Two papers in this issue of Gutfocus on RNA and protein levels of matrix metalloproteinases (MMPs) in diagnostic biopsies of patients with inflammatory bowel disease (see pages 57 and 63).1 ,2 Both reports are congruent in that in ulcerated lesions, MMP-3 mRNA, which encodes a key enzyme in matrix degradation, is highly increased (up to 15- and 100-fold, depending on the methodology used), paralleled by a more moderate increase in MMP-3 protein levels, whereas there is little change in levels of the physiological inhibitor of most MMPs, tissue inhibitor of metalloproteinases 1 (TIMP-1). Thus the elevated ratio of MMP-3 over TIMP-1 would favour matrix degradation. Similar data were obtained for MMP-1/TIMP-1 (degradation of fibrillar collagens), MMP-2/TIMP-2 (degradation of basement membrane collagen and denatured collagens), and for membrane-type MMP-1 (MMP-14) which can activate MMP-2.2

Whereas matrix dissolution is plausible for ulcerative colitis (UC), it comes as a surprise that comparable levels of expression for MMPs and MMP/TIMP-1 ratios were found in ulcers of Crohn's disease (CD) which rather leads to intestinal fibrosis. As expression of several collagens which are the major components of scar tissue was also not different between lesions of CD and UC,3 the factors that determine …