Connective Tissue Formation in Wound Healing - E-thesis

activate the latent MMP-2 by proteolysis. Excess TIMP-2 interferes with this activation
mechanism by binding and inhibiting all available MT1-MMP molecules (252).
The major physiologic inhibitors of the MMPs are the family of specific tissue inhibitor of
MMP (TIMP), and α-2 macroglobulin, which may be important in controlling overall
proteolytic activity (224). In addition, thrombospondins can inhibit MMP-2 and 9 activation
and induce their clearance through scavenger receptor-mediated endocytosis (278). The TIMP
family comprises at present four structurally related members, TIMP-1, 2, and 3, and 4. They
are composed of N-and C-terminal domains, which both are stabilized by three disulfide bonds
between six conserved cysteine residues. The larger N-terminal domain is important for MMP
inhibition. All TIMPs can inhibit most MMPs by tight non-covalent binding to their active site
in a 1:1 molar ratio resulting in loss of proteolytic activity (274). A unique characteristic of
MMP-2 and MMP-9 is the ability of their zymogens to form tight non-covalent and stable
complexes with TIMPs. It has been shown that pro-MMP-2 binds TIMP-2 and pro-MMP-9
binds TIMP-1 (279, 280). This interaction has been suggested to provide an extra level of
regulation by potentially preventing activation. Both TIMP-1 and -2 have mitogenic activities
on a number of cell types, whereas overexpression of these inhibitors reduces tumor cell
growth. TIMP-1 and -2 are secreted in soluble form, whereas TIMP-3 is associated with the
ECM (274).
The role of MMPs and TIMPs in wound healing
The degradation of extracellular matrix is required to remove damaged tissue and provisional
matrixes and to permit vessel formation and cell migration during wound healing. These
remodeling processes involve the action of extracellular proteinases (36). MMP-1 is present in
the wound environment and it is produced by fibroblasts, macrophages, and other cells within
the granulation tissue (281). Basal keratinocytes at the migrating front of re-epithelialization
are the predominant source of MMP-1 during active wound repair (282, 283). Keratinocytes
seems to be a major participant in the degradation of extracellular matrix during wound healing
and fibroblasts. Macrophages, and other cells within the dermis release MMP-1 only at certain
stages of repair (36). MMP-2 and MMP-9 may be important in detaching keratinocytes from
the basement membrane prior to lateral movement at the beginning of epithelial wound
healing, and both MMP-2 and MMP-9 are transiently seen in epidermal cells shortly after
wounding (284, 285). In chronic wounds, however, these gelatinases are not actively
synthesized by epidermal cells and are only occasionally expressed by either resident dermal or
40