Identification of the hypothalamic hormone that regulates proliferatin of lactorophs

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The serum-free culture system of anterior pituitary cells were validated for determining proliferation in vitro of alctortrphs. Estradiol at 10nM increased BrdU-labeling indices of lactotrophs in the presence of a low insulin concentration, but conversely decreased those of lactotrophs in the presence of a high insulin concentration. Dopamine was effective in decreasing BrdU-labeling indices regardless of its concentrations.Thyrotropin releasing hormone (TRH) at 10^<-6> M decreased BrdU-labeling indices of lactotrophs in the absence of bovine serum albumin (BSA). However, TRH ranging 10^<-11>-10^<-9> M was found to increase BrdU-labeling indices in the presence of BSA in serum-free medium. None of vasoactive intestinal peptide, oxytocin, or luteinizing hormone releasing hormone altered BrdU-labeling indices at the concentrations examined.Involvement of cyclic AMP,a second messenger molecule that is considered to mediate the mitogenic actions of the hypothalamic hormones, in the regulation of lactotroph proliferation and its interactions with other signal transduction pathways of extracellular signals were examined. The cyclic AMP-increasing agents forskolin and dibutyryl cyclic AMP (db-cyclic AMP) increased BrdU-labeling indices of lactotrophs in a dose-dependent manner. The stimulatory action of forkolin on lactotroph proliferation was attenuated by PD98059, a inhibitor of mitogen-activated protein kinase (MAPK) kinase, suggesting an interaction between cyclic AMP and signaling pathway for mitogenic growth factors. Furthermore, the result that pretreatment with the dopaminergic agonist bromocriptine blocked the stimulatory action of forskolin but not that of db-cyclic AMP supported the view that dopamine acts on lactortrophs to inhibit the synthesis of cyclic AMP leading to inhibition of proliferation.