ARACHIDONIC-ACID AND LIPOXYGENASE PRODUCTS STIMULATE GONADOTROPIN ALPHA-SUBUNIT MESSENGER-RNA LEVELS IN PITUITARY ALPHA-T3-1 CELL-LINE - ROLE IN GONADOTROPIN-RELEASING-HORMONE ACTION

The role of arachidonic acid (AA) and its lipoxygenase metabolites in gonadotropin releasing hormone (GnRH) induced cu-subunit gene expressi on was investigated in the transformed gonadotroph cell line alpha T3- 1. The stable analog [D-Trp(6)]GnRH (GnRHa) stimulated [H-3]AA release from prelabeled cells after a lag of 1-2 min. Addition of AA stimulat ed or-subunit mRNA levels in a dose-dependent manner, a significant ef fect being detected at 5 mu M AA. Among various lipoxygenase metabolit es of AA, only the 5-lipoxygenase products 5-hydroxyeicosatetraenoic a cid (5-HETE) and leukotriene C-4 (LTC(4)) stimulated alpha-subunit mRN A levels. However, while 5-HETE and LTC(4) (0.1 nM each) were active a lready after 30 min of incubation, similar to GnRHa, AA (20 mu M) stim ulated alpha-mRNA levels after 1 h of incubation. Addition of the phos pholipase A(2) inhibitor 4-bromophenacyl bromide (BPB) or the selectiv e 5-lipoxygenase inhibitor L-656,224 inhibited GnRHa elevation of alph a-subunit mRNA by 65%, while the cyclooxygenase inhibitor indomethacin had no effect. Addition of AA (20 mu M) or LTC(4) (0.1 nM) to normal cultured rat pituitary cells mimicked the rapid (30 min) stimulatory e ffect of GnRH (1 nM) upon alpha-subunit, LH beta, and FSH beta mRNA le vels, while 5-HETE (0.1 nM) stimulated only FSH beta mRNA levels at th is time point. Thus AA and selected 5-lipoxygenase products, in partic ular LTC(4), participate in GnRHa-induced alpha-subunit mRNA elevation .