Matrix metalloproteinase dependent and independent collagen degradation.

Cleavage of the fibrillar collagens occurs during physiological conditions, as well as pathological conditions. The resistance of the fibrillar collagens to degradation is due to their rigid and compact structures. There are only a limited number of proteinases that have the capability to initiate the cleavage of the fibrillar collagens. These include some of the matrix metalloproteinases (MMPs) and cathepsins, as well as a few serine proteinases. The MMPs have long been implicated in the collagen degradation and remodeling that occurs at physiological pHs. The cathepsins, on the other hand, have been implicated in the collagen cleavage that occurs at acidic pHs, particularly the collagen degradation that is mediated by osteoclasts. In addition to the MMPs, a few serine proteinases have been implicated in the collagen degradation that occurs at neutral pH. The characteristics that contribute to the resistance of the fibrillar collagens to cleavage are discussed along with the MMPs, cathepsins, and serine proteinases that can cleave these collagens.