Dependence of Gonadotropin-releasing Hormone-induced Neuronal MAPK Signaling on Epidermal Growth Factor Receptor Transactivation

Abstract

The hypothalamic decapeptide, gonadotropin-releasing hormone (GnRH), utilizes multiple signaling pathways to activate extracellularly regulated mitogen-activated protein kinases (ERK1/2) in normal and immortalized pituitary gonadotrophs and transfected cells expressing the GnRH receptor. In immortalized hypothalamic GnRH neurons (GT1-7 cells), which also express GnRH receptors, GnRH, epidermal growth factor (EGF), and phorbol 12-myristate 13-acetate (PMA) caused marked phosphorylation of ERK1/2. This action of GnRH and PMA, but not that of EGF, was primarily dependent on activation of protein kinase C (PKC), and the ERK1/2 responses to all three agents were abolished by the selective EGF receptor kinase inhibitor, AG1478. Consistent with this, both GnRH and EGF increased tyrosine phosphorylation of the EGF receptor. GnRH and PMA, but not EGF, caused rapid phosphorylation of the proline-rich tyrosine kinase, Pyk2, at Tyr 402 . This was reduced by Ca 2؉ chelation and inhibition of PKC, but not by AG1478. GnRH stimulation caused translocation of PKC␣ and -⑀ to the cell membrane and enhanced the association of Src with PKC␣ and PKC⑀, Pyk2, and the EGF receptor. The Src inhibitor, PP2, the C-terminal Src kinase (Csk), and dominant-negative Pyk2 attenuated ERK1/2 activation by GnRH and PMA but not by EGF. These findings indicate that Src and Pyk2 act upstream of the EGF receptor to mediate its transactivation, which is essential for GnRH-induced ERK1/2 phosphorylation in hypothalamic GnRH neurons.

Receptor tyrosine kinases positively regulate BACE activity and Amyloid-β production through enhancing BACE internalization

“…Following stimulation with the RTK agonist, the secreted Aβ 40 and Aβ 42 levels were enhanced to 1.967 ± 0.023 and 2.040 ± 0.140 fold in swHEK293 cells ( Figure 2A), and to 2.082 ± 0.219 and 2.145 ± 0.306 fold in swC6 cells ( Figure 2B). The intracellular Aβ level was also significantly increased ( Figure 2C-2F).…”

“…*, P<0.05 versus untreated cells. though being somewhat less potent, NGF stimulation in creased both the activity of BACE ( Figure 1E) and the resulting production of Aβ 40 and Aβ 42 ( Figure 2E results show that RTK activation enhances BACE activity and Aβ production in vitro and in vivo.…”

“…Upon stimulation, RTKs function by activating downstream signaling molecules such as c-Src, MAPK, and PKC [38][39][40] or by inducing the internalization of other …”

“…RTK is reported to transduce signals through various downstream effectors such as MAPK, PKC, PKA, PI3K and Src tyrosine kinases [38][39][40]. Therefore, we sought to determine which signal pathway is involved in mediating the RTK-enhanced BACE activity and Aβ production.…”

Constitutively Active Gα16 Stimulates STAT3 via a c-Src/JAK- and ERK-dependent Mechanism

“…Group I metabotropic glutamate receptors have been shown to activate ERK via CaMKII in striatal neurons (46), and CaMKII-dependent activation of ERK in vascular smooth muscle requires the participation of tyrosine kinases such as Pyk2 and Src (47). As for PKC-dependent activation of ERK, gonadotrophin-releasing hormone-induced phosphorylation of ERK in hypothalamic neurons also requires Pyk2 and Src (48). Gonadotrophin-releasing hormone stimulation caused the translocation of PKC␣ and PKC⑀ to the cell membrane and enhanced the association of Src with PKC␣ and PKC⑀.…”