A Cdc25A antagonizing K vitamin inhibits hepatocyte DNA synthesis in vitro and in vivo

2-Methoxyestradiol inhibits hepatocellular carcinoma cell growth by inhibiting Cdc25 and inducing cell cycle arrest and apoptosis

PURPOSE

2-Methoxyestradiol (2-ME) is a physiological metabolite of estrogen, which can inhibit growth of many types of tumor cells, including hepatocellular carcinoma, both in vitro and in vivo. The exact mechanisms of its action are still unclear. We have studied the mechanisms of growth inhibition of several of human and rat hepatoma and normal liver cells by 2-ME.

METHODS

Human (Hep3B, HepG2, PLC/PRF5) and rat (McA-RH7777, JM-1) hepatoma and normal rat (CRL-1439) and human (CRL-11233) liver cell lines were cultured in vitro, in presence of 2-ME, and its IC50s were determined. Cell cycle arrest, Cdc25 phosphatase inhibition and apoptosis induction were studied. Finally, the effect of 2-ME on the growth of JM-1 rat hepatoma cells in rat liver was determined in vivo.

RESULTS

The IC50 range for growth inhibition of hepatoma cells was found to be between 0.5 and 3 microM. In contrast, normal rat hepatocytes and liver cell lines were resistant to 2-ME up to 20 microM. JM-1 cells were arrested in the G2/M phase of the cell cycle. Cdc25A and Cdc25B, cell cycle controlling phosphatases, activities were inhibited in vitro and 2-ME was found to likely bind to their catalytic site cysteines. As a consequence, their cellular substrates Cdk1 and Cdk2 were tyrosine phosphorylated. Caspase-3 was cleaved suggesting apoptotic cell death. Moreover, growth of JM-1 tumors, which were transplanted into rat liver, was also inhibited by treatment with 2-ME in vivo.

CONCLUSIONS

2-Methoxyestradiol is a selective, potent and relatively non-toxic hepatoma growth inhibitor both in vitro and in vivo. Cell cycle arrest of hepatoma cells was likely mediated by binding and inactivation of the Cdc25 phosphatases and induction of apoptosis.

α-Thrombin inhibits DNA synthesis in rat hepatocytes but not in hepatoma cells by receptor activation and proteolysis

Prothrombin is a plasma protein, which after tissue injury is converted to alpha-thrombin and is mainly involved in blood clot formation. It has also been shown to have a mitogenic effect on primary endothelial cells, vascular smooth muscle cells, fibroblasts and some tumor cells, but is an inhibitor of rat hepatocyte DNA synthesis on fibronectin matrix in cell culture. We now report that prothrombin is converted to alpha-thrombin by primary cultures of normal adult rat hepatocytes and alpha-thrombin is also a potent inhibitor of hepatocytes DNA synthesis. In contrast, rat hepatoma cells cultured under similar conditions were resistant to alpha-thrombin mediated DNA synthesis inhibition. The inhibitory effect of alpha-thrombin on DNA synthesis was antagonized by hirudin and antithrombin, two specific alpha-thrombin inhibitors or by the presence of collagen-I matrix. A thrombin receptor activating peptide (TRAP6) also inhibited EGF-mediated rat hepatocyte DNA synthesis, suggesting a role of the thrombin receptors in this process. Matrix fibronectin was degraded by alpha-thrombin. However, no appreciable cell detachment was observed. These results suggest a role of alpha-thrombin as a potent growth inhibitor of normal hepatocytes, possibly through control of fibronectin or other matrix protein(s).

Growth inhibitory actions of prothrombin on normal hepatocytes: influence of matrix

Most hepatomas have a defect in prothrombin carboxylation, and can secrete under-carboxylated prothrombin or des-gamma-carboxy-prothrombin (DCP), the function of which is unknown. We considered that the prothrombin-DCP axis might also be involved in growth control. Hepatocytes and hepatoma cells were treated with prothrombin and DNA synthesis and cytoskeletal changes were studied. Prothrombin inhibited DNA synthesis in hepatocytes on fibronectin, but not collagen matrix. Hepatoma cell lines were not inhibited. We found that hepatoma cell matrix conferred resistance to hepatocytes. Prothrombin decreased fibronectin but not collagen amounts, but only in the presence of hepatocytes and not hepatoma cells, indicating that it has a differential action on matrix proteins. It also caused changes in cell shape and actin depolymerization. In vivo, there was a decrease in plasma prothrombin activity after a partial hepatectomy (PH), concomitant with the peak of DNA synthesis in the hepatocytes at 24h after PH. Injection of warfarin at the time of PH, further inhibited PT activity and enhanced this 24h peak of DNA synthesis. Furthermore, repeated injection of prothrombin lowered the peak DNA synthesis after PH. The data support the hypothesis that prothrombin can act as a hepatocyte growth inhibitor, likely at the level of fibronectin loss and result in cytoskeletal changes. Hepatomas resist this action, possibly due to their different matrix proteins. This represents a novel mechanism for growth regulation and provides a possible biological significance for the tumor marker DCP.

Fluorinated Cpd 5, a pure arylating K-vitamin derivative, inhibits human hepatoma cell growth by inhibiting Cdc25 and activating MAPK

We previously synthesized several K-vitamin derivatives, which are potent growth inhibitors of human tumor cells, including Hep3B human hepatoma cells. Among these, Cpd 5 was the most potent. However, being a quinone derivative, Cpd 5 has the potential for generating toxic reactive oxygen species (ROS). We therefore synthesized a fluorinated derivative of Cpd 5, F-Cpd 5. The calculated reduction potential of F-Cpd 5 was much higher than that for Cpd 5 and it was not predicted to generate ROS. This was supported by our observation that F-Cpd 5 generated significantly lower ROS than Cpd 5. F-Cpd 5 was three times more potent than Cpd 5 in inhibiting Hep3B cell growth. Interestingly, under identical culture conditions, F-Cpd 5 inhibited mitogen-induced DNA synthesis in normal rat hepatocytes 12-fold less potently than Hep3B cells. F-Cpd 5 was found to induce caspase-3 cleavage and nuclear DNA laddering, evidences for apoptosis. It preferentially inhibited the activities of the cell cycle controlling phosphatases Cdc25A and Cdc25B, by binding to their catalytic cysteines. Consequently, inhibitory tyrosine phosphorylation of the Cdc25 substrate kinases Cdk2 and Cdk4 were induced. F-Cpd 5 also induced phosphorylation of the MAPK proteins ERK1/2, JNK1/2 and p38 in Hep3B cells and the MAPK inhibitors (U0126, JNKI-II, and SB 203580) antagonized its growth inhibition. F-Cpd 5 inhibited the action of cytosolic ERK phosphatase activity, which likely caused the ERK phosphorylation. F-Cpd 5 thus differentially inhibited growth of normal and tumor cells by preferentially inhibiting the actions of Cdc25A and Cdc25B phosphatases and inducing MAPK phosphorylation.

PM-20, a novel inhibitor of Cdc25A, induces extracellular signal-regulated kinase 1/2 phosphorylation and inhibits hepatocellular carcinoma growth in vitro and in vivo

We have synthesized several new phenyl maleimide compounds, which are potent growth inhibitors of several human tumor cell lines. Among these, PM-20 was the most potent with an IC50 of 700 nmol/L for Hep3B human hepatoma cell growth. Two other derivatives, PM-26 and PM-38, did not inhibit Hep3B cell growth even at 100 micromol/L. Interestingly, under identical experimental conditions, PM-20 inhibited DNA synthesis of primary cultures of normal hepatocytes at a 10-fold higher concentration than that needed to inhibit the DNA synthesis of the Hep3B hepatoma cells. PM-20 affected two cellular signaling pathways in Hep3B cells: Cdc25 phosphatase and extracellular signal-regulated kinase (ERK) 1/2. It competitively inhibited the activity of Cdc25 (preferentially Cdc25A) by binding to the active site, likely through the catalytic cysteine, but did not inhibit PTP1B, CD45, or MKP-1 phosphatases. As a result of its action, tyrosine phosphorylation of the cellular Cdc25A substrates Cdk2 and Cdk4 was induced. It also induced strong and persistent phosphorylation of the Cdc25A substrate ERK1/2. Hep3B cell lysates were found to contain ERK2 phosphatase(s) activity, which was inhibited by the actions of PM-20. However, activity of exogenous dual-specificity ERK2 phosphatase MKP1 was not inhibited. Induction of ERK1/2 phosphorylation correlated with the potency of growth inhibition in tumor cell lines and inhibition of ERK1/2 phosphorylation by the mitogen-activated protein kinase (MAPK)/ERK kinase 1/2 inhibitor U0126 or overexpression of the cdc25A gene in Hep3B cells antagonized the growth inhibitory actions of PM-20. Growth of transplantable rat hepatoma cells in vivo was also inhibited by PM-20 action with a concomitant induction of pERK in the tumors. The mechanism(s) of growth inhibition of Hep3B hepatoma cells by the phenyl maleimide PM-20 involves prolonged ERK1/2 phosphorylation, likely resulting from inhibition of the ERK phosphatase Cdc25A. PM-20 thus represents a novel class of tumor growth inhibitor that inhibits mainly Cdc25A, is dependent on ERK activation, and has a considerable margin of selectivity for tumor cells compared with normal cells.

Phosphorylation regulates Myc expression via prolonged activation of the mitogen-activated protein kinase pathway

We previously showed that prolonged and strong ERK phosphorylation induced by Compound 5 (Cpd 5), a Cdc25A protein phosphatase inhibitor, was involved in its mechanism of cell growth inhibition. To study the relationship between ERK phosphorylation and cell growth inhibition, we used Cpd 5 as a tool to investigate ERK-regulated c-Myc expression in Hep3B hepatoma cells. We found that ERK phosphorylation caused by Cpd 5 induced c-Myc phosphorylation, but suppressed c-Myc expression at the mRNA and protein levels. Furthermore, Cpd 5 inhibited c-Myc transcriptional activity and DNA binding ability, and this inhibition was antagonized by ERK kinase (MEK) inhibitor U-0126, implying that the ERK pathway was involved in regulating c-Myc expression. Since the participation of c-Myc protein in transcription requires its dimerization with Max protein, we examined the Myc-Max association in Cpd 5-treated cells and found that Cpd 5 suppressed Myc-Max dimerization. Transfection of Hep3B cells with mutated ERK (T188A/Y190F), which has lost its dual-phosphorylation sites, attenuated the actions of Cpd 5 on Myc-Max association. To further demonstrate whether Myc phosphorylation by Cpd 5-induced ERK activation was able to directly regulate c-myc gene expression, a chromatin immunoprecipitation (ChIP) assay was used to examine the binding of phospho-Myc to the c-myc promoter region. We found that phospho-Myc induced by Cpd 5 had lost its ability to bind to the c-myc promoter, whereas MEK inhibitor U-0126 antagonized this inhibitory effect. These data suggest that an increase in c-Myc phosphorylation in response to prolonged ERK phosphorylation negatively auto-regulates c-Myc gene expression, leading to the suppression of its target gene expression and cell cycle block.

A novel cinnamic acid derivative that inhibits Cdc25 dual-specificity phosphatase activity

The Cdc25 dual-specificity phosphatases are key regulators of cell cycle progression through activation of cyclin-dependent kinases (Cdk). Three homologs exist in humans: Cdc25A, Cdc25B, and Cdc25C. Cdc25A and Cdc25B have oncogenic properties and are overexpressed in some types of tumors. Compounds that inhibit Cdc25 dual-specificity phosphatase activity might thus be potent anticancer agents. We screened several hundred compounds in a library using an in vitro phosphatase assay, with colorimetric measurement of the conversion of p-nitrophenyl phosphate (pNPP) to p-nitrophenol by the catalytic domain of recombinant human Cdc25, and discovered TPY-835, which inhibits Cdc25A and Cdc25B activity (IC50 = 5.1 and 5.7 microM, respectively). TPY-835 had mixed inhibition kinetics for Cdc25A and Cdc25B. TPY-835 caused cell cycle arrest in the G1 phase in human lung cancer cells (A549 and SBC-5) but not cell cycle arrest in the G2/M phase. After treatment with TPY-835, the activation of Cdk2 was suppressed and phosphorylation of the retinoblastoma (Rb) protein was decreased in SBC-5 cells. In addition, TPY-835 induced an increase of the sub-G1 phase cell population after 48-72 h treatment. The growth inhibitory effects of TPY-835 against cisplatin (CDDP)-, camptothecin- and 5-FU-resistant cell lines are comparable to the growth inhibitory effect on their parental lines, thus indicating that TPY-835 did not show cross-resistance to these cell lines. These results suggest that TPY-835 is a promising candidate for constructing a novel class of antitumor agents that can control the cell cycle progression of cancer cells.

Redox regulation of the Cdc25 phosphatases

The Cdc25 phosphatases are essential for cell-cycle control in eukaryotes under normal conditions and in response to DNA damage via checkpoint controls. Recent evidence indicates direct control of the Cdc25s, and therefore the cell cycle, in response to changes in cellular redox status. These redox changes may originate intracellularly from mitochondrial leakage or in response to specific external triggers leading to production of reactive oxygen species (ROS). This review shows that the known chemistry and biology of the Cdc25s favor a direct role for these phosphatases in temporarily blocking cell-cycle progression until favorable reducing conditions are restored. First, the Cdc25s contain a highly reactive cysteine at the active site that can react directly with ROS, leading to enzyme inactivation. Second, the ROS-inactivated form of Cdc25 is expected to prevent cell-cycle progression based on precedent from cellular responses to DNA damage. Third, ROS-mediated oxidation of the Cdc25s leads to an intramolecular disulfide that is readily reversible by the cellular reductant thioredoxin. Finally, in vivo data supporting a direct role for the Cdc25s in redox regulation are considered.

Novel hydroxyl naphthoquinones with potent Cdc25 antagonizing and growth inhibitory properties

Cdc25 phosphatases are important in cell cycle control and activate cyclin-dependent kinases (Cdk). Efforts are currently under way to synthesize specific small-molecule Cdc25 inhibitors that might have anticancer properties. NSC 95397, a protein tyrosine phosphatase antagonist from the National Cancer Institute library, was reported to be a potent Cdc25 inhibitor. We have synthesized two hydroxyl derivatives of NSC 95397, monohydroxyl-NSC 95397 and dihydroxyl-NSC 95397, which both have enhanced activity for inhibiting Cdc25s. The new analogues, especially dihydroxyl-NSC 95397, potently inhibited the growth of human hepatoma and breast cancer cells in vitro. They influenced two signaling pathways. The dual phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) was induced, likely due to inhibition of the ERK phosphatase activity in Hep 3B cell lysate but not the dual specificity ERK phosphatase MKP-1. They also inhibited Cdc25 enzymatic activities and induced tyrosine phosphorylation of the Cdc25 target Cdks. Addition of hydroxyl groups to the naphthoquinone ring thus enhanced the potency of NSC 95397. These two new compounds may be useful probes for the biological functions of Cdc25s and have the potential for disrupting the cell cycle of growing tumor cells.

Cdc25A and ERK interaction: EGFR-independent ERK activation by a protein phosphatase Cdc25A inhibitor, compound 5

Extracellular signal-regulated kinase (ERK) plays a central role in regulating cell growth, differentiation, and apoptosis. We previously found that 2-(2-mercaptoethanol)-3-methyl-1,4-napthoquinone or Compound 5 (Cpd 5), is a Cdc25A protein phosphatase inhibitor and causes prolonged, strong ERK phosphorylation which is triggered by epidermal growth factor receptor (EGFR) activation. We now report that Cpd 5 can directly cause ERK phosphorylation by inhibiting Cdc25A activity independently of the EGFR pathway. We found that Cdc25A physically interacted with and de-phosphorylated phospho-ERK both in vitro and in cell culture. Inhibition of Cdc25A activity by Cpd 5 resulted in ERK hyper-phosphorylation. Transfection of Hep3B human hepatoma cells with inactive Cdc25A mutant enhanced Cpd 5 action on ERK phosphorylation, whereas over-expression of Cdc25A attenuated this Cpd 5 action. Furthermore, endogenous Cdc25A knock-down by Cdc25A siRNA resulted in a constitutive-like ERK phosphorylation and Cpd 5 treatment further enhanced it. In EGFR-devoid NR6 fibroblasts and MEK (ERK kinase) mutated MCF7 cells, Cpd 5 treatment also resulted in ERK phosphorylation, providing support for the idea that Cpd 5 can directly act on ERK phosphorylation by inhibiting Cdc25A activity. These data suggest that phospho-ERK is likely another Cdc25A substrate, and Cpd 5-caused ERK phosphorylation is probably regulated by both EGFR-dependent and EGFR-independent pathways.

Involvement of c-Myc in growth inhibition of Hep 3B human hepatoma cells by a vitamin K analog

BACKGROUND/AIMS

A synthetic vitamin K analog, compound 5 (Cpd 5), is a potent inhibitor of cell growth. The aim was to investigate whether c-Myc was involved in Cpd 5-induced cell growth inhibition.

METHODS

Human hepotoma cells (Hep 3B) were cultured and treated with Cpd 5, and c-Myc protein expression and phosphorylation were investigated using Western blot analysis.

RESULTS

Cpd 5 was found to inhibit c-Myc protein expression and induce c-Myc phosphorylation in Hep 3B cells. The phosphorylation of c-Myc was induced by both Cpd 5-mediated persistent extracellular signal-regulated kinase (ERK) phosphorylation and Cpd 5 increased glycogen synthase kinase-3 (GSK-3) activity. When using GSK-3 inhibitor, SB216763, c-Myc phosphorylation was significantly decreased and c-Myc levels were restored in Cpd 5 treated cells, suggesting that Cpd 5-mediated increase of GSK-3 activity enhanced c-Myc degradation and resulted in reduction of c-Myc levels. The lower c-Myc levels were found to cause altered expression of two c-Myc target genes, growth arrest gene gadd45 and ornithine decarboxylase (ODC).

CONCLUSIONS

The results suggest that Cpd 5-mediated c-Myc phosphorylation resulted in enhanced c-Myc protein degradation and reduced c-Myc protein levels, which may contribute to cell growth inhibition by Cpd 5.

Differential effects of two growth inhibitory K vitamin analogs on cell cycle regulating proteins in human hepatoma cells

A comparison was made between two K vitamin analogs. Growth in vitro of Hep G2 hepatoma cells was inhibited both by Compound 5 (Cpd 5), a recently synthesized thioalkyl analog of vitamin K or 2-(2-mercaptoethanol)-3-methyl-1, 4-naphthoquinone, as well as by synthetic vitamin K3 (menadione). Using synchronized Hep G2 hepatoma cells, the actions of both Cpd 5 and vitamin K3 on cell cycle regulating proteins were examined. Cpd 5 decreased the levels of cyclin D1, Cdk4, p16, p21 and cyclin B1. By contrast, VK3 only decreased the level of cyclin D1, but had no effect on the levels of Cdk4, p16 or p21. Interestingly, both VK3 and VK2 increased the levels of p21. The naturally occurring K vitamins had little effect on cell growth and none on the cyclins or Cdks. Amounts and activity of the G1/S phase controlling Cdc25A were measured. We found that Cpd 5 directly inhibited both Cdc25A activity and its protein expression, whereas VK3 did not. Thus, the main effects of Cpd 5 were on G1 and S phase proteins, especially Cdk4 and Cdc25A amounts in contrast to VK3. Computer docking studies of Cpd 5 and VK3 to Cdc25A phosphatase showed three binding sites. In the best conformation, Cpd 5 was found to be closer to the enzyme active site than VK3. These findings show that Cpd 5 represents a new class of anticancer agent, being a protein tyrosine phosphatase (PTP) antagonist, that binds to Cdc25A with suppression of its activity. Tumors expressing high levels of oncogenic Cdc25A phosphatase may thus be susceptible to the growth inhibitory activities of this class of compound.

Persistent ERK phosphorylation negatively regulates cAMP response element-binding protein (CREB) activity via recruitment of CREB-binding protein to pp90RSK

Compound 5 (Cpd 5) or 2-(2-mercaptoethanol)-3-methyl-1,4-naphthoquinone, is an inhibitor of protein phosphatase Cdc25A and causes persistent activation of extracellular signal-regulated kinase (ERK) and cell growth inhibition. To study the mechanism(s) by which persistent ERK phosphorylation might induce cell growth inhibition, we used Cpd 5 as a tool to examine its effects on the activity of CREB (cAMP response element-binding protein) transcription factor in Hep3B human hepatoma cells. We found that CREB activity, including its DNA binding ability and phosphorylation on residue Ser-133, was strongly inhibited by Cpd 5, followed by suppression of CRE-mediated transcription of cyclin D1 and Bcl-2 genes. Cpd 5-mediated suppression of CREB phosphorylation and transcriptional activity was antagonized by mitogen-activated protein kinase kinase inhibitors PD 98059 and U-0126, implying that this inhibition of CREB activity was regulated at least in part by the ERK pathway. The phosphorylation of ribosomal S6 kinase (pp90(RSK)), a CREB kinase in response to mitogen stimulation, was also found to be inhibited by Cpd 5 action. This inhibition of pp90(RSK) phosphorylation is likely the result of its increased association with CREB-binding protein (CBP), which subsequently caused inhibition of CREB phosphorylation and activity. To support the hypothesis that Cpd 5 effects on Cdc25A inhibition with subsequent ERK activation could cause CREB inhibition, we examined the effects of Cdc25A inhibition without the use of Cpd 5. Hep3B cells were transfected with C430S Cdc25A mutant, and ERK was found to be phosphorylated in a constitutively activated manner, which was accompanied by decreased CREB phosphorylation and increased recruitment of CBP to pp90(RSK). These data provide evidence that CBP.RSK complex formation in response to persistent ERK phosphorylation by Cpd 5 down-regulates CREB activity, leading to inhibition of both cAMP response element-mediated gene expression and cell growth.