Matrix Metalloproteinase Inhibitors as Investigational and Therapeutic Tools in Unrestrained Tissue Remodeling and Pathological Disorders

Abstract

Matrix metalloproteinases (MMPs) are zinc-dependent proteolytic enzymes that degrade various proteins in the extracellular matrix (ECM). MMPs may also regulate the activity of membrane receptors and postreceptor signaling mechanisms and thereby affect cell function. The MMP family includes collagenases, gelatinases, stromelysins, matrilysins, membrane-type MMPs, and other MMPs. Inactive proMMPs are cleaved by other MMPs or proteases into active MMPs, which interact with various protein substrates in ECM and cell surface. MMPs regulate important biological processes such as vascular remodeling and angiogenesis and may be involved in the pathogenesis of cardiovascular disorders such as hypertension, atherosclerosis, and aneurysm. The role of MMPs is often assessed by measuring their mRNA expression, protein levels, and proteolytic activity using gel zymography. MMP inhibitors are also used to assess the role of MMPs in different biological processes and pathological conditions. MMP activity is regulated by endogenous tissue inhibitors of metalloproteinases (TIMPs), and the MMP/TIMP balance could determine the net MMP activity, ECM turnover, and tissue remodeling. Also, several synthetic MMP inhibitors have been developed. Synthetic MMP inhibitors include a large number of zinc-binding globulins (ZBGs), in addition to non-ZBGs and mechanism-based inhibitors. MMP inhibitors have been proposed as potential tools in the management of osteoarthritis, cancer, and cardiovascular disorders. However, most MMP inhibitors have broad-spectrum actions on multiple MMPs and could cause undesirable musculoskeletal side effects. Currently, doxycycline is the only MMP inhibitor approved by the Food and Drug Administration. New generation biological and synthetic MMP inhibitors may show greater MMP specificity and fewer side effects and could be useful in targeting specific MMPs, reducing unrestrained tissue remodeling, and the management of MMP-related pathological disorders.

Keywords: Aneurysm; Angiogenesis; Atherosclerosis; Extracellular matrix; Hypertension; Remodeling; TIMP.

Fig. 1

TIMP-MMP Interaction. TIMP is a ~190 aa protein, with an N-terminal domain (loops L1, 2, and 3) and C-terminal domain (loops L4, 5 and 6), which fold independently as a result of 6 disulfide bonds between 12 specific Cys residues. The N-terminal Cys1-Thr-Cys-Val4 and Glu67-Ser-Val-Cys70 are connected via a disulfide bond between Cys1 and Cys70 and are essential for MMP inhibition, as they enter the MMP active site and bidentately chelate the MMP Zn²⁺. The carbonyl oxygen and α-amino nitrogen in the TIMP Cys1 coordinate with the MMP Zn²⁺, which is localized in the MMP molecule via the 3 histidines in the HEXXHXXGXXH motif. The TIMP α-amino group then expels Zn²⁺-bound H₂O by binding the MMP H₂O binding site and forming a H bond with carboxylate oxygen from conserved MMP Glu202 (E in the HEXXHXXGXXH sequence). TIMP-1 and MMP-3 are used as prototypes. The amino acids involved in Zn²⁺- and pocket-binding may vary with different MMPs and TIMPs.

Fig. 2

MMP Inhibition by TIMP. TIMP Thr2 side chains enter the MMP S1’ pocket in a manner similar to that of a substrate P1’ substituent, largely determining the affinity to MMP. Thr2 –OH group could also interact with Glu202, further contributing to expelling Zn²⁺-bound H₂O and preventing substrate degradation. Additionally, the TIMP Cys3, Val4 and Pro5 interact with MMP S2’, S3’, and S4’ pockets in a P2’, P3’, and P4’-like manner, further preventing substrate binding or degradation. TIMP-1 and MMP-3 are used as prototypes. The amino acids involved in Zn²⁺- and pocket-binding may vary with different MMPs and TIMPs.