Glycosylation of human prolactin regulates hormone bioactivity and metabolic clearance

Abstract

To analyze the role of individual glycosylation pattern on PRL biopotency, monomeric prolactin (PRL), secreted by human prolactinoma cells in culture, was isolated by gel filtration and separated by affinity chromatography on Concanavalin A-Sepharose or Lentil-Agarose. These lectins allowed the isolation of PRL glycoforms containing either biantennary, mannose-rich or fucosylated complex carbohydrate structures, respectively. Endoglycosidase treatment and carbohydrate content of PRL was found to be consistent with N-linked oligosaccharides of mannose-rich structure and complex units terminated in sialic acid. Mannose-rich PRL and PRL with biantennary oligosaccharides promoted cell growth of rat lymphoma cells to a diminished extent compared to non-glycosylated PRL (NG-PRL), indicating that the two major types of carbohydrate structure are able to decrease the intrinsic bioactivity of PRL. Metabolic clearance of the various forms of PRL in rats was also found to be highly dependent upon hormone glycosylation. The various glycosylated forms (G-PRLs) proved to be totally eliminated from the circulation within 60 min, faster than NG-PRL 10% of which was still present at that time. Mannose-rich or biantennary G-PRLs were differently cleared in a biphasial fashion with a similar rapid phase of about 2 min followed by distinct slow phases of 12 and 27 min, respectively. The presence of fucose did not alter this distribution. In contrast, NG-PRL was eliminated with a half-time of approximately 5 min, followed by a very slow disappearance over several h. It thus appeared that glycosylation increased the metabolic clearance rate of PRL from 0.13 +/- 0.07 ml/min for NG-PRL to 0.47 +/- 0.12 ml/min for PRL with biantennary carbohydrate chains and 0.8 +/- 0.2 ml/min for the hormone with mannose-rich oligosaccharides. The distribution of PRL to target and elimination organs was also found to be different according to the carbohydrate structure present in the hormone. NG-PRL and mannose-rich G-PRL showed higher incorporation in liver than biantennary G-PRL which was preferentially eliminated by the kidney. Altogether, the current data show that addition of oligosaccharides to PRL as well as carbohydrate structure contribute to modulate both the duration of the hormone in the blood and its distribution to different organs. It is proposed that glycosylation may selectively down-regulate PRL action at individual target tissues.

Plasma Prolactin Concentrations and Risk of Postmenopausal Breast Cancer

“…First, several forms of prolactin circulate in human plasma, which appear to have different biological activities (45,46). The immunoassay used in this study, which identify most prolactin isoforms (47), cannot distinguish between them.…”

Association between Plasma Prolactin Concentrations and Risk of Breast Cancer among Predominately Premenopausal Women

“…First, several forms of prolactin circulate in human plasma, which may have different biological activities (52,53); further, a prolactin-binding protein has been identified in plasma that may alter prolactin activity (54), which we could not account for here. The immunoassay used in this study measures most prolactin isoforms (55); however, it cannot distinguish between them or determine what amount of prolactin is bound to the binding protein.…”

Prolactin involvement in breast cancer.

“…Glycosylation may selectively down-regulate PRL action in target tissues (Hoffman et al 1993). Glycosylated PRL, in the mammary casein synthesis and the Nb2 proliferation assays, has biological activity similar to or lower than that of the nonglycosylated form (Sinha et al 1991, Pellegrini et al 1992.…”