Increased levels of α6 integrins are associated with the metastatic phenotype of human breast cancer cells

Integrins play an important role in interactions between cells and the extracellular matrix, and thus have a potential role in metastasis. Expression levels of α6, β1 and β4 integrin sub-units were measured in a panel of human breast cancer cell lines by RT/PCR, immunoprecipitation and flow cytometry. All the lines expressed α6, with the highest levels in the MDA-MB-231 and MDA-MB-435 cells. These grew the most aggressively and were metastastic in nude mice. Low levels of α6 protein were measured in breast cancer cells that were poorly tumorigenic and non-metastatic in nude mice, and there was an inverse relationship between ER and α6 expression. RT/PCR revealed that all lines expressed the 2 isoforms of α6, with the α6A isoform generally more abundant than α6B isoform. Clones of MDA-MB-435 were isolated by sterile sorting for cells with high or low α6 expression, and two variants established from metastases in nude mice were found to differ in α6 expression. When injected into nude mice, the α6-high variants produced significantly more lung metastases than the α6-low variants. β1 was abundant in all lines, while β4 was not detected in MDA-MB-134 cells, and in the MDA-MB-435 cells an alternately spliced variant of β4 was identified. Sequencing of the alternate variant revealed a novel sequence from a splicing event in the cytoplasmic tail of β4. None of the cells with this variant mRNA expressed detectable levels of β4 protein. Our results suggest that high α6 expression in human breast cancer cells is associated with tumorigenicity and metastatic potential.