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.     

Interaction of retinoblastoma protein and D cyclins during cell-growth inhibition by hexamethylenebisacetamide in TM2H mouse epithelial cells

To explore the regulation and function of D-type cyclins in breast cancer cells, the mouse mammary hyperplastic epithelial cell line TM2H was treated with 5 mM hexamethylenebisacetamide (HMBA), a polar differentiation factor. The resulting growth-inhibitory effect of HMBA was completely reversible and was analyzed in terms of percent cells in G₁; association of D-type cyclins with cyclin-dependent kinase (cdk) 4 and cdk6; G₁ kinase activity; association of retinoblastoma protein (pRb) and phosphorylated pRb with D-type cyclins; and association of p16^INK4a, p15^INK4b, and p27^Kip1 with cdk4 and cdk6. Synchronized TM2H cells were examined at 0, 3, 5, 9, 12, and 24 h after exposure to 5 mM HMBA. Inhibition of DNA synthesis, as measured by thymidine uptake, was first observed at 5 h (40%) and peaked at 24 h (80%). Flow cytometry at 9 h showed treated cells to be in G₁ arrest. Western blot analysis showed weakly detectable cyclin D1 but readily detectable cyclin D2 and D3 proteins at 0 h; thereafter, cyclin D2 and D3 protein levels remained higher while cyclin D1 levels declined significantly in treated versus untreated cells. By 5 h (early G₁), HMBA had markedly inhibited cdk4 and cdk6 kinase activity (67% and 75%, respectively) in treated versus untreated cells. By 9 and 12 h, pRb levels had increased 3.4-fold in treated versus untreated cells. At 5 h, cyclin D–associated pRb was totally hypophosphorylated in treated cells and hyperphosphorylated in untreated cells. The levels of pRb associated with cyclin D2 and D3 increased 2.89-fold and 4.6-fold, respectively, in treated versus untreated cells. At 5 h, treated cells showed a fivefold increase in cdk4-associated p27^Kip1 and, at 9 h, a fourfold increase in cdk6-associated p27^Kip1 over control levels. In confirmation of these data, HMBA was found to inhibit the growth of Rb-positive Du/145Rb cells but not their Rb-negative parental Du/145 cells. The data suggest that HMBA-induced growth inhibition is due to multifactorial mechanisms involving decreases in total cyclin D1 and inhibition of cdk4 and cdk6 kinase activities through elevation of levels of cdk4- and cdk6-associated p27^Kip1 and concomitant increases in hypophosphorylated pRb and stable cyclin D2/pRb and cyclin D3/pRb complexes that help maintain pRb in a functional state. Mol. Carcinog. 22:128–143, 1998. © 1998 Wiley-Liss, Inc.     

Rhabdoid tumor growth is inhibited by flavopiridol.

PURPOSE: Rhabdoid tumors are aggressive and incurable pediatric malignancies. INI1/hSNF5, a tumor suppressor biallelically deleted/inactivated in rhabdoid tumors, directly represses cyclin D1. Rhabdoid tumors and cells are exquisitely dependent on cyclin D1 for genesis and survival, suggesting that targeting the cyclin/cyclin-dependent kinase (cdk) axis may be an effective therapeutic strategy for these tumors. Because cdk inhibitors have not been used for preclinical or clinical testing on rhabdoid tumors, we investigated the effect of flavopiridol, a pan-cdk inhibitor with promising clinical activity, on rhabdoid tumors. EXPERIMENTAL DESIGN: The effect of flavopiridol on rhabdoid cells was tested in vitro using survival, cell cycle, and apoptosis assays. Its effect was assessed in vivo using xenografted rhabdoid tumor models. Immunoblot and immunohistochemical analysis was used to assess the effect of flavopiridol on cyclin D1 and p21 expression in vitro and in vivo, respectively. RESULTS: Nanomolar concentrations of flavopiridol inhibited rhabdoid cell growth (IC(50) approximately 200 nmol/L), induced G(1) and G(2) arrest, and apoptosis in vitro in a concentration-dependent manner. These effects were correlated with the down-modulation of cyclin D1, up-regulation of p21, and induction of caspase 3/7 activities. Flavopiridol (at 7.5 mg/kg) significantly inhibited the growth of xenografted rhabdoid tumors, and its effect was correlated with the induction of p21 and down-modulation of cyclin D1. CONCLUSIONS: Flavopiridol is effective in inducing cell cycle arrest and cytotoxicity in rhabdoid tumors. Its effects are correlated with the down-regulation of cyclin D1 and the up-regulation of p21. Flavopiridol is potentially a novel chemotherapeutic agent for rhabdoid tumors.     

Involvement of p27Kip1 in G1 arrest by high dose 5 alpha-dihydrotestosterone in LNCaP human prostate cancer cells

The cell cycle is governed by cyclin dependent kinases (cdks), which are activated by binding of cyclins, inhibited by cdk inhibitors and regulated by phosphorylation and dephosphorylation. Exposure to high dose dihydrotestosterone (DHT) inhibits population growth of the human prostate carcinoma cell line, LNCaP. To determine the mechanism of growth arrest by high dose DHT, we assayed the changes in cell cycle profile and the cell cycle regulators that mediate these effects. Treatment of asynchronously growing LNCaP cells with 100 nM DHT caused a G1 arrest. The proportion of cells in S phase fell from 22 to 2%, while the G1 fraction rose from 74 to 92% by 24 h. Loss of phosphorylation of the retinoblastoma protein was noted and cdk4 and cyclin E/ cdk2 activities fell. Inhibition of these G1 cyclin dependent kinases was not due to loss of either cyclin or cdk proteins nor to increases in the cdk inhibitors p16INK4A and p21CiP1. p21Cip1 protein levels remained constant, and cyclin E-associated p21CiP1 fell, suggesting that p21CiP1 is not relevant to this form of cyclin E/cdk2 inhibition. Of note, total p27KiP1 levels and cyclin E-associated p27Kip1 increased as cells arrested and the amount of the CAK activated cdk2 bound to cyclin E decreased. p27KiP1 immunodepletion experiments demonstrated that the DHT-mediated increase in p27Kip1 was sufficient to fully saturate and inhibit target cyclin E/ cdk2. The inhibition of cyclin E/cdk2 by p27Kip1 contributes to G1 arrest of LNCaP following high dose DHT. p27KiP1 may be a key effector of androgen dependent growth modulation in prostate cancer cells.     

Fludarabine-induced apoptosis of B chronic lymphocytic leukemia cells includes early cleavage of p27kip1 by caspases.

B-cell chronic lymphocytic leukemia (B-CLL) is characterized by the accumulation of growth arrested clonal B lymphocytes that undergo apoptosis when treated with fludarabine. To further explore the mechanism for the cell cycle arrest, we examined the expression and activity of cyclin-dependent kinases and inhibitors in primary B-CLL cells. We observed high levels of p27kip1, cyclin D2, cyclin E, cdk2, and cdk4 expression in freshly isolated B-CLL cells. Despite high levels of cyclins and cdks, little cdk2 or cdk4 activity was observed with p27kip1 in complex with cyclinD2/cdk4 and cyclin E/cdk2. Remarkably, when B-CLL cells were treated in vitro with fludarabine, p27kip1 underwent caspase-specific degradation accompanied by an increase in cdk4 activity. We conclude that the G0/G1 arrest of B-CLL cells may protect against apoptosis and that the decrease in p27kip1 expression by caspase cleavage may be a key step in chemotherapy-induced apoptosis in B-CLL.     

Alteration of G1 cell-cycle protein expression and induction of p53 but not p21/waf1 by the DNA-modifying carcinogen 2-acetylaminofluorene in growth-stimulated hepatocytes in vitro

2-Acetylaminofluorene (AAF) is a potent tumor promoter in rat liver carcinogenesis models. In the resistant hepatocyte model, AAF is combined with a growth stimulus for efficient promotion of preneoplastic lesions. The promoting property of AAF in this model is closely associated with mito-inhibition of normal hepatocytes, an effect to which initiated cells are resistant. How AAF induces growth arrest is not known, but genotoxic as well as non-genotoxic effects have been implicated. To elucidate the mechanisms of AAF-induced mito-inhibition, we studied the expression of the tumor suppressor protein p53 and the cyclin-dependent kinase (cdk) complexes mediating G1 progression and S-phase entry. Hepatocytes were isolated from male Fisher 344 rats fed either a control diet or a diet supplemented with 0.02% AAF for 1 wk and cultured in a defined serum-free medium containing epidermal growth factor, insulin, and dexamethasone. Thymidine labeling revealed a profound inhibition of DNA synthesis in AAF-exposed cells compared with control cells. The retinoblastoma protein did not become hyperphosphorylated in AAF-exposed cells. Thus, inhibition of G1 cyclin-cdk activity was implied as a cause of growth arrest. Indeed, G1 cell-cycle arrest was accompanied by reduced induction and nuclear accumulation of the cyclin D1-cdk4 complex and inhibited nuclear translocation of cdk2. Furthermore, the growth arrest was not mediated through p21/waf1 upregulation, although nuclear levels of p53 were increased. Thus, carcinogen-induced mito-inhibition may be effected by altered levels and localization of G1 cyclin-cdk complexes, independent of the upregulation of cdk inhibitory proteins.     

p19ARF prevents G1 cyclin-dependent kinase activation by interacting with MDM2 and activating p53 in mouse fibroblasts.

p19ARF encoded by the INK4a tumor suppressor gene locus functions upstream of p53 to induce cell cycle arrest. p19ARF can interact with MDM2 and p53 in cells ectopically overexpressing these three components, but the biochemical cascades from p19ARF to cell cycle arrest has not been fully elucidated. In this study, we generated stably transfected NIH3T3 cells that express exogenous p19ARF under the control of a heavy metal-inducible metalothionine promoter. Cells arrested in G1 by ectopically expressed p19ARF contained considerably reduced G1 cyclin dependent kinase (cdk2 and cdk4) activities. The expression of cyclin A (a regulatory subunit of cdk2) markedly decreased, while cyclin D1, the major cdk4 partner in fibroblasts, expressed at a slightly higher level and formed complexes with cdk2 and cdk6 in addition to cdk4. Induction of p19ARF activated p53 by increasing its stability, and allowed the expression of p21Cip1, which bound to all of the cyclin D1-cdk complexes (cyclin D1-cdk2, -cdk4, and -cdk6) thereby inhibiting their kinase activities. p19ARF formed complexes with several cellular proteins including mouse MDM2. The majority of MDM2 was found in the complex with p19ARF, while no p53 was detected in association with p19ARF. Thus, we propose that p19ARF neutralizes MDM2 by sequestration from p53, which results in activation of p53, inhibition of G1 cyclin-cdk activities, and G1 arrest.     

UVB induced cell cycle checkpoints in an early stage human melanoma line, WM35

The activation of cell cycle checkpoints in response to genotoxic stressors is essential for the maintenance of genomic integrity. Although most prior studies of cell cycle effects of UV irradiation have used UVC, this UV range does not penetrate the earth's atmosphere. Thus, we have investigated the mechanisms of ultraviolet B (UVB) irradiation-induced cell cycle arrest in a biologically relevant target cell type, the early stage human melanoma cell line, WM35. Irradiation of WM35 cells with UVB resulted in arrests throughout the cell cycle: at the G1/S transition, in S phase and in G2. G1 arrest was accompanied by increased association of p21 with cyclin E/cdk2 and cyclin A/cdk2, increased binding of p27 to cyclin E/cdk2 and inhibition of these kinases. A loss of Cdc25A expression was associated with an increased inhibitory phosphotyrosine content of cyclin E- and cyclin A-associated cdk2 and may also contribute to G1 arrest following UVB irradiation. The association of Cdc25A with 14-3-3 was increased by UVB. Reduced cyclin D1 protein and increased binding of p21 and p27 to cyclin D1/cdk4 complexes were also observed. The loss of cyclin D1 could not be attributed to inhibition of either MAPK or PI3K/PKB pathways, since both were activated by UVB. Cdc25B levels fell and the remaining protein showed an increased association with 14-3-3 in response to UVB. Losses in cyclin B1 expression and an increased binding of p21 to cyclin B1/cdk1 complexes also contributed to inhibition of this kinase activity, and G2/M arrest. Oncogene (2000) 19, 4480 - 4490.     

A plant oxylipin, 12-oxo-phytodienoic acid,inhibits proliferation of human breast cancer cells by targeting cyclin D1

Cyclin D1 overexpression has been associated with poor prognosis and resistance to therapy in human breast cancer. Thus, the development of therapeutic agents that selectively target cyclin D1 activity is of clinical interest. This study demonstrates that 12-oxo-phytodienoic acid (OPDA), a phytohormone with critical functions in growth and development in plants, induces growth arrest in MDA-MB-231 and T47D breast cancer cells. In response to OPDA treatment, the human breast cancer cell lines exhibit a progressive decline in cyclin D1 expression, which is tightly associated with the accumulation of hypophosphorylated form of the retinoblastoma protein (Rb) and G1 arrest. The decrease in cyclin D1 protein expression accompanies a dramatic decline in nuclear but not membranous beta-catenin expression and activation of glycogen synthase kinase-3-beta (GSK3beta) caused by inhibition of its serine-9 phosphorylation. The proteasome inhibitor MG132 blocks OPDA-mediated decrease in cyclin D1. In addition, the overexpression of T286A, a cyclin D1 mutant which is refractory to phosphorylation by GSK3beta and proteosomal degradation, is resistant to OPDA-mediated Rb dephosphorylation as well as G(1) cell cycle arrest. Thus, our results demonstrate that degradation of cyclin D1 protein is a key event in OPDA induced growth inhibition in breast cancer cells. These data provide the basic foundation for future efforts to develop OPDA-based approaches in the prevention and treatment of breast cancer and other types of cancer.     

Inhibition of cdk2 kinase activity by methylselenocysteine in synchronized mouse mammary epithelial tumor cells

Methylselenocysteine (MSC), an organic selenium compound has significant anticarcinogenic activity against mammary tumorigenesis. Previous experiments have demonstrated that MSC and inorganic selenite inhibit mammary cell (TM6 cell line) growth through different pathways. The present investigation demonstrated that MSC arrested cells in S phase during the TM6 cell cycle, which was followed by cells entering apoptosis at 48 h. Methylselenocysteine specifically affected the cdk2 kinase activity of the TM6 cells (54% reduction) at 16 h after release from growth arrest. The cdk4 kinase activity did not change during the cell cycle, confirming that cells had passed the G1 checkpoint and had entered S phase. The amount of cyclin E associated with cdk2 was increased by MSC by the 12 h time point, thereby facilitating entry of cells into S phase. Afterwards, cyclin E and cyclin A associated with cdk2 did not change for the remainder of the cell cycle. The data demonstrate that inhibition of mammary cell growth by MSC is mediated by alterations in progression of cells through S phase. The decrease in cdk2 kinase activity is coincident with prolonged arrest in S phase. One consequence of prolonged arrest may be apoptosis.      

Deregulated expression of cyclin D1 overrides antimitogenic signals

Several types of epithelial neoplasms exhibit high expression of transforming growth factor beta1 (TGFbeta-1), indicating that they have acquired tolerance to this normally growth inhibitory cytokine. Since cyclin D1 is expressed at high levels in murine skin tumors coincident with high levels of TGFbeta-1 expression, we hypothesized that cyclin D1 may override TGFbeta-1 induced growth arrest. We observed that in primary murine keratinocytes treated with TGFbeta-1, cyclin D1 is quickly suppressed at both the mRNA and protein level. Since changes in other cell cycle proteins occur at a later time during TGFbeta-1 treatment, the early suppression of cyclin D1 suggests that this gene is a critical target for TGFbeta-1 growth suppression. Using primary keratinocytes from transgenic mice that overexpress cyclin D1 (K5-D1 mice), we observed partial resistance to TGFbeta-1 growth inhibition. This resistance involves changes in the cyclin/cdk/inhibitor complexes rather than differences in expression of the TGFbeta receptors or signaling. Comparison of cdk associated kinase activity between wild-type and K5-D1 cells shows differential regulation. We conclude that deregulated cyclin D1 and subsequent alterations in cell cycle machinery provides keratinocytes the ability to at least partially override growth inhibitory signals.     

Lycopene inhibition of cell cycle progression in breast and endometrial cancer cells is associated with reduction in cyclin D levels and retention of p27(Kip1) in the cyclin E-cdk2 complexes.

Numerous studies have demonstrated the anticancer activity of the tomato carotenoid, lycopene. However, the molecular mechanism of this action remains unknown. Lycopene inhibition of human breast and endometrial cancer cell growth is associated with inhibition of cell cycle progression at the G(1) phase. In this study we determined the lycopene-mediated changes in the cell cycle machinery. Cells synchronized in the G(1) phase by serum deprivation were treated with lycopene or vehicle and restimulated with 5% serum. Lycopene treatment decreased serum-induced phosphorylation of the retinoblastoma protein and related pocket proteins. This effect was associated with reduced cyclin-dependent kinase (cdk4 and cdk2) activities with no alterations in CDK protein levels. Lycopene caused a decrease in cyclin D1 and D3 levels whereas cyclin E levels did not change. The CDK inhibitor p21(Cip1/Waf1) abundance was reduced while p27(Kip1) levels were unaltered in comparison to control cells. Serum stimulation of control cells resulted in reduction in the p27 content in the cyclin E--cdk2 complex and its accumulation in the cyclin D1--cdk4 complex. This change in distribution was largely prevented by lycopene treatment. These results suggest that lycopene inhibits cell cycle progression via reduction of the cyclin D level and retention of p27 in cyclin E--cdk2, thus leading to inhibition of G(1) CDK activities.     

Cdc25-dependent activation of cyclin A/cdk2 is blocked in G2 phase arrested cells independently of ATM/ATR

Cyclin A/cdk2 is active during S and G2 phases of the cell cycle, but its regulation and function during G2 phase is poorly understood. In this study we have examined the regulation of cyclin A/cdk2 activity during normal G2 phase progression and in genotoxin-induced G2 arrest. We show that cyclin A/cdk2 is activated in early G2 phase by a cdc25 activity. In the G2 phase checkpoint arrest initiated in response to various forms of DNA damage, the cdc25-dependent activation of both cyclin A/cdk2 and cyclin B1/cdc2 is blocked. Ectopic expression of cdc25B, but not cdc25C, in G2 phase arrested cells efficiently activated both cyclin A/cdk2 and cyclin B1/cdc2. Finally, we demonstrate that the block in cyclin A/cdk2 activation in the G2 checkpoint arrest is independent of ATM/ATR. We speculate that the ATM/ATR-independent block in G2 phase cyclin A/cdk2 activation may act as a further layer of checkpoint control, and that blocking G2 phase cyclin A/cdk2 activation contributes to the G2 phase checkpoint arrest.     

Resveratrol causes WAF-1/p21-mediated G(1)-phase arrest of cell cycle and induction of apoptosis in human epidermoid carcinoma A431 cells.

Resveratrol (trans-3,4',5,-trihydroxystilbene), a phytoalexin found in grapes, nuts, fruits, and red wine, is a potent antioxidant with cancer-preventive properties. The mechanism by which resveratrol imparts cancer chemopreventive effects is not clearly defined. Here, we demonstrate that resveratrol, via modulations in cyclin-dependent kinase (cdk) inhibitor-cyclin-cdk machinery, results in a G(1)-phase arrest of the cell cycle followed by apoptosis of human epidermoid carcinoma (A431) cells. Resveratrol treatment (1-50 microM for 24 h) of A431 cells resulted in a dose-dependent (a) inhibition of cell growth as shown by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, (b) G(1)-phase arrest of the cell cycle as shown by DNA cell cycle analysis, and (c) induction of apoptosis as assessed by ELISA. The immunoblot analysis revealed that resveratrol treatment causes a dose- and time-dependent (a) induction of WAF1/p21; (b) decrease in the protein expressions of cyclin D1, cyclin D2, and cyclin E; and (c) decrease in the protein expressions of cdk2, cdk4, and cdk6. Resveratrol treatment was also found to result in a dose- and time-dependent decrease in kinase activities associated with all of the cdks examined. Taken together, our study suggests that resveratrol treatment of the cells causes an induction of WAF1/p21 that inhibits cyclin D1/D2-cdk6, cyclin D1/D2-cdk4, and cyclin E-cdk2 complexes, thereby imposing an artificial checkpoint at the G(1)-->S transition of the cell cycle. This series of events results in a G(1)-phase arrest of the cell cycle, which is an irreversible process that ultimately results in the apoptotic death of cancer cells. To our knowledge, this is the first systematic study showing the involvement of each component of cdk inhibitor-cyclin-cdk machinery during cell cycle arrest and apoptosis of cancer cells by resveratrol.