Atrial natriuretic peptide regulates lipid mobilization and oxygen consumption in human adipocytes by activating AMPK.

Atrial natriuretic peptide (ANP) has been shown to regulate lipid and carbohydrate metabolism providing a possible link between cardiovascular function and metabolism by mediating the switch from carbohydrate to lipid mobilization and oxidation. ANP exerts a potent lipolytic effect via cGMP-dependent protein kinase (cGK)-I mediated-stimulation of AMP-activated protein kinase (AMPK). Activation of the ANP/cGK signaling cascade also promotes muscle mitochondrial biogenesis and fat oxidation. Here we demonstrate that ANP regulates lipid metabolism and oxygen utilization in differentiated human adipocytes by activating the alpha2 subunit of AMPK. ANP treatment increased lipolysis by seven fold and oxygen consumption by two fold, both of which were attenuated by inhibition of AMPK activity. ANP-induced lipolysis was shown to be mediated by the alpha2 subunit of AMPK as introduction of dominant-negative alpha2 subunit of AMPK attenuated ANP effects on lipolysis. ANP-induced activation of AMPK enhanced mitochondrial oxidative capacity as evidenced by a two fold increase in oxygen consumption and induction of mitochondrial genes, including carnitine palmitoyltransferase 1A (CPT1a) by 1.4-fold, cytochrome C (CytC) by 1.3-fold, and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) by 1.4-fold. Treatment of human adipocytes with fatty acids and tumor necrosis factor α (TNFα) induced insulin resistance and down-regulation of mitochondrial genes, which was restored by ANP treatment. These results show that ANP regulates lipid catabolism and enhances energy dissipation through AMPK activation in human adipocytes.

Fibroblast growth factor 21 regulates energy metabolism by activating the AMPK-SIRT1-PGC-1alpha pathway.

Fibroblast growth factor 21 (FGF21) has been identified as a potent metabolic regulator. Administration of recombinant FGF21 protein to rodents and rhesus monkeys with diet-induced or genetic obesity and diabetes exerts strong antihyperglycemic and triglyceride-lowering effects and reduction of body weight. Despite the importance of FGF21 in the regulation of glucose, lipid, and energy homeostasis, the mechanisms by which FGF21 functions as a metabolic regulator remain largely unknown. Here we demonstrate that FGF21 regulates energy homeostasis in adipocytes through activation of AMP-activated protein kinase (AMPK) and sirtuin 1 (SIRT1), resulting in enhanced mitochondrial oxidative function. AMPK phosphorylation levels were increased by FGF21 treatment in adipocytes as well as in white adipose tissue from ob/ob mice. FGF21 treatment increased cellular NAD(+) levels, leading to activation of SIRT1 and deacetylation of its downstream targets, peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) and histone 3. Activation of AMPK and SIRT1 by FGF21 in adipocytes enhanced mitochondrial oxidative capacity as demonstrated by increases in oxygen consumption, citrate synthase activity, and induction of key metabolic genes. The effects of FGF21 on mitochondrial function require serine/threonine kinase 11 (STK11/LKB1), which activates AMPK. Inhibition of AMPK, SIRT1, and PGC-1alpha activities attenuated the effects of FGF21 on oxygen consumption and gene expression, indicating that FGF21 regulates mitochondrial activity and enhances oxidative capacity through an AMPK-SIRT1-PGC1alpha-dependent mechanism in adipocytes.

Notch signaling plays a key role in cardiac cell differentiation.

Results from lineage tracing studies indicate that precursor cells in the ventricles give rise to both cardiac muscle and conduction cells. Cardiac conduction cells are specialized cells responsible for orchestrating the rhythmic contractions of the heart. Here, we show that Notch signaling plays an important role in the differentiation of cardiac muscle and conduction cell lineages in the ventricles. Notch1 expression coincides with a conduction marker, HNK-1, at early stages. Misexpression of constitutively active Notch1 (NIC) in early heart tubes in chick exhibited multiple effects on cardiac cell differentiation. Cells expressing NIC had a significant decrease in expression of cardiac muscle markers, but an increase in expression of conduction cell markers, HNK-1, and SNAP-25. However, the expression of the conduction marker connexin 40 was inhibited. Loss-of-function study, using a dominant-negative form of Suppressor-of-Hairless, further supports that Notch1 signaling is important for the differentiation of these cardiac cell types. Functional studies show that the expression of constitutively active Notch1 resulted in abnormalities in ventricular conduction pathway patterns.