Inhibition of
ornithine decarboxylase (ODC), a key
enzyme in
polyamine biosynthesis, by the irreversible inhibitor
alpha-difluoromethylornithine (DFMO) has been shown to decrease the invasiveness of metastatic human
breast cancer cell lines. However, the mechanism by which DFMO acts to reduce invasiveness is unclear. Using the human
breast cancer cell line MDA-MB-435, the effect of DFMO on
metalloprotease gene expression was investigated. DFMO treatment decreases the expression of the
metalloprotease meprin alpha, while concurrent treatment with DFMO and the
polyamine putrescine partially restored
meprin alpha expression levels. Expression of MMP-7
mRNA was reduced by DFMO, while MMPs-1, -2, -3, -14, and
meprin beta were unaffected. Treatment of cells with a second inhibitor of
polyamine biosynthesis, the
S-adenosylmethionine decarboxylase (SAMDC) inhibitor
SAM486A, also resulted in a dosage dependent decrease in
meprin alpha and MMP-7
mRNA. In addition, DFMO treatment decreased
meprin alpha at the
protein level by 2 days of treatment, and MMP-7
protein levels at 4 and 6 days. Previous studies have shown that DFMO treatment increases ERK phosphorylation and signaling through the MAP
kinase pathway. The decrease in
meprin alpha expression was reversed with the
MEK inhibitor
PD98059, demonstrating that MAP
kinase signaling mediates the effect of DFMO and
SAM486A. MDA-MB-435 cells treated with the
meprin alpha inhibitor
actinonin (5 nM) were less invasive in vitro, indicating that
meprin alpha is mechanistically involved in invasion. The decrease in
meprin alpha expression in DFMO and SAM486A-treated cells indicates a means by which these compounds can decrease the invasiveness of metastatic
breast cancer cells.