The polycomb group protein B lymphoma Mo-MLV insertion region 1 homolog (Bmi-1) is dysregulated in various cancers, and its upregulation strongly correlates with an invasive phenotype and poor prognosis in patients with nasopharyngeal carcinomas. However, the underlying mechanism of Bmi-1-mediated invasiveness remains unknown. In the current study, we found that upregulation of Bmi-1 induced epithelialmesenchymal transition (EMT) and enhanced the motility and invasiveness of human nasopharyngeal epithelial cells, whereas silencing endogenous Bmi-1 expression reversed EMT and reduced motility.

The polycomb protein Bmi-1 represses the INK4a locus, which encodes the tumor suppressors p16 and p14 ARF . Here we report that Bmi-1 is downregulated when WI-38 human fibroblasts undergo replicative senescence, but not quiescence, and extends replicative life span when overexpressed. Life span extension by Bmi-1 required the pRb, but not p53, tumor suppressor protein. Deletion analysis showed that the RING finger and helix-turn-helix domains of Bmi-1 were required for life span extension and suppression of p16. Furthermore, a RING finger deletion mutant exhibited dominant negative activity, inducing p16 and premature senescence. Interestingly, presenescent cultures of some, but not all, human fibroblasts contained growtharrested cells expressing high levels of p16 and apparently arrested by a p53-and telomere-independent mechanism. Bmi-1 selectively extended the life span of these cultures. Low O 2 concentrations had no effect on p16 levels or life span extension by Bmi-1 but reduced expression of the p53 target, p21. We propose that some human fibroblast strains are more sensitive to stress-induced senescence and have both p16-dependent and p53/telomere-dependent pathways of senescence. Our data suggest that Bmi-1 extends the replicative life span of human fibroblasts by suppressing the p16-dependent senescence pathway.

The Bmi-1 oncoprotein regulates proliferation and oncogenesis in human cells. Its overexpression leads to senescence bypass in human fibroblasts and immortalization of human mammary epithelial cells. In this study, we report that compared with normal nasopharyngeal epithelial cells (NPEC), Bmi-1 is overexpressed in nasopharyngeal carcinoma cell lines. Importantly, Bmi-1 was also found to be overexpressed in 29 of 75 nasopharyngeal carcinoma tumors (38.7%) by immunohistochemical analysis. In contrast to nasopharyngeal carcinoma, there was no detectable expression of Bmi-1 in noncancerous nasopharyngeal epithelium. Moreover, high Bmi-1 expression positively correlated with poor prognosis of nasopharyngeal carcinoma patients. We also report that the overexpression of Bmi-1 leads to bypass of senescence and immortalization of NPECs, which normally express p16INK4a and exhibit finite replicative life span. Overexpression of Bmi-1 in NPECs led to the induction of human telomerase reverse transcriptase activity and reduction of p16INK4a expression. Mutational analysis of Bmi-1 showed that both RING finger and helix-turn-helix domains of it are required for immortalization of NPECs. Our findings suggest that Bmi-1 plays an important role in the development and progression of nasopharyngeal carcinoma, and that Bmi-1 is a valuable marker for assessing the prognosis of nasopharyngeal carcinoma patients. Furthermore, this study provides the first cellular proto-oncogene immortalized nasopharyngeal epithelial cell line, which may serve as a cell model system for studying the mechanisms involved in the tumorigenesis of nasopharyngeal carcinoma. (Cancer Res 2006; 66(12): 6225-32)

Protein tyrosine kinases (PTKs) coordinate a broad spectrum of cellular responses to extracellular stimuli and cell–cell interactions during development, tissue homeostasis, and responses to environmental challenges. Thus, an understanding of the regulatory mechanisms that ensure physiological PTK function and potential aberrations of these regulatory processes during diseases such as cancer are of broad interest in biology and medicine. Aside from the expected role of phospho-tyrosine phosphatases, recent studies have revealed a critical role of covalent modification of activated PTKs with ubiquitin as a critical mechanism of their negative regulation. Members of the Cbl protein family (Cbl, Cbl-b and Cbl-c in mammals) have emerged as dominant “activated PTK-selective” ubiquitin ligases. Structural, biochemical and cell biological studies have established that Cbl protein-dependent ubiquitination targets activated PTKs for degradation either by facilitating their endocytic sorting into lysosomes or by promoting their proteasomal degradation. This mechanism also targets PTK signaling intermediates that become associated with Cbl proteins in a PTK activation-dependent manner. Cellular and animal studies have established that the relatively broadly expressed mammalian Cbl family members Cbl and Cbl-b play key physiological roles, including their critical functions to prevent the transition of normal immune responses into autoimmune disease and as tumor suppressors; the latter function has received validation from human studies linking mutations in Cbl to human leukemia. These newer insights together with embryonic lethality seen in mice with a combined deletion of Cbl and Cbl-b genes suggest an unappreciated role of the Cbl family proteins, and by implication the ubiquitin-dependent control of activated PTKs, in stem/progenitor cell maintenance. Future studies of existing and emerging animal models and their various cell lineages should help test the broader implications of the evolutionarily-conserved Cbl family protein-mediated, ubiquitin-dependent, negative regulation of activated PTKs in physiology and disease.