Integrin αvß3 Upregulation in Response to Nutrient Stress Promotes Lung Cancer Cell Metabolic Plasticity.

Cancer stem/tumor-initiating cells display stress tolerance and metabolic flexibility to survive in a harsh environment with limited nutrient and oxygen availability. The molecular mechanisms underlying this phenomenon could provide targets to prevent metabolic adaptation and halt cancer progression. Here, we showed in cultured cells and live human surgical biopsies of non-small cell lung cancer that nutrient stress drives the expression of the epithelial cancer stem cell marker integrin αvß3 via upregulation of the ß3 subunit, resulting in a metabolic reprogramming cascade that allows tumor cells to thrive despite a nutrient-limiting environment. Although nutrient deprivation is known to promote acute, yet transient, activation of the stress sensor AMP-activated protein kinase (AMPK), stress-induced αvß3 expression via Src activation unexpectedly led to secondary and sustained AMPK activation. This resulted in the nuclear localization of peroxisome proliferator-activated receptor-gamma coactivator 1α (PGC1α) and upregulation of glutamine metabolism, the tricarboxylic acid cycle, and oxidative phosphorylation. Pharmacological or genetic targeting of this axis prevented lung cancer cells from evading the effects of nutrient stress, thereby blocking tumor initiation in mice following orthotopic implantation of lung cancer cells. These findings reveal a molecular pathway driven by nutrient stress that results in cancer stem cell reprogramming to promote metabolic flexibility and tumor initiation. SIGNIFICANCE: Upregulation of integrin αvß3, a cancer stem cell marker, in response to nutrient stress activates sustained AMPK/PGC1α signaling that induces metabolic reprogramming in lung cancer cells to support their survival. See related commentary by Rainero, p. 1543.

The association between previous use of anti-obesity medication and semaglutide weight loss outcomes.

AIMS: To compare weight loss outcomes between patients starting semaglutide who had previously been on another anti-obesity medication (AOM) compared to those who were AOM-naïve. MATERIALS AND METHODS: We performed a retrospective study in patients with overweight or obesity taking semaglutide for weight loss for a duration of 3 to 12 months. Our primary endpoint was assessment of percentage of total body weight loss (TBWL) in patients who started semaglutide as their first AOM (AOM-naïve) compared to those who started semaglutide and had previously taken another AOM (non-AOM-naïve). The secondary outcome was a comparison of the proportions of patients achieving ≥5%, ≥10%, ≥15% and ≥20% TBWL between the groups. Our endpoints were analysed using independent t-tests and ANOVA/ANCOVA for continuous variables and Pearson's test for categorical variables. RESULTS: This study included 305 patients. Outcomes of semaglutide treatment were superior in AOM-naïve patients (n = 231) compared to non-AOM-naïve patients (n = 74) at 3 (6.3% vs. 3.8%), 6 (10.6% vs. 6.7%), 9 (14.0% vs. 9.1%) and 12 months (14.3% vs. 10.6%; p < 0.0001 at 3, 6 and 9 months, and p = 0.01 at 12 months). A greater proportion of patients in the AOM-naïve group achieved a TBWL ≥ 15% (48% vs 21%; p = 0.02) and ≥20% (27% vs 4% p < 0.01) at 12 months. CONCLUSION: The use of semaglutide in patients with previous intake of other AOMs was associated with inferior weight loss outcomes in comparison to patients who were AOM-naïve.

Weight loss and cardiovascular disease risk outcomes of semaglutide: a one-year multicentered study.

BACKGROUND/OBJECTIVE: There are limited real-world studies assessing semaglutide weight loss and associated comorbidity and metabolic outcomes over periods ≥ 6 months. We aim to assess weight loss, metabolic, and cardiovascular outcomes of 12 months of semaglutide. SUBJECT/METHODS: We conducted a multicentered retrospective cohort study on semaglutide use. We included patients with a body-mass index (BMI) ≥ 27 kg/m2 who were prescribed weekly semaglutide subcutaneous injections. We excluded patients with bariatric surgeries, taking other anti-obesity medications, and with active malignancy or pregnancy. A total of 1023 patients had semaglutide prescription for obesity. INTERVENTION/METHODS: We assessed weight loss outcomes of subcutaneous semaglutide for 12 months. The primary endpoint was total body weight loss percentage (TBWL%) at 12 months. Secondary endpoints included proportion of patients achieving ≥5%, ≥10%, ≥15%, and ≥20% weight loss, and improvements in metabolic, cardiovascular, and comorbidities after 12 months of follow-up. RESULTS: We included 304 patients (73% female, 93% White, mean age 48.8 [12.4] years, BMI 40.9 [9.6] kg/m2) in the analysis. Patients achieved a TBWL of 13.4 (8.0)% at 12 months (p < 0.001 from baseline). Patients without T2DM achieved a TBWL of 16.9 (6.9)% compared to 9.9 (8.4)% in patients without T2DM at 12 months on the higher doses of semaglutide (p < 0.001 from baseline). In this cohort, 81% achieved ≥5%, 64% achieved ≥10%, 41% achieved ≥15%, and 22% achieved ≥20% TBWL at 12 months. Patients with overweight or obesity experienced significant improvements in metabolic, lipid profile, blood pressure, liver function tests, and cardiovascular disease risk outcomes. CONCLUSIONS: Semaglutide demonstrated notable improvement in obesity, metabolic, and cardiovascular disease risk outcomes in a clinical setting.

Type 2 Diabetes Remission in Patients with Heterozygous Variants in the Leptin-Melanocortin Pathway after Roux-en-Y Gastric Bypass: A Matched Case-Control Study.

BACKGROUND: Roux-en-Y gastric bypass (RYGB) is associated with a high rate of type 2 diabetes (T2D) remission. Carriers of heterozygous variants in the leptin-melanocortin pathway (LMP) are more likely to experience weight recurrence after RYGB. Our aim was to investigate if carrier status and associated weight regain affects the rate of T2D remission after RYGB. METHODS: Carriers of LMP variants with a diagnosis of T2D prior to RYGB (N = 16) were matched to non-carriers (N = 32) based on sex, age, and BMI. We assessed for post-operative T2D remission status post-surgery on a yearly basis, for up to 15 years. Our primary endpoint was the proportion of patients achieving T2D remission at 1 year. We conducted a survival analysis for all patients that achieved remission at least at one time-point to evaluate for maintenance of T2D remission by using a log-rank test. RESULTS: Both carriers and non-carriers had similar baseline and procedural characteristics. The proopiomelanocortin gene in the LMP pathway had the most variants (n = 5, 31%). Carriers had a lower total body weight loss percentage at nadir (28.7% ± 6.9) than non-carriers (33.7% ± 8.8, p = 0.04). The proportion of patients achieving T2D remission at 1 year was 68.8% for carriers and 71.9% for non-carriers (p = 1.0). Survival curves for maintenance of first remission were similar for both groups (p = 0.73), with a median survival of 8 years for both carriers and non-carriers. CONCLUSIONS: Despite inferior weight loss outcomes at nadir, carriers had similar T2D remission rates when compared to non-carriers. Weight-independent metabolic benefits of RYGB might contribute to this observation.

Pancreatic cancer cells upregulate LPAR4 in response to isolation stress to promote an ECM-enriched niche and support tumour initiation.

Defining drivers of tumour initiation can provide opportunities to control cancer progression. Here we report that lysophosphatidic acid receptor 4 (LPAR4) becomes transiently upregulated on pancreatic cancer cells exposed to environmental stress or chemotherapy where it promotes stress tolerance, drug resistance, self-renewal and tumour initiation. Pancreatic cancer cells gain LPAR4 expression in response to stress by downregulating a tumour suppressor, miR-139-5p. Even in the absence of exogenous lysophosphatidic acid, LPAR4-expressing tumour cells display an enrichment of extracellular matrix genes that are established drivers of cancer stemness. Mechanistically, upregulation of fibronectin via an LPAR4/AKT/CREB axis is indispensable for LPAR4-induced tumour initiation and stress tolerance. Moreover, ligation of this fibronectin-containing matrix via integrins α5ß1 or αVß3 can transfer stress tolerance to LPAR4-negative cells. Therefore, stress- or drug-induced LPAR4 enhances cell-autonomous production of a fibronectin-rich extracellular matrix, allowing cells to survive 'isolation stress' and compensate for the absence of stromal-derived factors by creating their own tumour-initiating niche.

Cardiovascular Risk and Diseases in Patients With and Without Leptin-Melanocortin Pathway Variants.

OBJECTIVE: To study differences in cardiovascular risk factors and diseases between patients with and without genetic variants in the leptin-melanocortin pathway. METHODS: A cross-sectional study of patients with a history of severe obesity genotyped in June 2019 as participants of the Mayo Clinic Biobank was conducted in March 2022 to assess differences in cardiovascular risk and diseases between carriers of a heterozygous variant in the leptin-melanocortin pathway and noncarriers. Cardiovascular risk factors included hypertension, diabetes, dyslipidemia, and smoking. Cardiovascular disease includes coronary artery disease, peripheral artery disease, and cerebrovascular accidents. Patients with a history of bariatric surgery were excluded. We used logistic regression models to estimate the odds ratio and 95% CI, adjusting for age, body mass index (BMI), and sex. RESULTS: Among a total of 168 carriers (8%; 121 [72%] female; mean [SD] age, 65.1 [14.9] years; BMI, 44.0 [7.4] kg/m2) and 2039 noncarriers (92%; 1446 [71%] female; mean [SD] age, 64.9 [14.4] years; BMI, 42.9 [6.6] kg/m2), carriers had higher prevalence odds of hypertension (odds ratio, 3.26; 95% CI, 2.31 to 4.61; P<.001) and reported higher number of cardiovascular risk factors compared with noncarriers (2.4 [1.1] vs 2.0 [1.1]; P<.001). There were no significant differences in the adjusted odds associated with diabetes, dyslipidemia, smoking, or cardiovascular disease. CONCLUSION: Despite having similar body weight and BMI, carriers of heterozygous variants in the leptin-melanocortin pathway had higher rates of hypertension than noncarriers. These findings point to an association between hypertension and leptin-melanocortin pathway variants.

Weight Loss Outcomes Associated With Semaglutide Treatment for Patients With Overweight or Obesity.

Importance: No retrospective cohort study has assessed the effectiveness of semaglutide at doses used in randomized clinical trials to treat obesity (ie, 1.7 and 2.4 mg). Objective: To study weight loss outcomes associated with semaglutide treatment at doses used in randomized clinical trials for patients with overweight or obesity. Design, Setting, and Participants: This cohort study, conducted at a referral center for weight management, retrospectively collected data on the use of semaglutide for adults with overweight or obesity between January 1, 2021, and March 15, 2022, with a follow-up of up to 6 months. A total of 408 patients with a body mass index (BMI) of 27 or more were prescribed weekly semaglutide subcutaneous injections for 3 months or more. Patients with a history of bariatric procedures, taking other antiobesity medications, and with an active malignant neoplasm were excluded. Exposures: Weekly 1.7-mg or 2.4-mg semaglutide subcutaneous injections for 3 to 6 months. Main Outcomes and Measures: The primary end point was the percentage of weight loss. Secondary end points were the proportion of patients achieving weight loss of 5% or more, 10% or more, 15% or more, and 20% or more after 3 and 6 months and the percentage of weight loss for patients with or without type 2 diabetes after 3 and 6 months. Results: The study included 175 patients (132 women [75.4%]; mean [SD] age, 49.3 [12.5] years; mean [SD] BMI, 41.3 [9.1]) in the analysis at 3 months and 102 patients at 6 months. The mean (SD) weight loss after 3 months was 6.7 (4.4) kg, equivalent to a mean (SD) weight loss of 5.9% (3.7%) (P < .001), and the mean (SD) weight loss after 6 months was 12.3 (6.6) kg, equivalent to a mean (SD) weight loss of 10.9% (5.8%) (P < .001 from baseline). Of the 102 patients who were followed up at 6 months, 89 (87.3%) achieved weight loss of 5% or more, 56 (54.9%) achieved weight loss of 10% or more, 24 (23.5%) achieved weight loss of 15% or more, and 8 (7.8%) achieved weight loss of 20% or more. Patients with type 2 diabetes had a lower mean (SD) percentage weight loss at 3 and 6 months compared with those without type 2 diabetes: 3.9% (3.1%) vs 6.3% (3.7%) at 3 months (P = .001) and 7.2% (6.3%) vs 11.8% (5.3%) at 6 months (P = .005). Conclusions and Relevance: The results of this cohort study suggest that weekly 1.7-mg and 2.4-mg doses of semaglutide were associated with weight loss similar to that seen in randomized clinical trials. Studies with longer periods of follow-up are needed to evaluate prolonged weight loss outcomes.

Effects of Heterozygous Variants in the Leptin-Melanocortin Pathway on Roux-en-Y Gastric Bypass Outcomes: a 15-Year Case-Control Study.

INTRODUCTION: Heterozygous variants in the leptin-melanocortin pathway are associated with obesity. However, their effect on the long-term outcomes after Roux-en-Y gastric bypass (RYGB) is still unknown. METHODS: In this matched case-control study, 701 participants from the Mayo Clinic Biobank with a history of RYGB were genotyped. Sixty-three patients had a heterozygous variant in the leptin-melanocortin pathway. After excluding patients with potential confounders, carriers were randomly matched (on sex, age, body mass index [BMI], and years since surgery) with two non-carrier controls. The electronic medical record of carriers and matched non-carriers was reviewed for up to 15 years after RYGB. RESULTS: A total of 50 carriers and 100 matched non-carriers with a history of RYGB were included in the study. Seven different genes (LEPR, PCSK1, POMC, SH2B1, SRC1, MC4R, and SIM1) in the leptin-melanocortin pathway were identified. At the time of surgery, the mean age was 50.8 ± 10.6 years, BMI 45.6 ± 7.3 kg/m2, and 79% women. There were no differences in postoperative years of follow-up, Roux limb length, or gastric pouch size between groups. Fifteen years after RYGB, the percentage of total body weight loss (%TBWL) in carriers was - 16.6 ± 10.7 compared with - 28.7 ± 12.9 in non-carriers (diff = 12.1%; 95% CI, 4.8 to 19.3) and the percentage of weight regain after maximum weight loss was 52.7 ± 29.7 in carriers compared with 29.8 ± 20.7 in non-carriers (diff = 22.9%; 95% CI, 5.3 to 40.5). The nadir %TBWL was lower - 32.1 ± 8.1 in carriers compared with - 36.8 ± 10.4 in non-carriers (diff = 4.8%; 95% CI 1.8 to 7.8). CONCLUSIONS: Carriers of a heterozygous variant in the leptin-melanocortin pathway have a progressive and significant weight regain in the mid- and long-term after RYGB. Genotyping patients experiencing significant weight regain after RYGB could help implement multidisciplinary and individualized weight loss interventions to improve weight maintenance after surgery.

[Infectious and inflammatory gastrointestinal manifestations of chronic granulomatous disease]. / Manifestaciones gastrointestinales inflamatorias e infecciosas de la enfermedad granulomatosa crónica.

Chronic granulomatous disease (CGD) is an inborn error of immunity caused by a defect in one of the components of the NADPH oxidase complex, which is responsible for generating reactive oxygen species (ROS) during the respiratory burst in phagocytes. The absence of ROS produced by NADPH oxidase in neutrophils and in macrophages leads to greater susceptibility to certain bacterial and fungal infections, and also to inflammatory manifestations due to a deregulated inflammatory response, which suggests that the ability to adequately regulate inflammatory signaling depends on ROS produced by NADPH oxidase. The disease course in patients with X-linked CGD is more severe, with recurrent invasive infections; in contrast, patients with non-classic CGD do not present invasive bacterial or fungal infections, but have more prominent inflammatory manifestations. The most frequent gastrointestinal manifestations are stomatitis, gingivitis, chronic diarrhea, liver abscesses that are similar to inflammatory bowel disease (IBD), and granulomas that can cause obstruction or stenosis in the esophagus, stomach or intestine. It has been observed that the deficiency of p40phox and ROS (non-classic CGD) are associated with greater susceptibility to colitis and the development of severe inflammation; therefore, it is presented that these proteins participate in the resolution of inflammation. In general, the inflammatory findings in CGD, including gastrointestinal manifestations, are seldom described. In international cohorts, manifestations that are similar to IBD are reported in up to 58% of patients with CGD; however, in the only Mexican cohort, its finding is described in only 4 out of 93 patients (4.3%). In this review, we summarize the gastrointestinal clinical findings of CGD, including infectious and inflammatory manifestations, emphasizing on the latter.

La enfermedad granulomatosa crónica (EGC) es un error innato de la inmunidad causado por un defecto en uno de los componentes del complejo NADPH oxidasa, responsable de generar especies reactivas de oxígeno (ERO) durante el estallido respiratorio en los fagocitos. La ausencia de ERO producidos por la NADPH oxidasa en los neutrófilos y en los macrófagos produce mayor susceptibilidad a infecciones bacterianas y fúngicas, además de manifestaciones inflamatorias por una respuesta inflamatoria desregulada, lo que sugiere que la capacidad para regular adecuadamente la señalización inflamatoria depende de las ERO derivadas de la NADPH oxidasa. Los pacientes con EGC ligada al cromosoma X tienen un curso de enfermedad más grave con infecciones invasivas recurrentes, a diferencia de los pacientes con EGC no clásica, quienes no presentan infecciones bacterianas o fúngicas invasivas, pero con manifestaciones inflamatorias más prominentes. Las manifestaciones gastrointestinales más frecuentes son estomatitis, gingivitis, diarrea crónica, abscesos hepáticos, similares a las de la enfermedad inflamatoria intestinal (EII) y granulomas, que pueden provocar obstrucción o estenosis en esófago, estómago o intestino. Se ha observado que la deficiencia de p40phox y EROS (EGC no clásica) se asocia a mayor susceptibilidad a colitis y al desarrollo de inflamación severa, por lo que se plantea que estas proteínas participan en la resolución de la inflamación. En general, los hallazgos inflamatorios en la EGC, incluyendo los gastrointestinales, han sido poco descritos. En las cohortes internacionales se reportan manifestaciones similares a EII hasta en 58 % de los pacientes con EGC; en cambio, en la única cohorte mexicana se describe su hallazgo solo en cuatro de 93 pacientes (4.3 %). En esta revisión resumimos los hallazgos clínicos gastrointestinales de la EGC, incluidas las manifestaciones infecciosas e inflamatorias, con énfasis en las últimas.

ERK5 signalling rescues intestinal epithelial turnover and tumour cell proliferation upon ERK1/2 abrogation.

The ERK1/2 MAPK signalling module integrates extracellular cues that induce proliferation and differentiation of epithelial lineages, and is an established oncogenic driver, particularly in the intestine. However, the interrelation of the ERK1/2 module relative to other signalling pathways in intestinal epithelial cells and colorectal cancer (CRC) is unclear. Here we show that loss of Erk1/2 in intestinal epithelial cells results in defects in nutrient absorption, epithelial cell migration and secretory cell differentiation. However, intestinal epithelial cell proliferation is not impeded, implying compensatory mechanisms. Genetic deletion of Erk1/2 or pharmacological targeting of MEK1/2 results in supraphysiological activity of the ERK5 pathway. Furthermore, targeting both pathways causes a more effective suppression of cell proliferation in murine intestinal organoids and human CRC lines. These results suggest that ERK5 provides a common bypass route in intestinal epithelial cells, which rescues cell proliferation upon abrogation of ERK1/2 signalling, with therapeutic implications in CRC.

Deletion of TAK1 in the myeloid lineage results in the spontaneous development of myelomonocytic leukemia in mice.

Previous studies of the conditional ablation of TGF-ß activated kinase 1 (TAK1) in mice indicate that TAK1 has an obligatory role in the survival and/or development of hematopoietic stem cells, B cells, T cells, hepatocytes, intestinal epithelial cells, keratinocytes, and various tissues, primarily because of these cells' increased apoptotic sensitivity, and have implicated TAK1 as a critical regulator of the NF-κB and stress kinase pathways and thus a key intermediary in cellular survival. Contrary to this understanding of TAK1's role, we report a mouse model in which TAK1 deletion in the myeloid compartment that evoked a clonal myelomonocytic cell expansion, splenomegaly, multi-organ infiltration, genomic instability, and aggressive, fatal myelomonocytic leukemia. Unlike in previous reports, simultaneous deletion of TNF receptor 1 (TNFR1) failed to rescue this severe phenotype. We found that the features of the disease in our mouse model resemble those of human chronic myelomonocytic leukemia (CMML) in its transformation to acute myeloid leukemia (AML). Consequently, we found TAK1 deletion in 13 of 30 AML patients (43%), thus providing direct genetic evidence of TAK1's role in leukemogenesis.

Critical role of monoubiquitination of histone H2AX protein in histone H2AX phosphorylation and DNA damage response.

DNA damage response is an important surveillance mechanism used to maintain the integrity of the human genome in response to genotoxic stress. Histone variant H2AX is a critical sensor that undergoes phosphorylation at serine 139 upon genotoxic stress, which provides a docking site to recruit the mediator of DNA damage checkpoint protein 1 (MDC1) and DNA repair protein complex to sites of DNA breaks for DNA repair. Here, we show that monoubiquitination of H2AX is induced upon DNA double strand breaks and plays a critical role in H2AX Ser-139 phosphorylation (γ-H2AX), in turn facilitating the recruitment of MDC1 to DNA damage foci. Mechanistically, we show that monoubiquitination of H2AX induced by RING finger protein 2 (RNF2) is required for the recruitment of active ataxia telangiectasia mutated to DNA damage foci, thus affecting the formation of γ-H2AX. Importantly, a defect in monoubiquitination of H2AX profoundly enhances ionizing radiation sensitivity. Our study therefore suggests that monoubiquitination of H2AX is an important step for DNA damage response and may have important clinical implications for the treatment of cancers.

The E3 ligase TRAF6 regulates Akt ubiquitination and activation.

Akt signaling plays a central role in many biological functions, such as cell proliferation and apoptosis. Because Akt (also known as protein kinase B) resides primarily in the cytosol, it is not known how these signaling molecules are recruited to the plasma membrane and subsequently activated by growth factor stimuli. We found that the protein kinase Akt undergoes lysine-63 chain ubiquitination, which is important for Akt membrane localization and phosphorylation. TRAF6 was found to be a direct E3 ligase for Akt and was essential for Akt ubiquitination, membrane recruitment, and phosphorylation upon growth-factor stimulation. The human cancer-associated Akt mutant displayed an increase in Akt ubiquitination, in turn contributing to the enhancement of Akt membrane localization and phosphorylation. Thus, Akt ubiquitination is an important step for oncogenic Akt activation.

E2 interaction and dimerization in the crystal structure of TRAF6.

Tumor necrosis factor (TNF) receptor-associated factor (TRAF)-6 mediates Lys63-linked polyubiquitination for NF-kappaB activation via its N-terminal RING and zinc finger domains. Here we report the crystal structures of TRAF6 and its complex with the ubiquitin-conjugating enzyme (E2) Ubc13. The RING and zinc fingers of TRAF6 assume a rigid, elongated structure. Interaction of TRAF6 with Ubc13 involves direct contacts of the RING and the preceding residues, and the first zinc finger has a structural role. Unexpectedly, this region of TRAF6 is dimeric both in the crystal and in solution, different from the trimeric C-terminal TRAF domain. Structure-based mutagenesis reveals that TRAF6 dimerization is crucial for polyubiquitin synthesis and autoubiquitination. Fluorescence resonance energy transfer analysis shows that TRAF6 dimerization induces higher-order oligomerization of full-length TRAF6. The mismatch of dimeric and trimeric symmetry may provide a mode of infinite oligomerization that facilitates ligand-dependent signal transduction of many immune receptors.

The RING domain and first zinc finger of TRAF6 coordinate signaling by interleukin-1, lipopolysaccharide, and RANKL.

TRAF6, a crucial adaptor molecule in innate and adaptive immunity, contains three distinct functional domains. The C-terminal TRAF domain facilitates oligomerization and sequence-specific interaction with receptors or other adaptor proteins. In conjunction with the dimeric E2 enzyme Ubc13-Uev1A, the N-terminal RING domain of TRAF6 functions as an E3 ubiquitin (Ub) ligase that facilitates its own site-specific ubiquitination through the generation of a Lys-63-linked poly-Ub chain. This modification does not cause its proteasomal degradation but rather serves as a scaffold to activate both the IKK and stress kinase pathways. Connecting the N-and C-terminal regions, the four internal zinc finger (ZF) motifs have yet to be functionally defined. In this study, we examined the role of the ZF domains in interleukin-1, lipopolysaccharide, and RANKL signaling by reconstitution of TRAF6-deficient cells with point mutations or deletions of these ZF motifs. Although ZF domains 2-4 are dispensable for activating IKK, p38, and JNK by interleukin-1 and lipopolysaccharide, the first ZF domain together with an intact RING domain of TRAF6 is essential for activating these pathways. Furthermore, TRAF6 autoubiquitination and its interaction with Ubc13 are dependent on ZF1 and an intact RING domain. Additionally, expression of TRAF6 lacking ZF2-4 in TRAF6-deficient monocytes rescues RANKL-mediated osteoclast differentiation and LPS-stimulated interleukin-6 production. These data provide evidence for the critical role of the Ub ligase activity of TRAF6, which is coordinated via the RING domain and ZF1 to supply the necessary elements in signaling by cytokines dependent upon TRAF6.

TRAF6 ubiquitin ligase is essential for RANKL signaling and osteoclast differentiation.

Tumor necrosis factor receptor-associated factor 6 (TRAF6), the crucial adaptor molecule of receptor activator of NF-kappaB (RANK), plays an essential role in governing the formation of multi-nucleated osteoclasts. TRAF6 is a RING-dependent ubiquitin (Ub) ligase that in conjunction with Ubc13/Uev1A catalyzes its own auto-ubiquitination via Lys63-linked poly-Ub chains. While the receptor-adaptor function of TRAF6 in RANK signaling is well understood, the significance of its Ub ligase activity in this process remains largely unknown. In this study, we show that retroviral expression of TRAF6, but not a RING mutant of TRAF6 was able to rescue TRAF6-deficient monocytes for the activation of IKK and osteoclast differentiation by RANKL. Furthermore, a catalytically inactive Ubc13 or stable knockdown of Ubc13 significantly prevents RANK-mediated TRAF6 ubiquitination and NF-kappaB and JNK activation. These data establish a signaling cascade in which regulated Lys63-linked TRAF6 auto-ubiquitination is the critical upstream mediator of osteoclast differentiation.

TRAF-interacting protein (TRIP) is a RING-dependent ubiquitin ligase.

TRAF-interacting protein (TRIP) was initially identified as a TRAF1- and TRAF2-binding partner that inhibited NF-kappaB activation without a known mechanism. Inspection of the TRIP sequence revealed an N-terminal RING domain, which is found in many E3 ubiquitin (Ub) ligases. We show that TRIP is a RING-dependent Ub ligase that undergoes auto-ubiquitination and requires an intact RING domain. Both TRIP and its RING mutant interact with TRAF1, 2, 3, 5, and 6, but failed to interact with CYLD and NIK. Stable expression of TRIP or a RING mutant did not affect IKK activation induced by TNF or IL-1 and had no affect on TNF-induced apoptosis. Similarly, RANKL-induced signaling and osteoclastogenesis were not affected by TRIP or its RING mutant. Interestingly, TRIP expression was down regulated during the late stages of osteoclastogenesis. Taken together, our results demonstrate that TRIP is a novel RING-dependent Ub ligase and a binding partner for TRAFs.

Site-specific Lys-63-linked tumor necrosis factor receptor-associated factor 6 auto-ubiquitination is a critical determinant of I kappa B kinase activation.

Tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) is a key mediator in proximal signaling of the interleukin-1/Toll-like receptor and the TNF receptor superfamily. Analysis of TRAF6-deficient mice revealed a fundamental role of TRAF6 in osteoclastogenesis; however, the molecular mechanism underlying TRAF6 signaling in this biological process is not understood. Recent biochemical evidence has indicated that TRAF6 possesses ubiquitin ligase activity that controls the activation of IKK and NF-kappaB. Because these studies are primarily based on cell-free systems, the role of the ubiquitin ligase activity of TRAF6 and its auto-ubiquitination to initiate the NF-kappaB pathway in vivo remain elusive. Here we show that an intact RING domain of TRAF6 in conjunction with the E2 enzyme Ubc13/Uev1A is necessary for Lys-63-linked auto-ubiquitination of TRAF6 and for its ability to activate IKK and NF-kappaB. Furthermore, a RING mutant of TRAF6 abolishes its ability to induce receptor activator of NF-kappaB-independent osteoclast differentiation and nuclear accumulation of the transcription factor NFATc1. Notably, we map the auto-ubiquitination site of TRAF6 to a single Lys residue, which if mutated renders TRAF6 unable to activate transforming growth factor-beta-activated kinase 1 and IKK and to cause spontaneous osteoclast differentiation. Additionally, we provide biochemical and in vivo evidence that TRAF6 serves as an E3 to directly ubiquitinate NEMO. Reconstituting TRAF6-deficent cells with various TRAF6 mutants, we clearly demonstrate the requirement for the TRAF6 RING domain and site-specific auto-ubiquitination of TRAF6 to activate IKK in response to interleukin-1. These data establish a signaling cascade in which regulated site-specific Lys-63-linked TRAF6 auto-ubiquitination is the critical upstream mediator of IKK.