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.