Surface Expressed Nucleolin Is Constantly Induced in Tumor Cells to Mediate Calcium-Dependent Ligand Internalization

Abstract

BackgroundNucleolin is one of the major proteins of the nucleolus, but it is also expressed on the cell surface where is serves as a binding protein for variety of ligands implicated in tumorigenesis and angiogenesis. Emerging evidence suggests that the cell-surface expressed nucleolin is a strategic target for an effective and nontoxic cancer therapy.Methodology/Principal FindingsBy monitoring the expression of nucleolin mRNA, and by measuring the level of nucleolin protein recovered from the surface and nucleus of cells, here we show that the presence of nucleolin at the cell surface is dependent on the constant induction of nucleolin mRNA. Indeed, inhibitors of RNA transcription or translation block expression of surface nucleolin while no apparent effect is observed on the level of nucleolin in the nucleus. The estimated half-life of surface nucleolin is less than one hour, whereas that of nuclear nucleolin is more than 8 hours. Nucleolin mRNA induction is reduced markedly in normal fibroblasts that reach confluence, while it occurs continuously even in post-confluent epithelial tumor cells consistent with their capacity to proliferate without contact inhibition. Interestingly, cold and heat shock induce nucleolin mRNA concomitantly to enhanced mRNA expression of the heat shock protein 70, thus suggesting that surface nucleolin induction also occurs in response to an environmental insult. At the cell surface, one of the main functions of nucleolin is to shuttle specific extracellular ligands by an active transport mechanism, which we show here to be calcium dependent.Conclusion/SignificanceOur results demonstrate that the expression of surface nucleolin is an early metabolic event coupled with tumor cell proliferation and stress response. The fact that surface nucleolin is constantly and abundantly expressed on the surface of tumor cells, makes them a preferential target for the inhibitory action of anticancer agents that target surface nucleolin.

Molecular tumor targeting of gelonin by fusion with F3 peptide

“…This result appears consistent with our previous reports that gelonin is quite stable in fusions due to its structure [26,32] , which consequently contributes to the retention of its activity even after fusion with F3 peptide. To elucidate any anti-cancer targeting effects of F3-Gel or gelonin alone, for the in vitro cell experiments, we used HeLa, LnCaP, 9L, and U87 MG cancer cells because these cell lines were fully examined in other previous studies related to the F3 peptide [18,33] . In contrast to the equipotent intrinsic activity compared with unmodified gelonin in the cell-free conditions (Figure 2), in the U87 MG cell lines, F3-Gel exhibited a significantly greater ability to reduce the protein level to less than 50% than that of the gelonin ( Figure 5).…”

“…To overcome the issues related to the short half-life and intrinsic toxicity at high doses of F3-Gel, further studies are underway to develop a nanoparticle-based drug delivery system to extend the plasma half-life and increase the tumor targeting efficiency at the lower dose. Based on reports that the interaction between nucleolin-targeted ligands and nucleolin is influenced by the isoelectric point [33,35] , it would be helpful to devise appropriate delivery vehicles for the F3-Gel by optimizing the physicochemical properties, such as surface charge, in the presence of F3 peptide to enable an efficient and safe therapeutic effect of F3-Gel.…”

Nucleolin and nucleophosmin: nucleolar proteins with multiple functions in DNA repair

“…Post-transcriptionally, NCL binding to mRNAs may effect expression of particular proteins, with NCL binding to the p53 mRNA 5'UTR resulting in translational activation, while its binding to gadd45α mRNA in response to arsenite treatment and K294-SUMOylation blocks the degradation of this transcript (Takagi et al 2005;Zhang et al 2015). Remarkably, NCL has even been reported to be constitutively displayed on the cell surface, where it may act as a ligand for particular growth-regulatory ligands and viruses (Fujiki et al 2014;Hovanessian et al 2010;Nisole et al 2002).…”

A Novel Post-translational Modification of Nucleolin, SUMOylation at Lys-294, Mediates Arsenite-induced Cell Death by Regulating gadd45α mRNA Stability

“…tion (18), arsenite (28,47,48,69), and inflammatory cytokines (70). Our previous findings showed that GADD45␣ was one of the crucial mediators of arsenite-induced apoptosis (47,48).…”