Collagen breakdown products as markers of fibrosis and cirrhosis

Collagens are extracellular matrix proteins that maintain the structural integrity of various tissues. Because normal collagen contained in connective tissues and bone is known to undergo metabolic alterations in a wide variety of diseases, there has been a great deal of interest in how measurements of protein fragments derived from collagen biosynthesis and breakdown could be used as markers of pathological collagen turnover. Immunological assays are now available for several proteins representing genetically distinct collagen types. Some of these, like the aminoterminal propeptide of type III procollagen (PIIINP), have reached a level where the measurements are part of normal routine in the treatment and assessment of patients with conditions involving excessive fibrogenesis. To date, PIIINP appears to be superior to the other markers available in terms of sensitivity and informative value for a clinician. New developments in this field may yield methods by which increased rates of collagen biosynthesis can be differentiated from collagen breakdown. Moreover, increasing knowledge on the endothelial cell function may open new avenues with respect to the interpretation of the assay results.