Expression and functional properties of antibodies to tissue inhibitors of metalloproteinases (TIMPs) in rheumatoid arthritis

The matrix metalloproteinases (MMPs) are a family of zinc-dependent endopeptidases regulating the breakdown of extracellular matrix and are thereby essential for physiological processes of embryonic development, morphogenesis, and tissue remodelling and resorption, but are also of crucial importance for pathological conditions including inflammation, tumour growth, and metastasis [1-3]. Extracellularly, the activity of MMPs is regulated by their endogenous inhibitors, tissue inhibitors of metalloproteinases (TIMPs) [4]. The TIMP family known at present consists of four distinct members (TIMPs 1 to 4) (Table 1). All of these except TIMP-4 are expressed in most tissues and body fluids. TIMP-4 has a tissue-specific distribution, being localized in brain, striated muscles, and ovaries. The expression of TIMPs is typically induced by external stimuli such as certain inflammatory cytokines (IL-6, IL-1β) and by certain growth factors.Extracellularly, TIMPs inhibit MMP activity by forming high-affinity noncovalent complexes with MMPs. The amino-terminal domain of TIMP binds the active site of MMPs, inhibiting their proteolytic activity. The carboxy-terminal domain of certain TIMPs has also the ability to form complexes with proenzymes (proMMPs) regulating the MMP activation process [4]. The balance between the inhibitory and activating properties of TIMP-1 and TIMP-2 defines their specificity regarding different MMPs. However, certain differences in TIMPs' specificities have been recognized. Indeed, TIMP-1 is a preferential inhibitor of soluble MMPs, while TIMP-2 and TIMP-3 are also efficient inhibitors of the membrane-bound MMPs. TIMP-3 stretches its inhibitory activity to include, besides MMPs, also some members of the ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) family, inhibiting aggrecanases and TNF-α-converting enzyme. Although TIMP-dependent inhibition of MMPs is the most-studied property of TIMPs, other, unexpected functions of these prote