GH3 cells transfected with gonadotropin-releasing hormone (GnRH) receptor complementary deoxyribonucleic acid release secretogranin-II through a constitutive pathway after GnRH analog-regulated synthesis: evidence that secretory proteins do not contain a sequence that obligates processing through a secretory granule or by regulated secretion.

Abstract

GGH3 cells (lactotropic GH3 cells transfected with rat GnRH receptor complementary DNA) respond to GnRH agonists with time- and dose-dependent release of secretogranin-II (SII), a molecule believed to be a marker of the regulated pathway of release and a component of secretory granules. Release requires both protein and RNA synthesis and is also stimulated by analogs of the cyclic nucleotides. Release, as such, appears to be determined by the availability of SII, because this process is constitutive once the molecule is synthesized, and intracellular levels are at the same steady state whether the receptor is occupied by an agonist or an antagonist or is unoccupied. This synthetic requirement is consistent with the lag in time from stimulation to release and the inhibitory actions of either cycloheximide or actinomycin-D on secretion as well as the morphologic observation that storage granules (i.e. secretory granules) are absent in these cells. SII, accordingly, can be released constitutively and does not require processing through the so-called regulated secretory pathway. These observations suggest, in addition, that proteins associated with release through regulated pathways do not have "sorting" domains that preclude release via constitutive routes or require processing through secretory granules.