Production and Characterization of Monoclonal Antibodies Directed against the Laminin Receptor Precursor

The Transition of the 37-Kda Laminin Receptor (Rpsa) to Higher Molecular Weight Species: Sumoylation or Artifact?

The 37-kDa laminin receptor (37LRP or RPSA) is a remarkable, multifaceted protein that functions in processes ranging from matrix adhesion to ribosome biogenesis. Its ability to engage extracellular laminin is further thought to contribute to cellular migration and invasion. Most commonly associated with metastatic cancer, RPSA is also increasingly found to be important in other pathologies, including microbial infection, neurodegenerative disease and developmental malformations. Importantly, it is thought to have higher molecular weight forms, including a 67-kDa species (67LR), the expression of which is linked to strong laminin binding and metastatic behavior. The composition of these larger forms has remained elusive and controversial. Homo- and heterodimerization have been proposed as events capable of building the larger species from the monomeric 37-kDa precursor, but solid evidence is lacking. Here, we present data suggesting that higher molecular weight species require SUMOylation to form. We also comment on the difficulty of isolating larger RPSA species for unambiguous identification and demonstrate that cell lines stably expressing tagged RPSA for long periods of time fail to produce tagged higher molecular weight RPSA. It is possible that higher molecular weight species like 67LR are not derived from RPSA.

Looking into laminin receptor: critical discussion regarding the non-integrin 37/67-kDa laminin receptor/RPSA protein

The 37/67-kDa laminin receptor (LAMR/RPSA) was originally identified as a 67-kDa binding protein for laminin, an extracellular matrix glycoprotein that provides cellular adhesion to the basement membrane. LAMR has evolutionary origins, however, as a 37-kDa RPS2 family ribosomal component. Expressed in all domains of life, RPS2 proteins have been shown to have remarkably diverse physiological roles that vary across species. Contributing to laminin binding, ribosome biogenesis, cytoskeletal organization, and nuclear functions, this protein governs critical cellular processes including growth, survival, migration, protein synthesis, development, and differentiation. Unsurprisingly given its purview, LAMR has been associated with metastatic cancer, neurodegenerative disease and developmental abnormalities. Functioning in a receptor capacity, this protein also confers susceptibility to bacterial and viral infection. LAMR is clearly a molecule of consequence in human disease, directly mediating pathological events that make it a prime target for therapeutic interventions. Despite decades of research, there are still a large number of open questions regarding the cellular biology of LAMR, the nature of its ability to bind laminin, the function of its intrinsically disordered C-terminal region and its conversion from 37 to 67 kDa. This review attempts to convey an in-depth description of the complexity surrounding this multifaceted protein across functional, structural and pathological aspects.

The metastasis-associated 67-kDa laminin receptor is involved in G-CSF–induced hematopoietic stem cell mobilization

The 67-kDa laminin receptor (67LR) is a nonintegrin cell-surface receptor with high affinity for laminin, which plays a key role in tumor invasion and metastasis. We investigated the role of 67LR in granulocyte colony-stimulating factor (G-CSF)–induced mobilization of CD34+ hematopoietic stem cells (HSCs) from 35 healthy donors. G-CSF–mobilized HSCs, including CD34+/CD38– cells, showed increased 67LR expression as compared with unstimulated marrow HSCs; noteworthy, also, is the fact that the level of 67LR expression in G-CSF–mobilized HSCs correlated significantly with mobilization efficiency. During G-CSF–induced HSC mobilization, the expression of laminin receptors switched from α6 integrins, which mediated laminin-dependent adhesion of steady-state human marrow HSCs, to 67LR, responsible for G-CSF–mobilized HSC adhesion and migration toward laminin. In vitro G-CSF treatment, alone or combined with exposure to marrow-derived endothelial cells, induced 67LR up-regulation in marrow HSCs; moreover, anti-67LR antibodies significantly inhibited transendothelial migration of G-CSF–stimulated marrow HSCs. Finally, G-CSF–induced mobilization in mice was associated with 67LR up-regulation both in circulating and marrow CD34+ cells, and anti-67LR antibodies significantly reduced HSC mobilization, providing the first in vivo evidence for 67LR involvement in stem-cell egress from bone marrow after G-CSF administration. In conclusion, 67LR up-regulation in G-CSF–mobilized HSCs correlates with their successful mobilization and reflects its increase in marrow HSCs, which contributes to the egress from bone marrow by mediating laminin-dependent cell adhesion and transendothelial migration.

Identification of genes up-regulated in urothelial tumors: the 67-kd laminin receptor and tumor-associated trypsin inhibitor

Studies investigating changes in gene expression in urothelial carcinoma have generally compared tumors of different stages and grades but comparisons between low-grade, noninvasive tumors and normal urothelium are needed to identify genes involved in early tumor development. We isolated the urothelium from a low-grade tumor and corresponding normal mucosa by laser capture microdissection on frozen sections. The RNA extracted was amplified to generate suppressive subtractive cDNA libraries. Random sequencing of cDNA clones identified approximately 100 unique species. Of these 83% were known genes, 15% had homology to genes with an unknown function in humans, and 2% did not show homology to any published gene sequence. Two of the known genes, the 67-kd laminin receptor (67LR) and tumor-associated trypsin inhibitor (TATI), had previously been associated with metastatic progression in many tumor types, although 67LR has not been investigated in urothelial tumors. Immunolabeling of the original tissue with antibodies against these two genes confirmed overexpression, validating our strategy: 67LR was not expressed in the normal urothelium but was present in the tumor, whereas TATI expression was confined to umbrella cells in the normal urothelium, but extended to all cell layers in the tumor. We investigated both markers further in a separate series of tumors of different stages and grades. TATI was more consistently overexpressed than 67LR in all tumor grades and stages. Levels of secreted TATI were significantly higher in urine samples from patients with tumors compared to controls. Our strategy, combining laser capture microdissection and cDNA library construction, has identified genes that may be involved in the early phases of urothelial tumor development rather than with disease progression, highlighting the importance of comparing tumor with normal rather than just tumors of different stages and grades.

p53-dependent downregulation of metastasis-associated laminin receptor

Based on observations suggesting a role for the tumor suppressor protein p53 in regulating expression of the 67-kDa laminin receptor precursor, 37LRP, we analysed the 37LRP promoter activity in a wild-type p53 (wt p53) ovarian carcinoma cell line and in a cisplatin-resistant subline with mutated p53. We observed an increased promoter activity in wt p53 cells as compared to the mutated-p53 line when the first intron of the 37LRP gene was present in the reporter construct. Cotransfection experiments showed that the promoter is downregulated by both wt and mutated p53. Deletion analysis of the first intron localized an enhancer activity in the first 5' 214 bp that upregulates both 37LRP and SV40 promoter activity and is repressed by both wt and mutant p53. Cotransfection, mutagenesis and gel-shift experiments identified a functional AP-2 cis-acting element in this intron region that is repressed by increased levels of both wt and mutated p53. Coimmunoprecipitation studies revealed AP-2 in physical association in vivo with both wt and mutated p53, indicating for the first time that interaction of p53 with AP-2 is involved in the repression mechanism and in the regulation of genes involved in cancer growth and progression.

Laminin receptors in differentiated thyroid tumors: restricted expression of the 67-kilodalton laminin receptor in follicular carcinoma cells

The expression of integrin laminin receptors was investigated in normal thyroid primary cultures; immortalized normal thyroid cells (TAD-2); papillary (NPA), follicular (WRO), and anaplastic (ARO) thyroid tumor cell lines; seven thyroid tumors (four papillary and three follicular carcinomas); and normal thyroid glands. The expression of alpha1beta1, alpha2beta1, alpha3beta1, alpha6beta1, and alpha6beta4 was found in all tumor specimens and in tumor cell lines, whereas normal thyroid cells and TAD-2 cells lacked the expression of alpha6beta4. Despite the presence of several integrin laminin receptors, adhesion of TAD-2, NPA, and ARO cells to immobilized laminin-1 was poor, whereas WRO cells and follicular carcinoma-derived cells displayed a strong adhesion. Indeed, WRO and follicular carcinoma-derived cells showed expression of a nonintegrin laminin receptor, the 67-kDa high affinity laminin receptor (67LR). TAD-2, NPA, and ARO cells as well as nodular goiter, toxic adenoma, follicular adenoma, and papillary carcinoma-derived cells did not express the 67LR. Adhesion of WRO and follicular carcinoma-derived cells to laminin-1 was specifically inhibited by a recombinant polypeptide containing laminin-binding domains of 67LR, demonstrating that this receptor confers to follicular carcinoma cells attachment capacity to laminin. Moreover, tissue specimens from follicular carcinomas expressed the 67LR, whereas follicular adenomas and normal thyroid tissues were negative. In thyroid tumors, integrin receptors, although abundant, participate weakly in adhesion to laminin. The expression in follicular carcinoma cells of a functional, high affinity 67LR together with nonfunctional integrin LM receptors could be responsible for the tendency of follicular carcinoma cells to metastasize by mediating stable contacts with basal membranes.

Cell surface and substrate distribution of the 67-kDa laminin-binding protein determined by using a ligand photoaffinity probe

BACKGROUND

Peptide 11, a nine-amino acid sequence from the beta1 chain of laminin-1, has been reported to inhibit tumor cell invasion of basement membranes, and to reduce tumor lung colonization (Iwamoto et al.: Science 238:1132-1134, 1987; Landowski et al.: Clin Exp Metastasis 13:357-372, 1995). The peptide is a ligand for the 32/67-kDa laminin-binding protein (LBP); however, the mechanism by which the 67-kDa LBP promotes invasion is unknown.

METHODS

We have synthesized a highly specific probe for the 67-kDa LBP by adding a biotinylated residue, and replacing the required tyrosine in peptide 11 with the photoactivatable bezophenone crosslinker, 4-benzoyl-L-phenylalanine. This probe was used to follow the distribution of the 67-kDa LBP by gel electrophoresis, fluorescence-activated cell scanning, and confocal microscopy techniques.

RESULTS

A single crosslinked protein, consistent with the high molecular weight form of the LBP, was found on Western blots of membrane detergent extracts from cells treated with the ligand probe. A CHO cell line, manipulated to overexpress the laminin-specific alpha6beta1 integrin, exhibited increased invasiveness, and expressed more cell surface 67-kDa LBP. Membrane-associated 67-kDa LBP was found in the vicinity of focal adhesion plaques and also associated with the matrix substrate. Studies on conditioned medium indicated that the matrix-associated LBP derived from material that was shed from the cells, with more being shed from the more invasive CHO variants.

CONCLUSIONS

These results demonstrate the utility of this novel probe in diverse experimental protocols, and suggest that shedding of the 67-kDa LBP may have a role in promoting tumor cell invasion.

TNF-alpha and IFN-gamma down-regulate the expression of the metastasis-associated bi-functional 37LRP/p40 gene and protein in transformed keratinocytes

The 37 LRP/p40 molecule is a bi-functional protein in which expression is increased in a large variety of cancers in association with their metastatic phenotype. Here we present the first data concerning the 37 LRP/p40 gene promoter activity and show that it is very active in a cervix carcinoma cell line. Interestingly, despite hallmarks of a housekeeping gene, we show that the 37 LRP/p40 gene promoter can be down-regulated by two potentially anticancerous cytokines, TNF-alpha and IFN-gamma. In addition, the dual fate of the protein, i.e., being intracellularly involved in the cell translation machinery and incorporated into a 67-kDa cell surface protein functioning as a laminin receptor (67LR), is differentially affected by the treatment. Our data suggest multiple regulation levels in the control of the 67LR/37LRP/p40 molecule expression and uncover new clues for the understanding of both the control of expression of this metastasis-associated molecule and the IFN-gamma and TNF-alpha anticancerous action.

Formation of the 67-kDa laminin receptor by acylation of the precursor

Even though the involvement of the 67-kDa laminin receptor (67LR) in tumor invasiveness has been clearly demonstrated, its molecular structure remains an open problem, since only a full-length gene encoding a 37-kDa precursor protein (37LRP) has been isolated so far. A pool of recently obtained monoclonal antibodies directed against the recombinant 37LRP molecule was used to investigate the processing that leads to the formation of the 67-kDa molecule. In soluble extracts of A431 human carcinoma cells, these reagents recognize the precursor molecule as well as the mature 67LR and a 120-kDa molecule. The recovery of these proteins was found to be strikingly dependent upon the cell solubilization conditions: the 67LR is soluble in NP-40-lysis buffer whereas the 37LRP is NP-40-insoluble. Inhibition of 67LR formation by cerulenin indicates that acylation is involved in the processing of the receptor. It is likely a palmitoylation process, as indicated by sensitivity of NP-40-soluble extracts to hydroxylamine treatment. Immunoblotting assays performed with a polyclonal serum directed against galectin3 showed that both the 67- and the 120-kDa proteins carry galectin3 epitopes whereas the 37LRP does not. These data suggest that the 67LR is a heterodimer stabilized by strong intramolecular hydrophobic interactions, carried by fatty acids bound to the 37LRP and to a galectin3 cross-reacting molecule.