Overexpression of cyclin D1 and cdk4 in tumorigenesis of sporadic hepatoblastomas

Drugging Hijacked Kinase Pathways in Pediatric Oncology: Opportunities and Current Scenario

Childhood cancer is considered rare, corresponding to ~3% of all malignant neoplasms in the human population. The World Health Organization (WHO) reports a universal occurrence of more than 15 cases per 100,000 inhabitants around the globe, and despite improvements in diagnosis, treatment and supportive care, one child dies of cancer every 3 min. Consequently, more efficient, selective and affordable therapeutics are still needed in order to improve outcomes and avoid long-term sequelae. Alterations in kinases' functionality is a trademark of cancer and the concept of exploiting them as drug targets has burgeoned in academia and in the pharmaceutical industry of the 21st century. Consequently, an increasing plethora of inhibitors has emerged. In the present study, the expression patterns of a selected group of kinases (including tyrosine receptors, members of the PI3K/AKT/mTOR and MAPK pathways, coordinators of cell cycle progression, and chromosome segregation) and their correlation with clinical outcomes in pediatric solid tumors were accessed through the R2: Genomics Analysis and Visualization Platform and by a thorough search of published literature. To further illustrate the importance of kinase dysregulation in the pathophysiology of pediatric cancer, we analyzed the vulnerability of different cancer cell lines against their inhibition through the Cancer Dependency Map portal, and performed a search for kinase-targeted compounds with approval and clinical applicability through the CanSAR knowledgebase. Finally, we provide a detailed literature review of a considerable set of small molecules that mitigate kinase activity under experimental testing and clinical trials for the treatment of pediatric tumors, while discuss critical challenges that must be overcome before translation into clinical options, including the absence of compounds designed specifically for childhood tumors which often show differential mutational burdens, intrinsic and acquired resistance, lack of selectivity and adverse effects on a growing organism.

Identification of Ten Core Hub Genes as Potential Biomarkers and Treatment Target for Hepatoblastoma

Background

This study aimed to systematically investigate gene signatures for hepatoblastoma (HB) and identify potential biomarkers for its diagnosis and treatment.

Materials and Methods

GSE131329 and GSE81928 were obtained from the Gene Expression Omnibus (GEO) database. Differentially expressed genes (DEGs) between hepatoblastoma and normal samples were identified using the Limma package in R. Then, the similarity of network traits between two sets of genes was analyzed by weighted gene correlation network analysis (WGCNA). Cytoscape was used to visualize and select hub genes. PPI network of hub genes was construed by Cytoscape. GO enrichment and KEGG pathway analyses of hub genes were carried out using ClueGO. The random forest classifier was constructed based on the hub genes using the GSE131329 dataset as the training set, and its reliability was validated using the GSE81928 dataset. The resulting core hub genes were combined with the InnateDB database to identify the innate core genes.

Results

A total of 4244 DEGs in HB were identified. WGCNA identified four modules that were significantly correlated with the disease status. A total of 114 hub genes were obtained within the top 20 genes of each node rank. 6982 relation pairs and 3700 nodes were contained in the PPI network of 114 hub genes. GO enrichment and KEGG pathway analyses of hub genes were focused on MAPK, cell cycle, p53, and other crucial pathways involved in HB. A random forest classifier was constructed using the 114 hub genes as feature genes, resulting in a 95.5% true positive rate when classifying HB and normal samples. A total of 35 core hub genes were obtained through the mean decrease in accuracy and mean decrease Gini of the random forest model. The classification efficiency of the random forest model was 81.4%. Finally, CDK1, TOP2A, ADRA1A, FANCI, XRCC1, TPX2, CCNB2, CDK4, GLYATL1, and CFHR3 were identified by cross-comparison with the InnateDB database.

Conclusion

Our study established a random forest classifier that identified 10 core genes in HB. These findings may be beneficial for the diagnosis, prediction, and targeted therapy of HB.

Chloroform fraction of Scutellaria barbata D. Don promotes apoptosis and suppresses proliferation in human colon cancer cells

Scutellaria barbata D. Don (SB) has long been used as a major component in numerous Chinese medical formulas to clinically treat various types of cancer. Previously, we reported that the extracts of SB were able to suppress colon cancer growth in vivo and in vitro, possibly by inducing cancer cell apoptosis and inhibiting cell proliferation and tumor angiogenesis. However, the anticancer mechanisms of its bioactive ingredients remain largely unclear. In the present study, using three human colon cancer cell lines SW620, HT-29 and HCT-8, the antitumor effect of different solvent fractions of SB were evaluated and the potential underlying molecular mechanisms were investigated. Using an 3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, it was revealed that the chloroform fraction of SB (ECSB) exhibited the most potent inhibitory effect on the growth of all three colon cancer cell lines and SW620 cells exhibited the most sensitive response to ECSB treatment (IC50=65 µg/ml). In addition, by performing fluorescence-activated cell sorting, transmission electron microscopy and colony formation assays, it was observed that ECSB significantly induced apoptosis and inhibited proliferation in SW620 cells in a dose-dependent manner. Furthermore, ECSB treatment resulted in the upregulation of the pro-apoptotic Bax/Bcl-2 ratio and a decrease in the expression of the pro-proliferative cyclin D1 and cyclin-dependent kinase 4. The results from the present study may provide a scientific foundation for the development of novel anticancer agents from the bioactive ingredients in the ECSB.

Deregulation of the p16-cyclin D1/cyclin-dependent kinase 4–retinoblastoma pathway involved in the rat bladder carcinogenesis induced by terephthalic acid-calculi

Prolonged cell proliferation in response to irritation by calculi may itself evoke malignant transformation of the urothelium. However, the molecular mechanisms underlying this process are still unknown. The aim of the present study was to investigate cell cycle regulatory mechanisms in bladder carcinogenesis induced by bladder calculi. Six-week-old Wistar rats were consecutively fed a diet containing 5% terephthalic acid (TPA), 5% TPA plus 4% sodium bicarbonate (NaHCO(3)), 4% NaHCO(3), or basal diet for 48 weeks. Animals were killed at weeks 12, 24, and 48. Treatment with 5% TPA caused high incidences of bladder calculi, preneoplastic lesions, and neoplastic lesions. Immunohistochemical examination revealed overexpression of cyclin D1, cyclin-dependent kinase 4 (Cdk4), retinoblastoma (Rb), and proliferating cell nuclear antigen (PCNA) in bladder preneoplastic and neoplastic lesions. In contrast, p16 expression was reduced or absent. These results were confirmed by immunoblotting analysis. Quantitation of mRNA by real-time reverse transcription-polymerase chain reaction (RT-PCR) showed a significant increase in cyclin D1 and PCNA mRNA in tumor cells. None of the 16 transitional cell carcinomas (TCCs) had ras mutations as examined by PCR-single strand conformational polymorphism (PCR-SSCP) analysis. These results suggested that deregulation of p16-cyclin D1/Cdk4-Rb pathway, but not oncogenic activation of ras, plays a crucial role in bladder tumorigenesis induced by bladder calculi.

Fascaplysin, a selective CDK4 inhibitor, exhibit anti-angiogenic activity in vitro and in vivo

PURPOSE

This study was to evaluate the correlation of two important strategies, namely, cell cycle proliferation arrest and anti-angiogenesis. We chose fascaplysin, a marine natural product with selective CDK4 selective inhibition activity, to study its potential anti-angiogenesis effects in vivo and in vitro.

METHODS

Chorioallantoic membrane (CAM) assay was initially used as an in vivo approach to evaluate anti-angiogenic activity of fascaplysin. In addition, human umbilical vein endothelial cell (HUVEC) line was used to further confirm the anti-angiogenic activity of fascaplysin in vitro. To explore the mechanism of anti-angiogenesis, we examined the effect of fascaplysin on vascular endothelial growth factor (VEGF) expression and secretion by hepatocarcinoma cells BeL-7402.

RESULTS

The results of CAM assay suggested fascaplysin inhibited capillary plexus formation in a dose-dependent manner and suppressed VEGF in cross section. Moreover, the in vitro assay also confirmed that fascaplysin provided selective inhibition of endothelial cells proliferation towards tumor cells in low concentration. The immunocytochemical staining and ELISA verified fascaplysin could inhibit VEGF expression and secretion by BeL-7402.

CONCLUSIONS

These findings strongly suggest that fascaplysin is a natural angiogenesis inhibitor.

Benzo[a]pyrene-induced cell cycle progression is through ERKs/cyclin D1 pathway and requires the activation of JNKs and p38 mapk in human diploid lung fibroblasts

Treatment of cells with carcinogen Benzo[a]pyrene (B[a]P) allows cells to evade G1 arrest and induces cells abnormal proliferation. However, the mechanisms of its action at cellular level are not well understood. To address this question, normal human embryo lung diploid fibroblasts (HELF) were selected in the present study. We found that exposure of cells with 2.5 microM of B[a]P for 24 h resulted in a decrease of G1 population by 11.9% (P < 0.05) and a increase of S population by 17.2% (P < 0.05). Treatment of cells with B[a]P also caused dose-related activation of MAPK and induction of cyclin D1 protein expression, whereas the CDK4 protein levels were not significantly affected by B[a]P. Overexpression of cyclin D1 protein stimulated by B[a]P was significantly inhibited by 50 microM AG126 (an inhibitor of ERK1/2), but not by 25 microM SP600125 (an inhibitor of JNK1/2) or 5 microM SB203580 (an inhibitor of p38 mapk), suggesting that B[a]P-induced cyclin D1 expression was only regulated by ERK1/2 pathway. However, AG126, SP600125 or SB203580 led to cell cycle significantly arrested in G1 phase, indicating that ERK1/2, JNK1/2 and p38 mapk pathways are all required for B[a]P-induced G1/S transition. In addition, HELF cells transfecting with antisense cyclin D1 cDNA or antisense CDK4 cDNA showed significantly G1 arrest after B[a]P stimulation. These results suggested that B[a]P exposure accelerated the G1-->S transition by activation of MAPK signaling pathways. Cyclin D1 and CDK4 are rate-limiting regulators of the G1-->S transition and expression of cyclin D1 is predominantly regulated by ERK1/2 pathway in HELF cells.

Regulation of survivin and CDK4 by Epstein-Barr virus encoded latent membrane protein 1 in nasopharyngeal carcinoma cell lines

Latent membrane protein 1 (LMP1), an important protein encoded by Epstein Barr virus (EBV), has been implied to link with the pathogenesis of nasopharyngeal carcinoma (NPC). Its dual effects of increasing cell proliferation and inhibiting cell apoptosis have been confirmed. In this study, we showed that the expression of Survivin and CDK4 protein in CNE-LMP1, a LMP1 positive NPC epithelial cell line, is higher than in LMP1 negative NPC epithelial cell line-CNE1, and the expression is LMP1 dosage-dependent. Although it was reported that Survivin specifically expressed in cell cycle G2/M phase, our studies suggested that LMP1 could promote the expression of Survivin in G0/G1, S and G2/M phase. It also showed that Survivin and CDK4 could be accumulated more in the nuclei triggered by LMP1. More interestingly, Survivin and CDK4 could form a protein complex in the nuclei of CNE-LMP1 rather than in that of CNE1, which demonstrated that the interaction between these two proteins could be promoted by LMP1. These results strongly suggested that the role of LMP1 in the regulation of Survivin and CDK4 may also shed some light on the mechanism research of LMP1 in NPC.

Elevated expression of Wnt antagonists is a common event in hepatoblastomas

Hepatoblastomas are the most frequent malignant liver tumors of childhood. A high frequency of activating beta-catenin mutations in hepatoblastomas indicates that the Wnt signaling pathway plays an important role in the development of this embryonic neoplasm. Stabilization of beta-catenin leads to an increased formation of nuclear beta-catenin-T-cell factor complexes and altered expression of Wnt-inducible target genes. In this study, we analyzed the mRNA expression levels of nine Wnt genes, including c-JUN, c-MYC, CYCLIN D1, FRA-1, NKD-1, ITF-2, MMP-7, uPAR, and beta-TRCP, by competitive reverse transcription-PCR. We analyzed 23 hepatoblastoma biopsies for which matching liver tissue was available, 6 hepatoblastoma cell lines, and 3 human fetal liver samples. beta-TRCP and NKD-1 were highly expressed in all hepatoblastoma samples, independent of the beta-catenin mutational status, in comparison with their nontumorous counterparts. beta-TRCP mRNA overexpression was associated with accumulation of intracytoplasmic and nuclear beta-TrCP protein. In human liver tumor cells without beta-catenin mutations, Nkd-1 inhibited the Wnt-3a-activated Tcf-responsive-luciferase reporter activity, whereas Nkd-1 in hepatoblastomas with beta-catenin mutations had no antagonistic effect. Our data emphasize the inhibitory effect of beta-TrCP and Nkd-1 on the Wnt signaling pathway in a manner analogous to Conductin (AXIN2) and Dkk-1, inhibitors shown previously to be up-regulated in hepatoblastomas. Our findings indicate that overexpression of the Wnt antagonists Nkd-1 and beta-TrCP reveals an activation of the Wnt signaling pathway as a common event in hepatoblastomas. We propose that Nkd-1 and beta-TrCP may be used as possible diagnostic markers for the activated Wnt signaling pathway in hepatoblastomas.

Mechanism targeted discovery of antitumor marine natural products

Antitumor drug discovery programs aim to identify chemical entities for use in the treatment of cancer. Many strategies have been used to achieve this objective. Natural products have always played a major role in anticancer medicine and the unique metabolites produced by marine organisms have increasingly become major players in antitumor drug discovery. Rapid advances have occurred in the understanding of tumor biology and molecular medicine. New insights into mechanisms responsible for neoplastic disease are significantly changing the general philosophical approach towards cancer treatment. Recently identified molecular targets have created exciting new means for disrupting tumor-specific cell signaling, cell division, energy metabolism, gene expression, drug resistance and blood supply. Such tumor-specific treatments could someday decrease our reliance on traditional cytotoxicity-based chemotherapy and provide new less toxic treatment options with significantly fewer side effects. Novel molecular targets and state-of-the-art, molecular mechanism-based screening methods have revitalized antitumor research and these changes are becoming an ever-increasing component of modern antitumor marine natural products research. This review describes marine natural products identified using tumor-specific mechanism-based assays for regulators of angiogenesis, apoptosis, cell cycle, macromolecule synthesis, mitochondrial respiration, mitosis, multidrug efflux and signal transduction. Special emphasis is placed on natural products directly discovered using molecular mechanism-based screening.

Synthesis of 1,7-annulated indoles and their applications in the studies of cyclin dependent kinase inhibitors

The synthesis of a novel series of 1,7-annulated indolocarbazoles 2 and 16 is described. These compounds were found to be potent cyclin dependent kinase inhibitors with good antiproliferative activity against two human carcinoma cell lines. These inhibitors also arrested tumor cells at the G1 phase and inhibited pRb phosphorylation.

Mutations and elevated transcriptional activity ofconductin (AXIN2) in hepatoblastomas

Hepatoblastoma (HB) is the most frequent malignant liver tumour of childhood. Most HBs develop sporadically but their incidence is highly elevated in patients with familial adenomatous polyposis coli (FAP). These patients carry germline mutations in the adenomatous polyposis coli (APC) tumour suppressor gene. APC forms a multi-protein complex involved in the WNT signalling pathway that controls the stability of beta-catenin, the central effector in this cascade. Whereas APC mutations are rare in sporadic HBs, a high frequency of beta-catenin mutations leading to overactivation of WNT signalling was previously found in these tumours. This pathway is negatively controlled by conductin (axin2), representing a further partner in this signalling complex. To investigate whether alterations in conductin may also be involved in the pathogenesis of sporadic HBs, 37 HBs and five HB cell lines were screened for mutations using single-strand conformation polymorphism (SSCP) analysis, reverse transcription-polymerase chain reaction (RT-PCR), and direct sequencing. In two cases, larger deletions (52 and 1624 bp) leading to frameshifts were found. In addition, one HB carried a somatic point mutation. Expression analysis by competitive RT-PCR in HBs revealed up-regulation of conductin mRNA compared with adjacent liver samples. This mRNA overexpression resulted in increased conductin protein levels demonstrated by western blot analysis. Tumours with activating beta-catenin mutations revealed higher levels of conductin mRNA transcripts. This finding indicates that conductin is a direct target gene of WNT signalling in HBs, as has been demonstrated in other tissues. In summary, conductin mutations may represent an alternative mechanism leading to activation of WNT signalling in HBs. The overexpression of conductin mRNA in HBs reflects activation of the WNT pathway because conductin represents a target gene of WNT signalling in liver tissue.

Over-expression of cyclin D1 regulates Cdk4 protein synthesis

Increased Cdk4 expression occurs coincident with over-expression of cyclin D1 in many human tumours and tumourigenic mouse models. Here, we investigate both in vivo and in vitro the mechanism by which Cdk4 expression is regulated in the context of cyclin D1 over-expression. Cdk4 mRNA levels in cyclin D1-over-expressing tissue and cultured cells were unchanged compared with controls. In contrast, Cdk4 protein levels were increased in cyclin D1-over-expressing tissue and cells versus their respective controls. This increase was not due to altered protein stability, but appeared to be due to an increase in Cdk4 protein synthesis. We also performed immunoprecipitation and in vitro kinase assays to demonstrate an increase in cyclin D1-Cdk4 complex formation and associated kinase activity. Blocking cyclin D1 expression resulted in diminished Cdk4 protein but not mRNA levels. These findings suggest a mechanism by which Cdk4 expression is increased in the context of cyclin D1 over-expression during tumourigenesis.

Studies on cyclin-dependent kinase inhibitors: indolo-[2,3- a ]pyrrolo[3,4- c ]carbazoles versus bis-indolylmaleimides

A series of indolo[2,3-a]pyrrolo[3,4-c]carbazoles and their bis-indolylmaleimides precursors have been prepared in order to compare their activity as D1-CDK4 inhibitors. Both enzymatic and antiproliferative assays have shown that the structurally more constrained indolo[2,3-a]pyrrolo[3,4-c]carbazoles are consistently more active (8-42-fold) in head-to-head comparison with their bis-indolylmaleimides counterparts. Cell-cycle analysis using flow cytometry have also shown that the indolocarbazoles are selective G1 blockers while the bis-indolylmaleimides arrest cells in the G2/M phase.

Aryl[ a ]pyrrolo[3,4- c ]carbazoles as selective cyclin D1-CDK4 inhibitors

The synthesis of new analogues of Arcyriaflavin A in which one indole ring is replaced by an aryl or heteroaryl ring is described. These new series of aryl[a]pyrrolo[3,4-c]carbazoles were evaluated as inhibitors of Cyclin D1-CDK4. A potent and selective D1-CDK4 inhibitor, 7a (D1-CDK4 IC(50)=45 nM), has been identified. The potency, selectivity profile against other kinases, and structure-activity relationship (SAR) trends of this class of compounds are discussed.

Novel, potent and selective cyclin D1/CDK4 inhibitors: indolo[6,7-a]pyrrolo[3,4-c]carbazoles

The synthesis and CDK inhibitory properties of a series of indolo[6,7-a]pyrrolo[3,4-c]carbazoles is reported. In addition to their potent CDK activity, the compounds display antiproliferative activity against two human cancer cell lines. These inhibitors also effect strong G1 arrest in these cell lines and inhibit Rb phosphorylation at Ser780 consistent with inhibition of cyclin D1/CDK4.

Hypermethylation of the p16 gene and lack of p16 expression in hepatoblastoma

Hepatoblastoma is the most frequent pediatric liver tumor that develops mostly in young children. Abnormal regulation of cell cycle regulatory genes including p16 has been described, displaying no p16 mRNA and p16 protein in hepatoblastomas. The inactivation of p16, leading to the disruption of cell cycle control is involved in many types of human malignancies. However, the mechanism of the p16 inactivation in hepatoblastomas has not yet been elucidated. In this present study, we examined the methylation status of the p16 gene promoter by using methylation-specific PCR in 24 cases of hepatoblastomas and in 20 cases of corresponding non-neoplastic liver tissue. Aberrant methylation of 5' CpG islands of p16 was present in 12 of 24 (50.0%) cases of hepatoblastoma. Clinicopathologic parameters were not associated with the methylation status of p16. To correlate the methylation status of p16 with the expression of p16, immunohistochemical staining was done in tumors and non-neoplastic liver tissue. All non-neoplastic liver tissues displayed moderate, but heterogeneous immunoreactivity for p16. Eight of 12 (66.6%) methylation-positive hepatoblastomas showed a complete lack of immunoreactivity for p16. The other 4 methylation-positive hepatoblastomas had heterogeneous immunoreactivity. Nine of 12 (75.0%) unmethylated cases of hepatoblastoma displayed diffuse immunoreactivity, whereas 3 cases of unmethylated hepatoblastoma were not immunostained for p16. Our data indicate that the hypermethylation of p16 is a major mechanism of the transcriptional repression of p16 in hepatoblastomas, and we suggest that the inactivation of p16, leading to the lack of p16, may play an important role in the tumorigenesis of hepatoblastomas.

Synthesis of quinolinyl/isoquinolinyl[a]pyrrolo [3,4-c] carbazoles as cyclin D1/CDK4 inhibitors

A novel series of pyrrolo[3,4-c] carbazoles fused with a quinolinyl/isoquinolinyl moiety were synthesized and their D1/CDK4 inhibitory and antiproliferative activity were evaluated. Compound 8H, 14H-isoquinolinyl[6,5-a]-pyrrolo[3,4-c]carbazole-7,9-dione (1d) was found to be a highly potent D1/CDK4 inhibitor with an IC(50) of 69 nM. Compound 1d also inhibited tumor cell growth, arrested tumor cells in G1 phase and inhibited pRb phosphorylation.

Gene discovery using the serial analysis of gene expression technique: implications for cancer research

Cancer is a genetic disease. As such, our understanding of the pathobiology of tumors derives from analyses of the genes whose mutations are responsible for those tumors. The cancer phenotype, however, likely reflects the changes in the expression patterns of hundreds or even thousands of genes that occur as a consequence of the primary mutation of an oncogene or a tumor suppressor gene. Recently developed functional genomic approaches, such as DNA microarrays and serial analysis of gene expression (SAGE), have enabled researchers to determine the expression level of every gene in a given cell population, which represents that cell population's entire transcriptome. The most attractive feature of SAGE is its ability to evaluate the expression pattern of thousands of genes in a quantitative manner without prior sequence information. This feature has been exploited in three extremely powerful applications of the technology: the definition of transcriptomes, the analysis of differences between the gene expression patterns of cancer cells and their normal counterparts, and the identification of downstream targets of oncogenes and tumor suppressor genes. Comprehensive analyses of gene expression not only will further understanding of growth regulatory pathways and the processes of tumorigenesis but also may identify new diagnostic and prognostic markers as well as potential targets for therapeutic intervention.

Alteration of cyclin D1 and CDK4 gene in carcinoma of uterine cervix

Amplification and overexpression of cyclin D1 and CDK4 genes in cervical carcinoma were studied by semi-quantitative differential polymerase chain reaction assay and an immunostaining technique, respectively. Amplifications of cyclin D1 and CDK4 genes were found in 24% (27/113) and 26% (29/112) of tumors, respectively. Overexpression of cyclin D1 and CDK4 was demonstrated in 32% (21/66) and 73% (45/62) of tumors, respectively. No tumor showed CDK4 gene mutation on single strand conformational polymorphism. Sixteen percent (8/49) of the tumor specimens showed neither amplification nor overexpression. Disease stage, tumor grade and overexpression of cyclin D1 were found to be independent poor prognostic factors in cervical carcinoma.

Selective in vivo and in vitro effects of a small molecule inhibitor of cyclin-dependent kinase 4

BACKGROUND

Cyclin-dependent kinase 4 (Cdk4) represents a prime target for the treatment of cancer because most human cancers are characterized by overexpression of its activating partner cyclin D1, loss of the natural Cdk4-specific inhibitor p16, or mutation(s) in Cdk4's catalytic subunit. All of these can cause deregulated cell growth, resulting in tumor formation. We sought to identify a small molecule that could inhibit the kinase activity of Cdk4 in vitro and to then ascertain the effects of that inhibitor on cell growth and tumor volume in vivo.

METHODS

A triaminopyrimidine derivative, CINK4 (a chemical inhibitor of Cdk4), was identified by screening for compounds that could inhibit Cdk4 enzyme activity in vitro. Kinase assays were performed on diverse human Cdks and on other kinases that were expressed in and purified from insect cells to determine the specificity of CINK4. Cell cycle effects of CINK4 on tumor and normal cells were studied by flow cytometry, and changes in phosphorylation of the retinoblastoma protein (pRb), a substrate of Cdk4, were determined by western blotting. The effect of the inhibitor on tumor growth in vivo was studied by use of tumors established through xenografts of HCT116 colon carcinoma cells in mice. Statistical tests were two-sided.

RESULTS

CINK4 specifically inhibited Cdk4/cyclin D1 in vitro. It caused growth arrest in tumor cells and in normal cells and prevented pRb phosphorylation. CINK4 treatment resulted in statistically significantly (P: =.031) smaller mean tumor volumes in a mouse xenograft model.

CONCLUSIONS

Like p16, the natural inhibitor of Cdk4, CINK4 inhibits Cdk4 activity in vitro and slows tumor growth in vivo. The specificity of CINK4 for Cdk4 raises the possibility that this small molecule or one with a similar structure could have therapeutic value.

p57(KIP2) is not mutated in hepatoblastoma but shows increased transcriptional activity in a comparative analysis of the three imprinted genes p57(KIP2), IGF2, and H19

Hepatoblastomas (HBs), representing malignant liver tumors of childhood, show frequent loss of heterozygosity (LOH) in the chromosomal region 11p15.5. This loss is of maternal origin suggesting the presence of a monoallelically expressed tumor suppressor gene in this region. p57(KIP2) (KIP2) located at 11p15.5 is predominantly expressed from the maternal allele and encodes a cyclin-dependent kinase inhibitor. We screened a series of 56 HB tumors and five HB cell lines for allelic loss (LOH) of the KIP2 locus by microsatellite analysis and KIP2 coding sequence mutations by single-strand conformation polymorphism analysis. Although LOH at the KIP2 locus occurred in 25% of the cases, no mutations were found. Analysis of KIP2 mRNA expression by competitive reverse transcriptase-polymerase chain reaction revealed up-regulation in nine of 12 HBs compared to matching liver samples. In contrast, mRNA levels of the putative suppressor gene H19 on 11p15.5 were decreased in 10 of 12 tumors, indicating that KIP2 and H19 are not co-regulated in HBs. IGF2 mRNA expression was increased in 11 of 12 HB samples. All HBs showed monoallelic KIP2 expression. However, the overexpression of KIP2 in HBs with maternal loss of 11p15.5 suggests a reactivation of the paternal allele in these cases. Overexpression of KIP2 in HBs argues against a role as a HB suppressor gene.

Inhibition of cyclin-dependent kinase 4 (Cdk4) by fascaplysin, a marine natural product

Small chemical molecules that interfere with biological proteins could be useful for gaining insight into the complex biochemical processes in mammalian cells. Cdk4 is a key protein whose activity is required not only for emergence of cells from quiescence but also at the G1/S transition in the cell cycle and which is misregulated in 60-70% of human cancers. We set out to identify chemical inhibitors of Cdk4 and discovered that, in vitro, fascaplysin specifically inhibited Cdk4. Molecular modelling based on the crystal structure of Cdk2 suggests that fascaplysin inhibits Cdk4 by binding to the ATP pocket of the kinase. Treatment of tumour (p16(-), pRb(+)) and normal (p16(+), pRb(+)) cell lines with fascaplysin caused G1 arrest and prevented pRb phosphorylation at sites implicated as being specific for Cdk4 kinase. Fascaplysin will therefore prove to be a useful tool in studying the consequence of Cdk4 inhibition, especially in cells containing inactivated p16.