CIN85 regulates ubiquitination and degradative endosomal sorting of the EGF receptor

E3 ligase-inactivation rewires CBL interactome to elicit oncogenesis by hijacking RTK-CBL-CIN85 axis

Casitas B-lineage lymphoma (CBL) is a ubiquitin ligase (E3) that becomes activated upon Tyr371-phosphorylation and targets receptor protein tyrosine kinases for ubiquitin-mediated degradation. Deregulation of CBL and its E3 activity is observed in myeloproliferative neoplasms and other cancers, including breast, colon, and prostate cancer. Here, we explore the oncogenic mechanism of E3-inactive CBL mutants identified in myeloproliferative neoplasms. We show that these mutants bind strongly to CIN85 under normal growth conditions and alter the CBL interactome. Lack of E3 activity deregulates CIN85 endosomal trafficking, leading to an altered transcriptome that amplifies signaling events to promote oncogenesis. Disruption of CBL mutant interactions with EGFR or CIN85 reduces oncogenic transformation. Given the importance of the CBL-CIN85 interaction in breast cancers, we examined the expression levels of CIN85, CBL, and the status of Tyr371-phosphorylated CBL (pCBL) in human breast cancer tissue microarrays. Interestingly, pCBL shows an inverse correlation with both CIN85 and CBL, suggesting that high expression of inactivated CBL could coordinate with CIN85 for breast cancer progression. Inhibition of the CBL-CIN85 interaction with a proline-rich peptide of CBL that binds CIN85 reduced the proliferation of MDA-MB-231 cells. Together, these results provide a rationale for exploring the potential of targeting the EGFR-CBL-CIN85 axis in CBL-inactivated mutant cancers.

The Role of the Human Cytomegalovirus UL133-UL138 Gene Locus in Latency and Reactivation

Human cytomegalovirus (HCMV) latency, the means by which the virus persists indefinitely in an infected individual, is a major frontier of current research efforts in the field. Towards developing a comprehensive understanding of HCMV latency and its reactivation from latency, viral determinants of latency and reactivation and their host interactions that govern the latent state and reactivation from latency have been identified. The polycistronic UL133-UL138 locus encodes determinants of both latency and reactivation. In this review, we survey the model systems used to investigate latency and new findings from these systems. Particular focus is given to the roles of the UL133, UL135, UL136 and UL138 proteins in regulating viral latency and how their known host interactions contribute to regulating host signaling pathways towards the establishment of or exit from latency. Understanding the mechanisms underlying viral latency and reactivation is important in developing strategies to block reactivation and prevent CMV disease in immunocompromised individuals, such as transplant patients.

Oral exposure to low-dose bisphenol A induces hyperplasia of dorsolateral prostate and upregulates EGFR expression in adult Sprague-Dawley rats

Prostate is sensitive to endocrine hormone level, and the synergetic effect of estrogen and androgen is critical in prostate growth. The change of signal pathways caused by the imbalance of estrogen and androgen might function in the occurrence of prostate diseases. As a well-known endocrine disruptor compound, bisphenol A (BPA) can disturb the normal function of endocrine hormone and affect prostate development. This study aims to investigate effects of BPA on the dorsolateral prostate (DLP) and the related gene expression of the tissue in adult Sprague-Dawley (SD) rats and to explore the mechanism for the effect of low-dose BPA on DLP hyperplasia. Three-month-old male SD rats were treated with BPA (10.0, 30.0, or 90.0 µg (kg.day)^-1, gavage) or vehicle (gavage) for 4 weeks. BPA significantly increased the DLP weight, the DLP organ coefficient, and the prostate epithelium height (p < 0.01) of rats dose-dependently. Microarray analysis and quantitative real-time polymerase chain reaction showed that BPA significantly upregulated the transcriptional levels of some genes, including pituitary tumor transforming gene 1, epidermal growth factor, Sh3kbp1, and Pcna. Furthermore, the expression of PCNA (p < 0.01), androgen receptor (p < 0.01), and EGF receptor (EGFR) (p < 0.001) in DLP was increased significantly by BPA treatment, and the expression of estrogen receptor alpha was also upregulated. The findings evidenced that low-dose BPA could induce DLP hyperplasia in adult rats, and the upregulated EGF/EGFR pathway that was responsive to estrogen and androgen might play an essential role in the DLP hyperplasia induced by low-dose BPA.

Human Cytomegalovirus UL135 Interacts with Host Adaptor Proteins To Regulate Epidermal Growth Factor Receptor and Reactivation from Latency

Human cytomegalovirus, HCMV, is a betaherpesvirus that establishes a lifelong latent infection in its host that is marked by recurrent episodes of reactivation. The molecular mechanisms by which the virus and host regulate entry into and exit from latency remain poorly understood. We have previously reported that UL135 is critical for reactivation, functioning in part by overcoming suppressive effects of the latency determinant UL138 We have demonstrated a role for UL135 in diminishing cell surface levels and targeting epidermal growth factor receptor (EGFR) for turnover. The attenuation of EGFR signaling promotes HCMV reactivation in combination with cellular differentiation. In this study, we sought to define the mechanisms by which UL135 functions in regulating EGFR turnover and viral reactivation. Screens to identify proteins interacting with pUL135 identified two host adaptor proteins, CIN85 and Abi-1, with overlapping activities in regulating EGFR levels in the cell. We mapped the amino acids in pUL135 necessary for interaction with Abi-1 and CIN85 and generated recombinant viruses expressing variants of pUL135 that do not interact with CIN85 or Abi-1. These recombinant viruses replicate in fibroblasts but are defective for reactivation in an experimental model for latency using primary CD34+ hematopoietic progenitor cells (HPCs). These UL135 variants have altered trafficking of EGFR and are defective in targeting EGFR for turnover. These studies demonstrate a requirement for pUL135 interactions with Abi-1 and CIN85 for regulation of EGFR and mechanistically link the regulation of EGFR to reactivation.IMPORTANCE Human cytomegalovirus (HCMV) establishes a lifelong latent infection in the human host. While the infection is typically asymptomatic in healthy individuals, HCMV infection poses life-threatening disease risk in immunocompromised individuals and is the leading cause of birth defects. Understanding how HCMV controls the lifelong latent infection and reactivation of replication from latency is critical to developing strategies to control HCMV disease. Here, we identify the host factors targeted by a viral protein that is required for reactivation. We define the importance of this virus-host interaction in reactivation from latency, providing new insights into the molecular underpinnings of HCMV latency and reactivation.

Biochemical and Structural Studies of the Interaction between ARAP1 and CIN85

Arf-GAP with Rho-GAP domain, ANK repeat and PH domain-containing protein 1 (ARAP1), Cbl-interacting protein of 85 kDa (CIN85), and casitas B-lineage lymphoma (Cbl) play important roles in epidermal growth factor receptor (EGFR) internalization and recycling. In previous studies, ARAP1 was found to interact with CIN85, and their interaction attenuated the ubiquitination of EGFR. However, the molecular mechanism was still unclear. In this study, we first biochemically and structurally characterized the interaction between ARAP1 and CIN85, and found that the CIN85 SH3B domain bound to the ARAP1 PXPXXRX (except P) XXR/H/K motif with high affinity and specificity. Based on this binding model, we further predicted other potential CIN85 binding partners and tested their interactions biochemically. Moreover, our swapping data and structure alignment analysis suggested that the β2-β3 loops of the CIN85 SH3 domains and the H87_ARAP1/E132_CIN85 interaction were critical for ARAP1 binding specificity. Finally, our competitive analytical gel-filtration chromatography and isothermal titration calorimetry (ITC) results showed that ARAP1 could compete with Cbl for CIN85 binding, which provides a biochemical basis for the regulatory roles of ARAP1 in the CIN85-mediated EGFR internalizing process.

Septins: Regulators of Protein Stability

Septins are small GTPases that play a role in several important cellular processes. In this review, we focus on the roles of septins in protein stabilization. Septins may regulate protein stability by: (1) interacting with proteins involved in degradation pathways, (2) regulating the interaction between transmembrane proteins and cytoskeletal proteins, (3) affecting the mobility of transmembrane proteins in lipid bilayers, and (4) modulating the interaction of proteins with their adaptor or signaling proteins. In this context, we discuss the role of septins in protecting four different proteins from degradation. First we consider botulinum neurotoxin serotype A (BoNT/A) and the contribution of septins to its extraordinarily long intracellular persistence. Next, we discuss the role of septins in stabilizing the receptor tyrosine kinases EGFR and ErbB2. Finally, we consider the contribution of septins in protecting hypoxia-inducible factor 1α (HIF-1α) from degradation.

CIN85 modulates TGFβ signaling by promoting the presentation of TGFβ receptors on the cell surface

Members of the transforming growth factor β (TGFβ) family initiate cellular responses by binding to TGFβ receptor type II (TβRII) and type I (TβRI) serine/threonine kinases, whereby Smad2 and Smad3 are phosphorylated and activated, promoting their association with Smad4. We report here that TβRI interacts with the SH3 domains of the adaptor protein CIN85 in response to TGFβ stimulation in a TRAF6-dependent manner. Small interfering RNA-mediated knockdown of CIN85 resulted in accumulation of TβRI in intracellular compartments and diminished TGFβ-stimulated Smad2 phosphorylation. Overexpression of CIN85 instead increased the amount of TβRI at the cell surface. This effect was inhibited by a dominant-negative mutant of Rab11, suggesting that CIN85 promoted recycling of TGFβ receptors. CIN85 enhanced TGFβ-stimulated Smad2 phosphorylation, transcriptional responses, and cell migration. CIN85 expression correlated with the degree of malignancy of prostate cancers. Collectively, our results reveal that CIN85 promotes recycling of TGFβ receptors and thereby positively regulates TGFβ signaling.

Nexus of signaling and endocytosis in oncogenesis driven by non-small cell lung cancer-associated epidermal growth factor receptor mutants

Epidermal growth factor receptor (EGFR) controls a wide range of cellular processes, and aberrant EGFR signaling as a result of receptor overexpression and/or mutation occurs in many types of cancer. Tumor cells in non-small cell lung cancer (NSCLC) patients that harbor EGFR kinase domain mutations exhibit oncogene addiction to mutant EGFR, which confers high sensitivity to tyrosine kinase inhibitors (TKIs). As patients invariably develop resistance to TKIs, it is important to delineate the cell biological basis of mutant EGFR-induced cellular transformation since components of these pathways can serve as alternate therapeutic targets to preempt or overcome resistance. NSCLC-associated EGFR mutants are constitutively-active and induce ligand-independent transformation in nonmalignant cell lines. Emerging data suggest that a number of factors are critical for the mutant EGFR-dependent tumorigenicity, and bypassing the effects of TKIs on these pathways promotes drug resistance. For example, activation of downstream pathways such as Akt, Erk, STAT3 and Src is critical for mutant EGFR-mediated biological processes. It is now well-established that the potency and spatiotemporal features of cellular signaling by receptor tyrosine kinases such as EGFR, as well as the specific pathways activated, is determined by the nature of endocytic traffic pathways through which the active receptors traverse. Recent evidence indicates that NSCLC-associated mutant EGFRs exhibit altered endocytic trafficking and they exhibit reduced Cbl ubiquitin ligase-mediated lysosomal downregulation. More recent work has shown that mutant EGFRs undergo ligand-independent traffic into the endocytic recycling compartment, a behavior that plays a key role in Src pathway activation and oncogenesis. These studies are beginning to delineate the close nexus between signaling and endocytic traffic of EGFR mutants as a key driver of oncogenic processes. Therefore, in this review, we will discuss the links between mutant EGFR signaling and endocytic properties, and introduce potential mechanisms by which altered endocytic properties of mutant EGFRs may alter signaling and vice versa as well as their implications for NSCLC therapy.

Cbl-family ubiquitin ligases and their recruitment of CIN85 are largely dispensable for epidermal growth factor receptor endocytosis

Members of the casitas B-lineage lymphoma (Cbl) family (Cbl, Cbl-b and Cbl-c) of ubiquitin ligases serve as negative regulators of receptor tyrosine kinases (RTKs). An essential role of Cbl-family protein-dependent ubiquitination for efficient ligand-induced lysosomal targeting and degradation is now well-accepted. However, a more proximal role of Cbl and Cbl-b as adapters for CIN85-endophilin recruitment to mediate ligand-induced initial internalization of RTKs is supported by some studies but refuted by others. Overexpression and/or incomplete depletion of Cbl proteins in these studies is likely to have contributed to this dichotomy. To address the role of endogenous Cbl and Cbl-b in the internalization step of RTK endocytic traffic, we established Cbl/Cbl-b double-knockout (DKO) mouse embryonic fibroblasts (MEFs) and demonstrated that these cells lack the expression of both Cbl-family members as well as endophilin A, while they express CIN85. We show that ligand-induced ubiquitination of EGFR, as a prototype RTK, was abolished in DKO MEFs, and EGFR degradation was delayed. These traits were reversed by ectopic human Cbl expression. EGFR endocytosis, assessed using the internalization of (125)I-labeled or fluorescent EGF, or of EGFR itself, was largely retained in Cbl/Cbl-b DKO compared to wild type MEFs. EGFR internalization was also largely intact in Cbl/Cbl-b depleted MCF-10A human mammary epithelial cell line. Inducible shRNA-mediated knockdown of CIN85 in wild type or Cbl/Cbl-b DKO MEFs had no impact on EGFR internalization. Our findings, establish that, at physiological expression levels, Cbl, Cbl-b and CIN85 are largely dispensable for EGFR internalization. Our results support the model that Cbl-CIN85-endophilin complex is not required for efficient internalization of EGFR, a prototype RTK.

The basic amino acids in the coiled-coil domain of CIN85 regulate its interaction with c-Cbl and phosphatidic acid during epidermal growth factor receptor (EGFR) endocytosis

Background

During EGFR internalization CIN85 bridges EGFR-Cbl complex, endocytic machinery and fusible membrane through the interactions of CIN85 with c-Cbl, endophilins and phosphatidic acid. These protein-protein and protein-lipid interactions are mediated or regulated by the positively charged C-terminal coiled-coil domain of CIN85. However, the details of CIN85-lipid interaction remain unknown. The present study suggested a possible electric interaction between the negative charge of phosphatidic acid and the positive charge of basic amino acids in coiled-coil domain.

Results

Mutations of the basic amino acids in the coiled-coil domain, especially K645, K646, R648 and R650, into neutral amino acid alanine completely blocked the interaction of CIN85 with c-Cbl or phosphatidic acid. However, they did not affect CIN85-endophilin interaction. In addition, CIN85 was found to associate with the internalized EGFR endosomes. It interacted with several ESCRT (Endosomal Sorting Complex Required for Transport) component proteins for ESCRT assembly on endosomal membrane. Mutations in the coiled-coil domain (deletion of the coiled-coil domain or point mutations of the basic amino acids) dissociated CIN85 from endosomes. These mutants bound the ESCRT components in cytoplasm to prevent them from assembly on endosomal membrane and inhibited EGFR sorting for degradation.

Conclusions

As an adaptor protein, CIN85 interacts with variety of partners through several domains. The positive charges of basic amino acids in the coiled-coil domain are not only involved in the interaction with phosphatidic acid, but also regulate the interaction of CIN85 with c-Cbl. CIN85 also interacts with ESCRT components for protein sorting in endosomes. These CIN85-protein and CIN85-lipid interactions enable CIN85 to link EGFR-Cbl endocytic complex with fusible membrane during EGFR endocytosis and subsequently to facilitate ESCRT formation on endosomal membrane for EGFR sorting and degradation.

Endocytosis of receptor tyrosine kinases

Endocytosis is the major regulator of signaling from receptor tyrosine kinases (RTKs). The canonical model of RTK endocytosis involves rapid internalization of an RTK activated by ligand binding at the cell surface and subsequent sorting of internalized ligand-RTK complexes to lysosomes for degradation. Activation of the intrinsic tyrosine kinase activity of RTKs results in autophosphorylation, which is mechanistically coupled to the recruitment of adaptor proteins and conjugation of ubiquitin to RTKs. Ubiquitination serves to mediate interactions of RTKs with sorting machineries both at the cell surface and on endosomes. The pathways and kinetics of RTK endocytic trafficking, molecular mechanisms underlying sorting processes, and examples of deviations from the standard trafficking itinerary in the RTK family are discussed in this work.

CIN85 phosphorylation is essential for EGFR ubiquitination and sorting into multivesicular bodies

This study provides new insights into the mechanisms by which CIN85 regulates targeting of the EGF receptor for degradation. It is the first to demonstrate that CIN85 is phosphorylated by src, phosphorylation of CIN85 is essential for ubiquitinylation of the EGFR, and CIN85 mediates EGFR sequestration into intraluminal vesicles.

Ubiquitination of the epidermal growth factor receptor (EGFR) by cbl and its cognate adaptor cbl-interacting protein of 85 kDa (CIN85) is known to play an essential role in directing this receptor to the lysosome for degradation. The mechanisms by which this ubiquitin modification is regulated are not fully defined, nor is it clear where this process occurs. In this study we show that EGFR activation leads to a pronounced src-mediated tyrosine phosphorylation of CIN85 that subsequently influences EGFR ubiquitination. Of importance, phospho-CIN85 interacts with the Rab5-positive endosome, where it mediates the sequestration of the ubiquitinated receptor into multivesicular bodies (MVBs) for subsequent degradation. These findings provide novel insights into how src- kinase–based regulation of a cbl adaptor regulates the fate of the EGFR.