The expression of the estrogen receptor β (ERβ) has been shown to play an important role in breast cancer. There is emerging hope that ERβ and its isoforms will be used as prognostic markers or as therapeutic targets in the clinical management of breast cancer. Many studies indicate that ERβ is down regulated during carcinogenesis. However, it is still unknown which signals can regulate ERβ expression. Basement membrane (BM) components have been shown to influence the expression levels of ERα and progesterone receptor. Therefore, we hypothesized that cell-matrix interactions can also affect the expression of ERβ and its isoforms. To test this we performed Matrigel assays using an ERα negative breast cancer cell line. MDA-MB-231 cells were plated on Matrigel, a reconstituted laminin-rich BM matrix, or on uncoated plastic culture plates. To investigate the effects of specific BM components we also cultured the cells on gels of purified collagen type IV and laminin-111. ERβ expression levels were investigated after 24, 48 and 72 h by RT-PCRs which allow to distinguish between different ERβ isoforms. MDA-MB-231 cells cultured on tissue culture plastic showed increased levels of ERβ1 mRNA after 48 h. However, in cells cultured on Matrigel signals for ERβ1 expression stayed at very low, nearly undetectable levels. Laminin-111 was identified to be the protein that represses ERβ1 expression at the mRNA stage. Collagen type IV showed no effect on ERβ expression. We further observed that MDA-MB-231 cells on Matrigel organize into cell aggregates which are connected in web-like structures that appear similar to lactiferous ducts. These data suggest that interactions of breast cancer cells with the BM protein laminin-111 suppress the expression of ERβ1 at the mRNA level. A laminin-111-rich microenvironment seems to keep ERβ1 at very low levels in breast cancer cells.

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