Matrix metalloproteinase inhibitors as investigative tools in the pathogenesis and management of vascular disease

Abstract

Matrix metalloproteinases (MMPs) are proteolytic enzymes that degrade various components of the extracellular matrix (ECM). MMPs could also regulate the activity of several non-ECM bioactive substrates and consequently affect different cellular functions. Members of the MMPs family include collagenases, gelatinases, stromelysins, matrilysins, membrane-type MMPs, and others. Pro-MMPs are cleaved into active MMPs, which in turn act on various substrates in the ECM and on the cell surface. MMPs play an important role in the regulation of numerous physiological processes including vascular remodeling and angiogenesis. MMPs may also be involved in vascular diseases such as hypertension, atherosclerosis, aortic aneurysm, and varicose veins. MMPs also play a role in the hemodynamic and vascular changes associated with pregnancy and preeclampsia. The role of MMPs is commonly assessed by measuring their gene expression, protein amount, and proteolytic activity using gel zymography. Because there are no specific activators of MMPs, MMP inhibitors are often used to investigate the role of MMPs in different physiologic processes and in the pathogenesis of specific diseases. MMP inhibitors include endogenous tissue inhibitors (TIMPs) and pharmacological inhibitors such as zinc chelators, doxycycline, and marimastat. MMP inhibitors have been evaluated as diagnostic and therapeutic tools in cancer, autoimmune disease, and cardiovascular disease. Although several MMP inhibitors have been synthesized and tested both experimentally and clinically, only one MMP inhibitor, i.e., doxycycline, is currently approved by the Food and Drug Administration. This is mainly due to the undesirable side effects of MMP inhibitors especially on the musculoskeletal system. While most experimental and clinical trials of MMP inhibitors have not demonstrated significant benefits, some trials still showed promising results. With the advent of new genetic and pharmacological tools, disease-specific MMP inhibitors with fewer undesirable effects are being developed and could be useful in the management of vascular disease.

Fig. 1

Structure of MMPs. Typically, MMPs consist of a propeptide of about 80 amino acids, a catalytic metalloproteinase domain of about 170 amino acids, a linker peptide (hinge region) of variable lengths and a hemopexin domain of about 200 amino acids. The catalytic domain contains the Zn²⁺ binding motif HEXXHXXGXXH. Matrilysins are exceptions as they lack the linker peptide and the hemopexin domain. Membrane-bound MMPs (MT-MMPs) have a furin-like proprotein convertase recognition sequence at the C-terminus of the propeptide.

Fig. 1

Structure of MMPs. Typically, MMPs consist of a propeptide of about 80 amino acids, a catalytic metalloproteinase domain of about 170 amino acids, a linker peptide (hinge region) of variable lengths and a hemopexin domain of about 200 amino acids. The catalytic domain contains the Zn²⁺ binding motif HEXXHXXGXXH. Matrilysins are exceptions as they lack the linker peptide and the hemopexin domain. Membrane-bound MMPs (MT-MMPs) have a furin-like proprotein convertase recognition sequence at the C-terminus of the propeptide.

Fig. 1

Structure of MMPs. Typically, MMPs consist of a propeptide of about 80 amino acids, a catalytic metalloproteinase domain of about 170 amino acids, a linker peptide (hinge region) of variable lengths and a hemopexin domain of about 200 amino acids. The catalytic domain contains the Zn²⁺ binding motif HEXXHXXGXXH. Matrilysins are exceptions as they lack the linker peptide and the hemopexin domain. Membrane-bound MMPs (MT-MMPs) have a furin-like proprotein convertase recognition sequence at the C-terminus of the propeptide.

Fig. 2

Collagenolytic activity of MMPs. Collagenases unwind triple helical collagen before they hydrolyze the peptide bonds breaking down collagen into 3/4 and 1/4 fragments. The MMPs hemopexin domains are essential for cleaving native fibrillar collagen.

Fig. 3

Endogenous inhibition of MMPs by TIMPs. The TIMP molecule wedges into the active-site cleft of MMP in a manner similar to that of the substrate. Cys1 is instrumental in chelating the active-site Zn²⁺ with its N-terminal α-amino group and carbonyl group, thereby expelling the water molecule bound to the catalytic Zn²⁺.