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

// Tetsuyuki Takahashi 1,* , Hisanori Uehara 1,* , Hirohisa Ogawa 1 , Hitomi Umemoto 1 , Yoshimi Bando 2 and Keisuke Izumi 1 1 Department of Molecular and Environmental Pathology, Institute of Health Biosciences, University of Tokushima Graduate School, Tokushima, Japan 2 Division of Pathology, Tokushima University Hospital, Tokushima, Japan * These authors contributed equally to this work Correspondence: Tetsuyuki Takahashi, email: // Keywords : E-prostanoid receptor, insulin-like growth factor receptor-1, protein kinase C-θ Received : October 21, 2014 Accepted : December 28, 2014 Published : December 31, 2014 Abstract 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-θ (PKC-θ) at the Thr538 position following the inhibition of EP2/EP4-mediated signaling. Inhibition of PKC-θ activity impaired cell viability compared with EP2/EP4-antagonized IGF-1-stimulated cells. PKC-θ kinase MAP4K3, which plays a pivotal role in PKC-θ 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 IGF-1-secreting pancreatic cancer cells, with increased phospho-PKC-θ 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-θ. These results indicate a novel signaling crosstalk between EP2/EP4 and IGF-1R in cancer cells, and suggest that the MAP4K3-PKC-θ axis is central and could be exploited as a molecular target for cancer therapy.