CYTOKERATIN EXPRESSION, FIBRILLAR ORGANIZATION, AND SUBTLE FUNCTION IN LIVER-CELLS

Cytokeratins (CKs) constitute a diverse group of intermediate filament (IF) proteins, expressed as pairs in keratinized and nonkeratinizing epithelial cells. Much is known now about the expression, assembly, an d function of CKs in keratinized epithelial cells, the main features b eing the tight coupling between CK pair switch and cell terminal diffe rentiation (protection barrier) and the vital role of CK Ifs in cell m echanical integrity. However, the picture about nonkeratinizing epithe lia, like the hepatic tissue, remains quite unclear. The Liver forms a multicellular system, where parenchymal cells (i.e., hepatocytes) exe rt diverse metabolic function(s) and nonparenchymal epithelial cells ( e.g., biliary epithelial cells) usually serve structural (or accessory ) purposes. In terms of differential CK gene expression, the data accu mulated so far demonstrated that parenchymal cells can contain as few as one single CK pair, whereas nonparenchymal cells contain more than two CKs, one of them being a representative of those found in epidermi s. Moreover, the distribution of the CK LF networks present in the dif ferent cell types varies a lot and can often be linked to the cell spe cialization. However, the function(s) played by these IF proteins in t his multicellular tissue remains a major issue. The use of new experim ental approaches, largely based on gene transfer technology, indicates that it is quite subtle.