Prolyl oligopeptidase induces angiogenesis both in vitro and in vivo in a novel regulatory manner

Background and purpose: A serine protease, prolyl oligopeptidase (POP) has been reported to be involved in the release of the pro-angiogenic tetrapeptide acetyl-N-Ser-Asp-Lys-Pro (Ac-SDKP) from its precursor, 43-mer thymosin β4 (Tß4). Recently, it was shown that both POP activity and the levels of Ac-SDKP are increased in the malignant tumours. The aim of this study was to clarify the release of Ac-SDKP, and test if POP and a POP inhibitor, KYP-2047, can affect the angiogenesis.
Experimental approach: We used HPLC for bioanalytical and enzyme immunoassay for pharmacological analysis. Human umbilical vein endothelial cells in “tube formation” assay and Matrigel plug assay in adult male rats were used as angiogenesis assays. Moreover, a colocalization of POP and blood vessels was studied.
Key results: We showed the sequential hydrolysis of Tβ4: the first step hydrolysis by proteases to <30-mer peptides is followed by an action of POP. Unexpectedly, POP inhibited the first hydrolysis step, revealing a novel regulation system. POP with Tβ4 significantly induced, while KYP-2047 effectively prevented angiogenesis in both models compared to Tβ4 addition itself. POP and endothelial cells were abundantly co-localized in vivo.
Conclusions and implications: We have now revealed that POP is a second step enzyme in the release of Ac-SDKP from Tβ4, and it has novel autoregulatory effect in the first step. Our results also stress a role of Ac-SDKP in angiogenesis, and suggest that POP has a pro-angiogenetic role via the release of Ac-SDKP from its precursor Tβ4 and POP inhibitors can block this action.