Inflammation-related neutrophil proteases, cathepsin G and elastase, function as insulin-like growth factor binding protein proteases

Over the past few years, several proteolytic enzymes have been identified as insulin-like growth factor binding protein (IGFBP) proteases. It has been suggested that proteolytic cleavage of IGFBPs is associated with regulation of the proliferative effects of IGFs on their target cells. In this study, we have demonstrated that two neutrophil proteases, cathepsin G and elastase, effectively cleave IGFBPs in vitro and in vivo at concentrations lower than previous described IGFBP proteases. Purified leukocyte cathepsin G and elastase cleaved all six well-characterized IGFBPs into distinct fragments in a concentration-dependent manner. Under similar experimental conditions, cathepsin G preferentially cleaved IGFBP-5, followed by BP-2, BP-3, BP-4, BP-1, and BP-6. In comparison, elastase equally preferred IGFBP-3 and IGFBP-4, followed by BP-1, BP-5, BP-6, and BP-2. Proteolysis of rh125I-IGFBP-3 by cathepsin G was blocked by α1-antichymotrypsin, while elastase proteolytic activity was blocked by α1-proteinase inhibitor as expected. Elastase, but not cathepsin G, cleaved free IGF-I into a smaller molecular weight fragment in vitro, possibly designating unique functions for each protease within the IGF axis. Sequence analysis of IGFBP-3 fragments produced by cathepsin G and elastase demonstrated that each protease cleaved IGFBP-3 at unique sites within its midregion. More importantly, extracts from purified neutrophils have demonstrated significant proteolytic cleavage of IGFBP-3 that resembles elastase proteolysis of IGFBP-3. Recent studies using a monocyte-like cell model have also shown significant cleavage of IGFBP-3. These in vitro and in vivo data suggest that the neutrophil proteases, cathepsin G and elastase, in addition to their previously described functions as extracellular matrix-degrading enzymes, may potentially act as IGFBP proteases involved in regulation of IGFs and IGFBPs during inflammation and wound healing.