Acquisition of Lubrol insolubility, a common step for growth hormone and prolactin in the secretory pathway of neuroendocrine cells

Rat prolactin in the dense cores of secretory granules of the pituitary gla nd is a Lubrol-insoluble aggregate. In GH(4)C(1) cells, newly synthesized r at prolactin and growth hormone were soluble, but after 30 min about 40% co nverted to a Lubrol-insoluble form. Transport from the endoplasmic reticulu m is necessary for conversion to Lubrol insolubility, since incubating cell s with brefeldin A or at 15 degreesC reduced formation of insoluble rat S-3 5-prolactin, Formation of Lubrol-insoluble aggregates has protein and cell specificity; newly synthesized human growth hormone expressed in AtT20 cell s underwent a 40% conversion to Lubrol insolubility with time, but albumin did not, and human growth hormone expressed in COS cells underwent less tha n 10% conversion to Lubrol insolubility. del32-46 growth hormone, a natural ly occurring form of growth hormone, and P89L growth hormone underwent conv ersion, although they were secreted more slowly, indicating that there is s ome tolerance in structural requirements for aggregation. An intracellular compartment with an acidic pH is not necessary for conversion to Lubrol ins olubility, because incubation with chloroquine or bafilomycin slowed, but d id not prevent, the conversion. GH(4)G(1) cells treated with estradiol, ins ulin, and epidermal growth factor accumulate more secretory granules and st ore more prolactin, but not more growth hormone, than untreated cells; Lubr ol-insoluble aggregates of prolactin and growth hormone formed to the same extent in hormone-treated or untreated GH(4)C(1) cells, but prolactin was r etained longer in hormone-treated cells. These findings indicate that aggre gation alone is not sufficient to cause retention of secretory granule prot eins, and there is an additional selective process.