Stimulation of Luteinizing Hormone‐Beta Messenger Ribonucleic Acid and Post‐Translational Modification of Luteinizing Hormone Isoforms by Second Messengers Mediating the Action of Gonadotrophin‐Releasing Hormone

Abstract

K~Y words second messengers, luteinizing hormone, luteinizing hormone pleiomorphisrn luteinizing hormone-/I messenger ritmnucleic acid. gonadotrophin-releasing hormone Abstract Several second messenger systems have been implicated in mediating the action of gonadotrophin-releasing hormone on the pituitary gonadotrophs and numeroLis studies have shown that activation of these systems induces luteinizing hormone (LH) secretion. However, it is not known how gonadotrophin-releasing hormone or the second messenger systems induce d e novo LH biosynthesis and post-translational modification of the hormone. In these experiments hemipituitary glands have been perifused with drugs which activate second messengers or stimulate protein kinase C directly. The LH secretory responses have been correlated with measurements of common CI and LH/I mRNA and t h e molecular species of LH which were present in t h e pituitary perifusate after exposure to the drugs.Gonadotrophin-releasing hormone (50 ng/ml, 42 nM), with and without the presence of extracellular Ca2'% the Ca2+ ionophore, A23187 (IOpM), and phorbol 12-myristate (1 pM) all stimulated an increase in LHP mRNA compared with controls and the appearance of a different isoform of LH to that found stored in and released from the unstimulated pituitary gland. Phospholipase C was without effect on LH/I mRNA levels and showed minimal efficacy in inducing the appearance of the different LH isoform.The intcraction of gonadotrophin-releasing hormone (GnKH) with specific receptors on the pituitary gonadotroph ultimately ind iiccs both luteinizing hormone (LH) release and synthesis. Thcse effects are mediated by second messenger systems, many of bcliich have been implicated in the LH secretory process. These include the Ca2+-ealmodulin system and phosphoinositide hydrolycij generating diacylglycerol and inositol trisphosphate and arachidonic acid (I).While it is clear that the action of GnRH involves more than a single second messenger system, the precise role of these second mcsacngers in mediating the initial rapid LH response to GnRH and o r the second phase protein synthesis-dependent rclease (2) has not yet been fully established. Even less is known about thc showcd that phorbol estcrs, like GnRH, can incrcase LHP mRNA levels in pituitary cell cultures and they suggest that protein kinase C may, in part, regulate LH biosynthesis. Furthermore, it has been shown that both CAMP and phorbol esters mimic G n R H in enhancing glycosylation of LH ( 5 , 6) thus indicating that both the transcriptional and post-translational effccts of G n R H arc mediated by similar second messenger pathways.In an attempt to establish intracellular mechanisms which may be involved in stimulating LH biosynthesis and post-translational modifications, we have perifused hemipituitary glands (in thc prcscncc of a protcase inhibitor) with a varicty of drugs known to activatc the Ca2+-calmodulin systcm or protein kinase C. The LH responses of the pituitary glands to these secretagogues have been measured and c...

GnRH-Stimulated Glycosylation (Proximal and Distal) of Luteinizing Hormone by Cultured Rat Pituitary Cells

“…The current results indicate that GnRH increased the amount of terminal sugar attached to LH molecule, which can be correlated with the studies demonstrating that the quantity as well as the quality of the secreted pituitary gonadotropins are GnRH-dependent [17][18][19]. The GnRH-induced changes on carbohydrate content may also explain the apparent increase in biological activity of LH [20][21][22], The present report provides additional clues on the site of hormonal regulation of LH glycosylation by establishing the specific effect of GnRH on the posttranslational terminal glycosylation.…”

Calcium mobilization is a prerequisite for the expression of phorbol ester-stimulated luteinizing hormone secretion from pituitaries of male and acutely ovariectomized rats

“…The differences include the fact that the ionomycin-induced responses obtained from pituitaries of cycling female rats were partially depen¬ dent on protein-synthesis (2). Additionally, pituitaries from intact females responded to PMA with increased LH secretion (2,5,10). Taken together, these observations make it tempting to speculate that ovariectomy appears to change some of the effects of the Ca2+ and PKC systems in gonadotropes of females, making them similar to the effects in gonadotropes of males.…”

“…A number of investigators have reported evidence for a possible involvement of PKC in GnRH-stimulated LH secretion (3,10,19,(21)(22)(23)(24)(25)(26). In contrast, several investigators (16,(27)(28)(29)(30)(31), using PKC inhibitors or cells depleted of PKC activity (following pretreatment with PMA), suggest that PKC does not mediate GnRHstimulated LH secretion.…”