GDF15 activates AMPK and inhibits gluconeogenesis and fibrosis in the liver by attenuating the TGF-ß1/SMAD3 pathway.

INTRODUCTION: The levels of the cellular energy sensor AMP-activated protein kinase (AMPK) have been reported to be decreased via unknown mechanisms in the liver of mice deficient in growth differentiation factor 15 (GDF15). This stress response cytokine regulates energy metabolism mainly by reducing food intake through its hindbrain receptor GFRAL. OBJECTIVE: To examine how GDF15 regulates AMPK. METHODS: Wild-type and Gdf15-/- mice, mouse primary hepatocytes and the human hepatic cell line Huh-7 were used. RESULTS: Gdf15-/- mice showed glucose intolerance, reduced hepatic phosphorylated AMPK levels, increased levels of phosphorylated mothers against decapentaplegic homolog 3 (SMAD3; a mediator of the fibrotic response), elevated serum levels of transforming growth factor (TGF)-ß1, as well as upregulated gluconeogenesis and fibrosis. In line with these observations, recombinant (r)GDF15 promoted AMPK activation and reduced the levels of phosphorylated SMAD3 and the markers of gluconeogenesis and fibrosis in the liver of mice and in mouse primary hepatocytes, suggesting that these effects may be independent of GFRAL. Pharmacological inhibition of SMAD3 phosphorylation in Gdf15-/- mice prevented glucose intolerance, the deactivation of AMPK and the increase in the levels of proteins involved in gluconeogenesis and fibrosis, suggesting that overactivation of the TGF-ß1/SMAD3 pathway is responsible for the metabolic alterations in Gdf15-/- mice. CONCLUSIONS: Overall, these findings indicate that GDF15 activates AMPK and inhibits gluconeogenesis and fibrosis by lowering the activity of the TGF-ß1/SMAD3 pathway.

CHOP upregulation and dysregulation of the mature form of the SNAT2 amino acid transporter in the placentas from small for gestational age newborns.

BACKGROUND: The placentas from newborns that are small for gestational age (SGA; birth weight < -2 SD for gestational age) may display multiple pathological characteristics. A key determinant of fetal growth and, therefore, birth weight is placental amino acid transport, which is under the control of the serine/threonine kinase mechanistic target of rapamycin (mTOR). The effects of endoplasmic reticulum (ER) stress on the mTOR pathway and the levels of amino acid transporters are not well established. METHODS: Placentas from SGA and appropriate for gestational age (AGA) newborns and the human placental BeWo cell line exposed to the ER stressor tunicamycin were used. RESULTS: We detected a significant increase in the levels of C/EBP homologous protein (CHOP) in the placentas from SGA newborns compared with those from AGA newborns, while the levels of other ER stress markers were barely affected. In addition, placental mTOR Complex 1 (mTORC1) activity and the levels of the mature form of the amino acid transporter sodium-coupled neutral amino acid transporter 2 (SNAT2) were also reduced in the SGA group. Interestingly, CHOP has been reported to upregulate growth arrest and DNA damage-inducible protein 34 (GADD34), which in turn suppresses mTORC1 activity. The GADD34 inhibitor guanabenz attenuated the increase in CHOP protein levels and the reduction in mTORC1 activity caused by the ER stressor tunicamycin in the human placental cell line BeWo, but it did not recover mature SNAT2 protein levels, which might be reduced as a result of defective glycosylation. CONCLUSIONS: Collectively, these data reveal that GADD34A activity and glycosylation are key factors controlling mTORC1 signaling and mature SNAT2 levels in trophoblasts, respectively, and might contribute to the SGA condition. Video Abstract.

Elafibranor upregulates the EMT-inducer S100A4 via PPARß/δ.

Elafibranor is a dual peroxisome proliferator-activated receptor (PPAR)α and ß/δ agonist that has reached a phase III clinical trial for the treatment of metabolic dysfunction-associated steatotic liver disease (MASLD). Here, we examined the effects of elafibranor in mice fed a choline-deficient high-fat diet (CD-HFD), a model of metabolic dysfunction-associated steatohepatitis (MASH) that presents obesity and insulin resistance. Our findings revealed that elafibranor treatment ameliorated steatosis, inflammation, and fibrogenesis in the livers of CD-HFD-fed mice. Unexpectedly, elafibranor also increased the levels of the epithelial-mesenchymal transition (EMT)-promoting protein S100A4 via PPARß/δ activation. The increase in S100A4 protein levels caused by elafibranor was accompanied by changes in the levels of markers associated with the EMT program. The S100A4 induction caused by elafibranor was confirmed in the BRL-3A rat liver cells and a mouse primary hepatocyte culture. Furthermore, elafibranor reduced the levels of ASB2, a protein that promotes S100A4 degradation, while ASB2 overexpression prevented the stimulating effect of elafibranor on S100A4. Collectively, these findings reveal an unexpected hepatic effect of elafibranor on increasing S100A4 and promoting the EMT program.

Design and Synthesis of AMPK Activators and GDF15 Inducers.

Targeting growth differentiation factor 15 (GDF15) is a recent strategy for the treatment of obesity and type 2 diabetes mellitus (T2DM). Here, we designed, synthesized, and pharmacologically evaluated in vitro a novel series of AMPK activators to upregulate GDF15 levels. These compounds were structurally based on the (1-dibenzylamino-3-phenoxy)propan-2-ol structure of the orphan ubiquitin E3 ligase subunit protein Fbxo48 inhibitor, BC1618. This molecule showed a better potency than metformin, increasing GDF15 mRNA levels in human Huh-7 hepatic cells. Based on BC1618, structural modifications have been performed to create a collection of diversely substituted new molecules. Of the thirty-five new compounds evaluated, compound 21 showed a higher increase in GDF15 mRNA levels compared with BC1618. Metformin, BC1618, and compound 21 increased phosphorylated AMPK, but only 21 increased GDF15 protein levels. Overall, these findings indicate that 21 has a unique capacity to increase GDF15 protein levels in human hepatic cells compared with metformin and BC1618.

A positive feedback loop between AMPK and GDF15 promotes metformin antidiabetic effects.

BACKGROUND AND AIMS: Metformin, the most prescribed drug for the treatment of type 2 diabetes mellitus, has been recently reported to promote weight loss by upregulating the anorectic cytokine growth differentiation factor 15 (GDF15). Since the antidiabetic effects of metformin are mostly mediated by the activation of AMPK, a key metabolic sensor in energy homeostasis, we examined whether the activation of this kinase by metformin was dependent on GDF15. METHODS: Cultured hepatocytes and myotubes, and wild-type and Gdf15-/- mice were utilized in a series of studies to investigate the involvement of GDF15 in the activation of AMPK by metformin. RESULTS: A low dose of metformin increased GDF15 levels without significantly reducing body weight or food intake, but it ameliorated glucose intolerance and activated AMPK in the liver and skeletal muscle of wild-type mice but not Gdf15-/- mice fed a high-fat diet. Cultured hepatocytes and myotubes treated with metformin showed AMPK-mediated increases in GDF15 levels independently of its central receptor GFRAL, while Gdf15 knockdown blunted the effect of metformin on AMPK activation, suggesting that AMPK is required for the metformin-mediated increase in GDF15, which in turn is needed to sustain the full activation of this kinase independently of the CNS. CONCLUSION: Overall, these findings uncover a novel mechanism through which GDF15 upregulation by metformin is involved in achieving and sustaining full AMPK activation by this drug independently of the CNS.

Study on immunostimulatory activity and extraction process optimization of polysaccharides from Caulerpa lentillifera.

The process of extracting polysaccharides from the green algae Caulerpa lentillifera was studied by single factor experiments and response surface methodology. Additionally, the immunostimulatory activity of Caulerpa lentillifera polysaccharides (CLP) on RAW264.7 mouse macrophage was evaluated by in vitro cell experiments. The results showed that the optimal extraction conditions consisted of ultrasonification for 30 min, extraction time of 9 h, extraction temperature of 100 °C, and a ratio of water to raw material of 40:1. RAW264.7 macrophage exhibited enhanced phagocytosis with no toxic effects after treatment with CLP. In addition, CLP effectively increased the synthesis and secretion of cytokines (IL-6, TNF-α, IL-1ß, and NO), whereby the secretion levels of IL-6, TNF-α, and IL-1ß were 1,840.32 ± 21.03 pg/mL (50 µg/mL), 197.17 ± 3.13 ng/mL (50 µg/mL), and 1,178.35 ± 78.82 pg/mL (25 µg/mL), respectively. The polysaccharides contained in Caulerpa lentillifera have potential value for further development due to their immunological activity.