Gonadotropin releasing hormone (GnRH) contributes to the maintenance of gon
adotrope function by increasing extracellular signal-regulated kinase (ERK)
activity subsequent to binding to its cognate G-protein-coupled receptor.
As the GnRH receptor exclusively interacts with G(q/11) proteins and as rec
eptor expression is regulated in a beta-arrestin-independent fashion, it re
presents a good model to systematically dissect underlying signaling pathwa
ys. In alpha T3-1 gonadotropes endogenously expressing the GnRH receptor, G
nRH challenge resulted in a rapid increase in ERK activity which was attenu
ated by the epidermal growth factor receptor (EGFR)-specific tyrosine kinas
e inhibitor AG1478, In COS-7 cells transiently expressing the human GnRH re
ceptor, agonist-induced ERK activation was independent of free G beta gamma
subunits but could be mimicked by shortterm phorbol ester treatment. Most
notably, G(q/11)-induced ERR activation was sensitive to N17-Ras and to exp
ression of the C-terminal Src kinase but also to other dominant negative mu
tants of signaling components localized upstream of Ras, like Shc and the E
GFR. GnRH as well as phorbol esters led to Ras activation in COS-7 and alph
a T3-1 cells, which was dependent on Src and EGFR tyrosine kinases, indicat
ing that both tyrosine kinases act downstream of protein kinase C (PHC) and
upstream of Ras, However, Src did not contribute to Shc tyrosine phosphory
lation, GnRH or phorbol ester challenge resulted in PKC-dependent EGFR auto
phosphorylation, Furthermore, a 5-min phorbol ester treatment was sufficien
t to trigger tyrosine phosphorylation of the platelet-derived growth factor
-beta receptor in L cells, Thus, in several cell systems PKC is able to sti
mulate Ras via activation of receptor tyrosine kinases.