Polyamine involvement in invasion and metastasis by human breast cancer cells

Increased cellular activity of ornithine decarboxylase (ODC) the first and rate-limiting enzyme in polyamine (PA) synthesis, is an independent adverse prognostic factor for overall survival in human breast cancer (Manni A, et al., Clin Cancer Res 2:1901, 1996), thus suggesting an important role for PA in tumor progression. These experiments were designed to investigate the role of PA in invasion and metastasis, using the highly aggressive MDA-MB435 and MDA-MB231 human breast cancer cell lines. Administration of α-difluoromethylornithine (DFMO), an irreversible inhibitor of ODC, significantly reduced in a dose-dependent manner the invasiveness in matrigel of both MDA-MB435 and MDA-MB231 cells by ∼70%. DFMO treatment also inhibited (p<0.0001) "stellate" colony formation (an indicator of aggressive phenotype) by MDA-MB435 cells plated in the matrigel outgrowth assay. Administration of DFMO (2% in drinking water) reduced the growth rate of both cell lines orthotopically implanted in nude mice. To evaluate the effect of DFMO separately from that on proliferation, DFMO-treated mice were sacrificed later to allow their tumors to reach the same size of the tumors in the control mice. The most striking finding was that DFMO, while ineffective in reducing local invasion by both cell lines and lymph nodal metastasis by MDA-MB231 cells, nearly totally abolished (p=0.0152) pulmonary metastasis in mice bearing MDA-MB-435 xenografts. Lung metastasis developed in all 6 control mice (grossly visible in 3, measuring 3.5 mm in 2 and 7 mm in 1) but in only one (a microscopic focus) of 5 DFMO-treated mice. These results support an important role of PA in promoting breast cancer aggressiveness, particularly with regard to the development of distant metastasis and suggest that their involvement is distal in the metastatic cascade.