Epidermal growth factor (EGF) receptor blockade inhibits the action of EGF, insulin-like growth factor I, and a protein kinase A activator on the mitogen-activated protein kinase pathway in prostate cancer cell lines

Epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I) are potent mitogens that regulate proliferation of prostate cancer cells via au tocrine and paracrine loops and promote tumor metastasis, They exert their action through binding to the corresponding cell surface receptors that ini tiate an intracellular phosphorylation cascade, leading to the activation o f mitogen-activated protein kinases (MAPKs), which recruit transcription fa ctors. We have studied the effects of EGF, IGF-I, and the protein kinase A (PKA) activator forskolin on the activation of p42/extracellular signal-reg ulated kinase (ERK) 2, which is a key kinase in mediation of growth factor- induced mitogenesis in prostate cancer cells, The activity of p42/ERK2 was determined by immune complex kinase assays and by immunoblotting using a ph ospho p44/p42 MAPK-specific antibody. EGF, IGF-I, and forskolin-induced PKA activity stimulate intracellular signaling pathways converging at the leve l of p42/ERK2, In the androgen-insensitive DU145 cell line, there is a cons titutive basal p42/ERK2 activity that is not present in androgen-sensitive LNCaP cells. Constitutive p42/ERK2 activity is abrogated by blockade of the EGF receptor. Hence, it Is obviously caused by an autocrine loop involving this receptor. The effects of EGF on p42/ERK2 are potentiated by forskolin in both cell lines, The blockade of PKA by the specific inhibitor H89 atte nuates this synergism. This finding is in contrast to those obtained in sev eral other systems studied thus far, in which PKA activators inhibited MAPK s, p42/ERK2 in DU145 cells is highly responsive to IGF-I stimulation, where as no effect of IGF-I on p42/ERK2 can be measured in LNCaP cells, Moreover, our results demonstrate that selective blockade of the EGF receptor in pro state cancer cells does not only inhibit the action of EGF, but also IGF-I- induced activation of the MAPK pathway and the interaction with the PKA pat hway. In conclusion, these findings offer new possibilities for a therapeut ical intervention in prostate cancer by targeting signaling pathways of gro wth factors and PKA.