The expression of matrix metalloproteinases (MMPs) in human odontoblasts

Matrix metalloproteinases (MMPs) are a group of enzymes that together can degrade practically all extracellular matrix proteins, including native collagen. They participate in normal tissue formation and remodeling, and the expression of different MMPs may be dramatically upregulated during various pathological conditions, e.g, inflammation. Knowledge of the expression of MMPs by the cells of the human dentin-pulp complex is still limited. Human odontoblasts express MMP-2 (gelatinase A) during tooth development, and MMP-2 is believed to participate in basement membrane degradation. Mature human odontoblasts express at least MMP-8 (collagenase-2), MMP-2, MMP-9 (gelatinase B) and MMP-20 (enamelysin). MMP-20 is particularly interesting, since presently it has been shown to be expressed only in dental cells (ameloblasts, odontoblasts and to a lesser extent in pulpal fibroblasts) and odontogenic tumors. In addition, growth factor TGF-beta1 seems to differentially regulate odontoblast MMP expression: MMP-9 expression is up-regulated, MMP-8 expression is dramatically down-regulated, while the effects on MMP-2 or -20 expression are minor or non-existent. Recent findings also indicate that human odontoblasts may express several other members of the MMP-family. MMP-2 and a yet unidentified collagenase have also been isolated from mineralized dentin matrix. Even though the physiological roles of MMPs expressed by odontoblasts are currently unknown, it may be speculated that they could participate in dentin matrix organization prior to mineralization, regulation of matrix mineralization, intratubular dentin formation, or remodelling of the pulp connective tissue. MMPs may also have a marked role in the pathogenesis of dentinal caries.