The Integrin a 6 ß 4 Is a Laminin Receptor

In this study, the putative laminin receptor function of the a6ß4 integrin was assessed . For this purpose, we used a human cell line, referred to as clone A, that was derived from a highly invasive, colon adenocarcinoma . This cell line, which expresses the a6ß4 integrin, adheres to the ES and not to the PI fragment of laminin . The adhesion of clone A cells to laminin is extremely rapid with half-maximal adhesion observed at 5 min after plating . Adhesion to laminin is blocked by GoH3, an a6 specific antibody (60% inhibition), as well as by A9, a 04 specific antibody (30% inhibition) . Most importantly, we demonstrate that a6ß4 binds specifically to lamininELL adhesion to laminin, as well as other extracellular matrix proteins, is mediated by multiple integrins (Hynes, 1987 ; Albeda and Buck, 1990) . To date, at least five different integrins a1ß1, a2ß1, 001, a6ß1, and avß3 are widely accepted as having laminin receptor function on the basis of two criteria : (a) they bind to laminin affinity columns in a divalent cation dependent manner; and (b) mAbs specific for their a and ß subunits block adhesion to laminin (reviewed in Mercurio and Shaw, 1991) . Many cells that have been examined express more than one of these laminin-binding integrins, though the functional significance of this apparent redundancy is unclear. Of all the known laminin binding integrins, a6ß1 appears to play a preeminent role in mediating laminin adhesion in a variety of cell types (Sonnenberg et al ., 1988, 1990 ; Shaw et al ., 1990 ; de Curtis et al ., 1991 ; Kramer et al ., 1990 ; Hall et al ., 1990 ; Elices et al ., 1991 ; Shimizu et al ., 1990 ; Cooper et al ., 1991) . The only ligand that has been identified for this integrin is laminin (Kramer et al ., 1990 ; Elices et al ., 1991) . In contrast, all ofthe other known laminin-binding integrins are capable of binding more than one matrix protein (Mercurio and Shaw, 1991) . Additional evidence for the key role of this integrin comes from studies on cells such as macrophages (Shaw et al ., 1990) and some types of neurons (de Curtis et al ., 1991) whose ability to adhere to laminin is regulated by physiological and developmental conditions . In such cells, it appears that a posttranslational mechanism regulates the laminin binding function of the a6ßl integrin (Shaw et al ., 1990 ; de Curtis et al ., 1991 ; Shimizu et al ., 1990 ; Neugebauer and Reichardt, 1991) . The nature of this mechanism is an issue of considerable interest at present . © The Rockefeller University Press, 0021-9525/92/05/671/8 $2 . 00 The Journal of Cell Biology, Volume 117, Number 3, May 1992 671-678 Sepharose columns in the presence of either Mgt+ or Mn2+ and it is eluted from these columns with EDTA but not with NaCl. The a6ß4 integrin does not bind to collagen-Sepharose, but the a2ß1 integrin does bind . Clone A cells do not express a6ß1 as evidenced by the following observations : (a) no ßl integrin is detected in ßl immunoblots of GoH3 immunoprecipitates ; and (b) no a6ß1 integrin is seen in GoH3 immunoprecipitates of clone A extracts that had been immunodepleted of all 04 containing integrin using the A9 antibody. These data establish that laminin is a ligand for the a6ß4 integrin and that this integrin can function as a laminin receptor independently of a6ß1 . The function of the a6 integrin subunit has been complicated by the finding that this subunit can also associate with a different 0 subunit, namely /34 (Hemler et al ., 1989 ; Kajiiji et al ., 1989) . The 04 subunit is expressed primarily on epithelial cells and their oncogenically transformed derivatives, although it also found in endothelial and some neuronal cells (reviewed in Quaranta and Jones, 1991) . An obvious question, based on the behavior of a6ß1, is whether a6ß4 can function as a laminin receptor, and, if so, whether this function differs from the regulated behavior of a6ß1 . This question has caused considerable controversy in the recent literature . Although we reported that a6ß4 can function as a laminin receptor on colon carcinoma cells based on the ability of an a6 specific antibody to block adhesion (Lotz et al ., 1990), this function for a6ß4 has not been widely accepted . Several papers have concluded, for example, that the ligand for a6ß4 must be distinct from that of a6ß1 (Sonnenberg et al ., 1990) and have emphasized that a6ß4 has not been shown to bind laminin affinity columns (Deluca et al ., 1990 ; Quaranta and Jones, 1991) . One tacit assumption in many of these studies has been that the a6-dependent adhesion of cells that express a6ß4 is mediated by low levels ofa6ß1 and not a6ß4 . Moreover, recent studies on a6ß4 have not addressed this laminin receptor function directly, but have focused on its function in stratified epithelial cells such as keratinocytes (Carter et al ., 1990 ; Quaranta and Jones, 1991 ; Sonnenberg et al ., 1991 ; Stepp et al ., 1990) . These groups have postulated that a6ß4 may play a critical role in the assembly and maintenance ofhemidesmosomes . This integrin is localized along the basal surface of keratinocytes suggesting its probable function in cell to basement membrane inter671 on N ovem er 0, 2010 jcb.rress. org D ow nladed fom Published May 1, 1992