Characterization of Cysteamine Induction of the 22K Prolactin Variant in the Rat Pituitary

Abstract

Glandular kallikrein (GK) is an estrogen-induced and dopamine-repressed lactotroph protease postulated to play a role in the processing of prolactin (PRL) to novel hormonal forms. Recent studies have shown that GK can process PRL in vitro from a 25K form to a 22K form in the presence of thiols -which appear to transform PRL into conformations (folding states) that are GK substrates. We and others have reported 22K PRL variants in the rat pituitary which can be increased by the administration of cysteamine (CSH), a biological thiol known to alter PRL conformation in vitro and in vivo. The present study further characterized CSH induction of the 22K PRL variant. Estrogen-primed rats were used in dose-response and time-course studies with CSH. CSH effects on 22K PRL levels were studied by Western blot analysis of reduced pituitary extracts (disulfides reduced with dithiothreitol before electrophoresis). Changes in PRL conformation were studied by Western blot analysis of nonreduced samples (thiols and disulfides trapped with iodoacetamide). CSH increased 22K PRL in a dose-dependent manner that was well correlated with the dose-response curve for changes in PRL conformation. CSH at 300 mg/kg produced 10- to 15-fold increases in pituitary levels of 22K PRL. In the time course study, 22K PRL levels peaked within 2 h, plateaued between 2 and 16 h, and approached control levels by 24 h after CSH dosing. CSH-elicited changes in PRL conformation peaked within 1 h, plateaued between 2 and 8 h, and reached control levels within 16 h of CSH dosing. Thus, changes in PRL conformation preceded the changes in 22K PRL levels. CSH actions were not mimicked by bromocriptine, a dopamine agonist that blocks PRL release. CSH-induction of 22K PRL was dependent upon estrogen pretreatment; however, CSH effects on PRL conformation were independent of estrogen treatment. The results support the hypothesis that CSH increases the 22K PRL variant by converting PRL into conformations which are GK substrates.