Staurosporine is a potent inhibitor of p34cdc2 and p34cdc2-like kinases

Chemical inhibitors of cyclin-dependent kinases

The eukaryotic cell division cycle is regulated by a family of protein kinases, the cyclin-dependent kinases (cdk's), constituted of at least two subunits, a catalytic subunit (cdk1-7) associated with a regulatory subunit (cyclin A-H). Transient activation of cdk's is responsible for transition through the different phases of the cell cycle. Major abnormalities of cdk's expression and regulation have been described in human tumours. Enzymatic screening is starting to uncover chemical inhibitors of cdk's with anti-mitotic activities. This review summarizes our knowledge of these first inhibitors, their mechanism of action, their effects on the cell cycle, and discusses the potential of such type of inhibitors as anti-tumour agents.

Early inhibition of DNA synthesis in the developing rat cerebral cortex by the purine analogues olomoucine and roscovitine

The effects of the cyclin-dependent kinase (CDK) inhibitors olomoucine and roscovitine on DNA synthesis were studied using short time incubation (30-90 minutes). Both purine analogues at concentrations from 1-100 microM decreased the DNA synthesis of rat brain cortex in a dose-dependent manner and the maximum effect occurred within 30 min of incubation. Staurosporine, another potent CDK inhibitor did not affect the DNA synthesis in the concentration range 1-250 nM. These results indicate that olomoucine and roscovitine block DNA synthesis by a mechanism independent of CDK inhibition. We propose that the cellular effects of olomuocine and roscovitine on the cell cycle are at least in part due to this early inhibitory effect on DNA synthesis.

An important role for the retinoblastoma protein in staurosporine-induced G1 arrest in murine embryonic fibroblasts

In this study, we investigated the molecular basis of the ability of staurosporine to induce G1 arrest in murine embryonic fibroblasts (MEFs). We used MEFs from transgenic mice lacking several negative regulators of the G1/S phase transition including cells from mice lacking p53, p21, retinoblastoma (Rb), or p16 genes. We found that p53 function was not essential for staurosporine-induced G1 arrest. In contrast, MEFs from mice lacking Rb genes showed approximately a 70% reduced capacity to arrest in the G1 phase following staurosporine treatment. In support of a role for Rb in staurosporine-induced G1 arrest, rat embryonic fibroblasts stably overexpressing cyclin D1/Cdk4(R24C) exhibited approximately a 50% reduced G1 arrest response to staurosporine. The role of Rb in determining the degree of staurosporine-induced G1 arrest did not depend on the function of the cyclin-dependent kinase inhibitors p16 or p21 because MEFs lacking either of these genes were still capable of undergoing G1 arrest following staurosporine exposure. Our studies provide evidence of an important role for the Rb protein in determining the degree of staurosporine-induced G1 arrest in the first cell cycle.

Uncoupling of the order of the S and M phases: effects of staurosporine on human cell cycle kinases

The protein kinase inhibitor staurosporine (SSP) was employed to study the involvement of kinases in human cell cycle progression. Thirty to 100 ng/ml SSP blocks entry into S phase and M phase. Lack of entry into S phase is due to impaired activity of the retinoblastoma protein kinase. The requirement for any of the SSP-sensitive kinases for cell cycle progression can be abrogated in tumour cells. Therefore, these kinases act in a checkpoint network negatively controlling the initiation of S phase, M phase and cytokinesis, rather than being inherent parts of a substrate-product chain required for the initiation of the cell cycle phases. As a consequence of the lack of certain checkpoint effectors, tumour cells may endoreduplicate or binucleate in the presence of SSP. The latter processes, as well as meiosis, are naturally occurring in specialized cell types, leading to the idea that this checkpoint network controls the order of the cell cycle phases in normal cells. A model is presented where the cell cycle is envisioned as two independently running cycles, the S and the M cycle, which are controlled by intra and intercycledependent checkpoints in human somatic cells. The model accounts for the dependency of S and M phase initiation on the successful completion of the previous M and S phase, respectively, as well as entry into a resting state.

Staurosporine-induced conformational changes of cAMP-dependent protein kinase catalytic subunit explain inhibitory potential

BACKGROUND

Staurosporine inhibits most protein kinases at low nanomolar concentrations. As most tyrosine kinases, along with many serine/threonine kinases, are either proto oncoproteins or are involved in oncogenic signaling, the development of protein kinase inhibitors is a primary goal of cancer research. Staurosporine and many of its derivatives have significant biological effects, and are being tested as anticancer drugs. To understand in atomic detail the mode of inhibition and the parameters of high-affinity binding of staurosporine to protein kinases, the molecule was cocrystallized with the catalytic subunit of cAMP-dependent protein kinase.

RESULTS

The crystal structure of the protein kinase catalytic subunit with staurosporine bound to the adenosine pocket shows considerable induced-fit rearrangement of the enzyme and a unique open conformation. The inhibitor mimics several aspects of adenosine binding, including both polar and nonpolar interactions with enzyme residues, and induces conformational changes of neighboring enzyme residues.

CONCLUSIONS

The results explain the high inhibitory potency of staurosporine, and also illustrate the flexibility of the protein kinase active site. The structure, therefore, is not only useful for the design of improved anticancer therapeutics and signaling drugs, but also provides a deeper understanding of the conformational flexibility of the protein kinase.

Increased pressure induces sustained protein kinase C-independent herbimycin A-sensitive activation of extracellular signal-related kinase 1/2 in the rabbit aorta in organ culture

The 42- and 44-kD mitogen-activated protein kinases, also referred to as extracellular signal-related kinase (ERK) 2 and 1, respectively, may be transiently activated by stretching vascular smooth muscle cells (VSMCs). Using an organ culture model of rabbit aorta, we studied short- and long-term ERK1/2 activation by intraluminal pressure (150 mm Hg). Activation of ERK1/2 was biphasic: it reached a maximum (217.5 +/- 8.4% of control) 5 minutes after pressurizing and decreased to 120.7 +/- 5.1% of control after 2 hours. Furthermore, after 24 hours of pressurizing, ERK1/2 activity was as high (241.8 +/- 14.7% of control) as in the acute phase. Long-term pressure-induced ERK1/2 activation correlated with stimulation of tyrosine phosphorylation of proteins in the 125- to 140-kD range. Neither protein kinase C inhibitors (1 mumol/L staurosporine or 50 mumol/L bisindolylmaleimide-I) nor tyrosine kinase inhibitors (50 mumol/L tyrphostin A48 or 50 mumol/L genistein) affected pressure-induced ERK1/2 activation. However, the Src-family tyrosine kinase inhibitor herbimycin A (500 nmol/L) did reduce both 5-minute (by 92 +/- 8%) and 24-hour (by 63 +/- 7%) pressure-induced ERK1/2 activation. Thus, our results demonstrate a sustained activation of ERK1/2 and tyrosine kinases by intraluminal pressure in the arterial wall. Pressure-induced ERK1/2 activation is PKC independent and Src-family tyrosine kinase dependent and possibly includes activation of extracellular matrix-associated tyrosine kinases.

Cell cycle-regulated expression, phosphorylation, and degradation of p55Cdc. A mammalian homolog of CDC20/Fizzy/slp1

p55Cdc is a mammalian protein that shows high homology to the cell cycle proteins Cdc20p of Saccharomyces cerevisiae and the product of the Drosophila fizzy (fzy) gene, both of which contain WD repeats and are thought to be required for the metaphase-anaphase transition. The fzy mutants exhibit a metaphase arrest phenotype, which is accompanied by stabilization of cyclins A and B, leading to the hypothesis that fzy function is required for cell cycle-regulated ubiquitin-mediated proteolysis. p55Cdc expression was initiated at the G1/S transition and steady state levels of p55Cdc were highest at M and lowest in G1. Inhibition of the 26 S proteasome prevented both mitotic exit and loss of p55Cdc at the M/G1 transition, suggesting that p55Cdc degradation was mediated by the cell cycle-regulated proteolytic pathway. Immune complexes of p55Cdc obtained at different cell cycle stages showed a variety of proteins with dramatic differences observed in the pattern of associated proteins during the transition from G2 to M. Immunolocalization of p55Cdc demonstrated dynamic changes in p55Cdc localization as the cells transit mitosis. p55Cdc appears to act as a regulatory protein interacting with several other proteins, perhaps via its seven WD repeats, at multiple points in the cell cycle.

Protein kinase inhibition by staurosporine revealed in details of the molecular interaction with CDK2

Staurosporine exhibits nanomolar IC50 values against a wide range of protein kinases. The structure of a CDK2 staurosporine complex explains the tight binding of this inhibitor, and suggests features to be exploited in the design of specific inhibitors of CDKs.

Staurosporine induces telophase arrest and apoptosis blocking mitosis exit in human Chang liver cells

Staurosporine is a protein kinase inhibitor that is known to block G1/S and G2/M cell cycle transitions, resulting in the accumulation of G1 or G2 interphase cells, depending on the applied dosage. It is, however, not known to block mitosis (M-phase) progression. We show here that a 15 min prepulse with 500 nM staurosporine had caused mitotic cell arrest in human Chang liver cells, accumulating telophase cells distinguished by their doublet nuclei and cytokinetic constrictions. The arrested telophase cells perished by apoptosis. Telophase-specific cell cycle arrest and apoptosis are novel. Staurosporine targeting cytokinesis could provide insights into late mitosis where control mechanisms remain largely unknown.

The dynamics of a pre-mRNA splicing factor in living cells

Pre-mRNA splicing is a predominantly co-transcriptional event which involves a large number of essential splicing factors. Within the mammalian cell nucleus, most splicing factors are concentrated in 20-40 distinct domains called speckles. The function of speckles and the organization of cellular transcription and pre-mRNA splicing in vivo are not well understood. We have investigated the dynamic properties of splicing factors in nuclei of living cells. Here we show that speckles are highly dynamic structures that respond specifically to activation of nearby genes. These dynamic events are dependent on RNA polymerase II transcription, and are sensitive to inhibitors of protein kinases and Ser/Thr phosphatases. When single genes are transcriptionally activated in living cells, splicing factors leave speckles in peripheral extensions and accumulate at the new sites of transcription. We conclude that one function of speckles is to supply splicing factors to active genes. Our observations demonstrate that the interphase nucleus is far more dynamic in nature than previously assumed.

Effect of protein kinase inhibitors on activity of mammalian small heat-shock protein (HSP25) kinase

The aim of this study was to investigate different protein kinase inhibitors (secondary metabolite-derived substances, synthetic compounds, and substrate-based peptides) for their potency to inhibit the mammalian small heat shock protein (HSP25) kinase (E.C. 2.7.1.37) isolated from Ehrlich ascites tumor cells. Among the secondary metabolite-derived inhibitors (staurosporine, K-252a, K-252b, KT5926, KT5720, erbstatin analog, and quercetin) and synthetic compounds (H-9, H-89, HA 1004, KN-62, ML-7, tyrphostin A25, and tyrphostin B42), KT5926, staurosporine, and K-252a inhibited HSP25 kinase most efficiently. Kinetic analysis revealed that inhibition by staurosporine (Ki = 32.4 nM) and K-252a (Ki = 13.7 nM) was competitive with ATP. Inhibition by KT5926 was competitive with the substrate peptide KKKALNRQLSVAA (Ki = 27.2 nM) and noncompetitive with respect to ATP (Ki = 38.8 nM). In comparison with other protein kinases, HSP25 kinase was relatively resistant to most of the inhibitors. KT5926 was the only tested inhibitor with certain preference for HSP25 kinase when compared with protein kinases A, C, and G. Among the tested substrate-based peptides, we identified one peptide (KKKALNRQLGVAA), which preferentially inhibited HSP25 kinase in comparison with protein kinases A and C and mitogen-activated protein kinase. This peptide inhibited HSP25 kinase competitively with the substrate peptide (Ki = 8.1 microM) and noncompetitively with ATP (Ki = 134 microM). A peptide (SRVLKEDKERWEDVK) derived from the putative autoinhibitory domain of the closely related human mitogen-activated protein kinase-activated protein kinase-2 did not inhibit HSP25 kinase activity, suggesting the existence of several species of HSP25 kinases. Furthermore, the data identified structural requirements for inhibitors of HSP25-kinase.

Biochemical differences between staurosporine-induced apoptosis and premature mitosis

Apoptosis is morphologically related to premature mitosis, an aberrant form of mitosis. Staurosporine, a potent protein kinase inhibitor, induces not only apoptotic cell death in a wide variety of mammalian cells but also premature initiation of mitosis in hamster cells that are arrested in S phase by DNA synthesis inhibitors. Here we report on the biochemical differences between the two phenomena commonly caused by staurosporine. Rat 3Y1 fibroblasts that had been arrested in S phase with hydroxyurea underwent apoptosis by treatment with staurosporine, whereas S-phase-arrested CHO cells initiated mitosis prematurely when similarly treated with a low concentration of staurosporine. Chromosome condensation occurred in both apoptosis (3Y1) and premature mitosis (CHO). However, neither formation of mitotic spindles nor mitosis-specific phosphorylation of MPM-2 antigens was observed in apoptosis of 3Y1 cells, unlike premature mitosis of CHO cells. The p34cdc2 kinase activated in normal and prematurely mitotic cells remained inactive in the apoptotic cells, probably because the active cyclin B/p34cdc2 complex was almost absent in the S-phase-arrested 3Y1 cells. The absence of intracellular activation of p34cdc2 in apoptosis was confirmed by immunohistochemical analyses using a specific antibody raised against Ser55-phosphorylated vimentin which is specifically phosphorylated by p34cdc2 during M phase. Furthermore, phosphorylation of histones H1 and H3, which is associated with mitotic chromosome condensation, did not occur in the apoptotic cells. These results indicate that the two phenomena, staurosporine-induced apoptosis and premature mitosis, are different in their requirement for p34cdc2 kinase activation and histone phosphorylation.

The response of renal tubular epithelial cells to physiologically and chemically induced growth arrest

Cells respond to a variety of stresses by activating the transcription of a battery of "acute phase" or "stress response" genes. The nature of this response is tailored to the nature of the stress. The extent to which physiologically and pathophysiologically induced growth arrest share common genomic responses is unclear. We therefore compared the effects of a physiologically induced (serum and nutrient depletion) and a chemically induced (2-Br-bis-(GSyl)HQ and 2-Br-6-(GSyl)HQ) stress in renal tubular epithelial cells (LLC-PK1). The response to physiological stress, induced by serum depletion, involves growth arrest characterized by an inhibition of DNA synthesis that occurs in the absence of a decrease in histone mRNA or an increase in gadd153 mRNA, one of the growth arrest and DNA damage inducible genes. In contrast, the chemical-induced stress involves growth arrest accompanied by a decrease in histone mRNA, particularly core histone H2B and H2A mRNA, and the induction of gadd153. Chemical-induced changes in histone mRNA inversely correlate to changes in the expression of a stress gene, hsp70, whose expression is dependent upon the maintenance of appropriate nucleosomal structure.

RNA-dependent phosphorylation of a nuclear RNA binding protein

The human C1 heterogeneous nuclear ribonucleoprotein particle protein (hnRNP protein) undergoes a cycle of phosphorylation-dephosphorylation in HeLa cell nuclear extracts that modulates the binding of this protein to pre-mRNA. We now report that hyperphosphorylation of the C1 hnRNP protein is mediated by a kinase activity in nuclear extracts that is RNA-dependent. Although the basal phosphorylation of the C1 hnRNP protein in nuclear extracts reflects a casein kinase II-type activity, its RNA-dependent hyperphosphorylation appears to be mediated by a different kinase. This is indicated by the unresponsiveness of the RNA-stimulated hyperphosphorylation to casein kinase II inhibitors, and the distinct glycerol gradient sedimentation profiles of the basal versus RNA-stimulated C1 hnRNP protein phosphorylation activities from nuclear extracts. RNA-dependent phosphorylation was observed both for a histidine-tagged recombinant human C1 hnRNP protein added to nuclear extracts and also for the endogenous C1 hnRNP protein. Additional results rule out protein kinase A, protein kinase C, calmodulin-dependent protein kinase II, and double-stranded RNA-activated protein kinase as the enzymes responsible for the RNA-dependent hyperphosphorylation of the C1 hnRNP protein. These results reveal the existence in nuclear extracts of an RNA-dependent protein kinase activity that hyperphosphorylates a known pre-mRNA binding protein, and define an additional element to be integrated into the current picture of how nuclear proteins are regulated by phosphorylation.

Inhibition of cyclin-dependent kinases by purine analogues: crystal structure of human cdk2 complexed with roscovitine

Cyclin-dependent kinases (cdk) control the cell division cycle (cdc). These kinases and their regulators are frequently deregulated in human tumours. A potent inhibitor of cdks, roscovitine [2-(1-ethyl-2-hydroxyethylamino)-6-benzylamino-9-isopropylpurin e], was identified by screening a series of C2,N6,N9-substituted adenines on purified cdc2/cyclin B. Roscovitine displays high efficiency and high selectivity (Meijer, L., Borgne, A., Mulner, O., Chong, J. P. J., Blow, J. J., Inagaki, N., Inagaki, M., Delcros, J.-G. & Moulinoux, J.-P. (1997) Eur. J. Biochem. 243, 527-536). It behaves as a competitive inhibitor for ATP binding to cdc2. We determined the crystal structure of a complex between cdk2 and roscovitine at 0.24-nm (2.4 A) resolution and refined to an Rfactor of 0.18. The purine portion of the inhibitor binds to the adenine binding pocket of cdk2. The position of the benzyl ring group of the inhibitor enables the inhibitor to make contacts with the enzyme not observed in the ATP-complex structure. Analysis of the position of this benzyl ring explains the specificity of roscovitine in inhibiting cdk2. The structure also reveals that the (R)-stereoisomer of roscovitine is bound to cdk2. The (R)-isomer is about twice as potent in inhibiting cdc2/cyclin B than the (S)-isomer. Results from structure/activity studies and from analysis of the cdk2/roscovitine complex crystal structure should allow the design of even more potent cdk inhibitors.

Biochemical and cellular effects of roscovitine, a potent and selective inhibitor of the cyclin-dependent kinases cdc2, cdk2 and cdk5

Cyclin-dependent kinases (cdk) play an essential role in the intracellular control of the cell division cycle (cdc). These kinases and their regulators are frequently deregulated in human tumours. Enzymatic screening has recently led to the discovery of specific inhibitors of cyclin-dependent kinases, such as butyrolactone I, flavopiridol and the purine olomoucine. Among a series of C2, N6, N9-substituted adenines tested on purified cdc2/cyclin B, 2-(1-ethyl-2-hydroxyethylamino)-6-benzylamino-9-isopropylpurine (roscovitine) displays high efficiency and high selectivity towards some cyclin-dependent kinases. The kinase specificity of roscovitine was investigated with 25 highly purified kinases (including protein kinase A, G and C isoforms, myosin light-chain kinase, casein kinase 2, insulin receptor tyrosine kinase, c-src, v-abl). Most kinases are not significantly inhibited by roscovitine. cdc2/cyclin B, cdk2/cyclin A, cdk2/cyclin E and cdk5/p35 only are substantially inhibited (IC50 values of 0.65, 0.7, 0.7 and 0.2 microM, respectively). cdk4/cyclin D1 and cdk6/cyclin D2 are very poorly inhibited by roscovitine (IC50 > 100 microM). Extracellular regulated kinases erk1 and erk2 are inhibited with an IC50 of 34 microM and 14 microM, respectively. Roscovitine reversibly arrests starfish oocytes and sea urchin embryos in late prophase. Roscovitine inhibits in vitro M-phase-promoting factor activity and in vitro DNA synthesis in Xenopus egg extracts. It blocks progesterone-induced oocyte maturation of Xenopus oocytes and in vivo phosphorylation of the elongation factor eEF-1. Roscovitine inhibits the proliferation of mammalian cell lines with an average IC50 of 16 microM. In the presence of roscovitine L1210 cells arrest in G1 and accumulate in G2. In vivo phosphorylation of vimentin on Ser55 by cdc2/cyclin B is inhibited by roscovitine. Through its unique selectivity for some cyclin-dependent kinases, roscovitine provides a useful antimitotic reagent for cell cycle studies and may prove interesting to control cells with deregulated cdc2, cdk2 or cdk5 kinase activities.

Screening of cell cycle inhibitors from microbial metabolites by a bioassay using a mouse cdc2 mutant cell line, tsFT210

We have established a bioassay system using a mouse cdc2 mutant cell line, tsFT210, to detect inhibitors of the mammalian cell cycle. When cultured at the high temperature, restrictive temperature at 39.4 degrees C, tsFT210 cells can be arrested at G2 phase and are large in size. Four hours after release from G2 arrest, the cells entered into the G1 phase. At this time, G1 phase cells were easily discriminated from the G2/M-cells by their size under microscopic observation. The cell-morphology-based bioassay utilizing tsFT210 cells is very simple and sensitive for detecting cdc2 kinase inhibitors and also G2/M-phase inhibitors of the mammalian cell cycle. To demonstrate the merits of this bioassay, the effects of protein kinase inhibitors isolated from actinomycetes were investigated. RK-286C and RK-1409, which are structurally related to staurosporine, inhibited cell cycle progression at the G2 phase in both G2-synchronized and nonsynchronized cultures of tsFT210 cells. Another kinase inhibitor, sangivamycin, inhibited cell cycle progression at the G2 phase of cells released from temperature arrest but did not inhibit that of the exponentially growing cells. Using the bioassay system, we carried out screening of the cell cycle inhibitors from the microbial metabolites and have discovered several new inhibitors, including novel compounds such as tryprostatins A, B and acetophthalidin. Thus, this bioassay allowed for the detection of cell cycle inhibitors and provided a convenient and useful method for the screening of new inhibitors from the microbial metabolites.

Stimulatory effect of reconstituted basement membrane components (matrigel) on the colony formation of a panel of human lung cancer cell lines in soft agar

Lung cancers have been distinguished into small-cell lung cancer (SCLC) and non-small cell-lung cancer (NSCLC) types on the basis of their clinical behaviors and their responses to treatment. Moreover, growth of most SCLC cell lines in liquid culture medium is nonadherent, while that of most NSCLC cell lines is adherent. In this study, we examined the effect of matrigel (reconstituted basement membrane components), which is known to have growth-stimulatory activity on various human tumor cell lines in immunodeficient mice, on soft-agar colony formation of a panel of SCLC and NSCLC cell lines to clarify its mechanism of growth stimulation of cancer cells. Matrigel enhanced colony formation of all 9 NSCLC cell lines and 4 of 9 SCLC cell lines. There was a statistically significant difference (P < 0.01) between colony formations with and without matrigel of NSCLC cell lines, but not for SCLC cell lines. In liquid culture medium, all 9 NSCLC lines and 3 of 9 SCLC lines adhered to plastic dishes, whereas the other SCLC lines did not. Matrigel enhanced colony formation of all 3 adherent-type SCLC lines and 1 of 6 nonadherent-type NSCLC lines. Matrigel enhanced colony formation of both of 2 adherent-type non-lung cancer cell lines and 1 of 2 nonadherent-type leukemia cell lines. Neither transforming growth factor beta, collagen type IV, fibronectin, nor laminin, which are components of matrigel, enhanced colony formation of an NSCLC cell line in soft agar. The increase in the colony number of the NSCLC cell line by matrigel was abrogated by the protein kinase inhibitors staurosporine and UCN-01.

Differential effects of staurosporine analogues on cell cycle, growth and viability in A549 cells

Staurosporine is a potent but non-specific kinase inhibitor. It has served as synthetic template for a variety of analogues, the indolocarbazoles, UCN-01 and CGP 41251, and the bisindolylmaleimides, Ro 31-8220 and GF 109203X, were investigated as growth inhibitors of human-derived A549 human lung adenocarcinoma cells. They were compared with respect to (1) effect on the cell cycle, (2) time dependency of growth arrest and (3) cytotoxic potency. Cells were exposed for 1, 2 and 4 days, or for 6, 12 and 24 h in the case of cycle-synchronised cells, to staurosporine analogues at concentrations at which they inhibited growth by 80% after 4 day exposure. Staurosporine and UCN-01 retarded cells in G0/1, and CGP 41251 appeared to inhibit cell growth without cell cycle specificity. Ro 31-8220 slowed progression of synchronised cells through the cycle; over a longer time period it induced a weak block in G2/M. GF 109203X induced potent G2/M arrest in synchronised cells. This was not so apparent in asynchronous cells, which by day 4 were slowed in G0/1 instead. Growth arrest induced by these inhibitors was more potent after incubation for 4 rather than 2 days. Incubation for 1 day followed by maintenance in drug-free medium for 3 days was sufficient to exert some cytostasis. The differences between cytotoxic and cytostatic concentrations, the former measured by release from cells of lactate dehydrogenase, were 15 000-fold for staurosporine, 300-fold for UCN-01, approximately 400-fold for CGP 41251, 25-fold for Ro 31-8220 and approximately 4-fold for GF 109203X. The results show that PKC-selective staurosporine analogues differ with respect to the mechanisms by which they interfere with the cell cycle. The necessity of long-term exposure for effective growth inhibition and the considerable margin between cytostatic and acute cytotoxic indolocarbazole concentrations are findings which might influence the planning and interpretation of clinical trials of these kinase inhibitors.

Understanding and controlling the cell cycle with natural products

Small molecule natural products have aided in the discovery and characterization of many proteins critical to the progression and maintenance of the cell cycle. Identification of the direct target of a natural product gives scientists a tool to control a specific aspect of the cell cycle, thus facilitating the study of the cell-cycle machinery.

Binding of the vesicle docking protein p115 to Golgi membranes is inhibited under mitotic conditions

The vesicle docking protein p115 showed saturable, high affinity binding to interphase Golgi membranes. The affinity of binding was up to 20-fold lower using membranes preincubated with mitotic cytosol. In contrast, binding was not affected by mitotic pretreatment of p115. The reduction in p115 binding was mediated by phosphorylation, could be induced by a cyclin-dependent kinase, and was fully reversible. A shift of p115 from membranes to cytosol was also found after fractionating mitotic cells. The functional significance of the decreased binding was addressed by in vitro mitotic incubations which disassemble Golgi cisternae, predominantly producing transport vesicles. The addition of excess p115 decreased loss of membrane from cisternae, indicating that p115's action is limiting while transport vesicles accumulate. The cessation of intra-Golgi traffic in mitosis has been hypothesized to result from an inhibition of membrane fusion while budding of transport vesicles continues. This process also contributes to mitotic Golgi disassembly. Our results imply that there is a mitotic modification to Golgi membranes leading to a reduction in the affinity of the p115 receptor. Reduced p115 binding may play a part in the inhibition of membrane fusion by preventing prior vesicle docking.

Decreased accumulation and dephosphorylation of the mitosis-specific form of nucleophosmin/B23 in staurosporine-induced chromosome decondensation

Nucleophosmin/B23 is highly phosphorylated by cdc2 kinase during mitosis, and this phosphorylation most probably has a role in initiating and controlling the entry of cells into mitosis [Peter, Nakagawa, Doree, Labbe and Nigg (1990) Cell 60, 791-801]. In the present study, the protein kinase inhibitor staurosporine has been used to examine possible changes in nucleophosmin/B23 at mitosis in HeLa cells. Addition of staurosporine to HeLa cells already arrested at mitosis by nocodazole causes: (i) decreased accumulation of the mitosis-specific form of nucleophosmin/B23, (ii) dephosphorylation of nucleophosmin/ B23, (iii) redistribution of nucleophosmin/B23 to the cytosol, and (iv) concomitant decondensation of chromosomes. These results suggest that the mitosis-specific phosphorylated form of nucleophosmin/B23 may play a role in maintaining mitotic chromosomes in their condensed state.

Evidence that the endogenous histone H1 phosphatase in HeLa mitotic chromosomes is protein phosphatase 1, not protein phosphatase 2A

Histone H1 is highly phosphorylated in mitotic HeLa cells, but is quickly dephosphorylated in vivo at the end of mitosis and in vitro following cell lysis. We show here that okadaic acid and microcystin-LR block the in vitro dephosphorylation of H1 and that they do so directly by inhibiting the histone H1 phosphatase rather than by some indirect mechanism. The concentrations of microcystin and okadaic acid required for inhibition strongly suggest that the histone H1 phosphatase is either PP1 or an unknown protein phosphatase with okadaic acid-sensitivity similar to PP1. The histone H1 phosphatase is predominantly located in chromosomes with at most one copy for every 86 nucleosomes. This tends to support its identification as PP1, since localization in mitotic chromosomes is a characteristic of PP1 but not of the other known okadaic acid-sensitive protein phosphatases. We also show that treatment of metaphase-arrested HeLa cells with staurosporine and olomoucine, inhibitors of p34cdc2 and other protein kinases, rapidly induces reassembly of interphase nuclei and dephosphorylation of histone H1 without chromosome segregation. This result indicates that protein kinase activity must remain elevated to maintain a mitotic block. Using this as a model system for the M- to G1-phase transition, we present evidence from inhibitor studies suggesting that the in vivo histone H1 phosphatase may be either PP1 or another phosphatase with similar okadaic acid-sensitivity, but not PP2A.

Inhibition of neuronal cyclin-dependent kinase-5 by staurosporine and purine analogs is independent of activation by Munc-18

Neuronal cdk5 can phosphorylate certain lys-ser-pro (KSP) motifs of neurofilaments and tau protein in the nervous system. We have immunoprecipitated the cdk5 from rat brain using a polyclonal antibody raised against the C-terminus of cdk5. The immunoprecipitate has phosphorylated a KSPXK peptide analog of NF-H, as well as histone H1 and a bacterially expressed rat NF-H protein. The kinase activity was inhibited by staurosporine, isopentanyladenine and olomoucine in a dose dependent manner. Kinetic studies indicated Ki values of 39 nM, 38 microM and 8 microM, respectively for staurosporine, isopentanyladenine and olomoucine. The inhibition by staurosporine was non-competitive with respect to phosphoryl acceptor acceptor substrates. Western blot analysis of the immunoprecipitate showed both cdk5 and p67 (Munc-18), a putative regulator molecule of the kinase. Addition of p67 fusion protein enhanced the kinase activity of the immunoprecipitate by 60% above the basal activity. P67 elevated Ki values for both staurosporine and olomoucine. The degree of inhibition at high concentrations of these inhibitors was unaltered by exogenous p67 indicating a lack of competitive interactions with p67. The high affinity of staurosporine for cdk5 suggests that cdk5 may be one of the targets for the neurotropic effect of staurosporine.

Reducing the radiation-induced G2 delay causes HeLa cells to undergo apoptosis instead of mitotic death

Cells exposed to radiation may undergo death through apoptosis or mitotic death. HeLa cells predominantly undergo mitotic death after irradiation. Treatment of these cells with caffeine has been shown to shorten the G2 delay after irradiation, and to decrease their survival. The kinase inhibitor staurosporine also decreases the radiation-induced G2 delay in HeLa cells. Here we extend these findings to show that the decrease in radiation-induced G2 delay mediated by caffeine or staurosporine is accompanied by a shift in the pathway of cell death from mitotic death to apoptotic death. The increase in apoptosis is further accompanied by decreased clonogenic survival after irradiation. Based on these findings we propose the hypothesis that one mechanism of enhancing cell killing by radiation is to trigger apoptosis by decreasing the G2 delay induced by irradiation.

Signal transduction for proliferation of glioma cells in vitro occurs predominantly through a protein kinase C-mediated pathway

Previous work has demonstrated that glioma cells have very high protein kinase C (PKC) enzyme activity when compared to non-malignant glia, and that their PKC activity correlates with their proliferation rate. The purpose of this study was to determine whether the elevated PKC activity in glioma is secondary to an autonomously active PKC isoform implying oncogenic transformation, or whether this activity is driven by upstream ligand-receptor tyrosine kinase interactions. We treated established human glioma cell lines A172, U563 or U251 with either the highly selective PKC inhibitor CGP 41 251, or with genistein, a tyrosine kinase inhibitor. The proliferation rate and PKC activity of all the glioma lines was reduced by CGP 41 251; the IC50 values for inhibiting cell proliferation corresponded to the IC50v values for inhibition of PKC activity. Genistein also inhibited cell proliferation, with IC50 proliferation values approximating those for inhibition of tyrosine kinase activity in cell free protein extracts. Importantly, in genistein-treated cells, downstream PKC enzyme activity was dose dependently reduced such that the correlation coefficient for effects of genistein on proliferation rate and PKC activity was 0.92. These findings suggest that upstream tyrosine kinase linked events, rather than an autonomously functioning PKC, result in the high PKC activity observed in glioma. Finally, fetal calf serum (FCS) evoked a strong mitogenic effect on glioma cell lines. This mitogenic activity was completely blocked by CGP 41 251, suggesting that although the many mitogens in FCS for glioma cells signal initially through genistein-inhibitable tyrosine kinases, they ultimately channel through a PKC-dependent pathway. We conclude that proliferative signal transduction in glioma cells occurs through a predominantly PKC-dependent pathway and that selectively targeting this enzyme provides an approach to glioma therapy.

Thrombin activates mitogen-activated protein kinase in primary astrocyte cultures

Thrombin is known to evoke numerous inflammatory and proliferative responses in a wide variety of its target cells. Recent studies have demonstrated morphoregulatory and mitogenic effects of thrombin on astroglial cells (astrocytes). The present study deals with thrombin-induced activation of mitogen-activated protein (MAP) kinase in primary cultures of rat astrocytes. Treatment of serum-starved astrocytes with thrombin resulted in a rapid activation of tyrosine (Tyr) phosphorylation of a set of proteins including a prominent one with a molecular mass of 42 kDa (p42). The identity of p42 with MAP kinase was confirmed by MAP kinase-immunoreactivity of isolated [i.e., immunoprecipitated with anti-phosphotyrosine (PY) antibodies] p42 and by increased myelin basic protein (MBP) kinase activity present in MAP kinase immunoprecipitates of thrombin-treated cultures. Pertussis toxin (PTX) pretreatment failed to inhibit thrombin stimulation of p42 phosphorylation, indicating the lack of involvement of PTX sensitive G proteins in the mechanism of activation of MAP kinase by thrombin. Chronic exposure of cultures to phorbol 12-myristate 13-acetate to down-regulate PKC resulted in an attenuation of thrombin-induced p42 Tyr phosphorylation, although H-7, a known PKC inhibitor, failed to block thrombin effect. However, staurosporine, a nonspecific protein kinase inhibitor, prevented the activation of p42 phosphorylation. It is concluded that thrombin induces MAP kinase activation in astrocytes by a mechanism involving a staurosporine-sensitive pathway.

Hierarchical analysis of the nerve growth factor-dependent and nerve growth factor-independent differentiation signaling pathways in PC12 cells with protein kinase inhibitors

The effects of a series of protein kinase inhibitors on nerve growth factor (NGF)-dependent and NGF-independent neurite outgrowth in PC12 cells have established an ordered relationship among those protein kinases sensitive to down regulation by bryostatin, stimulation by staurosporine, inhibition by sphingosine, or inhibition by 6-thioguanine (6-TG). Quantitation of the biphasic staurosporine effects on NGF-induced neurite outgrowth (Hashimoto and Hagino: J Neurochem 53:1675-1685, 1989) gave an IC50 of 2-4 nM for inhibition and an EC50 of 15-20 nM for induction of neurite extension. Both sphingosine and 6-TG inhibited neurite outgrowth induced by staurosporine and basic fibroblast derived growth factor (bFGF), as well as by NGF; therefore, sphingosine- and 6-TG-sensitive protein kinase steps occur after the convergence of the NGF, bFGF, and staurosporine signal pathways. Down regulation of protein kinase C by bryostatin chronic treatment, which inhibits NGF- and bFGF-induced neuritogenesis (Singh et al.: Biochemistry 33:542-551, 1994), did not inhibit the staurosporine-induced neurite outgrowth. Thus, the bryostatin-sensitive protein kinase C must occur subsequent to the convergence of the bFGF and NGF pathways, but before (or parallel to) staurosporine initiation of neurite outgrowth. In contrast, low concentrations of phorbol myristoyl acetate (PMA) or bryostatin, which activate protein kinase C activity, enhanced the staurosporine- or NGF-induced neurite extension. These data indicate that stimulation of one or more protein kinase C isozymes can synergistically interact with the signaling pathway to increase the rate of neuritogenesis. Inhibition by 5-7.5 nM staurosporine acted rapidly to arrest and decrease development of neurites up to 24 hr after NGF treatment, as did K252a and NGF polyclonal antibody addition. Our cellular data support the concept that staurosporine acts to inhibit the NGF receptor Trk (Nye et al.: Mol Biol Cell 3:677-686, 1992), but that downstream steps can be activated by the higher concentration of staurosporine to bypass Trk and lead to neurite generation. Effects of staurosporine, 6-TG, and sphingosine on c-fos gene induction with or without NGF were not correlated with the generation of neurites. The sequence of protein kinases sensitive to these effectors appears to be in the order (but not consecutive) bryostatin, staurosporine, sphingosine, and 6-TG.

Effect of retinoic acid on Nm/23 nucleoside diphosphate kinase and components of cyclic adenosine monophosphate-dependent signalling in human neuroblastoma cell lines

The effects of retinoic acid on components of the cAMP-dependent signalling system were examined in two related human neuroblastoma cell lines SK-N-SH-F (SHF) and SK-N-SH-N (SHN). Retinoid treatment for a week significantly increased the concentration of intracellular cAMP and the levels of activity of protein kinase A and adenylate cyclase in both cell lines. Retinoic acid treatment also caused a very marked translocation of nucleoside diphosphate kinase from the cytosol to the membrane fraction. The increases in cyclic nucleotide and protein kinase A activity were observed to occur as early as within 1 and 2 days respectively and preceded or were concurrent with the onset of observable morphological differentiation. Results also indicated that agents which elevated intracellular cAMP caused neuronal differentiation and blunted retinoic acid-induced melanocytic differentiation in SHF cells. However, increases in cAMP brought about by treatment of SHF cells with retinoic acid alone were several-fold smaller and thus insufficient to induce neuritogenesis in these cells. The results as a whole indicate that one overall effect of retinoic acid treatment is to upgrade the activity of components of the cAMP-dependent signalling system in both neuroblastoma cell lines. However, retinoic acid causes the SH-F and SH-N cell lines to differentiate along different routes which means that the upgrading responses may be related to more general aspects of differentiation rather than to specific phenotype expression.

Differential Cellular Distribution of Retinoic Acid during Staurosporine Potentiation of Retinoic Acid-Induced Granulocytic Differentiation in Human Leukemia HL-60 Cells

Pretreatment of cells with staurosporine, a protein kinase C (PKC) inhibitor, was found to potentiate the granulocytic differentiation induced by a brief (2 h) retinoic acid treatment. By cell cycle analysis, staurosporine was found to have little effect on the cell cycle. Retinoic acid was distributed equally in the nuclei (40%) and in the plasma membrane (40%) of staurosporine-pretreated cells while less than 20% of retinoic acid was found in the membrane of control non-staurosporine-pretreated cells during the retinoic acid-induced differentiation. These results indicate that the enhancing effect of staurosporine may be somehow associated with the localization of retinoic acid in the plasma membrane of the cell. Copyright 1995 S. Karger AG, Basel

Comparison of ability of protein kinase C inhibitors to arrest cell growth and to alter cellular protein kinase C localisation

Inhibitors of protein kinase C (PKC) such as the staurosporine analogues UCN-01 and CGP 41251 possess antineoplastic properties, but the mechanism of their cytostatic action is not understood. We tested the hypothesis that the ability of these compounds to arrest growth is intrinsically linked with their propensity to inhibit PKC. Compounds with varying degrees of potency and specificity for PKC were investigated in A549 and MCF-7 carcinoma cells. When the log values of drug concentration which arrested cell growth by 50% (IC50) were plotted against the logs of the IC50 values for inhibition of cytosolic PKC activity, two groups of compound could be distinguished. The group which comprised the more potent inhibitors of enzyme activity (calphostin C, staurosporine and its analogues UCN-01, RO 31-8220, CGP 41251) were the stronger growth inhibitors, whereas the weaker enzyme inhibitors (trimethylsphingosine, miltefosine, NPC-15437, H-7, H-7I) affected proliferation less potently. GF 109203X was exceptional in that it inhibited PKC with an IC50 in the 10(-8) M range, yet was only weakly cytostatic. To substantiate the role of PKC in the growth inhibition caused by these agents, cells were depleted of PKC by incubation with bryostatin 1 (1 microM). The susceptibility of these enzyme-depleted cells towards growth arrest induced by staurosporine, RO 31-8220, UCN-01 or H-7 was studied. The drug concentrations which inhibited incorporation of [3H]thymidine into PKC-depleted A549 cells by 50% were slightly, but not significantly, lower than significantly, lower than those observed in control cells. These results suggest that PKC is unlikely to play a direct role in the arrest of the growth of A549 and MCF-7 cells mediated by these agents. Staurosporine is not only a strong inhibitor of PKC but also mimics activators of this enzyme in that it elicits the cellular redistribution of certain PKC isoenzymes. The ability of kinase inhibitors other than staurosporine to exert a similar effect was investigated. Calphostin C, H-7, H-7I, miltefosine, staurosporine, UCN-01, RO 31-8220, CGP 41251 or GF 109203X were incubated for 30 min with A549 cells in the absence or presence of the PKC activator 12-O-tetradecanoyl phorbol-13-acetate. The subcellular distribution of PKC-alpha-, -epsilon and -zeta was measured by Western blot analysis. None of the agents affected PKC-alpha or -zeta.(ABSTRACT TRUNCATED AT 400 WORDS)

Paradoxical elevation of Ki-67 labeling with protein kinase inhibition in malignant gliomas

The monoclonal antibody Ki-67 recognizes a nuclear antigen expressed in the G1, S, G2, and M phase of the cell cycle and has been used extensively as an indicator of cellular proliferation in malignant gliomas, both in the laboratory and clinically. Recently, protein kinase C (PKC) inhibitors have been demonstrated to inhibit malignant glioma growth both in in vitro and in vivo. This study was undertaken to determine whether Ki-67 could function as an indicator of cellular proliferation rate after PKC inhibition in gliomas and to explore cell cycle specificity of such inhibition. Both established and low-passage malignant glioma cell lines have previously been shown to be sensitive to growth inhibition by the PKC inhibitors staurosporine and tamoxifen in vitro (IC50 in the nanomolar and micromolar ranges, respectively), as measured by cell numbers, [3H]thymidine uptake, and flow-cytometric DNA analysis. However, in the same cells that are inhibited by staurosporine and tamoxifen on these assays, and on the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl tetrazolium bromide (MTT) assay in the present study, the Ki-67 labeling index paradoxically increased in a dose-related manner with the same treatments, as measured by immunohistochemistry and confirmed by flow cytometry. For example, in established line U-87, a 20.5% decrease in thymidine uptake and a 28.5% decrease in absorbance on the MTT assay produced by tamoxifen at 1 microM was associated with an increase in Ki-67 labeling from 42% to 62%; staurosporine, which produces a 78.8% decrease in thymidine uptake in cell line A-172 at 10 nM, produced an increase in Ki-67 labeling from 19% to 32%. In this regard, Ki-67 labeling of glioblastoma tissue from a patient treated with high-dose tamoxifen yielded results within the range of 10% to 15% (consistent with values seen in untreated glioblastoma), despite tumor regression with treatment. The authors' interpretation of these results is that these PKC inhibitors are halting the cell cycle in the G1 phase or the G1-S transition (beyond G0 but before S-phase), resulting in a paradoxical increase in labeling while arresting growth. Two important implications from these observations are that Ki-67 is not a reliable indicator of cellular proliferation after treatment with PKC inhibitors and that these inhibitors used at the doses given above halt cell growth in a phase-specific manner.

Staurosporine affects calcium homeostasis in cultured bovine adrenal chromaffin cells

These studies show that the potent, non-specific, protein kinase inhibitor, staurosporine, disrupts Ca2+ homeostasis in cultured bovine adrenal chromaffin cells. Staurosporine treatment reduces basal and A23187-stimulated catecholamine release from chromaffin cells, but does not inhibit activated Ca2+ influx. Furthermore, pretreatment with staurosporine also reduces Ca(2+)-stimulated catecholamine release from digitonin-permeabilized cells (t1/2, 40.6 min; IC50, 66.0 nm). However, staurosporine does not inhibit the rise in intracellular Ca2+ ([Ca2+]i) in response to nicotine stimulation as measured by fura-2 photometry. These studies demonstrate that staurosporine interferes with the secretory process at some step at or after the rise in [Ca2+]i in adrenal chromaffin cells. Examination of the effects of staurosporine on 45Ca2+ movement shows that staurosporine produces a slowly developing basal 45Ca2+ accumulation; after 30 min no significant change is observed, but by 120 min, 45Ca2+ accumulation is increased by 29.5%. Thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBHQ), inhibitors of Ca(2+) ATPases, were used to determine whether staurosporine induced 45Ca2+ accumulation results from sequestration of 45Ca2+ within intracellular stores. While thapsigargin has no significant effect, concomitant treatment with tBHQ prevents the increase in 45Ca2+ uptake associated with staurosporine treatment. Therefore, the tBHQ-sensitive Ca2+ store, but not the thapsigargin/inositol 1,4,5-triphosphate-sensitive Ca2+ store, appears to be staurosporine-sensitive. Overall, these studies indicate that staurosporine reduces catecholamine release by interfering with Ca2+ homeostasis. Furthermore, this work suggests that a staurosporine-sensitive phosphoprotein(s) is involved with the regulation of Ca2+ homeostasis in bovine adrenal chromaffin cells.

Transcription of histone H4, H3, and H1 cell cycle genes: promoter factor HiNF-D contains CDC2, cyclin A, and an RB-related protein

Cell cycle-controlled human histone genes are coordinately expressed during S phase, and transcriptional regulation involves a series of trans-acting factors (HiNFs). The proliferation-specific factor HiNF-D interacts with multiple recognition motifs in histone H4, H3, and H1 promoters. Using gel shift immunoassays, we show that CDC2, cyclin A, and an RB-related protein are ubiquitous subunits of HiNF-D binding activity isolated from several cell types. HiNF-D levels in vivo are sensitive to okadaic acid and staurosporine, indicating that HiNF-D activity and/or assembly is influenced by phosphorylation status. Thus, HiNF-D appears to be a multicomponent phosphoprotein that participates in coordinate control of multiple histone H4, H3, and H1 genes during the cell cycle. The presence of cell cycle mediators in the HiNF-D complex suggests linkage between transcriptional control of histones, enzymes involved in DNA synthesis, and the onset of DNA replication during the G1/S phase transition.

Staurosporine induces tyrosine phosphorylation of a 145 kDa protein but does not activate gp140trk in PC12 cells

Staurosporine, a protein kinase C inhibitor, induces neurite outgrowth in PC12 cells similarly to nerve growth factor (NGF). Since NGF neurotropic effects are transduced by the 'trk' gene product 140 kDa tyrosine kinase receptor, gp140trk, we investigated the role of gp140trk and tyrosine phosphorylations in staurosporine neurotropic effects. A direct correlation between staurosporine neurotropic effects and a novel stimulation of tyrosine phosphorylation of a 145 kDa protein (p145) with the following characteristics has been discovered: (1) Staurosporine specifically induced, among indolcarbazoles-K252a derivatives, in a dose-dependent manner (5-100 nM), p145 tyrosine phosphorylation and neurite outgrowth. (2) Staurosporine-induced p145 tyrosine phosphorylation was selective compared to other neurotropic compounds such as 8-Br-cAMP, acidic and basic fibroblast growth factors and NGF. (3) Staurosporine stimulation of p145 tyrosine phosphorylation gradually increased during the first 8 h of staurosporine treatment coinciding with the initiation of neurotropic effects. (4) K252a, a selective inhibitor of NGF actions, and several tyrphostins did not block staurosporine-induced p145 tyrosine phosphorylation and neurotropic effects. (5) Staurosporine stimulation of p145 tyrosine phosphorylation and neurotropic effects are independent of PKC. (6) Staurosporine did not activate gp140trk-NGF receptor in PC12 cells. The present study proposes staurosporine as a pharmacological tool to study the role of tyrosine phosphorylation pathway(s), such as p145 phosphorylation, in the action of neurotropic agents.

Inhibition of cyclin-dependent kinases by purine analogues

While testing purines related to the non-specific protein kinase inhibitors N6-dimethylaminopurine and N6-(delta 2-isopentenyl)adenine as potential inhibitors of the p34cdc2/cyclin B kinase, we discovered a compound with high specificity, 2-(2-hydroxyethylamino)-6- benzylamino-9-methylpurine (olomoucine). Kinetic analysis of kinase inhibition reveals that olomoucine behaves as a competitive inhibitor for ATP and as a non-competitive inhibitor for histone H1 (linear inhibition for both substrates). The kinase specificity of this inhibition was investigated for 35 highly purified kinases (including p34cdk4/cyclin D1, p40cdk6/cyclin D3, cAMP-dependent and cGMP-dependent kinases, eight protein kinase C isoforms, calmodulin-dependent kinase II, myosin light-chain kinase, mitogen-activated S6 kinase, casein kinase 2, double-stranded RNA-activated protein kinase, AMP-stimulated kinase, eight tyrosine kinases). Most kinases are not significantly inhibited. Only the cell-cycle regulating p34cdc2/cyclin B, p33cdk2/cyclin A and p33cdk2/cyclin E kinases, the brain p33cdk5/p35 kinase and the ERK1/MAP-kinase (and its starfish homologue p44mpk) are substantially inhibited by olomoucine (IC50 values are 7, 7, 7, 3 and 25 microM, respectively). The cdk4/cyclin D1 and cdk6/cyclin D3 kinases are not significantly sensitive to olomoucine (IC50 values greater than 1 mM and 150 microM, respectively). N6-(delta 2-Isopentenyl)adenine is confirmed as a general kinase inhibitor with IC50 values of 50-100 microM for many kinases. The purine specificity of cyclin-dependent kinase inhibition was investigated: among 81 purine derivatives tested, only C2, N6 and N9-substituted purines exert a strong inhibitory effect on the p34cdc2/cyclin B kinase. An essentially similar sensitivity to this olomoucine family of compounds was observed for the brain-specific cdk5/p35 kinase. Structure/activity relationship studies allow speculation on the interactions of olomoucine and its analogues with the kinase catalytic subunit. Olomoucine inhibits in vitro M-phase-promoting factor activity in metaphase-arrested Xenopus egg extracts, inhibits in vitro DNA synthesis in Xenopus interphase egg extracts and inhibits the licensing factor, an essential replication factor ensuring that DNA is replicated only once in each cell cycle. Olomoucine inhibits the starfish oocyte G2/M transition in vivo. Through its unique selectivity olomoucine provides an anti-mitotic reagent that may preferentially inhibit certain steps of the cell cycle.

COP-coated vesicles are involved in the mitotic fragmentation of Golgi stacks in a cell-free system

Rat liver Golgi stacks fragmented when incubated with mitotic but not interphase cytosol in a process dependent on time, temperature, energy (added in the form of ATP) and cdc2 kinase. The cross-sectional length of Golgi stacks fell in the presence of mitotic cytosol by approximately 50% over 30 min without a corresponding decrease in the number of cisternae in the stack. The loss of membrane from stacked and single cisternae occurred with a half-time of approximately 20 min, and was matched by the appearance of both small (50-100 nm in diameter) and large (100-200 nm in diameter) vesicular profiles. Small vesicular profiles constituted more than 50% of the total membrane after 60 min of incubation and they were shown to be vesicles or very short tubules by serial sectioning. In the presence of GTP gamma S all of the small vesicles were COP-coated and both the extent and the rate at which they formed were sufficient to account for the production of small vesicles during mitotic incubation. The involvement of the COP-mediated budding mechanism was confirmed by immunodepletion of one of the subunits of COP coats (the coatomer) from mitotic cytosol. Vesicles were no longer formed but highly fenestrated networks appeared, an effect reversed by the readdition of purified coatomer. Together these experiments provide strong support for our hypothesis that the observed vesiculation of the Golgi apparatus during mitosis in animal cells is caused by continued budding of COP-coated transport vesicles but an inhibition of their fusion with their target membranes.

Signalling pathways as targets for anticancer drug development

Intracellular signalling pathways mediating the effects of oncogenes on cell growth and transformation offer novel targets for the development of anticancer drugs. With this approach, it may be sufficient to target a component of the signalling pathway activated by the oncogene rather than the oncogene product itself. In this review, the abilities of some antiproliferative drugs to inhibit signalling targets are considered. There are some anticancer drugs already in clinical trial that may act by inhibiting signalling targets, as well as drugs in preclinical development. Some problems that may be encountered in developing this new class of anticancer drugs are discussed.

Elongation factor-2 kinase: effective inhibition by the novel protein kinase inhibitor rottlerin and relative insensitivity towards staurosporine

The elongation factor-2 (eEF-2) is selectively phosphorylated by the eEF-2 kinase (calmodulin-dependent kinase III). This phosphorylation can be inhibited by calmodulin antagonists, such as CGS 9343B (IC50 = 4 microM). The novel protein kinase inhibitor rottlerin is shown to suppress eEF-2 phosphorylation with an IC50 of 5.3 microM. By contrast, the eEF-2 kinase is rather resistant towards the potent but non-selective protein kinase inhibitor staurosporine (IC50 > 50 microM) and thus can be differentiated from most other protein kinases that are suppressed by staurosporine in the nM range.

Hypoxia-induced accumulation of erythropoietin mRNA in isolated hepatocytes is inhibited by protein kinase C

To define the role of protein kinase C (PKC) in oxygen-dependent production of erythropoietin (EPO) in the liver, we have determined EPO messenger ribonucleic acid (mRNA) expression in primary cultures of juvenile rat hepatocytes incubated at different oxygen tensions in the absence and presence of phorbol esters, vasopressin, and structurally different kinase inhibitors. Upon reduction of oxygen concentrations from 40% to 3% EPO mRNA in cultured hepatocytes increased markedly within 1.25 h, reached maximal values after 2.5 h and remained elevated for up to 72 h. Treatment of hepatocytes during 1.25-5 h of hypoxic exposure with phorbol 12-myristate-13 acetate (PMA) attenuated hypoxia-induced EPO mRNA levels dose-dependently by a maximum of approximately 50%. This inhibitory effect of PMA disappeared upon treatment for more than 5 h and was completely lost after incubation for 9 and 18 h in the presence of 10(-6) M and 10(-7) M PMA, respectively. Phorbol 12,13-dibutyrate and vasopressin also inhibited EPO mRNA accumulation, whereas 4 alpha-phorbol 12,13-didecanoate was ineffective. Western blot analysis of PKC isozymes revealed the presence of PKC alpha, beta II, delta, epsilon and zeta and provided no evidence that the PMA-induced inhibition of EPO expression was associated with depletion of any of these isozymes. Conversely, PMA-induced inhibition of EPO mRNA accumulation was paralleled by translocation of PKC alpha from cytosol to membranes and the time- and dose-dependent attenuation of the inhibitory effect of PMA on EPO mRNA levels was paralleled by down-regulation of PKC alpha. A dose-dependent inhibition of EPO mRNA formation, independent of effects on total RNA synthesis, as determined by [3H]uridine incorporation, was also found in the presence of the kinase inhibitor staurosporine (ED50 approximately 2 x 10(-8) M) and three structurally related derivatives with increased selectivity for PKC (RO 317549, ED50 approximately 1 x 10(-6) M; RO 318220, ED50 approximately 1 x 10(-6) M and CGP 41251, ED50 approximately 4 x 10(-6) M). The markedly lower potency of the latter three compounds as compared to staurosporine suggests that this suppression of EPO gene induction was not mediated by inhibition of PKC. In summary the data indicate that PKC alpha is a negative modulator of EPO gene expression in hepatocytes. A kinase other than PKC, however, appears to be an essential element of hypoxic signalling.

Effect of UCN-01, a selective inhibitor of protein kinase C, on the cell-cycle distribution of human epidermoid carcinoma, A431 cells

UCN-01 (7-hydroxy-staurosporine), a selective inhibitor of protein kinase C (PKC), was shown to exhibit antitumor activity in murine and human tumor cell lines in vitro and in vivo. On the other hand, staurosporine, a non-selective protein kinase inhibitor, was not shown to exert antitumor activity in vivo despite its potent antiproliferative activity in vitro. To compare the modes of action of UCN-01 and staurosporine in vitro, the effects of both drugs on the cell cycle progression of human epidermoid carcinoma A431 cells were examined by flow cytometry using propidium iodide (PI) staining. At 50% growth inhibitory concentrations, both UCN-01 and staurosporine induced G1 phase accumulation in the cell cycle. At 80% growth inhibitory concentrations, UCN-01 also induced preferential G1 phase accumulation, but staurosporine mostly induced G2M phase accumulation. Staurosporine also induced higher DNA ploidy when the cells were exposed to the drug for more than one generation time of A431 cells. An analysis of cell kinetics by 5-bromo-2-deoxyuridine incorporation versus DNA content confirmed that the G1 phase block by UCN-01 and the G1 and G2M phase block by staurosporine at the respective doses, as was the case for PI staining. Additionally, DNA synthesis of the cells, which was determined by the uptake of 3H-TdR, was not suppressed at least 8 h after the treatment with UCN-01. These results suggested that UCN-01 could affect the G1 phase of cell cycle in A431 cells in quite different manners from staurosporine. The G1 phase block induced by UCN-01 might be important for the growth inhibitory activity of UCN-01 against A431 cells in vitro and in vivo.

Protein kinase inhibitor, staurosporine, induces a mature neuronal phenotype in SH-SY5Y human neuroblastoma cells through an alpha-, beta-, and zeta-protein kinase C-independent pathway

Previous studies have shown that the tumour-promoting phorbol ester 12-O-tetradecanoyl phorbol-13 acetate (TPA) induces both morphological and functional differentiation in SH-SY5Y human neuroblastoma cells (Påhlman et al., 1981). In order to investigate the role of protein kinase C (PKC) in TPA-induced maturation of SH-SY5Y cells, we have used staurosporine, which is a potent inhibitor of protein kinases including PKC. Treatment of SH-SY5Y cells with 25 nM staurosporine for 72 hours caused an appearance of long, neuritelike processes with varicosities, terminated by growth cones. The morphological differentiation was accompanied by a cessation of DNA synthesis, induction of growth associated protein 43 (GAP-43), and neuropeptide Y (NPY) mRNA. These effects of staurosporine were comparable to those elicited by TPA. Staurosporine further induced a time-dependent increase in the expression of tyrosine hydroxylase protein and a 30-fold increase in the concentration of noradrenaline. TPA only induced a marginal increase in tyrosine hydroxylase expression. Both TPA and staurosporine induced an appearance of voltage-gated Ca2+ channels in SH-SY5Y cells detected with single-cell fluorescent measurements using fura-2. The Ca2+ channels were found almost exclusively in growth cones and varicosities. Staurosporine inhibited both basal and a TPA-induced phosphorylation of an endogenous 80kDa PKC substrate (p80), and also blocked c-fos proto-oncogene mRNA expression induced by the phorbol ester. Bryostatin 1, a potent activator of PKC, has failed to induce morphological or functional differentiation in SH-SY5Y cells (Jalava et al., 1990). Incubation of SH-SY5Y cells in the presence of 100 nM bryostatin 1 for 24 hours caused a complete disappearance of all immunoreactive alpha-, beta-, and zeta-PKC. The level of epsilon-PKC decreased by 70%. Staurosporine induced a partial translocation of the epsilon-isoenzyme but it failed to cause down-regulation of epsilon-PKC. Bryostatin 1-treatment did not interfere in the ability of staurosporine to induce morphological differentiation, cessation of DNA synthesis, and GAP-43 and NPY mRNA expression. The ability of staurosporine to stimulate tyrosine hydroxylase expression and to increase cellular content of noradrenaline was also unaffected. Taken together the results of this study show that staurosporine induces a mature neuronal noradrenergic phenotype in SH-SY5Y cells through an alpha-, beta-, and zeta-PKC-independent pathway.

Multiple kinase arrest points in the G1 phase of nontransformed mammalian cells are absent in transformed cells

We have shown that nontransformed mammalian cells arrest early in the G1 phase of the cell cycle when treated with exceedingly low concentrations of the nonspecific kinase inhibitor staurosporine, whereas transformed cells continue to progress through the cell cycle. We have now treated normal or transformed human skin fibroblasts with four other kinase inhibitors. Three of these inhibitors are highly specific: KT5720 inhibits cAMP-dependent protein kinase, KT5823 inhibits cGMP-dependent protein kinase, and KT5926 inhibits myosin light-chain kinase. The fourth inhibitor K252b has a moderate specificity for protein kinase C but also inhibits the three kinases just mentioned. We have found that these inhibitors reversibly arrest normal human skin fibroblasts at different times in the G1 phase but do not affect the cell cycle progression of transformed cells. The times of arrest within the G1 phase can be divided into two categories. Two of the inhibitors, KT5926 and K252b, act at an early time that is approximately 4 h after the transition from G0 to G1. The cAMP- and cGMP-dependent protein kinase inhibitors KT5720 and KT5823 arrest cells at a later time that is approximately 6 h after the G0/G1 boundary. These data indicate that there are multiple kinase-mediated phosphorylations of different substrates that are essential for the progression of normal cells, but not transformed cells, through the G1 phase. These inhibitors provide us with a set of biochemical probes that should be invaluable in the study of the function of kinases during G1 phase progression of normal cells.