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, and function of CKs in keratinized epithelial cells, the main features being the tight coupling between CK pair switch and cell terminal differentiation (protection barrier) and the vital role of CK IFs in cell mechanical integrity. However, the picture about nonkeratinizing epithelia, like the hepatic tissue, remains quite unclear. The liver forms a multicellular system, where parenchymal cells (i.e., hepatocytes) exert 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 accumulated 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 epidermis. Moreover, the distribution of the CK IF networks present in the different cell types varies a lot and can often be linked to the cell specialization. However, the function(s) played by these IF proteins in this multicellular tissue remains a major issue. The use of new experimental approaches, largely based on gene transfer technology, indicates that it is quite subtle.