Adult adenohypophysial cells express β₁ integrins and prefer laminin during cell-substratum adhesion

β₁ Integrins are a family of structurally related heterodimeric cell surface receptors that are involved in adhesion to molecules in the extracellular matrix (ECM) such as laminin (LN), fibronectin (FN), and collagen. These receptors are expressed by many cell types and mediate a variety of processes such as cell-matrix and cell-to-cell adhesion, cell migration, growth, and differentiation. The purpose of these studies was to identify and partially characterize β₁ integrins on adenohypophyseal cells and to begin to elucidate their functional importance. Adenohypophyses were removed from adult male rats, dispersed using 0.25% trypsin, rinsed, and resuspended in a 1:1 mixture of Dulbecco's modified Eagle's medium and F12 medium containing 10% fetal bovine serum and antibiotics. Ten million cells were allowed to attach to each of five plastic culture dishes overnight. The next day, the adenohypophyseal cells were surface-labeled with¹²⁵I. The labeled cells were lysed and centrifuged. The supernatant was immunoprecipitated using preimmune IgGs (100 μg/ml) and was then incubated with a polyclonal antibody against the rat β₁ family of integrins or with a variety of immune IgGs directed against the α subunit of the receptor (anti α₁, anti α₂, anti α₃, and anti α₅ antibodies). The receptors were then immunoprecipitated by addition of protein A-Sepharose or IgG₁ Sepharose. After washing, the immunoprecipitates were subjected to sodium dodecyl sulfate polyacrylamide gel electrophoresis and autoradiography. Cultured adenohypophyseal cells expressed the β₁ integrin subunit, which was associated with the α₁, α₂, α₃, and α₅ integrin subunits. These integrins are known to have binding specificities for LN, FN, epiligrin, and several collagens. Immunocytochemical staining and confocal microscopy verified that these receptors were present on the cell surface in vitro. The addition of anti rat β₁ integrin antibodies to dispersed adenohypophyseal cells partially blocked their attachment to ECM ligands in cell adhesion assays. In addition, peptides containing Agr-Gly-Asp-Ser (RGDS) partially blocked adenohypophyseal cell attachment to FN and to a lesser extent to LN. These studies show for the first time that adult adenohypophyseal cells express several β₁ integrin dimers and attach to ECM ligands corresponding to their binding specificities. The fact that these interactions are only partially blocked by RGDS peptides and antibodies against the β₁ family of integrins may indicate that other cell-matrix receptors are also present. Additional studies are necessary to determine whether these interactions have a functional significance (such as an effect on hormone secretion) beyond their role in cell-matrix adhesion.