The muscle-specific laminin receptor alpha 7 beta 1 integrin negatively regulates alpha 5 beta 1 fibronectin receptor function

alpha 7 beta 1 is the major integrin complex expressed in differentiated mu scle cells where it functions as a laminin receptor, In this work we have e xpressed the alpha 7 integrin subunit in CHO cells to investigate the funct ional properties of this receptor. After transfection with alpha 7 CHO cell s acquired the ability to adhere and spread on laminin 1 consistent with th e laminin receptor activity of the alpha 7 beta 1, alpha 7 transfectants, h owever, showed a 70% reduction in the ability to adhere to fibronectin and were unable to assemble a fibronectin matrix. The degree of reduction was i nversely related to the level of alpha 7 expression. To define the mechanis ms underlying this adhesive defect we analyzed surface expression and funct ional properties of the alpha 5 beta 1 fibronectin receptor. Although cell surface expression of alpha 5 beta 1 was reduced by a factor of 20-25% in a lpha 7 transfectants compared to control untransfected cells, this slight r eduction was not sufficient to explain the dramatic reduction in cell adhes ion (70%) and matrix assembly (close to 100%). Binding studies showed that the affinity of I-125-fibronectin for its surface receptor was decreased by 50% in alpha 7 transfectants, indicating that the alpha 5 beta 1 integrin is partially inactivated in these cells. Inactivation can be reversed by Mn 2+, a cation known to increase integrin affinity for their ligands, In fact , incubation of cells with Mn2+ restored fibronectin binding affinity, adhe sion to fibronectin, and assembly of fibronectin matrix in alpha 7 transfec tants. These data indicate that alpha 7 expression leads to the functional down regulation of alpha 5 beta 1 integrin by decreasing ligand binding aff inity and surface expression. In conclusion, the data reported establish th e existence of a negative cooperativity between alpha 7 and alpha 5 integri ns that may be important in determining functional regulation of integrins during myogenic differentiation. (C) 1999 Academic Press.