Atrophin-1-interacting Protein 4/Human Itch Is a Ubiquitin E3 Ligase for Human Enhancer of Filamentation 1 in Transforming Growth Factor-β Signaling Pathways

Functional implications of paralog genes in polyglutamine spinocerebellar ataxias

Polyglutamine (polyQ) spinocerebellar ataxias (SCAs) comprise a group of autosomal dominant neurodegenerative disorders caused by (CAG/CAA)n expansions. The elongated stretches of adjacent glutamines alter the conformation of the native proteins inducing neurotoxicity, and subsequent motor and neurological symptoms. Although the etiology and neuropathology of most polyQ SCAs have been extensively studied, only a limited selection of therapies is available. Previous studies on SCA1 demonstrated that ATXN1L, a human duplicated gene of the disease-associated ATXN1, alleviated neuropathology in mice models. Other SCA-associated genes have paralogs (i.e., copies at different chromosomal locations derived from duplication of the parental gene), but their functional relevance and potential role in disease pathogenesis remain unexplored. Here, we review the protein homology, expression pattern, and molecular functions of paralogs in seven polyQ dominant ataxias-SCA1, SCA2, MJD/SCA3, SCA6, SCA7, SCA17, and DRPLA. Besides ATXN1L, we highlight ATXN2L, ATXN3L, CACNA1B, ATXN7L1, ATXN7L2, TBPL2, and RERE as promising functional candidates to play a role in the neuropathology of the respective SCA, along with the parental gene. Although most of these duplicates lack the (CAG/CAA)n region, if functionally redundant, they may compensate for a partial loss-of-function or dysfunction of the wild-type genes in SCAs. We aim to draw attention to the hypothesis that paralogs of disease-associated genes may underlie the complex neuropathology of dominant ataxias and potentiate new therapeutic strategies.

Atrophin-1 Function and Dysfunction in Dentatorubral-Pallidoluysian Atrophy

Dentatorubral-pallidoluysian atrophy (DRPLA) is a rare, incurable genetic disease that belongs to the group of polyglutamine (polyQ) diseases. DRPLA is the most common in the Japanese population; however, its global prevalence is also increasing due to better clinical recognition. It is characterized by cerebellar ataxia, myoclonus, epilepsy, dementia, and chorea. DRPLA is caused by dynamic mutation of CAG repeat expansion in ATN1 gene encoding the atrophin-1 protein. In the cascade of molecular disturbances, the pathological form of atrophin-1 is the initial factor, which has not been precisely characterized so far. Reports indicate that DRPLA is associated with disrupted protein-protein interactions (in which an expanded polyQ tract plays a crucial role), as well as gene expression deregulation. There is a great need to design efficient therapy that would address the underlying neurodegenerative process and thus prevent or alleviate DRPLA symptoms. An in-depth understanding of the normal atrophin-1 function and mutant atrophin-1 dysfunction is crucial for this purpose. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Itch regulation of innate and adaptive immune responses in mice and humans

The E3 ubiquitin ligase Itch has long been appreciated to be a critical suppressor of inflammation, first identified as a regulator of Th2 differentiation and lung inflammation. Recent studies have revealed novel roles for this protein in mouse and human disease, and it is now clear that Itch also limits the function of other lymphocytes, innate immune cells, and nonhematopoietic cells to regulate immunity. In addition to Th2 cells, Itch also regulates Th17 and regulatory T cells. Itch regulates humoral immunity through direct roles in T follicular helper cells and T follicular regulatory cells, and B cells. Furthermore, Itch limits innate immune responses, such as macrophage cytokine production. Through these cell-intrinsic functions, Itch regulates the interplay between innate and adaptive immune cells, resulting in profound autoinflammation in Itch-deficient mice. Whereas Itch deficiency was previously thought to be an extremely rare occurrence humans, whole exome sequencing of patients with unexplained autoimmune disease has revealed at least two additional cases of Itch deficiency in the last year alone, each caused by distinct mutations within the Itch gene. The recent identification of these patients suggests that Itch mutations may be more common than previously thought, and demonstrates the need to understand how this protein regulates inflammation and autoimmune disease.

ITCH as a potential therapeutic target in human cancers

The ITCH/AIP4 ubiquitin E3 ligase was discovered independently by two groups searching for atrophin-1 interacting proteins and studying the genetics of mouse coat color alteration, respectively. ITCH is classified as a NEDD4 family E3 ligase featured with the C-terminal HECT domain for E3 ligase function and WW domains for substrate recruiting. ITCH deficiency in the mouse causes severe multi-organ autoimmune disease. Its roles in maintaining a balanced immune response have been extensively characterized over the past two and a half decades. A wealth of reports demonstrate a multifaceted role of ITCH in human cancers. Given the versatility of ITCH in catalyzing both proteolytic and non-proteolytic ubiquitination of its over fifty substrates, ITCH's role in malignancies is believed to be context-dependent. In this review, we summarize the downstream substrates of ITCH, the functions of ITCH in both tumor cells and the immune system, as well as the implications of such functions in human cancers. Moreover, we describe the upstream regulatory mechanisms of ITCH and the efforts have been made to target ITCH using small molecule inhibitors.

The E3 ligase Itch in immune regulation and beyond

Itch or itchy E3 ubiquitin ligase was initially discovered by genetic studies on the mouse coat color changes, and its deletion results in an itchy phenotype with constant skin scratching and multi-organ inflammation. It is a member of the homologous to E6-associated protein C-terminus (HECT)-type family of E3 ligases, with the protein-interacting WW-domains for the recruitment of substrate and the HECT domain for the transfer of ubiquitin to the substrate. Since its discovery, numerous studies have demonstrated that Itch is involved in the control of many aspects of immune responses including T-cell activation and tolerance and T-helper cell differentiation. Itch is also implicated in other biological contexts such as tumorigenesis, development, and stress responses. Many signaling pathways are regulated by Itch-promoted ubiquitylation of diverse target proteins. Itch is also involved in human diseases. Here, we discuss the major progress in understanding the biological significance of Itch-promoted protein ubiquitylation in the immune and other systems and in Itch-mediated regulation of signal transduction.

HEF1 promotes epithelial mesenchymal transition and bone invasion in prostate cancer under the regulation of microRNA-145

The principal problem arising from prostate cancer (PCa) is its propensity to metastasize to bones, and it's crucial to understand the mechanism of tumor progression to metastasis in order to develop therapies that may reduce the morbidity and mortality of PCa patients. Although we had identified that microRNA(miR)-145 could repress bone metastasis of PCa via regulating epithelial-mesenchymal transition (EMT) in previous study, it is still unknown how miR-145 regulated EMT. In the present study, we constructed a luciferase reporter system and identified HEF1 as a direct target of miR-145. More importantly, HEF1 was shown to promote migration, invasion and EMT of PC-3 cells, a human PCa cell line originated from a bone metastatic PCa specimen. And HEF1 was also shown to partially mediate miR-145 suppression of EMT and invasion. Furthermore, inhibition of HEF1 repressed bone invasion of PC-3 cells in vivo. Expression of HEF1 was negatively correlated with miR-145 in primary PCa and bone metastatic specimens, but HEF1 was higher in samples which were more likely to commit to bone metastasis or those with higher free prostate-specific antigen (fPSA) levels and Gleason scores. Taken together, these findings indicate that HEF1 promotes EMT and bone invasion in prostate cancer by directly targeted by miR-145, and miR-145 suppresses EMT and invasion, at least in part, through repressing HEF1.

ITCH regulates degradation of mutant glucocerebrosidase: implications to Gaucher disease

Inability to properly degrade unfolded or misfolded proteins in the endoplasmic reticulum (ER) leads to ER stress and unfolded protein response. This is particularly important in cases of diseases in which the mutant proteins undergo ER-associated degradation (ERAD), as in Gaucher disease (GD). GD is a genetic, autosomal recessive disease that results from mutations in the GBA1 gene, encoding the lysosomal enzyme acid β-glucocerebrosidase (GCase). We have shown that mutant GCase variants undergo ERAD, the degree of which is a major determinant of disease severity. Most ERAD substrates undergo polyubiquitination and proteasomal degradation. Therefore, one expects that mutant GCase variants are substrates for several E3 ubiquitin ligases in different cells. We tested the possibility that ITCH, a known E3 ubiquitin ligase, with a pivotal role in proliferation and differentiation of the skin, recognizes mutant GCase variants and mediates their polyubiquitination and degradation. Our results strongly suggest that ITCH interacts with mutant GCase variants and mediates their lysine 48 polyubiquitination and degradation.

NSP-CAS Protein Complexes: Emerging Signaling Modules in Cancer

The CAS (CRK-associated substrate) family of adaptor proteins comprises 4 members, which share a conserved modular domain structure that enables multiple protein-protein interactions, leading to the assembly of intracellular signaling platforms. Besides their physiological role in signal transduction downstream of a variety of cell surface receptors, CAS proteins are also critical for oncogenic transformation and cancer cell malignancy through associations with a variety of regulatory proteins and downstream effectors. Among the regulatory partners, the 3 recently identified adaptor proteins constituting the NSP (novel SH2-containing protein) family avidly bind to the conserved carboxy-terminal focal adhesion-targeting (FAT) domain of CAS proteins. NSP proteins use an anomalous nucleotide exchange factor domain that lacks catalytic activity to form NSP-CAS signaling modules. Additionally, the NSP SH2 domain can link NSP-CAS signaling assemblies to tyrosine-phosphorylated cell surface receptors. NSP proteins can potentiate CAS function by affecting key CAS attributes such as expression levels, phosphorylation state, and subcellular localization, leading to effects on cell adhesion, migration, and invasion as well as cell growth. The consequences of these activities are well exemplified by the role that members of both families play in promoting breast cancer cell invasiveness and resistance to antiestrogens. In this review, we discuss the intriguing interplay between the NSP and CAS families, with a particular focus on cancer signaling networks.

The SH2 domain protein Shep1 regulates the in vivo signaling function of the scaffolding protein Cas

The members of the p130Cas (Cas) family are important scaffolding proteins that orchestrate cell adhesion, migration and invasiveness downstream of integrin adhesion receptors and receptor tyrosine kinases by recruiting enzymes and structural molecules. Shep1, BCAR3/AND-34 and NSP1 define a recently identified family of SH2 domain-containing proteins that constitutively bind Cas proteins through a Cdc25-type nucleotide exchange factor-like domain. To gain insight into the functional interplay between Shep1 and Cas in vivo, we have inactivated the Shep1 gene in the mouse through Cre-mediated deletion of the exon encoding the SH2 domain. Analysis of Cas tyrosine phosphorylation in the brains of newborn mice, where Shep1 is highly expressed, revealed a strong decrease in Cas substrate domain phosphorylation in knockout compared to wild-type brains. Src family kinases bind to Cas via their SH3 and SH2 domains, which contributes to their activation, and phosphorylate multiple tyrosines in the Cas substrate domain. These tyrosine-phosphorylated motifs represent docking sites for the Crk adaptor, linking Cas to the downstream Rac1 and Rap1 GTPases to regulate cell adhesion and actin cytoskeleton organization. Accordingly, we detected lower Cas-Crk association and lower phosphorylation of the Src activation loop in Shep1 knockout brains compared to wild-type. Conversely, Shep1 transfection in COS cells increases Cas tyrosine phosphorylation. The SH2 domain is likely critical for the effects of Shep1 on Cas and Src signaling because the knockout mice express Shep1 fragments that lack the amino-terminal region including the SH2 domain, presumably due to aberrant translation from internal ATG codons. These fragments retain the ability to increase Cas levels in transfected cells, similar to full-length Shep1. However, they do not affect Cas phosphorylation on their own or in the presence of co-transfected full-length Shep1. They also do not show dominant negative effects on the activity of full-length Shep1 in vivo because the heterozygous mice, which express the fragments, have a normal life span. This is in contrast to the homozygous knockout mice, most of which die soon after birth. These data demonstrate that Shep1 plays a critical role in the in vivo regulation of Src activity and Cas downstream signaling through Crk, and suggest that the SH2 domain of Shep1 is critical for these effects.

The WW-HECT protein Smurf2 interacts with the Docking Protein NEDD9/HEF1 for Aurora A activation

The multi-functional adaptor protein NEDD9/HEF1/Cas-L regulates cell motility, invasion and cell cycle progression, and plays key roles in cancer progression and metastasis. NEDD9 is localized to the centrosome and is required for activation of Aurora A kinase in mitosis. Here we demonstrate that the HECT-WW protein Smurf2 physically associates with NEDD9 and is required for the stability of NEDD9 protein. Smurf2 depletion results in a marked decrease in NEDD9 protein levels, by facilitating polyubiquitination and proteasomal degradation of NEDD9. Conversely, forced overexpression of Smurf2 results in upregulation of endogenous NEDD9 protein, confirming the role for Smurf2 in NEDD9 stability. Cells with Smurf2 depletion fail to activate Aurora A at the G₂/M boundary, leading to a marked delay in mitotic entry. These observations suggest that the stable complex of Smurf2 and NEDD9 is required for timely entry into mitosis via Aurora A activation.

Regulation of T-lymphocyte physiology by the Chat-H/CasL adapter complex

The Cas family of proteins consists of at least four members implicated in the regulation of diverse cellular processes such as cell proliferation, adhesion, motility, and cancer cell metastasis. Cas family members have conserved C-termini that mediate constitutive heterotypic interactions with members of a different group of proteins, the NSP family. Both the Cas and NSP proteins have conserved domains that mediate protein-protein interactions with other cytoplasmic intermediates. Signaling modules assembled by these proteins in turn regulate signal transduction downstream of a variety of receptors including integrin, chemokine, and antigen receptors. T lymphocytes express the NSP protein NSP3/Chat-H and the Cas protein Hef1/CasL, which are found in a constitutive complex in naive T cells. We recently showed that Chat-H and Hef1/CasL regulate integrin-mediated adhesion and promote T-cell migration and trafficking downstream of activated chemokine receptors. It is currently unclear if the Chat-H/CasL module also plays a role in antigen receptor signaling. Here we review our current knowledge of how Chat-H and Hef1/CasL regulate T-cell physiology and whether this protein complex plays a functional role downstream of T-cell receptor activation.

CAS proteins in normal and pathological cell growth control

Proteins of the CAS (Crk-associated substrate) family (BCAR1/p130Cas, NEDD9/HEF1/Cas-L, EFS/SIN and CASS4/HEPL) are integral players in normal and pathological cell biology. CAS proteins act as scaffolds to regulate protein complexes controlling migration and chemotaxis, apoptosis, cell cycle, and differentiation, and have more recently been linked to a role in progenitor cell function. Reflecting these complex functions, over-expression of CAS proteins has now been strongly linked to poor prognosis and increased metastasis in cancer, as well as resistance to first-line chemotherapeutics in multiple tumor types including breast and lung cancers, glioblastoma, and melanoma. Further, CAS proteins have also been linked to additional pathological conditions including inflammatory disorders, Alzheimer's and Parkinson's disease, as well as developmental defects. This review will explore the roles of the CAS proteins in normal and pathological states in the context of the many mechanistic insights into CAS protein function that have emerged in the past decade.

Phosphorylation of human enhancer of filamentation (HEF1) on serine 369 induces its proteasomal degradation

Human enhancer of filamentation 1 (HEF1) is a multi-domain docking protein of the p130 Cas family. HEF1 is present at focal adhesions and is involved in integrin signalling mediating cytoskeleton reorganization associated with cell migration, adhesion or apoptosis. HEF1 functions are regulated in part by phosphorylation on tyrosine residues. HEF1 is also phosphorylated on serines/threonines leading to two isoforms refered to as p105 and p115. In most cases, the serine/threonine kinase(s) responsible for HEF1 phosphorylation have not been identified. In the present study, we have investigated HEF1 ser/thr phosphorylation. In the HCT-116 cell line transiently overexpressing Flag-HEF1 we showed that Hesperadin, a synthetic indolinone displaying antiproliferative effect and described as an inhibitor of various kinases including Aurora-B, prevented HEF1 phosphorylation induced by the ser/thr phosphatase PP2A inhibitor: okadaic acid (OA). In addition we showed that conversion of endogenous HEF1 p105 to p115 in HaCaT cells was prevented upon treatment with Hesperadin, resulting in accumulation of p105HEF1. We also identified serine 369 as the target site of phosphorylation by this Hesperadin-inhibited kinase in HCT-116. Finally, we provide evidence that phosphorylation on serine 369 but not phosphorylation on serine 296, triggers HEF1 degradation by the proteasomal machinery. These data suggest that conversion of p105 to p115 results from a ser-369-dependent phosphorylation mediated by an Hesperadin-sensitive kinase and regulates the half-life of HEF1.

Atrophin proteins: an overview of a new class of nuclear receptor corepressors

The normal development and physiological functions of multicellular organisms are regulated by complex gene transcriptional networks that include myriad transcription factors, their associating coregulators, and multiple chromatin-modifying factors. Aberrant gene transcriptional regulation resulting from mutations among these elements often leads to developmental defects and diseases. This review article concentrates on the Atrophin family proteins, including vertebrate Atrophin-1 (ATN1), vertebrate arginine-glutamic acid dipeptide repeats protein (RERE), and Drosophila Atrophin (Atro), which we recently identified as nuclear receptor corepressors. Disruption of Atrophin-mediated pathways causes multiple developmental defects in mouse, zebrafish, and Drosophila, while an aberrant form of ATN1 and altered expression levels of RERE are associated with neurodegenerative disease and cancer in humans, respectively. We here provide an overview of current knowledge about these Atrophin proteins. We hope that this information on Atrophin proteins may help stimulate fresh ideas about how this newly identified class of nuclear receptor corepressors aids specific nuclear receptors and other transcriptional factors in regulating gene transcription, manifesting physiological effects, and causing diseases.

Transforming growth factor-beta signaling and ubiquitinators in cancer

Transforming growth factor-beta (TGF-beta) represents a large family of growth and differentiation factors that mobilize complex signaling networks to regulate cellular differentiation, proliferation, motility, adhesion, and apoptosis. TGF-beta signaling is tightly regulated by multiple complex mechanisms, and its deregulation plays a key role in the progression of many forms of cancer. Upon ligand binding, TGF-beta signals are transduced by Smad proteins, which in turn are tightly dependent on modulation by adaptor proteins such as embryonic liver fodrin, Smad anchor for receptor activation, filamin, and crkl. A further layer of regulation is imposed by ubiquitin-mediated targeting and proteasomal degradation of specific components of the TGF-beta signaling pathway. This review focuses on the ubiquitinators that regulate TGF-beta signaling and the association of these ubiquitin ligases with various forms of cancer. Delineating the role of ubiquitinators in the TGF-beta signaling pathway could yield powerful novel therapeutic targets for designing new cancer treatments.

The Nedd4-like family of E3 ubiquitin ligases and cancer

Accumulating evidence suggests that E3 ubiquitin ligases play important roles in cancer development. In this article, we provide a comprehensive summary of the roles of the Nedd4-like family of E3 ubiquitin ligases in human cancer. There are nine members of the Nedd4-like E3 family, all of which share a similar structure, including a C2 domain at the N-terminus, two to four WW domains in the middle of the protein, and a homologous to E6-AP COOH terminus domain at the C-terminus. The assertion that Nedd4-like E3s play a role in cancer is supported by the overexpression of Smurf2 in esophageal squamous cell carcinoma, WWP1 in prostate and breast cancer, Nedd4 in prostate and bladder cancer, and Smurf1 in pancreatic cancer. Because Nedd4-like E3s regulate ubiquitin-mediated trafficking, lysosomal or proteasomal degradation, and nuclear translocation of multiple proteins, they modulate important signaling pathways involved in tumorigenesis like TGFbeta, EGF, IGF, VEGF, SDF-1, and TNFalpha. Additionally, several Nedd4-like E3s directly regulate various cancer-related transcription factors from the Smad, p53, KLF, RUNX, and Jun families. Interestingly, multiple Nedd4-like E3s show ligase independent function. Furthermore, Nedd4-like E3s themselves are frequently regulated by phosphorylation, ubiquitination, translocation, and transcription in cancer cells. Because the regulation and biological output of these E3s is such a complex process, study of the role of these E3s in cancer development poses some challenges. However, understanding the oncogenic potential of these E3s may facilitate the identification and development of biomarkers and drug targets in human cancer.

Molecular basis for HEF1/NEDD9/Cas-L action as a multifunctional co-ordinator of invasion, apoptosis and cell cycle

Upregulation of the scaffolding protein HEF1, also known as NEDD9 and Cas-L, has recently been identified as a pro-metastatic stimulus in a number of different solid tumors, and has also been strongly associated with pathogenesis of BCR-Abl-dependent tumors. As the evidence mounts for HEF1/NEDD9/Cas-L as a key player in metastatic cancer, it is timely to review the molecular regulation of HEF1/NEDD9/Cas-L. Most of the mortality associated with cancer arises from uncontrolled metastases, thus a better understanding of the properties of proteins specifically associated with promotion of this process may yield insights that improve cancer diagnosis and treatment. In this review, we summarize the extensive literature regarding HEF1/NEDD9/Cas-L expression and function in signaling relevant to cell attachment, migration, invasion, cell cycle, apoptosis, and oncogenic signal transduction. The complex function of HEF1/NEDD9/Cas-L revealed by this analysis leads us to propose a model in which alleviation of cell cycle checkpoints and acquired resistance to apoptosis is permissive for a HEF1/NEDD9/Cas-L-promoted pro-metastatic phenotype.

Ubiquitin-dependent regulation of TGFbeta signaling in cancer

The transforming growth factorbeta (TGFbeta) superfamily regulates a broad spectrum of biological responses throughout embryonic development and adult life, including cell proliferation and differentiation, epithelial-to-mesenchymal transition, apoptosis, and angiogenesis. TGFbeta members initiate signaling by bringing together a complex of serine/threonine kinase receptors that transmit signals through intracellular Smad proteins. Genetic alterations in numerous components of the TGFbeta signaling pathway have been associated with several human cancers. In addition, tight regulation of TGFbeta signaling is pivotal to the maintenance of homeostasis and the prevention of carcinogenesis. The ubiquitin/proteosome system is one mechanism by which cells regulate the expression and activity of effectors of the TGFbeta signaling cascade. Mounting evidence also suggests that disruption of the ubiquitin-dependent degradation of components of the TGFbeta pathway leads to the development and progression of cancer. Therefore, understanding how these two pathways intertwine will contribute to the advancement of our knowledge of cancer development.

Cell adhesion regulates Ser/Thr phosphorylation and proteasomal degradation of HEF1

Human enhancer of filamentation 1 (HEF1), a multifunctional docking protein of the Cas family, participates in integrin and growth factor signaling pathways that regulate global cellular processes including growth, motility and apoptosis. HEF1 consists of two isoforms, p105 and p115, the larger molecular weight form resulting from Ser/Thr phosphorylation of p105HEF1. The molecular mechanisms that regulate the interconversion of the two HEF1 species as well as the function of HEF1 Ser/Thr phosphorylation are unknown. Our study reveals that cell adhesion and detachment regulate the interconversion of the two HEF1 isoforms. Experiments using various inhibitors of cytoskeletal organization indicated that disruption of actin microfilaments but not intermediate filaments or microtubules resulted in a complete conversion of p115HEF1 to p105HEF1. The conversion of p115HEF1 to p105HEF1 was prevented by inhibition of protein phosphatase 2A (PP2A), suggesting that cytoskeletal regulation of PP2A activity controlled the dephosphorylation of p115HEF1. Degradation of endogenous HEF1 was dependent on proteasomes with the p115 species of HEF1 being preferentially targeted for turnover. Dephosphorylation of HEF1 by suspending cells or disrupting actin filaments protected HEF1 from degradation. These results suggest that the adhesion-dependent actin organization regulates proteasomal turnover of HEF1 through the activity of PP2A.