Characterization of integrins in cultured human renal cortical tubule epithelial cells

We have characterized the integrins present on cultured tubule epithelial cells from human renal cortexes, enriched for proximal cells, using fluorescence microscopy, immunoprecipitation, and cell adhesion assays. By immunofluorescence, the α₃-integrin subunit stained most intensely and was present on all cells predominantly at cell-cell contacts. The α₆-subunit was present on all cells in a pattern consistent with extracellular matrix contacts. The α₅-subunit was present on most cells in a cell-matrix contact pattern; α_V-subunit was weakly positive and occasionally seen in cell-matrix contacts. The α₂-subunit was present on clusters of distal tubule cells, predominantly at cell-cell contacts. Immunoprecipitation revealed the predominant integrin to be α₃β₁ with some α₂β₁, presumably contributed by distal cells. The α₅β₁-, α₆β₁-, α₆β₄-, and α_Vβ₃-integrins, as well as trace amounts of α₁β₁-integrins, were also present. The α₄β₁-integrin was not detected. Initial attachment to fibronectin was mediated by α_Vβ₃- and α₅β₁-integrins; initial attachment to laminin was mediated by the α₆β₁- and α₃β₁-integrins and, in some preparations, by an unidentified integrin; and initial attachment to collagen type IV was mediated by α_Vβ₃-integrin and an unidentified β₁-integrin. After extensively immunodepleting membrane extracts with anti-α₁, -α₂, -α₃, -α₄, -α₅, -α₆, and -α_V antibodies, an anti-β₁ antibody still precipitated an integrin. Its electrophoretic mobility differs from the laminin-binding α₇β₁-integrin. Thus we have identified many of the integrins on cortical tubule cells and their role in mediating initial attachment to extracellular matrix. However, the cell adhesion assays and immunoprecipitations suggest the presence of an unidentified β₁-integrin that may mediate renal tubule cell attachment to laminin and collagen.