Impaired phosphatase activity contributes to the persistent activation of STAT3 in tumors. Given that STAT family members with various or even opposite functions are often phosphorylated or dephosphorylated by the same enzymes, the mechanism for STAT3-specific dephosphorylation in cells remains largely unknown. Here, we report that GdX (UBL4A) promotes STAT3 dephosphorylation via mediating the interaction between TC45 (the nuclear isoform of TC-PTP) and STAT3 specifically. GdX stabilizes the TC45-STAT3 complex to bestow upon STAT3 an efficient dephosphorylation by TC45. Inasmuch, GdX suppresses tumorigenesis and tumor development by reducing the level of phospho-STAT3 (p-STAT3), whereas deletion of GdX results in a high level of p-STAT3 and accelerated colorectal tumorigenesis induced by AOM/DSS. Thus, GdX converts TC45, a nonspecific phosphatase, into a STAT3-specific phosphatase by bridging an association between TC45 and STAT3.

IntroductionSignal transducer and activator of transcription 3 (STAT3) is over-activated or phosphorylated in breast cancers. The hyper-phosphorylation of STAT3 was attributed to either up-regulated phosphorylation by several tyrosine-kinases or down-regulated activity of phosphatases. Although several factors have been identified to phosphorylate STAT3, it remains unclear how STAT3 is dephosphorylated by PTPMeg2. The aim of this study was to determine the role of PTPMeg2 as a phosphatase in regulation of the activity of STAT3 in breast cancers.MethodsImmunoprecipitation assays were used to study the interaction of STAT3 with PTPMeg2. A series of biochemistry experiments were performed to evaluate the role of PTPMeg2 in the dephosphorylation of STAT3. Two breast cancer cell lines MCF7 (PTPMeg2 was depleted as it was endogenously high) and MDA-MB-231 (PTPMeg2 was overexpressed as it was endogenously low) were used to compare the level of phosphorylated STAT3 and the tumor growth ability in vitro and in vivo. Samples from breast carcinoma (n = 73) were subjected to a pair-wise Pearson correlation analysis for the correlation of levels of PTPMeg2 and phosphorylated STAT3.ResultsPTPMeg2 directly interacts with STAT3 and mediates its dephosphorylation in the cytoplasm. Over-expression of PTPMeg2 decreased tyrosine phosphorylation of STAT3 while depletion of PTPMeg2 increased its phosphorylation. The decreased tyrosine phosphorylation of STAT3 is coupled with suppression of STAT3 transcriptional activity and reduced tumor growth in vitro and in vivo. Levels of PTPMeg2 and phosphorylated STAT3 were inversely correlated in breast cancer tissues (P = 0.004).ConclusionsPTPMeg2 is an important phosphatase for the dephosphorylation of STAT3 and plays a critical role in breast cancer development.

The carboxyl terminus of Hsc70-interacting protein (CHIP, also named Stub1), a U-box containing E3 ubiquitin ligase, is involved in degradation of certain oncogenic proteins. Recent studies indicated that CHIP suppresses tumor progression in human cancers by targeting Src-3, hypoxia inducible factor 1α, NF-κB, ErbB2 and c-Myc. Here, we report that CHIP was downregulated, predominantly, in the late stages of human colorectal cancer (CRC), and that the CHIP promoter was hypermethylated in CRC specimens. Overexpression of CHIP in HCT-116 cells resulted in impaired tumor growth in nude mice and decreased abilities of tumor cell migration and invasion. Conversely, depletion of CHIP in HCT-116 cells promoted tumor growth and increased tumor cell migration and invasion. CHIP was further found to negatively regulate NF-κB signaling in HCT-116 cells by promoting ubiquitination and degradation of p65, a subunit of the NF-κB complex. The suppressive effect of CHIP led to decreased expression of NF-κB-targeted oncogenes including Cyclin D1, c-Myc, MMP-2, VEGF and IL-8. We proposed that CHIP inhibits the malignancy of CRC cells, possibly through targeting NF-κB signaling. This study provides functional evidence for CHIP as a potential tumor suppressor in CRC, and CHIP expression may be a marker for stages of CRC.

Uncoupling protein 1+ beige adipocytes are dynamically regulated by environment in rodents and humans; cold induces formation of beige adipocytes, whereas warm temperature and nutrient excess lead to their disappearance. Beige adipocytes can form through de novo adipogenesis; however, how “beiging” characteristics are maintained afterward is largely unknown. In this study, we show that beige adipocytes formed postnatally in subcutaneous inguinal white adipose tissue lost thermogenic gene expression and multilocular morphology at the adult stage, but cold restored their beiging characteristics, a phenomenon termed beige adipocyte renaissance. Ablation of these postnatal beige adipocytes inhibited cold-induced beige adipocyte formation in adult mice. Furthermore, we demonstrated that beige adipocyte renaissance was governed by liver kinase b1 and histone deacetylase 4 in white adipocytes. Although neither presence nor thermogenic function of uncoupling protein 1+ beige adipocytes contributed to metabolic fitness in adipocyte liver kinase b1–deficient mice, our results reveal an unexpected role of white adipocytes in maintaining properties of preexisting beige adipocytes.

Various physiological stimuli, such as cold environment, diet, and hormones, trigger brown adipose tissue (BAT) to produce heat through sympathetic nervous system (SNS)- and β-adrenergic receptors (βARs). The βAR stimulation increases intracellular cAMP levels through heterotrimeric G proteins and adenylate cyclases, but the processes by which cAMP modulates brown adipocyte function are not fully understood. Here we described that specific ablation of cAMP production in brown adipocytes led to reduced lipolysis, mitochondrial biogenesis, uncoupling protein 1 (Ucp1) expression, and consequently defective adaptive thermogenesis. Elevated cAMP signaling by sympathetic activation inhibited Salt-inducible kinase 2 (Sik2) through protein kinase A (PKA)-mediated phosphorylation in brown adipose tissue. Inhibition of SIKs enhanced Ucp1 expression in differentiated brown adipocytes and Sik2 knockout mice exhibited enhanced adaptive thermogenesis at thermoneutrality in an Ucp1-dependent manner. Taken together, our data indicate that suppressing Sik2 by PKA-mediated phosphorylation is a requisite for SNS-induced Ucp1 expression and adaptive thermogenesis in BAT, and targeting Sik2 may present a novel therapeutic strategy to ramp up BAT thermogenic activity in humans.

Vertical sleeve gastrectomy (VSG) is one of the most effective and durable therapies for morbid obesity and its related complications. Although bile acids (BAs) have been implicated as downstream mediators of VSG, the specific mechanisms through which BA changes contribute to the metabolic effects of VSG remain poorly understood. Here, we confirm that high fat diet-fed global farnesoid X receptor (Fxr) knockout mice are resistant to the beneficial metabolic effects of VSG. However, the beneficial effects of VSG were retained in high fat diet-fed intestine- or liver-specific Fxr knockouts, and VSG did not result in Fxr activation in the liver or intestine of control mice. Instead, VSG decreased expression of positive hepatic Fxr target genes, including the bile salt export pump (Bsep) that delivers BAs to the biliary pathway. This reduced small intestine BA levels in mice, leading to lower intestinal fat absorption. These findings were verified in sterol 27-hydroxylase (Cyp27a1) knockout mice, which exhibited low intestinal BAs and fat absorption and did not show metabolic improvements following VSG. In addition, restoring small intestinal BA levels by dietary supplementation with taurocholic acid (TCA) partially blocked the beneficial effects of VSG. Altogether, these findings suggest that reductions in intestinal BAs and lipid absorption contribute to the metabolic benefits of VSG.

SUMMARY Brown adipose tissue (BAT) thermogenesis is critical for thermoregulation and contributes to total energy expenditure. However, whether BAT has non-thermogenic functions is largely unknown. Here, we describe that BAT-specific liver kinase b1 knockout (Lkb1BKO) mice exhibited impaired BAT mitochondrial respiration and thermogenesis but reduced adiposity and liver triglyceride accumulation under high-fat-diet feeding at room temperature. Importantly, these metabolic benefits were also present in Lkb1BKO mice at thermoneutrality, where BAT thermogenesis was not required. Mechanistically, decreased mRNA levels of mtDNA-encoded electron transport chain (ETC) subunits and ETC proteome imbalance led to defective BAT mitochondrial respiration in Lkb1BKO mice. Furthermore, reducing mtDNA gene expression directly in BAT by removing mitochondrial transcription factor A (Tfam) in BAT also showed ETC proteome imbalance and the trade-off between BAT thermogenesis and systemic metabolism at room temperature and thermo-neutrality. Collectively, our data demonstrate that ETC proteome imbalance in BAT regulates systemic metabolism independently of thermogenesis.

Nuclear factor-kappa B (NF-κB) activation is critical for innate immune responses. However, cellular-intrinsic regulation of NF-κB activity during inflammatory diseases remains incompletely understood. Ubiquitin-like protein 4A (UBL4A, GdX) is a small adaptor protein involved in protein folding, biogenesis and transcription. Yet, whether GdX has a role during innate immune response is largely unknown. Methods: To investigate the involvement of GdX in innate immunity, we challenged GdX-deficient mice with lipopolysaccharides (LPS). To investigate the underlying mechanism, we performed RNA sequencing, real-time PCR, ELISA, luciferase reporter assay, immunoprecipitation and immunoblot analyses, flow cytometry, and structure analyses. To investigate whether GdX functions in inflammatory bowel disease, we generated dendritic cell (DC), macrophage (Mφ), epithelial-cell specific GdX-deficient mice and induced colitis with dextran sulfate sodium. Results: GdX enhances DC and Mφ-mediated innate immune defenses by positively regulating NF-κB signaling. GdX-deficient mice were resistant to LPS-induced endotoxin shock and DSS-induced colitis. DC- or Mφ- specific GdX-deficient mice displayed alleviated mucosal inflammation. The production of pro-inflammatory cytokines by GdX-deficient DCs and Mφ was reduced. Mechanistically, we found that tyrosine-protein phosphatase non-receptor type 2 (PTPN2, TC45) and protein phosphatase 2A (PP2A) form a complex with RelA (p65) to mediate its dephosphorylation whereas GdX interrupts the TC45/PP2A/p65 complex formation and restrict p65 dephosphorylation by trapping TC45. Conclusion: Our study provides a mechanism by which NF-κB signaling is positively regulated by an adaptor protein GdX in DC or Mφ to maintain the innate immune response. Targeting GdX could be a strategy to reduce over-activated immune response in inflammatory diseases.