Cleavage of Human Apolipoprotein(a) Coated on the Surface of Consituents of the Vascular Extracellular Matrix by Pancreatic Elastase and Metalloproteinase-12. Different Partitions Between Cleaved NTerminal and C-Terminal Domains
In solution phase studies we previously showed that enzymes of the elastase and metalloproteinase (MMP) families cleave human apolipoprotein(a), apo(a), in the linker region between kringles (K) IV-4 and IV-5 generating two fragments F1 and F2 , the N- and C-terminal domains, respectively. Since, apo(a) binds avidly to the surface of components of the extracellular matrix, ECM, we investigated its proteolytic cleavage after immobilization on the surface of selected proteoglycans (PG) and fibronectin and examined the partition of the resulting fragments between surface and solution phase.
Decorin, biglycan and fibronectin were coated onto microtiter plates and then overlayed with apo(a) to surface saturation. Subsequently, the system was subjected to limited proteolysis either by pancreatic elastase or MMP-12, and the plates eluted first with Tris buffered saline, (TBS) and then 6M guanidine hydrochloride (GdHCl) solution. By Western blot analyses the first TBS eluate and virtually less the subsequent ones, contained F1 but not F2. The latter was also absent in the concentrates of GdHCl eluates. We were also unable to elute F2 when it was directly coated on the PGs or fibronectin surfaces. None of the eluates contained immunodetectable PGs or fibronectin either intact or fragmented.
In conclusion, apo(a), immobilized on the surface of components of the vascular ECM is cleaved, as in solution phase, by representative enzymes of the elastase and MMP families. The cleaved C-terminal F2 remains attached to the surface whereas F1 is readily released into the aqueous phase. We speculate that as in vitro, Lp(a) binds in vivo to the ECM via the C-terminal domain of apo(a).
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