Preascitic sodium retention in cirrhosis: a role for disregulated proteolysis by proprotein convertases

Loss of effective arterial blood volume, secondary hyperaldosteronism, adrenergic activation and nonosmotic hypersecretion of vasopressin induce sodium and water retention in cirrhotic patients with ascites. The mechanisms of sodium retention that precede ascites formation remain elusive. In patients who are at the preascites stage of cirrhosis, no sign of reduced effective volaemia is found; nonetheless, tubular sodium retention is already present. Maturation and full functionality of epithelial sodium channels (ENaC) in distal segments of the nephron and, therefore, final control of sodium excretion are dependent on regulated proteolysis by proprotein convertases. Evidence of abnormal or incomplete maturation of ENaCs in preascitic cirrhosis exists, but the complex mechanisms of regulated proteolysis leading to ENaC maturation through sequential action of serine endopeptidases (i.e., furin, site-1 protease, prostasin, plasmin) have never been studied in liver cirrhosis. Also, the mechanisms of cirrhosis-associated immune dysfunction, which are characterised by systemic sterile inflammation and release of proinflammatory cytokines that profoundly influence renal function, remain largely unknown. Release of proinflammatory cytokines and functions of respective receptors are controlled through regulated proteolysis by cell membrane metallopeptidases (mainly ADAM-10 and -17). Once again, little is known in preascitic cirrhosis about potential disregulated proteolysis of proinflammatory cytokines that may trigger systemic inflammation and renal dysfunction. We advance a new hypothesis that (a) may link proprotein convertases to disregulated proteolysis of tubular sodium channels, renin-angiotensin system receptors and inflammatory mediators, and that (b) may shed light on the mechanisms of sodium retention before any sys