American journal of physiology. Gastrointestinal and liver physiology最新文献

DRA (Downregulated in adenoma, SLC26A3) is a major apical intestinal Cl^-/HCO₃^- exchanger, which is expressed in complex and hybrid N-glycosylated forms. Although the importance of N-glycosylation is evident from the significantly reduced transport activity of non-N-glycosylated DRA constructs (DRA-N0), the underlying molecular mechanisms are controversial. Therefore, plasma membrane expression and lipid raft localization of glycosylation-deficient DRA-N0 were analyzed in HEK cells. The activity of DRA-N0 was reduced by 70% compared with the wild-type construct. Absolute expression of DRA-N0 was significantly reduced by ∼57% in the cell lysate and by 34 and 45% in the plasma membrane and in plasma membrane-derived lipid rafts, respectively. These amounts are insufficient to account for the reduction in activity. Furthermore, the statistical analysis did not support a difference in the relative expression of DRA and DRA-N0 in the plasma membrane and in plasma membrane-derived lipid rafts, indicating that N-glycosylation does not affect transport activity through trafficking and localization in these cell compartments. To gain insight into potential intramolecular effects of N-glycosylation on DRA, its three-dimensional structure was predicted using AlphaFold3 with complex N-glycans covalently attached to N153, N161, and N164 in the transport domain. This revealed multiple inward- and outward-facing conformations of the protein. The number of interdomain contacts of the transport domain-bound glycans with the scaffold domain was higher in the inward-facing state. Because substrate release to the cytoplasm represents the rate-limiting step in many transport proteins, this suggests that in DRA, glycans stabilize the inward-facing state facilitating anion transport.NEW & NOTEWORTHY Deficient N-glycosylation decreases DRA transport activity but does not significantly affect trafficking to the plasma membrane or to lipid rafts. Meanwhile, molecular modeling predicts stabilizing interdomain contacts of the glycans, covalently attached to the transport domain, with the scaffold domain having more contacts in the inward-facing state. Favoring the inward-facing state may facilitate more efficacious anion transport, as substrate release from this state into the cytoplasm is a rate limiting step for numerous transport proteins.

The brain regulates liver metabolism through neuroendocrine and autonomic pathways, which can be disrupted in metabolic dysfunction-associated steatotic liver disease (MASLD). Although autonomic dysfunction, including liver neuropathy, has been reported in MASLD, the role of hepatic sympathetic signaling in disease progression remains unclear. Recent studies show that liver innervation is predominantly of a sympathetic nature, suggesting that adrenergic receptors in hepatocytes may influence the pathogenesis of MASLD. We previously identified adrenoceptor alpha-1b (ADRA1B) as the dominant hepatic adrenergic receptor. Here, we hypothesized that ADRA1B plays a protective role in MASLD progression. To test this, we generated hepatocyte-specific Adra1b knockout mice (Adra1b^LKO) and induced MASLD with the Gubra Amylin NASH diet for up to 32 wk. Liver pathology was quantified by automated image analysis (MorphoQuant), and metabolic phenotyping included glucose tolerance, insulin sensitivity, and bile acid composition. Hepatocyte-specific Adra1b deletion did not affect body weight, hepatic lipid accumulation, glucose tolerance, or insulin sensitivity. However, Adra1b^LKO mice exhibited significantly increased hepatic inflammation compared to wild-type controls. These changes were associated with higher hepatic expression of tumor necrosis factor (Tnf) and interleukin-1b (Il1b), as well as an increase in monocyte chemoattractant protein-1 (MCP-1) and interleukin-6 (IL-6). We also observed elevated transforming growth factor beta (TGF-β) and α-smooth muscle actin (Acta2) expression, suggesting activation of hepatic stellate cells. In addition, Adra1b^LKO mice displayed higher circulating bilirubin levels, with no significant alterations in albumin and bile acid pool composition. These findings reveal a previously unrecognized role for hepatic ADRA1B in restraining inflammatory responses in MASLD. Loss of Adra1b signaling promotes hepatic inflammation, highlighting a neuroimmune mechanism that may be targeted to prevent disease progression.NEW & NOTEWORTHY This study identifies the hepatic α1b adrenoceptor (ADRA1B) as a regulator of inflammation in metabolic dysfunction-associated steatotic liver disease (MASLD). Using a hepatocyte-specific knockout model, we show that loss of Adra1b exacerbates hepatic inflammatory responses without affecting steatosis or systemic metabolism. These findings reveal a previously unknown immune mechanism in liver disease progression.

α-1 Antitrypsin deficiency (AATD) is a genetic disorder characterized by accumulation of misfolded Z α-1 antitrypsin (ZAAT) in hepatocytes, leading to liver injury and metabolic dysfunction. There is no therapy to reduce ZAAT accumulation and restore proteostasis. Pioglitazone activates AMP-activated protein kinase (AMPK), enhance autophagy, and modulate endoplasmic reticulum stress responses, suggesting a potential effect on ZAAT clearance. Our objective is to examine whether pioglitazone can protect against AATD-mediated liver disease. Huh7.5 cells expressing ZAAT (HuhZ) and Pi*Z transgenic mice were used to investigate pioglitazone treatment on hepatic ZAAT accumulation, autophagy activation, and AMPK signaling. Histological, molecular, and metabolic analyses were conducted to assess changes in ZAAT content, autophagy markers, AMPK phosphorylation, and proteostasis. Pioglitazone significantly reduced intracellular ZAAT and decreased lipid droplet accumulation in HuhZ cells. Pioglitazone markedly lowered hepatic ZAAT content in Pi*Z mice, suggesting enhanced degradation. This reduction was mediated through the AMPK pathway, indicated by increased phosphorylation of AMPK and ULK1. Pioglitazone induced autophagy, shown by decreased p62 and increased ATG5 and LC3B-II. This is indicative of enhanced autophagy. Although total hepatic AAT levels were reduced, periodic acid-Schiff with diastase-positive ZAAT aggregates exhibited only a downward trend, suggesting these may be more resistant to clearance. These findings demonstrate pioglitazone reduces hepatic ZAAT accumulation by activating AMPK and inducing autophagy in AATD-associated liver disease, supporting its potential for therapeutic repurposing. As pioglitazone is FDA-approved with benefits for metabolic liver health, further studies are warranted to evaluate efficacy in restoring proteostasis and reducing hepatic ZAAT.NEW & NOTEWORTHY α-1 Antitrypsin deficiency (AATD)-mediated liver disease lacks therapies that reduce hepatic ZAAT accumulation and liver manifestations. We demonstrate that pioglitazone activates AMPK and induces autophagy, leading to decreased ZAAT and improved proteostasis in Pi*Z mouse livers and human hepatocyte models. As an FDA-approved drug with metabolic benefits, pioglitazone holds promise for repurposing in AATD-related liver disease. These findings offer a mechanistic rationale for targeting autophagy to alleviate hepatic injury in protein misfolding disorders.

During cholestasis, cholangiocytes become activated, promoting macrophage-associated periductal infiltration and fibrosis. The cholangiocyte-specific mechanisms responsible for these processes are unclear. To gain insight into the cholangiocyte signaling mechanisms contributing to these pathophysiologic processes, mice were fed a 3,5-diethoxycarbonyl-1,4-dihydro-collidine (DDC) diet for 10 days to induce liver injury and then switched to a chow diet to permit recovery, designated as R days. Profiling of isolated intrahepatic leukocytes by mass spectrometry revealed an abundant CX3CR1⁺ macrophage population on the DDC diet that declined during the recovery period. This observation was confirmed using Cx3cr1^GFP mice. Next, cholangiocytes were isolated from control, DDC, and R15 mice, and RNA sequencing (RNAseq) was performed. Cholangiocyte CX3CL1 expression, the cognate ligand for CX3CR1, increased in DDC-fed mice and returned to basal values by R15, implicating cholangiocytes in CX3CR1⁺ macrophage recruitment. Ingenuity pathway analysis (IPA) of the RNAseq data revealed upregulation of the pathogen-induced cytokine storm pathway in cholangiocytes activated from DDC fed mice, and resolution of this pathway in R15 isolated cholangiocytes. SCENIC regulon analysis identified that NF-Y, a transcription factor complex, was activated only on the DDC diet, but not in control or R15 mice. Finally, siRNA targeted suppression of NF-YA in normal human cholangiocytes (NHC) reduced cholangiocyte expression of the profibrogenic ligand TGFβ1. Consistent with this observation, Tgfβ1 was increased in cholangiocytes from DDC-fed animals that returned to control values at day R15. Collectively, these observations provide mechanistic insights into cholangiocyte pathobiology during cholestasis.NEW & NOTEWORTHY Cholangiocyte pathophysiological activation was examined in a model of murine cholestasis. CX3CR1⁺ macrophages are recruited to the periportal region, likely mediated by cholangiocyte expression of CX3CL1. Cholangiocyte transcriptomics from cholestatic mice display activation of a "pathogen-induced cytokine storm" pathway, and exhibit activation of the transcription factor NF-Y. In human cholangiocytes, NF-Y promotes expression of the profibrogenic ligand TGFβ1. These observations provide insights into the cholestatic cholangiocyte pathobiology contributing to periductal inflammation and fibrosis.

The role of N⁶-methyladenosine (m⁶A) RNA methylation in liver regeneration is unclear. This study aimed to determine the role of m⁶A methylation in liver regeneration after a 70% hepatectomy (HEPA) using liver-specific methyltransferase-like 14 (Mettl14) knockout (KO) male mice. Analysis was conducted on postoperative days 1, 3, or 7 (HEPA1, 3, or 7) in control (Flox) mice. In Flox mice, cyclin D1 protein expression was highest on postoperative day 3 (HEPA3) consistent with a dynamic increase in hepatocyte replication. The abundance of Mettl14 protein presented a similar pattern on HEPA3. Then, we performed hepatectomy in Mettl14 KOs (M14KO) and Flox controls and observed significantly higher postsurgical mortality in mutants. In Flox mice, cyclin D1 protein levels and Ki-67 were markedly increased on HEPA3 compared to sham operation, while being downregulated in M14KO. Characterizing the m⁶A epitranscriptomic changes in Flox mice after hepatectomy and contrasting them to hepatectomy in M14KO in HEPA3 revealed enrichment for gene ontology terms associated with endoplasmic reticulum, inflammation, and apoptosis. Differentially methylated genes in M14KO compared to Flox on HEPA3 were also enriched for peroxisome proliferator-activated receptor (PPAR) and AMPK signaling. Finally, we identified hypomethylated transcripts involved in fibrinogen synthesis, such as Fga, Fgb, and Fgg, by comparing differentially m⁶A-decorated genes in M14KO vs. Flox on HEPA3. Knockdown of fibrinogen leads to suppression of proliferation via activation of p21 protein in AML12 cells. Together, these data point to m⁶A RNA methylation being significant in decorating genes involved in fibrinogen synthesis in liver regeneration.NEW & NOTEWORTHY This study uncovers a previously unrecognized mechanism for regulation of the fibrinogen pathway in N⁶-methyladenosine (m⁶A) RNA methylation-mediated liver regeneration.

Metabolic dysfunction-associated steatohepatitis-driven hepatocellular carcinoma (MASH-HCC) incidence is rapidly rising worldwide. Lipid metabolic reprogramming is a hallmark of solid tumors to satisfy cancer's high metabolic demand. However, it may confer sensitivity to ferroptosis, a cell death mode driven by iron-dependent lipid peroxidation. In this report, we describe the lipid metabolic landscape in MASH-HCC and characterize long-chain acyl-CoA synthetases (ACSLs), a family of enzymes involved in the synthesis of cellular lipids. Bulk RNA sequencing, single-cell RNA sequencing, spatial transcriptomics, and immunohistochemistry analyses of human MASH-HCC were integrated to identify differentially expressed lipid metabolism genes. Ferroptosis in vitro was assessed in human HCC cell lines. The characterization of ACSLs was also conducted at the single-cell level in a diet-induced experimental murine model of MASH-HCC. Our analysis revealed that in human MASH-HCC, ACSLs exhibit a heterogeneous expression, with ACSL4 notably enriched in tumor tissues, contrasting with ACSL5 upregulation in noncancerous MASH. We identified a unique lipid metabolic gene signature of MASH-HCC, which included genes associated with ferroptosis vulnerability. In vitro, high ACSL4 expression was associated with increased ferroptosis sensitivity in human HCC cell lines. Finally, single-cell RNA sequencing revealed elevated ACSL4 expression in immune cells in a murine MASH-HCC model, suggesting a role of ACSL4 in shaping the tumor immune microenvironment. Overall, this report offers new insights into the lipid metabolic landscape and ferroptosis sensitivity for novel MASH-HCC treatments.NEW & NOTEWORTHY Our study examined healthy human MASH and MASH-associated hepatocellular carcinoma (MASH-HCC) livers using bulk and scRNA sequencing, spatial transcriptomics, and immunohistochemistry. We found that ACSLs displayed differential and spatially heterogeneous expression. ACSL4 was abundant in tumor tissues, whereas ACSL5 was elevated in noncancerous MASH tissues. ACSL4 was mainly found in immune cells like natural killer cells and natural killer T cells in murine MASH-HCC, suggesting its role in tumor immune microenvironment modulation.

Parenteral nutrition (PN) is a lifesaving intervention for patients unable to feed enterally but is often associated with parenteral nutrition-associated liver disease (PNALD), also called intestinal failure-associated liver disease (IFALD). This disease is characterized by steatosis, cholestasis, and elevated liver stress markers. Continuous PN induces hepatic injury through mechanisms including insulin resistance, lipotoxicity, systemic inflammation, and oxidative stress. Infusion cycling is known to ameliorate clinical markers of liver injury, but metabolic underpinnings have not been thoroughly investigated. Therefore, we modeled PN-induced liver injury in mice to investigate how differential infusion patterns impacted hepatic metabolism. Intermittent infusions protected against increased circulating alanine aminotransferase levels and improved histopathology to more closely resemble chow controls. Transcriptomic analyses revealed 804 differentially expressed genes between PN groups, highlighting pathways related to peroxisome proliferator-activated receptor signaling, fatty acid metabolism, and peroxisomes. Relative to the continuous group, intermittent PN infusion specifically downregulated Acaa1b, Aldh3a2, Inmt, and Acot4; transcripts involved in peroxisomal lipid oxidation, dicarboxylic acid synthesis, and one-carbon metabolism. This study suggests that infusion cycling may attenuate metabolic burden induced by alternate lipid oxidation pathways. Future work can therapeutically leverage these metabolic pathways to deepen our understanding of PNALD/IFALD and guide PN treatments to improve patient outcomes.NEW & NOTEWORTHY This work demonstrated that the infusion schedule, independent of nutrient and caloric concentration, is a modulator of hepatic lipid metabolism in a novel murine model of parenteral nutrition. This cyclic infusion paradigm attenuated transcripts involved in microsomal and peroxisomal lipid oxidation, which were upregulated in the continuous infusion group. These data support the clinical use of cyclic infusion to improve hepatic parameters known to be adversely affected by parenteral nutrition.

The mucus layer is an essential physical barrier that protects and lubricates mucosal surfaces in the body. The semipermeable nature of the mucus layer limits bacterial interactions with the epithelium while allowing nutrient absorption. Goblet cells (GCs) are specialized epithelial cells with a classical role to synthesize and secrete mucus to maintain the mucus layer. Emerging research has revealed the diverse nature of GC functions, including their interaction with the immune system through goblet cell-associated antigen passages to promote tolerance to dietary and bacterial antigens. Dysfunction of GCs or the mucus layer leaves the epithelium vulnerable to infection and is commonly associated with digestive disease. As such, there is a growing appreciation for the importance of GCs and the mucus layer to regulate mucosal homeostasis and protect against disease. Long-standing anatomical and pharmacological evidence indicates that the nervous system is a key regulator of GC function. However, the relative contribution from each division of the nervous system to the control of GC function is poorly defined. This is partly due to conflicting evidence from the literature and differences in experimental methods used. Furthermore, whether neurotransmitters influence GC functions and the associated mucus barrier directly or via indirect mechanisms, such as enhanced fluid secretion, remains unclear. The emergence of highly specific genetic approaches provides new opportunities to examine how specific nerve types can influence GC function. In this review, we consolidate the literature to date, with a focus on the stomach and lower gastrointestinal tract, and outline how current technologies may be useful to progress our fundamental understanding of neural-GC control.