Bone proteinases

This chapter surveys our knowledge of the functions of the proteinases expressed in bone. The osteoclast secretes abundant lysosomal cysteine proteinases, especially cathepsin K, and produces some of the neutral proteinases, e.g., matrix metalloproteinase-9. However, osteoblasts and osteocytes, like their related cells, fibroblasts, are able to secrete a host of proteinases, including neutral proteinases such as serine proteinases, plasminogen activators, and metalloproteinases such as matrix metalloproteinase-13, as well as lysosomal proteinases, e.g., cathepsins. Thus, osteoblasts and osteocytes, like fibroblasts, have the capacity not only to synthesize a range of matrix proteins, including type I collagen, but also to remodel their own extracellular matrix via the secretion of a range of proteinases.