DOWN-REGULATION OF LAMININ-5 IN BREAST-CARCINOMA CELLS

Background: Laminin-5 (ln-5), a large heterotrimeric glycoprotein cons isting of an alpha 3, beta 3, and gamma 2 chain, is a component of epi thelial cell basement membranes that functions as a ligand of the alph a 3 beta 1 and alpha 6 beta 4 integrins to regulate cell adhesion, mig ration, and morphogenesis. The ln-5 chains show tissue-specific patter ns of regulation in tumors derived from different tissues. For example , ln-5 is often up-regulated in gliomas, gastric carcinomas, and squam ous carcinomas and down-regulated in prostate and basal cell carcinoma s. Ln-5 expression patterns may represent useful tumor markers and hel p to elucidate the role of ln-5 in tumor progression in different tiss ue types. Materials and Methods: Mie have studied ln-5 expression patt erns in the breast. mRNA levels were examined in tumor and normal brea st epithelial cell lines, tissue samples, and immunomagnetically sorte d primary cultures using differential display, Northern blotting, and hybridization arrays. Protein levels were examined by immunoprecipitat ion. Gene integrity was assessed by Southern blotting of representativ e cell types. Results: Ln-5 alpha 3, beta 3, and gamma 2 mRNA expressi on was found to be markedly down-regulated in a panel of breast tumor cell lines when compared with normal breast epithelial cells. Ln-5 mRN A was expressed at relatively high levels in MCF-10A immortal normal b reast epithelial cells, long-term cultures of normal breast cells, and sorted primary cultures of normal breast luminal epithelial and myoep ithelial cells. Reduced, but detectable, levels of ln-5 tended to be e xpressed in cell lines derived from early-stage breast tumors, whereas expression was generally not detected in cell lines derived from late r-stage tumors. Zn breast tumor tissue specimens, expression of in alp ha 3 and beta 3 mRNAs tended to be reduced relative to levels observed in adjacent nontumor tissue, whereas in gamma 2 levels were elevated in specimens with increased amounts of myoepithelial cells. These ln-5 expression changes could not be attributed to large-scale mutations o r gene rearrangements. Ln-5 protein levels were found to reflect mRNA levels in representative cell lines. At senescence, a growth slate bel ieved to suppress tumorigenesis, expression of all three ln-5 mRNAs wa s up-regulated. Conclusion: The down-regulation of ln-5 mRNA expressio n in breast tumors cells provides a new molecular marker and suggests that ln-5 functions to control tumor progression in the breast.