Ziqiu Wang

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 (pp90RSK), a CREB kinase in response to mitogen stimulation, was also found to be inhibited by Cpd 5 action. This inhibition of pp90RSKphosphorylation 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 pp90RSK. 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.

JS‐K, a novel non‐ionic diazeniumdiolate derivative, inhibits Hep 3B hepatoma cell growth and induces c‐Jun phosphorylation via multiple MAP kinase pathways

JS‐K, a non‐ionic diazeniumdiolate derivative, is capable of arylating nucleophiles and spontaneously generating nitric oxide (NO) at physiological pH. This recently synthesized low molecular weight compound is shown here to be an inhibitor of cell growth with concomitant activation of mitogen‐activated protein kinase (MAPK) members ERK, JNK, and p38 and their downstream effectors c‐Jun and AP‐1. Inhibitors of these MAPK pathways abrogated the growth inhibitory actions of JS‐K. In addition to the well‐described actions of JNK as a kinase for c‐Jun, we show that c‐Jun is also an ERK target. Furthermore, JS‐K generated NO in culture and NO inhibitors antagonized both MAPK induction and the growth inhibitory effects of JS‐K. These results suggest two possible mechanisms for the mediation of JS‐K growth inhibitory actions, namely NO‐induction of MAPK pathway constituents as well as possible arylation reactions. The data support the idea that prolonged MAPK activation by JS‐K action is important in mediating its growth‐inhibitory actions. JS‐K thus represents a promising platform for novel growth inhibitory analog synthesis. J. Cell. Physiol. 197: 426–434, 2003© 2003 Wiley‐Liss, Inc.

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.

K vitamins, PTP antagonism, and cell growth arrest

The main function of K vitamins is to act as co‐factors for γ‐glutamyl carboxylase. However, they have also recently been shown to inhibit cell growth. We have chemically synthesized a series of K vitamin analogs with various side chains at the 2 or 3 position of the core naphthoquinone structure. The analogs with short thio‐ethanol side chains are found to be more potent growth inhibitors in vitro of various tumor cell lines. Cpd 5 or [2‐(2‐mercaptoethanol)‐3‐methyl‐1,4‐naphthoquinone] is one of the most potent. The anti‐proliferation activity of these compounds is antagonized by exogenous thiols but not by non‐thiol antioxidants. This suggests that the growth inhibition is mediated by sulfhydryl arylation of cellular glutathione and cysteine‐containing proteins and not by oxidative stress. The protein tyrosine phosphatases (PTP) are an important group of proteins that contain cysteine at their catalytic site. PTPs regulate mitogenic signal transduction and cell cycle progression. PTP inhibition by Cpd 5 results in prolonged tyrosine phosphorylation and activation of several kinases and transcription factors including EGFR, ERK1/2, and Elk1. Cpd 5 could activate ERK1/2 either by signaling from an activated EGFR, which is upstream in the signaling cascade, or by direct inhibition of ERK1/2 phosphatase(s). Prolonged ERK1/2 phosphorylation strongly correlates with Cpd 5‐mediated growth inhibition. Cpd 5 can also bind to and inhibit the Cdc25 family of dual specific phosphatases. As a result, several Cdc25 substrates (Cdk1, Cdk2, Cdk4) involved in cell cycle progression are tyrosine phosphorylated and thereby inhibited by its action. Cpd 5 could also inhibit both normal liver regeneration and hepatoma growth in vivo. DNA synthesis during rat liver regeneration following partial hepatectomy, transplantable rat hepatoma cell growth, and glutathione‐S‐transferase‐pi expressing hepatocytes after administration of the chemical carcinogen diethylnitrosamine, are all inhibited by Cpd 5 administration. The growth inhibitory effect during liver regeneration and transplantable tumor growth is also correlated with ERK1/2 phosphorylation induced by Cpd 5. Thus, Cpd 5‐mediated inhibition of PTPs, such as Cdc25 leads to cell growth arrest due to altered activity of key cellular kinases involved in signal transduction and cell cycle progression. This prototype K vitamin analog represents a novel class of growth inhibitor based upon its action as a selective PTP antagonist. It is clearly associated with prolonged ERK1/2 phosphorylation, which is in contrast with the transient ERK1/2 phosphorylation induced by growth stimulatory mitogens.

Involvement of hepatocyte epidermal growth factor receptor mediated activation of mitogen‐activated protein kinase signaling pathways in response to growth inhibition by a novel K vitamin

Compound 5 (Cpd 5), a synthetic K vitamin analogue, or 2‐(2‐mercaptoethanol)‐3‐methyl‐1,4‐naphthoquinone, is a potent inhibitor of epidermal growth factor (EGF)‐induced rat hepatocyte DNA synthesis and induces EGF receptor (EGFR) tyrosine phosphorylation. To understand the cellular responses to Cpd 5, its effects on the EGF signal transduction pathway were examined and compared to those of the stimulant, EGF. Cpd 5 induced a cellular response program that included the induction of EGFR tyrosine phosphorylation and the activation of the mitogen‐activated protein kinase (MAPK) cascade. EGFR tyrosine phosphorylation was induced by Cpd 5 in a time‐ and dose‐dependent manner. Coimmunoprecipitation studies demonstrated that both EGF and Cpd 5 induced tyrosine phosphorylation of EGFR was associated with increased amounts of adapter proteins Shc and Grb2, and the Ras GTP‐GDP exchange protein Sos, indicating the formation of functional EGFR complexes. Although EGFR phosphorylation was induced both by the stimulant EGF and the inhibitor Cpd 5, the timing and intensity of activation by EGF and Cpd 5 were different. EGF activated EGFR transiently, whereas Cpd 5 induced an intense and sustained activation. Cpd 5‐altered cells had a decreased ability to dephosphorylate tyrosine phosphorylated EGFR, providing evidence for an inhibition of tyrosine phosphatase activity. Both EGF and Cpd 5 caused an induction of phospho‐extracellular response kinase (ERK), which was also more sustained with Cpd 5. Moreover, whereas Cpd 5 induced a striking translocation of phosphorylated ERK from cytosol to the nucleus, no significant nuclear translocation occurred after stimulation with EGF. The data suggest that this novel compound causes growth inhibition through antagonism of EGFR phosphatases and consequent induction of EGFR and ERK phosphorylation. This is supported by experiments with PD 153035 and PD 098059, antagonists of phosphorylation of EGFR and MAP kinase kinase (MEK), respectively, which both antagonized Cpd 5‐induced phosphorylation and the inhibition of DNA synthesis. These results imply a mechanism of cell growth inhibition associated with intense and prolonged protein tyrosine phosphorylation. Protein tyrosine phosphatases may thus be a novel target for drugs designed to inhibit cell growth. J. Cell. Physiol. 183:338–346, 2000.

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.

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.

Vitamin K uptake in hepatocytes and hepatoma cells

Hepatocellular carcinoma (HCC) or hepatoma cells have impaired ability to perform vitamin K-dependent carboxylation reactions. Vitamin K can also inhibit growth of HCC cells in vitro. Both carboxylation and growth inhibition are vitamin K dose dependent. We used rat hepatocytes, a vitamin K-growth sensitive (MH7777) and a vitamin K-growth resistant (H4IIE) rat hepatoma cell line to examine vitamin K uptake and vitamin K-mediated microsomal carboxylation. We found that vitamin K is taken up by normal rat hepatocytes against a saturable concentration gradient. The relative rates of uptake by rat hepatocytes and the two rat cell lines MH7777 and H4IIE correlated with their sensitivity to vitamin K-mediated cell growth inhibition. Pooled hepatocytes from liver nodules from rats treated with the hepatocarcinogen diethylnitrosamine (DEN) also had a reduced rate of vitamin K uptake. However, using a cell-free system, microsomes from both normal rat hepatocytes and the two rat hepatoma cell lines had a similar ability to support carboxylation mediated by exogenously added vitamin K. The results support the hypothesis that different sensitivity of hepatoma cells to vitamin K may be due to differences in vitamin K uptake and may be unrelated to the actions of vitamin K on carboxylation.

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

Thioalkyl containing K vitamin analogs have been shown to be potent inhibitors of hepatoma cell growth and antagonizers of protein tyrosine phosphatase activity. We now show that they inhibit the activity of specific protein tyrosine phosphatases (PTP) in cell-free conditions in vitro, particularly the dual specificity phosphatase Cdc25A. Using primary cultures of adult rat hepatocytes that are in G0/G1 phase until stimulated into DNA synthesis by epidermal growth factor, we found that 2-(2-mercaptoethanol)-3-methyl-1,4-naphthoquinone or Compound 5 (Cpd 5) inhibited hepatocyte DNA synthesis and PTP activity in cell culture and in vivo after a two-thirds partial hepatectomy. We found a selective inhibition of Cdc25A activity in vitro, using both synthetic substrates and authentic cellular substrate, immunoprecipitated phospho-Cdk4. Intact Cpd 5-treated cells had decreased cellular Cdc25A activity and increased tyrosine phosphorylation of Cdk4, resulting in decreased phosphorylation of retinoblastoma (Rb). Loss of Cdk4 activity was confirmed using Cdk4 immunoprecipitates from either Cpd 5-treated or untreated cells and measuring its kinase activity using GST-Rb as target. We found a similar order of activity for inhibition of growth and Cdc25A activity using several thiol-containing analogs. Cdc25A inhibitors may thus be useful for defining biochemical pathways involving protein tyrosine phosphorylation that mediate cell growth inhibition.

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. J. Cell. Physiol.