Expression of the alpha7 integrin is developmentally regulated and is thought to be tissue-specific for both skeletal and cardiac muscles. We now report that alpha7 is also strongly and ubiquitously expressed by various types of smooth muscle, including vascular, gastrointestinal and genitourinary smooth muscles. In addition, alpha7 was surface-expressed by a number of smooth muscle cell lines that maintained their differentiated phenotype following adaptation to culture. Studies with the mouse 9E11G smooth muscle cell line showed that the alpha7 integrin mediated both adhesion and motility of these cells on laminin 1 substrates. Alpha7 expression appears to correlate with the smooth-muscle-differentiated phenotype. The multipotential P19 mouse embryonic stem cell line lacks alpha7 but uses the alpha6 integrin to adhere to laminin 1. Following retinoic acid-induced P19 differentiation predominantly to the smooth muscle cell lineage, high expression of alpha7 was detected along with partial dependence on alpha7 for binding to laminin. The expression of alpha7 paralleled the induction of smooth-muscle-specific alpha-actin, as revealed by dual-labeling flow cytometry. In contrast, alpha7, which initially was highly expressed on the surface of vascular smooth muscle cell explants, was rapidly downregulated in smooth muscle cell outgrowths as they dedifferentiated into their synthetic phenotype. The results indicate that the expression of alpha7 integrin in smooth muscle cells is associated with their differentiated phenotype and mediates their interaction with laminins.

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