Niacin Ameliorates Hepatic Steatosis by Inhibiting De Novo Lipogenesis Via a GPR109A-Mediated PKC-ERK1/2-AMPK Signaling Pathway in C57BL/6 Mice Fed a High-Fat Diet.

BACKGROUND: Nonalcoholic fatty liver disease (NAFLD) is the most common liver disease in the world. Hepatic de novo lipogenesis (DNL) has been suggested to contribute to the pathogenesis of NAFLD. Recent studies have demonstrated that niacin (NA) modulates hepatic DNL through GPR109A. However, the underlying mechanism remains largely unknown. OBJECTIVES: This study aims to elucidate the potential molecular mechanism by which GPR109A inhibits hepatic DNL. METHODS: C57BL/6 wild-type (WT) and Gpr109a knockout (KO) mice (male, 5 wk old) were fed a high-fat diet (60% energy from fat) firstly for 6 wk to generate a diet-induced obese model. Subsequently, they were randomly divided into 4 groups for the next 8-9 wk: WT mice with oral water [WT + vehile (VE)], WT mice with oral NA (50 mM, dissolved in water) (WT + NA), KO mice with oral water (KO + VE), and KO mice with oral NA (50 mM) (KO + NA). Mechanisms were examined in HepG2 cells. Body composition, liver histology, biomarkers of hepatic function, lipid accumulation, and lipid synthesis signals in HepG2 cells were measured. RESULTS: Upon activation, GPR109A apparently protected against obesity and hepatic steatosis (P < 0.05). The concentrations of hepatic Tnf-α in the WT + NA group were about 50% of those in the WT + VE group (P < 0.05). The activities of serum alanine transaminase and aspartate transaminase were 26.7% and 53.5% lower in the WT + NA group than in the WT + VE group, respectively (P < 0.05). In HepG2 cells, activation of GPR109A resulted in remarkable inhibition of oleic acid-induced lipid accumulation via a protein kinase C-extracellular signal-regulated kinase-1/2-AMP-activated protein kinase signaling pathway. CONCLUSIONS: NA inhibits hepatic lipogenesis in C57BL/6 mice through a GPR109A-mediated signaling pathway, consistent with the mechanistic studies in HepG2 cells, suggesting its potential for treatment of NAFLD and other fatty liver diseases.

Niacin fine-tunes energy homeostasis through canonical GPR109A signaling.

The incidence of overweight and obesity has become a global public health problem, constituting a major risk factor for numerous comorbidities. Despite tremendous efforts, effective pharmacological agents for the treatment of obesity are still limited. Here, we showed that in contrast to lactate receptor GPR81, niacin receptor GPR109A-deficient mice had progressive weight gain and hepatic fat accumulation. Using high-fat diet-induced mouse model of obesity, we demonstrated that niacin treatment apparently protected against obesity without affecting food intake in wild-type mice but not in GPR109A-deficient mice. Further investigation showed that niacin treatment led to a remarkable inhibition of hepatic de novo lipogenesis. Additionally, we demonstrated that niacin treatment triggered brown adipose tissue and/or white adipose tissue thermogenic activity via activation of GPR109A. Moreover, we observed that mice exposed to niacin exhibited a dramatic decrease in intestinal absorption of sterols and fatty acids. Taken together, our findings demonstrate that acting on GPR109A, niacin shows the potential to maintain energy homeostasis through multipathways, representing a potential approach to the treatment of obesity, diabetes and cardiovascular disease.-Ye, L., Cao, Z., Lai, X., Wang, W., Guo, Z., Yan, L., Wang, Y., Shi, Y., Zhou, N. Niacin fine-tunes energy homeostasis through canonical GPR109A signaling.

Activated niacin receptor HCA2 inhibits chemoattractant-mediated macrophage migration via Gßγ/PKC/ERK1/2 pathway and heterologous receptor desensitization.

The niacin receptor HCA2 is implicated in controlling inflammatory host responses with yet poorly understood mechanistic basis. We previously reported that HCA2 in A431 epithelial cells transduced Gßγ-protein kinase C- and Gßγ-metalloproteinase/EGFR-dependent MAPK/ERK signaling cascades. Here, we investigated the role of HCA2 in macrophage-mediated inflammation and the underlying mechanisms. We found that proinflammatory stimulants LPS, IL-6 and IL-1ß up-regulated the expression of HCA2 on macrophages. Niacin significantly inhibited macrophage chemotaxis in response to chemoattractants fMLF and CCL2 by disrupting polarized distribution of F-actin and Gß protein. Niacin showed a selected additive effect on chemoattractant-induced activation of ERK1/2, JNK and PI3K pathways, but only the MEK inhibitor UO126 reduced niacin-mediated inhibition of macrophage chemotaxis, while activation of ERK1/2 by EGF alone did not inhibit fMLF-mediated migration of HEK293T cells co-expressing HCA2 and fMLF receptor FPR1. In addition, niacin induced heterologous desensitization and internalization of FPR1. Furthermore, niacin rescued mice from septic shock by diminishing inflammatory symptoms and the effect was abrogated in HCA2-/- mice. These results suggest that Gßγ/PKC-dependent ERK1/2 activation and heterologous desensitization of chemoattractant receptors are involved in the inhibition of chemoattractant-induced migration of macrophages by niacin. Thus, HCA2 plays a critical role in host protection against pro-inflammatory insults.

Human Neuropeptide S Receptor Is Activated via a Gαq Protein-biased Signaling Cascade by a Human Neuropeptide S Analog Lacking the C-terminal 10 Residues.

Human neuropeptide S (NPS) and its cognate receptor regulate important biological functions in the brain and have emerged as a future therapeutic target for treatment of a variety of neurological and psychiatric diseases. The human NPS (hNPS) receptor has been shown to dually couple to Gαs- and Gαq-dependent signaling pathways. The human NPS analog hNPS-(1-10), lacking 10 residues from the C terminus, has been shown to stimulate Ca(2+)mobilization in a manner comparable with full-length hNPSin vitrobut seems to fail to induce biological activityin vivo Here, results derived from a number of cell-based functional assays, including intracellular cAMP-response element (CRE)-driven luciferase activity, Ca(2+)mobilization, and ERK1/2 phosphorylation, show that hNPS-(1-10) preferentially activates Gαq-dependent Ca(2+)mobilization while exhibiting less activity in triggering Gαs-dependent CRE-driven luciferase activity. We further demonstrate that both Gαq- and Gαs-coupled signaling pathways contribute to full-length hNPS-mediated activation of ERK1/2, whereas hNPS-(1-10)-promoted ERK1/2 activation is completely inhibited by the Gαqinhibitor UBO-QIC but not by the PKA inhibitor H89. Moreover, the results of Ala-scanning mutagenesis of hNPS-(1-13) indicated that residues Lys(11)and Lys(12)are structurally crucial for the hNPS receptor to couple to Gαs-dependent signaling. In conclusion, our findings demonstrate that hNPS-(1-10) is a biased agonist favoring Gαq-dependent signaling. It may represent a valuable chemical probe for further investigation of the therapeutic potential of human NPS receptor-directed signalingin vivo.

Agonist-induced activation of histamine H3 receptor signals to extracellular signal-regulated kinases 1 and 2 through PKC-, PLD-, and EGFR-dependent mechanisms.

The histamine H3 receptor (H3R), abundantly expressed in the central and the peripheral nervous system, has been recognized as a promising target for the treatment of various important CNS diseases including narcolepsy, Alzheimer's disease, and attention deficit hyperactivity disorder. The H3R acts via Gi/o -proteins to inhibit adenylate cyclase activity and modulate MAPK activity. However, the underlying molecular mechanisms for H3R mediation of the activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) remain to be elucidated. In this study, using HEK293 cells stably expressing human H3R and mouse primary cortical neurons endogenously expressing mouse H3R, we found that the H3R-mediated activation of ERK1/2 was significantly blocked by both the pertussis toxin and the MEK1/2 inhibitor U0126. Upon stimulation by H3R agonist histamine or imetit, H3R was shown to rapidly induce ERK1/2 phosphorylation via PLC/PKC-, PLDs-, and epidermal growth factor receptor (EGFR) transactivation-dependent pathways. Furthermore, it was also indicated that while the ßγ-subunits play a key role in H3R-activated ERK1/2 phosphorylation, ß-arrestins were not required for ERK1/2 activation. In addition, when the cultured mouse cortical neurons were exposed to oxygen and glucose deprivation conditions (OGD), imetit exhibited neuroprotective properties through the H3R. Treatment of cells with the inhibitor UO126 abolished these protective effects. This suggests a possible neuroprotective role of the H3R-mediated ERK1/2 pathway under hypoxia conditions. These observations may provide new insights into the pharmacological effects and the physiological functions modulated by the H3R-mediated activation of ERK1/2. Histamine H3 receptors are abundantly expressed in the brain and play important roles in various CNS physiological functions. However, the underlying mechanisms for H3R-induced activation of extracellular signal-regulated kinase (ERK)1/2 remain largely unknown. Here, we provide evidence that upon activation by an agonist, H3Rs trigger ERK1/2 activation via phospholipase C/protein kinase C (PLC/PKC)-, phospholipase D (PLD)s-, and matrix metallopeptidase/epidermal growth factor receptor (MMP/EGFR) transactivation-dependent pathways. Moreover, we demonstrate that H3Rs exhibit a neuroprotective effect on the cultured mouse cortical neurons under hypoxia conditions through the ERK1/2 pathway.

Melatonin receptor type 1 signals to extracellular signal-regulated kinase 1 and 2 via Gi and Gs dually coupled pathways in HEK-293 cells.

The pineal gland hormone melatonin exerts its regulatory roles in a variety of physiological and pathological responses through two G protein-coupled receptors, melatonin receptor type 1 (MT1) and melatonin receptor type 2 (MT2), which have been recognized as promising targets in the treatment of a number of human diseases and disorders. The MT1 receptor was identified nearly 20 years ago; however, the molecular mechanisms by which MT1-mediated signaling affects physiology remain to be further elucidated. In this study, using HEK293 cells stably expressing the human MT1 receptor, melatonin induced a concentration-dependent activation of extracellular signal-regulated kinase 1 and 2 (ERK1/2). The melatonin-mediated phosphorylation of ERK1/2 at later time points (≥5 min) was strongly suppressed by pretreatment with pertussis toxin, but only a slight, if any, inhibition of ERK1/2 activation at early time points (≤2 min) was detected. Further experiments demonstrated that the Gßγ subunit, phosphoinositide 3-kinase, and calcium-insensitive protein kinase C were involved in the MT1-mediated activation of ERK1/2 at later time points (≥5 min). Moreover, results derived from cAMP assays combined with a MT1 mutant indicated that the human MT1 receptor could also couple to Gs protein, stimulating intracellular cAMP formation, and that the MT1-induced activation of ERK1/2 at early time points (≤2 min) was mediated by the Gs/cAMP/PKA cascade. Our findings may provide new insights into the pharmacological effects and physiological functions modulated by the MT1-mediated activation of ERK1/2.