Evidence for a regulatory role of Cullin-RING E3 ubiquitin ligase 7 in insulin signaling

Post-translational modifications in diabetic cardiomyopathy

The increasing attention towards diabetic cardiomyopathy as a distinctive complication of diabetes mellitus has highlighted the need for standardized diagnostic criteria and targeted treatment approaches in clinical practice. Ongoing research is gradually unravelling the pathogenesis of diabetic cardiomyopathy, with a particular emphasis on investigating various post-translational modifications. These modifications dynamically regulate protein function in response to changes in the internal and external environment, and their disturbance of homeostasis holds significant relevance for the development of chronic ailments. This review provides a comprehensive overview of the common post-translational modifications involved in the initiation and progression of diabetic cardiomyopathy, including O-GlcNAcylation, phosphorylation, methylation, acetylation and ubiquitination. Additionally, the review discusses drug development strategies for targeting key post-translational modification targets, such as agonists, inhibitors and PROTAC (proteolysis targeting chimaera) technology that targets E3 ubiquitin ligases.

Identification of hypertrophy-modulating Cullin-RING ubiquitin ligases in primary cardiomyocytes

Cullin-RING ubiquitin ligases (CRL) regulate numerous biological processes in the heart and have been implicated in regulating cardiac hypertrophy. This study aimed to identify novel hypertrophy-modulating CRLs in cardiomyocytes (CM). A functional genomic approach using siRNA-mediated depletion and automated microscopy was employed to screen for cell size-modulating CRLs in neonatal rat CM. Screening hits were confirmed by ³H-isoleucine incorporation. Of 43 targets screened, siRNA-mediated depletion of Fbxo6, Fbxo45, and Fbxl14 resulted in decreased cell size, whereas depletion of Fbxo9, Fbxo25, Fbxo30, Fbxo32, Fbxo33, Cullin1, Roc1, Ddb1, Fbxw4, and Fbxw5 led to a markedly increased cell size under basal conditions. In CM stimulated with phenylephrine (PE), depletion of Fbxo6, Fbxo25, Fbxo33, Fbxo45, and Fbxw4 further augmented PE-induced hypertrophy. As a proof-of-concept, the CRL^Fbox25 was analysed by transverse aortic constriction (TAC) resulting in a 4.5-fold increase in Fbxo25 protein concentrations compared to control animals. In cell culture, siRNA-mediated depletion of Fbxo25 resulted in a ∼ 37% increase in CM cell size and ∼41% increase in ³H-isoleucine incorporation. Depleting Fbxo25 resulted in upregulation of Anp and Bnp. In summary, we identified 13 novel CRLs as positive or negative regulators of CM hypertrophy. Of these, CRL^Fbox25 was further characterized, as a potential modulator of cardiac hypertrophy.

RING-finger E3 ligases regulatory network in PI3K/AKT-mediated glucose metabolism

The phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway plays an essential role in glucose metabolism, promoting glycolysis and resisting gluconeogenesis. PI3K/AKT signaling can directly alter glucose metabolism by phosphorylating several metabolic enzymes or regulators of nutrient transport. It can indirectly promote sustained aerobic glycolysis by increasing glucose transporters and glycolytic enzymes, which are mediated by downstream transcription factors. E3 ubiquitin ligase RING-finger proteins are mediators of protein post-translational modifications and include the cullin-RING ligase complexes, the tumor necrosis factor receptor-associated family, the tripartite motif family and etc. Some members of the RING family play critical roles in regulating cell signaling and are involved in the development and progression of various metabolic diseases, such as cancer, diabetes, and dyslipidemia. And with the progression of modern research, as a negative or active regulator, the RING-finger adaptor has been found to play an indispensable role in PI3K/AKT signaling. However, no reviews have comprehensively clarified the role of RING-finger E3 ligases in PI3K/AKT-mediated glucose metabolism. Therefore, in this review, we focus on the regulation and function of RING ligases in PI3K/AKT-mediated glucose metabolism to establish new insights into the prevention and treatment of metabolic diseases.

Roles of Cullin-RING Ubiquitin Ligases in Cardiovascular Diseases

Maintenance of protein homeostasis is crucial for virtually every aspect of eukaryotic biology. The ubiquitin-proteasome system (UPS) represents a highly regulated quality control machinery that protects cells from a variety of stress conditions as well as toxic proteins. A large body of evidence has shown that UPS dysfunction contributes to the pathogenesis of cardiovascular diseases. This review highlights the latest findings regarding the physiological and pathological roles of cullin-RING ubiquitin ligases (CRLs), an essential player in the UPS, in the cardiovascular system. To inspire potential therapeutic invention, factors regulating CRL activities are also discussed.

Potential Roles of Endoplasmic Reticulum Stress and Cellular Proteins Implicated in Diabesity

The role of the endoplasmic reticulum (ER) has evolved from protein synthesis, processing, and other secretory pathways to forming a foundation for lipid biosynthesis and other metabolic functions. Maintaining ER homeostasis is essential for normal cellular function and survival. An imbalance in the ER implied stressful conditions such as metabolic distress, which activates a protective process called unfolded protein response (UPR). This response is activated through some canonical branches of ER stress, i.e., the protein kinase RNA-like endoplasmic reticulum kinase (PERK), inositol-requiring enzyme 1α (IRE1α), and activating transcription factor 6 (ATF6). Therefore, chronic hyperglycemia, hyperinsulinemia, increased proinflammatory cytokines, and free fatty acids (FFAs) found in diabesity (a pathophysiological link between obesity and diabetes) could lead to ER stress. However, limited data exist regarding ER stress and its association with diabesity, particularly the implicated proteins and molecular mechanisms. Thus, this review highlights the role of ER stress in relation to some proteins involved in diabesity pathogenesis and provides insight into possible pathways that could serve as novel targets for therapeutic intervention.

Hypoxia transactivates cholecystokinin gene expression in 3D-engineered muscle

At high altitudes, the hypoxic atmosphere decreases the oxygen partial pressure in the body, inducing several metabolic changes in tissues and cells. Furthermore, it exerts potent anorectic effects, thus causing an energy deficit. Two decades ago, a marked increase in the resting level of plasma cholecystokinin (CCK) was observed in humans at the Mt. Kanchenjunga basecamp, located at 5100 m above the sea level, compared to sea-level control values. Interestingly, acute exercise also raises plasma CCK and exerts potent anorectic effects under normoxic conditions. However, the transcriptional regulations of Cck gene underlying these effects have not yet been established. Here, we employed acute electrical pulse stimulation (EPS) followed by microarray analysis to discover novel myokines in 3D-engineered muscle. Acute EPS affects the contractile function, inducing a decline in the contractile force. Surprisingly, microarray analysis revealed an EPS-induced activation of cholecystokinin receptor (CCKR)-mediated signaling. Furthermore, Cck was constitutively upregulated in 3D-engineered muscle, and its expression increased under hypoxic conditions. Notably, a hypoxia-responsive element was detected in the Cck promoters of mice and humans. Our results suggested that hypoxia transactivated Cck expression in 3D-engineered muscle. Furthermore, the elevation in plasma CCK levels following acute exercise or at high altitude might be partly attributed to myogenic cells.

Role of UPP pathway in amelioration of diabetes-associated complications

Type 2 diabetes (T2D) is one of the most widely spread metabolic disorder also called as "life style" disease. Due to the alarming number of patients, there is great need to therapies targeting functions which can help in maintaining the homeostasis of glucose levels and improving insulin sensitivity. Detailed analysis was done through various research and review papers which was searched using MEDLINE, BIOSIS, and EMBASE using various keywords. This search retrieved the most appropriate content on these molecules targeting UPP pathway. From this extensive review involving UPP pathway, it was concluded that the role of ubiquitin's is not only limited to neurodegenerative disorders but also plays a critical role in progression of diabetes including obesity, insulin resistance, and various neurogenerative disorders but it also targets proteasomal degradation including mediation of cellular signaling pathways. Thus, drugs targeting UPP not only may show effect against diabetes but also are therapeutically beneficial in the treatment of diabetes-associated complications which may be obtained. Thus, based on the available information and data on UPP functions, it can be concluded that regulation of UPP pathway via downstream regulators mainly E1, E2, and E3 may bring promising results. Drugs targeting these transcriptional factors may emerge as a novel therapy in the treatment of diabetes and diabetes-associated complications.

Genetic ablation of Cullin-RING E3 ubiquitin ligase 7 restrains pressure overload-induced myocardial fibrosis

Fibrosis is a pathognomonic feature of structural heart disease and counteracted by distinct cardioprotective mechanisms, e.g. activation of the phosphoinositide 3-kinase (PI3K) / AKT pro-survival pathway. The Cullin-RING E3 ubiquitin ligase 7 (CRL7) was identified as negative regulator of PI3K/AKT signalling in skeletal muscle, but its role in the heart remains to be elucidated. Here, we sought to determine whether CRL7 modulates to cardiac fibrosis following pressure overload and dissect its underlying mechanisms. For inactivation of CRL7, the Cullin 7 (Cul7) gene was deleted in cardiac myocytes (CM) by injection of adeno-associated virus subtype 9 (AAV9) vectors encoding codon improved Cre-recombinase (AAV9-CMV-iCre) in Cul7^flox/flox mice. In addition, Myosin Heavy Chain 6 (Myh6; alpha-MHC)-MerCreMer transgenic mice with tamoxifen-induced CM-specific expression of iCre were used as alternate model. After transverse aortic constriction (TAC), causing chronic pressure overload and fibrosis, AAV9-CMV-iCre induced Cul7-/- mice displayed a ~50% reduction of interstitial cardiac fibrosis when compared to Cul7+/+ animals (6.7% vs. 3.4%, p<0.01). Similar results were obtained with Cul7^flox/floxMyh6-Mer-Cre-Mer^Tg(1/0) mice which displayed a ~30% reduction of cardiac fibrosis after TAC when compared to Cul7^+/+Myh6-Mer-Cre-Mer^Tg(1/0) controls after TAC surgery (12.4% vs. 8.7%, p<0.05). No hemodynamic alterations were observed. AKT^Ser473 phosphorylation was increased 3-fold (p<0.01) in Cul7-/- vs. control mice, together with a ~78% (p<0.001) reduction of TUNEL-positive apoptotic cells three weeks after TAC. In addition, CM-specific expression of a dominant-negative CUL7^1152stop mutant resulted in a 16.3-fold decrease (p<0.001) of in situ end-labelling (ISEL) positive apoptotic cells. Collectively, our data demonstrate that CM-specific ablation of Cul7 restrains myocardial fibrosis and apoptosis upon pressure overload, and introduce CRL7 as a potential target for anti-fibrotic therapeutic strategies of the heart.

Age-related copy number variations and expression levels of F-box protein FBXL20 predict ovarian cancer prognosis

About 70% of ovarian cancer (OvCa) cases are diagnosed at advanced stages (stage III/IV) with only 20–40% of them survive over 5 years after diagnosis. A reliably screening marker could enable a paradigm shift in OvCa early diagnosis and risk stratification. Age is one of the most significant risk factors for OvCa. Older women have much higher rates of OvCa diagnosis and poorer clinical outcomes. In this article, we studied the correlation between aging and genetic alterations in The Cancer Genome Atlas Ovarian Cancer dataset. We demonstrated that copy number variations (CNVs) and expression levels of the F-Box and Leucine-Rich Repeat Protein 20 (FBXL20), a substrate recognizing protein in the SKP1-Cullin1-F-box-protein E3 ligase, can predict OvCa overall survival, disease-free survival and progression-free survival. More importantly, FBXL20 copy number loss predicts the diagnosis of OvCa at a younger age, with over 60% of patients in that subgroup have OvCa diagnosed at age less than 60 years. Clinicopathological studies further demonstrated malignant histological and radiographical features associated with elevated FBXL20 expression levels. This study has thus identified a potential biomarker for OvCa prognosis.

The Insulin Receptor Adaptor IRS2 is an APC/C Substrate That Promotes Cell Cycle Protein Expression and a Robust Spindle Assembly Checkpoint

Insulin receptor substrate 2 (IRS2) is an essential adaptor that mediates signaling downstream of the insulin receptor and other receptor tyrosine kinases. Transduction through IRS2-dependent pathways is important for coordinating metabolic homeostasis, and dysregulation of IRS2 causes systemic insulin signaling defects. Despite the importance of maintaining proper IRS2 abundance, little is known about what factors mediate its protein stability. We conducted an unbiased proteomic screen to uncover novel substrates of the Anaphase Promoting Complex/Cyclosome (APC/C), a ubiquitin ligase that controls the abundance of key cell cycle regulators. We found that IRS2 levels are regulated by APC/C activity and that IRS2 is a direct APC/C target in G1 Consistent with the APC/C's role in degrading cell cycle regulators, quantitative proteomic analysis of IRS2-null cells revealed a deficiency in proteins involved in cell cycle progression. We further show that cells lacking IRS2 display a weakened spindle assembly checkpoint in cells treated with microtubule inhibitors. Together, these findings reveal a new pathway for IRS2 turnover and indicate that IRS2 is a component of the cell cycle control system in addition to acting as an essential metabolic regulator.

Cullin-RING E3 Ubiquitin Ligase 7 in Growth Control and Cancer

CRL7^Fbxw8 is an E3 ubiquitin ligase complex, containing cullin7 (CUL7) as a scaffold, the F-box protein Fbxw8 as a substrate receptor, the Skp1 adaptor, and the ROC1/Rbx1 RING finger protein for working with E2 enzyme to facilitate ubiquitin transfer. This chapter provides an update on studies linking CRL7^Fbxw8 to hereditary human growth retardation disease, as at least 64 cul7 germ line mutations were found in patients with autosomal recessive 3-M syndrome. CRL7^Fbxw8 interacts with two additional 3-M associated proteins OBSL1 and CCDC8, leading to subcellular localization of the E3 complex to regions including plasma membrane, centrosome, and Golgi. At least ten mammalian cellular proteins were identified or implicated as CRL7^Fbxw8 substrates. Discussion focuses on the possible impact of CRL7^Fbxw8-mediated proteolytic or non-proteolytic pathways in growth control and cancer.

Regulation of liver receptor homologue-1 by DDB2 E3 ligase activity is critical for hepatic glucose metabolism

Liver receptor homologue-1 (LRH-1) plays a critical role in hepatic metabolism and disease. Here we show that LRH-1 protein stability is regulated by the cullin 4 (CUL4) E3 ubiquitin ligase complex. We found that DNA damage-binding protein 2 (DDB2) directly interacts with LRH-1 and functions as a substrate recognition component of CUL4-DDB1 to promote LRH-1 ubiquitination and proteasomal degradation. In human hepatoma (HepG2) cells, we observed that protein levels of endogenous LRH-1 are increased by insulin without a change in mRNA levels of LRH-1. However, overexpression of DDB2 impaired the insulin-stimulated increase in LRH-1 levels. In addition, DDB2 overexpression decreased LRH-1 transcriptional activation and expression of target genes, such as glucokinase, whereas knockdown of DDB2 increased the expression of glucokinase. Finally, we demonstrated that DDB2 knockdown increases glucose uptake and intracellular levels of glucose-6-phosphate in HepG2 cells. Our study reveals a novel regulatory mechanism of LRH-1 activity and suggests a role for DDB2 in hepatic glucose metabolism.

CUL7 promotes cancer cell survival through promoting Caspase-8 ubiquitination

The Cullin 7 (CUL7) gene encodes a member of the cullin family of E3 ubiquitin ligases. Accumulated evidence suggests that CUL7 is oncogenic. However, the mechanism by which CUL7 improves cancer cell survival has not been fully elucidated. Here, we reported that CUL7 confers anti-apoptotic functions by interacting with Caspase-8. CUL7 prevents Caspase-8 activation by promoting Caspase-8 modification with non-degradative polyubiquitin chains at K215. CUL7 knockdown sensitized cancer cells to TRAIL-induced apoptosis in vitro and in nude mice. These results suggest that CUL7 limits extrinsic apoptotic signaling by promoting Caspase-8 ubiquitination.

Inhibition of C1-Ten PTPase activity reduces insulin resistance through IRS-1 and AMPK pathways

Insulin resistance causes type 2 diabetes; therefore, increasing insulin sensitivity is a therapeutic approach against type 2 diabetes. Activating AMP-activated protein kinase (AMPK) is an effective approach for treating diabetes, and reduced insulin receptor substrate-1 (IRS-1) protein levels have been suggested as a molecular mechanism causing insulin resistance. Thus, dual targeting of AMPK and IRS-1 might provide an ideal way to treat diabetes. We found that 15,16-dihydrotanshinone I (DHTS), as a C1-Ten protein tyrosine phosphatase inhibitor, increased IRS-1 stability, improved glucose tolerance and reduced muscle atrophy. Identification of DHTS as a C1-Ten inhibitor revealed a new function of C1-Ten in AMPK inhibition, possibly through regulation of IRS-1. These findings suggest that C1-Ten inhibition by DHTS could provide a novel therapeutic strategy for insulin resistance-associated metabolic syndrome through dual targeting of IRS-1 and AMPK.

Whole-Body Vibration Mimics the Metabolic Effects of Exercise in Male Leptin Receptor-Deficient Mice

Whole-body vibration (WBV) has gained attention as a potential exercise mimetic, but direct comparisons with the metabolic effects of exercise are scarce. To determine whether WBV recapitulates the metabolic and osteogenic effects of physical activity, we exposed male wild-type (WT) and leptin receptor-deficient (db/db) mice to daily treadmill exercise (TE) or WBV for 3 months. Body weights were analyzed and compared with WT and db/db mice that remained sedentary. Glucose and insulin tolerance testing revealed comparable attenuation of hyperglycemia and insulin resistance in db/db mice following TE or WBV. Both interventions reduced body weight in db/db mice and normalized muscle fiber diameter. TE or WBV also attenuated adipocyte hypertrophy in visceral adipose tissue and reduced hepatic lipid content in db/db mice. Although the effects of leptin receptor deficiency on cortical bone structure were not eliminated by either intervention, exercise and WBV increased circulating levels of osteocalcin in db/db mice. In the context of increased serum osteocalcin, the modest effects of TE and WBV on bone geometry, mineralization, and biomechanics may reflect subtle increases in osteoblast activity in multiple areas of the skeleton. Taken together, these observations indicate that WBV recapitulates the effects of exercise on metabolism in type 2 diabetes.

Role of E3 ubiquitin ligases in insulin resistance

E3 ubiquitin ligases are a large family of proteins that catalyse the ubiquitination of many proteins for degradation by the 26S proteasome. E3 ubiquitin ligases play pivotal roles in the process of insulin resistance and diabetes. In this review, we summarize the currently available studies to analyse the potential role of E3 ubiquitin ligases in the development of insulin resistance. We propose two mechanisms by which E3 ubiquitin ligases can affect the process of insulin resistance. First, E3 ubiquitin ligases directly degrade the insulin receptor, insulin receptor substrate and other key insulin signalling molecules via the UPS. Second, E3 ubiquitin ligases indirectly regulate insulin signalling by regulating pro-inflammatory mediators that are involved in the regulation of insulin signalling molecules, such as tumour necrosis factor-α, interleukin (IL)-6, IL-4, IL-13, IL-1β, monocyte chemoattractant protein-1 and hypoxia-inducible factor 1α. Determining the mechanism by which E3 ubiquitin ligases affect the development of insulin resistance can identify a novel strategy to protect against insulin resistance and diabetes.

Asparaginase treatment side-effects may be due to genes with homopolymeric Asn codons (Review-Hypothesis)

The present treatment of childhood T-cell leukemias involves the systemic administration of prokary-otic L-asparaginase (ASNase), which depletes plasma Asparagine (Asn) and inhibits protein synthesis. The mechanism of therapeutic action of ASNase is poorly understood, as are the etiologies of the side-effects incurred by treatment. Protein expression from genes bearing Asn homopolymeric coding regions (N-hCR) may be particularly susceptible to Asn level fluctuation. In mammals, N-hCR are rare, short and conserved. In humans, misfunctions of genes encoding N-hCR are associated with a cluster of disorders that mimic ASNase therapy side-effects which include impaired glycemic control, dislipidemia, pancreatitis, compromised vascular integrity, and neurological dysfunction. This paper proposes that dysregulation of Asn homeostasis, potentially even by ASNase produced by the microbiome, may contribute to several clinically important syndromes by altering expression of N-hCR bearing genes. By altering amino acid abundance and modulating ribosome translocation rates at codon repeats, the microbiomic environment may contribute to genome decoding and to shaping the proteome. We suggest that impaired translation at poly Asn codons elevates diabetes risk and severity.

The Novel Functions of High-Molecular-Mass Complexes Containing Insulin Receptor Substrates in Mediation and Modulation of Insulin-Like Activities: Emerging Concept of Diverse Functions by IRS-Associated Proteins

Insulin-like peptides, such as insulin-like growth factors (IGFs) and insulin, induce a variety of bioactivities, such as growth, differentiation, survival, increased anabolism, and decreased catabolism in many cell types and in vivo. In general, IGFs or insulin bind to IGF-I receptor (IGF-IR) or insulin receptor (IR), activating the receptor tyrosine kinase. Insulin receptor substrates (IRSs) are known to be major substrates of receptor kinases, mediating IGF/insulin signals to direct bioactivities. Recently, we discovered that IRSs form high-molecular-mass complexes (referred to here as IRSomes) even without IGF/insulin stimulation. These complexes contain proteins (referred to here as IRSAPs; IRS-associated proteins), which modulate tyrosine phosphorylation of IRSs by receptor kinases, control IRS stability, and determine intracellular localization of IRSs. In addition, in these complexes, we found not only proteins that are involved in RNA metabolism but also RNAs themselves. Thus, IRSAPs possibly contribute to modulation of IGF/insulin bioactivities. Since it is established that disorder of modulation of insulin-like activities causes various age-related diseases including cancer, we could propose that the IRSome is an important target for treatment of these diseases.

Insulin and metabolic stress stimulate multisite serine/threonine phosphorylation of insulin receptor substrate 1 and inhibit tyrosine phosphorylation

IRS1 and IRS2 are key substrates of the insulin receptor tyrosine kinase. Mass spectrometry reveals more than 50 phosphorylated IRS1 serine and threonine residues (Ser(P)/Thr(P) residues) in IRS1 from insulin-stimulated cells or human tissues. We investigated a subset of IRS1 Ser(P)/Thr(P) residues using a newly developed panel of 25 phospho-specific monoclonal antibodies (αpS/TmAb(Irs1)). CHO cells overexpressing the human insulin receptor and rat IRS1 were stimulated with insulin in the absence or presence of inhibitors of the PI3K → Akt → mechanistic target of rapamycin (mTOR) → S6 kinase or MEK pathways. Nearly all IRS1 Ser(P)/Thr(P) residues were stimulated by insulin and significantly suppressed by PI3K inhibition; fewer were suppressed by Akt or mTOR inhibition, and none were suppressed by MEK inhibition. Insulin-stimulated Irs1 tyrosine phosphorylation (Tyr(P)(Irs1)) was enhanced by inhibition of the PI3K → Akt → mTOR pathway and correlated with decreased Ser(P)-302(Irs1), Ser(P)-307(Irs1), Ser(P)-318(Irs1), Ser(P)-325(Irs1), and Ser(P)-346(Irs1). Metabolic stress modeled by anisomycin, thapsigargin, or tunicamycin increased many of the same Ser(P)/Thr(P) residues as insulin, some of which (Ser(P)-302(Irs1), Ser(P)-307(Irs1), and four others) correlated significantly with impaired insulin-stimulated Tyr(P)(Irs1). Thus, IRS1 Ser(P)/Thr(P) is an integrated response to insulin stimulation and metabolic stress, which associates with reduced Tyr(P)(Irs1) in CHO(IR)/IRS1 cells.

Inhibition of Cullin-RING E3 ubiquitin ligase 7 by simian virus 40 large T antigen

Simian virus 40 (SV40) large tumor antigen (LT) triggers oncogenic transformation by inhibition of key tumor suppressor proteins, including p53 and members of the retinoblastoma family. In addition, SV40 transformation requires binding of LT to Cullin 7 (CUL7), a core component of Cullin-RING E3 ubiquitin ligase 7 (CRL7). However, the pathomechanistic effects of LT-CUL7 interaction are mostly unknown. Here we report both in vitro and in vivo experimental evidence that SV40 LT suppresses the ubiquitin ligase function of CRL7. We show that SV40 LT, but not a CUL7 binding-deficient mutant (LT(Δ69-83)), impaired 26S proteasome-dependent proteolysis of the CRL7 target protein insulin receptor substrate 1 (IRS1), a component of the insulin and insulin-like growth factor 1 signaling pathway. SV40 LT expression resulted in the accumulation and prolonged half-life of IRS1. In vitro, purified SV40 LT reduced CRL7-dependent IRS1 ubiquitination in a concentration-dependent manner. Expression of SV40 LT, or depletion of CUL7 by RNA interference, resulted in the enhanced activation of IRS1 downstream signaling pathways phosphatidylinositol-3-kinase/AKT and Erk mitogen-activated pathway kinase, as well as up-regulation of the downstream target gene c-fos. Finally, SV40 LT-positive carcinoma of carcinoembryonic antigen 424/SV40 LT transgenic mice displayed elevated IRS1 protein levels and activation of downstream signaling. Taken together, these data suggest that SV40 LT protects IRS1 from CRL7-mediated degradation, thereby sustaining high levels of promitogenic IRS1 downstream signaling pathways.

Chromium-Insulin Reduces Insulin Clearance and Enhances Insulin Signaling by Suppressing Hepatic Insulin-Degrading Enzyme and Proteasome Protein Expression in KKAy Mice

JDS-chromium-insulin (CRI)-003 is a novel form of insulin that has been directly conjugated with chromium (Cr) instead of zinc. Our hypothesis was that CRI enhances insulin's effects by altering insulin-degrading enzyme (IDE) and proteasome enzymes. To test this hypothesis, we measured hepatic IDE content and proteasome parameters in a diabetic animal model. Male KKAy mice were randomly divided into three groups (n = 8/group); Sham (saline), human regular insulin (Reg-In), and chromium conjugated human insulin (CRI), respectively. Interventions were initiated at doses of 2 U insulin/kg body weight daily for 8-weeks. Plasma glucose and insulin were measured. Hepatic IDE, proteasome, and insulin signaling proteins were determined by western blotting. Insulin tolerance tests at week 7 showed that both insulin treatments significantly reduced glucose concentrations and increased insulin levels compared with the Sham group, CRI significantly reduced glucose at 4 and 6 h relative to Reg-In (P < 0.05), suggesting the effects of CRI on reducing glucose last longer than Reg-In. CRI treatment significantly increased hepatic IRS-1 and Akt1 and reduced IDE, 20S as well as 19S protein abundance (P < 0.01, P < 0.05, and P < 0.001, respectively), but Reg-In only significantly increased Akt1 (P < 0.05). Similar results were also observed in Reg-In- and CRI-treated HepG2 cells. This study, for the first time, demonstrates that CRI reduces plasma insulin clearance by inhibition of hepatic IDE protein expression and enhances insulin signaling as well as prevents degradation of IRS-1 and IRS-2 by suppressing ubiquitin-proteasome pathway in diabetic mice.