Objective: We previously demonstrated that secreted proform(s) of the neutrophil serine protease PR3 (proteinase 3) can down-modulate the fraction of normal human colony-forming unit granulocyte-macrophage (CFU-GM) in S-phase, whereas PR3 extracted from mature neutrophils lacks this ability. The objective of this study was to characterize the structural and functional dissociation between secreted proforms and granule-stored mature forms and to extend the investigation to other related hematopoietic serine proteases.
Materials and methods: Conditioned media containing secreted proteases from transfectant cell lines with stable expression of human PR3, neutrophil elastase, cathepsin G, azurocidin, and granzymes A, B, H, K, and M were tested for their ability to reduce the fraction of normal human CFU-GM in S phase. Furthermore, recombinant PR3, azurocidin, and granzyme B with defined N-terminal propeptides, and the respective mature forms without propeptide, were functionally characterized.
Results: In addition to PR3, secreted proforms of azurocidin and granzymes A, B, H, K, and M, but not cathepsin G or neutrophil elastase, have S-phase reducing activity. This activity is restricted to the dipeptide proforms, whereas mature forms without propeptide have no S-phase reducing activity. On the other hand, only the mature forms of PR3 and granzyme B could bind the serine protease inhibitor diisopropylfluorophosphate (DFP), or aprotinin in the case of azurocidin. We also demonstrate that granulocyte colony-stimulating factor-stimulated CD34+ cells and interleukin-2-stimulated lymphocytes secrete active proforms of PR3 and granzyme B, respectively.
Conclusion: These results demonstrate distinctive functional and conformational differences between proforms and mature forms of these hematopoietic serine proteases and suggest novel growth regulatory mechanisms in granulopoiesis.