Endocrinology
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The plasma membrane Na(+)/Ca(2+) exchanger (NCX) is a bidirectional transporter that mediates the exchange of Na(+) for Ca(2+) depending on the electrochemical gradients. Mammalian NCXs form a multigene family comprising NCX1, NCX2, and NCX3 isoforms. Although it has been known that NCX1 in rat osteoclasts is coupled with the Na(+)/ H(+) exchanger for regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)), it is unclear what kind of NCX1 variants are expressed and whether the other two NCX isoforms are also present in mouse osteoclasts. ⋯ NCX inhibitors also decreased the resorption pit area surrounding osteoclasts in a dose-dependent manner. Furthermore, small interference RNAs targeted against NCX1.3, NCX1.41, and NCX3.2 expressed in mouse osteoclasts suppressed osteoclastic pit formation. These results show that three NCX variants are expressed in mouse osteoclasts and play an important role for Ca(2+) transport and regulation during osteoclastic bone resorption.
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Leptin is an adipocyte-derived hormone that signals body energy status to the brain by acting on multiple neuronal subgroups in the hypothalamus, including those that express proopiomelanocortin (Pomc) and agouti-related protein (Agrp). Signal transducer and activator of transcription 3 (Stat3) is an important intracellular signaling molecule activated by leptin, and previous studies have shown that mice carrying a mutated leptin receptor that abolished Stat3 binding are grossly obese. To determine the extent to which Stat3 signaling in Pomc neurons was responsible for these effects, we constructed Pomc-specific Stat3 mutants using a Cre recombinase transgene driven by the Pomc promoter. ⋯ Pomc-specific Stat3 female mutant mice exhibit a 2-fold increase in fat pad mass but only a slight increase in total body weight. Mutant mice remain responsive to leptin-induced hypophagia and are not hypersensitive to a high-fat diet; however, mutant mice fail to mount a normal compensatory refeeding response. These results demonstrate a requirement for Stat3 in transcriptional regulation of Pomc but indicate that this circuit is only one of several components that underlie the neuronal response to leptin and the role of Stat3 in that response.
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The brainstem is essential for mediating energetic response to starvation. Brain stem TRH is synthesized in caudal raphe nuclei innervating brainstem and spinal vagal and sympathetic motor neurons. Intracisternal injection (ic) of a stable TRH analog RX77368 (7.5-25 ng) dose-dependently stimulated solid food intake by 2.4- to 3-fold in freely fed rats, an effect that lasted for 3 h. ⋯ Brain stem TRH mRNA and TRH receptor 1 mRNA increased by 57-58 and 33-35% in 24- and 48-h fasted rats and returned to the fed levels after a 3-h refeeding. Natural food intake in overnight fasted rats was significantly reduced by ic TRH antibody, ic Y1 antagonist, and peripheral atropine. These data establish a physiological role of brainstem TRH in vagal-ghrelin-mediated stimulation of food intake, which involves interaction with brainstem Y1 receptors.
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The aim of this study was to investigate the effects of 5-aminoimidasole-4-carboxamide-1-beta-d-ribofuranoside (AICAR)-induced AMP-activated protein kinase activation on glycogen metabolism in soleus (slow twitch, oxidative) and epitrochlearis (fast twitch, glycolytic) skeletal muscles. Isolated soleus and epitrochlearis muscles were incubated in the absence or presence of insulin (100 nM), AICAR (2 mM), and AICAR plus insulin. In soleus muscles exposed to insulin, glycogen synthesis and glycogen content increased 6.4- and 1.3-fold, respectively. ⋯ We also report the novel evidence that AICAR markedly reduced insulin-induced Akt-Thr308 phosphorylation after 15 and 30 min exposure to insulin, which coincided with a marked reduction in glycogen synthase kinase 3 (GSK)-3alpha/beta phosphorylation. Importantly, phosphorylation of glycogen synthase was increased by AICAR treatment 45 min after insulin stimulation. Our results indicate that AICAR-induced AMP-activated protein kinase activation caused a time-dependent reduction in Akt308 phosphorylation, activation of glycogen synthase kinase-3alpha/beta, and the inactivation of glycogen synthase, which are compatible with the acute reduction in insulin-stimulated glycogen synthesis in oxidative but not glycolytic skeletal muscles.
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Several proprotein convertase subtilisin kexin type 9 (PCSK9) mutations lead to familial hypercholesterolemia by virtue of its role as a negative modulator of the low-density lipoprotein receptor (LDLr). Here, we uncover that upon dietary challenge, the down-regulation of the LDLr is also a key mechanism by which PCSK9 modulates the hepatic production of apolipoprotein-B-containing lipoproteins. Thus, adenoviral-mediated overexpression of PCSK9 in 24-h fasted mice results in massive hyperlipidemia, due to a striking increase in very-low-density lipoprotein (VLDL) triglycerides and apolipoprotein B100 hepatic output. ⋯ This increased production of VLDL was associated with a concomitant reduction of intrahepatic lipid stores as well as a lack of down-regulation of peroxisome proliferator-activated receptor-alpha activity and target genes expression. Finally, we show that PCSK9 hepatic expression is inhibited by the hypotriglyceridemic peroxisome proliferator-activated receptor-alpha agonist fenofibrate. In summary, the negative modulation of LDLr expression by PCSK9, which decreases plasma LDL clearance, also promotes an overproduction of nascent VLDL in vivo upon fasting.