Do prorenin-synthesizing cells release active, 'open' prorenin?

Abstract

The function of prorenin, the inactive precursor of renin, remains unclear after many decades of research. The discovery of a (pro)renin receptor suggested that prorenin, by binding to this receptor, would become active, that is, obtain an 'open' conformation. However, the receptor only interacted with prorenin at levels that were many orders of magnitude above its normal levels, making such interaction in-vivo unlikely. Prorenin occurs in two conformations, an open, active form, and a closed, inactive form. Under physiological conditions (pH 7.4, 37 °C), virtually all prorenin occurs in the closed conformation. This study investigated to what degree prorenin-synthesizing cells release prorenin in an open conformation. Renin plus prorenin-synthesizing human mast cells, and prorenin-synthesizing HEK293 cells (transfected with the mammalian expression vector pRhR1100, containing human prorenin) and human decidua cells were incubated with the renin inhibitor VTP-27999. This inhibitor will trap open prorenin, as after VTP-27999 binding, prorenin can no longer return to its closed conformation, thus allowing its detection in a renin immunoradiometric assay. No evidence for the release of open prorenin was found. Moreover, incubating decidua cells with angiotensinogen yielded low angiotensin levels, corresponding with the activity of ≈1% of prorenin in the medium, that is, the amount of open prorenin expected based upon the equilibrium between open and closed prorenin under physiological conditions. Our study does not reveal evidence for the release of open, active prorenin by prorenin-synthesizing cells, at least under cell culture conditions. This argues against prorenin activity at the site of its release.