Cell Cycle-controlled Interaction of Nucleolin with the Retinoblastoma Protein and Cancerous Cell Transformation

Abstract

Retinoblastoma protein (Rb) is a multifunctional tumor suppressor, frequently inactivated in certain types of human cancer. Nucleolin is an abundant multifunctional phosphoprotein of proliferating and cancerous cells, recently identified as cell cycle-regulated transcription activator, controlling expression of human papillomavirus type 18 (HPV18) oncogenes in cervical cancer. Here we find that nucleolin is associated with Rb in intact cells in the G 1 phase of the cell cycle, and the complex formation is mediated by the growth-inhibitory domain of Rb. Association with Rb inhibits the DNA binding function of nucleolin and in consequence the interaction of nucleolin with the HPV18 enhancer, resulting in Rb-mediated repression of the HPV18 oncogenes. The intracellular distribution of nucleolin in epithelial cells is Rb-dependent, and an altered nucleolin localization in human cancerous tissues results from a loss of Rb. Our findings suggest that deregulated nucleolin activity due to a loss of Rb contributes to tumor development in malignant diseases, thus providing further insights into the molecular network for the Rb-mediated tumor suppression.Nucleolin is an abundant multifunctional phosphoprotein of proliferating and cancerous cells (1, 2) (reviewed in Ref.3). High levels of nucleolin expression are related to poor clinical prognosis for certain cancer types (2, 4). A role of nucleolin in activation and repression of gene transcription as well as in regulation of RNA metabolism has been reported (5-7). Nucleolin is a subunit of the transcription factor LR1, which activates expression of the c-myc gene and the EBNA-1 (Epstein-Barr virus nuclear antigen 1) gene in B-cell lymphomas (7-9). Nucleolin is also directly involved in post-transcriptional inhibition of the p53 gene expression (10). As activator of transcription of HPV18 2 oncogenes in cervical cancer cells, nucleolin has oncogenic potential (11). Nucleolin controls the chromatin structure of the HPV18 enhancer in vivo and is directly linked to HPV18-induced cervical cancer formation (11). Nucleolin is a substrate of cyclin-dependent kinases (CDKs), CDK1, CDK4, and CDK2 (12, 13), and its DNA binding activity is most prominent in the S phase of the cell cycle (11). Here we report a functional interaction of nucleolin with Rb, related to cell transformation.Rb is a prototypical tumor suppressor, frequently inactivated in certain types of human cancer, and it controls cell proliferation, differentiation, and survival (14). Therefore, it is important to work out molecular mechanisms of Rb-mediated tumor suppression to understand the cancer disease process (14, 15). Rb functions to constrain cell proliferation by providing a cell cycle checkpoint between the G 1 and S phases (16,17). Phosphorylation of Rb by cyclin-dependent kinases is initiated in mid-G 1 phase, and Rb phosphorylation is associated with G 1 /S transition and S phase progression (18,19). Hypophosphorylated Rb restrains cell proliferation, in part by targeting E2F transcription factors (20 -23)...

A Simple Approach to Cancer Therapy Afforded by Multivalent Pseudopeptides That Target Cell-Surface Nucleoproteins

“…In MCF-7 breast carcinoma cells and HeLa cells, a decrease in nucleolin level leads to an increase in p53 and apoptosis (22,23). Nucleolin associates with tumor suppressor retinoblastoma protein, leading to cancer cell transformation (24).…”

Nucleolin is important for Epstein–Barr virus nuclear antigen 1-mediated episome binding, maintenance, and transcription

“…NCL is also implicated in CMV UL44-mediated DNA replication, in HSV1 UL12-mediated nucleocapsid egress (40,41), and in HPV18, but not HPV16, E6, and E7 transcription in cervical cancer (42). Nucleolin also has important effects in cancer cell binding and stabilization of Hif-1α mRNA through ARE interactions (43).…”

The Nucleolus and Ribosomal Genes in Aging and Senescence

“…RB was initially reported to accumulate in the nucleolus and to have a repressive role in Pol I transcription (Cavanaugh et al, 1995;Hannan et al, 2000). Subsequently, Nucleolin (NCL), a multifunctional nucleolar protein essential for rRNA processing (Mongelard & Bouvet, 2007) was reported to associate with hypophosphorylated (active) RB (pRB) during the G1 phase of the cell cycle (Grinstein et al, 2006). pRB mediates a cell cycle checkpoint between G 1 and S phase (Bartek et al, 1996) by targeting members of the E2F family of transcriptional activators (Chellappan et al, 1991) that are essential for cellular proliferation.…”