Multiple cell surface receptors for the short arms of laminin: α1β1 integrin and RGD-dependent proteins mediate cell attachment only to domains III in murine tumor laminin

Abstract

Cell surface molecules that interact with the cross formed by the three short arms of murine tumor laminin were studied using thermal perturbation, antibody and peptide blocking, and affinity chromatography. Several potential receptors for the laminin short arms were revealed that differed from those mediating cell attachment to the E8 (long arm) fragment. Two cell lines, Rugli and L8 attached well to E1-X (short arm) fragments of laminin. This attachment was blocked by antibodies against α1 integrin chains. Other cells were unable to attach strongly to E1-X, but attached to P1. This attachment was unaffected by anti-β1 integrin antibodies, but specifically blocked by the peptide GRGDS. By contrast, binding of Rugli cells was RGD independent and blocked by anti-β integrin antibodies. G7 and C2C12 myoblasts were very sensitive to GRGDS (ID50 ∼2 μg·ml-1) for attachment to P1 which implied that a non-β1 series integrin, possibly αvβ3, was involved. On heat denaturation of P13 attachment remained sensitive to RGDS and ID50 was unchanged. On heat denaturation of E1-X, attachment remained sensitive to RGDS but the ID50 increased to ∼200 μg·ml-1. Cellular β1 integrins were retained on laminin affinity columns. A β1 integrin with an ∼190 kD α-chain could be isolated from Rugli cells whose attachment could be blocked by anti-α1 antibodies and not from cells blocked by RGDS peptides. Anti-α1 antibodies blocked Rugli attachment to native laminin, but only when the E8 cell binding sites on laminin were also blocked. Thus, a receptor related to α1β1 integrin can function simultaneously with a receptor for E8. Anti-α1 also blocked attachment to heated laminin, suggesting that the heat-stable attachment activity in laminin involved the E1-X binding site. Thus, at least two putative receptors mediate attachment to the short arms of laminin. One, related to α1β1 integrin, recognizes RGDS-independent sites in E1-X defined by P1 (within domains III, IIIa, IIIb), and one is an RGD-dependent molecule recognizing sites in P1, and is not a β1 integrin.