Surface expressed nucleolin is constantly induced in tumor cells to mediate calcium-dependent ligand internalization

• Published in: PloS one Vol. 5; no. 12; p. e15787
• Main Authors: Hovanessian, Ara G, Soundaramourty, Calaiselvy, El Khoury, Diala, Nondier, Isabelle, Svab, Josette, Krust, Bernard
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 23.12.2010; Public Library of Science (PLoS)
• Subjects: Active transport; Angiogenesis; Animals; Anticancer properties; Antitumor agents; Apoptosis; Biology; Biotinylation; Calcium; Calcium - chemistry; Cancer; Cell adhesion & migration; Cell Line, Tumor; Cell Membrane - metabolism; Cell Nucleus - metabolism; Cell proliferation; Cell surface; Cellular stress response; Cold shock; Confluence; Contact inhibition; Cytoplasm - metabolism; Development and progression; Fibroblasts; Gene expression; Gene Expression Regulation, Neoplastic; Glycoproteins; Heat shock proteins; HeLa Cells; HIV; Hsp70 protein; Human immunodeficiency virus; Humans; Internalization; Kinases; Ligands; Mice; Neoplasms - embryology; Neoplasms - metabolism; NIH 3T3 Cells; Nuclei; Nuclei (cytology); Nucleoli; Nucleolin; Phosphoproteins - biosynthesis; Phosphoproteins - metabolism; Protein binding; Proteins; Ribonucleic acid; RNA; RNA-Binding Proteins - biosynthesis; RNA-Binding Proteins - metabolism; Rodents; Transcription; Transcription (Genetics); Translation (Genetics); Tumor cells; Tumorigenesis; Tumors; France
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0015787

Nucleolin 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. By 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. Our 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.