The role of Plk3 in oncogenesis

The polo-like kinases (Plks) encompass a family of five serine/threonine protein kinases that play essential roles in many cellular processes involved in the control of the cell cycle, including entry into mitosis, DNA replication and the response to different types of stress. Plk1, which has been validated as a cancer target, came into the focus of many pharmaceutical companies for the development of small-molecule inhibitors as anticancer agents. Recently, FDA (Food and Drug Administration) has granted a breakthrough therapy designation to the Plk inhibitor BI 6727 (volasertib), which provided a survival benefit for patients suffering from acute myeloid leukemia. However, the various ATP-competitive inhibitors of Plk1 that are currently in clinical development also inhibit the activities of Plk2 and Plk3, which are considered as tumor suppressors. Plk3 contributes to the control and progression of the cell cycle while acting as a mediator of apoptosis and various types of cellular stress. The aberrant expression of Plk3 was found in different types of tumors. Recent progress has improved our understanding of Plk3 in regulating stress signaling and tumorigenesis. When using ATP-competitive Plk1 inhibitors, the biological roles of Plk1-related family members like Plk3 in cancer cells need to be considered carefully to improve treatment strategies against cancer.

Stamping out RAF and MEK1/2 to inhibit the ERK1/2 pathway: an emerging threat to anticancer therapy

The RAS-RAF-MEK1/2-ERK1/2 pathway is a key signal transduction pathway in the cells. Critically, it remains constitutively active in approximately 30% of human cancers, having key roles in cancer development, maintenance and progression, while being responsible for poorer prognosis and drug resistance. Consequently, the inhibition of this pathway has been the subject of intense research for >25 years. The advent of better patient screening techniques has increasingly shown that upstream regulators like RAS and RAF remain persistently mutated in many cancer types. These gain-of-function mutations, such as KRAS-4B(G12V/G13D/Q61K), NRAS(Q61L/Q61R) or BRAF(V600E), lead to tremendous increase in their activities, resulting in constitutively active extracellular signal-regulated kinase 1/2 (ERK1/2). They were not efficiently targeted by the first-generation inhibitors such as Lonafarnib or Sorafenib, which were essentially broad spectrum inhibitors targeting pan-RAS and pan-RAF, respectively. This triggered the development of the second-generation inhibitors selective against the mutated proteins. Second generation inhibitors such as Vemurafenib (Zelboraf) and Dabrafenib (Tafinlar) targeting BRAF(V600E), Trametinib (Mekinist) targeting MEK1/2 and the first generation pan-RAF inhibitor Sorafenib (Nexavar) have already been approved for treating renal, hepatocellular, thyroid cancers and BRAF(V600E/K) harboring metastatic melanoma. Others against RAF and MEK1/2 are presently undergoing clinical trials. Their success would depend on the better understanding of the acquired resistance mechanisms to these drugs in the cancer cells and the identification of predictive biomarkers for the proper administration of suitable inhibitor(s).

Human and Xenopus mo15 messenger-RNA are highly conserved but show different patterns of expression in adult tissues

Phosphorylation of human p34(cdc2) at Thr 161 seems to be necessary for its catalytic activity. CAK (cdk activating kinase) containing p40(MO15) from Xenopus egg extracts phosphorylates and activates p34(cdc2) in a cyclin dependent manner at Thr 161. We describe the cDNA sequence coding for human MO15, which predicts a serine/threonine kinase of 346 aa. Despite the high homology of 91% between the human and Xenopus proteins we observed a rather different mRNA distribution in adult tissues: In contrast to ubiquitously expressed human MO15-transcripts MO15-mRNA expression in Xenopus is restricted to oocytes indicating a different cellular role in these two phylogenetically distant species. By virtue of the homology to members of the family of cell cycle kinase genes we examined MO15 mRNA expression for its correlation to the proliferative activity of cells. Stimulation of lymphocytes showed MO15 mRNA expression to be independent of mitotic activity.

Human SAK related to the PLK/polo family of cell cycle kinases shows high mRNA expression in testis

We identified the nucleotide sequence of a cDNA encoding a polypeptide with a kinase domain that is related to the catalytic region of Drosophila melanogaster polo, Saccharomyces cerevisiae CDC5 as well as human FNK and PLK. The novel gene seems to represent the human counterpart of the mouse gene sak. The sequence of SAK predicts a serine/threonine kinase of 970 aa. The distribution of SAK mRNA in adult organs is restricted to certain tissues such as testis and thymus. Northern analyses of tumor tissues (lung, breast, brain) and corresponding normal tissues from the same patient did not reveal SAK expression. Comparing the mRNA distribution of the proliferation-associated polo-like kinase (PLK) with the expression of SAK we observed distinct differences. Thus, we suggest that these kinases have unique physiological roles in different cells or in response to different signals.

Pre-treatment haemoglobin level predicts response and survival after TPF induction polychemotherapy in advanced head and neck cancer patients

OBJECTIVE

To investigate the prognostic value of the pre-treatment haemoglobin level in patients with advanced squamous cell head and neck cancer treated with induction polychemotherapy.

DESIGN

Seventy-two patients with advanced squamous cell head and neck cancer received primary combination chemotherapy consisting of docetaxel 75 mg/m(2) on day 1, cisplatin 100 mg/m(2) on day 1, and 5-fluorouracil (5-FU) 1000 mg/m(2)/day on days 1-4 (total dose 4000 mg/m(2)), repeated on days 1, 22 and 43 followed by chemoradiation. The data collected included pre-treatment haemoglobin, response to treatment, disease-free and overall survival.

RESULTS

The pre-treatment haemoglobin level was found to be a significant predictor of response to induction chemotherapy (P = 0.01) and an independent predictor of overall survival [hazard ratio (HR) 0.77, 95% confidence interval (CI) 0.58-1.03, P = 0.0001] and disease free survival (HR 2.09, 95% CI 1.41-3.09, P = 0.0001). Furthermore N-stage was found to be a significant prognostic factor of overall survival (HR 9.24, 95% CI 6.90-21.34, P = 0.005). The Eastern Cooperative Oncology Group performance status scale was also found to be significant for disease free survival (HR 7.66, 95% CI 2.61-22.46, P = 0.003).

CONCLUSION

In patients with advanced squamous cell head and neck cancer, the haemoglobin level prior to induction chemotherapy is significantly related to outcome including response and survival.

Combinatorial application of nucleic acid-based agents targeting protein kinases for cancer treatment

The progress made in cancer biology, genetics and biotechnology has led to a major transition in cancer drug design and development, from an emphasis on non-specific, cytotoxic agents to specific, molecular-targeted smart cancer drugs. Many of these targeted agents have shown to have improved selectivity for cancer versus normal cells and are associated with better anti-tumor efficacy and lower toxicity. The new generation of anti-cancer drugs requires low concentrations and minimizes unwanted side effects. Their use leads to enhanced anti-cancer effects and to a reduction of chemotherapy resistance. Still, resistance to common chemotherapeutic agents is a major obstacle in cancer treatment. Silencing of cancer-relevant genes is a challenging strategy to reduce resistance and to sensitize cancer cells towards anti-neoplastic agents. Resistance can be an intrinsic problem of the tumor or can be acquired during the life time of the tumor. A fascinating species of anti-cancer drugs include antisense oligonucleotides (ASOs) or small interfering RNAs (siRNAs) which are able to specifically down-regulate the expression of the target genes. The combination of nucleic acid-based agents with anti-neoplastic drugs can induce synergistic induction of cell cycle arrest, apoptosis and reduced cell proliferation in vitro or tumor growth in vivo. These two strategies (ASOs and siRNAs) will help to improve current therapeutic regimens. In addition, the combination of targeted drugs with common chemotherapeutic agents might be able to make resistant cells again sensitive towards a chemotherapeutic agent.

Rational combinations of siRNAs targeting Plk1 with breast cancer drugs

Commonly used drugs for the treatment of breast cancer patients like paclitaxel and Herceptin often show severe side effects or induce resistance in clinical settings. Thus, we analysed a combination of Plk1 (polo-like kinase 1)-specific small interfering RNAs (siRNAs), a powerful tool to induce 'mitotic catastrophe' in cancer cells, together with these drugs to identify conditions for enhanced drug sensitivity. After transfection, the antineoplastic agents were added and cell proliferation, apoptosis and cell cycle distribution in breast cancer cells (MCF-7, SK-BR-3, MDA-MB-435 and BT-474) and in primary human mammary epithelial cells were determined. Downregulation of cellular Plk1 levels led to an elevated percentage of cells in G(2)/M phase. The percentage of apoptotic nuclei in MCF-7, MDA-MB-435, SK-BR-3 and BT-474 cells was clearly increased after incubation with Plk1-specific siRNAs and paclitaxel. Interestingly, the caspase pathway was activated after treatment with Plk1-specific siRNAs and paclitaxel or Herceptin. Treatment of breast cancer cells with siRNAs targeting Plk1 improved the sensitivity toward paclitaxel and Herceptin in a synergistic manner. In all experiments, very low concentrations across a wide range of clinically relevant concentrations were sufficient to induce an antiproliferative effect. The combination of Plk1-specific siRNAs with modern breast cancer drugs seems to represent rational combinations to be tested in preclinical trials.

Effect of enzastaurin on cell signaling, proliferation and apoptosis in breast cancer cell lines

14121

Background: Enzastaurin, an acyclic bisindolylmaleimide, is a potent selective serine/threonine kinase inhibitor that inhibits PKCβ, targets the PI3K/AKT pathway, and inhibits GSK3β phosphorylation. Enzastaurin induced apoptosis and decreased proliferation of various cancer lines, and decreased VEGF expression and microvessel density in human tumor xenografts. In animal models, enzastaurin had antitumor/antiangiogenic activity in non-small-cell lung, colon, renal cell, hepatocellular, and other cancers. Therefore, we sought to determine enzastaurin’s impact on cellular PKCβ-mediated signaling in breast cancer cells. Secondarily, we sought to determine the induction of the apoptotic cascade by enzastaurin. Methods: Breast cancer cell lines MCF-7, BT-474, MDA-MB-435 and SK-BR-3 were treated with differing enzastaurin concentrations. Western-Blot analyses were performed to examine PKCβ, phospho-GSK3β and caspase 9 expressions. The phenotype and proliferation of enzastaurin-treated cells were also monitored by fluorescence microscopy. Results: Treating all 4 cancer cell lines with ascending enzastaurin doses (0.1–10 μM) led to a significant downregulation of GSK3β phosphorylation (2–17%) compared to control cells. A 48–72 hr incubation with increasing enzastaurin doses also reduced the PKCβ expression significantly (5–50%). Moreover, a dose- dependent reduction of cell proliferation to levels of 15–40% compared to control cells with the highest enzastaurin concentration was detectable. We also saw a marked pro-caspase 9 reduction (0–30%) after enzastaurin compared to control cells. The microscopic inspection of treated cells phenotypically confirmed increasing apoptosis-induced cell death. Conclusions: Enzastaurin has a significant antiproliferative effect in different breast cancer cells. Moreover, enzastaurin suppresses GSK3β phosphorylation, suggesting that it may be a reliable pharmacodynamic marker for enzastaurin activity in breast cancer cells; however, more preclinical analysis is needed. Our study provides evidence for enzastaurin’s potential to directly suppress breast cancer cell proliferation and to induce tumor cell death by apoptotic induction.

No significant financial relationships to disclose.

Expression and function of the four and a half LIM-only protein 2 (FHL2) in breast cancer

10109

Background: FHL2 belongs to the expanding family of proteins that possess highly conserved double Zn-finger motifs, CX2CX16–23HX2CX2CX2CX16–21CX2 (C/H/D), and are involved in protein-protein interactions and transcriptional regulation. FHL2 was primarily described as “down regulated in rhabdomyosarcama cells LIM domain protein” (DRAL). Further studies also indicated a connection of FHL2 with the tumor biology. Its expression has been shown to be increased in tumors of epithelial and melanoma cell origin. A recent publication describes the interaction of FHL2 with the breast cancer susceptibility gene 1 (BRCA1) but to date little is known about the function of FHL2 in breast cancer. Methods: Tumor samples of 61 breast cancer patients and 8 normal breast tissues were lysed and subjected to Western blot analysis using a monoclonal FHL2-specific antibody; Tissue specimens of normal breast and breast cancer were histologically examined with the monoclonal antibody used for the Western blot analysis; Luciferase assays were carried out in MCF-7 breast cancer cells, using AP1-dependent reporter plasmids. Anchorage-independent growth was assayed by colony formation in soft agar. Results: FHL2 is overexpressed in almost all human breast cancer tissues tested but not in normal breast tissues and only low levels of FHL2 expression were present in four premalignant ductal carcinoma in situ (DCIS). Co-immunoprecipitation assays and GST-pull down experiments revealed that FHL2 interacts with the proto-oncogene c-jun in breast cancer cells. Reporter gene assays demonstrated that FHL2 represses c-jun-mediated activation of an idealized AP-1-regulated reporter gene in a dose-dependent manner. This repression was also seen on the naturally occurring AP-1-dependent IL2 promoter. The repression domain was identified within the N-terminal half of the FHL2 molecule. Finally, we show that stable expression of FHL2 leads to anchorage-independent growth of breast cancer cells in soft agar. Conclusions: Taken together, our data suggest that by modulating the activity of transcription factors such as c-jun, the expression of FHL2 in breast cancer might contribute to the malignant potential of breast tumors.

No significant financial relationships to disclose.

Downregulation of PLK1 elevates chemosensitivity of various breast cancer cell lines in vitro and in vivo

13169

Background: A central role for polo-like kinases (PLK) in regulating mitosis has been shown in different species. Overexpression of PLK1 is observed in various human tumors, and it is a negative prognostic factor in patients suffering from diverse cancers. In order to reduce side-effects exerted by commonly used anti-neoplastic agents and to enhance chemosensitivity of different breast cancer cell lines, we used phosphorothioate antisense oligonucleotides (ASOs) targeted against PLK1 together with Paclitaxel, Carboplatin and Herceptin. Methods: We used different HER2-positive and -negative breast cancer cell lines (BT-474, MCF-7, MDA-MB-435) to define the role of reduced PLK1 expression for the necessary dose of anti-neoplastic agents. After transfection with PLK1-specific ASOs these agents were added and cell proliferation, cell cycle distribution, and apoptosis were measured. Results: We observed synergistic effects after combination of very low doses of PLK1-specific ASOs with Paclitaxel and Herceptin. Using Carboplatin we could only observe a synergistic effect in MDA-MB-435 cells. Downregulation of PLK1 levels led to an elevated percentage of cells in G2/M. Apoptosis and G2/M arrest were increased after combination of PLK1-specific ASOs with Paclitaxel in MDA-MB-435 cells. In a human Xenograft experiment using MDA-MB-435 cells the combination of PLK1-ASOs with Paclitaxel led to synergistic reduction of tumor growth after three weeks treatment compared to either agent alone. Conclusion: This study suggests that antisense inhibitors against PLK1 at well tolerated doses may be considered as cancer therapeutic agents which elevate chemosensitivity especially against Paclitaxel in very low doses with a significant better outcome than each agent alone.

No significant financial relationships to disclose.

Stable gene silencing of cyclin B1 in tumor cells increases susceptibility to taxol and leads to growth arrest in vivo

Cyclin B1 is the regulatory subunit of cyclin-dependent kinase 1 (Cdk1) and is critical for the initiation of mitosis. Accumulating data indicate that the deregulation of cyclin B1 is tightly linked to neoplastic transformation. To study the phenotype and the potential preclinical relevance, we generated HeLa cell lines stably transfected with the plasmids encompassing short hairpin RNA (shRNA) targeting cyclin B1. We demonstrate that the reduction of cyclin B1 caused inhibition of proliferation by arresting cells in G2 phase and by inducing apoptosis. Cells, entering mitosis, were impaired in chromosome condensation and alignment. Importantly, HeLa cells with reduced cyclin B1 were more susceptible to the treatment of small interfering RNA targeting Polo-like kinase 1 (Plk1) and to the administration of the chemotherapeutic agent taxol. Finally, HeLa cells with reduced cyclin B1 showed inhibited tumor growth in nude mice compared to that of control cells. In summary, our data indicate that cyclin B1 is an essential molecule for tumor cell survival and aggressive proliferation, suggesting that the downregulation of cyclin B1, especially in combination with other molecular targets, might become an interesting strategy for antitumor intervention.

Immunohistochemical evaluation of endothelial nitric oxide synthase expression in primary breast cancer

To evaluate the clinical potential of endothelial nitric oxide synthase (e-NOS) in human breast cancer, we performed immunohistochemical (IHC) staining of paraffin-embedded primary breast cancer tissue of 163 patients for e-NOS using a monoclonal antibody. A correlation was found between e-NOS expression and both the classic prognostic factors and survival rates. Under half the patients were premenopausal (38.5%), 61.5% being postmenopausal. The median tumour size was 2 cm; in 41.7% of the patients there was involvement of the axillary lymph nodes. Most (84.1%) of the tumours were hormone receptor positive. e-NOS staining was positive in 62%, most of the positive tumours having weak (32.5%) or medium (21.5%) staining for e-NOS. The median follow-up time was 42 months, during which 46 (28%) patients had a local recurrence or metastatic disease. A positive correlation of e-NOS with the hormone receptor status was found (P=0.031). However, no impact on survival rates was observed.

Carcinomas unresponsive to either cisplatinum or anti-EGFR therapy can be growth inhibited by combination therapy of both agents

BACKGROUND

Previously we demonstrated that the antitumor efficacy of monoclonal antibodies against the EGFR (epidermal growth factor receptor) of human tumor xenografts mainly depends on the EGFR content of tumors rather than on the tumors' entity. In this study we wanted to elucidate whether the described cumulative effect of cisplatin and Anti-EGFR therapy also depends on the EGFR expression.

MATERIALS AND METHODS

Xenotransplanted carcinomas with different EGFR levels were treated with monoclonal antibodies against the EGFR (EMD 72000 and EMD 55900), cisplatinum and a combination of both.

RESULTS

Each monoclonal antibody alone led to an EGFR-dependent significant tumor growth reduction. Cisplatinum alone had no growth inhibitory effects on tumors with high content in contrast to those with low EGFR content. The combination of antibodies with cisplatinum resulted in an EGFR-independent tumor growth inhibition which was stronger than observed in the case of monotherapy.

DISCUSSION

The obtained results may address upcoming phase I/II trials to use Anti-EGFR/Cisplatinum therapy regardless of the EGFR content of tumors.

Molecular classification of breast cancer patients by gene expression profiling

For many tumors, pathological subclasses exist which have to be further defined by genetic markers to improve therapy and follow-up strategies. In this study, cDNA array analyses of breast cancers have been performed to classify tumors into categories based on expression patterns. Comparing purified normal ductal epithelial cells and corresponding tumour tissues, the expression of only a small fraction of genes was found to be significantly changed. A subset of genes repeatedly found to be differentially expressed in breast cancers was subsequently employed to perform a classification of 82 normal and malignant breast specimens by cluster analysis. This analysis identifies a subgroup of transcriptionally related tumours, designated class A, which can be further subdivided into A1 and A2. Correlation with classical clinicopathological parameters revealed that subgroup A1 was characterized by a high number of node-positive tumours (14 of 16). In this subgroup there was a disproportionate number of patients who had already developed distant metastases at the time of diagnosis (25% in this subgroup, compared with 5% among the rest of the samples). Taken together, the use of these differentially expressed marker genes in conjunction with sample clustering algorithms provides a novel molecular classification of breast cancer specimens, which facilitates the identification of patients with a higher risk of recurrence.

Adhesion induced expression of the serine/threonine kinase Fnk in human macrophages

Members of the polo subfamily of protein kinases play crucial roles in cell proliferation. To study the function of this family in more detail, we isolated the cDNA of human Fnk (FGF-inducible kinase) which codes for a serine/threonine kinase of 646 aa. Despite the homology to the proliferation-associated polo-like kinase (Plk), tissue distribution of Fnk transcripts and expression kinetics differed clearly. In contrast to Plk no correlation between cell proliferation and Fnk gene expression was found. Instead high levels of Fnk mRNA were detectable in blood cells undergoing adhesion. The transition of monocytes from peripheral blood to matrix bound macrophages was accompanied by increasing levels of Fnk with time in culture. Neither treatment of monocytes with inducers of differentiation nor withdrawal of serum did influence Fnk mRNA levels significantly, suggesting that cell attachment triggers the onset of Fnk gene transcription. The idea that Fnk is part of the signalling network controlling cellular adhesion was supported by the analysis of the cytoplasmic distribution of the Fnk protein and the influence of its overexpression on the cellular architecture. Fnk as fusion protein with GFP localized at the cellular membrane in COS cells. Dysregulated Fnk gene expression disrupted the cellular f-actin network and induced a spherical morphology. Furthermore, Fnk binds to the Ca2+/integrin-binding protein Cib in two-hybrid-analyses and co-immunoprecipitation in assays. Moreover, both proteins were shown to co-localize in mammalian cells. The homology of Cib with calmodulin and with calcineurin B suggests that Cib might be a regulatory subunit of polo-like kinases.

Identification and functional characterization of the human and murine fibroblast growth factor receptor 4 promoters

Fibroblast growth factor receptors (FGFRs) play crucial roles in signal transduction of adult tissues and during embryonic development. To study the transcriptional control, we isolated and characterized the promoter of human FGFR4. Two transcription initiation sites were identified. The deletion analysis in different cell types defined a core promoter reaching from -9 to -198, lacking TATA and CCAAT boxes but displaying high GC content (77%) in a stretch of 300 bp upstream of the major mRNA start. This region harbors multiple binding motifs for transcription factors. Moreover, the region between -1085 and -1140 contains a potential repressor element, which downregulates transcriptional activity. To identify conserved regulatory elements, we isolated and analyzed also the murine FGFR4 promoter. Only one transcription start was identified using RNase protection assays. Sequence alignment of human and mouse shows a striking similarity in the core promoter region of both genes, encompassing conserved transcription factor binding sites and a splice acceptor site. Furthermore, the region containing the putative repressor element is also conserved suggesting a functional role for gene expression.

Morphological alteration of X-ray induced partially transformed human cells by transfection with a small c-myc DNA sequence

During attempts to transform a normal human fibroblast strain (GM730) by X-irradiation, we obtained a partially transformed cell strain (GM730pt) which demonstrates several aspects of the transformed phenotype including morphological changes, increased saturation density, growth in soft agar, and focus formation in long-term cultures. When GM730pt cells were transfected with the feline c-myc gene, morphology of the cells changed dramatically following seven days of expression. Transfection of other plasmid DNAs or oncogenes such as pUC8, pSV2neo, src, sis, and H-ras had little or no effects on the phenotype of GM730pt cells. On the other hand, a gel purified, small fragment of c-myc DNA had a complete cell alteration activity. Furthermore, Bal 31 deletion and M13 sequencing experiments showed that the alteration seen in GM730pt cells is delimited to a 24 nucleotide stretch (active myc element) from the second intron of the feline c-myc gene that contains a T-rich sequence.

Polo-like kinase: a novel marker of proliferation: correlation with estrogen-receptor expression in human breast cancer

Previous data have shown that the mRNA-expression of the serin/threonine-kinase polo-like kinase (PLK) is closely correlated with the survival of patients suffering from a subset of malignant tumors. PLK-mRNA and protein-expression are restricted to cells in the cell cycle. PLK-mRNA-transcripts are highly abundant in proliferating cells; no gene expression is found in G0-phase cells. Here we investigated the mRNA- and protein-expression of PLK- and estrogen-receptor (ER) in human breast-carcinoma by northern-blotting, RT-PCR and immunohistochemistry. The expression of MIB-I was determined on serial sections. Analysis of the immunohistochemical data revealed a close correlation between the ER and PLK-expression (r = 0.677; p = 0.001, n = 30). No relationship between the mRNA-expression of ER and PLK was found. Furthermore, no correlation for the protein expression of PLK and MIB-I exists. The influence of estrogen (ES) is known to have proliferative potential. The expression of ER correlates with the ES-plasma-level. In addition, the hormone cycle of premenopausal women undergoes rapid vacillations with varying effects on the proliferating tumor cells, e.g., growth induction. Our results therefore show that ER-expression is not only of therapeutic value for the clinician, but it may also be a tool for determining the tumor proliferation index more precisely by integrating the hormone-mediated proliferation stimulus.

Expression of p16 and lack of pRB in primary small cell lung cancer

The retinoblastoma protein (pRB), p16, and cyclin D1 are major components of the RB pathway, which controls the G1 checkpoint of the cell cycle. Proper regulation of this pathway is crucial for normal cell proliferation. Abnormal forms of these proteins have been found in various types of malignant tumours. In the present report, immunohistochemical techniques were applied to study the expression of pRB, p16, and cyclin D1 in 161 samples of primary small cell lung cancer (SCLC) and 20 samples of non-small cell lung cancer (NSCLC). While pRB and cyclin D1 staining was negative in 161 specimens of SCLC, expression of p16 was observed in 153 samples. In contrast to SCLC, 16 out of 20 NSCLC cases exhibited pRB expression and 15 showed cyclin D1 expression, but only very weak p16 staining was found in five samples. These observations could provide additional criteria for the distinction between SCLC and NSCLC. Furthermore, these findings, based on primary tissues, implicate different mechanisms in the tumourigenesis of SCLC and NSCLC.

The polo-like protein kinases Fnk and Snk associate with a Ca(2+)- and integrin-binding protein and are regulated dynamically with synaptic plasticity

In order to stabilize changes in synaptic strength, neurons activate a program of gene expression that results in alterations of their molecular composition and structure. Here we demonstrate that Fnk and Snk, two members of the polo family of cell cycle associated kinases, are co-opted by the brain to serve in this program. Stimuli that produce synaptic plasticity, including those that evoke long-term potentiation (LTP), dramatically increase levels of both kinase mRNAs. Induced Fnk and Snk proteins are targeted to the dendrites of activated neurons, suggesting that they mediate phosphorylation of proteins in this compartment. Moreover, a conserved C-terminal domain in these kinases is shown to interact specifically with Cib, a Ca(2+)- and integrin-binding protein. Together, these studies suggest a novel signal transduction mechanism in the stabilization of long-term synaptic plasticity.

Prognostic significance of polo-like kinase (PLK) expression in squamous cell carcinomas of the head and neck

Previously, we demonstrated that the mammalian polo-like kinase (PLK), which participates in the regulation of the cell cycle, is a novel marker of cellular proliferation. Because current prognostic tools for the evaluation of patients with head and neck squamous cell cancer (HNSCC) need to be improved, we analyzed 89 patients and found elevated PLK expression in most tumors. Nodal stage as a crucial prognostic factor in HNSCC also correlated to PLK transcript levels (P = 0.0043). A Kaplan-Meier analysis showed that HNSCC patients with moderate versus high PLK expression survived significantly longer (5-year survival rates, 43% versus 12%; P = 0.0047). Interestingly, a combination of nodal stage and PLK expression contributed to discriminate patients with a better prognosis in the pN(0/1) and pN(2/3) groups, which could improve the definition of a suitable therapy.

PDZ-domain-mediated interaction of the Eph-related receptor tyrosine kinase EphB3 and the ras-binding protein AF6 depends on the kinase activity of the receptor

Eph-related receptor tyrosine kinases (RTKs) have been implicated in intercellular communication during embryonic development. To elucidate their signal transduction pathways, we applied the yeast two-hybrid system. We could demonstrate that the carboxyl termini of the Eph-related RTKs EphA7, EphB2, EphB3, EphB5, and EphB6 interact with the PDZ domain of the ras-binding protein AF6. A mutational analysis revealed that six C-terminal residues of the receptors are involved in binding to the PDZ domain of AF6 in a sequence-specific fashion. Moreover, this PDZ domain also interacts with C-terminal sequences derived from other transmembrane receptors such as neurexins and the Notch ligand Jagged. In contrast to the association of EphB3 to the PDZ domain of AF6, the interaction with full-length AF6 clearly depends on the kinase activity of EphB3, suggesting a regulated mechanism for the PDZ-domain-mediated interaction. These data gave rise to the idea that the binding of AF6 to EphB3 occurs in a cooperative fashion because of synergistic effects involving different epitopes of both proteins. Moreover, in NIH 3T3 and NG108 cells endogenous AF6 is phosphorylated specifically by EphB3 and EphB2 in a ligand-dependent fashion. Our observations add the PDZ domain to the group of conserved protein modules such as Src-homology-2 (SH2) and phosphotyrosine-binding (PTB) domains that regulate signal transduction through their ability to mediate the interaction with RTKs.

Tyrosine-614, the major autophosphorylation site of the receptor tyrosine kinase HEK2, functions as multi-docking site for SH2-domain mediated interactions

HEK2 belongs to the family of EPH-related receptor tyrosine kinases (RTK) which are involved in axonal pathfinding and the formation of the embryonic body plan. The knowledge about intracellular pathways of signal transduction mediated by EPH-related receptors is still limited. Many of the known key players of cellular signalling contain Src homology 2 (SH2) domains, which recognize phosphotyrosine motifs in RTKs. Thus, we examined the interactions of various SH2-containing molecules like PLC-gamma1, rasGAP, p85 subunit of PI3-kinase, Src, Fyn, Crk, Nck, Grb2 and Shc with HEK2 using in vitro binding assays, immunoprecipitations and yeast Two-Hybrid assays. We found that rasGAP, Crk and Fyn bind in a SH2-dependent manner to autophosphorylated HEK2. rasGAP, which contains two SH2- and one SH3-domain, was shown to associate with its N-terminal SH2-domain to HEK2. Furthermore, we demonstrated that a single amino acid substitution (Y614F) clearly reduces the phosphotyrosine content of HEK2 and abrogates its ability to bind rasGAP, Crk and Fyn indicating that this residue functions as major phosphorylation and multi-docking site. The conservation of this predicted binding site among various EPH-related RTKs provides evidence that Fyn, Crk and rasGAP are key players in signal transduction of at least a subset of these receptors.

Nef proteins of distinct HIV-1 or -2 isolates differ in their binding properties for HCK: isolation of a novel Nef binding factor with characteristics of an adaptor protein

The Nef gene of the human and simian immunodeficiency viruses HIV and SIV has been implicated in pathogenicity; however, the mechanism by which Nef induces disease is still unknown. An impact on signal transduction in cells has been suggested by the interaction of Nef from an HIV-1 strain and tyrosine kinases like HCK and LCK as well as serine/threonine kinases. We have confirmed the binding of HCK to HIV-1 subtype B Nef and demonstrated an equally strong interaction with a subtype E Nef protein but weaker binding to Nef of HIV-2 subtype A (HIV-2D194). No binding, however, was observed to HIV-2 subtype B Nef (HIV-2D205). Instead, this protein bound to a novel cellular protein, Nefin 1, with characteristics of an adaptor protein and strong expression in all human hematopoietic tissues. Nefin 1 binds through an amino-terminal domain, which is related to SH3 domains. For interaction of Nef with Nefin 1, the PxxP motif and the three-dimensional conformation of the molecule appear necessary. In conclusion, this study demonstrates that Nef proteins of divergent strains of HIV-1 and HIV-2 may use different elements of signal transduction pathways for the induction of pathogenicity in vivo.

Polo-like kinase, a novel marker for cellular proliferation

PLK (polo-like kinase) belongs to a family of serine/threonine kinases and represents the human counterpart of polo in Drosophila melanogaster and of CDC5 in Saccharomyces cerevisiae. It is strongly involved in spindle formation and chromosome segregation during mitosis. We have shown previously that PLK mRNA expression correlates with the mitotic activity of cells and the prognosis of lung cancer patients. In this report, the level of PLK protein was analyzed using immunohistochemical techniques. PLK protein was found expressed in the nuclei of tumor cells from lung and breast cancer as well as in several tumor cell lines. Furthermore, in peripheral lymphocytes treated with phytohemagglutinin, elevated proliferative activity of the cells correlated with the up-regulation of PLK protein expression. In contrast, in U937 and HL-60 cells after induction of differentiation with phorbol ester, PLK immunostaining disappeared under conditions of terminal differentiation. Most of the PLK protein was found in the nucleus of proliferating cells with diffuse but distinct staining also in the cytoplasm. Taken together, high levels of PLK protein are associated with cellular proliferation. Combined with other proliferative and oncogene markers, PLK may be useful for improved prediction of the clinical prognosis of cancer patients and for early cancer diagnosis. Due to its activity late in the cell cycle, it may be a target for cancer chemotherapy.

Prognostic significance of polo-like kinase (PLK) expression in non-small cell lung cancer

Our previous data indicate that the expression of the PLK gene which codes for a serine/threonine kinase is restricted to proliferating cells. In Northern blot experiments PLK mRNA expression was at the limit of detection in normal lung tissue but elevated in most samples of non-small cell lung cancer (NSCLC). A very low frequency of PLK transcripts was only found in bronchiolo-alveolar carcinomas. NSCLC patients whose tumors showed moderate PLK expression survived significantly longer (5 year survival rate=51.8%) than those with high levels of PLK transcripts (24.2%, P=0.001). No statistically significant correlation was found between PLK mRNA expression and age, sex, TNM status, histological type or degree of differentiation. Interestingly, the prognosis of patients in post-surgical stages I and II was correlated with PLK expression (5 year survival rates in stage I: 69.1% (moderate PLK) - 43.5% (high PLK), P=0.03 or in stage II: 51.9% (moderate PLK) - 9.9% (high PLK), P=0.006). These results suggest that PLK mRNA expression provides a new independent prognostic indicator for patients with NSCLC.

Identification of genes induced by factor deprivation in hematopoietic cells undergoing apoptosis using gene-trap mutagenesis and site-specific recombination

A strategy employing gene-trap mutagenesis and site-specific recombination (Cre/loxP) has been developed to isolate genes that are transcriptionally activated during programmed cell death. Interleukin-3 (IL-3)-dependent hematopoietic precursor cells (FDCP1) expressing a reporter plasmid that codes for herpes simplex virus-thymidine kinase, neomycin phosphotransferase, and murine IL-3 were transduced with a retroviral gene-trap vector carrying coding sequences for Cre-recombinase (Cre) in the U3 region. Activation of Cre expression from integrations into active genes resulted in a permanent switching between the selectable marker genes that converted the FDCP1 cells to factor independence. Selection for autonomous growth yielded recombinants in which Cre sequences in the U3 region were expressed from upstream cellular promoters. Because the expression of the marker genes is independent of the trapped cellular promoter, genes could be identified that were transiently induced by IL-3 withdrawal.

Cell-cell adhesion mediated by binding of membrane-anchored ligand LERK-2 to the EPH-related receptor human embryonal kinase 2 promotes tyrosine kinase activity

Human embryonal kinase 2 (HEK2) is a protein-tyrosine kinase that is a member of the EPH family of receptors. Transcripts for HEK2 have a wide tissue distribution. Recently, a still growing family of ligands, which we have named LERKs, for ligands of the eph-related kinases, has been isolated. In order to analyze functional effects between the LERKs and the HEK2 receptor, we expressed HEK2 cDNA in an interleukin-3-dependent progenitor cell line 32D that grows as single cells in culture. Within the group of LERKs, LERK-2 and -5 were shown to bind to HEK2. Membrane-bound and soluble forms of LERK-2 were demonstrated to signal through HEK2 as judged by receptor phosphorylation. Coincubation of HEK2 and LERK-2 expressing cells induced cell-cell adhesion and formation of cell aggregates. This interaction could be inhibited by preincubation of HEK2 expressing cells with soluble LERK-2. Coexpression of HEK2 and LERK-2 in 32D cells showed reduced kinase activity and autophosphorylation of HEK2 compared with the juxtacrine stimulation, which seems to be due to a reduced sensitivity of the receptor.

Eph receptors and ligands comprise two major specificity subclasses and are reciprocally compartmentalized during embryogenesis

We report that the many Eph-related receptor tyrosine kinases, and their numerous membrane-bound ligands, can each be grouped into only two major specificity subclasses. Receptors in a given subclass bind most members of a corresponding ligand subclass. The physiological relevance of these groupings is suggested by viewing the collective distributions of all members of a subclass. These composite distributions, in contrast with less informative patterns seen with individual members of the family, reveal that the developing embryo is subdivided into domains defined by reciprocal and apparently mutually exclusive expression of a receptor subclass and its corresponding ligands. Receptors seem to encounter their ligands only at the interface between these domains. This reciprocal compartmentalization implicates the Eph family in the formation of spatial boundaries that may help to organize the developing body plan.

Identification and functional characterization of the human and murine polo-like kinase (Plk) promoter

The Plk gene encodes a serine/theronine kinase which is located in the nucleus. Northern blot analysis linked Plk expression to the proliferative activity of cells and tissues. To analyse the transcriptional regulation of the Plk gene we have isolated several human genomic clones containing the Plk promoter. RNAse protection assays revealed three major transcription start sites within a 40 bp region centered around the 5' end of the known human cDNA and 6 minor Cap sites. A genomic fragment of 2.3 kb located 5' to the translation start sites drives the expression of the CAT-reporter in transient transfections in human (EPLC, HeLa) and mouse (NIH3T3, 32D) cell lines in an orientation dependent fashion. The 2.3 kb genomic fragment contains a CCAAT motif located 30-70 bp upstream of the Cap sites and two overlapping Sp1 sites 20 bp further upstream. Additional sequence motif homologues to binding sites of known transcription factors could be identified. In addition to the human Plk promoter, the mouse Plk promoter was isolated. The sequence alignment of the human and murine promoter revealed three regions with extensive sequence homology within a region of 300 bp immediately upstream of the Cap sites. A fourth region of homology encompassing 90 bp about 2.1 kb 5' of the Cap sites was identified as well. Deletion of various regions within the 2.3 kb promoter fragment identified several domains involved in the regulation of the human Plk promoter. The 300 bp region immediately 5' of the Cap sites which is highly conserved between mouse and man is essential for promoter activity. 3' deletions including the CCAAT site abolished promoter activity. Growing 5' deletions within the core region of the promoter reduces transcriptional activity. Furthermore, using deletion clones we identified regions 5' of the core region which enhance or silence the transcriptional activity of the core promoter.

Induction and down-regulation of PLK, a human serine/threonine kinase expressed in proliferating cells and tumors

We have identified the nucleotide sequence of the cDNA encoding the human counterpart of the mouse gene Plk (polo-like kinase). The sequence of the human gene, PLK, predicts a serine/threonine kinase of 603 aa. Expression of PLK mRNA appeared to be strongly correlated with the mitotic activity of cells. Resting peripheral lymphocytes did not express the gene at all. When primary T cells were activated by phytohemagglutinin, a high level of PLK transcripts resulted within 2-3 days. In some cases, addition of interleukin 2 to these cells increased the expression of PLK mRNA further. In contrast, primary cultures of human peripheral macrophages, which were not dividing under the culture conditions applied, showed very little or no PLK mRNA. Stimulation of these cells by bacterial lipopolysaccharide, an inducer of several cytokines in macrophages, totally abrogated the expression of PLK mRNA. In line with a function of PLK mRNA expression in mitotically active cells is our finding that six immortalized cell lines examined expressed the gene. In A-431 epidermoid carcinoma cells this expression was down-regulated by serum starvation and enhanced after serum was added again. Tumors of various origin (lung, colon, stomach, smooth muscle, and esophagus as well as non-Hodgkin lymphomas) expressed high levels of PLK transcripts in about 80% of the samples studied, whereas PLK mRNA was absent in surrounding tissue, except for colon. The only normal tissues where PLK mRNA expression was observed were colon and placenta, both known to be mitotically active. No PLK transcripts were found in normal adult lung, brain, heart, liver, kidney, skeletal muscle, and pancreas. In Northern blot experiments with RNA from lymphocytes which were treated with phytohemagglutinin and cycloheximide, PLK transcripts were not detectable, suggesting that PLK is not an early growth-response gene.

Oncogenic alterations in primary human lung-tumors (review)

Lung cancer is the leading cause of death from cancer in Western countries. For improved diagnosis and refined therapeutical approaches it is of major importance to understand by what mechanisms carcinoma of the lung develop. The analysis of primary lung cancer revealed specific chromosomal alterations and allelic losses of the short arm of chromosome 3. Genetic aberrations have been observed in proto-oncogenes such as H-ras, K-ras, C-myc and raf-1 as well as in the tumor suppressor genes Rb and p53. Rearrangement of rlf and elevated expression in certain lung tumors have also been reported. The development of lung cancer also involves the altered activation of genes coding for growth factors such as TGF beta 2 and certain growth factor receptor genes such as c-erbB-2, HEK2 and FGFR-4.

Structure, expression and chromosomal mapping of TKT from man and mouse: a new subclass of receptor tyrosine kinases with a factor VIII-like domain

Using a polymerase chain reaction-mediated approach we have characterized cDNAs from human and mouse origin representing a novel type of receptor protein tyrosine kinase (RTK). The deduced amino acid sequence (855 amino acids) of the longest open reading frame has a unique extracellular region encompassing a factor VIII-like domain, not previously described for RTKs. The most closely related RTKs are members of the neurotrophin receptors (TRK), which showed 47-49% homology with the kinase domain of the new RTK. Therefore, the new gene has been called TKT (Tyrosine-Kinase related to TRK). TKT orthologs from man and mouse were 98% similar. In both species a major transcript of 10 kb was found to be expressed at high levels in heart and lung. Low levels of this mRNA-species were detected in human brain, placenta, liver, skeletal muscle, kidney and in mouse brain and testis. Analysing human/mouse somatic cell hybrids we demonstrated that TKT segregates with human chromosome 1.

PCR mediated detection of a new human receptor-tyrosine-kinase, HEK 2

We have previously amplified cDNA subfragments of protein-tyrosine-kinases (PTKs) by using the polymerase chain reaction (PCR) and specific sets of oligonucleotide primers derived from nucleotide sequences of their kinase domain. In this study we have used a more directed approach to identify new members of the EPH/elk-family by PCR of human embryonic cDNA: we utilized oligonucleotide primers specifically designed to a highly conserved N-terminal motif and the kinase region of EPH/elk-PTKs in RNA-PCRs. The 5' and 3' elongation of the primary PCR product was achieved by the RACE (rapid amplification of cDNA ends)-technique. Sequence analysis of 3.8 kb of overlapping PCR products allowed to identify a novel receptor-PTK, HEK 2 (human embryo kinase 2), as an additional member of this family, without the need to screen a cDNA library. This approach should be useful for the rapid isolation of other PTK-genes as well. Analysis of genomic DNA placed HEK 2 on chromosome 3. Northern blot analysis demonstrated the expression of a 4.6 kb HEK 2-mRNA in lung, brain, pancreas, liver, placenta, kidney, skeletal muscle, heart and several human cells. In a protein kinase assay with HEK 2-specific immunoprecipitates from the human epidermoid carcinoma cell line A431, a protein of 130 kDa was found phosphorylated.

Characterization of the human CSK locus

The CSK-gene encodes an intracellular protein-tyrosine kinase (PTK). In contrast to members of the src-family, an autophosphorylation site corresponding to Tyr416, as well as the equivalent of the regulatory Tyr527 in p60c-src are missing in the amino acid sequence deduced from the gene. CSK phosphorylates other members of the src-family of tyrosine kinases at their regulatory carboxy-terminus. By regulating the activity of these kinases, CSK may play an important role in cell growth and development. Here we describe the structure of the human CSK gene. The entire coding region spans a genomic distance of only 4.9 kb. It encompasses 12 exons ranging between 66 and 220 bp in size. The introns between coding exons vary between 76 and 920 bp in length. An exon coding for the 5'-untranslated region of CSK is separated from the first coding exon by an intron of more than 6400 bp. Based on comparisons of sequence homologies within the catalytic domains, the intracellular PTKs are divided into the src-family, the fes/fer- and the abl/arg-group. The genomic structure of four members of the SRC-family revealed nearly identical exon/intron boundaries within the catalytic domain of this family. They differ from those described for FES. Comparing the genomic structure of CSK with the exon/intron organisation of both, it is obvious that the exon/intron boundaries are in common either with those of the SRC-type or the FES boundaries. This intermediate exon/intron structure of CSK between FES and the SRC-family agrees with the position of CSK in a phylogenetic tree based on sequence homology within the kinase domain.

A strong protein-tyrosine kinase activity is associated with a baculovirus-expressed chicken tkl gene

We have previously described a gene named tkl (tyrosine kinase related to lck). It belongs to the src family of protein-tyrosine kinases and among these it has significant homology to the lck gene (lymphoide cell kinase). The tkl gene product may represent the avian homolog of Lck, which is believed to participate in a lymphocyte-specific signal transduction pathway by association with a membrane receptor. To study the biochemical properties of the protein, a nearly complete tkl gene (isolated from a cDNA library from chicken spleen cells) was expressed in a baculovirus system. Approximately 10% of the extracted protein consisted of the soluble 51-kDa Tkl protein (p51tkl) at 40 h post-infection. This protein was found to be phosphorylated on tyrosine and serine residues at a ratio of 5:1. As expected, glycosylation or myristoylation could not be detected. Immunocomplex kinase assays indicated strong autophosphorylation of p51tkl at tyrosine residues and phosphorylation of exogenous substrates such as D-glyceraldehyde-3-phosphate dehydrogenase (GAPDH), histones H2b and H4, and casein. This protein-tyrosine kinase activity also exhibited a marked preference for Mn2+ compared to Mg2+. The high level expression of enzymatically active Tkl should provide an excellent tool to further study the biological functions of this class of enzymes.

The genomic locus of the human hemopoietic-specific cell protein tyrosine kinase (PTK)-encoding gene (HCK) confirms conservation of exon-intron structure among human PTKs of the src family

Protein tyrosine kinases (PTKs) are implicated in the control of cell growth by virtue of their frequent appearance as products of retroviral oncogenes, as intracellular signal transducers, and as growth factor receptors or components thereof. The knowledge of the structure and sequence of family genes encoding PTKs is still limited. To date, the complete genomic structure of human src family members is only available for the C-FGR gene (encoding p55 Fgr, PTK). Sequence analysis and characterization of the intron/exon organization of the human HCK gene, encoding a hemopoietic-specific cell PTK of the src-related family, revealed a length of over 16 kb for the seven 3'-exons. All intron/exon splice junctions agree with the GT/AG rule. In each case where a boundary occurs at a Gly codon, GGG or GGA, the triplet is split between the first and second nucleotide (nt). A total of eight complete and one partial Alu repeats were identified within the introns. The nt sequence of the genomic clones resolves existing discrepancies among two published sequences of HCK cDNAs. Human HCK, C-SRC (encoding p60 Src PTK), C-FGR and LCK (encoding p56 Lck, PTK) genes thus share very similar exon/intron structures for the conserved exons. These results provide additional evidence that the different PTKs of the src-like family most likely arose by duplication of an ancestral src-like gene.

Isolation and characterization of a human gene that encodes a new subclass of protein tyrosine kinases

We describe the isolation and cDNA sequence of a novel human gene, which is distinct from all known members of the human src family of proto-oncogenes. In contrast to these, an autophosphorylation site corresponding to Tyr416, as well as the equivalent of Tyr527 in p60c-src, are missing in the amino acid (aa) sequence deduced from this gene. Furthermore, no N-terminal myristylation site is found. Our human clone is 98% identical at the aa level to a gene which was isolated independently from neonatal rat brain and was termed csk for c-src kinase. We, therefore, propose to designate the present human gene CSK. In Northern blot experiments, CSK was found to be expressed in human lung and macrophages. Due to its extreme conservation across species barriers, the CSK product is likely to exert important regulatory functions. On the basis of its expression in tissues, not typically expressing high c-src levels, it can be assumed that its regulatory role is more general and may also involve other tyrosine kinases.