Inhibition of PLC-γ1 activity converts nerve growth factor from an anti-mitogenic to a mitogenic signal in CHO cells

Nerve growth factor (NGF) treatment of Chinese hamster ovary fibroblast (CHO) cells exogenously expressing 2.5x10(5) TrkA receptors (CHO/TrkA) results in inhibition of serum and insulin-like growth factor-I (IGF-I) stimulated cell proliferation in a dose-dependent manner, Furthermore, NGF does not stimulate [H-3]thymidine incorporation and inhibits IGF-I mediated DNA synthesis in CHO/TrkA cells. NGF and IGF-I induce extracellular-signal regulated kinase 1 (ERK1) and ERK2 activation, but NGF is able to stimulate a higher and more sustained activation of these enzymes compared with IGF-I, Cotreatment with NGF and IGF-I yields an ERK1/2 activity profile similar to that of NGF treatment alone. While pretreatment with mitogen activated protein kinase kinase (MKK) inhibitor PD98059 (30 mu M) results in 100% inhibition of IGF-I stimulated MAPK phosphorylation (IC50 < 1 mu M), NGF mediated MAPK phosphorylation is only decreased by 50% (IC50 = 3 mu M). NGF, but not IGF-I, stimulates tyrosine phosphorylation and activation of PLC-gamma 1 which can be inhibited in a dose-dependent manner by phosphoinositide-specific phospholipase C (PI-PLC) inhibitor U73122 (IC50 = 4 mu M). Pretreatment with U73122 (IC50 = 7 mu M) results in an 87% inhibition of NGF mediated MAPK phosphorylation, while cotreatment with PD98059 and U73122 results in 97% inhibition. U73122 pretreatment has no effect on NGF stimulated Akt activation. NGF, but not IGF-I, stimulates the tyrosine phosphorylation of Suc1-associated neurotrophic factor-induced tyrosine phosphorylation target (SNT-1) fibroblast growth factor receptor substrate 2 (FRS2) which can be completely prevented by pretreatment with 10 mu M U73122, Finally, inhibition of PI-PLC results in NGF's ability to stimulate DNA synthesis in the absence and presence of IGF-I.