PRCP: a key to blood vessel homeostasis

In this issue of Blood, Adams and coworkers identify a critical role of the endothelial peptidase PRCP in the maintenance of the physiologic state of the vasculature.¹ 

Prolylcarboxypeptidase or angiotensinase C (PRCP) is a serine protease that has an established biologic role in the generation of kallikrein from prekallikrein, as well as in the metabolism of bradykinin and angiotensin II.^2,3  However, the in vivo functions of PRCP have been poorly understood. PRCP may participate in the development of the vasculature; its transcripts are up-regulated during the active growth phase of the chick extra-embryonic vasculature.⁴  It shows a preferential expression in the endothelium⁴  and is abundantly present at the surface of cultured endothelial cells.² 

Light was first shed on the in vivo function of PRCP in 2009 when gene trap (gt) mice with greatly reduced PRCP levels were generated.⁵  Interestingly, Prcp^gt/gt mice had significantly reduced body weight and fat compared with control mice. This unexpected effect most likely is mediated by the proteolytic activity of the enzyme in the central nervous system, where it controls levels of α-MSH (α-melanocyte-stimulating hormone), a hormone implicated in food intake. From this first report of the PRCP-depleted mouse, it seemed that the predominant in vivo function of PRCP was not related to its role as a modulator of enzymes important for coagulation and the regulation of blood pressure.

However, in the present study, Adams et al took a closer look at the phenotype of Prcp^gt/gt mice. In a series of clearly designed experiments, they show that absence of PRCP has significant effects on vital physiologic systems such as blood pressure and coagulation.

What is the evidence for elevated blood pressure in the Prcp^gt/gt mice? Histologic examination of the gene trap mice showed thickening of the Bowman capsule in the kidney, a sign of hypertension. Direct evidence came from telemetric measurements of blood pressure over several days. Prcp^gt/gt mice have significantly elevated pressure levels, especially during their active phase in the night. Importantly, this phenotype could be reversed by administrating an effective mitochondrial antioxidant, suggesting that generation of reactive oxygen species (ROS) might be implicated in the hypertension phenotype generated by lack of PRCP.

The second important observation with potential consequences for the clinic is perturbation of thrombosis when PRCP is not present as evidenced by shortened arterial occlusion times. Either Prcp^gt/gt mice or wild-type mice treated with a PRCP inhibitor exhibited similar prothrombotic reactions. Interestingly, pharmacologic treatment of plasma kallikrein also shortens arterial occlusion times in wild-type mice. The exact mechanism of the increased arterial thrombosis risk remains unknown; however, Prcp^gt/gt vessels as well as siRNA-mediated knockdown of PRCP in endothelial cells show an increase in ROS. These latter cells also have reduced protein C activation and uncoupled eNOS. In vessels, lack of PRCP induces changes in vascular gene expression, including reduction of regulators such as krüppel-like transcription factor 2 and 4 (KLF2, KLF4), which in turn regulate levels of anticoagulant genes such as THBD (thrombomodulin) and NOS3 (nitric oxide synthase 3 [eNOS], see figure).

Prolyl peptidases such as PRCP are excellent candidates for the design of small molecule inhibitors.⁶  The crystal structure of PRCP has been recently resolved, which provides essential information for the design of specific modulators of PRCP activity.⁷  Novel inhibitors of PRCP have been developed and tested successfully in an animal model of obesity.⁸  These authors claim that no side effects including elevated blood pressure have been observed after PRCP inhibition in wild-type or PRCP knockout mice.

The obvious question that arises from the work of Adams et al is how safe pharmacologic inhibition of PRCP will be if applied as a regulator of food intake to fight obesity. Even though this pharmacologic approach is appealing, further work with special emphasis on changes in blood pressure and coagulation during treatment with PRCP inhibitors is needed.

Another open question is the role of PRCP in physiologic or pathologic angiogenesis. To date, no angiogenic defect has been reported in the Prcp^gt/gt mice. It may be that there are only slight changes that are not of physiologic relevance, or that changes will only become obvious in environmental metabolic changes such as hypoxia or during tumor development.