Extraribosomal functions associated with the C terminus of the 37/67 kDa laminin receptor are required for maintaining cell viability

The 37/67 kDa laminin receptor (LAMR) is a multifunctional protein, acting as an extracellular receptor, localizing to the nucleus, and playing roles in rRNA processing and ribosome assembly. LAMR is important for cell viability; however, it is unclear which of its functions are essential. We developed a silent mutant LAMR construct, resistant to siRNA, to rescue the phenotypic effects of knocking down endogenous LAMR, which include inhibition of protein synthesis, cell cycle arrest, and apoptosis. In addition, we generated a C-terminal-truncated silent mutant LAMR construct structurally homologous to the Archaeoglobus fulgidus S2 ribosomal protein and missing the C-terminal 75 residues of LAMR, which displays more sequence divergence. We found that HT1080 cells stably expressing either silent mutant LAMR construct still undergo arrest in the G1 phase of the cell cycle when treated with siRNA. However, the expression of full-length silent mutant LAMR rescues cell viability, whereas the expression of the C-terminal-truncated LAMR does not. Interestingly, we also found that both silent mutant constructs restore protein translation and localize to the nucleus. Our findings indicate that the ability of LAMR to regulate viability is associated with its C-terminal 75 residues. Furthermore, this function is distinct from its role in cell proliferation, independent of its ribosomal functions, and may be regulated by a nonnuclear localization

Green Tea Polyphenol EGCG Sensing Motif on the 67-kDa Laminin Receptor

BACKGROUND: We previously identified the 67-kDa laminin receptor (67LR) as the cell-surface receptor conferring the major green tea polyphenol (-)-epigallocatechin-3-O-gallate (EGCG) responsiveness to cancer cells. However, the underlying mechanism for interaction between EGCG and 67LR remains unclear. In this study, we investigated the possible role of EGCG-67LR interaction responsible for its bioactivities. METHODOLOGY/PRINCIPAL FINDINGS: We synthesized various peptides deduced from the extracellular domain corresponding to the 102-295 region of human 67LR encoding a 295-amino acid. The neutralizing activity of these peptides toward EGCG cell-surface binding and inhibition of cancer cell growth were assayed. Both activities were inhibited by a peptide containing the 10-amino acid residues, IPCNNKGAHS, corresponding to residues 161-170. Furthermore, mass spectrometric analysis revealed the formation of a EGCG-LR161-170 peptide complex. A study of the amino acid deletion/replacement of the peptide LR161-170 indicated that the 10-amino acid length and two basic amino acids, K(166) and H(169), have a critical role in neutralizing EGCG's activities. Moreover, neutralizing activity against the anti-proliferation action of EGCG was observed in a recombinant protein of the extracellular domain of 67LR, and this effect was abrogated by a deletion of residues 161-170. These findings support that the 10 amino-acid sequence, IPCNNKGAHS, might be the functional domain responsible for the anti-cancer activity of EGCG. CONCLUSIONS/SIGNIFICANCE: Overall, our results highlight the nature of the EGCG-67LR interaction and provide novel structural insights into the understanding of 67LR-mediated functions of EGCG, and could aid in the development of potential anti-cancer compounds for chemopreventive or therapeutic uses that can mimic EGCG-67LR interactions

Interactions Between Laminin Receptor and the Cytoskeleton During Translation and Cell Motility

Human laminin receptor acts as both a component of the 40S ribosomal subunit to mediate cellular translation and as a cell surface receptor that interacts with components of the extracellular matrix. Due to its role as the cell surface receptor for several viruses and its overexpression in several types of cancer, laminin receptor is a pathologically significant protein. Previous studies have determined that ribosomes are associated with components of the cytoskeleton, however the specific ribosomal component(s) responsible has not been determined. Our studies show that laminin receptor binds directly to tubulin. Through the use of siRNA and cytoskeletal inhibitors we demonstrate that laminin receptor acts as a tethering protein, holding the ribosome to tubulin, which is integral to cellular translation. Our studies also show that laminin receptor is capable of binding directly to actin. Through the use of siRNA and cytoskeletal inhibitors we have shown that this laminin receptor-actin interaction is critical for cell migration. These data indicate that interactions between laminin receptor and the cytoskeleton are vital in mediating two processes that are intimately linked to cancer, cellular translation and migration

Co-regulation and physical association of the 67-kDa monomeric laminin receptor and the alpha6beta4 integrin.

The interactions between tumor cells and laminin or other components of the extracellular matrix have been shown to play an important role in tumor invasion and metastasis. However, the role of the monomeric 67-kDa laminin receptor (67LR) remains unclear. We analyzed the regulation of 67LR expression under different culture conditions with respect to the expression of other well characterized laminin receptors. In A431 cells treated with laminin for different time periods, the regulation of 67LR expression correlated with expression of the alpha6 integrin subunit but not with the expression of other laminin receptors. Moreover, cytokine treatment resulted in down-modulated expression of the alpha6 integrin subunit and the 67LR. Co-regulation of the expression of the two receptors was further suggested by the observation that specific down-modulation of the alpha6-chain by antisense oligonucleotides was accompanied by a proportional decrease in the cell surface expression of 67LR. Biochemical analyses indicated co-immunoprecipitation of 67LR and the alpha6 subunit with an anti-alpha6 but not an anti-beta1 monoclonal antibody. Co-regulation of 67LR and alpha6 subunit expression, together with the physical association between the two receptors, supports the hypothesis that 67LR is an auxiliary molecule involved in regulating or stabilizing the interaction of laminin with the alpha6beta4 integrin

Peptide G, containing the binding site of the 67-kDa laminin receptor, increases and stabilizes laminin binding to cancer cells.

We investigated the effect of peptide G, a synthetic peptide derived from the sequence of the 37-kDa laminin receptor precursor, on the interaction of laminin in two tumor cell lines one of which produces laminin and one of which does not. Addition of peptide G to the culture medium induced a significant increase in the amount of endogenous laminin detectable on the cell membrane of both cell lines. Moreover, pretreatment of exogenous laminin with peptide G dramatically increased laminin binding on both cell lines. Kinetics analysis of membrane-bound labeled laminin revealed a 3-fold decrease in the kd of peptide G-treated laminin compared with untreated or unrelated or scrambled peptide-treated laminin. Moreover, the affinity constant of peptide G-treated laminin increased 2-fold, with a doubling of the number of laminin binding sites, as determined by Scatchard analysis. Expression of the VLA6 integrin receptor on the cell membrane increased after incubation with peptide G-treated laminin. However, the lower binding inhibition of peptide G-treated laminin after anti-VLA6 antibody or cation chelation treatment indicates that membrane molecules in addition to integrin receptors are involved in the recognition of peptide G-modified laminin. These "new" laminin-binding proteins also mediated cell adhesion to laminin, the first step in tumor invasion. Together, the data suggest that peptide G increases and stabilizes laminin binding on tumor cells, involving surface receptors that normally do not take part in this interaction. This might explain the abundant clinical and experimental data suggesting a key role for the 67-kDa laminin receptor in the interaction between cancer cells and the basement membrane glycoprotein laminin during tumor invasion and metastasis

Production and characterization of monoclonal antibodies directed against the laminin receptor precursor.

The 67-kDa laminin receptor (67LR) is an important tumor marker whose molecular structure has not yet been fully elucidated. To shed new light on this molecule, we raised a series of eight new monoclonal antibodies, designated MPLR1 to 8, directed against the 37-kDa recombinant laminin receptor precursor (37LRP). Cross-competition experiments demonstrated that the epitopes recognized by MPLR2, 4 and 5 partially overlap, since MPLR4 and 5 compete with labelled MPLR2 for the binding to recombinant 37LRP. These three antibodies belong to the IgG1 class, whereas the other ones are all IgM. Presumably due to the fact that they are directed against partially unfolded antigenic determinants expressed on the recombinant protein, MPLRs did not recognize the native protein. Indeed, they showed no reactivity at the membrane level in cytofluorimetric analysis and they did not work in immunoprecipitation experiments. In contrast, these reagents are valuable tools in immunoblotting, since they clearly identify a 67-kDa protein (the mature laminin receptor) in addition to the 37-kDa precursor form. MPLRs are thus a new powerful tool which could help in the characterization of the still enigmatic 67LR molecule