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Thyroid carcinomas are the most frequent malignancy of endocrine organs. They can be classified in four entities of various differentiation and prognosis. Follicular, papillary, anaplastic and medullary carcinomas. An important factor of cancer progression was recognized in the capacity of thyroid carcinoma cells to angiogenesis, procuring them an unlimited growth and the invasion of the surrounding tissue. For a great number of carcinomas the angiogenesis is a capital pathophysiological step. For this reason the antiangiogen therapy places itself more and more in the center of scientific interest.
The aim of this thesis was to give a survey of the interaction of integrins, VEGF and the extracellular matrix (ECM) by using a cell culture model of different thyroid carcinomas. A better understanding of the interaction of these factors essential to the angiogenesis is the precondition for the development of effective therapy strategies.
In the test series the Hurthle cell line XTC, the follicular cell line HTC+, the anaplastic cell line and the cell line Hth74 as well as a cell line isolated from natural thyroid tissue (Thy1) were examined. The spectrum of the integrins of the oncogenic (XTC, HTC+, Hth74) and the normal cells (Thy1) was found out by immunhistochemical tests.
All cells expressed a great spectrum of integrins whereas in comparison the cell line of normal tissue contains the smallest spectrum. Histiotypical associated similarities appear among the cancer cell lines, out of which the undifferentiated (Hth74) shows the largest spectrum. These results suggest that the thyroid cancer cells dispose of an enlarged possibility of adhesion to ECM. It is assured that the spectrum and the extent of the integrin expression of cancer cells correlate with their abilities of proliferation and invasion. This change of their integrin expression has radical effects on their biological properties. In this context the linking to Fibronectin seems to play an important role.
Two cancer cell lines (XTC, HTC+) were incubated on different matrix components and the VEGF expression was examined via ELISA. Fibronectin (FN) was the only of the tested matrix proteins that induced an increase of VEGF secretion. The stimulation of the VEGF secretion by FN confirms the important role of FN in angiogenesis which is also described in the literature. The incubation of the two cancer cell lines (XTC, HTC+) without matrix components with a choice of the best examined and available integrin antibodies at this time had no effect on the VEGF secretion. Since the blockade of integrins had no reducing effect on the high VEGF basic output of thyroid cancer cells, the regulation of the VEGF secretion is assumed to be very complex and does not only depend on integrin signalling.
In a further examination of XTC cells, integrin antibodies and integrin blocking RGD sequences (RGDS) were used to suppress the stimulating effect of FN on VEGF. The RGD sequences increased the intra- and extra cellular VEGF concentration of the XTC cells proportional to the RGDS-concentration. The FN specific integrin antibodies had no influence on the VEGF secretion. Since the RGDS led to an increase of the VEGF concentration and not to a suppression, as expected, it must be discussed, that they dispose of an intrinsic activity with initiation of intracellular signal pathways in the sense of an “Outside-in” signalling.
The VEGF-synthesis of thyroid cancers seems thus to be influenced by the extra cellular matrix (FN) via integrins. The influence of VEGF on the integrin spectrum of XTC cells was analysed by FACS. There was shown no change of the integrin expression by VEGF. The importance of growth factors on the integrin expression is demonstrated in the literature. The examination of the influence of VEGF on the integrin expression of the XTC cells in the sense of an “Inside-out“ control loop could not prove an autocrine control loop in spite of the well known close interaction of integrins and VEGF receptors.
It could be shown that thyroid cancers similar to other cancers depend on the expression of integrins and their interaction with the extra cellular matrix (ECM) in order to use angiogenesis.
This thesis represents a detailed examination of a great number of integrins and is one of the first which has tried to exert influence on the VEGF secretion of thyroid cancer cells via RGDS and integrin antibodies. Further examinations are needed to explore these detailed interactions. Inhibitors of the integrin mediated angiogenesis like Volociximab® will be a focus of the medical cancer therapy in the future. Today they are already a useful option of therapy besides the chemotherapy.