Yan Liu

The nuclear factor κB (NF-κB) signaling pathway plays a critical role in activating macrophages in the pathogenesis of many inflammatory diseases. Tissue mechanical properties are important in modulating key cellular proinflammatory responses. Here, we investigated how the mechanosensitive membrane cation channel TRPV4 (transient receptor potential vanilloid 4) limits macrophage proinflammatory responses in bacterial pneumonia. We found that TRPV4 suppressed proinflammatory gene expression in alveolar macrophages in response to Pseudomonas aeruginosa pneumonia in mice and in response to agonists of various Toll-like receptors (TLRs) in vitro. TRPV4 suppressed proinflammatory gene expression in macrophages by decreasing the activity of the NF-κB subunit p65. Upon stimulation of macrophages with bacterial lipopolysaccharide, a fraction of TRPV4 translocated from the endoplasmic reticulum to the plasma membrane, releasing p65 for nuclear translocation. TRPV4 interacted with p65 through an N-terminal cytoplasmic ankyrin repeat domain (ANKRD) that shares sequence homology with the p65-binding ANKRD of the NF-κB inhibitor IκBα. Given the diverse roles of TRPV4 and NF-κB in various cell types, our identification of cross-talk between a mechanosensitive channel and p65 in macrophages suggests application to many NF-κB–dependent diseases, such as cancer and atherosclerosis.

Purpose of review: The aim of this study is to highlight recent studies that have advanced our understanding of apolipoprotein F (ApoF) and its role in lipid metabolism. Recent findings: Previous studies showed that ApoF hepatic mRNA levels are suppressed by fat-enriched diets. Recent studies show this downregulation is mediated by agonist-induced binding of liver X receptor (LXR) and PPARalpha to a regulatory element in the ApoF promoter. First-of-kind in-vivo studies show ApoF lowers low-density lipoprotein levels and enhances reverse cholesterol transport in fat-fed hamsters. Summary: Diverse studies collectively provide compelling evidence that cholesteryl ester transfer protein (CETP) plays an important role in regulating lipid metabolism. Inhibiting CETP raises HDL cholesterol. However, considering the recent failures of pharmacological inhibitors of CETP in clinical trials, it does not seem likely that global inhibition of CETP will be beneficial. ApoF is a minor apolipoprotein that functions as a natural inhibitor of CETP. However, ApoF is not a general inhibitor of CETP, but rather it preferentially inhibits CETP activity with LDL. Therefore, ApoF tailors CETP activity so that less tissue-derived cholesterol traffics from HDL into the LDL compartment. Lower LDL cholesterol levels have recognized clinical benefit for reduced cardiovascular disease.

De novo lipogenesis is tightly regulated by insulin and nutritional signals to maintain metabolic homeostasis. Excessive lipogenesis induces lipotoxicity, leading to nonalcoholic fatty liver disease (NAFLD) and type 2 diabetes. Genetic lipogenic programs have been extensively investigated, but epigenetic regulation of lipogenesis is poorly understood. Here, we identified Slug as an important epigenetic regulator of lipogenesis. Hepatic Slug levels were markedly upregulated in mice by either feeding or insulin treatment. In primary hepatocytes, insulin stimulation increased Slug expression, stability, and interactions with epigenetic enzyme lysine-specific demethylase-1 (Lsd1). Slug bound to the fatty acid synthase (Fasn) promoter where Slug-associated Lsd1 catalyzed H3K9 demethylation, thereby stimulating Fasn expression and lipogenesis. Ablation of Slug blunted insulin-stimulated lipogenesis. Conversely, overexpression of Slug, but not a Lsd1 binding-defective Slug mutant, stimulated Fasn expression and lipogenesis. Lsd1 inhibitor treatment also blocked Slug-stimulated lipogenesis. Remarkably, hepatocyte-specific deletion of Slug inhibited the hepatic lipogenic program and protected against obesity-associated NAFLD, insulin resistance, and glucose intolerance in mice. Conversely, liver-restricted overexpression of Slug, but not the Lsd1 binding-defective Slug mutant, had the opposite effects. These results unveil an insulin/Slug/Lsd1/H3K9 demethylation lipogenic pathway that promotes NAFLD and type 2 diabetes.

Reparative hepatocyte replication is impaired in chronic liver disease, contributing to disease progression; however, the underlying mechanism remains elusive. Here, we identify Map3k14 (also known as NIK) and its substrate Chuk (also called IKKα) as unrecognized suppressors of hepatocyte replication. Chronic liver disease is associated with aberrant activation of hepatic NIK pathways. We found that hepatocyte-specific deletion of Map3k14 or Chuk substantially accelerated mouse hepatocyte proliferation and liver regeneration following partial-hepatectomy. Hepatotoxin treatment or high fat diet feeding inhibited the ability of partial-hepatectomy to stimulate hepatocyte replication; remarkably, inactivation of hepatic NIK markedly increased reparative hepatocyte proliferation under these liver disease conditions. Mechanistically, NIK and IKKα suppressed the mitogenic JAK2/STAT3 pathway, thereby inhibiting cell cycle progression. Our data suggest that hepatic NIK and IKKα act as rheostats for liver regeneration by restraining overgrowth. Pathological activation of hepatic NIK or IKKα likely blocks hepatocyte replication, contributing to liver disease progression.

Insulin stimulates lipogenesis but insulin resistance is also associated with increased hepatic lipogenesis in obesity. However, the underlying mechanism remains poorly characterized. Here, we show a noncanonical insulin-Snail1 pathway that suppresses lipogenesis. Insulin robustly upregulates zinc-finger protein Snail1 in a PI 3-kinase-dependent manner. In obesity, the hepatic insulin-Snail1 cascade is impaired due to insulin resistance. Hepatocyte-specific deletion of Snail1 enhances insulin-stimulated lipogenesis in hepatocytes, exacerbates dietary NAFLD in mice, and attenuates NAFLD-associated insulin resistance. Liver-specific overexpression of Snail1 has the opposite effect. Mechanistically, Snail1 binds to the fatty acid synthase promoter and recruits HDAC1/2 to induce deacetylation of H3K9 and H3K27, thereby repressing fatty acid synthase promoter activity. Our data suggest that insulin pathways bifurcate into canonical (lipogenic) and noncanonical (anti-lipogenesis by Snail1) two arms. The noncanonical arm counterbalances the canonical arm through Snail1-elicited epigenetic suppression of lipogenic genes. Impairment in the insulin-Snail1 arm may contribute to NAFLD in obesity.

Background: As a monoamine neurotransmitter, 5‐hydroxytryptamine (5‐HT) or serotonin modulates mood, appetite, and sleep. Besides, 5‐HT also has important peripheral functions. 5‐HT receptor 2B (5‐HT2BR) plays a key role in cardiovascular diseases, such as pulmonary arterial hypertension and cardiac valve disease. Percutaneous intervention has been used to restore blood flow in occlusive vascular disease. However, restenosis remains a significant problem. Herein, we investigated the role of 5‐HT2BR in neointimal hyperplasia, a key pathological process in restenosis. Methods and Results: The expression of 5‐HT2BR was upregulated in wire‐injured mouse femoral arteries. In addition, BW723C86, a selective 5‐HT2BR agonist, promoted the injury response during restenosis. 5‐HT and BW723C86 stimulated migration and proliferation of rat aortic smooth muscle cells. Conversely, LY272015, a selective antagonist, attenuated the 5‐HT–induced smooth muscle cell migration and proliferation. In vitro study showed that the promigratory effects of 5‐HT2BR were mediated through the activation of mammalian target of rapamycin (mTOR)/p70S6K signaling in a β‐arrestin2–dependent manner. Inhibition of mammalian target of rapamycin or p70S6K mitigated 5‐HT2BR–mediated smooth muscle cell migration. Mice with deficiency of 5‐HT2BR showed significantly reduced neointimal formation in wire‐injured arteries. Conclusions: These results demonstrated that activation of 5‐HT2BR and β‐arrestin2–biased downstream signaling are key pathological processes in neointimal formation, and 5‐HT2BR may be a potential target for the therapeutic intervention of vascular restenosis.