Role of ornithine decarboxylase in regulation of estrogen receptor alpha expression and growth in human breast cancer cells

Our previous studies demonstrated that specific polyamine analogues, oligoamines, down-regulated the activity of a key polyamine biosynthesis enzyme, ornithine decarboxylase (ODC), and suppressed expression of estrogen receptor alpha (ER alpha) in human breast cancer cells. However, the mechanism underlying the potential regulation of ER alpha expression by polyamine metabolism has not been explored. Here, we demonstrated that RNAi-mediated knockdown of ODC (ODC KD) down-regulated the polyamine pool, and hindered growth in ER alpha-positive MCF7 and T47D and ER alpha-negative MDA-MB-231 breast cancer cells. ODC KD significantly induced the expression and activity of the key polyamine catabolism enzymes, spermine oxidase (SMO) and spermidine/spermine N (1)-acetyltransferase (SSAT). However, ODC KD-induced growth inhibition could not be reversed by exogenous spermidine or overexpression of antizyme inhibitor (AZI), suggesting that regulation of ODC on cell proliferation may involve the signaling pathways independent of polyamine metabolism. In MCF7 and T47D cells, ODC KD, but not DFMO treatment, diminished the mRNA and protein expression of ER alpha. Overexpression of antizyme (AZ), an ODC inhibitory protein, suppressed ER alpha expression, suggesting that ODC plays an important role in regulation of ER alpha expression. Decrease of ER alpha expression by ODC siRNA altered the mRNA expression of a subset of ER alpha response genes. Our previous analysis showed that oligoamines disrupt the binding of Sp1 family members to an ER alpha minimal promoter element containing GC/CA-rich boxes. By using DNA affinity precipitation and mass spectrometry analysis, we identified ZBTB7A, MeCP2, PARP-1, AP2, and MAZ as co-factors of Sp1 family members that are associated with the ER alpha minimal promoter element. Taken together, these data provide insight into a novel antiestrogenic mechanism for polyamine biosynthesis enzymes in breast cancer.