrhosis and portal hypertension are associated with an increased risk of developing liver cancer. However, it is unknown how changes in the cellular mechanical microenvironment induced by portal hypertension affect the occurrence and development of liver cancer. The aim of this study was to determine the effect of tensile strain on the proliferation of a human liver cancer cell line (HepG2 cells) using methods such as flow cytometry, Cell Counting Kit-8 and 5-bromodeoxyuridine assays, and to examine the changes in microRNA (miRNA/miR) expression using microarray, reverse transcription-quantitative (RT-q)PCR and bioinformatics analyses. It was demonstrated that cyclic tensile force promoted the proliferation of HepG2 cells. The most suitable research conditions were as follows: Tensile strain force loading amplitude 15%; frequency 1 Hz; and time 24 h. After loading the HepG2 cells under such conditions, the differentially expressed miRNAs were screened out using an Agilent Human miRNA Microarray, identifying seven miRNAs with significant differences (expression difference >2 times and P<0.05). A total of five were upregulated, including hsa-miR-296-5p, hsa-miR-6752-5p, hsa-miR-6794-5p, hsa-miR-6889-5p and hsa-miR-7845-5p; and two were downregulated, hsa-miR-4428 and hsa-miR-503-5p. The results of RT-qPCR also further confirmed the expression changes of these miRNAs. Gene Ontology and pathway analyses showed the involvement of these miRNAs in numerous important physiological processes. These findings may provide novel miRNA-based information, thus enhancing the understanding of the pathophysiological processes leading to liver cancer.

Protein O-fucosyltransferase 1 promotes trophoblast cell proliferation through activation of MAPK and PI3K/Akt signaling pathways

Protein O-fucosylation is an important glycosylation modification and plays an important role in embryonic development. Protein O-fucosyltransferase 1 (poFUT1) is an essential enzyme that catalyzes the synthesis of protein O-fucosylation. Our previous studies showed that poFUT1 promoted trophoblast cell migration and invasion at the fetal-maternal interface, but the role of poFUT1 in trophoblast cells proliferation remains unclear. Here, immunohistochemistry data showed that poFUT1 and PCNA levels were decreased in abortion patient's trophoblasts compared with women with normal pregnancies. Our results also showed that poFUT1 promoted trophoblast cell proliferation by CCK-8 assay and cell cycle analysis. PoFUT1 increased the phosphorylation of ERK1/2, p38 MAPK, and PI3K/Akt, while inhibitors of ERK1/2(PD98059), p38 MAPK(SB203580), and PI3K (LY294002) prevented ERK1/2, p38 MAPK, and Akt phosphorylation. Moreover, poFUT1 stimulation of trophoblast cells proliferation correlated with increased cell cycle progression by promoting cells into S-phase. The underlying mechanism involved increased cyclin D1, cyclin E, CDK 2, CDK 4, and pRb expression and decreased levels of the cyclin-dependent kinase inhibitors p21 and p27, which were blocked by inhibitors of the upstream signaling molecules MAPK and PI3K/Akt. In conclusion, poFUT1 promotes trophoblast cell proliferation by activating MAPK and PI3K/Akt signaling pathways.

Role of apoptosis-related miRNAs in resveratrol-induced breast cancer cell death

Breast cancer is the most frequently diagnosed cancer in women, and one of the leading causes of cancer-related deaths worldwide. Recent evidences indicate that dietary agents such as resveratrol may inhibit cancer progression through modulation of microRNAs (miRNAs). We demonstrate that resveratrol regulates apoptotic and cell cycle machinery in breast cancer cells by modulating key tumor-suppressive miRNAs including miR-125b-5p, miR-200c-3p, miR-409-3p, miR-122-5p and miR-542-3p. Resveratrol-mediated miRNA modulation regulates key anti-apoptotic and cell cycle proteins including Bcl-2, X-linked inhibitor of apoptosis protein and CDKs, which are critical for its activity. Modulating miRNAs with mimics or inhibitors further validated a key role for miR-542-3p in MCF-7 and miR-122-5p in MDA-MB-231 breast cancer cell death in response to resveratrol. In conclusion, this study reveals novel miRNAs modulated by resveratrol that have a key role in breast cancer cell death.

The best of both worlds - managing the cancer, saving the bone

In the context of breast cancer, the importance of the skeleton in the regulation of primary tumour development and as a site for subsequent metastasis is well characterized. Our understanding of the contributions made by the host bone and bone marrow cells increasingly demonstrates the extent of the interaction between tumour cells and normal host cells. As a result, the need to develop and utilize therapies that can impede the growth and/or function of tumour cells while sparing normal host bone and bone marrow cells is immense and expanding. The need for these new treatments is, however, superimposed on the orthopaedic management of patients' quality of life, where pain control and continued locomotion are paramount. Indeed, the majority of the anticancer therapies used to date often result in direct or indirect damage to bone. Thus, although the bone microenvironment regulates tumour cell growth in bone, cells within the bone marrow niche also mediate many of the orthopaedic consequences of tumour progression as well as resistance to the antitumour effects of existing therapies. In this Review, we highlight the effects of existing cancer treatments on bone and the bone marrow microenvironment as well as the mechanisms mediating these effects and the current utility of modern orthopaedic interventions.

Extra precision docking, free energy calculation and molecular dynamics simulation studies of CDK2 inhibitors

Molecular docking, free energy calculation and molecular dynamics (MD) simulation studies have been performed, to explore the putative binding modes of 3,5-diaminoindazoles, imidazo(1,2-b)pyridazines and triazolo(1,5-a) pyridazines series of Cyclin-dependent kinase (CDK2) inhibitors. To evaluate the effectiveness of docking protocol in flexible docking, we have selected crystallographic bound compound to validate our docking procedure as evident from root mean square deviations (RMSDs). We found different binding sites namely catalytic, inhibitory phosphorylation, cyclin binding and CKS-binding site of the CDK2 contributing towards the binding of these compounds. Moreover, correlation between free energy of binding and biological activity yielded a statistically significant correlation coefficient. Finally, three representative protein-ligand complexes were subjected to molecular dynamics simulation to determine the stability of the predicted conformations. The low value of the RMSDs between the initial complex structure and the energy minimized final average complex structure suggests that the derived docked complexes are close to equilibrium. We suggest that the phenylacetyl type of substituents and cyclohexyl moiety make the favorable interactions with a number of residues in the active site, and show better inhibitory activity to improve the pharmacokinetic profile of compounds against CDK2. The structure-based drug design strategy described in this study will be highly useful for the development of new inhibitors with high potency and selectivity.

UCN-01 induces S and G2/M cell cycle arrest through the p53/p21(waf1) or CHK2/CDC25C pathways and can suppress invasion in human hepatoma cell lines

BACKGROUND

UCN-01 (7-hydroxystaurosporine), a protein kinase inhibitor, has attracted a great deal of attention as a potent antitumour agent. Several clinical trials of UCN-01 alone or in combination with other agents for different tumour types are currently underway, and some of these trials have had positive results. Hepatocellular carcinoma has high incidence rates and is associated with poor prognosis and high mortality rates.

METHODS

Three different hepatoma cell lines (Huh7, HepG2, and Hep3B) were treated with different concentrations of UCN-01, and the anti-tumour effects of UCN-01 were evaluated. Following UCN-01 treatment, cell growth was measured using an MTT assay, cell cycle arrest was assayed using flow cytometry, and the mechanisms of cell cycle arrest and invasion inhibition were investigated through western blotting and a Matrigel invasion assay.

RESULTS

After a 72-h UCN-01 treatment, the growth of different hepatoma cell lines was significantly inhibited in a dose-dependent manner, with IC50 values ranging from 69.76 to 222.74 nM. Flow cytometry results suggested that UCN-01 inhibits proliferation in the hepatoma cells by inducing S and G2/M phase arrest, but not G1/S arrest, which differs from previous reports that used other tumour cell lines. Western blot results illustrated that UCN-01 induces a G2/M phase arrest, regardless of the status of the p53/P21(waf1) pathway, whereas the CHK2/CDC25C pathway and the p53/p21(waf1)pathway were involved in the UCN-01-induced S phase arrest. UCN-01 remarkably inhibited Huh7 cell invasion in a time-dependent manner. Suppression of Huh7 cell invasion may be due to the down-regulation of phosphorylated β-catenin by UCN-01.

CONCLUSIONS

These findings suggest that UCN-01 induces hepatoma cell growth inhibition by regulating the p53/p21(waf1) and CHK2/CDC25 pathways. Suppression of Huh7 cell invasion by UCN-01 may be due to the down-regulation of phosphorylated β-catenin. These data lend support for further studies on UCN-01 as a promising anti-HCC candidate.

H-RN, a peptide derived from hepatocyte growth factor, inhibits corneal neovascularization by inducing endothelial apoptosis and arresting the cell cycle

BACKGROUND

The goal of this study was to investigate the anti-angiogenic activity of a novel peptide H-RN, derived from the hepatocyte growth factor kringle 1 domain (HGF K1), in a mouse model of corneal neovascularization. The anti-angiogenic effect of H-RN on vascular endothelial growth factor (VEGF)-stimulated cell proliferation, cell migration and endothelial cell tube formation was assessed in vitro using Human Umbilical Vein Endothelial Cells (HUVECs) and in vivo using a mouse cornea micropocket assay. Apoptosis and cell cycle arrest were assessed by flow cytometry. A scrambled peptide was used as a negative control.

RESULTS

H-RN effectively inhibited VEGF-stimulated HUVEC proliferation, migration and tube formation on Matrigel, while a scrambled peptide exerted no effect. In the mouse model of corneal angiogenesis, VEGF-stimulated angiogenesis was significantly inhibited by H-RN compared to a scrambled peptide that had no such activity. VEGF protected HUVECs from apoptosis, while H-RN inhibited this protective effect of VEGF. VEGF significantly increased the proportion of cells in the S phase compared to control treated cells (p<0.05). Treatment with H-RN (1.5 mM) induced the accumulation of cells in G0/G1 phase, while the proportion of cells in the S phase and G2/M phase decreased significantly compared to control group (p<0.05).

CONCLUSIONS

H-RN has anti-angiogenic activity in HUVECs and in a mouse model of VEGF-induced corneal neovascularization. The anti-angiogenic activity of H-RN was related to apoptosis and cell cycle arrest, indicating a potential strategy for anti-angiogenic treatment in the cornea.

Constitutive CCND1/CDK2 activity substitutes for p53 loss, or MYC or oncogenic RAS expression in the transformation of human mammary epithelial cells

Cancer develops following the accumulation of genetic and epigenetic alterations that inactivate tumor suppressor genes and activate proto-oncogenes. Dysregulated cyclin-dependent kinase (CDK) activity has oncogenic potential in breast cancer due to its ability to inactivate key tumor suppressor networks and drive aberrant proliferation. Accumulation or over-expression of cyclin D1 (CCND1) occurs in a majority of breast cancers and over-expression of CCND1 leads to accumulation of activated CCND1/CDK2 complexes in breast cancer cells. We describe here the role of constitutively active CCND1/CDK2 complexes in human mammary epithelial cell (HMEC) transformation. A genetically-defined, stepwise HMEC transformation model was generated by inhibiting p16 and p53 with shRNA, and expressing exogenous MYC and mutant RAS. By replacing components of this model, we demonstrate that constitutive CCND1/CDK2 activity effectively confers anchorage independent growth by inhibiting p53 or replacing MYC or oncogenic RAS expression. These findings are consistent with several clinical observations of luminal breast cancer sub-types that show elevated CCND1 typically occurs in specimens that retain wild-type p53, do not amplify MYC, and contain no RAS mutations. Taken together, these data suggest that targeted inhibition of constitutive CCND1/CDK2 activity may enhance the effectiveness of current treatments for luminal breast cancer.

Stat6 cooperates with Sp1 in controlling breast cancer cell proliferation by modulating the expression of p21(Cip1/WAF1) and p27 (Kip1)

BACKGROUND

The signal transducer and activator of transcription 6 (Stat6), a member of the family of DNA-binding proteins, has been identified as a critical cell differentiation modulator in breast cancer cells. As of yet, the mechanisms underlying this function have remained largely unknown. To further elucidate the role of Stat6 in breast cancer development, we investigated the consequences of exogenous Stat6 expression.

METHODS

Proliferation assays and flow cytometry assays were conducted to evaluate the putative role of Stat6 on cell proliferation. To this end, we produced synchronized cells after a double thymidine block, as confirmed by FACS analysis. mRNA levels of Stat6 were measured by RNase protection analysis. To confirm the interaction among proteins, we employed GST pull-down assays and immunoprecipitation assays. Luciferase assays and ChIP assays were used to assess the transcriptional activity.

RESULTS

Compared to control breast cancer cells, we found that exogenous Stat6 expression plays a critical role in controlling cell proliferation. Also in different breast tumor cell lines, endogenous Stat6 expression was found to be positively related to a lower proliferation rate. Interestingly, in human breast cancer cells Stat6 functions in G1/S cell cycle progression, and the growth-inhibitory effect of Stat6 was shown to be mediated by induction of the G1 cyclin-dependent kinase inhibitors p21(Cip1/WAF1) (p21) and p27(Kip1) (p27). Simultaneously, G1-related cyclin/cyclin-dependent kinase activities and pRB phosphorylation were markedly reduced, and cell cycle progression was blocked in the G1 phase. Stat6 knockdown resulted in enhanced cell proliferation and a decrease in p21 and p27 mRNA levels in the steroid-responsive and non-responsive T-47D and MDA-MB-231 cell lines, respectively. In addition, the stimulatory effect of Stat6 on p21 and p27 gene transcription was found to be associated with interaction of Stat6 with the transcription factor Sp1 at the proximal Sp1-binding sites in their respective promoters.

CONCLUSIONS

Together, these results identify Stat6 as an important cell differentiation regulatory protein functioning, at least in part, by interacting with Sp1 to activate the p21 and p27 gene promoters in breast cancer cells.

Euphorbia mauritanica and Kedrostis hirtella extracts can induce anti-proliferative activities in lung cancer cells

Cancer is a public health problem in the world accounting for most of the deaths. Currently, common treatment of cancer such as chemotherapy works by killing fast-growing cancer cells. Unfortunately, chemotherapy cannot tell the difference between cancer cells and fast-growing healthy cells, including red and white blood cells. As a result, one of the most serious potential side effects of some types of chemotherapy is a low white blood cell count that makes it unreliable (Parkin et al. [34]; Pauk et al. [3]). Even though intense research has been going on in recent years, successful therapeutic targets against this disease have been elusive. In this study, we evaluate the anti-proliferative activity of Euphorbia mauritanica and Kedrostis hirtella in lung cancer. In our assessment it was observed that E. mauritanica and K. hirtella were able to induce cell death at 5 μg/ml in A549 cells over 22 h and at 10 μg/ml over 24 h in the Lqr1 cell line. Molecular analysis of DNA fragmentation and Annexin V were used to examine the type of cell death induced by E. mauritanica and K. hirtella extracts. These results showed an increase in necrotic and apoptotic characteristics with both nuclear DNA fragmentation and smear. Therefore, these results suggest that E. mauritanica and K. hirtella may play a role in inducing cell death in lung cancer cells. However, further studies need to be conducted to ascertain these results.

Activation of JNK/c-Jun is required for the proliferation, survival, and angiogenesis induced by EET in pulmonary artery endothelial cells

Pulmonary artery endothelial plexiform lesion is responsible for pulmonary vascular remodeling (PVR), a basic pathological change of pulmonary arterial hypertension (PAH). Recent evidence suggests that epoxyeicosatrienoic acid (EET), which is derived from arachidonic acid by cytochrome p450 (CYP) epoxygenase, has an essential role in PAH. However, until now, most research has focused on pulmonary vasoconstriction; it is unclear whether EET produces mitogenic and angiogenic effects in pulmonary artery endothelial cells (PAEC). Here we found that 500 nM/l 8,9-EET, 11,12-EET, and 14,15-EET markedly augmented JNK and c-Jun activation in PAECs and that the activation of c-Jun was mediated by JNK, but not the ERK or p38 MPAK pathway. Moreover, treatment with 8,9-EET, 11,12-EET, and 14,15-EET promoted cell proliferation and cell-cycle transition from the G0/G1 phase to S phase and stimulated tube formation in vitro. All these effects were reversed after blocking JNK with Sp600125 (a JNK inhibitor) or JNK1/2 siRNA. In addition, the apoptotic process was alleviated by three EET region isomers through the JNK/c-Jun pathway. These observations suggest that 8,9-EET, 11,12-EET, and 14,15-EET stimulate PAEC proliferation and angiogenesis, as well as protect the cells from apoptosis, via the JNK/c-Jun pathway, an important underlying mechanism that may promote PAEC growth and angiogenesis during PAH.

Sox2 targets cyclinE, p27 and survivin to regulate androgen-independent human prostate cancer cell proliferation and apoptosis

OBJECTIVES

Sox2 is a major transcription factor and the transforming growth factor-α (TGF-α)/EGFR autocrine loop is a hallmark of prostate cancer progression. In this study, we have evaluated the effects and potential mechanisms of Sox2 on cell proliferation and apoptosis, and investigated effects of TGF-α on expression of Sox2 on androgen-independent human prostate cancer cells.

MATERIALS AND METHODS

Expression of Sox2 has been determined by RT-PCR, western blot analysis and immunocytochemistry, using RNAi and over-expression strategy to study functions of Sox2 in DU145 and PC-3 cells. Changes in level of proliferation, cell cycle and apoptosis profiles were measured by MTT, colony-forming, bromodeoxyuridine incorporation assays, cell cycle and annexin V analysis.

RESULTS

Sox2 was expressed in six human prostate cancer cell lines, and its inhibition reduced cell proliferation and induced apoptosis in DU145 cells. We have shown that knock-down of Sox2 inhibited G(1) to S phase transition concomitantly with down-regulation of cyclin E and up-regulation of p27 proteins. Conversely, over-expression of Sox2 led to the opposite effect in PC-3 cells but its inhibition induced apoptosis by down-regulation of survivin in DU145 cells. We also found that TGF-α up-regulated Sox2 and survivin protein expression via the EGFR/PI3K/AKT pathway.

CONCLUSIONS

Sox2 expression is necessary for cell proliferation and evasion of apoptosis in prostate cancer cells and TGF-α could regulate Sox2 and survivin expression by activating the EGFR/PI3K/AKT pathway.

Effects of an Indolocarbazole-Derived CDK4 Inhibitor on Breast Cancer Cells

Introduction: Cyclin D1 (D1) binds to cyclin-dependent kinases (CDK) 4 or 6 to form a holoenzyme that phosphorylates the Rb protein to promote cell cycle progression from G1 to S phase. Therefore, targeting CDK4/6 may be a good strategy for chemotherapy of cancer. We performed a proof-of-principle study to determine the effect of Naphtho [2, 1-α] pyrrolo [3, 4-c] carbazole-5, 7 (6H, 12H)-dione (NPCD), a novel CDK4 inhibitor, on breast cancer cell lines.

Methods: NPCD was synthesized and purified to over 99% purity verified by HPLC. MCF7, MB231, MCF15, T47D and GI101Ap human breast cancer cells were analyzed for the efficacy of NPCD with MTT and clonogenic assays, with FACS and staining for ethidium bromide and acridine orange for cell death and cell cycle profile. Western blot, reverse transcription and PCR were used for studies of gene expression, and co-immunoprecipitation for protein-complex formation.

Results: MTT assay showed that NPCD caused growth arrest and apoptosis of MCF7, MDA-MB231, T47D, MCF15 and GI101Ap cells with an IC50 ranging between 3 to 8 µM given as a single dose. The growth arrest persisted for many days after cessation of the treatment, as shown in a clonogenic assay. NPCD could induce or reduce the D1 and CDK4 protein levels, depending on the cell line, but this effect was not correlated with its efficacy. Phosphorylation of D1 at Thr286 was decreased but it unexpectedly did not correlate with the change in D1 level in the cell lines studied. Phosphorylation of the Rb protein was decreased as expected whereas the p27kip1 protein level was decreased unexpectedly. Protein levels of p21cip1, CDK2 and cyclin E were also decreased in some, but not all, of the cell lines, whereas the mRNA levels of D1, CDK4, cyclin E, CDK2, p27kip1 and p21cip1 were increased in different cell lines.

Conclusions: NPCD can cause long-lasting growth arrest and cell death of breast cancer cell lines at an IC50 of 3-8 µM. Decreased phosphorylation of Rb by D1-CDK4/6 and decreased p27kip1 protein level may be part of the underlying mechanism.

Overexpression of fucosyltransferase IV promotes A431 cell proliferation through activating MAPK and PI3K/Akt signaling pathways

Lewis Y (LeY) is a carbohydrate tumor-asssociated antigen. The majority of cancer cells derived from epithelial tissue express LeY type difucosylated oligosaccharide. Fucosyltransferase IV (FUT4) is an essential enzyme that catalyzes the synthesis of LeY oligosaccharide. Our previous studies have shown that FUT4 overexpression promotes A431 cell proliferation, but the mechanism is still largely unknown. Herein, we investigated the role of the mitogen-activated protein kinases (MAPKs) and phosphoinositide-3 kinase (PI3K)/Akt signaling pathways on FUT4-induced cell proliferation. Results show that overexpression of FUT4 increases the phosphorylation of ERK1/2, p38 MAPK, and PI3K/Akt. Inhibitors of PI3K (LY294002 and Wortmannin) prevented the phosphorylation of ERK1/2, p38 MAPK, and Akt PI3K). Moreover, phosphorylation of Akt is abolished by inhibitors of ERK1/2 (PD98059) and p38 MAPK (SB203580). These data suggested that FUT4 not only activates MAPK and PI3K/Akt signals, but also promotes the crosstalk among these signaling pathways. In addition, FUT4-induced stimulation of cell proliferation correlates with increased cell cycle progression by promoting cells into S-phase. The mechanism involves in increased expression of cyclin D1, cyclin E, CDK 2, CDK 4, and pRb, and decreased level of cyclin-dependent kinases inhibitors p21 and p27, which are blocked by the inhibitors of upstream signal molecules, MAPK and PI3K/Akt. In conclusion, these studies suggest that FUT4 regulates A431 cell growth through controlling cell cycle progression via MAPK and PI3K/Akt signaling pathways.

Cyclin-dependent kinase inhibitor SU9516 enhances sensitivity to methotrexate in human T-cell leukemia Jurkat cells

Methotrexate (MTX) has been used to treat various hematological malignancies. Since MTX prevents tumor cells from proliferating by inhibiting dihydrofolate reductase (DHFR), DHFR expression is a key determinant of resistance to MTX in malignant hematological tumor cells. The antiproliferative effect of MTX was significantly enhanced by the knockdown of DHFR expression by siRNA in Jurkat cells. Therefore, a novel strategy down-regulating DHFR expression seems promising for enhancing sensitivity to MTX. We found that SU9516, a cyclin-dependent kinase inhibitor, reduced the expression of both DHFR mRNA and protein. Moreover, we found that DHFR promoter activity was attenuated by SU9516 dependent on the E2F site. Finally, pretreatment with SU9516 significantly enhanced sensitivity to MTX in a colony formation assay. We conclude that a combination of cyclin-dependent kinase inhibitors and MTX may be useful for overcoming resistance to MTX.

The Bmi-1 polycomb protein antagonizes the (-)-epigallocatechin-3-gallate-dependent suppression of skin cancer cell survival

The polycomb group (PcG) proteins are epigenetic regulators of gene expression that enhance cell survival. This regulation is achieved via action of two multiprotein PcG complexes--PRC2 (EED) and PRC1 [B-cell-specific Moloney murine leukemia virus integration site 1 (Bmi-1)]. These complexes modulate gene expression by increasing histone methylation and reducing acetylation--leading to a closed chromatin conformation. Activity of these proteins is associated with increased cell proliferation and survival. We show increased expression of key PcG proteins in immortalized keratinocytes and skin cancer cell lines. We examine the role of two key PcG proteins, Bmi-1 and enhancer of zeste homolog 2 (Ezh2), and the impact of the active agent in green tea, (-)-epigallocatechin-3-gallate (EGCG), on the function of these regulators. EGCG treatment of SCC-13 cells reduces Bmi-1 and Ezh2 level and this is associated with reduced cell survival. The reduction in survival is associated with a global reduction in histone H3 lysine 27 trimethylation, a hallmark of PRC2 complex action. This change in PcG protein expression is associated with reduced expression of key proteins that enhance progression through the cell cycle [cyclin-dependent kinase (cdk)1, cdk2, cdk4, cyclin D1, cyclin E, cyclin A and cyclin B1] and increased expression of proteins that inhibit cell cycle progression (p21 and p27). Apoptosis is also enhanced, as evidenced by increased caspase 9, 8 and 3 cleavage and increased poly(adenosine diphosphate ribose) polymerase cleavage. EGCG treatment also increases Bax and suppresses Bcl-xL expression. Vector-mediated enhanced Bmi-1 expression reverses these EGCG-dependent changes. These findings suggest that green tea polyphenols reduce skin tumor cell survival by influencing PcG-mediated epigenetic regulatory mechanisms.

Oligosaccharides from human milk induce growth arrest via G2/M by influencing growth-related cell cycle genes in intestinal epithelial cells

Oligosaccharides are present in human milk in large amounts and in a high variety. We have previously shown that these oligosaccharides are strong inhibitors of proliferation and inducers of differentiation in intestinal cell lines. To elucidate the molecular mechanism, we investigated the influence on cell cycle events via flow cytometry and expression levels by using quantitative real-time RT-PCR. Human intestinal cells, i.e. HT-29, HIEC and Caco-2 cells, were exposed to neutral or acidic human milk oligosaccharides. Both fractions induced a concentration-dependent G2/M arrest. Cell cycle analysis for HT-29 revealed 37 % of cells in G1 and 35 % in G2/M (neutral oligosaccharides) and incubation with acidic oligosaccharides led to 42 % cells in G1 and 40 % in G2/M. In control experiments without oligosaccharides we found 71 % of cells to be in G1 and 17 % in G2/M. This G2/M arrest was associated with changes in mRNA expression of cyclin A and B. A G2/M arrest with concomitant alterations of cell cycle gene expression could also be shown for HIEC and Caco-2 cells. Analysing the expression of cyclin-dependent kinase inhibitors p21(cip1) and p27(kip1) and the tumour suppressor p53 we observed that the expression of p21(cip1) was p53-independent and necessary for arresting cells in the G2/M phase, while p27(kip1) was associated with differentiation effects. Both neutral and acidic human milk oligosaccharides were able to induce epidermal growth factor receptor, extracellular signal-regulated kinase 1/2 and p38 phosphorylation. These results suggest that oligosaccharides from human milk inhibited intestinal cell proliferation and altered cell cycle dynamics by affecting corresponding regulator genes and mitogen-activated protein kinase signalling.

Identifying the differential effects of silymarin constituents on cell growth and cell cycle regulatory molecules in human prostate cancer cells

Prostate cancer (PCa) is the leading cause of cancer-related deaths in men; urgent measures are warranted to lower this deadly malignancy. Silymarin is a known cancer chemopreventive agent, but the relative anticancer efficacy of its constituents is still unknown. Here, we compared the efficacy of 7 pure flavonolignan compounds isolated from silymarin, namely silybin A, silybin B, isosilybin A, isosilybin B, silydianin, isosilydianin, silychristin and isosilychristin, in advanced human PCa PC3 cells. Silybin A, silybin B, isosilybin A, isosilybin B, silibinin and silymarin strongly inhibited the colony formation by PC3 cells (p < 0.001), while silydianin, silychristin and isosilychristin had marginal effect (p < 0.05). Using cell growth and death assays, we identified isosilybin B as the most effective isomer. FACS analysis for cell cycle also showed that silybin A, silybin B, isosilybin A, isosilybin B, silibinin and silymarin treatment resulted in strong cell cycle arrest in PC3 cells after 72 hr of treatment, while the effect of silydianin, silychristin and isosilychristin was marginal (if any). Western blot analysis also showed the differential effect of these compounds on the levels of cell cycle regulators-cyclins (D, E, A and B), CDKs (Cdk2, 4 and Cdc2), CDKIs (p21 and p27) and other cell cycle regulators (Skp2, Cdc25A, B, C and Chk2). This study provided further evidence for differential anticancer potential among each silymarin constituent, which would have potential implications in devising better formulations of silymarin against prostate and other cancers.

Synthesis and biological evaluation of 3,5-diaminoindazoles as cyclin-dependent kinase inhibitors

A novel series of 3,5-diaminoindazoles were prepared and found to be CDK inhibitors. Potent inhibitors against CDK1 and CDK2 were obtained by introduction of 1lambda(6)-isothiazolidine-1,1-dioxide at 5-position of indazole. Anti-proliferative activities of compounds were evaluated using EJ, HCT116, SW620, and A549 cancer cell lines.

Molecular-targeted therapies: lessons from years of clinical development

Over the past decade, molecular-targeted therapies have been added to cytotoxic and anti-endocrine drugs in the treatment of cancer, with the aim to target the molecular pathways that underlie the carcinogenic process and maintain the cancer phenotype. Success with some of these agents has suggested that identification and validation of the drug target is the starting point for the route of development of active, safe and effective drugs. Main molecular targets used to the development of anticancer drugs are cell surface receptors, signal transduction pathways, gene transcription targets, ubiquitin-proteasome/heat shock proteins and tumour microenvironment components (especially antiangiogenic agents). Here, we review the development of the main molecular targeted non-cytotoxic agents studied in cancer, highlighting lessons derived from the development of these novel drugs and proposing new horizons for the clinical development of molecular-targeted therapies.

Synthesis and activity of quinolinyl-methylene-thiazolinones as potent and selective cyclin-dependent kinase 1 inhibitors

A novel series of quinolinyl-methylene-thiazolinones has been identified as potent and selective cyclin-dependent kinase 1 (CDK1) inhibitors. Their synthesis and structure activity relationships (SAR) are described. Representative compounds from this class reversibly inhibit CDK1 activity in vitro, and block cell cycle progression in human tumor cell lines, suggesting a potential use as antitumor agents.

3-Hydroxychromones as cyclin-dependent kinase inhibitors: synthesis and biological evaluation

A novel series of 3-hydroxychromones were prepared and found to be CDK inhibitors. Isothiazolidine 1,1-dioxide analogues showed potent CDK1 and CDK2 inhibitory activities and inhibited proliferation of EJ, HCT116, SW620, and MDAMB468 cancer cells.

Design, synthesis, and antiproliferative and CDK2-cyclin a inhibitory activity of novel flavopiridol analogues

The design and synthesis of a small library of 8-amidoflavone, 8-sulfonamidoflavone, 8-amido-7-hydroxyflavone, and heterocyclic analogues of flavopiridol is reported. The potential activity of these compounds as kinase inhibitors was evaluated by cytotoxicity studies in MCF-7 and ID-8 cancer cell lines and inhibition of CDK2-Cyclin A enzyme activity in vitro. The antiproliferative and CDK2-Cyclin A inhibitory activity of these analogues was significantly lower than the activity of flavopiridol. Molecular docking simulations were carried out and these studies suggested a different binding orientation inside the CDK2 binding pocket for these analogues compared to flavopiridol.

Cheminformatics analysis and learning in a data pipelining environment

Workflow technology is being increasingly applied in discovery information to organize and analyze data. SciTegic's Pipeline Pilot is a chemically intelligent implementation of a workflow technology known as data pipelining. It allows scientists to construct and execute workflows using components that encapsulate many cheminformatics based algorithms. In this paper we review SciTegic's methodology for molecular fingerprints, molecular similarity, molecular clustering, maximal common subgraph search and Bayesian learning. Case studies are described showing the application of these methods to the analysis of discovery data such as chemical series and high throughput screening results. The paper demonstrates that the methods are well suited to a wide variety of tasks such as building and applying predictive models of screening data, identifying molecules for lead optimization and the organization of molecules into families with structural commonality.

A study of cytotoxic synergy of UCN-01 and flavopiridol in syngeneic pair of cell lines

Flavopiridol and UCN-01 are two novel protein kinase inhibitors with diverse cellular effects that may complement each other with regards to induction of apoptosis. HeLa cells engineered to overexpress human survivin (HeLa-S) were at least approximately 4.8-fold resistant to UCN-01 relative to proliferation observed in control HeLa cells (HeLa-V). Flavopiridol cytotoxicity as measured using the MTT assay was unaffected in HeLa-S cells when compared with HeLa-V cells. Similarly, simultaneous treatment of HeLa-V cells with flavopiridol and UCN-01 for 72 hours did not result in synergistic inhibition of proliferation; however, in HeLa-S cells, this combination resulted in synergistic inhibition of cell proliferation. Flavopiridol and UCN-01 augmented apoptosis in HeLa-S cells (as compared with HeLa-V cells) as measured by caspase-3 cellular activity assay, DNA fragmentation and PARP cleavage by western blot. In HeLa-V and -S cells, combination treatment resulted in caspase-8 cleavage. Caspase-9 was expressed in HeLa-V cells; however, there was a marked reduction of caspase-9 content in HeLa-S cells only. Combination treatment resulted in a significant reduction in survivin abundance in HeLa-S and SKBR3-UR cells, but not in their respective parental lines. The synergy of Flavopiridol and UCN-01 are selectively toxic to survivin-overexpressing cell lines and the mechanism of toxicity involves caspase-dependent cell death.

Discovery of a potent and selective inhibitor of cyclin-dependent kinase 4/6

A pharmacological approach to inhibition of cyclin-dependent kinases 4 and 6 (Cdk4/6) using highly selective small molecule inhibitors has the potential to provide novel cancer therapies for clinical use. Achieving high levels of selectivity for Cdk4/6, versus other ATP-dependent kinases, presents a significant challenge. The pyrido[2,3-d]pyrimidin-7-one template provides an effective platform for the inhibition of a broad cross-section of kinases, including Cdks. It is now demonstrated that the modification of pyrido[2,3-d]pyrimidin-7-ones to include a 2-aminopyridine side chain at the C2-position provides inhibitors with exquisite selectivity for Cdk4/6 in vitro. This selectivity profile is recapitulated in cells where the most selective inhibitors create a G(1) block at concentrations up to 100-fold the IC(50) for cell proliferation. On the basis of its selectivity profile and pharmacokinetic profile, compound 43 (PD 0332991) was identified as a drug candidate for the treatment of cancer.

Cyclin G1 and cyclin G2 are expressed in the periimplantation mouse uterus in a cell-specific and progesterone-dependent manner: evidence for aberrant regulation with Hoxa-10 deficiency

Because uterine cell-specific proliferation, differentiation, and apoptosis are differentially regulated during the periimplantation period, we speculated that negative cell cycle regulators are also operative in the uterus during this period. This prompted us to examine the roles of two negative growth-regulatory genes, cyclin G1 and cyclin G2, in the periimplantation mouse uterus. We show that cyclin G1 and cyclin G2 genes are differentially regulated in the uterus during this period (d 1-8 of pregnancy) in a spatiotemporal manner. The results suggest that cyclin G1 is primarily associated with epithelial cell differentiation before implantation and stromal cell proliferation and differentiation during decidualization, whereas cyclin G2 is associated with terminal differentiation and apoptosis of the luminal epithelial and stromal cells at the site of blastocyst after implantation. Pharmacological and genetic studies provide evidence that the expression of cyclin G1, not cyclin G2, is regulated by progesterone via its nuclear receptor. Furthermore, the expression of these genes is aberrantly up-regulated in homeo box A-10 mutant uteri, suggesting that cyclin G1 and cyclin G2 genes act as downstream targets of homeobox A-10 and negatively impact uterine cell proliferation. Collectively, our present and previous studies suggest that negative cell cycle regulators collaborate with growth-promoting regulators in regulating uterine cell-specific proliferation, differentiation, and apoptosis relevant to implantation and decidualization.

Pyrido[2,3-d]pyrimidin-7-ones as Specific Inhibitors of Cyclin-Dependent Kinase 4

Inhibition of the cell cycle kinase, cyclin-dependent kinase-4 (Cdk4), is expected to provide an effective method for the treatment of proliferative diseases such as cancer. The pyrido[2,3-d]pyrimidin-7-one template has been identified previously as a privileged structure for the inhibition of ATP-dependent kinases, and good potency against Cdks has been reported for representative examples. Obtaining selectivity for individual Cdk enzymes, particularly Cdk4, has been challenging. Here, we report that the introduction of a methyl substituent at the C-5 position of the pyrido[2,3-d]pyrimidin-7-one template is sufficient to confer excellent selectivity for Cdk4 vs other Cdks and representative tyrosine kinases. Further optimization led to the identification of highly potent and selective inhibitors of Cdk4 that exhibit potent antiproliferative activity against human tumor cells in vitro. The most selective Cdk4 inhibitors were evaluated for antitumor activity against MDA-MB-435 human breast carcinoma xenografts in mice.

Preclinical and clinical development of the cyclin-dependent kinase inhibitor flavopiridol

Flavopiridol is the first potent inhibitor of cyclin-dependent kinases (cdks) to reach clinical trial. In the majority of solid tumor cell lines and xenografts, flavopiridol induces cell cycle arrest and tumor growth inhibition. This is reflected in clinical outcomes: across multiple Phase II trials there are subsets of patients with prolonged stable disease, although few responses have been observed. Flavopiridol displays sequence-dependent cytotoxic synergy with chemotherapy agents. These effects are most marked when chemotherapy precedes flavopiridol. In the case of DNA-damaging agents that impose S-phase delay, flavopiridol-mediated cdk inhibition disrupts the phosphorylation of E2F-1, leading to inappropriate persistence of its activity, inducing apoptotic pathways. This mechanism has been exploited in a Phase I trial of sequential gemcitabine and flavopiridol that has produced promising results. Flavopiridol is also synergistic with taxanes. Inhibition of cyclin B-cdk1 by flavopiridol accelerates exit from an abnormal mitosis associated with taxane-induced cell death and reduces the phosphorylation of survivin, preventing its stabilization and the cellular protection it affords after taxane exposure. The sequential combination of docetaxel and flavopiridol has been investigated in a Phase I trial in patients with advanced non-small cell lung cancer, and a randomized Phase II study is under way. Initial schedules of flavopiridol used prolonged continuous infusions that produced nanomolar levels of drug thought to be capable of achieving cdk inhibition based on results in tumor cell lines. Recently, it has been discovered that micromolar concentrations are likely to be more effective, and shorter infusions that achieve a higher C(max) have now been adopted. Loading followed by maintenance infusions are also under development, designed to achieve sustained micromolar drug levels. Clinical trials remain complicated by the absence of pharmacodynamic end points to confirm target inhibition.

Cyclin-dependent kinase inhibition by the KLF6 tumor suppressor protein through interaction with cyclin D1

Kruppel-like factor 6 (KLF6) is a tumor suppressor gene inactivated in prostate and colon cancers, as well as in astrocytic gliomas. Here, we establish that KLF6 mediates growth inhibition through an interaction with cyclin D1, leading to reduced phosphorylation of the retinoblastoma protein (Rb) at Ser(795). Furthermore, introduction of KLF6 disrupts cyclin D1-cyclin-dependent kinase (cdk) 4 complexes and forces the redistribution of p21(Cip/Kip) onto cdk2, which promotes G(1) cell cycle arrest. Our data suggest that KLF6 converges with the Rb pathway to inhibit cyclin D1/cdk4 activity, resulting in growth suppression.

Protein kinase inhibitors as a therapeutic modality

Most of the signal transduction pathways are mediated by protein kinases regulating every aspect of cell function. Mutations which deregulate their expression or their function or both result in cancers. Therefore, protein kinase inhibitors has become the focus of development of new therapies for cancer. Almost all 120 protein tyrosine kinases are involved in signaling, whereas only a handful of Ser/Thr kinases are involved. Thus, most of the effort is directed toward the development of tyrosine phosphorylation inhibitors. The success of Gleevec in the treatment of chronic myeloid leukemia and of Iressa for lung cancer validates the approach.

The use of cyclin-dependent kinase inhibitors alone or in combination with established cytotoxic drugs in cancer chemotherapy

Cyclin-dependent kinase (CDK) inhibitors are small molecule inhibitors of the kinases required for the orderly progression of cells, both normal and neoplastic, through the cell cycle. Because cell cycle dysregulation is such a common occurrence in neoplasia, the search for agents that might block cell cycle traverse has been the focus of intense interest. These efforts have led to the identification of a broad array of compounds that interfere directly with the function of CDKs. Two of these agents (flavopiridol and UCN-01) have now entered the clinical arena, and others are scheduled to do so in the near future. In preclinical studies, CDK inhibitors have shown the ability not only to block neoplastic cell proliferation, but also to induce, through a variety of mechanisms, programmed cell death. The latter capacity may stem from the diverse effects that CDK inhibitors exert on multiple kinases and apoptotic regulatory molecules. In addition, there is abundant preclinical evidence that CDK inhibitors can potentiate, generally in a dose- and sequence-dependent manner, the anti-tumor effects of many established cytotoxic agents. In clinical studies in humans, flavopiridol and UCN-01 have been shown to be tolerable, although clear evidence of single agent activity or enhancement of the efficacy of established agents has not yet emerged. This may reflect a failure to optimize drug schedules/pharmacokinetics, or to identify the critical molecular targets of these agents. Finally, in recent years, a rationale has emerged for combining CDK inhibitors with other molecularly targeted agents (i.e. differentiation-inducers and signal transduction modulators). Current research has basically two goals: (a). to identify CDK inhibitor concentrations and schedules that inhibit the growth of and induce apoptosis in specific tumor cell types; and (b). to establish a rational basis for combining CDK inhibitors with more conventional cytotoxic agents to enhance antitumor efficacy. This review gives a brief summary of such efforts, with an emphasis on agents and combinations that are in or near clinical development.

Synthesis and evaluation of indenopyrazoles as cyclin-dependent kinase inhibitors. 3. Structure activity relationships at C3(1,2)

The identification of indeno[1,2-c]pyrazol-4-ones as inhibitors of cyclin-dependent kinases (CDKs) has led to the discovery of a series of novel and potent compounds. Herein, we report the effects of substitutions at C3 of the indeno[1,2-c]pyrazol-4-one core with alkyls, heterocycles, and substituted phenyls. Substitutions at the para position of the phenyl ring at C3 were generally well-tolerated; however, larger groups were generally inactive. For alkyls directly attached to C3, longer chain substituents were not tolerated; however, shorter alkyl groups and cyclic alkyls were acceptable. In general, the heterocycles at C3 gave the most potent analogues. One such heterocycle, 24j, was examined in detail and was determined to have a biological profile consistent with CDK inhibition. An X-ray crystal structure of one of the alkyl compounds, 13q, complexed with CDK2 was determined and showed the inhibitor residing in the adenosine 5'-triphosphate pocket of the enzyme.

Plant cytokinin analogues with inhibitory activity on cyclin-dependent kinases exert their antiproliferative effect through induction of apoptosis initiated by the mitochondrial pathway: determination by a multiparametric flow cytometric analysis

OBJECTIVE

Regulation of the cell cycle by cyclin-dependent kinase (CDK) activity occurs at multiple levels and is often altered in human cancers. Therefore, CDK activity has been targeted for drug discovery, and a number of small molecules have now been identified as CDK inhibitors. Plant cytokinin analogues with CDK inhibitory activity and antiproliferative effects were studied to characterize the cellular basis of the cytotoxic effect.

METHODS

The IC(50) value (concentration at which 50% of the cell proliferation is inhibited) and AC(50) value (concentration at which 50% of the cell population is apoptotic) were determined by flow cytometry and microscopy, respectively. A new multiparametric flow cytometric analysis was used to study the sequence of different apoptotic events. In this assay, analysis of phosphatidylserine exposure, mitochondrial membrane depolarization, activation of caspases and DNA condensation were combined.

RESULTS

Treatment of Jurkat and KG1 cells with the CDK inhibitors results in a decrease of viable cells and a parallel increase in percentage of apoptotic cells. Apoptosis was accompanied by a rapid decrease of mitochondrial membrane potential, which precedes DNA condensation, exposure of phosphatidylserine and activation of caspases.

CONCLUSIONS

The main cellular mechanism of the antiproliferative effect of plant cytokinin analogues with CDK inhibitory activity is the induction of apoptosis. The multiparametric flow cytometric technique allowed to follow the kinetics of various aspects of apoptotic cell changes and demonstrated that cytokinin analogue-induced apoptosis starts through the mitochondrial pathway. This technique could also become of value for the rapid screening of pro-apoptotic properties of chemotherapeutic compounds.

p42/p44 MAPKs are intracellular targets of the CDK inhibitor purvalanol

Chemical inhibitors of cyclin-dependent kinases (CDKs) have a great therapeutic potential against various proliferative and neurodegenerative disorders. Intensive screening of a combinatorial chemistry library of 2,6,9-trisubstituted purines has led to the identification of purvalanol, one of the most potent and selective CDK inhibitors to date. In preliminary studies, this compound demonstrates definite anti-mitotic properties, consistent with its nanomolar range efficiency towards purified CDK1 and CDK2. However, the actual intracellular targets of purvalanol remain to be identified, and a method for the determination of its in vivo selectivity was developed. In this technique, cell extracts were screened for purvalanol-interacting proteins by affinity chromatography on immobilized inhibitor. In addition to CDK1, p42/p44 MAPK were found to be two major purvalanol-interacting proteins in five different mammalian cell lines (CCL39, PC12, HBL100, MCF-7 and Jurkat cells), suggesting the generality of the purvalanol/p42/p44 MAPK interaction. The Chinese hamster lung fibroblast cell line CCL39 was used as a model to investigate the anti-proliferative properties of purvalanol. The compound inhibited cell growth with a GI(50) value of 2.5 microM and induced a G2/M block when added to exponentially growing cells. It did not appear to trigger massive activation of caspase. We next tested whether CDKs and p42/p44 MAPK were actually targeted by the compound in vivo. p42/p44 MAPK activity was visualized using an Elk-Gal4 luciferase reporter system and CDK1 activity was detected by the phosphonucleolin level. When cells were treated with purvalanol, p42/p44 MAPK and CDK1 activities were inhibited in a dose-dependent manner. Furthermore, purvalanol inhibited the nuclear accumulation of p42/p44 MAPK, an event dependent on the catalytic activity of these kinases. We conclude that the anti-proliferative properties of purvalanol are mediated by inhibition of both p42/p44 MAPK and CDKs. These observations highlight the potency of moderate selectivity compounds and encourage the search for new therapeutics which simultaneously target distinct but relevant pathways of cell proliferation.

Identifying in vivo targets of cyclin-dependent kinase inhibitors by affinity chromatography

Cyclin-dependent kinases (CDKs) regulate the cell division cycle, apoptosis, transcription, differentiation and many functions in the nervous system. The frequent deregulation of CDKs in cancers and in numerous other pathologies justifies the active search for chemical inhibitors capable of reversibly and selectively inhibiting this class of enzymes. Intensive screening of collections of natural and synthetic compounds has led to the identification of several families of ATP competitive CDK inhibitors. As the therapeutic potential of the most promising compounds is currently being evaluated in preclinical and clinical trials, their mechanism of action is still unclear. In particular, the real spectrum of their intracellular targets remains largely unknown. Determination of the selectivity of the compounds and identification of their intracellular targets constitute a prerequisite to understand their cellular effects and to improve their efficiency on a rational basis. The classical method for the determination of a compound's selectivity consists in testing the compound in a panel of purified kinases. However, the selectivity study is then restricted to the panel's enzymes. As a consequence, many, if not most other potential targets are not evaluated. As an alternative way to investigate the range of true targets of CDK inhibitors, we propose an affinity chromatography approach based on immobilized inhibitors. Briefly, the inhibitor is covalently bound to a resin and cellular extracts are batch loaded on this inhibitor matrix. After extensive washing, the bound proteins are resolved by SDS-PAGE and identified by microsequencing. In addition to confirming the interaction of CDK inhibitors with CDKs, this method has led to the identification of additional, sometimes unexpected, targets. We here illustrate the potential of this technique through a few examples.

Phase I and pharmacokinetic study of E7070, a novel chloroindolyl sulfonamide cell-cycle inhibitor, administered as a one-hour infusion every three weeks in patients with advanced cancer

PURPOSE

The objectives were to determine the maximum-tolerated dose, the recommended dose, the dose-limiting toxicity, the pharmacokinetics, and the activity of E7070, a novel cell-cycle inhibitor.

PATIENTS AND METHODS

E7070 was given as a 1-hour intravenous infusion every 3 weeks in two groups of patients with advanced solid tumors who met prespecified eligibility criteria (group A) or who met the same eligibility criteria but in addition were less heavily pretreated and had more favorable liver functions (group B).

RESULTS

Forty patients (31 patients in group A and nine patients in group B) were entered. Dose escalation proceeded through eight levels (range, 50 to 1,000 mg/m(2)). In group A, neutropenia and thrombocytopenia were dose-limiting toxicities occurring during the first cycle in two of seven patients treated at the doses of 700 mg/m(2) and two of four patients treated at 800 mg/m(2). Identical dose-limiting toxicities were observed in zero of six and two of three patients from group B at doses of 800 and 1,000 mg/m(2), respectively. Other toxicities included acne-like skin eruption, mucositis, conjunctivitis, nausea, fatigue, and alopecia. At doses greater than 400 mg/m(2), the area under the concentration-time curve increased disproportionately to the administered dose. Tumor stabilization lasting > or = 6 months was observed in six assessable patients.

CONCLUSION

The recommended doses of E7070 in this schedule were 700 mg/m(2) (group A) and 800 mg/m(2) in patients who were less heavily pretreated (group B) with a moderate tumor burden. Prolonged disease stabilization observed in this study might warrant further investigation of E7070 in selected tumor types.

Targeting the cell cycle for cancer therapy

Most if not all neoplasias show a directly or indirectly deregulated cell cycle. Targeting its regulatory molecules, the cyclin-dependent kinases, as a therapeutic mode to develop new anticancer drugs, is being currently explored in both academia and pharmaceutical companies. The development of new compounds is being focused on the many features of the cell cycle with promising preclinical data in most fields. Moreover, a few compounds have entered clinical trials with excellent results maintaining the high hopes. Thus, although too early to provide a cell cycle target based new commercial drug, there is no doubt that it will be an excellent source of new anticancer compounds.

Intracellular Targets of Paullones. Identification following affinity purification on immobilized inhibitor

Numerous inhibitors of cyclin-dependent kinases and glycogen synthase kinase-3 (GSK-3) are being developed in view of their potential applications against cancers and neurodegenerative disorders. Among these, paullones constitute a family of potent and apparently selective cyclin-dependent kinase and GSK-3 inhibitors. However, their actual intracellular targets remain to be identified. To address this issue we have immobilized a paullone, gwennpaullone, on an agarose matrix. Extracts from various cell types and tissues were screened for proteins interacting with this matrix. This approach validated GSK-3alpha and GSK-3beta as major intracellular paullone targets and also mitochondrial, but not cytoplasmic, malate dehydrogenase (MDH). Mitochondrial MDH was indeed inhibited by micromolar concentrations of paullones. Mitochondrial MDH was the major paullone-binding protein in the parasitic protozoon Leishmania mexicana, and paullones inhibited growth of the parasite. This simple batchwise affinity chromatography approach constitutes a straightforward method for the identification of intracellular targets of this particular class of novel anti-mitotic compounds. It has revealed an unexpected target, mitochondrial MDH, the inhibition of which may participate in the pharmacological effects of paullones.