Diabetes Accelerates Steatohepatitis in Mice: Liver Pathology and Single-Cell Gene Expression Signatures.

Glucose lowering independently reduces liver fibrosis in human nonalcoholic fatty liver disease. This study investigated the impact of diabetes on steatohepatitis and established a novel mouse model for diabetic steatohepatitis. Male C57BL/6J mice were fed a 60% high-fat diet (HFD) and injected with carbon tetrachloride (CCl4) and streptozotocin (STZ) to induce diabetes. The HFD+CCl4+STZ group showed more severe liver steatosis, hepatocyte ballooning, and regenerative nodules compared with other groups. Diabetes up-regulated inflammatory cytokine-associated genes and increased the M1/M2 macrophage ratios in the liver. Single-cell RNA sequencing analysis of nonparenchymal cells in the liver showed that diabetes reduced Kupffer cells and increased bone marrow-derived recruited inflammatory macrophages, such as Ly6Chi-RM. Diabetes globally reduced liver sinusoidal endothelial cells (LSECs). Furthermore, genes related to the receptor for advanced glycation end products (RAGE)/Toll-like receptor 4 (TLR4) were up-regulated in Ly6Chi-RM and LSECs in mice with diabetes, suggesting a possible role of RAGE/TLR4 signaling in the interaction between inflammatory macrophages and LSECs. This study established a novel diabetic steatohepatitis model using a combination of HFD, CCl4, and STZ. Diabetes exacerbated steatosis, hepatocyte ballooning, fibrosis, regenerative nodule formation, and the macrophage M1/M2 ratios triggered by HFD and CCl4. Single-cell RNA sequencing analysis indicated that diabetes activated inflammatory macrophages and impairs LSECs through the RAGE/TLR4 signaling pathway. These findings open avenues for discovering novel therapeutic targets for diabetic steatohepatitis.

Selenoprotein P deficiency protects against immobilization-induced muscle atrophy by suppressing atrophy-related E3 ubiquitin ligases.

The quality of skeletal muscle is maintained by a balance between protein biosynthesis and degradation. Disruption in this balance results in sarcopenia. However, its underlying mechanisms remain underinvestigated. Selenoprotein P (SeP; encoded by Selenop in mice) is a hepatokine that is upregulated in type 2 diabetes and aging and causes signal resistances via reductive stress. We created immobilized muscle atrophy model in Selenop knockout (KO) mice. Immobilization (IMM) significantly reduced cross-sectional areas and the size of skeletal muscle fibers, which were ameliorated in KO mice. IMM upregulated the genes encoding E3 ubiquitin ligases and their upstream FoxO1, FoxO3, and KLF15 transcription factors in the skeletal muscle, which were suppressed in KO mice. These findings suggest a possible involvement of SeP-mediated reductive stress in physical inactivity-mediated sarcopenia, which may be a therapeutic target against sarcopenia.NEW & NOTEWORTHY Selenoprotein P (SeP) is a hepatokine that is upregulated in type 2 diabetes and aging and causes signal resistances via reductive stress. Immobilization (IMM) significantly reduced skeletal muscle mass in mice, which was prevented in SeP knockout (KO) mice. IMM-induced Foxos/KLF15-atrogene upregulation was suppressed in the skeletal muscle of KO mice. These findings suggest that SeP-mediated reductive stress is involved in and may be a therapeutic target for physical inactivity-mediated muscle atrophy.

Forkhead box protein O1 (FoxO1) knockdown accelerates the eicosapentaenoic acid (EPA)-mediated Selenop downregulation independently of sterol regulatory element-binding protein-1c (SREBP-1c) in H4IIEC3 hepatocytes.

Selenoprotein P is upregulated in type 2 diabetes, causing insulin and exercise resistance. We have previously reported that eicosapentaenoic acid (EPA) negatively regulates Selenop expression by suppressing Srebf1 in H4IIEC3 hepatocytes. However, EPA downregulated Srebf1 long before downregulating Selenop. Here, we report additional novel mechanisms for the Selenop gene regulation by EPA. EPA upregulated Foxo1 mRNA expression, which was canceled with the ERK1/2 inhibitor, but not with the PKA inhibitor. Foxo1 knockdown by siRNA initiated early suppression of Selenop, but not Srebf1, by EPA. However, EPA did not affect the nuclear translocation of the FoxO1 protein. Neither ERK1/2 nor PKA inhibitor affected FoxO1 nuclear translocation. In summary, FoxO1 knockdown accelerates the EPA-mediated Selenop downregulation independent of SREBP-1c in hepatocytes. EPA upregulates Foxo1 mRNA via the ERK1/2 pathway without altering its protein and nuclear translocation. These findings suggest redundant and conflicting transcriptional networks in the lipid-induced redox regulation.

PAC1 Deficiency Protects Obese Male Mice From Immobilization-Induced Muscle Atrophy by Suppressing FoxO-Atrogene Axis.

Muscle atrophy is the cause and consequence of obesity. Proteasome dysfunction mediates obesity-induced endoplasmic reticulum (ER) stress and insulin resistance in the liver and adipose tissues. However, obesity-associated regulation of proteasome function and its role in the skeletal muscles remains underinvestigated. Here, we established skeletal muscle-specific 20S proteasome assembly chaperone-1 (PAC1) knockout (mPAC1KO) mice. A high-fat diet (HFD) activated proteasome function by ∼8-fold in the skeletal muscles, which was reduced by 50% in mPAC1KO mice. mPAC1KO induced unfolded protein responses in the skeletal muscles, which were reduced by HFD. Although the skeletal muscle mass and functions were not different between the genotypes, genes involved in the ubiquitin proteasome complex, immune response, endoplasmic stress, and myogenesis were coordinately upregulated in the skeletal muscles of mPAC1KO mice. Therefore, we introduced an immobilization-induced muscle atrophy model in obesity by combining HFD and immobilization. mPAC1KO downregulated atrogin-1 and MuRF1, together with their upstream Foxo1 and Klf15, and protected against disused skeletal muscle mass reduction. In conclusion, obesity elevates proteasome functions in the skeletal muscles. PAC1 deficiency protects mice from immobilization-induced muscle atrophy in obesity. These findings suggest obesity-induced proteasome activation as a possible therapeutic target for immobilization-induced muscle atrophy.