Inhibition of EP2/EP4 signaling abrogates IGF-1R-mediated cancer cell growth: Involvement of protein kinase C-θ activation

Associations between growth factor receptor-mediated cell signaling and cancer cell growth have been previously characterized. Receptors for prostaglandin E-2, such as EP2, and EP4, play roles in cancer growth, progression and invasion. Thus, we examined the interactions between EP2/EP4- and IGF-1R-mediated cellular signaling in human pancreatic cancer cells. Selective antagonists against EP2 and EP4 abrogated IGF-1-stimulated cell growth and suppressed MEK/ERK phosphorylation. In subsequent experiments, phospho-antibody arrays indicated increased phosphorylation levels of protein kinase C-theta (PKC-theta) at the Thr538 position following the inhibition of EP2/EP4-mediated signaling. Inhibition of PKC-theta activity impaired cell viability compared with EP2/EP4-antagonized IGF-1-stimulated cells. PKC-theta kinase MAP4K3, which plays a pivotal role in PKC-theta activation, also affected growth signaling in the presence of EP2/EP4 antagonists. Administration of EP2 and EP4 antagonists significantly inhibited the growth of an orthotopic xenograft of IGF1- secreting pancreatic cancer cells, with increased phospho-PKC-theta and decreased phospho-ERK. Clinico-pathological analyses showed that 17.4% of surgical pancreatic cancer specimens were quadruple-positive for IGF-1R, EP2 (or EP4), MAP4K3, and PKC-theta. These results indicate a novel signaling crosstalk between EP2/EP4 and IGF1R in cancer cells, and suggest that the MAP4K3-PKC-theta axis is central and could be exploited as a molecular target for cancer therapy.