Human luteinizing hormone and chorionic gonadotropin are targeted to a regulated secretory pathway in GH₃ cells

LH is a dimeric glycoprotein hormone that is stored in the anterior pituitary and is released in response to GnRH, while the placental hormone, human CG (hCG), sharing the same alpha-subunit and a related β-subunit, is secreted constitutively. In search of a determinant that allows sorting of LH into a regulated secretory pathway, the genes encoding the common α- and LH/CG β-subunits were expressed in the GH₃ rat pituitary tumor cell line, which contains a regulated secretory pathway. Steady state labeling and subsequent chase experiments showed that not only LH but also hCG can be sorted to a regulated secretory pathway; after an initial period of constitutive secretion, the mature forms of both hormones containing processed oligosaccharides were stored intracellularly, and their release was stimulated by either forskolin or KCl depolarization. In Chinese hamster ovary cells, which lack a regulated pathway and are devoid of storage granules, only hormones containing unprocessed N-linked oligosaccharides were found. In GH₃ cells the LH β-subunit was partially retained in an endoglycosidase H-sensitive form, presumably in the endoplasmic reticulum; the enzyme-resistant fraction was secreted through a regulated secretory pathway. A large fraction of the hCG β-subunit was released constitutively, although some mature hCG β-subunit accumulated in secretory granules and was released by forskolin. The common α-subunit was secreted constitutively with little intracellular accumulation of the mature forms. We conclude that the LH β-subunit contains sufficient information to direct LH to a regulated pathway, and α:LH β assembly is not a prerequisite for this targeting. The sorting of hCG to a regulated pathway in GH₃ cells presumably reflects a structural similarity between LH and hCG. In addition, we have shown that GH₃ cells can recognize the N-linked oligosaccharides on the gonadotropin subunits as substrates for sulfation.