Decreased protein levels of the c-Cbl protooncogene in murine AIDS

Ubiquitin E3 Ligase c-Cbl Is a Host Negative Regulator of Nef Protein of HIV-1

Nef is an accessory protein encoded by human immunodeficiency virus type-1 (HIV-1) and plays important roles in regulating HIV-1 infection and viral replication. Interestingly, HIV-1 Nef can promote degradation of numerous host proteins to disrupt cellular antiviral immune response. However, how HIV-1 Nef is degraded by host factors remains largely unexplored. Here, we identified c-Cbl as a host ubiquitin E3 ligase of HIV-1 Nef. We found that c-Cbl interacts with Nef and reduces protein levels of HIV-1 Nef. Further studies demonstrated that c-Cbl promoted Lys48-linked polyubiquitination of HIV-1 Nef, thus attenuating protein stability of HIV-1 Nef. Importantly, cellular c-Cbl ubiquitinated and degraded Nef proteins produced by HIV-1 NL4-3 virions, and ultimately attenuated HIV-1 virulence for infection of THP1 cells. This study reveals a ubiquitination and proteasome-dependent degradation mechanism of HIV-1 Nef protein, and could provide potential strategies for fighting against HIV-1.

Degradation of cofilin is regulated by Cbl, AIP4 and Syk resulting in increased migration of LMP2A positive nasopharyngeal carcinoma cells

Expression of cofilin is directly associated with metastatic activity in many tumors. Here, we studied the role of Latent Membrane Protein 2 A (LMP2A) of Epstein-Barr Virus (EBV) in the accumulation of cofilin observed in nasopharyngeal cancer (NPC) tumor cells. We used LMP2A transformed NPC cell lines to analyze cofilin expression. We used mutation analysis, ectopic expression and down-regulation of Cbl, AIP4 and Syk in these cell lines to determine the effect of the LMP2A viral protein on cofilin degradation and its role in the assembly of a cofilin degrading protein complex. The LMP2A of EBV was found to interfer with cofilin degradation in NPC cells by accelerating the proteasomal degradation of Cbl and Syk. In line with this, we found significantly higher cofilin expression in NPC tumor samples as compared to the surrounding epithelial tissues. Cofilin, as an actin severing protein, influences cellular plasticity, and facilitates cellular movement in response to oncogenic stimuli. Thus, under relaxed cellular control, cofilin facilitates tumor cell movement and dissemination. Interference with its degradation may enhance the metastatic potential of NPC cells.

c-Cbl and Cbl-b ubiquitin ligases: substrate diversity and the negative regulation of signalling responses

The activation of signalling pathways by ligand engagement with transmembrane receptors is responsible for determining many aspects of cellular function and fate. While these outcomes are initially determined by the nature of the ligand and its receptor, it is also essential that intracellular enzymes, adaptor proteins and transcription factors are correctly assembled to convey the intended response. In recent years, it has become evident that proteins that regulate the amplitude and duration of these signalling responses are also critical in determining the function and fate of cells. Of these, the Cbl family of E3 ubiquitin ligases and adaptor proteins has emerged as key negative regulators of signals from many types of cell-surface receptors. The array of receptors and downstream signalling proteins that are regulated by Cbl proteins is diverse; however, in most cases, the receptors have a common link in that they either possess a tyrosine kinase domain or they form associations with cytoplasmic PTKs (protein tyrosine kinases). Thus Cbl proteins become involved in signalling responses at a time when PTKs are first activated and therefore provide an initial line of defence to ensure that signalling responses proceed at the desired intensity and duration.

c-Cbl is a suppressor of the neu oncogene

A rodent oncogenic mutant of the Neu receptor tyrosine kinase is a useful experimental model because overexpression of the respective receptor, namely HER2/ErbB-2, in human malignancies is associated with relatively aggressive diseases. Here we show that the oncogenic form of Neu is constitutively associated with the product of the c-cbl proto-oncogene and is part of a large complex that includes the phosphoinositide 3-kinase and Shc. Ectopic expression of c-Cbl, a ubiquitin-protein isopeptide ligase specific to activated tyrosine kinases, causes rapid removal of Neu from the cell surface and severely reduces signaling downstream of oncogenic Neu. c-Cbl-induced down-regulation of Neu involves covalent attachment of ubiquitin molecules and requires the carboxyl-terminal domain of Neu. The negative effect of c-Cbl is antagonized by v-Cbl, a virus-encoded oncogenic truncated form of c-Cbl. In an in vivo model, infection of a Neu-transformed neuroblastoma with a c-Cbl-encoding retrovirus caused enhanced down-regulation of Neu and correlated with tumor retardation. Our results implicate c-Cbl in negative regulation of Neu and offer a potential target for treatment of HER2/ErbB-2-positive human malignancies.