Apoptosis in Leukemia Cells Is Accompanied by Alterations in the Levels and Localization of Nucleolin

Abstract

Molecular defects in apoptotic pathways are thought to often contribute to the abnormal expansion of malignant cells and their resistance to chemotherapy. Therefore, a comprehensive knowledge of the mechanisms controlling induction of apoptosis and subsequent cellular disintegration could result in improved methods for prognosis and treatment of cancer. In this study, we have examined apoptosis-induced alterations in two proteins, nucleolin and poly(ADP-ribose) polymerase-1 (PARP-1), in U937 leukemia cells. Nucleolin is expressed at high levels in malignant cells, and it is a multifunctional and mobile protein that can shuttle among the nucleolus, nucleoplasm, cytoplasm, and plasma membrane. Here, we report our findings that UV irradiation or camptothecin treatment of U937 cells induced apoptosis and caused a significant change in the levels and localization of nucleolin within the nucleus. Additionally, nucleolin levels were dramatically decreased in extracts containing the cytoplasm and plasma membrane. These alterations could be abrogated by pre-incubation with an inhibitor of PARP-1 (3-aminobenzamide), and our data support a potential role for nucleolin in removing cleaved PARP-1 from dying cells. Furthermore, both nucleolin and cleaved PARP-1 were detected in the culture medium of cells undergoing apoptosis, associated with particles of a size consistent with apoptotic bodies. These results indicate that nucleolin plays an important role in apoptosis, and could be a useful marker for assessing apoptosis or detecting apoptotic bodies. In addition, the data provide a possible explanation for the appearance of nucleolin and PARP-1 autoantibodies in some autoimmune diseases.

Identification of Nucleolin as an AU-rich Element Binding Protein Involved in bcl-2 mRNA Stabilization

“…It is possible that other anticancer drugs which inhibit cell proliferation or promote cellular differentiation may also induce apoptosis via nucleolin down-regulation. For example, induction of apoptosis in U937 leukemia cells is accompanied by notable decreases in the levels of nuclear and cytoplasmic nucleolin (55). Under these conditions less nucleolin would be available in the cytoplasm to stabilize bcl-2 mRNA.…”

Rearrangement of nuclear ribonucleoprotein (RNP)‐containing structures during apoptosis and transcriptional arrest

“…Some autoantigens, such as ribosomal RNPs, previously described to resist proteolysis (Wu et al, 2001) have recently been shown to be cleaved and expressed at the cell surface: six proteins (S15, P0, L5, L6, L36a, L41) have been found to follow this pathway (Nishida et al, 2002). Nucleolin is one of the most studied protein in the nucleolus and it has recently been reported (Mi et al, 2003) to decrease in UV-induced apoptosis, in parallel with PARP-1 degradation.Interestingly, nucleolin and several other nuclear protein accumulate within blebs which are extruded at the cell sur- …”

Nucleolin Protein Interacts with Microprocessor Complex to Affect Biogenesis of MicroRNAs 15a and 16

“…Evidence suggests that it may also have a role as an accessory protein in miRNA biogenesis. Nucleolin is predominantly a nucleolus-localized protein; however, in a number of different cancers, nucleolin is found largely in the cytoplasm (21)(22)(23)(24). In the cytoplasm, nucleolin functions to stabilize the mRNA of bcl-2, thereby inhibiting apoptosis (21).…”