¹Section of Molecular Gastroenterology, Leeds Institute of Molecular Medicine, University of Leeds, St James's University Hospital, Leeds LS9 7TF, UK

Correspondence: Dr G Hawcroft, Section of Molecular Gastroenterology, Leeds Institute of Molecular Medicine, University of Leeds, St James's University Hospital, Leeds LS9 7TF, UK. E-mail: medgha@leeds.ac.uk

Received 15 May 2006; Revised 2 October 2006; Accepted 6 October 2006; Published online 20 November 2006.

Abstract

The predominant product of cyclooxygenase (COX) activity in the colon, prostaglandin (PG) E₂ promotes intestinal tumorigenesis. Expression of the PGE₂ receptor EP4 is upregulated during colorectal carcinogenesis. Therefore, we investigated the role of elevated PGE₂-EP4 receptor signalling in the protumorigenic activity of PGE₂ by increasing EP4 receptor expression in HT-29 human colorectal cancer (CRC) cells (HT-29-EP4) by stable transfection. Elevated PGE₂-induced EP4 receptor activity in HT-29 cells increased resistance to spontaneous apoptosis and promoted anchorage-independent growth, but had no effect on proliferation of HT-29-EP4 cells. EP4 receptor activation by PGE₂ in HT-29-EP4 cells also led to development of fluid-filled cysts, which was associated with increased tight junction protein (occludin and zonula occludens-1) expression. Overexpression of the EP4 receptor in HT-29 cells led to basal EP4 receptor signalling in the absence of exogenous PGE₂, which was explained by autocrine activity of endogenous, COX-2-derived PGE₂ and constitutive, ligand-independent EP4 receptor activity. The predominant signalling pathway mediating antiapoptotic activity downstream of PGE₂-EP4 receptor activation in HT-29-EP4 cells was elevation of cyclic adenosine monophosphate (cAMP) levels, which was associated with phosphorylation of cAMP-response element binding protein. EP4 receptor activation led to a small increase in phosphorylated extracellular signal-regulated kinase (ERK) 2 protein levels but inhibition of ERK phosphorylation did not abrogate the antiapoptotic activity of PGE₂. However, PGE₂-EP4 receptor signalling did not lead to trans-activation of the epidermal growth factor receptor in HT-29 cells. Inhibition of protumorigenic PGE₂-EP4 receptor signalling represents a potential strategy for anti-CRC therapy that may avoid the toxicity associated with systemic COX inhibition.