{beta}-Arrestin is crucial for ubiquitination and down-regulation of the insulin-like growth factor-1 receptor by acting as adaptor for the MDM2 E3 ligase

Myeloid beta-arrestin 2 depletion attenuates metabolic dysfunction-associated steatohepatitis via the metabolic reprogramming of macrophages

Macrophage-mediated inflammation has been implicated in the pathogenesis of metabolic dysfunction-associated steatohepatitis (MASH); however, the immunometabolic program underlying the regulation of macrophage activation remains unclear. Beta-arrestin 2, a multifunctional adaptor protein, is highly expressed in bone marrow tissues and macrophages and is involved in metabolism disorders. Here, we observed that β-arrestin 2 expression was significantly increased in the liver macrophages and circulating monocytes of patients with MASH compared with healthy controls and positively correlated with the severity of metabolic dysfunction-associated steatotic liver disease (MASLD). Global or myeloid Arrb2 deficiency prevented the development of MASH in mice. Further study showed that β-arrestin 2 acted as an adaptor protein and promoted ubiquitination of immune responsive gene 1 (IRG1) to prevent increased itaconate production in macrophages, which resulted in enhanced succinate dehydrogenase activity, thereby promoting the release of mitochondrial reactive oxygen species and M1 polarization. Myeloid β-arrestin 2 depletion may be a potential approach for MASH.

Arrestins: A Small Family of Multi-Functional Proteins

The first member of the arrestin family, visual arrestin-1, was discovered in the late 1970s. Later, the other three mammalian subtypes were identified and cloned. The first described function was regulation of G protein-coupled receptor (GPCR) signaling: arrestins bind active phosphorylated GPCRs, blocking their coupling to G proteins. It was later discovered that receptor-bound and free arrestins interact with numerous proteins, regulating GPCR trafficking and various signaling pathways, including those that determine cell fate. Arrestins have no enzymatic activity; they function by organizing multi-protein complexes and localizing their interaction partners to particular cellular compartments. Today we understand the molecular mechanism of arrestin interactions with GPCRs better than the mechanisms underlying other functions. However, even limited knowledge enabled the construction of signaling-biased arrestin mutants and extraction of biologically active monofunctional peptides from these multifunctional proteins. Manipulation of cellular signaling with arrestin-based tools has research and likely therapeutic potential: re-engineered proteins and their parts can produce effects that conventional small-molecule drugs cannot.

Extravascular administration of IGF1R antagonists protects against aortic aneurysm in rodent and porcine models

An abdominal aortic aneurysm (AAA) is a life-threatening cardiovascular disease. We identified plasma insulin-like growth factor 1 (IGF1) as an independent risk factor in patients with AAA by correlating plasma IGF1 with risk. Smooth muscle cell- or fibroblast-specific knockout of Igf1r, the gene encoding the IGF1 receptor (IGF1R), attenuated AAA formation in two mouse models of AAA induced by angiotensin II infusion or CaCl2 treatment. IGF1R was activated in aortic aneurysm samples from human patients and mice with AAA. Systemic administration of IGF1C, a peptide fragment of IGF1, 2 weeks after disease development inhibited AAA progression in mice. Decreased AAA formation was linked to competitive inhibition of IGF1 binding to its receptor by IGF1C and modulation of downstream alpha serine/threonine protein kinase (AKT)/mammalian target of rapamycin signaling. Localized application of an IGF1C-loaded hydrogel was developed to reduce the side effects observed after systemic administration of IGF1C or IGF1R antagonists in the CaCl2-induced AAA mouse model. The inhibitory effect of the IGF1C-loaded hydrogel administered at disease onset on AAA formation was further evaluated in a guinea pig-to-rat xenograft model and in a sheep-to-minipig xenograft model of AAA formation. The therapeutic efficacy of IGF1C for treating AAA was tested through extravascular delivery in the sheep-to-minipig model with AAA established for 2 weeks. Percutaneous injection of the IGF1C-loaded hydrogel around the AAA resulted in improved vessel flow dynamics in the minipig aorta. These findings suggest that extravascular administration of IGF1R antagonists may have translational potential for treating AAA.

β-arrestin1 is an E3 ubiquitin ligase adaptor for substrate linear polyubiquitination

G protein-coupled receptor (GPCR) signaling and trafficking are regulated by multiple mechanisms, including posttranslational modifications such as ubiquitination by E3 ubiquitin ligases. E3 ligases have been linked to agonist-stimulated ubiquitination of GPCRs via simultaneous binding to βarrestins. In addition, βarrestins have been suggested to assist E3 ligases for ubiquitination of key effector molecules, yet mechanistic insight is lacking. Here, we developed an in vitro reconstituted system and show that βarrestin1 (βarr1) serves as an adaptor between the effector protein signal-transducing adaptor molecule 1 (STAM1) and the E3 ligase atrophin-interacting protein 4. Via mass spectrometry, we identified seven lysine residues within STAM1 that are ubiquitinated and several types of ubiquitin linkages. We provide evidence that βarr1 facilitates the formation of linear polyubiquitin chains at lysine residue 136 on STAM1. This lysine residue is important for stabilizing the βarr1:STAM1 interaction in cells following GPCR activation. Our study identifies atrophin-interacting protein 4 as only the second E3 ligase known to conjugate linear polyubiquitin chains and a possible role for linear ubiquitin chains in GPCR signaling and trafficking.

Competing Engagement of β-arrestin Isoforms Balances IGF1R/p53 Signaling and Controls Melanoma Cell Chemotherapeutic Responsiveness

Constraints on the p53 tumor suppressor pathway have long been associated with the progression, therapeutic resistance, and poor prognosis of melanoma, the most aggressive form of skin cancer. Likewise, the insulin-like growth factor type 1 receptor (IGF1R) is recognized as an essential coordinator of transformation, proliferation, survival, and migration of melanoma cells. Given that β-arrestin (β-arr) system critically governs the anti/pro-tumorigenic p53/IGF1R signaling pathways through their common E3 ubiquitin-protein ligase MDM2, we explore whether unbalancing this system downstream of IGF1R can enhance the response of melanoma cells to chemotherapy. Altering β-arr expression demonstrated that both β-arr1-silencing and β-arr2-overexpression (-β-arr1/+β-arr2) facilitated nuclear-to-cytosolic MDM2 translocation accompanied by decreased IGF1R expression, while increasing p53 levels, resulting in reduced cell proliferation/survival. Imbalance towards β-arr2 (-β-arr1/+β-arr2) synergizes with the chemotherapeutic agent, dacarbazine, in promoting melanoma cell toxicity. In both 3D spheroid models and in vivo in zebrafish models, this combination strategy, through dual IGF1R downregulation/p53 activation, limits melanoma cell growth, survival and metastatic spread. In clinical settings, analysis of the TCGA-SKCM patient cohort confirms β-arr1-/β-arr2+ imbalance as a metastatic melanoma vulnerability that may enhance therapeutic benefit. Our findings suggest that under steady-state conditions, IGF1R/p53-tumor promotion/suppression status-quo is preserved by β-arr1/2 homeostasis. Biasing this balance towards β-arr2 can limit the protumorigenic IGF1R activities while enhancing p53 activity, thus reducing multiple cancer-sustaining mechanisms. Combined with other therapeutics, this strategy improves patient responses and outcomes to therapies relying on p53 or IGF1R pathways.

IMPLICATIONS

Altogether, β-arrestin system bias downstream IGF1R is an important metastatic melanoma vulnerability that may be conductive for therapeutic benefit.

2021 update on thyroid-associated ophthalmopathy

PURPOSE

Our understanding of thyroid-associated ophthalmopathy (TAO, A.K.A Graves' orbitopathy, thyroid eye disease) has advanced substantially, since one of us (TJS) wrote the 2010 update on TAO, appearing in this journal.

METHODS

PubMed was searched for relevant articles.

RESULTS

Recent insights have resulted from important studies conducted by many different laboratory groups around the World. A clearer understanding of autoimmune diseases in general and TAO specifically emerged from the use of improved research methodologies. Several key concepts have matured over the past decade. Among them, those arising from the refinement of mouse models of TAO, early stage investigation into restoring immune tolerance in Graves' disease, and a hard-won acknowledgement that the insulin-like growth factor-I receptor (IGF-IR) might play a critical role in the development of TAO, stand out as important. The therapeutic inhibition of IGF-IR has blossomed into an effective and safe medical treatment. Teprotumumab, a β-arrestin biased agonist monoclonal antibody inhibitor of IGF-IR has been studied in two multicenter, double-masked, placebo-controlled clinical trials demonstrated both effectiveness and a promising safety profile in moderate-to-severe, active TAO. Those studies led to the approval by the US FDA of teprotumumab, currently marketed as Tepezza for TAO. We have also learned far more about the putative role that CD34+ fibrocytes and their derivatives, CD34+ orbital fibroblasts, play in TAO.

CONCLUSION

The past decade has been filled with substantial scientific advances that should provide the necessary springboard for continually accelerating discovery over the next 10 years and beyond.

The roles of mouse double minute 2 (MDM2) oncoprotein in ocular diseases: A review

Mouse double minute 2 (MDM2), an E3 ubiquitin ligase and the primary negative regulator of the tumor suppressor p53, cooperates with its structural homolog MDM4/MDMX to control intracellular p53 level. In turn, overexpression of p53 upregulates and forms an autoregulatory feedback loop with MDM2. The MDM2-p53 axis plays a pivotal role in modulating cell cycle control and apoptosis. MDM2 itself is regulated by the PI3K-AKT and RB-E2F-ARF pathways. While amplification of the MDM2 gene or overexpression of MDM2 (due to MDM2 SNP T309G, for instance) is associated with various malignancies, numerous studies have shown that MDM2/p53 alterations may also play a part in the pathogenetic process of certain ocular disorders (Fig. 1). These include cancers (retinoblastoma, uveal melanoma), fibrocellular proliferative diseases (proliferative vitreoretinopathy, pterygium), neovascular diseases, degenerative diseases (cataract, primary open-angle glaucoma, age-related macular degeneration) and infectious/inflammatory diseases (trachoma, uveitis). In addition, MDM2 is implicated in retinogenesis and regeneration after optic nerve injury. Anti-MDM2 therapy has shown potential as a novel approach to treating these diseases. Despite major safety concerns, there are high expectations for the clinical value of reformative MDM2 inhibitors. This review summarizes important findings about the role of MDM2 in ocular pathologies and provides an overview of recent advances in treating these diseases with anti-MDM2 therapies.

IGF-1R is a molecular determinant for response to p53 reactivation therapy in conjunctival melanoma

As the p53 tumor suppressor is rarely mutated in conjunctival melanoma (CM), we investigated its activation as a potential therapeutic strategy. Preventing p53/Mdm2 interaction by Nutlin-3, the prototypical Mdm2 antagonist, or via direct siRNA Mdm2 depletion, increased p53 and inhibited viability in CM cell lines. The sensitivity to Nutlin-3 p53 reactivation with concomitant Mdm2 stabilization was higher than that achieved by siRNA, indicative of effects on alternative Mdm2 targets, identified as the cancer-protective IGF-1R. Nutlin-3 treatment increased the association between IGF-1R and β-arrestin1, the adaptor protein that brings Mdm2 to the IGF-1R, initiating receptor degradation in a ligand-dependent manner. Controlled expression of β-arrestin1 augmented inhibitory Nutlin-3 effects on CM survival through enhanced IGF-1R degradation. Yet, the effect of IGF-1R downregulation on cell proliferation is balanced by β-arrestin1-induced p53 inhibition. As mitomycin (MMC) is a well-established adjuvant treatment for CM, and it triggers p53 activation through genotoxic stress, we evaluated how these alternative p53-targeting strategies alter the cancer-relevant bioactivities of CM. In 2D and 3D in vitro models, Nutlin-3 or MMC alone, or in combination, reduces the overall cell tumor growth ~30%, with double treatment inhibition rate only marginally higher than single-drug regimens. However, histopathological evaluation of the 3D models revealed that Nutlin-3 was the most effective, causing necrotic areas inside spheroids and complete loss of nuclear staining for the proliferative marker Ki67. These findings were further validated in vivo; zebrafish xenografts demonstrate that Nutlin-3 alone has higher efficacy in restraining CM tumor cell growth and preventing metastasis. Combined, these results reveal that β-arrestin1 directs Mdm2 toward different substrates, thus balancing IGF-1R pro-tumorigenic and p53-tumor suppressive signals. This study defines a potent dual-hit strategy: simultaneous control of a tumor-promoter (IGF-1R) and tumor-suppressor (p53), which ultimately mitigates recurrent and metastatic potential, thus opening up targeted therapy to CM.

Resistance mechanisms to inhibitors of p53-MDM2 interactions in cancer therapy: can we overcome them?

Since the discovery of the first MDM2 inhibitors, we have gained deeper insights into the cellular roles of MDM2 and p53. In this review, we focus on MDM2 inhibitors that bind to the p53-binding domain of MDM2 and aim to disrupt the binding of MDM2 to p53. We describe the basic mechanism of action of these MDM2 inhibitors, such as nutlin-3a, summarise the determinants of sensitivity to MDM2 inhibition from p53-dependent and p53-independent points of view and discuss the problems with innate and acquired resistance to MDM2 inhibition. Despite progress in MDM2 inhibitor design and ongoing clinical trials, their broad use in cancer treatment is not fulfilling expectations in heterogenous human cancers. We assess the MDM2 inhibitor types in clinical trials and provide an overview of possible sources of resistance to MDM2 inhibition, underlining the need for patient stratification based on these aspects to gain better clinical responses, including the use of combination therapies for personalised medicine.

Glucagon's Metabolic Action in Health and Disease

Discovered almost simultaneously with insulin, glucagon is a pleiotropic hormone with metabolic action that goes far beyond its classical role to increase blood glucose. Albeit best known for its ability to directly act on the liver to increase de novo glucose production and to inhibit glycogen breakdown, glucagon lowers body weight by decreasing food intake and by increasing metabolic rate. Glucagon further promotes lipolysis and lipid oxidation and has positive chronotropic and inotropic effects in the heart. Interestingly, recent decades have witnessed a remarkable renaissance of glucagon's biology with the acknowledgment that glucagon has pharmacological value beyond its classical use as rescue medication to treat severe hypoglycemia. In this article, we summarize the multifaceted nature of glucagon with a special focus on its hepatic action and discuss the pharmacological potential of either agonizing or antagonizing the glucagon receptor for health and disease. © 2021 American Physiological Society. Compr Physiol 11:1759-1783, 2021.

Overexpression of β-Arrestins inhibits proliferation and motility in triple negative breast cancer cells

β-Arrestins (βArrs) are intracellular signal regulating proteins. Their expression level varies in some cancers and they have a significant impact on cancer cell function. In general, the significance of βArrs in cancer research comes from studies examining GPCR signalling. Given the diversity of different GPCR signals in cancer cell regulation, contradictory results are inevitable regarding the role of βArrs. Our approach examines the direct influence of βArrs on cellular function and gene expression profiles by changing their expression levels in breast cancer cells, MDA-MB-231 and MDA-MB-468. Reducing expression of βArr1 or βArr2 tended to increase cell proliferation and invasion whereas increasing their expression levels inhibited them. The overexpression of βArrs caused cell cycle S-phase arrest and differential expression of cell cycle genes, CDC45, BUB1, CCNB1, CCNB2, CDKN2C and reduced HER3, IGF-1R, and Snail. Regarding to the clinical relevance of our results, low expression levels of βArr1 were inversely correlated with CDC45, BUB1, CCNB1, and CCNB2 genes compared to normal tissue samples while positively correlated with poorer prognosis in breast tumours. These results indicate that βArr1 and βArr2 are significantly involved in cell cycle and anticancer signalling pathways through their influence on cell cycle genes and HER3, IGF-1R, and Snail in TNBC cells.

Insulin-Like Growth Factor Pathway and the Thyroid

The insulin-like growth factor (IGF) pathway comprises two activating ligands (IGF-I and IGF-II), two cell-surface receptors (IGF-IR and IGF-IIR), six IGF binding proteins (IGFBP) and nine IGFBP related proteins. IGF-I and the IGF-IR share substantial structural and functional similarities to those of insulin and its receptor. IGF-I plays important regulatory roles in the development, growth, and function of many human tissues. Its pathway intersects with those mediating the actions of many cytokines, growth factors and hormones. Among these, IGFs impact the thyroid and the hormones that it generates. Further, thyroid hormones and thyrotropin (TSH) can influence the biological effects of growth hormone and IGF-I on target tissues. The consequences of this two-way interplay can be far-reaching on many metabolic and immunologic processes. Specifically, IGF-I supports normal function, volume and hormone synthesis of the thyroid gland. Some of these effects are mediated through enhancement of sensitivity to the actions of TSH while others may be independent of pituitary function. IGF-I also participates in pathological conditions of the thyroid, including benign enlargement and tumorigenesis, such as those occurring in acromegaly. With regard to Graves' disease (GD) and the periocular process frequently associated with it, namely thyroid-associated ophthalmopathy (TAO), IGF-IR has been found overexpressed in orbital connective tissues, T and B cells in GD and TAO. Autoantibodies of the IgG class are generated in patients with GD that bind to IGF-IR and initiate the signaling from the TSHR/IGF-IR physical and functional protein complex. Further, inhibition of IGF-IR with monoclonal antibody inhibitors can attenuate signaling from either TSHR or IGF-IR. Based on those findings, the development of teprotumumab, a β-arrestin biased agonist as a therapeutic has resulted in the first medication approved by the US FDA for the treatment of TAO. Teprotumumab is now in wide clinical use in North America.

Inhibition of G Protein-Coupled Receptor Kinase 2 Promotes Unbiased Downregulation of IGF1 Receptor and Restrains Malignant Cell Growth

The ability of a receptor to preferentially activate only a subset of available downstream signal cascades is termed biased signaling. Although comprehensively recognized for the G protein-coupled receptors (GPCR), this process is scarcely explored downstream of receptor tyrosine kinases (RTK), including the cancer-relevant insulin-like growth factor-1 receptor (IGF1R). Successful IGF1R targeting requires receptor downregulation, yet therapy-mediated removal from the cell surface activates cancer-protective β-arrestin-biased signaling (β-arr-BS). As these overlapping processes are initiated by the β-arr/IGF1R interaction and controlled by GPCR-kinases (GRK), we explored GRKs as potential anticancer therapeutic targets to disconnect IGF1R downregulation and β-arr-BS. Transgenic modulation demonstrated that GRK2 inhibition or GRK6 overexpression enhanced degradation of IGF1R, but both scenarios sustained IGF1-induced β-arr-BS. Pharmacologic inhibition of GRK2 by the clinically approved antidepressant, serotonin reuptake inhibitor paroxetine (PX), recapitulated the effects of GRK2 silencing with dose- and time-dependent IGF1R downregulation without associated β-arr-BS. In vivo, PX treatment caused substantial downregulation of IGF1R, suppressing the growth of Ewing's sarcoma xenografts. Functional studies reveal that PX exploits the antagonism between β-arrestin isoforms; in low ligand conditions, PX favored β-arrestin1/Mdm2-mediated ubiquitination/degradation of IGF1R, a scenario usually exclusive to ligand abundancy, making PX more effective than antibody-mediated IGF1R downregulation. This study provides the rationale, molecular mechanism, and validation of a clinically feasible concept for "system bias" targeting of the IGF1R to uncouple downregulation from signaling. Demonstrating system bias as an effective anticancer approach, our study reveals a novel strategy for the rational design or repurposing of therapeutics to selectively cross-target the IGF1R or other RTK. SIGNIFICANCE: This work provides insight into the molecular and biological roles of biased signaling downstream RTK and provides a novel "system bias" strategy to increase the efficacy of anti-IGF1R-targeted therapy in cancer.

Investigating higher-order interactions in single-cell data with scHOT

Single-cell genomics has transformed our ability to examine cell fate choice. Examining cells along a computationally ordered 'pseudotime' offers the potential to unpick subtle changes in variability and covariation among key genes. We describe an approach, scHOT-single-cell higher-order testing-which provides a flexible and statistically robust framework for identifying changes in higher-order interactions among genes. scHOT can be applied for cells along a continuous trajectory or across space and accommodates various higher-order measurements including variability or correlation. We demonstrate the use of scHOT by studying coordinated changes in higher-order interactions during embryonic development of the mouse liver. Additionally, scHOT identifies subtle changes in gene-gene correlations across space using spatially resolved transcriptomics data from the mouse olfactory bulb. scHOT meaningfully adds to first-order differential expression testing and provides a framework for interrogating higher-order interactions using single-cell data.

New Insights from IGF-IR Stimulating Activity Analyses: Pathological Considerations

Insulin-like growth factor-I (IGF-I) and insulin-like growth factor-II (IGF-II) play a crucial factor in the growth, differentiation and survival of cells in health and disease. IGF-I and IGF-II primarily activate the IGF-I receptor (IGF-IR), which is present on the cell surface. Activation of the IGF-IR stimulates multiple pathways which finally results in multiple biological effects in a variety of tissues and cells. In addition, activation of the IGF-IR has been found to be essential for the growth of cancers. The conventional view in the past was that the IGF-IR was exclusively a tyrosine kinase receptor and that phosphorylation of tyrosine residues, after binding of IGF-I to the IGF-IR, started a cascade of post-receptor events. Recent research has shown that this view was too simplistic. It has been found that the IGF-IR also has kinase-independent functions and may even emit signals in the unoccupied state through some yet-to-be-defined non-canonical pathways. The IGF-IR may further form hybrids with the insulin receptors but also with receptor tyrosine kinases (RTKs) outside the insulin-IGF system. In addition, the IGF-IR has extensive cross-talk with many other receptor tyrosine kinases and their downstream effectors. Moreover, there is now emerging evidence that the IGF-IR utilizes parts of the G-protein coupled receptor (GPCR) pathways: the IGF-IR can be considered as a functional RTK/GPCR hybrid, which integrates the kinase signaling with some IGF-IR mediated canonical GPCR characteristics. Like the classical GPCRs the IGF-IR can also show homologous and heterologous desensitization. Recently, it has been found that after activation by a ligand, the IGF-IR may be translocated into the nucleus and function as a transcriptional cofactor. Thus, in recent years, it has become clear that the IGF-IR signaling pathways are much more complex than first thought. Therefore a big challenge for the (near) future will be how all the new knowledge about IGF-IR signaling can be translated into the clinical practice and improve diagnosis and treatment of diseases.

ARRB1-Promoted NOTCH1 Degradation Is Suppressed by OncomiR miR-223 in T-cell Acute Lymphoblastic Leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is a type of aggressive leukemia with inferior prognosis. Although activating mutations of NOTCH1 are observed in most T-ALL cases, these mutations alone are not sufficient to drive the full development of T-ALL. β-Arrestins (ARRB) are versatile and multifunctional adapter proteins that regulate diverse cellular functions, including promoting the development of cancer. However, the role of ARRBs in T-ALL has largely remained elusive. In this study, we showed that ARRB1 is expressed at low levels in assayed T-ALL clinical samples and cell lines. Exogenous ARRB1 expression inhibited T-ALL proliferation and improved the survival of T-ALL xenograft animals. ARRB1 facilitated NOTCH1 ubiquitination and degradation through interactions with NOTCH1 and DTX1. Mechanistically, the oncogenic miRNA (oncomiR) miR-223 targets the 3'-UTR of ARRB1 (BUTR) and inhibits its expression in T-ALL. Furthermore, overexpression of the ARRB1-derived miR-223 sponge suppressed T-ALL cell proliferation and induced apoptosis. Collectively, these results demonstrate that ARRB1 acts as a tumor suppressor in T-ALL by promoting NOTCH1 degradation, which is inhibited by elevated miR-223, suggesting that ARRB1 may serve as a valid drug target in the development of novel T-ALL therapeutics.Significance: These findings highlight a novel tumor suppressive function of the adaptor protein β-arrestin1 in T-ALL.

A Fungal Arrestin Protein Contributes to Cell Cycle Progression and Pathogenesis

To survive under unpredictable conditions, all organisms must adapt to stressors by regulating adaptive cellular responses. Arrestin proteins are conserved regulators of adaptive cellular responses in eukaryotes. Studies that have been limited to mammals and model fungi have demonstrated that the disruption of arrestin-regulated pathways is detrimental for viability. The human fungal pathogen Cryptococcus neoformans causes more than 180,000 infection-related deaths annually, especially among immunocompromised patients. In addition to being genetically tractable, C. neoformans has a small arrestin family of four members, lending itself to a comprehensive characterization of its arrestin family. This study serves as a functional analysis of arrestins in a pathogen, particularly in the context of fungal fitness and virulence. We investigate the functions of one arrestin protein, Ali1, and define its novel contributions to cytokinesis. We additionally explore the virulence contributions of the C. neoformans arrestin family and find that they contribute to disease establishment and progression.

Arrestins, a structurally specialized and functionally diverse group of proteins, are central regulators of adaptive cellular responses in eukaryotes. Previous studies on fungal arrestins have demonstrated their capacity to modulate diverse cellular processes through their adaptor functions, facilitating the localization and function of other proteins. However, the mechanisms by which arrestin-regulated processes are involved in fungal virulence remain unexplored. We have identified a small family of four arrestins, Ali1, Ali2, Ali3, and Ali4, in the human fungal pathogen Cryptococcus neoformans. Using complementary microscopy, proteomic, and reverse genetics techniques, we have defined a role for Ali1 as a novel contributor to cytokinesis, a fundamental cell cycle-associated process. We observed that Ali1 strongly interacts with proteins involved in lipid synthesis, and that ali1Δ mutant phenotypes are rescued by supplementation with lipid precursors that are used to build cellular membranes. From these data, we hypothesize that Ali1 contributes to cytokinesis by serving as an adaptor protein, facilitating the localization of enzymes that modify the plasma membrane during cell division, specifically the fatty acid synthases Fas1 and Fas2. Finally, we assessed the contributions of the C. neoformans arrestin family to virulence to better understand the mechanisms by which arrestin-regulated adaptive cellular responses influence fungal infection. We observed that the C. neoformans arrestin family contributes to virulence, and that the individual arrestin proteins likely fulfill distinct functions that are important for disease progression.

Arrestin-3 interaction with maternal embryonic leucine-zipper kinase

Maternal embryonic leucine-zipper kinase (MELK) overexpression impacts survival and proliferation of multiple cancer types, most notably glioblastomas and breast cancer. This makes MELK an attractive molecular target for cancer therapy. Yet the molecular mechanisms underlying the involvement of MELK in tumorigenic processes are unknown. MELK participates in numerous protein-protein interactions that affect cell cycle, proliferation, apoptosis, and embryonic development. Here we used both in vitro and in-cell assays to identify a direct interaction between MELK and arrestin-3. A part of this interaction involves the MELK kinase domain, and we further show that the interaction between the MELK kinase domain and arrestin-3 decreases the number of cells in S-phase, as compared to cells expressing the MELK kinase domain alone. Thus, we describe a new mechanism of regulation of MELK function, which may contribute to the control of cell fate.

β-arrestin-2 enhances intestinal epithelial apoptosis in necrotizing enterocolitis

Apoptosis among intestinal epithelial cells contributes to necrotizing enterocolitis (NEC), a severe intestinal disease that particularly affects premature infants. β-arrestin-2, an important regulator of G-protein-coupled receptors, is expressed in intestinal epithelial cells, where its activation promotes apoptosis. We found that β-arrestin-2 was overexpressed in both human and murine NEC samples. β-arrestin-2-deficient mice were protected from endoplasmic reticulum stress and NEC development. The endoplasmic reticulum-resident chaperone BiP was found to promote intestinal epithelial cell survival. Pretreatment of intestinal epithelial cells or mice with the BiP inhibitor HA15 increased cell apoptosis and promoted NEC development. β-arrestin-2 bound to BiP and promoted its polyubiquitination and degradation, thereby facilitating the release of the pro-apoptotic molecule BIK from BiP. Silencing β-arrestin-2 downregulated apoptosis by increasing BiP levels, which suppressed endoplasmic reticulum stress. This study suggests that β-arrestin-2 induces NEC development by inhibiting BiP, thereby triggering apoptosis in response to endoplasmic reticulum stress. Thus, novel therapeutic strategies to inhibit β-arrestin-2 may enhance the treatment of NEC.

Below the Surface: IGF-1R Therapeutic Targeting and Its Endocytic Journey

Ligand-activated plasma membrane receptors follow pathways of endocytosis through the endosomal sorting apparatus. Receptors cluster in clathrin-coated pits that bud inwards and enter the cell as clathrin-coated vesicles. These vesicles travel through the acidic endosome whereby receptors and ligands are sorted to be either recycled or degraded. The traditional paradigm postulated that the endocytosis role lay in signal termination through the removal of the receptor from the cell surface. It is now becoming clear that the internalization process governs more than receptor signal cessation and instead reigns over the entire spatial and temporal wiring of receptor signaling. Governing the localization, the post-translational modifications, and the scaffolding of receptors and downstream signal components established the endosomal platform as the master regulator of receptor function. Confinement of components within or between distinct organelles means that the endosome instructs the cell on how to interpret and translate the signal emanating from any given receptor complex into biological effects. This review explores this emerging paradigm with respect to the cancer-relevant insulin-like growth factor type 1 receptor (IGF-1R) and discusses how this perspective could inform future targeting strategies.

β-Arrestin-1 inhibits glucocorticoid receptor turnover and alters glucocorticoid signaling

Glucocorticoids are among the most widely used drugs to treat many autoimmune and inflammatory diseases. Although much research has been focused on investigating glucocorticoid activity, it remains unclear how glucocorticoids regulate distinct processes in different cells. Glucocorticoids exert their effects through the glucocorticoid receptor (GR), which, upon glucocorticoid binding, interacts with regulatory proteins, affecting its activity and function. These protein-protein interactions are necessary for the resolution of glucocorticoid-dependent physiological and pharmacological processes. In this study, we discovered a novel protein interaction between the glucocorticoid receptor and β-arrestin-1, a scaffold protein with a well-established role in G protein-coupled receptor signaling. Using co-immunoprecipitation and in situ proximity ligation assays in A549 cells, we observed that β-arrestin-1 and unliganded GR interact in the cytoplasm and that, following glucocorticoid binding, the protein complex is found in the nucleus. We show that siRNA-mediated β-arrestin-1 knockdown alters GR protein turnover by up-regulating the E3 ubiquitin ligase Pellino-1, which catalyzes GR ubiquitination and thereby marks the receptor for proteasomal degradation. The enhanced GR turnover observed in β-arrestin-1-deficient cells limits the duration of the glucocorticoid response on GR target genes. These results demonstrate that β-arrestin-1 is a crucial player for the stability of the glucocorticoid receptor. The GR/β-arrestin-1 interaction uncovered here may help unravel mechanisms that contribute to the cell type-specific activities of glucocorticoids.

New Routes in GPCR/β-Arrestin-Driven Signaling in Cancer Progression and Metastasis

Tumor cells acquire invasive and metastatic behavior by sensing changes in the localization and activation of signaling pathways, which in turn determine changes in actin cytoskeleton. The core-scaffold machinery associated to β-arrestin (β-arr) is a key mechanism of G-protein coupled receptors (GPCR) to achieve spatiotemporal specificity of different signaling complexes driving cancer progression. Within different cellular contexts, the scaffold proteins β-arr1 or β-arr2 may now be considered organizers of protein interaction networks involved in tumor development and metastatic dissemination. Studies have uncovered the importance of the β-arr engagement with a growing number of receptors, signaling molecules, cytoskeleton regulators, epigenetic modifiers, and transcription factors in GPCR-driven tumor promoting pathways. In many of these molecular complexes, β-arrs might provide a physical link to active dynamic cytoskeleton, permitting cancer cells to adapt and modify the tumor microenvironment to promote the metastatic spread. Given the complexity and the multidirectional β-arr-driven signaling in cancer cells, therapeutic targeting of specific GPCR/β-arr molecular mechanisms is an important avenue to explore when considering future new therapeutic options. The focus of this review is to integrate the most recent developments and exciting findings of how highly connected components of β-arr-guided molecular connections to other pathways allow precise control over multiple signaling pathways in tumor progression, revealing ways of therapeutically targeting the convergent signals in patients.

Personalized Medicine in Malignant Melanoma: Towards Patient Tailored Treatment

Despite enormous international efforts, skin melanoma is still a major clinical challenge. Melanoma takes a top place among the most common cancer types and it has one of the most rapidly increasing incidences in many countries around the world. Until recent years, there have been limited options for effective systemic treatment of disseminated melanoma. However, lately, we have experienced a rapid advancement in the understanding of the biology and molecular background of the disease. This has led to new molecular classifications and the development of more effective targeted therapies adapted to distinct melanoma subtypes. Not only are these treatments more effective but they can be rationally prescribed to the patients standing to benefit. As such, melanoma management has now become one of the most developed for personalized medicine. The aim of the present paper is to summarize the current knowledge on melanoma molecular classification, predictive markers, combination therapies, as well as emerging new treatments.

β-arrestin1-medieated inhibition of FOXO3a contributes to prostate cancer cell growth in vitro and in vivo

Recently, β-arrestin1 has been indicated as a prostate cancer promoter through promoting cell proliferation and epithelial to mesenchymal transition, but its underlying mechanism remains unclear. Here, our data revealed that β-arrestin1 could promote cell growth through inhibiting the transcriptional activity and expression of FOXO3a in prostate cancer cells in vitro and in vivo. We found that β-arrestin1 could promote the cell and tumor growth of prostate cancer, and β-arrestin1 expression represented a negative correlation with FOXO3a expression but not FOXO1 expression in prostate cancer cell lines and tissues. In addition, forced expression of β-arrestin1 induced a significant decrease of FOXO3a expression but had no clear effect on FOXO1 expression. Mechanistically, β-arrestin1 could interact with FOXO3a and MDM2, respectively, and promote the interaction between FOXO3a and MDM2, whereas it had no obvious interaction with FOXO1. Furthermore, β-arrestin1 could inhibit the transcriptional activity of FOXO3a via Akt and ERK1/2 pathways. Together, our results revealed a novel mechanism for β-arrestin1 in the regulation of the prostate cancer procession through inhibiting FOXO3a.

Advances in Membrane Trafficking and Endosomal Signaling of G Protein-Coupled Receptors

The integration of GPCR signaling with membrane trafficking, as a single orchestrated system, is a theme increasingly evident with the growing reports of GPCR endosomal signaling. Once viewed as a mechanism to regulate cell surface heterotrimeric G protein signaling, the endocytic trafficking system is complex, highly compartmentalized, yet deeply interconnected with cell signaling. The organization of receptors into distinct plasma membrane signalosomes, biochemically distinct endosomal populations, endosomal microdomains, and its communication with distinct subcellular organelles such as the Golgi provides multiple unique signaling platforms that are critical for specifying receptor function physiologically and pathophysiologically. In this chapter I discuss our emerging understanding in the endocytic trafficking systems employed by GPCRs and their novel roles in spatial control of signaling. Given the extensive roles that GPCRs play in vivo, these evolving models are starting to provide mechanistic understanding of distinct diseases and provide novel therapeutic avenues that are proving to be viable targets.

Unique Roles of β-Arrestin in GPCR Trafficking Revealed by Photoinducible Dimerizers

Intracellular trafficking of G protein-coupled receptors (GPCRs) controls their localization and degradation, which affects a cell's ability to adapt to extracellular stimuli. Although the perturbation of trafficking induces important diseases, these trafficking mechanisms are poorly understood. Herein, we demonstrate an optogenetic method using an optical dimerizer, cryptochrome (CRY) and its partner protein (CIB), to analyze the trafficking mechanisms of GPCRs and their regulatory proteins. Temporally controlling the interaction between β-arrestin and β2-adrenergic receptor (ADRB2) reveals that the duration of the β-arrestin-ADRB2 interaction determines the trafficking pathway of ADRB2. Remarkably, the phosphorylation of ADRB2 by G protein-coupled receptor kinases is unnecessary to trigger clathrin-mediated endocytosis, and β-arrestin interacting with unphosphorylated ADRB2 fails to activate mitogen-activated protein kinase (MAPK) signaling, in contrast to the ADRB2 agonist isoproterenol. Temporal control of β-arrestin-GPCR interactions will enable the investigation of the unique roles of β-arrestin and the mechanism by which it regulates β-arrestin-specific trafficking pathways of different GPCRs.

Mdm2 regulates cardiac contractility by inhibiting GRK2-mediated desensitization of β-adrenergic receptor signaling

The oncoprotein Mdm2 is a RING domain-containing E3 ubiquitin ligase that ubiquitinates G protein-coupled receptor kinase 2 (GRK2) and β-arrestin2, thereby regulating β-adrenergic receptor (βAR) signaling and endocytosis. Previous studies showed that cardiac Mdm2 expression is critical for controlling p53-dependent apoptosis during early embryonic development, but the role of Mdm2 in the developed adult heart is unknown. We aimed to identify if Mdm2 affects βAR signaling and cardiac function in adult mice. Using Mdm2/p53-KO mice, which survive for 9-12 months, we identified a critical and potentially novel role for Mdm2 in the adult mouse heart through its regulation of cardiac β1AR signaling. While baseline cardiac function was mostly similar in both Mdm2/p53-KO and wild-type (WT) mice, isoproterenol-induced cardiac contractility in Mdm2/p53-KO was significantly blunted compared with WT mice. Isoproterenol increased cAMP in left ventricles of WT but not of Mdm2/p53-KO mice. Additionally, while basal and forskolin-induced calcium handling in isolated Mdm2/p53-KO and WT cardiomyocytes were equivalent, isoproterenol-induced calcium handling in Mdm2/p53-KO was impaired. Mdm2/p53-KO hearts expressed 2-fold more GRK2 than WT. GRK2 polyubiquitination via lysine-48 linkages was significantly reduced in Mdm2/p53-KO hearts. Tamoxifen-inducible cardiomyocyte-specific deletion of Mdm2 in adult mice also led to a significant increase in GRK2, and resulted in severely impaired cardiac function, high mortality, and no detectable βAR responsiveness. Gene delivery of either Mdm2 or GRK2-CT in vivo using adeno-associated virus 9 (AAV9) effectively rescued β1AR-induced cardiac contractility in Mdm2/p53-KO. These findings reveal a critical p53-independent physiological role of Mdm2 in adult hearts, namely, regulation of GRK2-mediated desensitization of βAR signaling.

Identification of Phosphorylation Codes for Arrestin Recruitment by G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) mediate diverse signaling in part through interaction with arrestins, whose binding promotes receptor internalization and signaling through G protein-independent pathways. High-affinity arrestin binding requires receptor phosphorylation, often at the receptor's C-terminal tail. Here, we report an X-ray free electron laser (XFEL) crystal structure of the rhodopsin-arrestin complex, in which the phosphorylated C terminus of rhodopsin forms an extended intermolecular β sheet with the N-terminal β strands of arrestin. Phosphorylation was detected at rhodopsin C-terminal tail residues T336 and S338. These two phospho-residues, together with E341, form an extensive network of electrostatic interactions with three positively charged pockets in arrestin in a mode that resembles binding of the phosphorylated vasopressin-2 receptor tail to β-arrestin-1. Based on these observations, we derived and validated a set of phosphorylation codes that serve as a common mechanism for phosphorylation-dependent recruitment of arrestins by GPCRs.

Enhanced response of melanoma cells to MEK inhibitors following unbiased IGF-1R down-regulation

Due to its ability to compensate for signals lost following therapeutic MAPK-inhibition, insulin-like growth factor type 1 receptor (IGF-1R) co-targeting is a rational approach for melanoma treatment. However IGF-1R conformational changes associated with its inhibition can preferentially activate MAPK-pathway in a kinase-independent manner, through a process known as biased signaling. We explored the impact of biased IGF-1R signaling, on response to MAPK inhibition in a panel of skin melanoma cell lines with differing MAPK and p53 mutation statuses. Specific siRNA towards IGF-1R down-regulates the receptor and all its signaling in a balanced manner, whilst IGF-1R targeting by small molecule Nutlin-3 parallels receptor degradation with a transient biased pERK1/2 activity, with both strategies synergizing with MEK1/2 inhibition. On the other hand, IGF-1R down-regulation by a targeted antibody (Figitumumab) induces a biased receptor conformation, preserved even when the receptor is exposed to the balanced natural ligand IGF-1. This process sustains MAPK activity and competes with the MEK1/2 inhibition. Our results indicate that IGF-1R down-regulation offers an approach to increase the sensitivity of melanoma cells to MAPK inhibition, and highlights that controlling biased signaling could provide greater specificity and precision required for multi-hit therapy.

Unbalancing p53/Mdm2/IGF-1R axis by Mdm2 activation restrains the IGF-1-dependent invasive phenotype of skin melanoma

Melanoma tumors usually retain wild-type p53; however, its tumor-suppressor activity is functionally disabled, most commonly through an inactivating interaction with mouse double-minute 2 homolog (Mdm2), indicating p53 release from this complex as a potential therapeutic approach. P53 and the tumor-promoter insulin-like growth factor type 1 receptor (IGF-1R) compete as substrates for the E3 ubiquitin ligase Mdm2, making their relative abundance intricately linked. Hence we investigated the effects of pharmacological Mdm2 release from the Mdm2/p53 complex on the expression and function of the IGF-1R. Nutlin-3 treatment increased IGF-1R/Mdm2 association with enhanced IGF-1R ubiquitination and a dual functional outcome: receptor downregulation and selective downstream signaling activation confined to the mitogen-activated protein kinase/extracellular signal-regulated kinase pathway. This Nutlin-3 functional selectivity translated into IGF-1-mediated bioactivities with biphasic effects on the proliferative and metastatic phenotype: an early increase and late decrease in the number of proliferative and migratory cells, while the invasiveness was completely inhibited following Nutlin-3 treatment through an impaired IGF-1-mediated matrix metalloproteinases type 2 activation mechanism. Taken together, these experiments reveal the biased agonistic properties of Nutlin-3 for the mitogen-activated protein kinase pathway, mediated by Mdm2 through IGF-1R ubiquitination and provide fundamental insights into destabilizing p53/Mdm2/IGF-1R circuitry that could be developed for therapeutic gain.

Functional antagonism of β-arrestin isoforms balance IGF-1R expression and signalling with distinct cancer-related biological outcomes

With very similar 3D structures, the widely expressed β-arrestin isoforms 1 and 2 play at times identical, distinct or even opposing roles in regulating various aspects of G protein-coupled receptors (GPCR) expression and signalling. Recent evidence recognizes the β-arrestin system as a key regulator of not only GPCRs, but also receptor tyrosine kinases, including the highly cancer relevant insulin-like growth factor type 1 receptor (IGF-1R). Binding of β-arrestin1 to IGF-1R leads to ligand-dependent degradation of the receptor and generates additional MAPK/ERK signalling, protecting cancer cells against anti-IGF-1R therapy. Because the interplay between β-arrestin isoforms governs the biological effects for most GPCRs, as yet unexplored for the IGF-1R, we sought to investigate specifically the regulatory roles of the β-arrestin2 isoform on expression and function of the IGF-1R. Results from controlled expression of either β-arrestin isoform demonstrate that β-arrestin2 acts in an opposite manner to β-arrestin1 by promoting degradation of an unstimulated IGF-1R, but protecting the receptor against agonist-induced degradation. Although both isoforms co-immunoprecipitate with IGF-1R, the ligand-occupied receptor has greater affinity for β-arrestin1; this association lasts longer, sustains MAPK/ERK signalling and mitigates p53 activation. Conversely, β-arrestin2 has greater affinity for the ligand-unoccupied receptor; this interaction is transient, triggers receptor ubiquitination and degradation without signalling activation, and leads to a lack of responsiveness to IGF-1, cell cycle arrest and decreased viability of cancer cells. This study reveals contrasting abilities of IGF-1R to interact with each β-arrestin isoform, depending on the presence of the ligand and demonstrates the antagonism between the two β-arrestin isoforms in controlling IGF-1R expression and function, which could be developed into a practical anti-IGF-1R strategy for cancer therapy.

Non-coding RNAs: the cancer genome dark matter that matters!

Protein-coding genes comprise only 3% of the human genome, while the genes that are transcribed into RNAs but do not code for proteins occupy majority of the genome. Once considered as biological darker matter, non-coding RNAs are now being recognized as critical regulators in cancer genome. Among the many types of non-coding RNAs, microRNAs approximately 20 nucleotides in length are best characterized and their mechanisms of action are well generalized. microRNA exerts oncogenic or tumor suppressor function by regulation of protein-coding genes via sequence complementarity. The expression of microRNA is aberrantly regulated in all cancer types, and both academia and biotech companies have been keenly pursuing the potential of microRNA as cancer biomarker for early detection, prognosis, and therapeutic response. The key involvement of microRNAs in cancer also prompted interest on exploration of therapeutic values of microRNAs as anticancer drugs and drug targets. MRX34, a liposome-formulated miRNA-34 mimic, developed by Mirna Therapeutics, becomes the first microRNA therapeutic entering clinical trial for the treatment of hepatocellular carcinoma, renal cell carcinoma, and melanoma. In this review, we presented a general overview of microRNAs in cancer biology, the potential of microRNAs as cancer biomarkers and therapeutic targets, and associated challenges.

Chapter Three - Ubiquitination and Protein Turnover of G-Protein-Coupled Receptor Kinases in GPCR Signaling and Cellular Regulation

G-protein-coupled receptors (GPCRs) are responsible for regulating a wide variety of physiological processes, and distinct mechanisms for GPCR inactivation exist to guarantee correct receptor functionality. One of the widely used mechanisms is receptor phosphorylation by specific G-protein-coupled receptor kinases (GRKs), leading to uncoupling from G proteins (desensitization) and receptor internalization. GRKs and β-arrestins also participate in the assembly of receptor-associated multimolecular complexes, thus initiating alternative G-protein-independent signaling events. In addition, the abundant GRK2 kinase has diverse "effector" functions in cellular migration, proliferation, and metabolism homeostasis by means of the phosphorylation or interaction with non-GPCR partners. Altered expression of GRKs (particularly of GRK2 and GRK5) occurs during pathological conditions characterized by impaired GPCR signaling including inflammatory syndromes, cardiovascular disease, and tumor contexts. It is increasingly appreciated that different pathways governing GRK protein stability play a role in the modulation of kinase levels in normal and pathological conditions. Thus, enhanced GRK2 degradation by the proteasome pathway occurs upon GPCR stimulation, what allows cellular adaptation to chronic stimulation in a physiological setting. β-arrestins participate in this process by facilitating GRK2 phosphorylation by different kinases and by recruiting diverse E3 ubiquitin ligase to the receptor complex. Different proteolytic systems (ubiquitin-proteasome, calpains), chaperone activities and signaling pathways influence the stability of GRKs in different ways, thus endowing specificity to GPCR regulation as protein turnover of GRKs can be differentially affected. Therefore, modulation of protein stability of GRKs emerges as a versatile mechanism for feedback regulation of GPCR signaling and basic cellular processes.

Chapter Nine - Cellular Roles of Beta-Arrestins as Substrates and Adaptors of Ubiquitination and Deubiquitination

β-Arrestin1 and β-arrestin2 are homologous adaptor proteins that are ubiquitously expressed in mammalian cells. They belong to a four-member family of arrestins that regulate the vast family of seven-transmembrane receptors that couple to heterotrimeric G proteins (7TMRs or GPCRs), and that modulate 7TMR signal transduction. β-Arrestins were originally identified in the context of signal inhibition via the 7TMRs because they competed with and thereby blocked G protein coupling to 7TMRs. Currently, in addition to their role as desensitizers of signaling, β-arrestins are appreciated as multifunctional adaptors that mediate trafficking and signal transduction of not only 7TMRs, but a growing list of additional receptors, ion channels, and nonreceptor proteins. β-Arrestins' interactions with their multifarious partners are based on their dynamic conformational states rather than particular domain-domain interactions. β-Arrestins adopt activated conformations upon 7TMR association. In addition, β-arrestins undergo various posttranslational modifications that are choreographed by activated 7TMRs, including phosphorylation, ubiquitination, acetylation, nitrosylation, and SUMOylation. Ubiquitination of β-arrestins is critical for their high-affinity interaction with 7TMRs as well as with endocytic adaptor proteins and signaling kinases. β-Arrestins also function as critical adaptors for ubiquitination and deubiquitination of various cellular proteins, and thereby affect the longevity of signal transducers and the intensity of signal transmission.

Chapter Seven - When Phosphorylation Encounters Ubiquitination: A Balanced Perspective on IGF-1R Signaling

Cell-surface receptors govern the critical information passage from outside to inside the cell and hence control important cellular decisions such as survival, growth, and differentiation. These receptors, structurally grouped into different families, utilize common intracellular signaling-proteins and pathways, yet promote divergent biological consequences. In rapid processing of extracellular signals to biological outcomes, posttranslational modifications offer a repertoire of protein processing options. Protein ubiquitination was originally identified as a signal for protein degradation through the proteasome system. It is now becoming increasingly recognized that both ubiquitin and ubiquitin-like proteins, all evolved from a common ubiquitin structural superfold, are used extensively by the cell and encompass signal tags for many different cellular fates. In this chapter we examine the current understanding of the ubiquitin regulation surrounding the insulin-like growth factor and insulin signaling systems, major members of the larger family of receptor tyrosine kinases (RTKs) and key regulators of fundamental physiological and pathological states.

Ubiquitin-Related Roles of β-Arrestins in Endocytic Trafficking and Signal Transduction

The non-visual arrestins, β-arrestin1, and β-arrestin2 were originally identified as proteins that bind to seven-transmembrane receptors (7TMRs, also called G protein-coupled receptors, GPCRs) and block heterotrimeric G protein activation, thus leading to desensitization of transmembrane signaling. However, as subsequent discoveries have continually demonstrated, their functionality is not constrained to desensitization. They are now recognized for their critical roles in mediating intracellular trafficking of 7TMRs, growth factor receptors, ion transporters, ion channels, nuclear receptors, and non-receptor proteins. Additionally, they function as crucial mediators of ubiquitination of 7TMRs as well as other receptors and non-receptor proteins. Recently, emerging studies suggest that a class of proteins with predicted structural features of β-arrestins regulate substrate ubiquitination in yeast and higher mammals, lending support to the idea that the adaptor role of β-arrestins in protein ubiquitination is evolutionarily conserved. β-arrestins also function as scaffolds for kinases and transduce signals from 7TMRs through pathways that do not require G protein activation. Remarkably, the endocytic and scaffolding functions of β-arrestin are intertwined with its ubiquitination status; the dynamic and site specific ubiquitination on β-arrestin plays a critical role in stabilizing β-arrestin-7TMR association and the formation of signalosomes. This review summarizes the current findings on ubiquitin-dependent regulation of 7TMRs as well as β-arrestins and the potential role of reversible ubiquitination as a "biological switch" in signal transduction. J. Cell. Physiol. 231: 2071-2080, 2016. © 2016 Wiley Periodicals, Inc.

Building the Case for Insulin-Like Growth Factor Receptor-I Involvement in Thyroid-Associated Ophthalmopathy

The pathogenesis of orbital Graves' disease (GD), a process known as thyroid-associated ophthalmopathy (TAO), remains incompletely understood. The thyrotropin receptor (TSHR) represents the central autoantigen involved in GD and has been proposed as the thyroid antigen shared with the orbit that could explain the infiltration of immune cells into tissues surrounding the eye. Another cell surface protein, insulin-like growth factor-I receptor (IGF-IR), has recently been proposed as a second antigen that participates in TAO by virtue of its interactions with anti-IGF-IR antibodies generated in GD, its apparent physical and functional complex formation with TSHR, and its necessary involvement in TSHR post-receptor signaling. The proposal that IGF-IR is involved in TAO has provoked substantial debate. Furthermore, several studies from different laboratory groups, each using different experimental models, have yielded conflicting results. In this article, we attempt to summarize the biological characteristics of IGF-IR and TSHR. We also review the evidence supporting and refuting the postulate that IGF-IR is a self-antigen in GD and that it plays a potentially important role in TAO. The putative involvement of IGF-IR in disease pathogenesis carries substantial clinical implications. Specifically, blocking this receptor with monoclonal antibodies can dramatically attenuate the induction by TSH and pathogenic antibodies generated in GD of proinflammatory genes in cultured orbital fibroblasts and fibrocytes. These cell types appear critical to the development of TAO. These observations have led to the conduct of a now-completed multicenter therapeutic trial of a fully human monoclonal anti-IGF-IR blocking antibody in moderate to severe, active TAO.

The emerging roles of β-arrestins in fibrotic diseases

β-Arrestins and β-arrestin2 are important adaptor proteins and signal transduction proteins that are mainly involved in the desensitization and internalization of G-protein-coupled receptors. Fibrosis is characterized by accumulation of excess extracellular matrix (ECM) molecules caused by chronic tissue injury. If highly progressive, the fibrotic process leads to organ malfunction and, eventually, death. The incurable lung fibrosis, renal fibrosis and liver fibrosis are among the most common fibrotic diseases. Recent studies show that β-arrestins can activate signaling cascades independent of G-protein activation and scaffold many intracellular signaling networks by diverse types of signaling pathways, including the Hedgehog, Wnt, Notch and transforming growth factor-β pathways, as well as downstream kinases such as MAPK and PI3K. These signaling pathways are involved in the pathological process of fibrosis and fibrotic diseases. This β-arrestin-mediated regulation not only affects cell growth and apoptosis, but also the deposition of ECM, activation of inflammatory response and development of fibrotic diseases. In this review, we survey the involvement of β-arrestins in various signaling pathways and highlight different aspects of their regulation of fibrosis. We also discuss the important roles of β-arrestins in the process of fibrotic diseases by regulating the inflammation and deposit of ECM. It is becoming more evident that targeting β-arrestins may offer therapeutic potential for the treatment of fibrotic diseases.

β-Arrestin2 Contributes to Cell Viability and Proliferation via the Down-Regulation of FOXO1 in Castration-Resistant Prostate Cancer

β-Arrestin2 has been identified to act as a corepressor of androgen receptor (AR) signaling by binding to AR and serving as a scaffold to affect the activity and expression of AR in androgen-dependent prostate cancer cells; however, little is known regarding its role in castration-resistant prostate cancer (CRPC) progression. Here, our data demonstrated that β-arrestin2 contributes to the cell viability and proliferation of CRPC via the downregulation of FOXO1 activity and expression. Mechanistically, in addition to its requirement for FOXO1 phosphorylation induced by IGF-1, β-arrestin2 could inhibit FOXO1 activity in an Akt-independent manner and delay FOXO1 dephosphorylation through the inhibition of PP2A phosphatase activity and the attenuation of the interaction between FOXO1 and PP2A. Furthermore, β-arrestin2 could downregulate FOXO1 expression via ubiquitylation and proteasomal degradation. Together, our results identified a novel role for β-arrestin2 in the modulation of the CRPC progress through FOXO1. Thus, the characterization of β-arrestin2 may represent an alternative therapeutic target for CRPC treatment.

The dichotomy of the Insulin-like growth factor 1 receptor: RTK and GPCR: friend or foe for cancer treatment?

The prime position of the insulin-like growth factor 1 receptor (IGF-1R), at the head of the principle mitogenic and anti-apoptotic signalling cascades, along with the resilience to transformation of IGF-1R deficient cells fuelled great excitement for its anti-cancer targeting. Yet its potential has not been fulfilled, as clinical trial results fell far short of expectations. Advancements in understanding of other receptors' function have now begun to shed light on this incongruity, with the now apparent parallels highlighting the immaturity of our understanding of IGF-1R biology, with the model used for drug development now recognised as having been too simplistic. Gathering together the many advancements of the field of IGF-1R research over the past decade, alongside those in the GPCR field, advocates for a major paradigm shift in our appreciation of the subtle workings of this receptor. This review will emphasise the updating of the IGF-1R's classification from an RTK, to an RTK/GPCR functional hybrid, which integrates both canonical kinase signalling with many functions characteristic of a GPCR. Recognition of the shortcomings of IGF-1R inhibitor drug development programs and the models used not only allows us to reignite the initial interest in the IGF-1R as an anti-cancer therapeutic target, but also points to the possibility of biased ligand therapeutics, which together may hold a very powerful key to unlocking the true potential of IGF-1R modulation.

Targeting the IGF-1R: The Tale of the Tortoise and the Hare

The insulin-like growth factor type 1 receptor (IGF-1R) plays a key role in the development and maintenance of cancer. Since the first links between growth factor receptors and oncogenes were noted over three decades ago, targeting the IGF-1R has been of great interest. This review follows the progress from inception through intense pharmaceutical development, disappointing clinical trials and recent updates to the signaling paradigm. In light of major developments in signaling understanding and activation complexities, we examine reasons for failure of first line targeting approaches. Recent findings include the fact that the IGF-1R can signal in the absence of the ligand, in the absence of kinase activity, and utilizes components of the GPCR system. With recognition of the unappreciated complexities that this first wave of targeting approaches encountered, we advocate re-recognition of IGF-1R as a valid target for cancer treatment and look to future directions, where both research and pharmaceutical strengths can lend themselves to finally unearthing anti-IGF-1R potential.

Insulin/Insulin-like growth factors in cancer: new roles for the aryl hydrocarbon receptor, tumor resistance mechanisms, and new blocking strategies

The insulin-like growth factor 1 receptor (IGF1R) and the insulin receptor (IR) are receptor tyrosine kinases that are expressed in cancer cells. The results of different studies indicate that tumor proliferation and survival is dependent on the IGF1R and IR, and that their inhibition leads to reductions in proliferation and increases in cell death. Molecular targeting therapies that have been used in solid tumors include anti-IGF1R antibodies, anti-IGF1/IGF2 antibodies, and small molecule inhibitors that suppress IGF1R and IR kinase activity. New advances in the molecular basis of anti-IGF1R blocking antibodies reveal they are biased agonists and promote the binding of IGF1 to integrin β3 receptors in some cancer cells. Our recent reports indicate that pharmacological aryl hydrocarbon receptor (AHR) ligands inhibit breast cancer cell responses to IGFs, suggesting that targeting AHR may have benefit in cancers whose proliferation and survival are dependent on insulin/IGF signaling. Novel aspects of IGF1R/IR in cancer, such as biased agonism, integrin β3 signaling, AHR, and new therapeutic targeting strategies will be discussed.

Regulation of SOD2 and β-arrestin1 by interleukin-6 contributes to the increase of IGF-1R expression in docetaxel resistant prostate cancer cells

Although several mechanisms behind resistance to docetaxel in castration-refractory prostate cancer (CRPC) have been investigated, molecular determinants of evolved resistance are still not entirely understood. Proteomics-based analysis in this study revealed that SOD2, associated with downregulation of reactive oxygen species (ROS), was significantly up-regulated in docetaxel-resistant (PC3/Doc) cells if compared to sensitive cells, and the expression of redox-regulated genes such as IGF-1R, CXCR4, and BCL2 was increased as well. Forced expression of SOD2 in sensitive cells led to the increase of IGF-1R and association with drug resistance, whereas silencing of SOD2 resulted in the decrease of IGF-1R at the protein level in resistant cells. Further study revealed that SOD2 acted as a negative regulator of β-arrestin1 that is an important adaptor responsible for degradation of IGF-1R via the changes in ROS, as evidenced by observations that an antioxidant agent substantially attenuated β-arrestin1 expression in vitro and in vivo. Finally, we found that blocking of IL6 that was up-regulated in resistant cells resulted in attenuation of SOD2 and STAT3, and simultaneously in increased expression of β-arrestin1. The modulation consequently led to the decreased IGF-1R at both protein and transcription levels. Together, our data provide a novel explanation that high level of IL6 stimulated SOD2 expression that, at least partially, contributed to the low level of ROS that would likely result in a sustained increase in the expression of IGF-1R through abolishment of β-arrestin1 in docetaxel resistant cells.

Arrestins as regulatory hubs in cancer signalling pathways

Non-visual arrestins were initially appreciated for the roles they play in the negative regulation of G protein-coupled receptors through the processes of desensitisation and endocytosis. The arrestins are also now known as protein scaffolding platforms that act downstream of multiple types of receptors, ensuring relevant transmission of information for an appropriate cellular response. They function as regulatory hubs in several important signalling pathways that are often dysregulated in human cancers. Interestingly, several recent studies have documented changes in expression and localisation of arrestins that occur during cancer progression and that correlate with clinical outcome. Here, we discuss these advances and how changes in expression/localisation may affect functional outputs of arrestins in cancer biology.

Ligand-mediated endocytosis and trafficking of the insulin-like growth factor receptor I and insulin receptor modulate receptor function

The insulin-like growth factor system and its two major receptors, the IGF receptor I (IGF-IR) and IR, plays a central role in a variety of physiological cellular processes including growth, differentiation, motility, and glucose homeostasis. The IGF-IR is also essential for tumorigenesis through its capacity to protect cancer cells from apoptosis. The IR is expressed in two isoforms: the IR isoform A (IR-A) and isoform B (IR-B). While the role of the IR-B in the regulation of metabolic effects has been known for several years, more recent evidence suggests that the IR, and in particular the IR-A, may be involved in the pathogenesis of cancer. Ligand-mediated endocytosis of tyrosine-kinases receptors plays a critical role in modulating the duration and intensity of receptors action but while the signaling pathways induced by the IGF-IR and IR are quite characterized, very little is still known about the mechanisms and proteins that regulate ligand-induced IGF-IR and IR endocytosis and trafficking. In addition, how these processes affect receptor downstream signaling has not been fully characterized. Here, we discuss the current understanding of the mechanisms and proteins regulating IGF-IR and IR endocytosis and sorting and their implications in modulating ligand-induced biological responses.

Arrestin interaction with E3 ubiquitin ligases and deubiquitinases: functional and therapeutic implications

Arrestins constitute a small family of four homologous adaptor proteins (arrestins 1-4), which were originally identified as inhibitors of signal transduction elicited by the seven-transmembrane G protein-coupled receptors. Currently arrestins (especially arrestin2 and arrestin3; also called β-arrestin1 and β-arrestin2) are known to be activators of cell signaling and modulators of endocytic trafficking. Arrestins mediate these effects by binding to not only diverse cell-surface receptors but also by associating with a variety of critical signaling molecules in different intracellular compartments. Thus, the functions of arrestins are multifaceted and demand interactions with a host of proteins and require an array of selective conformations. Furthermore, receptor ligands that specifically induce signaling via arrestins are being discovered and their physiological roles are emerging. Recent evidence suggests that the activity of arrestin is regulated in space and time by virtue of its dynamic association with specific enzymes of the ubiquitination pathway. Ubiquitin-dependent, arrestin-mediated signaling could serve as a potential platform for developing novel therapeutic strategies to target transmembrane signaling and physiological responses.