EP₃ prostanoid receptor isoforms utilize distinct mechanisms to regulate ERK 1/2 activation

Prostaglandin-E₂ (PGE₂) is a hormone derived from the metabolism of arachidonic acid whose functions include regulation of platelet aggregation, fever and smooth muscle contraction/relaxation. PGE₂ mediates its physiological and pathophysiological effects through its binding to four G-protein coupled receptor subtypes, named EP₁, EP₂, EP₃ and EP₄. The EP₃ prostanoid receptor is unique in that it has multiple isoforms generated by alternative mRNA splicing. These splice variants display differences in tissue expression, constitutive activity and regulation of signaling molecules. To date there are few reports identifying differential activities of EP₃ receptor isoforms and their effects on gene regulation. We generated HEK cell lines expressing the human EP_3-Ia, EP_3-II or EP_3-III isoforms. Using immunoblot analysis we found that nM concentrations of PGE₂ strongly stimulated the phosphorylation of ERK 1/2 by the EP_3-II and EP_3-III isoforms; whereas, ERK 1/2 phosphorylation by the EP_3-Ia isoform was minimal and only occurred at μM concentrations of PGE₂. Furthermore, the mechanisms of the PGE₂ mediated phosphorylation of ERK 1/2 by the EP_3-II and EP_3-III isoforms were different. Thus, PGE₂ stimulation of ERK 1/2 phosphorylation by the EP_3-III isoform involves activation of a Gα_i/PI3K/PKC/Src and EGFR-dependent pathway; while for the EP_3-II isoform it involves activation of a Gα_i/Src and EGFR-dependent pathway. These differences result in unique differences in the regulation of reporter plasmid activity for the downstream effectors ELK1 and AP-1 by the EP_3-II and EP_3-III prostanoid receptor isoforms.