Butyrolactone I induces cyclin B1 and causes G2/M arrest and skipping of mitosis in human prostate cell lines

C-4-Modified Isotetrones Prevent Biofilm Growth and Persister Cell Resuscitation in Mycobacterium smegmatis

Hyperphosphorylated nucleotide (p)ppGpp, synthesized by Rel protein, regulates the stringent response pathway responsible for biofilm and persister cell growth in mycobacteria. The discovery of vitamin C as an inhibitor of Rel protein activities raises the prospect of tetrone lactones to prevent such pathways. The closely related isotetrone lactone derivatives are identified herein as inhibitors of the above processes in a mycobacterium. Synthesis and biochemical evaluations show that an isotetrone possessing phenyl substituent at C-4 inhibit the biofilm formation at 400 μg mL^-1, 84 h post-exposure, followed by moderate inhibition by the isotetrone possessing the p-hydroxyphenyl substituent. The latter isotetrone inhibits the growth of persister cells at 400 μg mL^-1 f.c. when monitored for 2 weeks, under PBS starvation. Isotetrones also potentiate the inhibition of antibiotic-tolerant regrowth of cells by ciprofloxacin (0.75 μg mL^-1) and thus act as bioenhancers. Molecular dynamics studies show that isotetrone derivatives bind to the Rel_Msm protein more efficiently than vitamin C at a binding site possessing serine, threonine, lysine, and arginine.

Morin Sensitizes MDA-MB-231 Triple-Negative Breast Cancer Cells to Doxorubicin Cytotoxicity by Suppressing FOXM1 and Attenuating EGFR/STAT3 Signaling Pathways

Considerable emphasis is being placed on combinatorial chemotherapeutic/natural treatments for breast cancer. This study reveals the synergistic anti-tumor activity of morin and Doxorubicin (Dox) co-treatment on MDA-MB-231 triple-negative breast cancer (TNBC) cell proliferation. Morin/Dox treatment promoted Dox uptake and induced DNA damage and formation of nuclear foci of p-H2A.X. Furthermore, DNA repair proteins, RAD51 and survivin, and cell cycle proteins, cyclin B1 and forkhead Box M1 (FOXM1), were induced by Dox alone but attenuated by morin/Dox co-treatment. In addition, Annexin V/7-AAD analysis revealed that necrotic cell death after co-treatment and apoptotic cell death by Dox alone were associated with the induction of cleaved PARP and caspase-7 without Bcl-2 family involvement. FOXM1 inhibition by thiostrepton showed that co-treatment caused FOXM1-mediated cell death. Furthermore, co-treatment downregulated the phosphorylation of EGFR and STAT3. Flow cytometry showed that the accumulation of cells in the G2/M and S phases might be linked to cellular Dox uptake, p21 upregulation, and cyclin D1 downregulation. Taken together, our study shows that the anti-tumor effect of morin/Dox co-treatment is due to the suppression of FOXM1 and attenuation of EGFR/STAT3 signaling pathways in MDA-MB-231 TNBC cells, which suggests that morin offers a means of improving therapeutic efficacy in TNBC patients.

Chemical Investigation of Marine-Derived Fungus Aspergillus flavipes for Potential Anti-Inflammatory Agents

The marine fungus, Aspergillus flavipes (MTCC 5220), was isolated from the pneumatophore of a mangrove plant Acanthus ilicifolius found in Goa, India. The crude extract of A. flavipes was found to show anti-inflammatory activity. It blocked interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) production in lipopolysaccharide (LPS)-activated THP-1 cells with IC₅₀ of 2.69±0.5 μM and 6.64±0.4 μM, respectively. The chemical investigation led to the isolation of optically inactive 4β-[(1E)-propen-1-yl]cyclopentane-1β,2β-diol (1) along with a new optically active diastereoisomeric compound, 4β-[(1E)-propen-1-yl]cyclopentane-1β,2α-diol (2). In addition, the fungus also produced known compounds (+)-terrein (3), butyrolactone I (4) and butyrolactone II (5) in high yields. Among these, (+)-terrein (3) exhibited IL-6 and TNF-α inhibition activity with IC₅₀ of 8.5±0.68 μM and 15.76±0.18 μM, respectively, while butyrolactone I (4) exhibited IC₅₀ of 12.03±0.85 μM (IL-6) and 43.29±0.76 μM (TNF-α) inhibition activity with low toxicity to host cells in LPS stimulated THP-1 cells. This is the first report of the isolation and characterization of 4β-[(1E)-propen-1-yl]cyclopentane-1β,2α-diol (2). The structures of all the isolated compounds were elucidated on the basis of extensive detailed NMR spectroscopic data. Anti-inflammatory activity of the fungi A. flavipes is presented here for the first time, which was due to (+)-terrein and butyrolactone I, as the major constituents and they can be further explored in the therapeutic area.

Synthesis of Butenolides via a Horner-Wadsworth-Emmons Cascading Dimerization Reaction

The efficient synthesis of a range of structurally related butenolides has been observed while we were exploring the substrate-scope of a Horner-Wadsworth-Emmons (HWE) reaction. While aliphatic aldehydes gave the expected HWE product, aromatic aldehydes furnished butenolides, resulting from the dimerization of the HWE product during desilylation of the initially formed HWE adduct. In addition to isolating butenolides in a high yield, we have also determined precisely when dimerization occurs.

A new benzophenone with biological activities from metabolites of butyrolactone I in rat faeces

Butyrolactone I, one of the major secondary metabolites of fungus Aspergillus terreus, is a selective cdc2 kinase inhibitor. In the present study, the metabolism of butyrolactone I in male Wistar rats was investigated by characterising metabolites excreted into faeces. Following an oral dose of 40 mg/kg butyrolactone I, two phase I metabolites were isolated from the faeces of rat. The new structure was identified on the spectroscopic data analysis. The new compound V1 and known compound V2 were tested for their cytotoxicity, antimicrobial and antioxidant activity. V1 and V2 showed significant free radical scavenging ability. V2 also showed strong inhibitory activity against Staphylococcus aureus.

Water enables an asymmetric cross reaction of α-keto acids with α-keto esters for the synthesis of quaternary isotetronic acids

A water promoted asymmetric aldol/lactonization/enolization cascade reaction of α-keto acids and α-keto esters affords chiral quaternary isotetronic acids with excellent enantioselectivity.

Dereplication and targeted isolation of bioactive sulphur compound from bacteria isolated from a hydrothermal field

Marine micro-organisms in the deep-sea hydrothermal vent systems are considered as potential sources of bioactive natural products. Sixteen bacterial strains were isolated from a deep-sea hydrothermal field and screened for bioactive metabolism studies. After the strains were subjected to bioactive testing at different culture media, chemical dereplication by HPLC coupled to high-resolution mass spectrometer was performed to analyse or determine the main secondary metabolisms in those strains. Strain 06204 was large-scale fermented with relative optimal media, for isolating the desired sulphur compound. Butyrolactone I 3-sulphate was isolated and structurally identified from the extract, guided by dereplication and showed moderate antivirus activities against H3N2 and EV71 viruses. Our study suggests that deep-sea hydrothermal bacteria are good sources of sulphur natural products. Meanwhile, the described approach, mainly bioactive screening, dereplication and targeted isolation, is effective and efficient to discover interesting bioactive compounds in hydrothermal bacteria.

Phenylisotertronic acids from the TCM endophytic fungus Phyllosticta sp

Four new phenylisotertronic acids (1a/1b, 2a, and 3a) were isolated from a TCM endophytic fungal strain Phyllosticta sp. J13-2-12Y obtained from the leaves of Acorus tatarinowii, along with two known ones (2b and 3b). Compounds 1-3 all existed as mixtures of enantiomers, and their corresponding optically pure enantiomers were successfully isolated by chiral HPLC. The structures of isolated compounds were determined by comprehensive spectroscopic analyses and X-ray diffraction. Their absolute configurations were determined by ECD experiments and quantum chemical calculations. In addition, the antimicrobial activities and the cytotoxicities of these three pairs of optically pure enantiomers (1a/1b, 2a/2b, and 3a/3b) had been evaluated.

γ-Butyrolactones from Aspergillus Species: Structures, Biosynthesis, and Biological Activities

Recently, numerous metabolites possessing uncommon structures and potent bioactivity have been isolated from strains of fungi collected from diverse environments. The genus Aspergillus is known as a rich source of γ-butyrolactones. These are a group of fungal secondary metabolites, consisting of a five-membered lactone bearing two aromatic rings, which shows a great variety of biological activities. This review summarizes the research on the biosynthesis, source, and biological activities of the naturally occurring γ-butyrolactones that have been isolated from Aspergillus species published over the last decades. More than 75 compounds are described and 65 references are cited.

Bioactive compounds of Aspergillus terreus-F7, an endophytic fungus from Hyptis suaveolens (L.) Poit

The compounds terrein (1), butyrolactone I (2), and butyrolactone V (3) were isolated from the ethyl acetate extract (EtOAc) of the endophytic fungus Aspergillus terreus-F7 obtained from Hyptis suaveolens (L.) Poit. The extract and the compounds presented schistosomicidal activity against Schistosoma mansoni; at 100 µg/mL for EtOAc extract, 1297.3 µM for compound 1, 235.6 µM for compound 2, and 454.1 µM for compound 3, they killed 100% of the parasites after 72 h of treatment. Compounds 1, 2, and 3 exerted moderate leishmanicidal activity against Leishmania amazonensis (IC₅₀ ranged from 23.7 to 78.6 µM). At 235.6 and 227.0 µM, compounds 2 and 3, respectively, scavenged 95.92 and 95.12% of the DPPH radical (2,2-diphenyl-1-picryl-hydrazyl), respectively. Regarding the cytotoxicity against the breast tumor cell lines MDA-MB-231 and MCF-7, compound 2 gave IC₅₀ of 34.4 and 17.4 µM, respectively, while compound 3 afforded IC₅₀ of 22.2 and 31.9 µM, respectively. At 117.6 µM, compound 2 inhibited the growth of and killed the pathogen Escherichia coli (ATCC 25922). Compounds 1, 2, and 3 displayed low toxicity against the normal line of human lung fibroblasts (GM07492A cells), with IC₅₀ of 15.3 × 10³, 3.4 × 10³, and 5.8 × 10³ µM, respectively. This is the first report on (i) the in vitro schistosomicidal and leishmanicidal activities of the EtOAc extract of A. terreus-F7 and compounds 1, 2, and 3; and (ii) the antitumor activity of compounds 2 and 3 against MDA-MB-231 and MCF-7 cells.

Three New Butenolides from the Fungus Aspergillus sp. CBS-P-2

Three new butenolides aspernolides H–J (1–3) together with seven known ones (4–10) were isolated from the fungus Aspergillus sp. CBS-P-2. Their chemical structures were established on the basis of 1D- and 2D-NMR spectroscopic data, HR-ESI-MS analysis, and their absolute configuration were determined by circular dichroism (CD) analysis. All the compounds were evaluated for the antioxidant effects by DPPH and ABTS methods, the antitumor activities against four human tumor cell lines (HL-60, ASPC1, HCT-116 and PC-3) and antimicrobial activities. Compounds 4–10 showed significant activity against DPPH (IC₅₀ = 15.9–34.3 μM) and compounds 1–10 exhibited significant ABTS free radical scavenging activity (IC₅₀ = 2.8–33.1 μM). Compounds 2, 5 and 11 showed potent cytotoxic activities against HL-60 cell lines with IC₅₀ values of 39.4, 13.2 and 16.3 μM, respectively. Compound 10 showed good antimicrobial activity against Staphylococcus aureus with minimum inhibitory concentration (MIC) of 21.3 μM.

Isotetronic acids from an oxidative cyclization

Oxidation of methyl enones with selenium dioxide leads to a cascade of reactions culminating in the formation of isotetronic acids.

Butyrolactone I Quantification from Lovastatin Producing Aspergillus terreus Using Tandem Mass Spectrometry-Evidence of Signalling Functions

Aspergillus terreus is an industrially important filamentous fungus producing a wide spectrum of secondary metabolites, including lovastatin and itaconic acid. It also produces butyrolactone I which has shown potential as an antitumour agent. Additionally, butyrolactone I has been implicated to have a regulating role in the secondary metabolism and morphology of A. terreus. In this study, a quantitative time-course liquid chromatography—electrospray ionisation—tandem mass spectrometry (LC-ESI-MS-MS) analysis of butyrolactone I is reported for the first time in nine-day long submerged cultures of A. terreus. Butyrolactone I was fragmented in the mass analysis producing a reproducible fragmentation pattern of four main daughter ions (m/z 307, 331, 363 and 393) in all the samples tested. Supplementing the cultures with 100 nM butyrolactone I caused a statistically significant increase (up to two-fold) in its production, regardless of the growth stage but was constitutive when butyrolactone I was added at high cell density during the stationary phase. Furthermore, the extracellular butyrolactone I concentration peaked at 48 h post inoculation, showing a similar profile as has been reported for bacterial quorum sensing molecules. Taken together, the results support the idea of butyrolactone I as a quorum sensing molecule in A. terreus.

The HLJ1-targeting drug screening identified Chinese herb andrographolide that can suppress tumour growth and invasion in non-small-cell lung cancer

HLJ1 is a novel tumour suppressor and is a potential druggable target for non-small-cell lung cancer (NSCLC). In this report, using a promoter-containing enhancer region as the HLJ1-targeting drug-screening platform, we identified several herbal compounds from a Chinese herbal bank with the capacity to enhance HLJ1 promoter activity and suppress tumour growth and invasion of NSCLC. Among the herbal drugs identified, the andrographolide (from Andrographis paniculata [Burm. f.] Nees.) most significantly induced HLJ1 expression and suppressed tumorigenesis both in vitro and in vivo. The andrographolide upregulates HLJ1 via JunB activation, which modulates AP-2α binding at the MMP-2 promoter and represses the expression of MMP-2. In addition, silencing of HLJ1 partially reverses the inhibition of cancer-cell invasion by andrographolide. Microarray transcriptomic analysis was performed to comprehensively depict the andrographolide-regulated signalling pathways. We showed that andrographolide can affect 939 genes (analysis of variance, false discovery rate < 0.05) that are dominantly involved in the cell cycle, apoptosis and adhesion-related biological signalling, including mitogen-activated protein kinase, focal adhesion and tight junction pathways, indicating the diverse effects of andrographolide on anticancer invasion and proliferation. In conclusion, the HLJ1-targeting drug-screening platform is useful for screening of novel anticancer compounds. Using this platform, we identified andrographolide is a promising new anticancer agent that could suppress tumour growth and invasion in NSCLC.

Asperjinone, a nor-neolignan, and terrein, a suppressor of ABCG2-expressing breast cancer cells, from thermophilic Aspergillus terreus

Breast cancer cells express ABCG2 transporters, which mediate multidrug resistance. Discovering a novel compound that can suppress ABCG2 expression and restore drug sensitivity could be the key to improving breast cancer therapeutics. In the current work, one new nor-neolignan, asperjinone (1), as well as 12 other known compounds, was isolated from Aspergillus terreus. The structure of the new isolate was determined by spectroscopic methods. Among these isolates, terrein (2) displayed strong cytotoxicity against breast cancer MCF-7 cells. Treatment with terrein (2) significantly suppressed growth of ABCG2-expressing breast cancer cells. This suppressive effect was achieved by inducing apoptosis via activating the caspase-7 pathway and inhibiting the Akt signaling pathway, which led to a decrease in ABCG2-expressing cells and a reduction in the side-population phenotype.

Four butyrolactones and diverse bioactive secondary metabolites from terrestrial Aspergillus flavipes MM2: isolation and structure determination

The chemical constituents and biological activities of the terrestrial Aspergillus flavipes MM2 isolated from Egyptian rice hulls are reported. Seven bioactive compounds were obtained, of which one sterol: ergosterol (1), four butyrolactones: butyrolactone I (2), aspulvinone H (3), butyrolactone-V (6) and 4,4'-diydroxypulvinone (7), along with 6-methylsalicylic acid (4) and the cyclopentenone analogue; terrien (5). Structures of the isolated compounds were deduced by intensive studies of their 1D & 2D NMR, MS data and comparison with related structures. The strain extract and the isolated compounds (1-7) were biologically studied against number of microbial strains, and brine shrimp for cytotoxicity. In this article, the taxonomical characterization of A. flavipes MM2 along with its upscale fermentation, isolation and structural assignment of the obtained bioactive metabolites, and evaluate their antimicrobial and cytotoxic activities were described.

Aspernolides A and B, butenolides from a marine-derived fungus Aspergillus terreus

Two aromatic butenolides, aspernolides A and B along with the known metabolites, butyrolactone I, terrein and physcion were isolated from the fermentation broth of a soft coral derived fungus Aspergillus terreus. The structures of these metabolites were assigned on the basis of detailed spectroscopic analysis. The absolute stereochemistry of aspernolides A (1) and B (2) was established by their preparation from the known butyrolactone I. Biogenetically aspernolides A and B must be derived from butyrolactone I, a well known specific inhibitor of cyclin dependent kinase (cdk) from A. terreus. When tested, aspernolide A exhibited mild cytotoxicity against cancer cell lines.

Chloramphenicol induces abnormal differentiation and inhibits apoptosis in activated T cells

Chloramphenicol is a broad-spectrum antibiotic used for the treatment of many infectious diseases and has become one of the major seafood contaminants. Hematologic disorders such as aplastic anemia and leukemia induced by chloramphenicol are a major concern. However, the mechanism underlying chloramphenicol-induced leukemogenesis is not known. By investigating the effects of chloramphenicol on the activation of mouse T cells stimulated with anti-CD3 antibody or staphylococcal enterotoxin B, we found that chloramphenicol induces the differentiation of activated T cells into lymphoblastic leukemia-like cells, characterized by large cell size, multiploid nuclei, and expression of CD7, a maker for immature T cells and T-cell lymphocytic leukemia, thus phenotypically indicating differentiation toward leukemogenesis. High expression of cyclin B1, but not p53, c-myc, and CDC25A, was detected in chloramphenicol-treated activated T cells, which may relate to abnormal cell differentiation. Chloramphenicol inhibited the activation-induced cell death of mouse and human T-cell receptor-activated T cells by down-regulating the expression of Fas ligand. Our findings show that abnormal cell differentiation and inhibition of apoptosis may contribute to the development of leukemia associated with clinical applications of chloramphenicol.

Butyrolactone I derivatives from Aspergillus terreus carrying an unusual sulfate moiety

In our ongoing search for new bioactive metabolites from microbial resources, Aspergillus terreus (HKI0499) was examined by chemical metabolite profiling. Together with the known butyrolactone I ( 3), the unusual sulfate derivatives butyrolactone I 3-sulfate ( 1) and butyrolactone I 4''-sulfate ( 2) were discovered. The chemical structures were determined by NMR and MS data analyses. All compounds were tested on CDK1/cyclin B, CDK5/p25, DYRK1A, CK1, and GSK-3alpha/beta kinases; compounds 2 and 3 were also evaluated for their cytotoxic and antiproliferative activities. Butyrolactone I 3-sulfate ( 1) exhibited specific inhibitory activity against CDK1/cyclin B and CDK5/p25, yet 15-30-fold less than butyrolactone I ( 3). Likewise, butyrolactone I 3-sulfate ( 1) exhibited moderate cytotoxicity solely against HeLa cells (CC 50 = 80.7 microM).

Alteration of subcellular and cellular expression patterns of cyclin B1 in renal cell carcinoma is significantly related to clinical progression and survival of patients

Cyclin B1, identified as a regulator of late cell cycle, is involved in the development and progression of a variety of human malignancies. To clarify the role of cyclin B1 in the pathogenesis and prognosis of renal cell carcinoma (RCC), protein expression was compared with clinicopathological characteristics of patients as well as the long-term survival after surgical therapy. Expression analysis was carried out by immunohistochemistry and tissue microarray analysis. The microarrays that represented the primary tumors, their invasion front and normal peritumoral renal parenchyma contained 753 tissue cores obtained from 251 randomly selected nephrectomy specimens. Immunopositivity within the primary tumors was significantly associated with tumor stage (pT) (p < 0.01), lymph node status (pN) (p < 0.01) as well as the presence of systemic metastatic disease (p = 0.01). Subcellular expression in the cytoplasm of tumor cells significantly correlated with pT (p = 0.02) and pN (p = 0.03). When peritumoral tissue samples exhibited a relative amount of <10% of positively reacting epithelial cells, cyclin B positivity was identified to predict long-term survival of patients in univariate analysis (p < 0.01) whereas borderline significance was observed in multivariate statistical analysis (p = 0.05). Increased intratumoral cyclin B1 positivity and aberrant localization of signals within the cytoplasm of tumor cells is positively correlated with the tendency towards tumor progression, indicating the significant role of cyclin B1 in the development and pathogenesis of RCC. The result of uni- and multivariate statistical analysis suggests the prognostic value of cyclin B1 for RCC patients.

Synthesis and biological evaluation of analogues of butyrolactone I and molecular model of its interaction with CDK2

A series of analogues of butyrolactone I, a natural product isolated from Aspergillus terreus that selectively inhibits the CDK2 and CDK1 kinases and that has been found to exhibit an interesting antiproliferative activity, have been synthesized. Its antitumor activity has been tested. Molecular models of the complex between butyrolactone I and the CDK2 active site have been built using a combination of conformational search and automated docking techniques. The stability of the resulting complexes has been assessed by molecular dynamics simulations and the experimental results obtained for the synthesized analogues are rationalized based on the molecular models.

Prominent induction of cyclin B1 in G2/M renal cancer cells with butyrolactone 1

BACKGROUND

Butyrolactone 1 (BL) is a cyclin dependent kinase (CDK) inhibitor derived from Aspergillus terreus. None of the present drugs are effective for the treatment of renal cell carcinoma. The use of BL is expected to promote a new type therapy of renal cancer.

METHODS

We investigated three human renal cancer cell lines: ACHN, OS-RC-2 and RCC10RGB, using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and two-color flow cytometry. Simultaneous measurements of DNA content and cyclin expression allowed us to perform cell cycle specific analysis. Western blot analysis was performed using ACHN to represent cell lines.

RESULTS

BL inhibited cell proliferation and caused cell accumulation at G2/M phase associated with the emergence of the third peak. Moreover, BL induced cyclin B1 over-expression in G2/M cells. These changes were quite definite, whereas cyclins D1, E and A showed no changes at all. Cyclin B1 accumulation was confirmed by western blot analysis. The chronological observation revealed that the emergence of the third peak preceded the regression of the increased cyclin B1 positive G2/M cells. These results suggested that BL accelerated cyclin B1 accumulation in G2/M cells, which then shifted to G1 phase without cell division. New G1 cells started DNA synthesis most likely as endoreduplication to form the third peak and the mechanism of cyclin B1 accumulation converted into down-regulation.

CONCLUSION

BL induced significant cell kinetic interference in the tested human renal carcinoma cell lines. This might indicate the possibility of a new medical treatment modality for renal cancer.

Distinct effects of dinuclear ruthenium(III) complexes on cell proliferation and on cell cycle regulation in human and murine tumor cell lines

We have examined the biological and antitumor activity of a series of dinuclear ruthenium complexes. The aim of this study was to compare the in vitro effects of these new compounds on cell proliferation, cell distribution among cell cycle phases, and the expression of some proteins involved in cell cycle regulation. Results obtained show a mild cytotoxic activity against human and murine cell lines, more evident after prolonged exposure of cell challenge. Two of the eight dinuclear complexes [namely, compounds D3 (Na(2)[(RuCl(4)(dmso-S))(2)(mu-bipy)]) and D7 ([NH(4)][(RuCl(4)(dmso-S))(mu-pyz)(RuCl(3)(dmso-S)(dmso-O))]) modify cell cycle distribution similarly to imidazolium trans-imidazoledimethylsulfoxidetetrachlororuthenate (NAMI-A), whereas the others have a low or negligible effect on this parameter. If we correlate the induction of cell cycle modifications with ruthenium uptake by tumor cells and with the modulation of proteins regulating cell cycle, we may stress that the induction of G(2)-M cell cycle arrest is related to the achievement of a threshold concentration of ruthenium inside the cells, which is dependent on the cell line being used, and that only cyclin B, among cell cycle regulating proteins examined by immunoblotting assays, appears to be significantly modified. This in vitro study shows that dinuclear ruthenium complexes may have a behavior similar to that of the monomer NAMI-A. These results encourage the future experimentation of their pharmacological properties in in vivo models.

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.

Mxi1, a Myc antagonist, suppresses proliferation of DU145 human prostate cells

BACKGROUND

Mxi1, an antagonist of c-Myc, maps to human chromosome 10q24-q25, a region altered in a substantial fraction of prostate tumors. Mice deficient for Mxi1 exhibit significant prostate hyperplasia. We studied the ability of Mxi1 to act as a growth suppressor in prostate tumor cells.

METHODS

We infected DU145 prostate carcinoma cells with an Mxi1-expressing adenovirus (AdMxi1) in vitro, and measured Mxi1 expression, cell proliferation, soft agar colony formation, and cell cycle distribution. To explore mechanisms of Mxi1-induced growth arrest, we performed gene expression analysis.

RESULTS

AdMxi1 infection resulted in reduced cell proliferation, reduced soft agar colony formation, and a higher proportion of cells in the G(2)/M phase of the cell cycle. This G(2)/M growth arrest was associated with elevated levels of cyclin B, and reduced levels of c-MYC and MDM2.

CONCLUSIONS

The ability of AdMxi1 to suppress prostate tumor cell proliferation supports a role for Mxi1 loss in the pathogenesis of a subset of human prostate cancers. Prostate 47:194-204, 2001.

Overexpression of Akt inhibits NGF-induced growth arrest and neuronal differentiation of PC12 cells

To investigate the role of Akt in nerve growth factor (NGF)-induced neuronal differentiation, PC12 cells ectopically expressing wild-type or dominant-inhibitory forms of Akt were analyzed. NGF-induced neurite outgrowth was greatly accelerated in cells expressing dominant-inhibitory Akt, compared to parental PC12 cells, but was almost completely blocked in cells expressing wild-type Akt. Since neuronal differentiation requires an arrest of cell growth, several aspects of cell growth of the different cell lines were compared. Cells expressing wild-type Akt were not susceptible to the growth-arresting effect of NGF, whereas parental PC12 cells and notably cells expressing mutant Akt were so affected. Accompanying this, the expressions of CDKs and p21(WAF1) were down- and up-regulated, respectively, in both parental PC12 cells and cells expressing mutant Akt. When treated with some growth arrest-inducing agents such as sodium nitroprusside, forskolin and butyrolactone I, cells expressing wild-type Akt regained their responsiveness to the effects of NGF on differentiation. In summary, our results indicate that Akt overrides the growth-arresting effect of NGF and thereby, negatively regulates neuronal differentiation.

Cyclin-dependent kinase inhibitors: novel anticancer agents

In current models of cell cycle control, the transition between different cell cycle states is regulated at checkpoints. Transition through the cell-cycle is induced by a family of protein kinase holoenzymes, the cyclin-dependent kinases (CDKs) and their heterodimeric cyclin partner. Orderly progression through the cell-cycle involves co-ordinated activation of the CDKs, which in the presence of an associated CDK-activating kinase, phosphorylate target substrates including members of the 'pocket protein' family. This family includes the product of the retinoblastoma susceptibility gene (the pRb protein) and the related p107 and p130 proteins. Activity of these holoenzymes is regulated by post-translational modification. Phosphorylation of inhibitory sites on a conserved threonine residue within the activation segment is regulated by CDK7/cyclin H, referred to as CDK-activating kinase [1]. In addition, the cdc25 phosphatases activate the CDKs by dephosphorylating their inhibitory tyrosine and threonine phosphorylated residues [2,3]. Among the many roles for endogenous inhibitors (CDKIs), including members of the p21(CIP1/Waf1) family and the p16 family, one role is to regulate cyclin activity. Cellular neoplastic transformation is accompanied by loss of regulation of cell cycle checkpoints in conjunction with aberrant expression of CDKs and/or cyclins and the loss or mutation of the negative regulators (the CDKIs or the pocket protein pRb). One strategy to inhibit malignant cellular proliferation involves inhibiting CDK activity or enhancing function of the CDKI. Novel inhibitors of CDKs showing promise in the clinic include flavopiridol and UCN-01, which show early evidence of human tolerability in clinical trials. This review examines pertinent advances in the field of CDK inhibitors.