Inflammatory breast cancer (IBC) is the most aggressive form of locally advanced breast cancer (LABC) characterized by rapid growth and aggressive invasion with no selective therapies developed to treat IBC. Cyclooxygenase-2 (Cox-2), which produces prostaglandin E2 (PGE2) is known to be upregulated in primary IBC tumors and metastatic lesions, however the use of selective Cox-2 inhibitors has diminished due to cardiovascular side effects. One alternative approach to targeting Cox-2 enzyme activity is to block binding of the PGE2 ligand to its prostanoid (EP) receptors, which are designated as EP1, EP2, EP3, and EP4 and are members of a subfamily of G protein coupled receptors (GPCRs). While SUM149 IBC tumor cells and MCF-7 non-IBC breast tumor cells produce both EP2 and EP4 receptors, the invasive MDA-MB-231 non-IBC breast tumor cells produced low but detectable levels of these receptors. PGE2 and the EP4 agonist, PGE2 alcohol, stimulated significantly increased (p < 0.05) levels of proliferation and invasion by SUM149 IBC tumor cells, with no effect on proliferation of either of the two non-IBC breast tumor cell lines. In contrast, the EP2 agonist butaprost had no effect on proliferation or invasion of any cell line examined. The selective EP4 antagonist, GW627368X, induced inhibition of proliferation and invasion of human SUM149 IBC tumor cells beginning at 0.1 microM, with inhibition of proliferation and invasion by MDA-MB-231 non-IBC cells at higher concentrations of GW627368X. Molecular knockdown of the EP4 receptor was accomplished by stable transfection of an EP4 short hairpin RNA (shRNA) construct, with a clonally derived cell line designated as SUM149/Clone 1 exhibiting significantly slowed proliferation and diminished invasion compared to SUM149/Vector 5 which contained a scrambled shRNA control vector. This is the first report using both a selective pharmacologic inhibitor and a molecular shRNA knockdown approach to demonstrate that EP4 is directly involved in regulation of proliferation and invasion of IBC cells.