Der pRB/E2F-Signalweg ist ein wichtiger Schlüsselpunkt für die Wachstumskontrolle in Säugerzellen und in vielen Tumoren sind Komponenten dieses Signalweges dereguliert. Durch die Nullmutation von E2F3 in Mausembryonalen Fibroblasten (MEFs) und Mäusen konnte gezeigt werden, dass E2F3 essentiell für das zelluläre Wachstum ist und in der Maus organspezifisch sowohl als Tumorsuppressor als auch Onkogen agieren kann. Jedoch sind dafür die zugrunde liegenden Mechanismen noch nicht genau geklärt. Möglicherweise tragen verschiedene Signalwege, die durch den Verlust von E2F3 dereguliert werden, zu den Defekten bei. In dieser Arbeit wurde TGFbeta1, ein wichtiger Wachstumsregulator, in den E2f3-/- MEFs untersucht und es konnte zum ersten Mal eine direkte Verbindung zwischen der E2F3-Expression und der TGFbeta1-Signalwirkung gezeigt werden. Durch den Verlust von E2F3 werden Tgfb1 und die TGFbeta1-regulierten Gene PAI-1, p21, Vimentin und Fibronectin in MEFs dereprimiert. Darüber hinaus werden MEFs und humane Lungenkarzinomzellen durch den Verlust von E2F3 gegenüber TGFbeta1 sensibilisiert und reagieren verstärkt auf TGFbeta1-induzierte Genexpression und Prozesse wie Wachstumsarrest und EMT. Somit wird E2F3 nicht nur durch TGFbeta1 reguliert, sondern kann auch auf TGFbeta1 und die TGFbeta1-Signalwirkung Einfluss nehmen, was für die Tumorprogression weit reichende Auswirkung haben kann. Um die tumorsuppressiven Eigenschaften von E2F3 besser zu verstehen, wurden im zweiten Teil dieser Arbeit murine medulläre Schilddrüsentumore mit unterschiedlichem metastatischen Potential miteinander verglichen und es konnten neue E2F-Zielgene identifiziert werden. Die Untersuchung von humanen Struma nodosa-Biopsien und metastatischen medullären Schilddrüsentumoren ergab, dass die in den Mäusen gefundenen Gene künftig auch als humane Metastasemarker Verwendung finden können. / The pRB/E2F-pathway plays a key role in growth control and it is deregulated in many tumors. Previously, by analysing E2f3 deficient mouse embryonic fibroblasts (MEFs) and mice it has been shown that E2F3, a key downstream target of pRB, is essential for cellular proliferation and can act either as an oncogene or tumorsuppressor in mice depending on the organ. However, the underlying mechanism is still unclear. We suggest that specific pathways which are deregulated due to the deletion of E2F3 contribute to these defects. TGFbeta1, which is one of the most potent growth regulators for mammalian cells was analysed in E2f3-/- MEFs. In this study, we could establish a direct link between E2F3 expression and TGFbeta1 signalling. Loss of E2F3 in MEFs leads to de-repression of Tgfb1 and TGFbeta1-regulated genes like PAI-1, p21, vimentin and fibronectin. Moreover, loss of E2F3 in MEFs or in human lung carcinoma cells results in an increased sensitivity to TGFbeta1-induced gene expression and processes like growth arrest and epithelial mesenchymal transition. These data suggest that not only TGFbeta1 can act on E2F3 but also E2F3 can affect TGFbeta1 and the outcome of TGFbeta1-induced signalling. In order to understand the tumor suppressive properties of E2F3, we compared gene expression profiles of murine medullary thyroid carcinomas (MTCs) of different metastatic potential and could identify novel E2F-target genes. Analysis of human struma nodosa biopsies and human metastatic medullary thyroid tumors showed that the genes identified in the mouse model can also be used as metastasis markers in human tumors.

The term "pattern formation" refers to the process by which order unfolds in development. The present thesis deals with a particular aspect of molecular pattern formation during vertebrate embryogenesis. The model system in the focus of this study is the zebrafish, Danio rerio. In the early developmental phases of the zebrafish, Fibroblast growth factors (Fgfs) are involved in the molecular patterning of various tissues, including two regions of the brain, the forebrain and the midbrain-hindbrain region, affecting cellular processes as diverse as cell proliferation, differentiation, and axonal targeting. The goal of this study was to better understand the mechanisms by which Fgf signaling regulates pattern formation and embryogenesis. I addressed this question on several levels, investigating the extent of intracellular signaling (MAPK activation) relative to sources of Fgf expression, and the transcriptional responses of cells to Fgf signaling during embryogenesis. By a macroarray analysis, I identified putative transcriptional targets of Fgf signaling in late gastrulation, providing a set of molecules that are likely to act as functional players in relaying the patterning information encoded by Fgf signals. Among those are the secreted signaling molecules Chordin and Wnt8, as well as Isthmin, a novel secreted molecule that I found capable to interfere with anterior embryonic patterning. In addition, I identified two ETS domain transcription factors, Erm and Pea3, which constitute bona fide integrators of FgfR signaling. By gain- and loss-of-function studies, I demonstrate that transcript levels of erm and pea3 are tightly regulated by Fgf signaling. Detailed analysis of the expression patterns of erm and pea3 along with other Fgf target genes also provides evidence for a differential read-out of Fgf concentration in the embryo, consistent with a role of Fgf as a vertebrate morphogen. The discovery of novel molecular components downstream of Fgf receptor activity paves a way to characterize previously unknown or underestimated developmental roles of Fgfs in the molecular patterning of the forebrain, the eye and parts of the neural crest.

The term "pattern formation" refers to the process by which order unfolds in development. The present thesis deals with a particular aspect of molecular pattern formation during vertebrate embryogenesis. The model system in the focus of this study is the zebrafish, Danio rerio. In the early developmental phases of the zebrafish, Fibroblast growth factors (Fgfs) are involved in the molecular patterning of various tissues, including two regions of the brain, the forebrain and the midbrain-hindbrain region, affecting cellular processes as diverse as cell proliferation, differentiation, and axonal targeting. The goal of this study was to better understand the mechanisms by which Fgf signaling regulates pattern formation and embryogenesis. I addressed this question on several levels, investigating the extent of intracellular signaling (MAPK activation) relative to sources of Fgf expression, and the transcriptional responses of cells to Fgf signaling during embryogenesis. By a macroarray analysis, I identified putative transcriptional targets of Fgf signaling in late gastrulation, providing a set of molecules that are likely to act as functional players in relaying the patterning information encoded by Fgf signals. Among those are the secreted signaling molecules Chordin and Wnt8, as well as Isthmin, a novel secreted molecule that I found capable to interfere with anterior embryonic patterning. In addition, I identified two ETS domain transcription factors, Erm and Pea3, which constitute bona fide integrators of FgfR signaling. By gain- and loss-of-function studies, I demonstrate that transcript levels of erm and pea3 are tightly regulated by Fgf signaling. Detailed analysis of the expression patterns of erm and pea3 along with other Fgf target genes also provides evidence for a differential read-out of Fgf concentration in the embryo, consistent with a role of Fgf as a vertebrate morphogen. The discovery of novel molecular components downstream of Fgf receptor activity paves a way to characterize previously unknown or underestimated developmental roles of Fgfs in the molecular patterning of the forebrain, the eye and parts of the neural crest.