HUMAN LUTEINIZING-HORMONE AND CHORIONIC-GONADOTROPIN ARE TARGETED TO A REGULATED SECRETORY PATHWAY IN GH(3) CELLS

LH is a dimeric glycoprotein hormone that is stored in the anterior pi tuitary and is released in response to GnRH, while the placental hormo ne, human CG (hCG), sharing the same alpha-subunit and a related beta- subunit, is secreted constitutively. In search of a determinant that a llows sorting of LH into a regulated secretory pathway, the genes enco ding the common alpha- and LH/CG beta-subunits were expressed in the G H, rat pituitary tumor cell line, which contains a regulated secretory pathway. Steady state labeling and subsequent chase experiments showe d that not only LH but also hCG can be sorted to a regulated secretory pathway; after an initial period of constitutive secretion, the matur e forms of both hormones containing processed oligosaccharides were st ored intracellularly, and their release was stimulated by either forsk olin 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 GH, c ells the LH beta-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 beta-subunit was released constitutively, although some mature hCG beta-subunit accumulated in secretory granul es and was released by forskolin. The common alpha-subunit was secrete d constitutively with little intracellular accumulation of the mature forms. We conclude that the LH beta-subunit contains sufficient inform ation to direct LH to a regulated pathway, and alpha:LH beta assembly is not a prerequisite for this targeting. The sorting of hCG to a regu lated pathway in GH(3) cells presumably reflects a structural similari ty between LH and hCG. In addition, we have shown that GH(3) cells can recognize the N-linked oligosaccharides on the gonadotropin subunits as substrates