Arachidonic acid and Lipoxygenase Products Stimulate Gonadotropin .alpha.-Subunit mRNA 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 alpha-subunit gene expression was investigated in the transformed gonadotroph cell line alpha T3-1. The stable analog [D-Trp6]GnRH (GnRHa) stimulated [3H]AA release from prelabeled cells after a lag of 1-2 min. Addition of AA stimulated alpha-subunit mRNA levels in a dose-dependent manner, a significant effect being detected at 5 microM AA. Among various lipoxygenase metabolites of AA, only the 5-lipoxygenase products 5-hydroxyeicosatetraenoic acid (5-HETE) and leukotriene C4 (LTC4) stimulated alpha-subunit mRNA levels. However, while 5-HETE and LTC4 (0.1 nM each) were active already after 30 min of incubation, similar to GnRHa, AA (20 microM) stimulated alpha-mRNA levels after 1 h of incubation. Addition of the phospholipase A2 inhibitor 4-bromophenacyl bromide (BPB) or the selective 5-lipoxygenase inhibitor L-656,224 inhibited GnRHa elevation of alpha-subunit mRNA by 65%, while the cyclooxygenase inhibitor indomethacin had no effect. Addition of AA (20 microM) or LTC4 (0.1 nM) to normal cultured rat pituitary cells mimicked the rapid (30 min) stimulatory effect of GnRH (1 nM) upon alpha-subunit, LH beta, and FSH beta mRNA levels, while 5-HETE (0.1 nM) stimulated only FSH beta mRNA levels at this time point. Thus AA and selected 5-lipoxygenase products, in particular LTC4, participate in GnRHa-induced alpha-subunit mRNA elevation.