Secretion of glycosylated pro-B-type natriuretic peptide from normal cardiomyocytes

B-type natriuretic peptide (BNP), a key cardiac hormone in cardiorenal homeostasis, is produced as a 108 amino acid prohormone, proBNP1-108, which is converted to a biologically active peptide BNP1-32 and an inactive N-terminal (NT)-proBNP1-76. The widely accepted model is that the normal heart releases a proteolytically processed BNP1-32 and NT-proBNP, whereas the diseased heart secretes high amounts of unprocessed/glycosylated proBNP1-108 or inappropriately processed BNPs. In contrast, circulating proBNP1-108 has recently been identified in healthy individuals, indicating that the normal heart also secretes unprocessed proBNP1-108. However, the mechanism of proBNP1-108 secretion from the normal heart remains elusive. Our goal was to determine the molecular mechanisms underlying proBNP1-108 intracellular trafficking and secretion from the normal heart.

We expressed preproBNP in cardiomyocytes, and determined the subcellular localization and dominant intracellular and extracellular forms of BNP.

Intracellular immunoreactive BNPs were first accumulated in the Golgi apparatus, and then distributed throughout the cytoplasm as secretory vesicles. The predominant intracellular form of BNP was nonglycosylated proBNP1-108, rather than BNP1-32. Glycosylated proBNP1-108, but not nonglycosylated proBNP1-108, was detected as the major extracellular form in the culture supernatants of preproBNP-expressing cell lines and primary human cardiomyocytes. Ablation of O-glycosylation of proBNP1-108 at T71 residue, near the convertase recognition site, reduced the extracellular proBNP1-108 and increased extracellular BNP1-32.

Intracellular proBNP trafficking occurs through a conventional Golgi-endoplasmic reticulum pathway. Glycosylation of proBNP1-108 controls the stability and processing of extracellular proBNP1-108. Our data establish a new BNP secretion model in which the normal cardiac cells secrete glycosylated proBNP1-108.