Propionate suppresses hepatic gluconeogenesis via GPR43/AMPK signaling pathway

Short-chain fatty acids (SCFAs) such as acetate, propionate, and butyrate are generated by gut microbial fermentation of dietary fiber. SCFAs may exert multiple beneficial effects on human lipid and glucose metabolism. However, their actions and underlying mechanisms are not fully elucidated. In this study, we examined the direct effects of propionate on hepatic glucose and lipid metabolism using human HepG2 hepatocytes. Here, we demonstrate that propionate at a physiologically-relevant concentration effectively suppresses palmitate-enhanced glucose production in HepG2 cells but does not affect intracellular neutral lipid levels. Our results indicated that propionate can decline in gluconeogenesis by down-regulation of glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK) through activation of AMP-activated protein kinase (AMPK), which is a major regulator of the hepatic glucose metabolism. Mechanistic studies also revealed that propionate-stimulated AMPK phosphorylation can be ascribed to Ca²⁺/calmodulin-dependent protein kinase kinase β (CaMKKβ) activation in response to an increase in intracellular Ca²⁺ concentration. Moreover, siRNA-mediated knockdown of the propionate receptor GPR43 prevented propionate-inducible activation of AMPK and abrogates the gluconeogenesis-inhibitory action. Thus, our data indicate that the binding of propionate to hepatic GPR43 elicits CaMKKβ-dependent activation of AMPK through intracellular Ca²⁺ increase, leading to suppression of gluconeogenesis. The present study suggests the potential efficacy of propionate in preventive and therapeutic management of diabetes.

ApoA-1 improves glucose tolerance by increasing glucose uptake into heart and skeletal muscle independently of AMPKα2

OBJECTIVE

Acute administration of the main protein component of high-density lipoprotein, apolipoprotein A-I (ApoA-1), improves glucose uptake in skeletal muscle. The molecular mechanisms mediating this are not known, but in muscle cell cultures, ApoA-1 failed to increase glucose uptake when infected with a dominant-negative AMP-activated protein kinase (AMPK) virus. We therefore investigated whether AMPK is necessary for ApoA-1-stimulated glucose uptake in intact heart and skeletal muscle in vivo.

METHODS

The effect of injection with recombinant human ApoA-1 (rApoA-1) on glucose tolerance, glucose-stimulated insulin secretion, and glucose uptake into skeletal and heart muscle with and without block of insulin secretion by injection of epinephrine (0.1 mg/kg) and propranolol (5 mg/kg), were investigated in 8 weeks high-fat diet-fed (60E%) wild-type and AMPKα2 kinase-dead mice in the overnight-fasted state. In addition, the effect of rApoA-1 on glucose uptake in isolated skeletal muscle ex vivo was studied.

RESULTS

rApoA-1 lowered plasma glucose concentration by 1.7 mmol/l within 3 h (6.1 vs 4.4 mmol/l; p < 0.001). Three hours after rApoA-1 injection, glucose tolerance during a 40-min glucose tolerance test (GTT) was improved compared to control (area under the curve (AUC) reduced by 45%, p < 0.001). This was accompanied by an increased glucose clearance into skeletal (+110%; p < 0.001) and heart muscle (+100%; p < 0.001) and an increase in glucose-stimulated insulin secretion 20 min after glucose injection (+180%; p < 0.001). When insulin secretion was blocked during a GTT, rApoA-1 still enhanced glucose tolerance (AUC lowered by 20% compared to control; p < 0.001) and increased glucose clearance into skeletal (+50%; p < 0.05) and heart muscle (+270%; p < 0.001). These improvements occurred to a similar extent in both wild-type and AMPKα2 kinase-dead mice and thus independently of AMPKα2 activity in skeletal- and heart muscle. Interestingly, rApoA-1 failed to increase glucose uptake in isolated skeletal muscles ex vivo.

CONCLUSIONS

In conclusion, ApoA-1 stimulates in vivo glucose disposal into skeletal and heart muscle independently of AMPKα2. The observation that ApoA-1 fails to increase glucose uptake in isolated muscle ex vivo suggests that additional systemic effects are required.

Clusterin alleviates Cr(VI)-induced mitochondrial apoptosis in L02 hepatocytes via inhibition of Ca2+-ROS-Drp1-mitochondrial fission axis

Hexavalent chromium [Cr(VI)] is ubiquitous in the environment and is commonly used in various industrial processes. Clusterin (CLU) is an extracellular chaperone protein which exerts the anti-apoptotic function. In this study, we aimed to explore the effect of CLU on Cr(VI)-induced mitochondrial fission and apoptosis. We revealed that the apoptosis rate of L02 hepatocytes treated with Cr (VI) was increased. CLU over-expression could protect the hepatocytes from Cr(VI)-induced mitochondrial apoptosis. Furthermore, Cr(VI) triggered the intracellular calcium overload, resulting in the activation of xanthine oxidase (XO). Cr(VI) induced reactive oxygen species (ROS) overproduction, led to dynamin-related protein 1 (Drp1) translocation to mitochondria and the subsequent mitochondrial fission, contributing to the caspase-3-dependent mitochondrial apoptosis as evidenced by higher mitochondrial permeability transition pore (mPTP) opening rate, lower mitochondrial membrane potential (MMP), and more alanine transaminase (ALT)/aspartate transaminase (AST) leakage into the culture medium. However, CLU over-expression could trigger the AMP-activated protein kinase (AMPK) pathway, which was followed by the increase of sarcoplasmic reticulum Ca²⁺-ATPase (SERCA2a) expression. CLU-induced AMPK/SERCA2a activation attenuated calcium overload, caspase-3 activation, and ultimate mitochondrial apoptosis. All in all, the present study demonstrated that Cr(VI) induced hepatocytes apoptosis via Ca²⁺-ROS-Drp1-mitochondrial fission axis and CLU alleviated the mitochondrial apoptosis through activation of the AMPK/SERCA2a pathway.

A triterpenoid Nrf2 activator, RS9, promotes LC3-associated phagocytosis of photoreceptor outer segments in a p62-independent manner

Daily phagocytosis of shed photoreceptor outer segments (POS) by the retinal pigment epithelium (RPE) is required to sustain the visual function. Recent reports revealed that POS phagocytosis is progressed with LC3-associated manner. Patients with age-related macular degeneration (AMD) had impaired autophagic degradation in the RPE. Nrf2 is a key antioxidant transcriptional regulator that ameliorates oxidative stress which is another contributor to AMD pathogenesis. Nrf2 activation also induces the autophagy receptor protein, p62. However, the role of the Nrf2-p62 pathway in LC3-associated phagocytosis of POS is poorly understood. Here, we investigated the relationships between Nrf2 activation and POS phagocytosis progression. A triterpenoid Nrf2 activator, RS9, facilitated POS uptake into phagolysosomes in RPE cells. RS9 also induced the expression of the autophagy-related proteins, LC3-II and p62, as well as phase-2 antioxidant enzymes. The effect of RS9 on POS phagocytosis was abolished by autophagy inhibition. Unexpectedly, p62 knockdown did not inhibit the effect of RS9 on POS phagocytosis, although, RS9-mediated LC3-II induction by RS9 was inhibited in p62 knockdown RPE cells. We also found that RS9 activated the AMPKα-mTOR signaling pathway earlier than p62 induction. Knockdown of AMPKα₁, but not α₂, inhibited the RS9-mediated activation of LC3-associated phagocytosis and RS9-mediated induction of LC3-II. Furthermore, intravitreal treatment of RS9 to adult mice decreased the size of POS phagolysosomes after light exposure. Collectively, these results showed that RS9-mediated activation of POS phagocytosis was mainly ascribed to the enhancement of autophagy via AMPKα₁ activation. Our findings reveal novel effects of Nrf2 and AMPK α₁ activation that contribute to the maintenance of the RPE function via LC3-associated POS phagocytosis.

Optimization of WZ4003 as NUAK inhibitors against human colorectal cancer

NUAK, the member of AMPK (AMP-activated protein kinase) family of protein kinases, is phosphorylated and activated by the LKB1 (liver kinase B1) tumor suppressor protein kinase. Recent work has indicated that NUAK1 is a key component of the antioxidant stress response pathway, and the inhibition of NUAK1 will suppress the growth and survival of colorectal tumors. As a promising target for anticancer drugs, few inhibitors of NUAK were developed. With this goal in mind, based on NUAK inhibitor WZ4003, a series of derivatives has been synthesized and evaluated for anticancer activity. Compound 9q, a derivative of WZ4003 by removing a methoxy group, was found to be the most potential one with stronger inhibitory against NUAK1/2 enzyme activity, tumor cell proliferation and inducing apoptosis of tumor cells. By in vivo efficacy evaluations of colorectal SW480 xenografts, 9q suppresses tumor growth more effectively with an excellent safety profile in vivo and is therefore seen as a suitable candidate for further investigation.

Carbon monoxide-loaded red blood cells ameliorate metabolic dysfunction-associated steatohepatitis progression via enhancing AMP-activated protein kinase activity and inhibiting Kupffer cell activation

Metabolic dysfunction-associated steatohepatitis (MASH) is a progressive form of nonalcoholic fatty liver disease characterised by fat accumulation, inflammation, oxidative stress, fibrosis, and impaired liver regeneration. In this study, we found that heme oxygenase-1 (HO-1) is induced in both MASH patients and in a MASH mouse model. Further, hepatic carbon monoxide (CO) levels in MASH model mice were >2-fold higher than in healthy mice, suggesting that liver HO-1 is activated as MASH progresses. Based on these findings, we used CO-loaded red blood cells (CO-RBCs) as a CO donor in the liver, and evaluated their therapeutic effect in methionine-choline deficient diet (MCDD)-induced and high-fat-diet (HFD)-induced MASH model mice. Intravenously administered CO-RBCs effectively delivered CO to the MASH liver, where they prevented fat accumulation by promoting fatty acid oxidation via AMP-activated protein kinase (AMPK) activation and peroxisome proliferator-activated receptor induction. They also markedly suppressed Kupffer cell activation and their corresponding anti-inflammatory and antioxidative stress activities in MASH mice. CO-RBCs also helped to restore liver regeneration in mice with HFD-induced MASH by activating AMPK. We confirmed the underlying mechanisms by performing in vitro experiments in RAW264.7 cells and palmitate-stimulated HepG2 cells. Taken together, CO-RBCs show potential as a promising cellular treatment for MASH.

An ADAM17 selective inhibitor promotes glucose uptake by activating AMPK

AMPK activation promotes glucose and lipid metabolism. Here, we found that our previously reported ADAM17 inhibitor SN-4 activates AMPK and promotes membrane translocation and sugar uptake of GLUT4. AMPK inhibitor dorsomorphin reversed this effect of SN-4, confirming that the effect is mediated by AMPK activation. In addition, SN-4 inhibited lipid accumulation in HepG2 under high glucose conditions by promoting lipid metabolism and inhibiting lipid synthesis. Although lactic acidosis is a serious side effect of biguanides such as metformin, SN-4 did not affect lactate production. Furthermore, SN-4 was confirmed to inhibit the release of TNF-α, a causative agent of insulin resistance, from adipocytes. In diabetes treatment, it is important to not only regulate blood sugar levels but also prevent complications. Our findings reveal the therapeutic potential of SN-4 as a new antidiabetic drug that can also help prevent future complications.