Abstract 1028: Prostaglandin E2 activates the mTORC1 pathway through an EP4/cAMP/PKA and EP1/calcium-mediated mechanisms in human pancreatic carcinoma cells

Abstract

Abstract Obesity, a known risk factor for pancreatic cancer, is associated with inflammation and insulin resistance. The pro-inflammatory prostaglandin E2 (PGE2) that signals via EP receptors and cAMP, and insulin-like growth factor 1 (IGF-1) that is elevated in insulin resistant states and activates the Akt/mammalian target of rapamycin complex 1 (mTORC1) signaling module, are both shown to play critical roles in pancreatic cancer progression. We aimed at exploring the potential signaling crosstalk between the PGE2/EP/cAMP and IGF-1/Akt/mTORC1 pathways in pancreatic cancer, which may be a key to unraveling the obesity-cancer link. In multiple human pancreatic cancer cell lines, PGE2 exposure increased intracellular cAMP levels, indicating the activation of Gαs-coupled receptors EP2 and/or EP4. In PANC-1 cells, which showed the greatest cAMP response, PGE2 dose- and time-dependently increased the phosphorylation of mTORC1 downstream targets p70S6 kinase (p70S6K) and S6 ribosomal protein (S6rp), suggesting a crosstalk between PGE2/cAMP and mTORC1. Accordingly, the mTORC1 inhibitor rapamycin suppressed PGE2-induced p70S6K and S6rp phosphorylation (Ser235/236 and Ser240/244). Also, the effect of PGE2 on p-S6rp was mimicked by forskolin, a cAMP stimulator. Interestingly, PGE2 and forskolin had no effect on p-Akt, suggesting a link downstream of Akt. Subsequent analysis using EP2 and EP4 antagonists/ agonists revealed that the PGE2-induced responses are mainly mediated by EP4. Further, PGE2-induced mTORC1 activation paralleled an increase in the phosphorylation of cAMP response element-binding protein (CREB), a substrate of protein kinase A (PKA). Pharmacological and genetic inhibition of PKA reduced baseline and PGE2-activated p-S6rp, indicating a role of PKA in the crosstalk. Additionally, calcium signaling seemed to be also involved in PGE2-induced mTORC1 activation, as p-S6rp (Ser235/236 and Ser240/244) was significantly decreased in cells pre-incubated with BAPTA-AM, a cell permeable Ca2+ chelator. Moreover, knockdown of EP1, a receptor known to mediate PGE2-induced Ca2+ response, attenuated PGE2-stimulated mTORC1 activation. Importantly, PGE2 enhanced the effect of IGF-1 on mTORC1 activation, suggesting a positive reinforcement by the interaction between the two pathways. Together, our data reveal a previously unrecognized mechanism of PGE2-stimulated mTORC1 activation mediated by EP4/cAMP/PKA and EP1/Ca2+ signaling in pancreatic cancer cells, which may be of great importance in elucidating the promoting effects of obesity in pancreatic cancer. Ultimately, a precise understanding of these molecular links may provide novel targets for efficacious interventions devoid of adverse effects. Citation Format: Hui-Hua Chang, Steven Young, James Sinnett-Smith, Caroline Ei Ne Chou, Oscar Joe Hines, Enrique Rozengurt, Guido Eibl. Prostaglandin E2 activates the mTORC1 pathway through an EP4/cAMP/PKA and EP1/calcium-mediated mechanisms in human pancreatic carcinoma cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1028. doi:10.1158/1538-7445.AM2015-1028