Differential sorting of lutropin and the free α-subunit in cultured bovine pituitary cells

The glycoprotein hormones lutropin (LH) and follitropin (FSH) are both synthesized by gonadotrophs in the anterior pituitary but are stored in separate secretory granules prior to secretion. Despite having highly homologous β-subunits and α-subunits with the identical amino acid sequence, the Asn-linked oligosaccharides on LH terminate with SO₄-GalNAc while those on FSH terminate with sialic acid-Gal. In addition to LH and FSH, gonadotrophs secrete uncombined (free) α-subunit which bears the same sulfated oligosaccharides as LH. We have examined the synthesis and secretion of LH and free α-subunit in primary cultures of bovine pituitary cells in order to determine if the sulfated oligosaccharides have any impact on sorting. Our results show that newly synthesized free α-subunit is secreted exclusively via the constitutive pathway with a t_1/2 of 1.8 h and is never found in dense-core secretory granules. In contrast, LH dimer is secreted by both the constitutive and the regulated pathways. Constitutive secretion and arrival in a dense secretory granule both occur with t_1/2 values of 1-1.5 h for newly synthesized LH. Sulfation occurs immediately prior to arrival of LH in the secretory granule and is followed by a period of 1-1.5 h before the LH-containing granules become sensitive to release by gonadotropin releasing hormone. As a result the t_1/2 for LH secretion in the presence of gonadotropin releasing hormone is 3.5 h. Sulfation of the free α-subunit oligosaccharides is not, therefore, sufficient to direct the α-subunit to secretory granules, and the information required for directing LH to granules must reside either in the β-subunit or the αβ-complex.