ABSTRACT
Apoptin, a protein of the chicken anemia virus (CAV), consists of 121 amino acids (aa) and represents a novel, potentially tumor-specific therapeutic and diagnostic agent. The C-terminal part of Apoptin (aa 81-121) is believed to contain a bipartite nuclear localization signal (NLS) (NLS1: aa 82-88 and NLS2: aa 111-121), which is only active in tumor cells after phosphorylation of threonine(108) by tumor-specific cytoplasmic phosphokinases. Furthermore, a nuclear export signal (NES) (aa 97-105) seems to enable nuclear export of Apoptin only in healthy cells. The specificity for tumor cell nuclei also applies to the truncated C-terminal part of Apoptin (aa 81-121), which therefore represents a highly attractive peptide sequence for peptide synthesis. Here we describe for the first time the synthesis of fluorescein isothiocyanate (FITC)- and Dansyl-labelled conjugates containing this C-terminal part of Apoptin, with either phosphorylated or nonphosphorylated threonine(108). The phosphorylated conjugates were synthesized in an attempt to achieve nuclear accumulation in healthy cells, which lack cytoplasmic tumor-specific phosphokinases. Surprisingly, all the conjugates accumulated rapidly within the cell nuclei of both tumor and non-tumor cells from the bladder, brain and prostate and led to cell death. By coupling Apoptin(81-121) to FITC and DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) at either the C- or N-terminus we could exlude that the coupling site is decisive for tumor cell-specific nuclear localization. The labels FITC, DOTA and Dansyl were not responsible for cell death in healthy cells because cell death was not prevented by using an unlabelled Apoptin(81-121) peptide. Cellular and nuclear uptake of the FITC-labelled Apoptin(81-121) peptide was almost completely abolished after altering the NLS2 (replacement of five arginines with serines).
Subject(s)
Apoptosis/drug effects , Capsid Proteins , Peptide Fragments , Amino Acid Sequence , Astrocytes/ultrastructure , Brain/ultrastructure , Capsid Proteins/chemistry , Cell Line, Tumor , Cell Nucleus/metabolism , Cells, Cultured , Dansyl Compounds/chemistry , Flow Cytometry , Fluorescein-5-isothiocyanate/chemistry , Glioma/diagnosis , Heterocyclic Compounds, 1-Ring/chemistry , Humans , Male , Microscopy, Confocal , Microscopy, Fluorescence , Nuclear Export Signals/physiology , Nuclear Localization Signals/physiology , Phosphothreonine/chemistry , Phosphothreonine/metabolism , Prostate/ultrastructure , Prostatic Neoplasms/diagnosis , Urinary Bladder/ultrastructure , Urinary Bladder Neoplasms/diagnosis , Urothelium/ultrastructureABSTRACT
We synthesized several novel compounds to evaluate the different effects of non-iodinated and mono- or diiodinated benzoic acid on the cellular and nuclear uptake of the SV 40 T antigen nuclear localization sequence (NLS) in human LN18 and U373 glioma cells. The skeletal structure of all the conjugates contained the fluorescein isothiocyanate (FITC)-labeled NLS of the SV 40T antigen, to which either benzoic acid, mono- or diiodobenzoic acid was coupled. As shown by confocal laser scanning microscopy (CLSM) and fluorescence-activated cell sorting (FACS), the basic FITC-labeled NLS alone was taken up by the nuclei of only a few glioma cells which remained intact. The coupling of non-iodinated benzoic acid (BA) did not result in a markedly larger number of nuclearly stained cells. A very marked increase in cells with nuclear staining was found with the conjugate containing monoiodobenzoic acid (MIBA). This was also associated with a high cell death rate. Similar results were obtained with the conjugate containing diiodobenzoic acid (DIBA). However, coincubation with free mono- or diiodobenzoic acid and the basic FITC-labeled NLS did not result in a marked change in the number of strongly stained cells or cell viability compared to the results of incubation with the FITC-labeled NLS alone. Surprisingly, FITC-labeled MIBA- and DIBA-conjugates containing a scrambled SV 40 T antigen NLS were also taken up by the cell nuclei of LN18 and U373 glioma cells and led to cell death. Such mono- or diiodobenzoic acid conjugates may therefore have potential in the development of new non-radioactive drugs against malignant glioma cells.
