UCN-01 abrogates G2 arrest through a Cdc2-dependent pathway that is associated with inactivation of the Wee1Hu kinase and activation of the Cdc25C phosphatase

Specificity and mechanism of action of some commonly used protein kinase inhibitors

The specificities of 28 commercially available compounds reported to be relatively selective inhibitors of particular serine/threonine-specific protein kinases have been examined against a large panel of protein kinases. The compounds KT 5720, Rottlerin and quercetin were found to inhibit many protein kinases, sometimes much more potently than their presumed targets, and conclusions drawn from their use in cell-based experiments are likely to be erroneous. Ro 318220 and related bisindoylmaleimides, as well as H89, HA1077 and Y 27632, were more selective inhibitors, but still inhibited two or more protein kinases with similar potency. LY 294002 was found to inhibit casein kinase-2 with similar potency to phosphoinositide (phosphatidylinositol) 3-kinase. The compounds with the most impressive selectivity profiles were KN62, PD 98059, U0126, PD 184352, rapamycin, wortmannin, SB 203580 and SB 202190. U0126 and PD 184352, like PD 98059, were found to block the mitogen-activated protein kinase (MAPK) cascade in cell-based assays by preventing the activation of MAPK kinase (MKK1), and not by inhibiting MKK1 activity directly. Apart from rapamycin and PD 184352, even the most selective inhibitors affected at least one additional protein kinase. Our results demonstrate that the specificities of protein kinase inhibitors cannot be assessed simply by studying their effect on kinases that are closely related in primary structure. We propose guidelines for the use of protein kinase inhibitors in cell-based assays.

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.

Inhibition of CDKs as a therapeutic modality

Altered cell cycle control has emerged as a recurring theme in neoplasia. Strategies that would return toward normal the altered cell cycle control present in tumor cells have appeal as novel approaches to cancer treatment. Cyclin-dependent kinases (CDKs) control the progression through the cell cycle, operating at the transition from the G2 to M and G1 to S phases, and progression through S. CDKs are regulated by a complex set of mechanisms, including the presence of activating cyclins, regulatory phosphorylations, and endogenous CDK inhibitors at "checkpoints." This overview focuses on progress in defining compounds that can antagonize directly the action of CDKs. These have emerged as various types of ATP site-directed inhibitors, including flavopiridol, N-substituted adenine derivatives, the natural product butyrolactone, staurosporine derivatives, and, more recently, the synthetic paullones. Paullones appear to be of interest in that one of the most active members of the class, 9-nitropaullone (alsterpaullone), requires relatively brief periods of exposure to living cells in order to effect lasting effects on cellular and proliferative potential. Two of these compounds, flavopiridol and UCN-01 (7-hydroxy-staurosporine), have entered early clinical trials and achieved concentrations that might potentially modulate CDK function. In the case of UCN-01, unexpected human plasma protein binding might prevent direct inhibition of CDKs but allow drug concentrations to be achieved that indirectly affect CDKs by checkpoint abrogation. Further studies with CDK inhibitors should define the expected end point of CDK inhibition more clearly in preclinical models and clinical systems, including cytostasis, apoptosis, or differentiation.

UCN-01 enhances the in vitro toxicity of clinical agents in human tumor cell lines

UCN-01 is undergoing Phase I evaluation and is a candidate for combination strategies in the clinic. UCN-01 has been shown to have a variety of effects on cellular targets and the cell cycle. It has also been reported to sensitize cells to several clinical drugs in vitro, possibly in a manner related to p53 status. Thus, combinations of UCN-01 with a series of clinical agents in variety of cell lines have been investigated in vitro. Certain cell lines demonstrated synergistic interactions with combinations of UCN-01 (20-150 nM) and thiotepa, mitomycin C, cisplatin, melphalan, topotecan, gemcitabine, fludarabine or 5-fluorouracil. In contrast, UCN-01 combinations with the antimitotic agents, paclitaxel and vincristine, or topoisomerase II inhibitors, adriamycin and etoposide, did not result in synergy, only in additive toxicity. Cells with non-functional p53 were significantly more susceptible to the supra-additive effects of certain DNA-damaging agents and UCN-01 combinations, than cells expressing functional p53 activity. In contrast, there was no significant relationship between p53 status and susceptibility to synergy between antimetabolites and UCN-01. The mechanism behind the observed synergy appeared unrelated to effects on protein kinase C or abrogation of the cell cycle in G2. Moreover, increased apoptosis did not fully explain the supradditive response. These data indicate that UCN-01 sensitizes a variety of cell lines to certain DNA-damaging agents (frequently covalent DNA-binding drugs) and antimetabolites in vitro, but the mechanism underlying this interaction remains undefined.

UCN-01, a protein kinase C inhibitor, inhibits endothelial cell proliferation and angiogenic hypoxic response

Angiogenesis is required for tumor formation and growth; inhibition of angiogenesis is a promising new approach in cancer therapy. UCN-01, a protein kinase C (PKC) inhibitor, induces growth arrest and apoptosis in cancer cells and was recently introduced in a phase I clinical trial. We demonstrate that UCN-01, at concentrations lower than those necessary to inhibit cancer cell growth, inhibit proliferation of human endothelial cells in vitro. Moreover, UCN-01, at concentrations as low as 32 nM, prevent microvessel outgrowth from explant cultures of rat aortic rings. Since hypoxia activates hypoxia-inducible factor (HIF-1)-dependent transcription in cancer cells that, in a paracrine fashion, drive tumor angiogenesis, we investigated the effects of UCN-01 on HIF-1-responsive promoter constructs. We report that, in addition to direct inhibitory effects on endothelial cell growth, UCN-01 abrogates hypoxia-mediated transactivation of HIF-1-responsive promoters in a prostate cancer cell line. We conclude that UCN-01, at clinically relevant concentrations, exerts an anti-neovascularization effect by blocking two important steps in vessel formation: (1) the response of cancer cells to hypoxia, and (2) endothelial cell proliferation.

Ionizing radiation-induced mitogen-activated protein (MAP) kinase activation in DU145 prostate carcinoma cells: MAP kinase inhibition enhances radiation-induced cell killing and G2/M-phase arrest

These studies examine the role(s) played by the mitogen-activated protein kinase (MAPK) pathway after exposure of DU145 prostate carcinoma cells to radiation. Radiation (2 Gy) was found to cause both immediate primary (0-30 min) and prolonged secondary activations (90-1440 min) of the MAPK pathway. These activations of the MAPK pathway were abolished by inhibition of epidermal growth factor receptor (EGFR) function. The secondary activation was also abolished by addition of a neutralizing monoclonal antibody against transforming growth factor alpha (TGFA). Activation of the MAPK pathway could be induced in nonirradiated cells by the transfer of medium from irradiated cultures. Neutralizing antibody to TGFA blocked this effect, indicating that radiation causes secondary activation of the MAPK pathway by release of TGFA in DU145 cells. Radiation induced a transient G(2)/M-phase growth arrest that was prolonged for up to 24 h by inhibition of the MAPK pathway. Inhibition of the MAPK pathway significantly increased the ability of radiation to cause apoptosis 24 h after exposure. The ability of DU145 cells to proliferate after irradiation became dependent on MAPK signaling. When cells were subjected to single doses or fractionated radiation exposure, continuous inhibition of the MAPK pathway significantly decreased clonogenic survival. Only a small fraction of this cell killing could be accounted for by apoptosis within the first 96 h. Thus inhibition of the MAPK pathway increased radiation-induced cell killing likely by both apoptotic and nonapoptotic mechanisms. Collectively, our findings indicate that disruption of the TGFA/EGFR/MAPK pathway may represent a strategy that could be exploited to manipulate prostate carcinoma growth and cell survival after irradiation.

UCN-01 selectively enhances mitomycin C cytotoxicity in p53 defective cells which is mediated through S and/or G(2) checkpoint abrogation

We have previously reported that UCN-01 (7-hydroxystaurosporine), a protein kinase inhibitor that is under clinical trials as an anti-cancer agent in the USA and Japan, enhanced the anti-tumor activity of mitomycin C (MMC) in vitro and in vivo. Subsequent studies from other laboratories revealed that UCN-01 could selectively enhance cytotoxicity of DNA damaging agents in p53 defective cells and that this was mediated by abrogation of S and /or G(2) arrest by UCN-01. In this study, we report that UCN-01 selectively enhances the cytotoxicity of MMC in human p53 mutant cell lines. In contrast, UCN-01 showed little, if any, effect on MMC cytotoxicity in human p53 wild-type cell lines. Terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-nick end-labeling (TUNEL) assay revealed that the combination of MMC with UCN-01 increased DNA breaks consistent with apoptosis in p53 defective A431 epidermoid carcinoma cells. In p53 wild-type MCF-7 breast carcinoma cells, the cyclin-dependent kinase inhibitor protein p21/WAF1 was markedly induced after the treatment with MMC alone, although this response was significantly delayed from the time of MMC treatment. Detailed cell-cycle studies revealed that UCN-01 abrogated S and G(2) phase accumulation induced by MMC in p53 defective cells and to a lesser extent in p53 wild-type cell lines. The abrogation of arrest in p53 wild-type cells was observed prior to significant induction of the p53 response. Since MMC was less effective against p53 defective cell lines than against p53 wild-type cell lines and UCN-01 selectively enhanced MMC cytotoxicity in p53 defective cell lines, UCN-01 may provide a new modality of MMC-based cancer chemotherapy, particularly in p53 defective cancer patients.

The Chk1 protein kinase and the Cdc25C regulatory pathways are targets of the anticancer agent UCN-01

A checkpoint operating in the G(2) phase of the cell cycle prevents entry into mitosis in the presence of DNA damage. UCN-01, a protein kinase inhibitor currently undergoing clinical trials for cancer treatment, abrogates G(2) checkpoint function and sensitizes p53-defective cancer cells to DNA-damaging agents. In most species, the G(2) checkpoint prevents the Cdc25 phosphatase from removing inhibitory phosphate groups from the mitosis-promoting kinase Cdc2. This is accomplished by maintaining Cdc25 in a phosphorylated form that binds 14-3-3 proteins. The checkpoint kinases, Chk1 and Cds1, are proposed to regulate the interactions between human Cdc25C and 14-3-3 proteins by phosphorylating Cdc25C on serine 216. 14-3-3 proteins, in turn, function to keep Cdc25C out of the nucleus. Here we report that UCN-01 caused loss of both serine 216 phosphorylation and 14-3-3 binding to Cdc25C in DNA-damaged cells. In addition, UCN-01 potently inhibited the ability of Chk1 to phosphorylate Cdc25C in vitro. In contrast, Cds1 was refractory to inhibition by UCN-01 in vitro, and Cds1 was still phosphorylated in irradiated cells treated with UCN-01. Thus, neither Cds1 nor kinases upstream of Cds1, such as ataxia telangiectasia-mutated, are targets of UCN-01 action in vivo. Taken together our results identify the Chk1 kinase and the Cdc25C pathway as potential targets of G(2) checkpoint abrogation by UCN-01.

Protein kinase C targeting in antineoplastic treatment strategies

Neoplastic cell survival is governed by a balance between pro-apoptotic and anti-apoptotic signals. Noteworthy among several anti-apoptotic signaling elements is the protein kinase C (PKC) isoenzyme family, which mediates a central cytoprotective effect in the regulation of cell survival. Activation of PKC, and subsequent recruitment of numerous downstream elements such as the mitogen-activated protein kinase (MAPK) cascade, opposes initiation of the apoptotic cell death program by diverse cytotoxic stimuli. The understanding that the lethal actions of numerous antineoplastic agents are, in many instances, antagonized by cytoprotective signaling systems has been an important stimulus for the development of novel antineoplastic strategies. In this regard, inhibition of PKC, which has been shown to initiate apoptosis in a variety of malignant cell types, has recently been the focus of intense interest. Furthermore, there is accumulating evidence that selective targeting of PKC may prove useful in improving the therapeutic efficacy of established antineoplastic agents. Such chemosensitizing strategies can involve either (a) direct inhibition of PKC (e.g., following acute treatment with relatively specific inhibitors such as the synthetic sphingoid base analog safingol, or the novel staurosporine derivatives UCN-01 and CGP-41251) or (b) down-regulation (e.g., following chronic treatment with the non-tumor-promoting PKC activator bryostatin 1). In preclinical model systems, suppression of the cytoprotective function(s) of PKC potentiates the activity of cytotoxic agents (e.g., cytarabine) as well as ionizing radiation, and efforts to translate these findings into the clinical arena in humans are currently underway. Although the PKC-driven cytoprotective signaling systems affected by these treatments have not been definitively characterized, interference with PKC activity has been associated with loss of the mitogen-activated protein kinase (MAPK) response. Accordingly, recent pre-clinical studies have demonstrated that pharmacological disruption of the primary MEK-ERK module can mimic the chemopotentiating and radiopotentiating actions of PKC inhibition and/or down-regulation.

The disappearance of cyclins A and B and the increase in activity of the G(2)/M-phase cellular kinase cdc2 in herpes simplex virus 1-infected cells require expression of the alpha22/U(S)1.5 and U(L)13 viral genes

In uninfected cells the G(2)/M transition is regulated by cyclin kinase complex containing cdc2 and, initially, cyclin A, followed by cyclin B. cdc2 is downregulated through phosphorylation by wee-1 and myt-1 and upregulated by cdc-25C phosphatase. We have examined the accumulation and activities of these proteins in cells infected with wild type and mutants of herpes simplex virus 1. The results were as follows. (i) Cyclin A and B levels were reduced beginning 4 h after infection and were undetectable at 12 to 16 h after infection. (ii) cdc2 protein also decreased in amount but was detectable at all times after infection. In addition, a fraction of cdc2 protein from infected cells exhibited altered electrophoretic mobility in denaturing gels. (iii) The levels of cdk7 or myt-1 proteins remained relatively constant throughout infection, whereas the level of wee-1 was significantly decreased. (iv) cdc-25C formed novel bands characterized by slower electrophoretic mobility that disappeared after treatment with phosphatase. In addition, one phosphatase-sensitive band reacted with MPM-2 antibody that recognizes a phosphoepitope phosphorylated exclusively in M phase. (v) cdc2 accumulating in infected cells exhibited kinase activity. The activity of cdc2 was higher in infected cell lysates than those of corresponding proteins present in lysates of mock-infected cells even though cyclins A and B were not detectable in lysates of infected cells. (vi) The decrease in the levels of cyclins A and B, the increase in activity of cdc2, and the hyperphosphorylation of cdc-25C were mediated by U(L)13 and alpha22/U(S)1.5 gene products. In light of its normal functions, the activated cdc2 kinase may play a role in the changes in the morphology of the infected cell. These results are consistent with the accruing evidence that herpes simplex virus scavenges the cell for useful cell cycle proteins and subverts them for its own use.

DNA damage-induced cell cycle checkpoints and DNA strand break repair in development and tumorigenesis

Several newly identified tumor suppressor genes including ATM, NBS1, BRCA1 and BRCA2 are involved in DNA double-strand break repair (DSBR) and DNA damage-induced checkpoint activation. Many of the gene products involved in checkpoint control and DSBR have been studied in great detail in yeast. In addition to evolutionarily conserved proteins such as Chk1 and Chk2, studies in mammalian cells have identified novel proteins such as p53 in executing checkpoint control. DSBR proteins including Mre11, Rad50, Rad51, Rad54, and Ku are present in yeast and in mammals. Many of the tumor suppressor gene products interact with these repair proteins as well as checkpoint regulators, thus providing a biochemical explanation for the pleiotropic phenotypes of mutant cells. This review focuses on the proteins mediating G1/S, S, and G2/M checkpoint control in mammalian cells. In addition, mammalian DSBR proteins and their activities are discussed. An intricate network among DNA damage signal transducers, cell cycle regulators and the DSBR pathways is illustrated. Mouse knockout models for genes involved in these processes have provided valuable insights into their function, establishing genomic instability as a major contributing factor in tumorigenesis.

Roles for basal and stimulated p21(Cip-1/WAF1/MDA6) expression and mitogen-activated protein kinase signaling in radiation-induced cell cycle checkpoint control in carcinoma cells

We investigated the role of the cdk inhibitor protein p21(Cip-1/WAF1/MDA6) (p21) in the ability of MAPK pathway inhibition to enhance radiation-induced apoptosis in A431 squamous carcinoma cells. In carcinoma cells, ionizing radiation (2 Gy) caused both primary (0-10 min) and secondary (90-240 min) activations of the MAPK pathway. Radiation induced p21 protein expression in A431 cells within 6 h via secondary activation of the MAPK pathway. Within 6 h, radiation weakly enhanced the proportion of cells in G(1) that were p21 and MAPK dependent, whereas the elevation of cells present in G(2)/M at this time was independent of either p21 expression or MAPK inhibition. Inhibition of the MAPK pathway increased the proportion of irradiated cells in G(2)/M phase 24-48 h after irradiation and enhanced radiation-induced apoptosis. This correlated with elevated Cdc2 tyrosine 15 phosphorylation, decreased Cdc2 activity, and decreased Cdc25C protein levels. Caffeine treatment or removal of MEK1/2 inhibitors from cells 6 h after irradiation reduced the proportion of cells present in G(2)/M phase at 24 h and abolished the ability of MAPK inhibition to potentiate radiation-induced apoptosis. These data argue that MAPK signaling plays an important role in the progression/release of cells through G(2)/M phase after radiation exposure and that an impairment of this progression/release enhances radiation-induced apoptosis. Surprisingly, the ability of irradiation/MAPK inhibition to increase the proportion of cells in G(2)/M at 24 h was found to be dependent on basal p21 expression. Transient inhibition of basal p21 expression increased the control level of apoptosis as well as the abilities of both radiation and MEK1/2 inhibitors to cause apoptosis. In addition, loss of basal p21 expression significantly reduced the capacity of MAPK inhibition to potentiate radiation-induced apoptosis. Collectively, our data argue that MAPK signaling and p21 can regulate cell cycle checkpoint control in carcinoma cells at the G(1)/S transition shortly after exposure to radiation. In contrast, inhibition of MAPK increases the proportion of irradiated cells in G(2)/M, and basal expression of p21 is required to maintain this effect. Our data suggest that basal and radiation-stimulated p21 may play different roles in regulating cell cycle progression that affect cell survival after radiation exposure.

Discovering novel chemotherapeutic drugs for the third millennium

There is enormous potential for the discovery of innovative cancer drugs with improved efficacy and selectivity for the third millennium. In this review we show how novel mechanism-based agents are being discovered by focusing on the molecular targets and pathways that are causally involved in cancer formation, maintenance and progression. We also show how new technologies, from genomics through high through-put bioscience, combinatorial chemistry, rational drug design and molecular pharmacodynamic and imaging techniques, are accelerating the pace of cancer drug discovery. The process of contemporary small molecule drug discovery is described and progress and current issues are reviewed. New and potential targets and pathways for therapeutic intervention are illustrated. The first examples of a new generation of molecular therapeutics are now entering hypothesis-testing clinical trials and showing activity. The early years of the new millennium will see a range of exciting new agents moving from bench to bedside and beginning to impact on the management and cure of cancer.

CDK inhibition and cancer therapy

The cell-division cycle is a tightly controlled process that is regulated by the cyclin/CDK family of protein kinase complexes. Stringent control of this process is essential to ensure that DNA synthesis and subsequent mitotic division are accurately and coordinately executed. There is now strong evidence that CDKs, their regulators, and substrates are the targets of genetic alteration in many human cancers. As a result of this, the CDKs have been targeted for drug discovery and a number of small molecule inhibitors of CDKs have been identified.