Previous results showed that gonadotropin-releasing hormone (GnRH) signaling is altered in cells from rat luteinized ovarian tumors (Tumor), since it did not activate the phospholipase C (PLC) pathway, in contrast to control ovarian cells from superovulated prepubertal rats (SPO). In the present work, alternate GnRH-induced second messengers such as phospholipase A₂ (PLA₂) and phospholipase D (PLD) activation, cAMP production, extracellular signal regulated kinase 1/2 (ERK1/2) phosphorylation and the presence of G proteins were evaluated to determine GnRH mechanism of action in tumor cells. G proteins examined were present in both cell types. Buserelin, a GnRH agonist, (1, 10 and 100 ng/ml) increased phosphatidylethanol in SPO, indicating PLD activation. Only 100 ng/ml Buserelin induced a significant response in Tumor. Buserelin (100 ng/ml) increased ³H-arachidonic acid in culture media in SPO, indicating PLA₂ activation; no effect was observed in Tumor. Buserelin (100 and 1000 ng/ml) induced Pertussis toxin insensitive cAMP increases in both cell types, with similar potencies. In Tumor, Buserelin (100 ng/ml) inhibited hCG-induced cAMP and progesterone; this effect was protein kinase C (PKC) dependent (inhibited by GF109203X, a PKC inhibitor). Buserelin (100 and 1000 ng/ml) induced ERK1/2 phosphorylation in both cell kinds. Buserelin-induced ERK1/2 activation was G_i/0 independent and PKC dependent. Only in Tumor, Buserelin-induced ERK1/2 activation was cAMP dependent (abolished by SQ 22536, the adenylyl cyclase inhibitor). Furthermore, dibutiryl cAMP-induced ERK1/2 activation in Tumor was PKC dependent (inhibited by GF109203X).

In conclusion, activation of phospholipases in tumor cells do not seem to mediate GnRH effects. GnRH signaling seems to involve adenylyl cyclase activation, PKC stimulation and ERK1/2 phosphorylation.