Yusuke Nakatsu

Activation of AMP-activated protein kinase by tributyltin induces neuronal cell death

AMP-activated protein kinase (AMPK), a member of the metabolite-sensing protein kinase family, is activated by energy deficiency and is abundantly expressed in neurons. The environmental pollutant, tributyltin chloride (TBT), is a neurotoxin, and has been reported to decrease cellular ATP in some types of cells. Therefore, we investigated whether TBT activates AMPK, and whether its activation contributes to neuronal cell death, using primary cultures of cortical neurons. Cellular ATP levels were decreased 0.5 h after exposure to 500 nM TBT, and the reduction was time-dependent. It was confirmed that most neurons in our culture system express AMPK, and that TBT induced phosphorylation of AMPK. Compound C, an AMPK inhibitor, reduced the neurotoxicity of TBT, suggesting that AMPK is involved in TBT-induced cell death. Next, the downstream target of AMPK activation was investigated. Nitric oxide synthase, p38 phosphorylation and Akt dephosphorylation were not downstream of TBT-induced AMPK activation because these factors were not affected by compound C, but glutamate release was suggested to be controlled by AMPK. Our results suggest that activation of AMPK by TBT causes neuronal death through mediating glutamate release.

Lactobacillus casei strain Shirota protects against nonalcoholic steatohepatitis development in a rodent model

Gut microbiota alterations are associated with various disorders. In this study, gut microbiota changes were investigated in a methionine-choline-deficient (MCD) diet-induced nonalcoholic steatohepatitis (NASH) rodent model, and the effects of administering Lactobacillus casei strain Shirota (LcS) on the development of NASH were also investigated. Mice were divided into three groups, given the normal chow diet (NCD), MCD diet, or the MCD diet plus daily oral administration of LcS for 6 wk. Gut microbiota analyses for the three groups revealed that lactic acid bacteria such as Bifidobacterium and Lactobacillus in feces were markedly reduced by the MCD diet. Interestingly, oral administration of LcS to MCD diet-fed mice increased not only the L. casei subgroup but also other lactic acid bacteria. Subsequently, NASH development was evaluated based on hepatic histochemical findings, serum parameters, and various mRNA and/or protein expression levels. LcS intervention markedly suppressed MCD-diet-induced NASH development, with reduced serum lipopolysaccharide concentrations, suppression of inflammation and fibrosis in the liver, and reduced colon inflammation. Therefore, reduced populations of lactic acid bacteria in the colon may be involved in the pathogenesis of MCD diet-induced NASH, suggesting normalization of gut microbiota to be effective for treating NASH.

Long-term exposure to endogenous levels of tributyltin decreases GluR2 expression and increases neuronal vulnerability to glutamate

Tributyltin (TBT), an endocrine-disrupting chemical, has been used commercially as a heat stabilizer, agricultural pesticide and component of antifouling paints. In this study, we investigated the effect of long-term exposure to endogenous levels of TBT on neuronal glutamate receptors. Cultured rat cortical neurons were exposed to 1-50 nM TBT for 9 days (from day 2 to day 10 in vitro). The number of neurons was reduced by long-term exposure to 50 nM TBT, but not to 1-20 nM TBT. Long-term exposure to 20 nM TBT decreased the mRNA expression of glutamate receptors NR1, NR2A, GluR1 and GluR2, and increased that of NR2B, GluR3 and GluR4. GluR2 protein was also reduced by long-term exposure to TBT. Because AMPA receptor lacking GluR2 exhibits Ca2+ permeability, we investigated whether Ca2+ influx or glutamate toxicity was affected. Indeed, glutamate-induced Ca2+ influx was increased in TBT-treated neurons. Consistent with this, neurons became more susceptible to glutamate toxicity as a result of long-term exposure to TBT and this susceptibility was abolished by an antagonist of GluR2-lacking AMPA receptor. Thus, it is suggested that long-term exposure to endogenous levels of TBT induces a decrease of GluR2 protein, causing neurons become more susceptible to glutamate toxicity.

Resistin-Like Molecule β Is Abundantly Expressed in Foam Cells and Is Involved in Atherosclerosis Development

Objective—Resistin-like molecule (RELM) &bgr; is a secretory protein homologous to resistin and reportedly contributes to local immune response regulation in gut and bronchial epithelial cells. However, we found that activated macrophages also express RELM&bgr; and thus investigated the role of RELM&bgr; in the development of atherosclerosis. Approach and Results—It was demonstrated that foam cells in atherosclerotic lesions of the human coronary artery abundantly express RELM&bgr;. RELM&bgr; knockout (−/−) and wild-type mice were mated with apolipoprotein E–deficient background mice. RELM&bgr;−/− apolipoprotein E–deficient mice exhibited less lipid accumulation in the aortic root and wall than RELM&bgr;+/+ apolipoprotein E–deficient mice, without significant changes in serum lipid parameters. In vitro, RELM&bgr;−/− primary cultured peritoneal macrophages (PCPMs) exhibited weaker lipopolysaccharide-induced nuclear factor-&kgr;B classical pathway activation and inflammatory cytokine secretion than RELM&bgr;+/+, whereas stimulation with RELM&bgr; upregulated inflammatory cytokine expressions and increased expressions of many lipid transporters and scavenger receptors in PCPMs. Flow cytometric analysis revealed inflammatory stimulation–induced RELM&bgr; in F4/80(+) CD11c(+) PCPMs. In contrast, the expressions of CD11c and tumor necrosis factor were lower in RELM&bgr;−/− PCPMs, but both were restored by stimulation with recombinant RELM&bgr;. Conclusions—RELM&bgr; is abundantly expressed in foam cells within plaques and contributes to atherosclerosis development via lipid accumulation and inflammatory facilitation.

Pin1 Associates with and Induces Translocation of CRTC2 to the Cytosol, Thereby Suppressing cAMP-responsive Element Transcriptional Activity

Pin1 is a unique regulator, which catalyzes the conversion of a specific phospho-Ser/Thr-Pro-containing motif in target proteins. Herein, we identified CRTC2 as a Pin1-binding protein by overexpressing Pin1 with Myc and FLAG tags in mouse livers and subsequent purification of the complex containing Pin1. The association between Pin1 and CRTC2 was observed not only in overexpression experiments but also endogenously in the mouse liver. Interestingly, Ser136 in the nuclear localization signal of CRTC2 was shown to be involved in the association with Pin1. Pin1 overexpression in HepG2 cells attenuated forskolin-induced nuclear localization of CRTC2 and cAMP-responsive element (CRE) transcriptional activity, whereas gene knockdown of Pin1 by siRNA enhanced both. Pin1 also associated with CRTC1, leading to their cytosol localization, essentially similar to the action of CRTC2. Furthermore, it was shown that CRTC2 associated with Pin1 did not bind to CREB. Taken together, these observations indicate the association of Pin1 with CRTC2 to decrease the nuclear CBP·CRTC·CREB complex. Indeed, adenoviral gene transfer of Pin1 into diabetic mice improved hyperglycemia in conjunction with normalizing phosphoenolpyruvate carboxykinase mRNA expression levels, which is regulated by CRE transcriptional activity. In conclusion, Pin1 regulates CRE transcriptional activity, by associating with CRTC1 or CRTC2.

DPP-IV inhibitor anagliptin exerts anti-inflammatory effects on macrophages, adipocytes, and mouse livers by suppressing NF-κB activation

Dipeptidyl peptidase IV (DPP-IV) expression in visceral adipose tissue is reportedly increased in obese patients, suggesting an association of DPP-IV with inflammation. In this study, first, lipopolysaccharide (LPS)- or palmitate-induced elevations of inflammatory cytokine mRNA expressions in RAW264.7 macrophages were shown to be significantly suppressed by coincubation with a DPP-IV inhibitor, anagliptin (10 μM), despite low DPP-IV expression in the RAW264.7 cells. Regarding the molecular mechanism, LPS-induced degradation of IκBα and phosphorylations of p65, JNK, and p38, as well as NF-κB and AP-1 promoter activities, were revealed to be suppressed by incubation with anagliptin, indicating suppressive effects of anagliptin on both NF-κB and AP-1 signaling pathways. Anagliptin also acted on 3T3-L1 adipocytes, weakly suppressing the inflammatory cytokine expressions induced by LPS and TNFα. When 3T3-L1 and RAW cells were cocultured and stimulated with LPS, the effects of anagliptin on the suppression of cytokine expressions in 3T3-L1 adipocytes were more marked and became evident at the 10 μM concentration. Anti-inflammatory effects of anagliptin were also observed in vivo on the elevated hepatic and adipose expressions and serum concentrations of inflammatory cytokines in association with the suppression of hepatic NF-κB transcriptional activity in LPS-infused mice. Taking these observations together, the anti-inflammatory properties of anagliptin may be beneficial in terms of preventing exacerbation of diabetes and cardiovascular events.

Nedd4-induced monoubiquitination of IRS-2 enhances IGF signalling and mitogenic activity

Insulin-like growth factors (IGFs) induce proliferation of various cell types and play important roles in somatic growth and cancer development. Phosphorylation of insulin receptor substrate (IRS)-1/2 by IGF-I receptor tyrosine kinase is essential for IGF action. Here we identify Nedd4 as an IRS-2 ubiquitin ligase. Nedd4 monoubiquitinates IRS-2, which promotes its association with Epsin1, a ubiquitin-binding protein. Nedd4 recruits IRS-2 to the membrane, probably through promoting Epsin1 binding, and enhances IGF-I receptor-induced IRS-2 tyrosine phosphorylation. In thyroid FRTL-5 cells, activation of the cyclic AMP pathway increases the association of Nedd4 with IRS-2, thereby enhancing IRS-2-mediated signalling and cell proliferation induced by IGF-I. The Nedd4 and IRS-2 association is also required for maximal activation of IGF-I signalling and cell proliferation in prostate cancer PC-3 cells. Nedd4 overexpression accelerates zebrafish embryonic growth through IRS-2 in vivo. We conclude that Nedd4-induced monoubiquitination of IRS-2 enhances IGF signalling and mitogenic activity.

Long-term Forskolin Stimulation Induces AMPK Activation and Thereby Enhances Tight Junction Formation in Human Placental Trophoblast BeWo Cells

BeWo cells, derived from human choriocarcinoma, have been known to respond to forskolin or cAMP analogues by differentiating into multinucleated cells- like syncytiotrophoblasts on the surfaces of chorionic villi of the human placenta. In this study, we demonstrated that long-term treatment with forskolin enhances the tight junction (TJ) formation in human placental BeWo cells. Interestingly, AMPK activation and phosphorylation of acetyl-CoA carboxylase (ACC), a molecule downstream from AMPK, were induced by long-term incubation (>12h) with forskolin, despite not being induced by acute stimulation with forskolin. In addition, co-incubation with an AMPK inhibitor, compound C, as well as overexpression of an AMPK dominant negative mutant inhibited forskolin-induced TJ formation. Thus, although the molecular mechanism underlying AMPK activation via the forskolin stimulation is unclear, the TJ formation induced by forskolin is likely to be mediated by the AMPK pathway. Taking into consideration that TJs are present in the normal human placenta, this mechanism may be important for forming the placental barrier system between the fetal and maternal circulations.

Role of Uric Acid Metabolism-Related Inflammation in the Pathogenesis of Metabolic Syndrome Components Such as Atherosclerosis and Nonalcoholic Steatohepatitis

Uric acid (UA) is the end product of purine metabolism and can reportedly act as an antioxidant. However, recently, numerous clinical and basic research approaches have revealed close associations of hyperuricemia with several disorders, particularly those comprising the metabolic syndrome. In this review, we first outline the two molecular mechanisms underlying inflammation occurrence in relation to UA metabolism; one is inflammasome activation by UA crystallization and the other involves superoxide free radicals generated by xanthine oxidase (XO). Importantly, recent studies have demonstrated the therapeutic or preventive effects of XO inhibitors against atherosclerosis and nonalcoholic steatohepatitis, which were not previously considered to be related, at least not directly, to hyperuricemia. Such beneficial effects of XO inhibitors have been reported for other organs including the kidneys and the heart. Thus, a major portion of this review focuses on the relationships between UA metabolism and the development of atherosclerosis, nonalcoholic steatohepatitis, and related disorders. Although further studies are necessary, XO inhibitors are a potentially novel strategy for reducing the risk of many forms of organ failure characteristic of the metabolic syndrome.

Mosapride citrate improves nonalcoholic steatohepatitis with increased fecal lactic acid bacteria and plasma glucagon-like peptide-1 level in a rodent model

Several lines of evidence have suggested a role of gut microbiota in the etiology of nonalcoholic steatohepatitis (NASH). NASH subjects reportedly showed a prolonged orocecal transit time coexistent with small intestinal bacterial overgrowth. We considered the possibility that enhanced gastrointestinal motility would influence gut microbiota and thus investigated the effects of the gastroprokinetic agent mosapride citrate (MC) on gut microbiota and the development of NASH using a methionine-choline deficient (MCD) diet-fed rodent model. Mice were divided into three groups, given the normal chow diet (NCD), the MCD diet, or the MCD diet containing 10 mg·kg(-1)·day(-1) of MC (MCD plus MC) for 6 wk. NASH development was evaluated based on hepatic histochemical findings, serum parameters and various mRNA and/or protein expression levels. MC treatment suppressed MCD diet-induced NASH development, with reduced serum lipopolysaccharide and increased plasma glucagon-like peptide-1 (GLP-1) concentrations. Calculation of the relative abundance of each strain based on gut microbiota analyses indicated lactic acid bacteria specifically, such as Bifidobacterium and Lactobacillus, in feces to be decreased in the MCD, compared with the NCD group. Interestingly, the reduction in lactic acid bacteria in the MCD diet group was reversed in the MCD plus MC group. In addition, colon inflammation observed in the MCD diet group was reduced in the MCD plus MC group. Therefore, MC showed a protective effect against MCD diet-induced NASH development in our rodent model, with possible involvements of increased fecal lactic acid bacteria, protection against colon inflammation and elevated plasma GLP-1.