The EGFR (epidermalgrowthfactorreceptor) has been suggested as a promising target since up to 70% of ovarian cancers are EGFR-positive and EGFR-overexpression is developed during cancer progression and correlated to poor prognosis [2,3]. The efficacy of EGFR-inhibition has been studied with monoclonal antibodies and low molecular weight tyrosinekinase inhibitors (TKI). The chimeric monoclonal anti-EGFR-antibody Cetuximab binds to the extracellular domain of the EGF-receptor, prevents EGFR-signaling and enhances receptor internalization. In vitro Cetuximab inhibited cell growth of ovarian cancer cell lines and acted synergistically with cytostatic agents . Further, Cetuximab is able to potentiate apoptosis, to inhibit angiogenesis and impairs tumour cell invasion and metastasis . However, in clinical trials Cetuximab has failed to show relevant clinical activity as monotherapy or in combination with chemotherapy in ovarian cancer so far [6–8]. Small molecules as tyrosinekinase inhibitors act intracellularly by competing with ATP binding and prevent further intracellular receptor signaling. In several phase I-II-studies of ovarian cancer the tyrosinekinase inhibitor Erlotinib (Tarceva ® ) did not effectively contribute to a therapeutic improvement neither as a single agent nor combined with chemotherapy or with the anti-VEGF-antibody Bevacizumab [9–12]. Single TKI-inhibition with Gefitinib (Iressa ® ) reached only limited responses [13,14]. Preclinical data revealed that Gefitinib could potentiate cytostatic antitumoural effects , which might be also of clinical benefit . Vandetanib (ZD6474, Zactima™), which inhibits VEGFR2 and EGFR signaling, had no clinical activity in monotherapy in recurrent ovarian cancer . These studies show that, so far, EGFR-targeting in ovarian cancer has not reached sufficient clinical benefit.
Receptortyrosine kinases (RTKs) are a subclass of signaling receptors anchored at the cell surface and have intrinsic tyrosinekinase activity triggering signal transduction in response to ligand binding. RTKs are generally activated through ligand-induced oligomerization, typically dimerization, which leads to autophosphorylation of tyrosine residues in the kinase activation loop or the juxtamembrane region . These phosphotyrosine residues are important docking sites for a plethora of intracellular downstream signaling molecules and are typically bound by Src homology-2 (SH2) or phosphotyrosine-binding (PTB) domains . The epidermalgrowthfactorreceptor (EGFR) is a member of the ErbB family of receptors, a subclass of RTKs, and is expressed in all epidermal cells as well as stromal, glia and smooth muscle cells . EGFR signaling is one of the most important pathways that regulate growth, survival, proliferation and differentiation in mammalian cells . Thus, EGFR signaling is also critical for the development of many types of cancer. Mutations that lead to EGFR overexpression or overactivity have been associated with a number of cancers, including lung cancer, anal cancers and the glioblastoma multiforme [5, 6]. Mutations involving the EGFR may lead to its constant activation, which results in uncontrolled cell division. Consequently, mutations of the EGFR have been identified in several types of cancer and it is the target of an expanding class of anticancer therapies [7- 9].
Treatment of SKOV-3 cells with Herceptin or tyrosinekinase inhibitors prior to the addition of paclitaxel as well as HER-2 downregulation by ribozyme-targeting render the cells more resistant to paclitaxel. These results indicate that reduced proliferation may be the underlying cellular effect that determines the increased paclitaxel resistance of SKOV-3 cells. This could merely be because cells take a longer time to reach mitosis, where the lethal event upon paclitaxel treatment occurs. The active paclitaxel might be metabolized in this time and therefore the slower growing cells may be subjected to a lower effective dose by the time they reach mitosis. The correlation between proliferation rate and sensitivity towards paclitaxel is further supported by the fact that serum-starving conditions attenuate paclitaxel- induced cell death in this cell system as well. On the other hand, doxorubicin or cisplatin cytotoxicities are independent of HER-2 levels as shown here and previously  demonstrating that HER-2 targeting does not result in a general increase in resistance towards all drugs only due to reduced cell proliferation. Taken together, these data indicate that HER-2 has an effect on cell cycle progression and proliferation which determines the cytotoxicity of paclitaxel, but not of doxorubicin or cisplatin.
Activating EGFR mutations are located in the tyrosinekinase domain of EGFR and result in increased enzyme activity. Typically they are heterozygous and comprise in-frame deletions and single-nucleotide substitutions.[9, 10, 69] Within the EGFR gene, activating mutations reside in the first four exons of the kinase domain (exons 18-21) that encode the N-terminal lobe (N-lobe) and the 5´ portion of the C-terminal lobe (C-lobe) of the tyrosinekinase domain. The adenosine triphosphate (ATP)-binding pocket is located in the cleft between the N-lobe and the C-lobe of the tyrosinekinase.[69, 73] This binding cleft is also targeted by small molecule inhibitors such as gefitinib or erlotinib (discussed in detail in section 1.3.1).[74, 75] The most frequently occurring activating EGFR mutations, accounting for around 90% of all mutation-positive NSCLCs, are exon 19 deletions and the exon 21 L858R substitution mutation. Deletions in exon 19 make up at least 45% of EGFR mutations. They refer to in- frame deletions around the LREA motif which comprises amino acid residues 747 to 750 in exon 19 and result in the elimination of the amino acids leucine-arginine-glutamic acid- alanine.[69, 73, 76] Some of these deletions additionally involve a single missense mutation as a result of a newly arising codon at the deletion breakpoint.[9, 76] Crystallographic imaging suggests that deletions in exon 19 shorten the loop structure at the side of the αC- helix in the N-lobe, potentially causing a downwards shift of the αC-helix and thereby narrowing the angle of the ATP-binding-pocket (figure 6).
Typically, upon the binding of EphrinA1 in a “lock and key” mechanism, EphA2 receptor becomes tyrosine phosphorylated and interacts with several adapter proteins to elicit downstream signaling transduction [34, 39]. The key downstream molecules of this signaling are phosphatidyl inositol 3’ kinases (PI3K), Src family kinases, Rho and Rac1 GTPase, mitogen activated protein kinases (MAPK) and integrins along with the crosstalk of other oncogenic receptors (e.g. epidermalgrowthfactorreceptor, EGFR) that regulate cell adhesion, proliferation, migration, and modulation of cytoskeleton architecture, and development of vascular network . However, there is a unique characteristic that makes EphA2 different from most RTKs in that ligand binding and phosphorylation of the conserved tyrosine may not be necessary for the kinase activity of EphA2 [41, 42]. In fact, EphA2 may possess ligand-independent kinase activity in tumor cells and trigger EphrinA1 independent signaling in malignancy in a non-tyrosine-phosphorylated state [41, 43]. In addition, EphA2 participates in feedback loops that may switch between different outputs depending on the state of other cellular signaling networks. This makes the signaling pathway of EphA2 extremely complex . Examples include EphA2─H-Ras─Erk negative feedback loop: Activation of the H-Ras─Erk pathway increases EphA2 expression through Mek1 and decreases EphrinA1 expression. In turn, EphrinA1-dependent EphA2 activation inhibits H-Ras─Erk signaling and also downregulates EphA2 levels by causing receptor internalization and degradation; EphA2─E-cadherin positive feedback loop: E-cadherin expression increases EphA2 expression, surface localization, interaction with EphrinA1 and consequently forward signaling. In turn, EphA2 signaling enhances E-cadherin-mediated adhesion.
Lung cancer is the leading cause of cancer deaths worldwide, having a great impact on personal and socioeconomic health. Current treatment regimes still leave a great percentage of patients with a poor outcome. To improve the outcome of patients by increasing the efficacy of radiation therapy, which represents a major component of cancer treatment, especially in late stage disease, new treatment strategies such as inhibition of the epidermalgrowthfactorreceptor (EGFR) are currently in development (Ansari et al., 2009; Sangha et al., 2009). While the EGFR inhibition is a promising strategy, not all patients are expected to benefit from a supporting inhibitory therapy in addition to irradiation. This indicates the need for predictive markers that can be evaluated for each cancer patient to initiate EGFR inhibitory treatment only in those for whom it will be beneficial. To study the value of a KRAS mutation in NSCLC cell lines as a predictive marker for response to a combined treatment with the tyrosinekinase inhibitor (TKI) erlotinib and irradiation, the presented experiments were conducted.
cell lines. A reduction of global tyrosine phosphorylation was detected by 4G10 mAb upon pre-BCR cross-linking in ALL Lyn-knockdown cells (Figure 22). In this regard, we analyzed the activation of several downstream proteins upon pre-BCR cross-linking in Lyn-knockdown ALL cells (Figure 23). Unexpectedly, a different activation profile was observed in Nalm6 cells (high Lyn-expressing cells) as compared to CALL3 (low Lyn-expressing cells). On one hand, repression of Lyn in Nalm6 cells noticeably affected the phosphorylation status of the SFK detected by p-Src family antibody. On the other hand, repression of Lyn in CALL3 cells resulted in a sustained activation of the SFK for a longer period (up to 40 minutes after pre-BCR cross-linking) as compared to CALL3 control cells. These striking differences observed in CALL3 cells led to conclusion that the SFK activation detected in CALL3 Lyn-knockdown cells is due to other SFK members present in CALL3 cells. The analysis using the p-SRC family antibody showed only one band in Lyn-knockdown CALL3 cells, whereas two bands in CALL3 control cells were detected. This result strongly suggested the presence of SFK other than Lyn. Furthermore, analyses of the PTK in this cell line revealed that Blk is the most prominent SFK expressed in CALL3 cells (Figure 15), and therefore, BLK might be able to compensate for Lyn functions. A recent study in splenic B-cells, purified from Lyn -/- mice, showed an enhancement of SFK activation after BCR ligation in absence of Lyn 110 .
Preclinical cancer research has relied heavily upon cell lines grown in culture and then xenografted for growth in mice. However, recent work has shown that human cancers placed directly into the mouse (tumorgrafts) maybe a more appropriate model that is better at predicting response to drugs in humans (144). Drs Jenny Chang and Mike Lewis have recently developed several new tumorgraft models of human TNBC. We screened three of these for activity of the IGF-IR by both immunohistochemistry (IHC) and immunoblotting (IB). Figure 16A shows that all tumorgrafts expressed various levels of IGF-IR and had divergent levels of active phosphorylated pY-IGF-IR. Protein lysates from the same tumorgrafts as in Figure 16A confirmed that MC1 had the highest activation of IGF-IR (Figure 16B). Activation of downstream signaling molecules varied among the tumorgraft models. MC1 had high levels of IRS1 and activated AKT whereas the tumorgraft 2665A showed activation of MAPK. We isolated RNA, generated gene expression data from all of the tumorgrafts, and calculated an IGF signature t-score for each tumorgraft. Analysis of these profiles showed that tumorgraft MC1 (145) had the highest level of IGF-IR, pY-IGF-IR, and in addition had a high IGF signature T-score.
Die L6-Zellen schienen aus zwei Gründen geeignet zu sein, um downstream-Gene von FGFR2 zu identifizieren. Zum einen exprimieren die Zellen ihre FGF-Rezeptoren nicht. Dadurch ist nach Transfektion der Zellen mit einer spezifischen FGFR-cDNA die Untersuc hung der Biologie dieses speziellen FGFR-Subtyps möglich. Aus diesem Grund wurden und werden L6-Zellen häufig verwendet, um bestimmte Aspekte der FGFR-Wirkung aufzuklären. Die folgenden Beispiele stellen nur eine Auswahl dar. So wurde an FGFR2-transfizierten L6-Zellen die Bedeutung der IgIII-Domäne der FGFRs für die Spezifität der Ligandenbindung ermittelt (Dell und Williams 1992), an FGFR1-transfizierten L6-Zellen beobachtet, dass die Aktivierung des Grb2-Sos-Ras- Signalweges über das Adapterprotein Shc erfolgt (Klint et al. 1995), dass die FGF2- Behandlung FGFR1-exprimierender L6-Zellen zur Aktivierung der p70-S6-Kinase führt (Kanda et al. 1997), dass ein erhöhter Lipidmetabolismus in FGF2-behandelten FGFR1-exprimierenden L6-Zellen stattfindet (van Dijk und van Blitterswijk 1998), dass verschiedene FGFR1 -Subtypen das Adapterprotein FRS2 unterschiedlich stark aktivieren (Lopez und Korc 2000) oder dass die Ligandenbindungsspezifität und - affinität der IgIIIb-Variante von FGFR1 für FGF1 und FGF2 verschieden ist (Beer et al. 2000). Diese Studien zeigen, dass FGFRs in L6-Zellen biologisch wirksam sind. Sie binden ihre Liganden und sie beeinflussen die Aktivität von Signaltransduktions- komponenten. Die Annahme war daher begründet, dass die Aktivierung von FGFR2 in L6-Zellen über die Aktivierung der Signaltransduktionskaskade das Expressions- muster einiger Gene verändern würde. Dies war der zweite Grund, weswegen L6- Zellen für die Identifizierung FGFR2-regulierter Gene geeignet schienen.
Bei den über sechs Monate mit NNM behandelten Tieren der Hauptgruppen 4 – 6 bot die Leber analog der in der Literatur beschriebenen chemischen Hepatokarzinogenese ein morphologisch heterogenes Bild mit dem Auftreten nicht nur klarzelliger und gemischtzelliger, sondern auch basophiler Präneoplasien sowie hepatozellulärer Adenome und Karzinome. Bei diesen Gruppen traten auch vereinzelt Hämangiosarkome der Leber auf. Die mit den Literaturangaben konsistente Tumorentwicklung, die insbesondere in den Arbeiten der Arbeitsgruppe von Bannasch beschrieben werden (10; 12; 14; 40), zeigt, dass die längerfristige niedrig dosierte Applikation von NNM einen reproduzierbaren Weg der chemischen Hepatokarzinogenese darstellt. Weitere Untersuchungen, beispielsweise die Aktivierung intrazellulärer Signalkaskaden wie der schon erwähnten MAP-Kinase durch IGF-1R sind somit zukünftig zuverlässig durchführbar und mit den vorliegenden Daten vergleichbar.
The presence of this conserved motif raises the question at which point during evolution this sequence appeared. The development of enzymes with tyrosinekinase activity in eukaryotes is from an evolutionary point of view a rather recent process. It is believed that the driving force in the establishment of these enzymes was the transition from uni- to multicellular organisms approx. 600 million years ago (Miller 2012). This is probably due to the fact that multicellularity requires sophisticated signaling networks for cell-cell communication. Consequently, a great variety of tyrosine kinases is found in metazoans (Figure 4.3A). In the past decades, many efforts have been done in sequencing the whole genome of various organisms. This has made available the complete deciphering of the kinome of humans (Manning et al. 2002a), sea urchins (Bradham et al. 2006), fruit flies (Manning et al. 2002b), and Nematodes (Plowman et al. 1999). As Syk-family kinases (of which Zap70 is a member) are not present throughout all Metazoans, I have decided to look at the sequence of SFKs which are more ancient and hence allow me to follow their evolution throughout all Metazoan. As already mentioned above, the human kinome comprises 91 tyrosine kinases. The kinome of the sea urchin (Strongylocentrotus purpuratus) comprises in total 353 kinases of which 49 are classical tyrosine kinases (Bradham et al. 2006). Amongst these, 7 are members of the SFKs and 6 of them harbor the motif (Figure 4.3B). SFK1b, the only SFK without the motif, appears to be a truncated kinase lacking the whole C-terminal section of the kinase domain. Interestingly, of the 49 tyrosine kinases in S. purpuratus, 33 possess (~67%) the whole Mx (2) CWx (6) R motif and additional 7 have one or two substitutions in the
The principal effector cells in the IgE-related allergy are mast cells (MC) and basophilic granulocytes (BA). Both cell types bind immunoglobulin E (IgE) through its high affinity receptor (FcεRI). Signalling cascade, associated with FcεRI, is initiated by a cross-linking of the IgE-preloaded receptor with an allergen. Thus, activated FcεRI-related signalling pathway lead to the degranulation of MC and BA. During the process of degranulation, pre- formed madiators are released from these activated effector cells. Activation of MC and BA also triggers de novo synthesis of various cytokines and lipid mediators that are important contributors to allergic reactions. Protein tyrosinekinase-associated signaling contributes essentially to the activation of MC and BA. The activation of SRC kinase family members is an important initial event in the signal transduction cascade activated by cross-linking FcεRI on MC and BA. The Bruton´s tyrosinekinase (BTK) may be considered as a constitutive and essential part of signalling pathway that mediates MC and BA activation induced by allergens. Moreover, the activation of BTK is controlled by SRC family members such as LYN or FYN. Thus, BTK may be a potential therapeutic target in FcεRI cross-linked BA. In this study, I tested BTK-specific inhibitors: ibrutinib, dasatinib, AVL-292, CNX-774. The inhibitory effects of BTK specific drugs were tested on signal transduction in BA. The effects of BTK inhibitors on activation of human BA were evaluated in functional assays: IgE- dependent activation and histamine secretion in human basophilic granulocytes. IgE-related activation of BA was determined by flow cytometry measuring the upregulation of activation markers (CD13, CD63, CD164 and CD203c) after challenging with anti-IgE or allergen. I examined the effects of BTK-specific inhibitors on growth of the MC line HMC-1 and the BA line KU812. In these experiments, growth and survival of HMC-1 and KU812 cells were determined by measuring incorporation of 3 H-thymidine into the cells. In addition, to examine
Das HCC entwickelt sich im Laufe von vielen Jahren aus dysplastischen Knoten bei chroni- schem Leberschaden. Neben Ätiologie-spezifischen molekularen Mechanismen kommt es in der entzündeten oder bereits fibrotischen/zirrhotischen Leber zur Überexpression von Zytoki- nen wie Tumornekrosefaktor (TNF)-α und Interferon (INF)-γ, zu einem Nitritoxid (NO)-An- stieg und dadurch zu DNA-Schädigung sowie zu Inaktivierung von Tumorsuppressorgenen und Aktivierung von Protoonkogenen (9). Aus einem dysplastischen Fokus mit zunehmend entdif- ferenzierten und funktionslosen Hepatozyten und Regenerationsarealen entsteht ein dysplasti- scher Knoten und anschließend ein frühes differenziertes Karzinom, welches sich zu einem fortgeschrittenen, wenig differenzierten Karzinom mit Tendenz zu weiterem Wachstum, Meta- stasierung und Invasion entwickelt (10). Die Dysregulierung von Wachstumsfaktor-Signalwe- gen wie insuline-linke growthfactor (IGF), epidermalgrowthfactor (EGF), vascular endothe- lial growthfactor (VEGF) und fibroblast growthfactor (FGF) und das komplexe Zusammen- spiel der Tumorzellen mit dem umliegenden Gewebe spielen dabei eine große Rolle. Das HCC kann anhand der betroffenen Signalwege und Gendysregulationen in viele verschiedene, hete- rogene Subklassen eingeteilt werden (11). Auf die spezifische Rolle des VEGF-Signalweges und der Angiogenese wird in Kapitel 5.1.3 näher eingegangen.
through production of IL-4 as well as other inflammatory mediators and enhanced reactivity to various stimuli (de Mora et al., 1993; de Mora et al., 2006; Hammerberg et al., 2001). Treatment of canine AD includes topical medications, antihistamines, glucocorticoids and non- steroidal anti-inflammatory drugs (Bloom, 2013; Marsella & Olivry, 2001; Olivry & Bizikova, 2013; Olivry et al., 2010). In addition to topical or systemic therapy, avoidance of flare factors and a good skin care may reduce signs of AD (Bloom, 2013; Olivry et al., 2010). A novel, effective targeted therapy for canine pruritus associated with allergic dermatitis is application of oclacitinib, a Janus kinase 1 (JAK1) inhibitor (Cosgrove et al., 2013; Gonzales et al., 2014). Oclacitinib is selectively inhibiting JAK1-dependent pathways responsible for itch and inflammation (Gadeyne et al., 2014; Gonzales et al., 2014). JAK1 is a key molecule in the signal transduction pathway of many pro-inflammatory cytokines known to be involved in canine pruritus, such as IL-2, IL-4, IL-6, IL-13, and IL-31 (Cosgrove et al., 2013; Felsburg, 2002; Gonzales et al., 2013; Ong & Leung, 2006). Recently, a study with 123 dogs suffering from AD reported that oclacitinib is comparable to the glucocorticoid prednisolone in effectively reducing pruritus and clinical symptoms (Gadeyne et al., 2014). More recently, a clinical study including dogs with AD revealed that the anti-canine IL-31 antibody, called lokivetmab, reduced pruritus and inflammatory skin lesions of the canine patients (Michels et al., 2016). Beyond that, a study with a young dog suffering from cutaneous mastocytosis reported that therapy with lokivetmab provided an effective reduction of pruritus without any adverse effects (Meichner et al., 2019).
Background: Oncoproteins encoded by the early region of adenoviruses have been shown to be powerful tools to study gene regulatory mechanisms, which affect major cellular events such as proliferation, differentiation, apoptosis and oncogenic transformation. They are possesing a key role to favor viral replication via their interaction with multiple cellular proteins. In a yeast two-hybrid screen we have identified Sprouty1 (Spry1) as a target of adenoviral E1A Oncoproteins. Spry proteins are central and complex regulators of the receptortyrosinekinase (RTK) signalling pathway. The deregulation of Spry family members is often associated with alterations of the RTK signalling and its downstream effectors, leading to the ERK pathway.
Despite being viable, many of the knockout mice show phenotypes. If the mice were stressed they showed further homeostasis defects in previously apparently normal tissues. For AR-deficient mice for example only a defect in mammary gland development was reported initially (Luetteke et al, 1999). Further studies however revealed impaired liver regeneration (Berasain et al, 2005a; Berasain et al, 2005b) and significantly less growth of tibial trabecular bone than wild-type mice (Qin et al, 2005). Notably, several of the phenotypes observed in mice lacking individual ErbB1 ligands such as for example hair follicle and eye lid closure defects in TGF-α deficient mice (Luetteke et al, 1999; Mann et al, 1993) or heart-valve defects and lung immaturity in HB-EGF deficient mice (Iwamoto et al, 2003; Jackson et al, 2003) are found in mice with an ErbB1-receptor knockout (Chen et al, 2000; Miettinen et al, 1995; Sibilia & Wagner, 1995; Threadgill et al, 1995) indicating that the ErbB1 receptor primarily mediates the signaling of these ErbB ligands.
experiments in which TrkB deficient mice (TrkB –/– ) were needed as controls, brains of newly born TrkB wildtype mice (TrkB +/+ ) and TrkB deficient mice (TrkB –/– ) were used since TrkB( –/– ) mice barely survive one week (Klein et al. 1993). After decapitation of the mice the brains were removed from the skulls and immediately homogenized in a Dounce homogenizer (Weaton, Teflon pestle, 2 ml, 5 ml or 10 ml) on ice. In the first cross-linking experiments using TrkB peptides for coupling to the cross-linker Sulfo-SBED brain ho- mogenate was prepared either in PBS with the protease inhibitors Complete TM or in lysis buffer 1 (see 3.2). Then the homogenate was centrifuged for 15 min at 1,000 x g and 4 °C before adding Triton X-100 at a final concentration of 1%. For later cross-linking experi- ments and proteolysis assays, mice brains were homogenized at 4 °C in lysis buffer inhibiting dephosphorylation (P+) or inhibiting tyrosinekinase phosphorylation (P–). The lysis condition inhibiting dephosphorylation (P+) is characterized by the presence of pro- tein tyrosine phosphatase and protease inhibitors in the lysis buffer (Haier and Nicolson, 2000; Hyuer et al., 1997). For proteolysis assays and co-immunoprecipitations (see 5.7), nuclei were removed by centrifugation for 15 min, at 600 x g and 4 °C after homogeniza- tion. After re-centrifugation of the supernatant for 45 min at 25,000 x g and 4 °C, the membrane pellet was resuspended in RPMI medium (for proteolysis assays) or in P+ lysis buffer (for co-immunoprecipitations). For detection of the interaction between full-length TrkB and NCAM180-ID using the cross-linking approach (see 5.6) and co- immunoprecipitations (see 5.7), the P+ lysis buffer additionally contained 20 µM of the matrix metalloprotease inhibitor GM 6001 and 1 µM of the γ-secretase inhibitor DAPT (Calbiochem, La Jolla, California, USA). The other lysis buffer (P–) inhibiting Trk phos- phorylation contained the specific Trk inhibitor K252a (Calbiochem) (Tapley et al., 1992).
Cetuximab (Erbitux ®) ist ein chimärer monoklonaler IgG1 Antikörper, der von einer rekombi- nanten Zelllinie in Maus-Myelomzellen produziert wird . Der Antikörper richtet sich gegen den epidermalen Wachstumsfaktorrezeptor (EGFR = EpidermalGrowth Faktor Receptor). Die- ser Rezeptor wird in gesunden Epithelzellen exprimiert. In Tumorzellen des kolorektalen Karzi- noms ist der EGFR-Rezeptor sogar oft überexprimiert (25 - 80 % der Fälle) . Die EGFR- Signalübertragung ist beteiligt an der Kontrolle der Überlebensfähigkeit von Tumorzellen, an der Zellzyklusprogression und der Angiogenese . Grundsätzlich wird die EGFR-Expression mit einer stärkeren Agressivität und einer schlechteren Prognose des kolorektalen Karzinoms in Ver- bindung gebracht . Cetuximab hemmt durch die Bindung an den EGF-Rezeptor dessen Funktion. Es induziert die Internalisierung und damit die Downregulierung des EGF-Rezeptors. Dies führt schließlich zu einer Verhinderung von Zellproliferation und Angiogenese und zu der Einleitung von Apoptose-induzierenden Mechanismen . Der Wirkungsmechanimus des mo- noklonalen Antikörper Cetuximab wird als „Targeted Therapieˮ (molekular gezielte Therapie), da Cetuximab selektiv an den EGF-Rezeptor bindet und somit dessen Funktion inhibiert.