Human cytokeratins : their use as targets in cancer management
Abstract: Cytokeratins, biochemically related to intermediate filaments (IF), form an intracellular network of filaments which contributes to the mechanical stabilizing of the cell. 19 individual polypeptides, divided into two groups, constitute the cytokeratin family.Each type of epithelial cell can be characterized by its content of cytokeratin polypeptides since the expression pattern varies with the type of epithelium. During the transformation of epithelial cells into tumours, the cytokeratin patterns are usually maintained. This property has enabled cytokeratins to be used as tumour markers, especially for tumours not easily classified.In order to evaluate cytokeratins as tumour markers, we have generated a battery of monoclonal antibodies (MAbs) against cytokeratins extracted from carcinomas. Five antibodies were further characterized. All reacted with cytokeratin 8 (CK 8) amongst others, but only one, TS 1, was specific for this keratin.CK 8 is one of the most abundant keratins in carcinomas. It is released into necrotic areas and can be found intratumourally and in the blood, circulating as partially degraded complexes and can as such be used as a tumour marker. The deposits of cytokeratins in tumours enable these structures to be used as targets for the management of tumours, i.e. by radioimmunodetection and radioimmunotherapy.To assess the value of circulating CK 8 as a serological tumour marker, an enzyme-linked immunosorbent assay (ELISA) was developed, employing two MAbs reactive with different epitopes on CK 8, i.e. TS 3 and TS 4. Serological determinations of CK 8 in cancer of the colon, pancreas and ovary showed that such measurements may be of value for the management of these types of cancer.For in vivo studies of cytokeratins as tumour markers, a nude mouse model carrying HeLa cell tumours was used to evaluate whether cytokeratins can play a role in radioimmunodetection and radioimmunotherapy. In such experiments, TS 1 demonstrated its usefulness in both techniques. The accumulation in the tumour was visualized by conjugated radionuclides and the biological half-life for the antibody was estimated to over 600 h which makes this MAb a suitable reagent for immunotherapy.Immunotherapy experiments in the same animal model showed that a modified approach should be considered if cytokeratins are to be used as the targets. Since antibodies accumulate mainly in necrotic areas, beta-emitters can not be used as in other studies on potential therapy methods. Instead, 125-1 which emits Auger electrons and has a relatively low energy gamma-radiation was investigated. Tumour growth was retarded with both 131-1- and 125-1- conjugated MAbs studied.This thesis indicates that simple epithelial cytokeratins (CK 7, 8, 18, 19) may be useful targets both for radioimmunodetection and radioimmuno-therapy of cancer.
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