SUMOylation regulates AKT1 activity

SUMO modifies GβL and mediates mTOR signaling

The mechanistic target of rapamycin (mTOR) signaling is influenced by multiple regulatory proteins and post-translational modifications; however, underlying mechanisms remain unclear. Here, we report a novel role of small ubiquitin-like modifier (SUMO) in mTOR complex assembly and activity. By investigating the SUMOylation status of core mTOR components, we observed that the regulatory subunit, GβL (G protein β-subunit-like protein, also known as mLST8), is modified by SUMO1, 2, and 3 isoforms. Using mutagenesis and mass spectrometry, we identified that GβL is SUMOylated at lysine sites K86, K215, K245, K261, and K305. We found that SUMO depletion reduces mTOR-Raptor (regulatory protein associated with mTOR) and mTOR-Rictor (rapamycin-insensitive companion of mTOR) complex formation and diminishes nutrient-induced mTOR signaling. Reconstitution with WT GβL but not SUMOylation-defective KR mutant GβL promotes mTOR signaling in GβL-depleted cells. Taken together, we report for the very first time that SUMO modifies GβL, influences the assembly of mTOR protein complexes, and regulates mTOR activity.

AKT1 induces Nanog promoter in a SUMOylation-dependent manner in different pluripotent contexts

AKT/PKB is a kinase crucial for pluripotency maintenance in pluripotent stem cells. Multiple post-translational modifications modulate its activity. We have previously demonstrated that AKT1 induces the expression of the pluripotency transcription factor Nanog in a SUMOylation-dependent manner in mouse embryonic stem cells. Here, we studied different cellular contexts and main candidates that could mediate this induction. Our results strongly suggest the pluripotency transcription factors OCT4 and SOX2 are not essential mediators. Additionally, we concluded that this induction takes place in different pluripotent contexts but not in terminally differentiated cells. Finally, the cross-matching analysis of ESCs, iPSCs and MEFs transcriptomes and AKT1 phosphorylation targets provided new clues about possible factors that could be involved in the SUMOylation-dependent Nanog induction by AKT.

SUMOylation and the oncogenic E17K mutation affect AKT1 subcellular distribution and impact on Nanog-binding dynamics to chromatin in embryonic stem cells

AKT/PKB is a kinase involved in the regulation of a plethora of cell processes. Particularly, in embryonic stem cells (ESCs), AKT is crucial for the maintenance of pluripotency. Although the activation of this kinase relies on its recruitment to the cellular membrane and subsequent phosphorylation, multiple other post-translational modifications (PTMs), including SUMOylation, fine-tune its activity and target specificity. Since this PTM can also modify the localization and availability of different proteins, in this work we explored if SUMOylation impacts on the subcellular compartmentalization and distribution of AKT1 in ESCs. We found that this PTM does not affect AKT1 membrane recruitment, but it modifies the AKT1 nucleus/cytoplasm distribution, increasing its nuclear presence. Additionally, within this compartment, we found that AKT1 SUMOylation also impacts on the chromatin-binding dynamics of NANOG, a central pluripotency transcription factor. Remarkably, the oncogenic E17K AKT1 mutant produces major changes in all these parameters increasing the binding of NANOG to its targets, also in a SUMOylation dependent manner. These findings demonstrate that SUMOylation modulates AKT1 subcellular distribution, thus adding an extra layer of regulation of its function, possibly by affecting the specificity and interaction with its downstream targets.

The role of SUMOylation in the neurovascular dysfunction after acquired brain injury

Acquired brain injury (ABI) is the most common disease of the nervous system, involving complex pathological processes, which often leads to a series of nervous system disorders. The structural destruction and dysfunction of the Neurovascular Unit (NVU) are prominent features of ABI. Therefore, understanding the molecular mechanism underlying NVU destruction and its reconstruction is the key to the treatment of ABI. SUMOylation is a protein post-translational modification (PTM), which can degrade and stabilize the substrate dynamically, thus playing an important role in regulating protein expression and biological signal transduction. Understanding the regulatory mechanism of SUMOylation can clarify the molecular mechanism of the occurrence and development of neurovascular dysfunction after ABI and is expected to provide a theoretical basis for the development of potential treatment strategies. This article reviews the role of SUMOylation in vascular events related to ABI, including NVU dysfunction and vascular remodeling, and puts forward therapeutic prospects.

SENP2 is vital for optimal insulin signaling and insulin-stimulated glycogen synthesis in human skeletal muscle cells

Sentrin-specific protease (SENP) 2 has been suggested as a possible novel drug target for the treatment of obesity and type 2 diabetes mellitus after observations of a palmitate-induced increase in SENP2 that lead to increased fatty acid oxidation and improved insulin sensitivity in skeletal muscle cells from mice. However, no precedent research has examined the role of SENP2 in human skeletal muscle cells. In the present work, we have investigated the impact of SENP2 on fatty acid and glucose metabolism as well as insulin sensitivity in human skeletal muscle using cultured primary human myotubes. Acute (4 ​h) oleic acid oxidation was reduced in SENP2-knockdown (SENP2-KD) cells compared to control cells, with no difference in uptake. After prelabeling (24 ​h) with oleic acid, total lipid content and incorporation into triacylglycerol was decreased, while incorporation into other lipids, as well as complete oxidation and β-oxidation was increased in SENP2-KD cells. Basal glucose uptake (i.e., not under insulin-stimulated conditions) was higher in SENP2-KD cells, whereas oxidation was similar to control myotubes. Further, basal glycogen synthesis was not different in SENP2-KD myotubes, but both insulin-stimulated glycogen synthesis and Akt^Ser473 phosphorylation was completely blunted in SENP2-KD cells. In conclusion, SENP2 plays an important role in fatty acid and glucose metabolism in human myotubes. Interestingly, it also appears to have a pivotal role in regulating myotube insulin sensitivity. Future studies should examine the role of SENP2 in regulation of insulin sensitivity in other tissues and in vivo, defining the potential for SENP2 as a drug target.

RSK1 SUMOylation is required for KSHV lytic replication

RSK1, a downstream kinase of the MAPK pathway, has been shown to regulate multiple cellular processes and is essential for lytic replication of a variety of viruses, including Kaposi's sarcoma-associated herpesvirus (KSHV). Besides phosphorylation, it is not known whether other post-translational modifications play an important role in regulating RSK1 function. We demonstrate that RSK1 undergoes robust SUMOylation during KSHV lytic replication at lysine residues K110, K335, and K421. SUMO modification does not alter RSK1 activation and kinase activity upon KSHV ORF45 co-expression, but affects RSK1 downstream substrate phosphorylation. Compared to wild-type RSK1, the overall phosphorylation level of RxRxxS*/T* motif is significantly declined in RSK1K110/335/421R expressing cells. Specifically, SUMOylation deficient RSK1 cannot efficiently phosphorylate eIF4B. Sequence analysis showed that eIF4B has one SUMO-interacting motif (SIM) between the amino acid position 166 and 170 (166IRVDV170), which mediates the association between eIF4B and RSK1 through SUMO-SIM interaction. These results indicate that SUMOylation regulates the phosphorylation of RSK1 downstream substrates, which is required for efficient KSHV lytic replication.

Targeting Akt in cancer for precision therapy

Biomarkers-guided precision therapeutics has revolutionized the clinical development and administration of molecular-targeted anticancer agents. Tailored precision cancer therapy exhibits better response rate compared to unselective treatment. Protein kinases have critical roles in cell signaling, metabolism, proliferation, survival and migration. Aberrant activation of protein kinases is critical for tumor growth and progression. Hence, protein kinases are key targets for molecular targeted cancer therapy. The serine/threonine kinase Akt is frequently activated in various types of cancer. Activation of Akt promotes tumor progression and drug resistance. Since the first Akt inhibitor was reported in 2000, many Akt inhibitors have been developed and evaluated in either early or late stage of clinical trials, which take advantage of liquid biopsy and genomic or molecular profiling to realize personalized cancer therapy. Two inhibitors, capivasertib and ipatasertib, are being tested in phase III clinical trials for cancer therapy. Here, we highlight recent progress of Akt signaling pathway, review the up-to-date data from clinical studies of Akt inhibitors and discuss the potential biomarkers that may help personalized treatment of cancer with Akt inhibitors. In addition, we also discuss how Akt may confer the vulnerability of cancer cells to some kinds of anticancer agents.

Neuronal activity-induced SUMOylation of Akt1 by PIAS3 is required for long-term potentiation of synaptic transmission

Neuronal activity regulates spatial distribution of the SUMOylation system in cytosolic and dendritic sites, which has been implicated in learning, memory, and underlying synaptic structural and functional remodeling in the hippocampus. However, the functional target proteins for activated small ubiquitin-like modifiers (SUMOs) and downstream molecular consequences behind long-term potentiation (LTP) of synaptic plasticity remain to be elucidated. In this study, we showed that N-methyl-D-aspartate receptor-mediated neuronal activity induced the covalent modification of cytosolic Akt1 by small ubiquitin-like modifier 1 (SUMO1) in rat cortical and hippocampal CA1 neurons. Protein inhibitor of activated STAT3 (PIAS3) was involved in the activity-induced Akt1 SUMO1-ylation, and K64 and K276 residues were major SUMOylated sites. Importantly, Akt1 SUMOylation at K64 and K276 enhanced its enzymatic activity and facilitated T308 phosphorylation. Furthermore, the N-terminal SAP domain of PIAS3 bound Akt1 directly. The disruption of Akt1-PIAS3 interaction by Tat-SAP, a synthetic Tat-fused cell-permeable peptide containing PIAS3 SAP domain, inhibited neuronal activity-induced Akt1 SUMOylation and impaired LTP expression and late phase LTP maintenance in the hippocampus. Correlatedly, Tat-SAP not only blocked the LTP-related extracellular signal-regulated kinase (ERK)1/2-Elk-1-brain-derived neurotrophic factor (BDNF)/Arc signaling, but also disrupted mammalian target of rapamycin (mTOR)-eIF4E-binding protein 1 (4E-BP1) pathway. These findings reveal an activity-induced Akt1 SUMOylation by PIAS3 that contributes to ERK1/2-BDNF/Arc and mTOR-4E-BP1 cascades, and in turn, long-lasting excitatory synaptic responses.

Wrestling and Wrapping: A Perspective on SUMO Proteins in Schwann Cells

Schwann cell development and peripheral nerve myelination are finely orchestrated multistep processes; some of the underlying mechanisms are well described and others remain unknown. Many posttranslational modifications (PTMs) like phosphorylation and ubiquitination have been reported to play a role during the normal development of the peripheral nervous system (PNS) and in demyelinating neuropathies. However, a relatively novel PTM, SUMOylation, has not been studied in these contexts. SUMOylation involves the covalent attachment of one or more small ubiquitin-like modifier (SUMO) proteins to a substrate, which affects the function, cellular localization, and further PTMs of the conjugated protein. SUMOylation also regulates other proteins indirectly by facilitating non-covalent protein–protein interaction via SUMO interaction motifs (SIM). This pathway has important consequences on diverse cellular processes, and dysregulation of this pathway has been reported in several diseases including neurological and degenerative conditions. In this article, we revise the scarce literature on SUMOylation in Schwann cells and the PNS, we propose putative substrate proteins, and we speculate on potential mechanisms underlying the possible involvement of this PTM in peripheral myelination and neuropathies.

SUMO conjugation susceptibility of Akt/protein kinase B affects the expression of the pluripotency transcription factor Nanog in embryonic stem cells

Akt/PKB is a kinase involved in the regulation of a wide variety of cell processes. Its activity is modulated by diverse post-translational modifications (PTMs). Particularly, conjugation of the small ubiquitin-related modifier (SUMO) to this kinase impacts on multiple cellular functions, such as proliferation and splicing. In embryonic stem (ES) cells, this kinase is key for pluripotency maintenance. Among other functions, Akt is known to promote the expression of Nanog, a central pluripotency transcription factor (TF). However, the relevance of this specific PTM of Akt has not been previously analyzed in this context. In this work, we study the effect of Akt1 variants with differential SUMOylation susceptibility on the expression of Nanog. Our results demonstrate that both, the Akt1 capability of being modified by SUMO conjugation and a functional SUMO conjugase activity are required to induce Nanog gene expression. Likewise, we found that the common oncogenic E17K Akt1 mutant affected Nanog expression in ES cells also in a SUMOylatability dependent manner. Interestingly, this outcome takes places in ES cells but not in a non-pluripotent heterologous system, suggesting the presence of a crucial factor for this induction in ES cells. Remarkably, the two major candidate factors to mediate this induction, GSK3-β and Tbx3, are non-essential players of this effect, suggesting a complex mechanism probably involving non-canonical pathways. Furthermore, we found that Akt1 subcellular distribution does not depend on its SUMOylatability, indicating that Akt localization has no influence on the effect on Nanog, and that besides the membrane localization of E17K Akt mutant, SUMOylation is also required for its hyperactivity. Our results highlight the impact of SUMO conjugation in the function of a kinase relevant for a plethora of cellular processes, including the control of a key pluripotency TF.

SLX4IP promotes RAP1 SUMOylation by PIAS1 to coordinate telomere maintenance through NF-κB and Notch signaling

The maintenance of telomere length supports repetitive cell division and therefore plays a central role in cancer development and progression. Telomeres are extended by either the enzyme telomerase or the alternative lengthening of telomeres (ALT) pathway. Here, we found that the telomere-associated protein SLX4IP dictates telomere proteome composition by recruiting and activating the E3 SUMO ligase PIAS1 to the SLX4 complex. PIAS1 SUMOylated the telomere-binding protein RAP1, which disrupted its interaction with the telomere-binding protein TRF2 and facilitated its nucleocytoplasmic shuttling. In the cytosol, RAP1 bound to IκB kinase (IKK), resulting in activation of the transcription factor NF-κB and its induction of Jagged-1 expression, which promoted Notch signaling and the institution of ALT. This axis could be targeted therapeutically in ALT-driven cancers and in tumor cells that develop resistance to antitelomerase therapies. Our results illuminate the mechanisms underlying SLX4IP-dependent telomere plasticity and demonstrate the role of telomere proteins in directly coordinating intracellular signaling and telomere maintenance dynamics.

Cleavage of Syndecan-1 Promotes the Proliferation of the Basal-Like Breast Cancer Cell Line BT-549 Via Akt SUMOylation

Basal-like breast cancer is characterized by an aggressive clinical outcome and presence of metastasis, for which effective therapies are unavailable. We have previously shown that chondroitin 4-O-sulfotransferase-1 (C4ST-1) controls the invasive properties of the basal-like breast cancer cell line BT-549 by inducing matrix metalloproteinase (MMP) expression through the N-cadherin/β-catenin pathway. Here we report that C4ST-1 controls the proliferation of BT-549 cells via the MMP-dependent cleavage of syndecan-1. Syndecan-1 is a membrane-bound proteoglycan associated with an aggressive phenotype and poor prognosis in breast cancer. In addition, the cleavage of syndecan-1 at a specific juxtamembrane cleavage site is implicated in the pathophysiological response in breast cancer. Knockout of C4ST-1 remarkably suppressed both the cleavage of syndecan-1 and proliferation of BT-549 cells. Kinases (AKT1, ERK1/2, PI3K, and STAT3) comprising cancer proliferative pathways are phosphorylated in C4ST-1 knockout cells at a level similar to that in parental BT-549 cells, whereas levels of phosphorylated S6 kinase and SUMOylated AKT (hyperactivated AKT observed in breast cancer) decreased in C4ST-1 knockout cells. An MMP inhibitor, GM6001, suppressed the small ubiquitin-like modifier (SUMO) modification of AKT, suggesting that cleavage of syndecan-1 by MMPs is involved in the SUMO modification of AKT. Forced expression of the cytoplasmic domain of syndecan-1, which is generated by MMP-dependent cleavage, increased the SUMO modification of AKT and global protein SUMOylation. Furthermore, syndecan-1 C-terminal domain-expressing BT-549 cells were more proliferative and sensitive to a potent SUMOylation inhibitor, tannic acid, compared with BT-549 cells transfected with an empty expression vector. These findings assign new functions to the C-terminal fragment of syndecan-1 generated by MMP-dependent proteolysis, thereby broadening our understanding of their physiological importance and implying that the therapeutic inhibition of syndecan-1 cleavage could affect the progression of basal-like breast cancer.

SUMOylation modulates the stability and function of PI3K-p110β

Class I PI3K are heterodimers composed of a p85 regulatory subunit and a p110 catalytic subunit involved in multiple cellular functions. Recently, the catalytic subunit p110β has emerged as a class I PI3K isoform playing a major role in tumorigenesis. Understanding its regulation is crucial for the control of the PI3K pathway in p110β-driven cancers. Here we sought to evaluate the putative regulation of p110β by SUMO. Our data show that p110β can be modified by SUMO1 and SUMO2 in vitro, in transfected cells and under completely endogenous conditions, supporting the physiological relevance of p110β SUMOylation. We identify lysine residue 952, located at the activation loop of p110β, as essential for SUMOylation. SUMOylation of p110β stabilizes the protein increasing its activation of AKT which promotes cell growth and oncogenic transformation. Finally, we show that the regulatory subunit p85β counteracts the conjugation of SUMO to p110β. In summary, our data reveal that SUMO is a novel p110β interacting partner with a positive effect on the activation of the PI3K pathway.

Inhibition of SENP2-mediated Akt deSUMOylation promotes cardiac regeneration via activating Akt pathway

Post-translational modification (PTM) by small ubiquitin-like modifier (SUMO) is a key regulator of cell proliferation and can be readily reversed by a family of SUMO-specific proteases (SENPs), making SUMOylation an ideal regulatory mechanism for developing novel therapeutic strategies for promoting a cardiac regenerative response. However, the role of SUMOylation in cardiac regeneration remains unknown. In the present study, we assessed whether targeting protein kinase B (Akt) SUMOylation can promote cardiac regeneration. Quantitative PCR and Western blotting results showed that small ubiquitin-like modifier-specific protease 2 (SENP2) is up-regulated during postnatal heart development. SENP2 deficiency promoted P7 and adult cardiomyocyte (CM) dedifferentiation and proliferation both in vitro and in vivo. Mice with SENP2 deficiency exhibited improved cardiac function after MI due to CM proliferation and angiogenesis. Mechanistically, the loss of SENP2 up-regulated Akt SUMOylation levels and increased Akt kinase activity, leading to a decrease in GSK3β levels and subsequently promoting CM proliferation and angiogenesis. In summary, inhibition of SENP2-mediated Akt deSUMOylation promotes CM differentiation and proliferation by activating the Akt pathway. Our results provide new insights into the role of SUMOylation in cardiac regeneration.

Increased SUMOylation of TCF21 improves its stability and function in human endometriotic stromal cells†

Endometriosis is an estrogen-dependent disease. Our previous study demonstrated that elevated levels of transcription factor 21 (TCF21) in endometriotic tissues enhanced steroidogenic factor-1 (SF-1) and estrogen receptor β (ERβ) expression by forming a heterodimer with upstream stimulatory factor 2 (USF2), allowing these TCF21/USF2 complexes to bind to the promoters of SF-1 and ERβ. Furthermore, TCF21 contributed to the increased proliferation of endometriotic stromal cells (ESCs), suggesting that TCF21 may play a vital role in the pathogenesis of endometriosis. SUMOylation is a posttranslational modification that has emerged as a crucial molecular regulatory mechanism. However, the mechanism regulating TCF21 SUMOylation in endometriosis is incompletely characterized. Thus, this study aimed to explore the effect of TCF21 SUMOylation on its expression and regulation in ovarian endometriosis. We found that the levels of SUMOylated TCF21 were increased in endometriotic tissues and stromal cells compared with eutopic endometrial tissues and stromal cells and enhanced by estrogen. Treatment with the SUMOylation inhibitor ginkgolic acid and the results of a protein half-life assay demonstrated that SUMOylation can stabilize the TCF21 protein. A coimmunoprecipitation assay showed that SUMOylation probably increased its interaction with USF2. Further analyses elucidated that SUMOylation of TCF21 significantly increased the binding activity of USF2 to the SF-1 and ERβ promoters. Moreover, the SUMOylation motifs in TCF21 affected the proliferation ability of ESCs. The results of this study suggest that SUMOylation plays a critical role in mediating the high expression of TCF21 in ESCs and may participate in the development of endometriosis.

Inhibitors in AKTion: ATP-competitive vs allosteric

Aberrant activation of the PI3K pathway is one of the commonest oncogenic events in human cancer. AKT is a key mediator of PI3K oncogenic function, and thus has been intensely pursued as a therapeutic target. Multiple AKT inhibitors, broadly classified as either ATP-competitive or allosteric, are currently in various stages of clinical development. Herein, we review the evidence for AKT dependence in human tumours and focus on its therapeutic targeting by the two drug classes. We highlight the future prospects for the development and implementation of more effective context-specific AKT inhibitors aided by our increasing knowledge of both its regulation and some previously unrecognised non-canonical functions.

The implication of the SUMOylation pathway in breast cancer pathogenesis and treatment

Breast cancer is the most commonly diagnosed malignancy in woman worldwide, and is the second most common cause of death in developed countries. The transformation of a normal cell into a malignant derivate requires the acquisition of diverse genomic and proteomic changes, including enzymatic post-translational modifications (PTMs) on key proteins encompassing critical cell signaling events. PTMs occur on proteins after translation, and regulate several aspects of proteins activity, including their localization, activation and turnover. Deregulation of PTMs can potentially lead to tumorigenesis, and several de-regulated PTM pathways contribute to abnormal cell proliferation during breast tumorigenesis. SUMOylation is a PTM that plays a pivotal role in numerous aspects of cell physiology, including cell cycle regulation, protein trafficking and turnover, and DNA damage repair. Consistently with this, the deregulation of the SUMO pathway is observed in different human pathologies, including breast cancer. In this review we will describe the role of SUMOylation in breast tumorigenesis and its implication for breast cancer therapy.

Modification of the base excision repair enzyme MBD4 by the small ubiquitin-like molecule SUMO1

The base excision repair DNA N-glycosylase MBD4 (also known as MED1), an interactor of the DNA mismatch repair protein MLH1, plays a central role in the maintenance of genomic stability of CpG sites by removing thymine and uracil from G:T and G:U mismatches, respectively. MBD4 is also involved in DNA damage response and transcriptional regulation. The interaction with other proteins is likely critical for understanding MBD4 functions. To identify novel proteins that interact with MBD4, we used tandem affinity purification (TAP) from HEK-293 cells. The MBD4-TAP fusion and its co-associated proteins were purified sequentially on IgG and calmodulin affinity columns; the final eluate was shown to contain MLH1 by western blotting, and MBD4-associated proteins were identified by mass spectrometry. Bands with molecular weight higher than that expected for MBD4 (˜66 kD) yielded peptides corresponding to MBD4 itself and the small ubiquitin-like molecule-1 (SUMO1), suggesting that MBD4 is sumoylated in vivo. MBD4 sumoylation was validated by co-immunoprecipitation in HEK-293 and MCF7 cells, and by an in vitrosumoylation assay. Sequence and mutation analysis identified three main sumoylation sites: MBD4 is sumoylated preferentially on K137, with additional sumoylation at K215 and K377. Patterns of MBD4 sumoylation were altered, in a DNA damage-specific way, by the anti-metabolite 5-fluorouracil, the alkylating agent N-Methyl-N-nitrosourea and the crosslinking agent cisplatin. MCF7 extract expressing sumoylated MBD4 displays higher thymine glycosylase activity than the unmodified species. Of the 67 MBD4 missense mutations reported in The Cancer Genome Atlas, 14 (20.9%) map near sumoylation sites. These results indicate that MBD4 is sumoylated in vivo in a DNA damage-specific manner, and suggest that sumoylation serves to regulate its repair activity and could be compromised in cancer. This study expands the role played by sumoylation in fine-tuning DNA damage response and repair.

Regulation of miR-34b/c-targeted gene expression program by SUMOylation

The miR-34 family of microRNAs suppresses the expression of proteins involved in pluripotency and oncogenesis. miR-34 expression is frequently reduced in cancers; however, the regulation of their expression is not well understood. We used genome-wide miRNA profiling and mechanistic analysis to show that SUMOylation regulates miR-34b/c expression, which impacts the expression of c-Myc and other tested miR-34 targets. We used site-directed mutagenesis and other methods to show that protein kinase B (also known as Akt) phosphorylation of FOXO3a plays an important role in SUMOylation-dependent expression of miR-34b/c. This study reveals how the miR-34-targeted gene expression program is regulated by SUMOylation and shows that SUMOylation need not regulate target proteins through direct modification, but instead can act through the expression of their targeting miRNAs.

Insulin-dependent GLUT4 trafficking is not regulated by protein SUMOylation in L6 myocytes

Type-II Diabetes Mellitus (T2DM) is one of the fastest growing public health issues today, consuming 12% of worldwide health budgets and affecting an estimated 400 million people. One of the key pathological traits of this disease is insulin resistance at ‘glucose sink’ tissues (mostly skeletal muscle), and this remains one of the features of this disease most intractable to therapeutic intervention. Several lines of evidence have implicated the post-translational modification, SUMOylation, in insulin signalling and insulin resistance in skeletal muscle. In this study, we examined this possibility by manipulation of cellular SUMOylation levels using multiple different tools, and assaying the effect on insulin-stimulated GLUT4 surface expression in differentiated L6 rat myocytes. Although insulin stimulation of L6 myocytes produced a robust decrease in total cellular SUMO1-ylation levels, manipulating cellular SUMOylation had no effect on insulin-responsive GLUT4 surface trafficking using any of the tools we employed. Whilst we cannot totally exclude the possibility that SUMOylation plays a role in the insulin signalling pathway in human health and disease, our data strongly argue that GLUT4 trafficking in response to insulin is not regulated by protein SUMOylation, and that SUMOylation does not therefore represent a viable therapeutic target for the treatment of insulin resistance.

SUMOylation pathway alteration coupled with downregulation of SUMO E2 enzyme at mucosal epithelium modulates inflammation in inflammatory bowel disease

Post-translational modification pathways such as SUMOylation are integral to all cellular processes and tissue homeostasis. We investigated the possible involvement of SUMOylation in the epithelial signalling in Crohn's disease (CD) and ulcerative colitis (UC), the two major forms of inflammatory bowel disease (IBD). Initially in a murine model of IBD, induced by dextran–sulfate–sodium (DSS mice), we observed inflammation accompanied by a lowering of global SUMOylation of colonic epithelium. The observed SUMOylation alteration was due to a decrease in the sole SUMO E2 enzyme (Ubc9). Mass-spectrometric analysis revealed the existence of a distinct SUMOylome (SUMO-conjugated proteome) in DSS mice with alteration of key cellular regulators, including master kinase Akt1. Knocking-down of Ubc9 in epithelial cells resulted in dramatic activation of inflammatory gene expression, a phenomenon that acted via reduction in Akt1 and its SUMOylated form. Importantly, a strong decrease in Ubc9 and Akt1 was also seen in endoscopic biopsy samples (N = 66) of human CD and UC patients. Furthermore, patients with maximum disease indices were always accompanied by severely lowered Ubc9 or SUMOylated-Akt1. Mucosal tissues with severely compromised Ubc9 function displayed higher levels of pro-inflammatory cytokines and compromised wound-healing markers. Thus, our results reveal an important and previously undescribed role for the SUMOylation pathway involving Ubc9 and Akt1 in modulation of epithelial inflammatory signalling in IBD.

The significance of SUMOylation of angiogenic factors in cancer progression

Angiogenesis is the process of endothelial cell migration and proliferation induced by angiogenic factors, which is essential for the development of tumors. In recent years, studies have reported that SUMOylation acts on tumor angiogenesis by targeting angiogenic factors as one of post-translational modifications of proteins. Anti-angiogenic therapy is a new treatment method for tumor treatment following radiotherapy and chemotherapy, and it inhibits tumor growth by blocking tumor blood vessels. Therefore, SUMOylation may become a potential target for anti-angiogenesis therapy. This article focuses on the effect of SUMOylation on vascular growth factors, important signaling pathways proteins, and the migration and function of endothelial cells, in order to provide a new research idea for the anti-angiogenic therapy of tumors.

Overexpression of SENP1 reduces the stemness capacity of osteosarcoma stem cells and increases their sensitivity to HSVtk/GCV

Osteosarcoma stem cells are able to escape treatment with conventional chemotherapeutic drugs, as the majority of them are in a quiescent state. Recent reports have suggested that small ubiquitin-like modifiers (SUMOs) serve important roles in the maintenance of cancer stem cell stemness. Therefore, a potential strategy to increase the effectiveness of chemotherapeutic agents is to interfere with SUMO modification of proteins associated with the maintenance of stemness in osteosarcoma stem cells. The present study revealed a significant decrease in the expression of SUMO1 specific peptidase 1 (SENP1) in osteosarcoma tissues and osteosarcoma cell lines, and SENP1 expression was much lower in osteosarcoma stem cells than in non-cancer stem cells. Further experiments indicated that the low levels of SENP1 were essential for maintenance of stemness in osteosarcoma stem cells. Overexpression of SENP1 resulted in a marked decrease in the maintenance of stemness, but only slightly induced apoptosis of osteosarcoma cells, which is crucial to reduce the side effects of drugs on normal precursor cells. Finally, SENP1 overexpression led to a significant increase in the sensitivity of osteosarcoma stem cells to the herpes simplex virus 1 thymidine kinase gene in combination with ganciclovir in vitro and in vivo. In conclusion, the present study described a novel method to increase the sensitivity of osteosarcoma stem cells to chemotherapeutic drugs. Notably, this approach may significantly reduce the required dose of conventional chemotherapeutic drugs and reduce side effects.

Silencing Ubc9 expression suppresses osteosarcoma tumorigenesis and enhances chemosensitivity to HSV-TK/GCV by regulating connexin 43 SUMOylation

The ability of herpes simplex virus thymidine kinase/ganciclovir (HSV-TK/GCV) systems to kill tumor cells is partially dependent on the integrity of gap junction intercellular communication (GJIC) of targeted tumor cells. Recent studies have suggested that connexin 43 (Cx43), which serves a role in gap junction-mediated intercellular communication, is regulated by small ubiquitin-like modifiers (SUMOs). However, the roles of these post-translational modifications remain to be elucidated. The present study demonstrated overexpression of SUMO‑conjugating enzyme Ubc9 (Ubc9) protein in osteosarcoma. Silencing Ubc9 by siRNA inhibited osteosarcoma cell proliferation and migration, and significantly increased the sensitivity of cells to HSV-TK/GCV systems both in vitro and in vivo. Further experimentation demonstrated that silencing Ubc9 induced decoupling of SUMO1 from Cx43, generating increased free Cx43 levels, which is important for reconstructing GJIC and recovering cellular functions. In conclusion, the present study revealed a novel method for the effective restoration of GJIC in osteosarcoma cells, which may increase their sensitivity to conventional chemotherapy.

SUMOylation regulates germinal vesicle breakdown and the Akt/PKB pathway during mouse oocyte maturation

SUMOylation, a process of posttranslational modification of proteins by the small ubiquitin-related modifier (SUMO) family of proteins, is known to be involved in yeast and mammalian somatic cell-cycle regulation. However, the identities of the SUMO-modified oocyte targets are largely unknown and the functional role(s) for SUMOylation during mammalian oocyte maturation remains unclear. On the basis of studies in non-germline cells, protein kinase B/Akt is a potential SUMOylation target in the mouse oocyte, where it plays an essential role in cell-cycle resumption and progression during maturation. This study investigated the temporal patterns and prospective role(s) for interactions between SUMOylation and Akt serine-phosphorylation during oocyte meiotic resumption. Pharmacological inhibition of SUMOylation significantly decreased follicular fluid meiosis-activating sterol-induced cell-cycle resumption in oocytes matured in vitro and negatively affected the phosphorylation and nuclear translocation of Akt. Similarly, nuclear localization of cyclin D1, a downstream target of Akt activation, was significantly decreased following SUMOylation inhibition. Together these data show that SUMO and the posttranslational process of SUMOylation are involved in cell-cycle resumption during murine oocyte maturation and exert a regulatory influence on the Akt pathway during germinal vesicle breakdown.

SUMOylation regulates nuclear accumulation and signaling activity of the soluble intracellular domain of the ErbB4 receptor tyrosine kinase

Erb-B2 receptor tyrosine kinase 4 (ErbB4) is a kinase that can signal via a proteolytically released intracellular domain (ICD) in addition to classical receptor tyrosine kinase-activated signaling cascades. Previously, we have demonstrated that ErbB4 ICD is posttranslationally modified by the small ubiquitin-like modifier (SUMO) and functionally interacts with the PIAS3 SUMO E3 ligase. However, direct evidence of SUMO modification in ErbB4 signaling has remained elusive. Here, we report that the conserved lysine residue 714 in the ErbB4 ICD undergoes SUMO modification, which was reversed by sentrin-specific proteases (SENPs) 1, 2, and 5. Although ErbB4 kinase activity was not necessary for the SUMOylation, the SUMOylated ErbB4 ICD was tyrosine phosphorylated to a higher extent than unmodified ErbB4 ICD. Mutation of the SUMOylation site compromised neither ErbB4-induced phosphorylation of the canonical signaling pathway effectors Erk1/2, Akt, or STAT5 nor ErbB4 stability. In contrast, SUMOylation was required for nuclear accumulation of the ErbB4 ICD. We also found that Lys-714 was located within a leucine-rich stretch, which resembles a nuclear export signal, and could be inactivated by site-directed mutagenesis. Furthermore, SUMOylation modulated the interaction of ErbB4 with chromosomal region maintenance 1 (CRM1), the major nuclear export receptor for proteins. Finally, the SUMO acceptor lysine was functionally required for ErbB4 ICD-mediated inhibition of mammary epithelial cell differentiation in a three-dimensional cell culture model. Our findings indicate that a SUMOylation-mediated mechanism regulates nuclear localization and function of the ICD of ErbB4 receptor tyrosine kinase.

A functional SUMO-motif in the active site of PIM1 promotes its degradation via RNF4, and stimulates protein kinase activity

The PIM1 serine/threonine protein kinase mediates growth factor and survival signalling, and cooperates potently with c-MYC during tumorigenesis. PIM1 is overexpressed in many human cancers and is a promising target for drug development. PIM1 levels are regulated mainly through cytokine-induced transcription and protein degradation, but mechanisms regulating its activity and levels remain largely unexplored. Here, we show that PIM1 is modified in vitro and in cultured cells by the Small ubiquitin-like modifier (SUMO) on two independent sites: K169, within a consensus SUMOylation motif (IK¹⁶⁹DE¹⁷¹) in the active site of PIM1, and also at a second promiscuous site. Alanine substitution of E171 (within the consensus motif) abolished SUMOylation, significantly increased the half-life of PIM1, and markedly reduced its ubiquitylation. Mechanistically, SUMOylation promoted ubiquitin-mediated degradation of PIM1 via recruitment of the SUMO-targeted ubiquitin ligase, RNF4. Additionally, SUMOylated PIM1 showed enhanced protein kinase activity in vitro. Interestingly, the E171A mutant was active in vitro but displayed altered substrate specificity in cultured cells, consistent with the idea that SUMOylation may govern PIM1 substrate specificity under certain contexts. Taken together, these data demonstrate that the protein kinase activity and levels of PIM1 can be regulated by a covalent post-translational modification.

The Role of PIAS SUMO E3-Ligases in Cancer

SUMOylation modifies the interactome, localization, activity, and lifespan of its target proteins. This process regulates several cellular machineries, including transcription, DNA damage repair, cell-cycle progression, and apoptosis. Accordingly, SUMOylation is critical in maintaining cellular homeostasis, and its deregulation leads to the corruption of a plethora of cellular processes that contribute to disease states. Among the proteins involved in SUMOylation, the protein inhibitor of activated STAT (PIAS) E3-ligases were initially described as transcriptional coregulators. Recent findings also indicate that they have a role in regulating protein stability and signaling transduction pathways. PIAS proteins interact with up to 60 cellular partners affecting several cellular processes, most notably immune regulation and DNA repair, but also cellular proliferation and survival. Here, we summarize the current knowledge about their role in tumorigenesis and cancer-related processes. Cancer Res; 77(7); 1542-7. ©2017 AACR.

Metabolic Reprogramming by the PI3K-Akt-mTOR Pathway in Cancer

In the past decade, there has been a resurgence of interest in elucidating how metabolism is altered in cancer cells and how such dependencies can be targeted for therapeutic gain. At the core of this research is the concept that metabolic pathways are reprogrammed in cancer cells to divert nutrients toward anabolic processes to facilitate enhanced growth and proliferation. Importantly, physiological cellular signaling mechanisms normally tightly regulate the ability of cells to gain access to and utilize nutrients, posing a fundamental barrier to transformation. This barrier is often overcome by aberrations in cellular signaling that drive tumor pathogenesis by enabling cancer cells to make critical cellular decisions in a cell-autonomous manner. One of the most frequently altered pathways in human cancer is the PI3K-Akt-mTOR signaling pathway. Here, we describe mechanisms by which this signaling network is responsible for controlling cellular metabolism. Through both the post-translational regulation and the induction of transcriptional programs, the PI3K-Akt-mTOR pathway coordinates the uptake and utilization of multiple nutrients, including glucose, glutamine, nucleotides, and lipids, in a manner best suited for supporting the enhanced growth and proliferation of cancer cells. These regulatory mechanisms illustrate how metabolic changes in cancer are closely intertwined with oncogenic signaling pathways that drive tumor initiation and progression.

High-Content Imaging Platform for Profiling Intracellular Signaling Network Activity in Living Cells

Essential characteristics of cellular signaling networks include a complex interconnected architecture and temporal dynamics of protein activity. The latter can be monitored by Förster resonance energy transfer (FRET) biosensors at a single-live-cell level with high temporal resolution. However, these experiments are typically limited to the use of a couple of FRET biosensors. Here, we describe a FRET-based multi-parameter imaging platform (FMIP) that allows simultaneous high-throughput monitoring of multiple signaling pathways. We apply FMIP to monitor the crosstalk between epidermal growth factor receptor (EGFR) and insulin-like growth factor-1 receptor signaling, signaling perturbations caused by pathophysiologically relevant EGFR mutations, and the effects of a clinically important MEK inhibitor (selumetinib) on the EGFR network. We expect that in the future the platform will be applied to develop comprehensive models of signaling networks and will help to investigate the mechanism of action as well as side effects of therapeutic treatments.

Regulation of Ebola virus VP40 matrix protein by SUMO

The matrix protein of Ebola virus (EBOV) VP40 regulates viral budding, nucleocapsid recruitment, virus structure and stability, viral genome replication and transcription, and has an intrinsic ability to form virus-like particles. The elucidation of the regulation of VP40 functions is essential to identify mechanisms to inhibit viral replication and spread. Post-translational modifications of proteins with ubiquitin-like family members are common mechanisms for the regulation of host and virus multifunctional proteins. Thus far, no SUMOylation of VP40 has been described. Here we demonstrate that VP40 is modified by SUMO and that SUMO is included into the viral like particles (VLPs). We demonstrate that lysine residue 326 in VP40 is involved in SUMOylation, and by analyzing a mutant in this residue we show that SUMO conjugation regulates the stability of VP40 and the incorporation of SUMO into the VLPs. Our study indicates for the first time, to the best of our knowledge, that EBOV hijacks the cellular SUMOylation system in order to modify its own proteins. Modulation of the VP40-SUMO interaction may represent a novel target for the therapy of Ebola virus infection.

SUMOylation of large tumor suppressor 1 at Lys751 attenuates its kinase activity and tumor-suppressor functions

Large tumor suppressor (Lats) plays a critical role in maintaining cellular homeostasis and is the core to mediate Hippo growth-inhibitory signaling pathway. SUMOylation is a reversible and dynamic process that regulates a variety of cell functions. Here, we show that SUMOylation of Lats1 affects its kinase activity specifically towards Hippo signaling. Small ubiquitin-like modifier (SUMO) 1 interacts with and directly SUMOylates Lats1, whereas loss of SUMOylation pathway function disrupts Lats1 SUMOylation. Among potential SUMOylation sites on hLats1, K751 and K830 are conversed and essential for maintaining the transcriptional output of Hippo signaling, whereas K751 mutation more significantly abolishes SUMO1-induced Lats1 SUMOylation than K830 mutation. Though Lats1 SUMOylation at K751 affects neither its subcellular distribution nor its interactions with YAP and TAZ, it significantly destabilizes the phosphorylated Lats1 (Thr1079 but not Ser909), resulting in the attenuation of Lats1 kinase activity and inhibition of Hippo signaling. Moreover, HepG2 hepatocellular carcinoma cells express significantly more SUMOylated Lats1 than LO2 normal human hepatic cells, and in HepG2 cells or HepG2 cells xenografts, Lats1 SUMOylation at K751 consistently attenuates Lats1 kinase activity and subsequently suppresses Hippo signaling, resulting in not only the promotion of cell proliferation and colony formation but also the suppression of cell apoptosis. Together, we demonstrate that Lats1 SUMOylation at K751 suppresses its kinase activity and subsequently attenuates its tumor-suppressor functions. Thus, this study provides additional insight into how Hippo signaling is regulated and highlights the potentially critical role of Lats1 SUMOylation in tumor development.

SUMOylation of sPRDM16 promotes the progression of acute myeloid leukemia

Background

In addition to genetic and epigenetic alteration, post-translational modification of proteins plays a critical role in the initiation, progression and maturation of acute myeloid leukemia (AML).

Methods

The SUMOylation site of sPRDM16 at K568 was mutated to arginine by site-directed mutagenesis. THP-1 acute myeloid leukemia cells were transduced with a lentivirus containing wild type or K568 mutant sPRDM16. Proliferation, self-renewal and differentiation of transduced THP-1 cells were analyzed both in vitro cell culture and in mouse xenografts. Gene expression profiles were analyzed by RNA-seq.

Results

Overexpression of sPRDM16 promoted proliferation, enhanced self-renewal capacity, but inhibited differentiation of THP-1 acute myeloid leukemia cells. We further confirmed that K568 is a bona fide SUMOylation site on sPRDM16. Mutation of the sPRDM16 SUMOylation site at K568 partially abolished the capacity of sPRDM16 to promote proliferation and inhibit differentiation of acute myeloid leukemia cells both in vitro and in mouse xenografts. Furthermore, THP-1 cells overexpressing sPRDM16-K568R mutant exhibited a distinct gene expression profile from wild type sPRDM16 following incubation with PMA.

Conclusions

Our results suggest that K568 SUMOylation of sPRDM16 plays an important role in the progression of acute myeloid leukemia.

Histone methylation modifiers in cellular signaling pathways

Histone methyltransferases and demethylases epigenetically regulate gene expression by modifying histone methylation status in numerous cellular processes, including cell differentiation and proliferation. These modifiers also control methylation levels of various non-histone proteins, such as effector proteins that play critical roles in cellular signaling networks. Dysregulated histone methylation modifiers alter expression of oncogenes and tumor suppressor genes and change methylation states of effector proteins, frequently resulting in aberrant cellular signaling cascades and cellular transformation. In this review, we summarize the role of histone methylation modifiers in regulating the following signaling pathways: NF-κB, RAS/RAF/MEK/MAPK, PI3K/Akt, Wnt/β-catenin, p53, and ERα.

SUMO modification of Akt regulates global SUMOylation and substrate SUMOylation specificity through Akt phosphorylation of Ubc9 and SUMO1

SUMOylation is an important post-translational modification, and Akt SUMOylation was found to regulate cell proliferation, tumorigenesis and cell cycle, but the molecular mechanism of Akt SUMOylation is less well known. Here, we show both endogenous and ectopic Akt SUMOylation and Lys276 is the major SUMO acceptor on Akt. Further, Akt SUMOylation is Akt phosphorylation dependent and Akt SUMOylation increases Akt kinase activity without affecting the phosphorylation level of Akt. Moreover, endogenous Akt SUMOylation is enhanced by insulin treatment and this is Akt activity dependent. Heat-shock stimulus also increases Akt SUMOylation and it is also Akt activity dependent. Endogenous Akt SUMOylation is also found in the rat brain and it is enhanced by insulin-like growth factor-1 stimulation. In addition, Akt directly phosphorylates Ubc9 at Thr35 and phosphorylates SUMO1 at Thr76. Ubc9 phosphorylation at Thr35 promotes Ubc9 thioester bond formation and SUMO1 phosphorylation at Thr76 stabilizes the SUMO1 protein. Through these distinct mechanisms, Akt SUMOylation regulates global SUMOylation, including Akt and Ubc9 SUMOylation, and substrate SUMOylation specificity, including STAT1 and CREB SUMOylation, in different manners. Akt SUMOylation also enhances phosphatase and tensin homolog (PTEN) SUMOylation through Akt phosphorylation of Ubc9 and SUMO1, which serves as an endogenous mechanism to stop the positive feedback loop resulted from Akt activation. Further, Akt SUMOylation increases cyclin D1 expression and cell proliferation, and these effects are also mediated through Ubc9 phosphorylation at Thr35 and SUMO1 phosphorylation at Thr76. Here, we have identified a novel mechanism for SUMOylation regulation. Because of the important role Akt plays in tumorigenesis, this mechanism may also be involved in Akt-regulated tumorigenesis.