ReviewClinical utility of cytokeratins as tumor markers
Introduction
Filamentous structures have long been observed in the cell cytoplasm and nucleus, but only during recent years has knowledge regarding the actual function of the complex cytoskeleton dramatically increased. In eukaryotic cells, the cytoskeleton is composed of three different types of morphologically distinct filamentous structures: microfilaments, intermediate filaments (IF), and microtubules [1]. The integrated cytoskeletal network formed by the three filament systems is responsible for the mechanical integrity of the cell and is a critical participant in several cellular processes, such as cell division, motility, and cell–cell contact. The intermediate filament (IF) protein family is by far the most complex of the filamentous structures, including several hundred different members. Based on their characteristics, for example, sequence similarities and expression, a classification system divided into several groups has been implemented [1]. Intermediate filament types I and II constitute the cytokeratins (acidic and basic proteins, respectively), while the type III IF group includes desmin, vimentin, and glial fibrillary acidic proteins. Type IV includes the neurofilament proteins (NF-L, NF-M, and NF-H) and internexin, while type V proteins are known as nuclear lamins, exclusive to the cell nuclei. The remaining IF proteins, sometimes called type VI, include filensin and phakinin [2], [3].
The epithelial cytokeratins (IF types I and II) are conserved phylogenetically and are closely related, biochemically and immunologically. At present, more than 20 different cytokeratins are known and are divided into types I and II based on sequence homology [2], [4], [5]. Cytokeratins 1–8 constitute the type II group (53–68 kDa, neutral to basic protein components), while cytokeratins 9–20 constitute the type I group (40–56 kDa, acidic proteins).
This dual nature of the cytokeratins is functionally important as the cytokeratin proteins assemble into obligate noncovalent heterodimers containing one cytokeratin protein of type I and one cytokeratin protein of type II in stochiometric amounts. A common example of the heteropolymer complex is the combination of cytokeratins 8 and 18 [6]. The heterodimers are further organized into filamentous structures by alignment side-by-side to form tetramers and through further end-to-end associations higher cytokeratin polymers with coiled-coil dimeric structures [1], [3], [7], [8]. In knockout mice, it has been shown that cytokeratin 18 can be replaced by cytokeratin 19 and, together with cytokeratin 8, provide a normal cytoskeleton [2].
The cytokeratins are encoded by a large multigene family of approximately 50 different members [8]. Amino acid sequence analysis of the encoded individual filament proteins reveals a relatively weak relationship between the cytokeratins and the other intermediate filament proteins [9]. But as for all other IF proteins, the cytokeratins exhibit a characteristic structure harboring three major domains: a nonhelical N-terminal region, a predominantly helical central rod, and a nonhelical C-terminal segment [3]. The helical rodlike domain (mostly of alpha-helical structure) constitutes a conserved sequence of about 300–320 amino acid residues and can be subdivided into four different domains: coil 1A, 1B, 2A, and 2B. The amino acid composition of the helical domains appears to be almost constant in size and contains repeated sequences of amino acid residues with a similar distribution of apolar amino acids and alternating charged amino acid residues. The helical segments are separated by significantly less conserved short linker regions, named L1, L1-2, and L2 [5], [8].
Section snippets
Expression of cytokeratins
The expression of cytokeratins varies with epithelial cell type, extent of differentiation, and development of the tissue. During the transformation of normal cells into malignant cells, the cytokeratin patterns are usually maintained, and this property has enabled cytokeratins to be applied as tumor markers [10], [11]. Posttranslational modifications of the central rodlike domain are relatively rare, but a number of such modifications have been reported for both the N- and the C-terminal
Cytokeratins in apoptosis
Recently, there has been considerable interest in the role of cytokeratin proteins during apoptosis, the well-choreographed sequence of events leading to structural and biochemical changes and eventually to phagocytosis [15]. Failure of cells to undergo appropriate apoptotic cell death is involved in a variety of human diseases including cancer.
Apoptosis can be initiated by different complex pathways, the majority involving caspase activation. Caspases-2, -8, -9, and -10 are initially activated
Cytokeratin antibodies
The immunoreactivity patterns of 30 different monoclonal antibodies with epitope specificity directed towards cytokeratins 8, 18, and 19 have recently been studied [21]. Six antibodies bound selectively to cytokeratin 8, while another 14 and 10 antibodies bound to cytokeratins 18 and 19, respectively. Immunometric assays using different antibody combinations were used to study the topography of antibody binding sites individual cytokeratin proteins isolated from the cytoskeleton on purified
Cytokeratins as serum tumor markers
The clinical usefulness of cytokeratin tumor markers is well established for monitoring patients with epithelial cell carcinomas. The cytokeratins reflect tumor cell activity. Thus, by following patients with repeated assays of a cytokeratin marker in combination with a marker that reflects tumor burden, the oncologist can obtain critical information about tumor growth activity. This applies particularly to cases where the tumor is already clinically confirmed. The ability of cytokeratin
Cytokeratin markers in clinical application
The cytokeratin tumor markers assays described above have been examined in many different types of epithelial cell carcinomas. Although based on detection of the same type of proteins in serum, the individual cytokeratin immunoassays may give different profiles of reactivity. This reflects each assay's uniqueness, concerning both the different detector antibodies employed and the fact that release of cytokeratin fragments into the circulation may differ from one cytokeratin to another. Thus, as
Conclusion
In summary, cytokeratins are very useful tumor markers in oncology. Their prime utility is in monitoring treatment and in providing early indications on recurrence and tumor progression. Using cytokeratins in the oncology clinic enables earlier and more effective treatment.
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