Concentration-dependent positive and negative regulation of a MAP kinase by a MAP kinase kinase

There are at least three distinct MAP kinase signaling modules in mammalian cells, distinguished by the family of kinases (Erk, SAPK/JNK, or p38) that is ultimately activated. Many input signals activate multiple MAP kinase cascades, and the mechanisms that control the specificity of signal output are not well understood. We show that SEK1/MKK4, a MAP kinase kinase proposed to activate SAPK/JNK, is a very potent inhibitor of p54 SAPK beta/JNK3 both in vitro and in vivo if present at equimolar or higher ratios. In contrast SEK can activate SAPK when present in substoichiometric amounts, but this activation is slow, consistent with the rate-limiting step in activation being the dissociation of an inactive SEK:SAPK complex. The N-terminal unique region of SEK is both necessary and partially sufficient for inhibition of SAPK, and is also necessary for activation of SAPK by SEK in vitro. We have also used the p38 MAP kinase and its activator MKK6 to examine the regulatory relationships among different kinases involved in stress responses. We show using purified kinases that inhibitory activity is specific for the combination of SEK and SAPK: SEK can activate but not inhibit p38, and MKK6 can activate but not inhibit SAPK beta and p38. These results reveal a potential mechanism for regulating stress-activated kinases, adding to a growing body of evidence suggesting that MAP kinases are controlled by relatively stable interactions with their activators.     

丝裂原活化蛋白激酶激酶4与肿瘤转移

丝裂原活化蛋白激酶激酶4(MKK4)是丝裂原活化蛋白激酶(MAPK)信号转导通路组成的成员之一.研究发现,MKK4与肿瘤的发生和转移密切相关,作用机制复杂,能抑制肿瘤发生,近5%不同组织来源的肿瘤均有MKK4基因的功能突变缺失.MKK4基因也是一个转移抑制基因,能抑制多种肿瘤的转移.     

Role of mitogen-activated protein kinase kinase 4 in cancer

Mitogen-activated protein (MAP) kinase kinase 4 (MKK4) is a component of stress activated MAP kinase signaling modules. It directly phosphorylates and activates the c-Jun N-terminal kinase (JNK) and p38 families of MAP kinases in response to environmental stress, pro-inflammatory cytokines and developmental cues. MKK4 is ubiquitously expressed and the targeted deletion of the Mkk4 gene in mice results in early embryonic lethality. Further studies in mice have indicated a role for MKK4 in liver formation, the immune system and cardiac hypertrophy. In humans, it is reported that loss of function mutations in the MKK4 gene are found in approximately 5% of tumors from a variety of tissues, suggesting it may have a tumor suppression function. Furthermore, MKK4 has been identified as a suppressor of metastasis of prostate and ovarian cancers. However, the role of MKK4 in cancer development appears complex as other studies support a pro-oncogenic role for MKK4 and JNK. Here we review the biochemical and functional properties of MKK4 and discuss the likely mechanisms by which it may regulate the steps leading to the formation of cancers.     

Redundant kinase activation and resistance of EGFR-tyrosine kinase inhibitors

Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) have shown dramatic effects against that tumors harboring EGFR activating mutations in the EGFR intracytoplasmic tyrosine kinase domain and resulted in cell apoptosis. Unfortunately, a number of patients ultimately developed resistance by multiple mechanisms. Thus, elucidation of the mechanism of resistance to EGFR-TKIs can provide strategies for blocking or reversing the situation. Recent studies suggested that redundant kinase activation plays pivotal roles in escaping from the effects of EGFR-TKIs. Herein, we aimed to characterize several molecular events involved in the resistance to EGFR-TKIs mediated by redundant kinase activation.     

Role of mitogen-activated protein kinase kinase kinases in signal integration

Mitogen-activated protein kinases (MAPKs) are members of a dynamic protein kinase network through which diverse stimuli regulate the spatio-temporal activities of complex biological systems. MAPKs regulate critical cellular functions required for homeostasis such as the expression of cytokines and proteases, cell cycle progression, cell adherence, motility and metabolism. MAPKs therefore influence cell proliferation, differentiation, survival, apoptosis and development. In vertebrates, five MAPK families are regulated by MAPK kinase kinase-MAPK kinase-MAPK (MKKK-MKK-MAPK) phosphorelay systems. There are at least 20 MKKKs that selectively phosphorylate and activate different combinations of the seven MKKs, resulting in a specific activation profile of members within the five MAPK families. MKKKs are differentially activated by upstream stimuli including cytokines, antigens, toxins and stress insults providing a mechanism to integrate the activation of different MAPKs with the cellular response to each stimulus. Thus, MKKKs can be considered as 'signaling hubs' that regulate the specificity of MAPK activation. In this review, we describe how the MKKK 'hub' function regulates the specificity of MAPK activation, highlighting MKKKs as targets for therapeutic intervention in cancer and other diseases.     

Xenopus Polo-like kinase Plx1: a multifunctional mitotic kinase

The Xenopus Polo-like kinase Plx1 plays multiple roles in mitosis. Accumulating evidence shows that Plx1 is the trigger kinase for the G2/M transition that phosphorylates and activates the phosphatase Cdc25C, which subsequently dephosphorylates Cdc2/cyclin B and initiates a positive feedback loop between Cdc25C and Cdc2/cyclin B. Recent findings indicate that Plx1 itself is also in a positive feedback loop. It phosphorylates and activates the protein kinase xPlkk1, which itself then phosphorylates and further activates Plx1. Plx1 functions on the centrosome to promote bipolar spindle formation. Plx1 associates with the anaphase-promoting complex/cyclosome (APC/C) and is required to activate the APC/C for degradation of mitotic regulators required for sister chromatid separation and exit from mitosis. Plx1 is also required for cytokinesis and is localized on the midbody of the contractile ring. All known functions of Plx1 require not only its kinase activity but also an intact polo box domain in the C-terminus.     

Tyrosine kinase inhibitors

Membrane receptors with tyrosine kinase activity and cytoplasmic tyrosine kinases have emerged as important potential targets in oncology. Starting from basic structures such as anilino-quinazoline, numerous compounds have been synthesised, with the help of tyrosine kinase crystallography, which has allowed to optimise protein-ligand interactions. The catalytic domains of all kinases present similar three-dimensional structures, which explains that it may be difficult to identify molecules having a high specificity for a given tyrosine kinase. Some tyrosine kinase inhibitors are relatively specific for epidermal growth factor receptor (EGFR) such as géfitinib and erlotinib; other are mainly active against platelet-derived growth factor receptor (PDGFR) and the receptor KIT, such as imatinib or nilotinib, and other against vascular endothelial growth factor (VEGF) receptors involved in angiogenesis, such as sunitinib and sorafenib. The oral formulation of tyrosine kinase inhibitors is well accepted by the patients but may generate sometimes compliance problems requiring pharmacokinetic monitoring. This chemical family is in full expansion and several dozens of compounds have entered clinical trials.     

The amino acid sequence of a mammalian MAP kinase kinase.

The amino acid sequence of the dual specificity mitogen-activated protein kinase kinase (MAPKK) has been determined by cDNA cloning and amino acid sequencing. MAPKK (393 residues, Mr 43,330) is a new member of the protein kinase subclass that comprises byr1 and STE7 that are involved in pheromone dependent signal transduction in yeast, wis1 a mitotic regulator in S. pombe and PBS2, which confers antibiotic resistance in S. cerevisiae.     

Protein kinase CK2 subunits are positive regulators of AKT kinase

Protein kinase CK2 is a highly conserved serine/threonine kinase ubiquitously distributed in all investigated eukaryotes and involved in several intracellular pathways which control, among others, cell cycle, proliferation, apoptosis and transformation. Similarly, the PI3K-AKT pathway deregulation has been shown to play an important role in tumor development and tumor response to anti-cancer treatment. In this study, it is shown that the specific inhibition of the PI3K-AKT signaling pathway in combination with depletion of CK2 subunits by antisense oligodeoxynucleotides leads to an enhanced drug-induced apoptotic response. Moreover, in vitro as well as in vivo studies show that the individual CK2 subunits interact with AKT kinase and that the complex formation is not modulated by the phosphorylation status of AKT. Furthermore, the fact that the interaction between CK2 subunits and AKT enhances AKT kinase activity identifies a novel molecular mechanism that leads to modulation of AKT activation raising the possibility that CK2 and AKT might be implicated in common pathways that control cell proliferation and survival.     

Targeting mitogen-activated protein kinase kinase (MEK) in solid tumors

The Raf-mitogen activated protein kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) protein kinase signaling cascade is an important intracellular pathway whose activation influences many fundamental cellular processes and whose aberrancy is associated with cancer cell growth. In addition to activation from within by, for example, Raf mutations, this pathway is frequently activated from above by mutated Ras or epidermal growth factor receptor (EGFR). Given the near ubiquity of derangements affecting at least part of this network in cancer, there is a strong and clear rationale for interrupting it. In recent times, in colorectal and lung cancer, Ras and EGFR mutant status have been shown to be critically important and mutually exclusive predictors of response to anti-EGFR therapies. These developments underline the importance of targeting downstream effectors, and MEK inhibition has been the subject of intense scientific and clinical research for some time now. This article reviews the current status of MEK inhibitors with regard to their clinical development.     

CaM kinase kinase beta-mediated activation of the growth regulatory kinase AMPK is required for androgen-dependent migration of prostate cancer cells

While patients with advanced prostate cancer initially respond favorably to androgen ablation therapy, most experience a relapse of the disease within 1-2 years. Although hormone-refractory disease is unresponsive to androgen-deprivation, androgen receptor (AR)-regulated signaling pathways remain active and are necessary for cancer progression. Thus, both AR itself and the processes downstream of the receptor remain viable targets for therapeutic intervention. Microarray analysis of multiple clinical cohorts showed that the serine/threonine kinase Ca2+/calmodulin-dependent protein kinase kinase β (CaMKKβ) is both highly expressed in the prostate and further elevated in prostate cancers. Using cellular models of prostate cancer, we have determined that androgens (a) directly increase the expression of a CaMKKβ splice variant and (b) increase functional CaMKKβ protein levels as determined by the phosphorylation of both CaMKI and AMP-activated protein kinase (AMPK), two of CaMKKβ's primary substrates. Importantly, inhibition of the CaMKKβ-AMPK, but not CaMKI, signaling axis in prostate cancer cells by pharmacological inhibitors or siRNA-mediated knockdown blocks androgen-mediated migration and invasion. Conversely, overexpression of CaMKKβ alone leads to both increased AMPK phosphorylation and cell migration. Given the key roles of CaMKKβ and AMPK in the biology of prostate cancer cells, we propose that these enzymes are potential therapeutic targets in prostate cancer.     

Association between mitogen-activated protein kinase kinase kinase 1 rs889312 polymorphism and breast cancer risk: evidence from 59,977 subjects

Published data on the association between mitogen-activated protein kinase kinase kinase 1 (MAP3K1) gene rs889312 polymorphism and breast cancer risk are inconclusive. To derive a more precise estimation of the relationship, a meta-analysis was performed. Crude ORs with 95% CIs were used to assess the strength of association between them. A total of seven eligible articles including 26,015 cases and 33,962 controls based on the search criteria were involved in this meta-analysis. We observed that the MAP3K1 rs889312 polymorphism was significantly correlated with breast cancer risk from the fixed effects model when all studies were pooled into the meta-analysis (the allele contrast model: OR 1.09, 95% CI 1.07–1.12; the homozygote codominant: OR 1.22, 95% CI 1.15–1.29; the heterozygote codominant: OR 1.07, 95% CI 1.04–1.11; the dominant model: OR 1.10, 95% CI 1.06–1.13; the recessive model: OR 1.18, 95% CI 1.12–1.25). No significant association was found in the BRCA1 mutation carriers in all genetic models. When stratified by BRCA2 mutation carriers status, statistically significantly elevated risk was found in this meta-analysis (C vs. A: OR 1.12, 95% CI 1.01–1.23; CC vs. AA: OR 1.35, 95% CI 1.06–1.71; the recessive model: OR 1.31, 95% CI 1.05–1.65). There was no evidence for significant association between MAP3K1 rs889312 polymorphism and breast cancer risk in BRCA1 and BRCA2 positive cohort for all comparison models. In conclusion, this meta-analysis suggests that the MAP3K1 rs889312 C allele is a low-penetrant risk factor for developing breast cancer, and there is limited evidence to indicate that MAP3K1 rs889312 polymorphism is associated with increased risk of breast cancer in BRCA1 mutation carriers.     

Uridine kinase, adenylate kinase, and guanase in human lung tumors

In pulmonary neoplasms, the uridine kinase concentration was higher (2- to 20-fold) than in the noninvolved lung portions of each of the 12 subjects studied. The extent of elevation of uridine kinase in the different tumors showed a significant positive correlation with the rises (1.5- to 30-fold) in thymidine kinase, suggesting that neoplastic transformation in human lung involved coordinated increases in the capacity for the reutilization of different nucleoside phosphates. Adenylate kinase was always at lower levels in neoplasms compared to noninvolved areas of the same lung, and the extent of this loss in the different tumors correlated inversely with the gain in uridine kinase and thymidine kinase. Normal fetal human lung was also deficient in adenylate kinase, while its uridine kinase and thymidine kinase (and also guanase) activities were above the adult levels. The guanase activities of the different neoplasms, unrelated to their uridine kinase or thymidine kinase content, correlated with the activities in the subjects' noninvolved lung. These individual differences were much more striking than those between the neoplastic and control samples. Variations in guanase activity thus appear to be "random," whereas observations on the three other enzymes attest to the orderly nature of biochemical differences among individual tumors and between normal and neoplastic lung.     

Arabinosylguanine is phosphorylated by both cytoplasmic deoxycytidine kinase and mitochondrial deoxyguanosine kinase

The prodrug of 9-beta-D-arabinosylguanine (ara-G), nelarabine, demonstrated efficacy against T-cell acute lymphoblastic leukemia, and its effectiveness correlated with the accumulation of the triphosphate form (ara-GTP). Although in vitro investigations using purified deoxycytidine kinase (dCK) or deoxyguanosine kinase (dGK) suggested that ara-G is a substrate for both enzymes, controversy exists in regard to the role of these enzymes in whole cells. In this work, we used a CEM mutant cell line containing low endogenous levels of dGK and deficient in dCK (dCK-) to assess the role of these kinases in ara-G phosphorylation. Using a retroviral vector system, we infected the dCK- mutant cell line to obtain cell lines with overexpression of dCK (dCK+) or dGK (dGK+). Only the dCK+ cell line phosphorylated 1-beta-D-arabinofuranosylcytosine (used as a substrate for dCK) in a cell-free system; phosphorylation of this compound by dGK+ was below the limit of detection. Again in in vitro assays, the dCK-and dCK+ cell lines phosphorylated dGuo to similar levels (0.91 +/- 0.15 and 0.93 +/- 0.19 pmol/mg/min, respectively), whereas dGK+ phosphorylated dGuo more efficiently (150 pmol at 60 min). When ara-G was used as a substrate in a cell-free system, the maximum accumulation of phosphorylated product was observed in dGK+ extracts at low ara-G levels (10 microM) and in dCK+ extracts at high concentrations of ara-G (100 microM). Thus, both dCK and dGK can phosphorylate ara-G, but at low ara-G concentrations, dGK seems to predominate, whereas at higher ara-G concentrations, dCK seems to be the preferred enzyme. In whole-cell systems after a 3-h incubation with 10 microM ara-G, both dCK+ and dGK+ cells accumulated ara-GTP; however, the levels were significantly (P = 0.0008) higher in dGK+ cells. In contrast, at 100 microM ara-G, intracellular ara-GTP accumulated to similar levels (P = 0.5529) in these cell types; 25 +/- 3.7 microM in dCK+, and 27.8 +/- 2.7 microM in the dGK+ cells. These results from whole-cell experiments are consistent with those from the cell-free system and strongly suggest that ara-G is phosphorylated by both kinases, and at low substrate concentrations, dGK is preferred enzyme. Evaluation of the expression of each of these kinases in primary leukemia cells may reveal a biochemical basis for the pharmacological differences in the accumulation of ara-GTP.     

Protein kinase C inhibitors

Protein kinase C (PKC) is a family of serine-threonine protein kinases that are involved in signal transduction pathways that regulate growth factor response, proliferation, and apoptosis. Its central role in these processes, which are closely involved in tumor initiation, progression, and response to antitumor agents, makes it an attractive therapeutic target in cancer. Despite initial activity seen in melanoma (bryostatin and UCN-01), non-Hodgkin’s lymphoma (ISIS 3521, bryostatin, and UCN-01), and ovarian carcinoma (ISIS 3521 and bryostatin) in phase I studies, single-agent activity in those phase II studies reported to date has been limited. Preclinical data highlight a role for PKC in modulation of drug resistance and synergy with conventional cytotoxic drugs. A randomized phase III study of ISIS 3521 in combination with carboplatin and paclitaxel, compared with chemotherapy alone, in advanced non-small-cell lung cancer is underway. This paper reviews the rationale for using PKC inhibitors in cancer therapy, the challenges for clinical trial design, and the recent clinical experience with modulators of PKC activity.     

Tyrosine kinase inhibitor--hematological malignancies

STI571 selectively inhibits the ABL-tyrosine kinase, the activity of which is activated by the formation of chimeric BCR/ABL. A phase I study in the USA showed STI571 to be remarkably effective in cases of interferon-refractory chronic myeloid leukemia, with almost no adverse effects. STI571 may become the first choice drug prior to stem cell transplantation and interferon treatment.