The role of the cytoplasmic domain of alpha 6 integrin in the assembly and function of alpha 6 beta 1 and alpha 6 beta 4

PI3Kγ stimulates a high molecular weight form of myosin light chain kinase to promote myeloid cell adhesion and tumor inflammation

Myeloid cells play key roles in cancer immune suppression and tumor progression. In response to tumor derived factors, circulating monocytes and granulocytes extravasate into the tumor parenchyma where they stimulate angiogenesis, immune suppression and tumor progression. Chemokines, cytokines and interleukins stimulate PI3Kγ-mediated Rap1 activation, leading to conformational changes in integrin α4β1 that promote myeloid cell extravasation and tumor inflammation Here we show that PI3Kγ activates a high molecular weight form of myosin light chain kinase, MLCK210, that promotes myosin-dependent Rap1 GTP loading, leading to integrin α4β1 activation. Genetic or pharmacological inhibition of MLCK210 suppresses integrin α4β1 activation, as well as tumor inflammation and progression. These results demonstrate a critical role for myeloid cell MLCK210 in tumor inflammation and serve as basis for the development of alternative approaches to develop immune oncology therapeutics.

Myeloid cell recruitment during tumor inflammation depends on the VCAM-1 receptor integrin α4β1. Here the authors show that a high molecular weight form of myosin light chain kinase, MLCK210, is required for myeloid cell integrin α4β1 activation and adhesion and that MLCK210 inhibition reduces tumor growth and inflammation in preclinical cancer models.

Functional characterization of integrin alpha6beta4 adhesion interactions using soluble integrin constructs reveals the involvement of different functional domains in the beta4 subunit

Integrin alpha6beta4-mediated adhesion interactions play key roles in keratinocyte and epithelial tumor cell biology. In order to evaluate how alpha6beta4 adhesion interactions contribute to these important cellular processes, the authors generated soluble versions of the integrin by recombinant expression of the subunit ectodomains fused to a human immunoglobulin G (IgG) Fc constant domain. Coexpression of the appropriate subunits enabled dimerization, secretion and purification of stable Fc-containing alpha6beta4 heterodimers. The soluble proteins exhibited the same metal ion and ligand dependency in their binding characteristics as intact alpha6beta4. Using these reagents in combination with anti-beta4 antibodies, the authors identified two distinct functional epitopes on the beta4 subunit. They demonstrated the involvement of one epitope in adhesion interactions and the other in regulating adhesion-independent growth in alpha6beta4-expressing tumor cell lines. The availability of these soluble integrin reagents and the data provided herein help to further delineate the structure-function relationships regulating alpha6beta4 signaling biology.

Generation of a minimal alpha5beta1 integrin-Fc fragment

The tertiary structure of the integrin heterodimer is currently unknown, although several predictive models have been generated. Detailed structural studies of integrins have been consistently hampered for several reasons, including the small amounts of purified protein available, the large size and conformational flexibility of integrins, and the presence of transmembrane domains and N-linked glycosylation sites in both receptor subunits. As a first step toward obtaining crystals of an integrin receptor, we have expressed a minimized dimer. By using the Fc dimerization and mammalian cell expression system designed and optimized by Stephens et al. (Stephens, P. E., Ortlepp, S., Perkins, V. C., Robinson, M. K., and Kirby, H. (2000) Cell. Adhes. Commun. 7, 377-390), a series of recombinant soluble human alpha(5)beta(1) integrin truncations have been expressed as Fc fusion proteins. These proteins were examined for their ligand-binding properties and for their expression of anti-integrin antibody epitopes. The shortest functional alpha(5)-subunit truncation contained the N-terminal 613 residues, whereas the shortest beta(1)-subunit was a fragment containing residues 121-455. Each of these minimally truncated integrins displayed the antibody binding characteristics of alpha(5)beta(1) purified from human placenta and bound ligand with the same apparent affinity as the native receptor.

The integrin alpha 7 cytoplasmic domain regulates cell migration, lamellipodia formation, and p130CAS/Crk coupling

The integrin alpha(7)beta(1) is the major laminin-binding integrin in skeletal, heart, and smooth muscle and is a receptor for laminin-1 and -2. It mediates myoblast migration on laminin-1 and -2 and thus might be involved in muscle development and repair. Previously we have shown that alpha(7)B as well as the alpha(7)A and -C splice variants induce cell motility on laminin when transfected into nonmotile HEK293 cells. In this study we have investigated the role of the cytoplasmic domain of alpha(7) in the laminin-induced signal transduction of alpha(7)beta(1) integrin regulating cell adhesion and migration. Deletion of the cytoplasmic domain did not affect assembly of the mutated alpha(7)Deltacyt/beta(1) heterodimer on the cell surface or adhesion of alpha(7)Deltacyt-transfected cells to laminin. The motility of these cells on the laminin-1/E8 fragment, however, was significantly reduced to the level of mock-transfected cells; lamellipodia formation and polarization of the cells were also impaired. Adhesion to the laminin-1/E8 fragment induced tyrosine phosphorylation of the focal adhesion kinase, paxillin, and p130(CAS) as well as the formation of a p130(CAS)-Crk complex in wild-type alpha(7)B-transfected cells. In alpha(7)BDeltacyt cells, however, the extent of p130(CAS) tyrosine formation was reduced and formation of the p130(CAS)-Crk complex was impaired, with unaltered levels of p130(CAS) and Crk protein levels. These findings indicate adhesion-dependent regulation of p130(CAS)/Crk complex formation by the cytoplasmic domain of alpha(7)B integrin after cell adhesion to laminin-1/E8 and imply alpha(7)B-controlled lamellipodia formation and cell migration through the p130(CAS)/Crk protein complex.

Talin controls the exit of the integrin alpha 5 beta 1 from an early compartment of the secretory pathway

Talin is a major cytosolic protein that links the intracellular domains of beta1 and beta3 integrins to the cytoskeleton. It is required for focal adhesion assembly. However, its downregulation not only slows down cell spreading and organization of focal adhesions but also impairs the maturation of some beta1 integrins, including the fibronectin receptor alpha5beta1. To investigate this, we characterized the beta1 integrin synthesized in cells expressing talin anti-sense RNA (AT22 cells). We identified a large intracellular pool of beta1 integrins that is abnormally accumulated in an earlier compartment of the secretory pathway. In this report, we show that in talin-deficient AT22 cells, the aberrant glycosylation of integrin receptors is accompanied by a delay in the export of the integrin alpha5beta1. In normal cells, talin was found associated with beta1 integrins in an enriched membrane fraction containing Golgi and endoplasmic reticulum. Finally, microinjection of anti-talin antibodies resulted in accumulation of the integrins within the cells. These data strongly suggest that talin plays a specific role in the export of newly synthesized integrins. We propose that talin binding to the integrin may disclose a diphenylalanine export signal, which is present in the membrane-proximal GFFKR motif conserved in all integrin alpha chains.

Expression of a soluble functional form of the integrin alpha4beta1 in mammalian cells

The integrin alpha4beta1(VLA4) has been expressed as a soluble, active, heterodimeric immunoglobulin fusion protein. cDNAs encoding the extracellular domains of the human alpha4 and beta1 subunits were fused to the genomic DNA encoding the human gamma1 immunoglobulin Fc domain and functional integrin fusion protein was expressed as a secreted, soluble molecule from a range of mammalian cell lines. Specific mutations were introduced into the Fc region of the molecules to promote alpha4beta1 heterodimer formation. The soluble alpha4beta1-Fc fusion protein exhibited divalent cation dependent binding to VCAM-1, which was blocked by the appropriate function blocking antibodies. The apparent Kd for VCAM-1 binding were similar for both the soluble and native forms of alpha4beta1. In addition, the integrin-Fc fusion was shown to stain cells expressing VCAM-1 on their surface by FACs analysis. This approach for expressing soluble alpha4beta1 should be generally applicable to a range of integrins.

Isolated deficient alpha6beta4 integrin expression in the gut associated with intractable diarrhea

BACKGROUND

An infant born with pyloric atresia had development of intractable diarrhea and was found to have total epithelial detachment of gastric and small and large bowel mucosa. She had no skin abnormalities. Parental consanguinity and pyloric atresia in a sibling who died without autopsy suggest an inherited origin for this disorder. The purpose of this study was to examine defects in intestinal and skin cell adhesion.

METHODS

Histologic, immunohistochemical, and ultrastructural characteristics of the skin and gut of the patient were compared with that of normal control subjects. Distribution of adhesion molecules was determined.

RESULTS

Immunofluorescent analysis of the digestive mucosa showed alpha6beta4 integrin expression deficiency at the epithelial cell-lamina propria junction. Ultrastructural examination of the digestive mucosa revealed a complete epithelial detachment with a cleavage plane lying between the lamina densa and the basal pole of the enterocytes. Consistent with the absence of skin blistering, integrin alpha6beta4 was expressed at the dermal-epidermal junction. Electron micrographs of skin biopsy specimens showed the presence of normal hemidesmosomes and the absence of dermal-epidermal dysadhesion.

CONCLUSION

It was postulated that this patient had protracted diarrhea related to epithelial detachment of the digestive mucosa as a consequence of a deficiency of an integrin alpha6beta4 isoform specific to the gut.

Increased levels of alpha6 integrins are associated with the metastatic phenotype of human breast cancer cells

Integrins play an important role in interactions between cells and the extracellular matrix, and thus have a potential role in metastasis. Expression levels of alpha6, beta1 and beta4 integrin sub-units were measured in a panel of human breast cancer cell lines by RT/PCR, immunoprecipitation and flow cytometry. All the lines expressed alpha6, with the highest levels in the MDA-MB-231 and MDA-MB-435 cells. These grew the most aggressively and were metastastic in nude mice. Low levels of alpha6 protein were measured in breast cancer cells that were poorly tumorigenic and non-metastatic in nude mice, and there was an inverse relationship between ER and alpha6 expression. RT/PCR revealed that all lines expressed the 2 isoforms of alpha6, with the alpha6A isoform generally more abundant than alpha6B isoform. Clones of MDA-MB-435 were isolated by sterile sorting for cells with high or low alpha6 expression, and two variants established from metastases in nude mice were found to differ in alpha6 expression. When injected into nude mice. the alpha6-high variants produced significantly more lung metastases than the alpha6-low variants. beta1was abundant in all lines, while beta4 was not detected in MDA-MB- 134 cells, and in the MDA-MB-435 cells an alternately spliced variant of beta4 was identified. Sequencing of the alternate variant revealed a novel sequence from a splicing event in the cytoplasmic tail of beta4. None of the cells with this variant mRNA expressed detectable levels of beta4 protein. Our results suggest that high alpha6 expression in human breast cancer cells is associated with tumorigenicity and metastatic potential.

The two phenylalanines in the GFFKR motif of the integrin alpha6A subunit are essential for heterodimerization

The membrane-proximal domain of the integrin alpha subunit contains a conserved motif of five amino acid residues, GFFKR. We deleted this motif from the human alpha6A subunit and found that in COS-7 cells this mutant cannot associate with the beta1 subunit and is retained in the endoplasmic reticulum. Point mutations in the GFFKR motif of the glycine residue or the two highly charged amino acids, or deletion of the lysine and arginine residues, had no effect on the ability of alpha6 to interact with beta1 and to be expressed at the cell surface. In contrast, by replacing either of the two phenylalanines with alanine, or by deletion of both of these residues, alpha6 was incapable of associating with beta1. The alpha6 point mutants that associated with beta1 were expressed in K562 cells and their responsiveness to integrin-activating factors was determined. None of these transfectants bound spontaneously to laminin-1, but binding could be induced by either PMA or the stimulating anti-beta1 antibody TS2/16 to the same extent as that of the wild-type transfectant. The ability of these mutants to initiate focal-contact formation in CHO cells plated on laminin-1 substrates also appeared to be unaltered. Thus the behaviour of alpha6 mutants involving the glycine, lysine or arginine residues was indistinguishable from that of wild-type alpha6 both in inside-out and outside-in signalling. In contrast, deletion of the cytoplasmic domain of alpha6 C-terminal of the GFFKR motif resulted in a loss of responsiveness of alpha6beta1 to PMA stimulation and formation of focal contacts on laminin-1. However, this mutant was targeted to focal contacts formed by other integrins, even when they had not bound ligand. Together, these results suggest that the two phenylalanine residues of the GFFKR motif provide a site for interaction of the alpha6A subunit with beta1, whereas the cytoplasmic domain C-terminal of this motif is involved in the regulation of bidirectional signalling via alpha6Abeta1.