The multidomain protooncogenic protein c-Cbl binds to tubulin and stabilizes microtubules

TULA Proteins in Men, Mice, Hens, and Lice: Welcome to the Family

The two members of the UBASH3/STS/TULA protein family have been shown to critically regulate key biological functions, including immunity and hemostasis, in mammalian biological systems. Negative regulation of signaling through immune receptor tyrosine-based activation motif (ITAM)- and hemITAM-bearing receptors mediated by Syk-family protein tyrosine kinases appears to be a major molecular mechanism of the down-regulatory effect of TULA-family proteins, which possess protein tyrosine phosphatase (PTP) activity. However, these proteins are likely to carry out some PTP-independent functions as well. Whereas the effects of TULA-family proteins overlap, their characteristics and their individual contributions to cellular regulation also demonstrate clearly distinct features. Protein structure, enzymatic activity, molecular mechanisms of regulation, and biological functions of TULA-family proteins are discussed in this review. In particular, the usefulness of the comparative analysis of TULA proteins in various metazoan taxa, for identifying potential roles of TULA-family proteins outside of their functions already established in mammalian systems, is examined.

Knockout of Sorbin And SH3 Domain Containing 2 (Sorbs2) in Cardiomyocytes Leads to Dilated Cardiomyopathy in Mice

Background

Sorbin and SH3 domain containing 2 (Sorbs2) protein is a cytoskeletal adaptor with an emerging role in cardiac biology and disease; yet, its potential relevance to adult‐onset cardiomyopathies remains underexplored. Sorbs2 global knockout mice display lethal arrhythmogenic cardiomyopathy; however, the causative mechanisms remain unclear. Herein, we examine Sorbs2 dysregulation in heart failure, characterize novel Sorbs2 cardiomyocyte‐specific knockout mice (Sorbs2‐cKO), and explore associations between Sorbs2 genetic variations and human cardiovascular disease.

Methods and Results

Bioinformatic analyses show myocardial Sorbs2 mRNA is consistently upregulated in humans with adult‐onset cardiomyopathies and in heart failure models. We generated Sorbs2‐cKO mice and report that they develop progressive systolic dysfunction and enlarged cardiac chambers, and they die with congestive heart failure at about 1 year old. After 3 months, Sorbs2‐cKO mice begin to show atrial enlargement and P‐wave anomalies, without dysregulation of action potential–associated ion channel and gap junction protein expressions. After 6 months, Sorbs2‐cKO mice exhibit impaired contractility in dobutamine‐treated hearts and skinned myofibers, without dysregulation of contractile protein expressions. From our comprehensive survey of potential mechanisms, we found that within 4 months, Sorbs2‐cKO hearts have defective microtubule polymerization and compensatory upregulation of structural cytoskeletal and adapter proteins, suggesting that this early intracellular structural remodeling is responsible for contractile dysfunction. Finally, we identified genetic variants that associate with decreased Sorbs2 expression and human cardiac phenotypes, including conduction abnormalities, atrial enlargement, and dilated cardiomyopathy, consistent with Sorbs2‐cKO mice phenotypes.

Conclusions

Our studies show that Sorbs2 is essential for maintaining structural integrity in cardiomyocytes, likely through strengthening the interactions between microtubules and other cytoskeletal proteins at cross‐link sites.

TULA proteins as signaling regulators

Two members of the UBASH3/STS/TULA family exhibit a unique protein domain structure, which includes a histidine phosphatase domain, and play a key role in regulating cellular signaling. UBASH3A/STS-2/TULA is mostly a lymphoid protein, while UBASH3B/STS-1/TULA-2 is expressed ubiquitously. Dephosphorylation of tyrosine-phosphorylated proteins by TULA-2 and, probably to a lesser extent, by TULA critically contribute to the molecular basis of their regulatory effect. The notable differences between the effects of the two family members on cellular signaling and activation are likely to be linked to the difference between their specific enzymatic activities. However, these differences might also be related to the functions of their domains other than the phosphatase domain and independent of their phosphatase activity. The down-regulation of the Syk/Zap-70-mediated signaling, which to-date appears to be the best-studied regulatory effect of TULA family, is discussed in detail in this publication.

MicroRNA-155 regulates the proliferation, cell cycle, apoptosis and migration of colon cancer cells and targets CBL

MicroRNA-155 (miR-155) is a well-studied miR and acts as an oncomiR in numerous cancer types. However, the biological functions of miR-155 in colon cancer as well as its target genes have remained to be fully elucidated. In order to investigate the biological functions of miR-155, MTT, colony formation and wound healing assays, cell cycle analysis and detection of apoptosis were performed. The results demonstrated that miR-155 promoted the proliferation of colon cancer cells and enhanced their colony formation capacity, promoted their cell cycle progression and inhibited apoptosis. miR-155 also promoted the migration of colon cancer cells. In the present study, casitas B-lineage lymphoma was identified as a novel target of miR-155. The present study suggested that miR-155 functions as an oncomiR in colon cancer cells and may become a promising therapeutic target for colon cancer therapy.

Modulation of host microtubule dynamics by pathogenic bacteria

The eukaryotic cytoskeleton is a vulnerable target of many microbial pathogens during the course of infection. Rearrangements of host cytoskeleton benefit microbes in various stages of their infection cycle such as invasion, motility, and persistence. Bacterial pathogens deliver a number of effector proteins into host cells for modulating the dynamics of actin and microtubule cytoskeleton. Alteration of the actin cytoskeleton is generally achieved by bacterial effectors that target the small GTPases of the host. Modulation of microtubule dynamics involves direct interaction of effector proteins with the subunits of microtubules or recruiting cellular proteins that affect microtubule dynamics. This review will discuss effector proteins from animal and human bacterial pathogens that either destabilize or stabilize host micro-tubules to advance the infectious process. A compilation of these research findings will provide an overview of known and unknown strategies used by various bacterial effectors to modulate the host microtubule dynamics. The present review will undoubtedly help direct future research to determine the mechanisms of action of many bacterial effector proteins and contribute to understanding the survival strategies of diverse adherent and invasive bacterial pathogens.

c-Cbl and Cbl-b act redundantly to protect osteoclasts from apoptosis and to displace HDAC6 from beta-tubulin, stabilizing microtubules and podosomes

c-Cbl and Cbl-b are highly conserved adaptor proteins that participate in integrin signaling, regulating cytoskeletal organization, motility, and bone resorption. Deletion of both c-Cbl and Cbl-b in mice leads to embryonic lethality, indicating that the two proteins perform essential redundant functions. To examine the redundant actions of c-Cbl and Cbl-b in osteoclasts, we depleted c-Cbl in Cbl-b(-/-) osteoclasts by using a short hairpin RNA. Depleting both Cbl proteins disrupted both the podosome belt and the microtubule network and decreased bone-resorbing activity. Stabilizing the microtubules with paclitaxel or inhibiting histone deacetylase 6 (HDAC6), which destabilizes microtubules by deacetylating beta-tubulin, protected both the microtubule network and the podosome belt. Examination of the mechanism involved demonstrated that the conserved four-helix bundle of c-Cbl's tyrosine kinase binding domain bound to beta-tubulin, and both c-Cbl and Cbl-b displaced HDAC6. In addition to the effects on microtubules and the podosome belt, depleting both Cbls significantly increased the levels of the proapoptotic protein Bim and apoptosis relative to the levels induced by eliminating either protein alone. Thus, both c-Cbl and Cbl-b promote bone resorption via the stabilization of microtubules, allowing the formation of the podosome belt in osteoclasts, and by promoting osteoclast survival.

Rapid microtubule bundling and stabilization by the Streptococcus pneumoniae neurotoxin pneumolysin in a cholesterol-dependent, non-lytic and Src-kinase dependent manner inhibits intracellular trafficking

Streptococcus pneumoniae is the most frequent cause of bacterial meningitis, leading to permanent neurological damage in 30% and lethal outcome in 25% of patients. The cholesterol-dependent cytolysin pneumolysin is a major virulence factor of S. pneumoniae. It produces rapid cell lysis at higher concentrations or apoptosis at lower concentrations. Here, we show that sublytic amounts of pneumolysin produce rapid bundling and increased acetylation of microtubules (signs of excessive microtubule stabilization) in various types of cells--neuroblastoma cells, fibroblasts and primary astrocytes. The bundling started perinuclearly and extended peripherally towards the membrane. The effect was not connected to pneumolysin's capacity to mediate calcium influx, macropore formation, apoptosis, or RhoA and Rac1 activation. Cellular cholesterol depletion and neutralization of the toxin by pre-incubation with cholesterol completely inhibited the microtubule phenotype. Pharmacological inhibition of Src-family kinases diminished microtubule bundling, suggesting their involvement in the process. The relevance of microtubule stabilization to meningitis was confirmed in an experimental pneumococcal meningitis animal model, where increased acetylation was observed. Live imaging experiments demonstrated a decrease in organelle motility after toxin challenge in a manner comparable to the microtubule-stabilizing agent taxol, thus proposing a possible pathogenic mechanism that might contribute to the CNS damage in pneumococcal meningitis.

Up-regulation of the Cbl family of ubiquitin ligases is involved in ATRA and bufalin-induced cell adhesion but not cell differentiation

The Casitas B-lineage Lymphoma (Cbl) family of ubiquitin ligases is multifunctional proteins that play important roles in different cell signaling pathways. It has been reported that c-Cbl and Cbl-b mRNAs are up-regulated during TPA-induced U937 and HL-60 cell differentiation. But the mechanism of the up-regulation and the roles of the Cbl family of ubiquitin ligases still remain unclear. In the present study, we demonstrated that bufalin enhanced all-trans retinoic acid (ATRA) induced differentiation of HL-60 cells, accompanied by up-regulation of the Cbl family of ubiquitin ligases. CsA, an inhibitor of calcium mobilization, reversed this up-regulation. Pretreatment with CsA and PS-341 did not affect the expression of CD11b, but suppressed the percentage of adherent cells. Lipid raft localization of Cbl-b enhanced cell adhesion, while C-terminal deletion partially suppressed the effect. Moreover, the expression of the adhesion-related kinases Pyk2 and Paxillin was up-regulated in parallel with the increase of Cbl proteins. These results suggested that up-regulation of c-Cbl and Cbl-b was involved in the regulation of ATRA and bufalin-induced HL-60 cell adhesion rather than cell differentiation, which might be mediated by lipid raft localization, ubiquitin ligase activity and C-terminal structure of Cbl proteins. Meanwhile, up-regulation of proline-rich tyrosine kinase (Pyk2) and Paxillin might also be implicated in this regulation.

c-Cbl-facilitated cytoskeletal effects in v-Abl-transformed fibroblasts are regulated by membrane association of c-Cbl

The multi-functional protein c-Cbl is an important modulator of actin cytoskeletal dynamics in diverse biological systems. We had previously reported that c-Cbl facilitates cell spreading and adhesion and suppresses anchorage-independent growth of v-Abl-transformed fibroblasts. To assess the importance of membrane localization of c-Cbl for the observed effects of c-Cbl in v-Abl-3T3 cells, we first mapped the membrane interactive domain(s) of c-Cbl. Our studies indicate that localization of c-Cbl to the membrane is likely to be mediated by the tyrosine kinase binding (TKB) domain and the proline-rich region of c-Cbl, whereas C-terminal tyrosine phosphorylation does not play a role. The association of v-Cbl, which encompasses the TKB domain, with the membrane was unusual as it was not entirely dependent on SH2-phosphotyrosine interactions. Our studies further demonstrate that Src-like adaptor protein (SLAP), which binds to v-Cbl in a tyrosine phosphorylation-independent manner, facilitates membrane association of Cbl. The interaction between c-Cbl and SLAP in v-Abl-3T3 cells positively influenced c-Cbl-mediated spreading and adhesion of these cells. SLAP appears to exert its effects not simply by increasing the amount of c-Cbl in the membrane but by facilitating binding of p85-phosphatidylinositol-3-kinase (PI3K) with membrane-associated c-Cbl.

The Cbl family proteins: ring leaders in regulation of cell signaling

The proto-oncogenic protein c-Cbl was discovered as the cellular form of v-Cbl, a retroviral transforming protein. This was followed over the years by important discoveries, which identified c-Cbl and other Cbl-family proteins as key players in several signaling pathways. c-Cbl has donned the role of a multivalent adaptor protein, capable of interacting with a plethora of proteins, and has been shown to positively influence certain biological processes. The identity of c-Cbl as an E3 ubiquitin ligase unveiled the existence of an important negative regulatory pathway involved in maintaining homeostasis in protein tyrosine kinase (PTK) signaling. Recent years have also seen the emergence of novel regulators of Cbl, which have provided further insights into the complexity of Cbl-influenced pathways. This review will endeavor to provide a summary of current studies focused on the effects of Cbl proteins on various biological processes and the mechanism of these effects. The major sections of the review are as follows: Structure and genomic organization of Cbl proteins; Phosphorylation of Cbl; Interactions of Cbl; Localization of Cbl; Mechanism of effects of Cbl: (a) Ubiquitylation-dependent events: This section elucidates the mechanism of Cbl-mediated downregulation of EGFR and details the PTK and non-PTKs targeted by Cbl. In addition, it addresses the functional requirements for E3 Ubiquitin ligase activity of Cbl and negative regulation of Cbl-mediated downregulation of PTKs, (b) Adaptor functions: This section discusses the mechanisms of adaptor functions of Cbl in mitogen-activated protein kinase (MAPK) activation, insulin signaling, regulation of Ras-related protein 1 (Rap1), PI-3' kinase signaling, and regulation of Rho-family GTPases and cytoskeleton; Biological functions: This section gives an account of the diverse biological functions of Cbl and includes the role of Cbl in transformation, T-cell signaling and thymus development, B-cell signaling, mast-cell degranulation, macrophage functions, bone development, neurite growth, platelet activation, muscle degeneration, and bacterial invasion; Conclusions and perspectives.

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.