Targeting IRE1α and PERK in the endoplasmic reticulum stress pathway attenuates fatty acid-induced insulin resistance in bovine hepatocytes

The role of glutathione S-transferase mu 2 in mitigating fatty acid-induced hepatic inflammation in dairy cows

Fatty liver is a major metabolic disorder in perinatal dairy cows, characterized by elevated plasma concentrations of nonesterified fatty acids (NEFA) and hepatic inflammation. Glutathione S-transferase mu 2 (GSTM2), a phase II detoxification enzyme, regulates cellular antioxidant and detoxification processes in nonruminants. However, its involvement in NEFA-induced hepatic inflammation in dairy cows with fatty liver remains unclear. This study aimed to elucidate the role of GSTM2 in mediating hepatic inflammation caused by elevated NEFA levels in dairy cows with severe fatty liver. An in vivo study was conducted using 10 healthy cows (hepatic triacylglycerol [TG] content <1%) and 10 cows with severe fatty liver (hepatic TG content >10%), matched for the number of lactations (median = 3, range = 2-4) and DIM (median = 9 d, range = 3-15 d). Liver tissue and blood samples were collected before feeding. Compared with healthy cows, cows with severe fatty liver had higher plasma concentrations of NEFA, BHB, haptoglobin (HP), plasma amyloid A (SAA), and lower plasma concentration of glucose. These cows also showed significantly lower abundance of hepatic GSTM2 and overactivated hepatic inflammatory pathways, as indicated by increased abundance of phosphorylated inhibitor of κB (IκB)α and nuclear factor κB (NF-κB) p65, NLR family pyrin domain containing 3 (NLRP3), apoptosis-associated speck-like protein containing CARD (ASC), and caspase-1 (CASP1), as well as mRNA levels of tumor necrosis factor α (TNFA), IL6, and IL1B. In vitro, hepatocytes isolated from 5 healthy calves (1 d old, fasted female, 30-40 kg of BW) were used to determine the effects of GSTM2 on hepatic inflammation. First, hepatocytes were treated with NEFA (1.2 mM) for varying durations (0.5, 1, 3, 6, 9, 12, 15, or 18 h). The NEFA treatment significantly increased the phosphorylation of IκBα and NF-κB p65, protein abundance of NLRP3, ASC and CASP1, and mRNA levels of TNFA, IL6 and IL1B, peaking at 9 and 12 h. Second, hepatocytes were treated with different concentrations of NEFA (0, 0.6, 1.2, or 2.4 mM) for 9 h, which decreased GSTM2 protein and mRNA abundance. Meanwhile, GSTM2 was silenced using small interfering RNA or overexpressed using adenovirus for 48 h in hepatocytes, followed by NEFA treatment. Silencing GSTM2 augmented the NEFA-induced increase in phosphorylation of IκBα and NF-κB p65, as well as protein abundance of NLRP3, ASC and CASP1, and mRNA levels of TNFA, IL6 and IL1B. Conversely, overexpression of GSTM2 mitigated these inflammatory signals upon NEFA treatment. In summary, these findings indicate that GSTM2 plays a crucial role in modulating NEFA-induced hepatic inflammation. Targeting GSTM2 may offer new strategies to treat or prevent fatty liver disease in dairy cows.

Caveolin 1 in bovine liver is associated with fatty acid-induced lipid accumulation and the endoplasmic reticulum unfolded protein response: Role in fatty liver development

Disruption of endoplasmic reticulum (ER) homeostasis (i.e., ER stress) is intrinsically linked with lipid metabolism disorders in dairy cows. Caveolin 1 (CAV1) is a ubiquitously expressed membrane-associated scaffolding protein involved in regulating the secretory pathway within the ER. Whether inhibiting the activity of CAV1 affects the ER and its potential role in hepatic lipid deposition in dairy cows is unknown. Biopsies of liver tissue from Holstein cows (median DIM = 13 d, range = 5-21 d) diagnosed as healthy (n = 6; hepatic triacylglycerol (TAG) levels <1%; median milk production = 38.9 kg/d, interquartile range = 38.0-40.8 kg/d) or suffering from fatty liver (n = 6; hepatic TAG levels >5%; median milk production = 36.6 kg/d, interquartile range = 35.7-38.1 kg/d) revealed that fatty liver was associated with lower abundance of the CAV1 gene and protein, higher phosphorylation (p) levels of PERK and IRE1α, and increased abundance of the ATF6, GRP78, and CHOP proteins, and several unfolded protein response (UPR) genes (ATF4, sXBP1, and GRP78). Proteins related to de novo fatty acid synthesis, including ACC1, SREBP-1c, PPARγ, and downstream targets genes of SREBP1 (ACACA and FASN) also had greater abundance. This in vivo analysis highlighted a mechanistic link between CAV1 protein abundance, ER stress, and lipid metabolism in fatty liver. A mechanistic study was then performed in vitro with primary hepatocytes isolated from 5 healthy calves (weight = 40-45 kg; 1 d old). Initially, hepatocytes were treated with free fatty acid (FFA; 1.2 mM) for 1, 3, 6, or 12 h. Treatment with FFA reduced CAV1 protein abundance linearly while reducing abundance of ER stress-related proteins, phosphorylated [p-] IRE1α, p-PERK, GRP78, ATF6, and CHOP. Proteins related to de novo fatty acid synthesis (ACC1, SREBP-1c, PPARγ) also increased linearly, and lipid droplets accumulated progressively over time following FFA treatment. Subsequently, to assess the role of CAV1 in FFA-induced ER stress and de novo fatty acid synthesis, hepatocytes were transfected with pCMV-CAV1 (cattle)-3 × FLAG-Neo (plasmid construct [pc-]CAV1) plasmid to overexpress CAV1 or with siRNA to silence CAV1 (siCAV1) transcription. Overexpression of CAV1 alleviated ER stress by reducing levels of p-PERK and p-IRE1α, as well as protein abundance of ATF6, GRP78, CHOP, and several UPR genes (GRP78, ATF4, and sXBP1). Similarly, CAV1 overexpression decreased protein abundance of ACC1, SREBP-1c, PPARγ, and downstream targets genes of SREBP1 (ACACA and FASN). Conversely, silencing CAV1 exacerbated FFA-induced ER stress and de novo fatty acid synthesis. Considering the negative role of FFA-induced ER stress on lipid accumulation in hepatocytes, a second in vitro experiment involved hepatocytes treated with 0.5 μg/mL tunicamycin (TM; a typical ER stress inducer) for 24 h with or without overexpressing CAV1 (pc-CAV1). Overexpressing CAV1 reversed TM-induced increases in mRNA and protein associated with ER stress and de novo fatty acid synthesis. Furthermore, use of hepatocytes transfected with pc-CAV1 for 48 h and subjected to co-immunoprecipitation revealed that CAV1 interacts with IRE1α and ATF6. Overall, the data suggest that CAV1 may help reduce hepatic ER stress and mitigate fatty acid synthesis by binding to and inhibiting IRE1α and ATF6 signaling.

UFL1 regulates cellular homeostasis by targeting endoplasmic reticulum and mitochondria in NEFA-stimulated bovine mammary epithelial cells via the IRE1α/XBP1 pathway

Elevated levels of NEFA caused by negative energy balance in transition cows induce cellular dyshomeostasis. Ubiquitin-like modifier 1 ligating enzyme 1 (UFL1) can maintain cellular homeostasis and act as a critical regulator of stress responses besides functioning in the ubiquitin-like system. The objective of this study was to elucidate the UFL1 working mechanism on promoting cellular adaptations in bovine mammary epithelial cells (BMECs) in response to NEFA challenge, with an emphasis on the ER and mitochondrial function. The results showed that exogenous NEFA and UFL1 depletion resulted in the disorder of ER and mitochondrial homeostasis and the damage of BMEC integrity, overexpression of UFL1 effectively alleviated the NEFA-induced cellular dyshomeostasis. Mechanistically, our study found that UFL1 had a strong interaction with IRE1α and could modulate the IRE1α/XBP1 pathway of unfolded protein response in NEFA-stimulated BMECs, thereby contributing to the modulation of cellular homeostasis. These findings imply that targeting UFL1 may be a therapeutic alternative to relieve NEB-induced metabolic changes in perinatal dairy cows.

Inositol-requiring enzyme 1α and c-Jun N-terminal kinase axis activation contributes to intracellular lipid accumulation in calf hepatocytes

During the perinatal period, dairy cows undergo negative energy balance, resulting in elevated circulating levels of nonesterified fatty acids (NEFA). Although increased blood NEFA concentrations are a physiological adaptation of early lactation, excessive NEFA in dairy cows is a major cause of fatty liver. Aberrant lipid metabolism leads to hepatic lipid accumulation and subsequently the development of fatty liver. Both inositol-requiring enzyme 1α (IRE1α) and c-Jun N-terminal kinase (JNK) have been validated for their association with hepatic lipid accumulation, including their regulatory functions in calf hepatocyte insulin resistance, oxidative stress, and apoptosis. Meanwhile, both IRE1α and JNK are involved in lipid metabolism in nonruminants. Therefore, the aim of this study was to investigate how IRE1α and JNK regulate lipid metabolism in bovine hepatocytes. An experiment was conducted on randomly selected 10 healthy cows (hepatic triglyceride [TG] content <1%) and 10 cows with fatty liver (hepatic TG content >5%). Liver tissue and blood samples were collected from experimental cows. Serum concentrations of NEFA and β-hydroxybutyrate (BHB) were greater, whereas serum concentrations of glucose and milk production were lower in cows with fatty liver. The western blot results revealed that dairy cows with fatty liver had higher phosphorylation levels of JNK, c-Jun, and IRE1α in the liver tissue. Three in vitro experiments were conducted using primary calf hepatocytes isolated from 5 healthy calves (body weight: 30-40 kg; 1 d old). First, hepatocytes were treated with NEFA (1.2 mM) for 0.5, 1, 2, 3, 5, 7, 9, or 12 h, which showed that the phosphorylated levels of JNK, c-Jun, and IRE1α increased in both linear and quadratic effects. In the second experiment, hepatocytes were treated with high concentrations of NEFA (1.2 mM) for 12 h with or without SP600125, a canonical inhibitor of JNK. Western blot results showed that SP600125 treatment could decrease the expression of lipogenesis-associated proteins (PPARγ and SREBP-1c) and increase the expression of fatty acid oxidation (FAO)-associated proteins (CPT1A and PPARα) in NEFA-treated hepatocytes. The perturbed expression of lipogenesis-associated genes (FASN, ACACA, and CD36) and FAO-associated gene ACOX1 were also recovered by JNK inhibition, indicating that JNK reduced excessive NEFA-induced lipogenesis and FAO dysregulation in calf hepatocytes. Third, short hairpin RNA targeting IRE1α (sh-IRE1α) was transfected into calf hepatocytes to silence IRE1α, and KIRA6 was used to inhibit the kinase activity of IRE1α. The blockage of IRE1α could at least partially suppressed NEFA-induced JNK activation. Moreover, the blockage of IRE1α downregulated the expression of lipogenesis genes and upregulated the expression of FAO genes in NEFA-treated hepatocytes. In conclusion, these findings indicate that targeting the IRE1α-JNK axis can reduce NEFA-induced lipid accumulation in bovine hepatocytes by modulating lipogenesis and FAO. This may offer a prospective therapeutic target for fatty liver in dairy cows.

Alterations in the gut microbiota and its metabolites contribute to metabolic maladaptation in dairy cows during the development of hyperketonemia

Metabolic maladaptation in dairy cows after calving can lead to long-term elevation of ketones, such as β-hydroxybutyrate (BHB), representing the condition known as hyperketonemia, which greatly influences the health and production performance of cows during the lactation period. Although the gut microbiota is known to alter in dairy cows with hyperketonemia, the association of microbial metabolites with development of hyperketonemia remains unknown. In this study, we performed a multi-omics analysis to investigate the associations between fecal microbial community, fecal/plasma metabolites, and serum markers in hyperketonemic dairy cows during the transition period. Dynamic changes in the abundance of the phyla Verrucomicrobiota and Proteobacteria were detected in the gut microbiota of dairy cows, representing an adaptation to enhanced lipolysis and abnormal glucose metabolism after calving. Random forest and univariate analyses indicated that Frisingicoccus is a key bacterial genus in the gut of cows during the development of hyperketonemia, and its abundance was positively correlated with circulating branched-chain amino acid levels and the ketogenesis pathway. Taurodeoxycholic acid, belonging to the microbial metabolite, was strongly correlated with an increase in blood BHB level, and the levels of other secondary bile acid in the feces and plasma were altered in dairy cows prior to the diagnosis of hyperketonemia, which link the gut microbiota and hyperketonemia. Our results suggest that alterations in the gut microbiota and its metabolites contribute to excessive lipolysis and insulin insensitivity during the development of hyperketonemia, providing fundamental knowledge about manipulation of gut microbiome to improve metabolic adaptability in transition dairy cows.IMPORTANCEAccumulating evidence is pointing to an important association between gut microbiota-derived metabolites and metabolic disorders in humans and animals; however, this association in dairy cows from late gestation to early lactation is poorly understood. To address this gap, we integrated longitudinal gut microbial (feces) and metabolic (feces and plasma) profiles to characterize the phenotypic differences between healthy and hyperketonemic dairy cows from late gestation to early lactation. Our results demonstrate that cows underwent excessive lipid mobilization and insulin insensitivity before hyperketonemia was evident. The bile acids are functional readouts that link gut microbiota and host phenotypes in the development of hyperketonemia. Thus, this work provides new insight into the mechanisms involved in metabolic adaptation during the transition period to adjust to the high energy and metabolic demands after calving and during lactation, which can offer new strategies for livestock management involving intervention of the gut microbiome to facilitate metabolic adaptation.

The Complex Interplay of Insulin Resistance and Metabolic Inflammation in Transition Dairy Cows

During the transition period, dairy cows exhibit heightened energy requirements to sustain fetal growth and lactogenesis. The mammary gland and the growing fetus increase their demand for glucose, leading to the mobilization of lipids to support the function of tissues that can use fatty acids as energy substrates. These physiological adaptations lead to negative energy balance, metabolic inflammation, and transient insulin resistance (IR), processes that are part of the normal homeorhetic adaptations related to parturition and subsequent lactation. Insulin resistance is characterized by a reduced biological response of insulin-sensitive tissues to normal physiological concentrations of insulin. Metabolic inflammation is characterized by a chronic, low-level inflammatory state that is strongly associated with metabolic disorders. The relationship between IR and metabolic inflammation in transitioning cows is intricate and mutually influential. On one hand, IR may play a role in the initiation of metabolic inflammation by promoting lipolysis in adipose tissue and increasing the release of free fatty acids. Metabolic inflammation, conversely, triggers inflammatory signaling pathways by pro-inflammatory cytokines, thereby leading to impaired insulin signaling. The interaction of these factors results in a harmful cycle in which IR and metabolic inflammation mutually reinforce each other. This article offers a comprehensive review of recent advancements in the research on IR, metabolic inflammation, and their intricate interrelationship. The text delves into multiple facets of physiological regulation, pathogenesis, and their consequent impacts.

Partial Deletion of Perk Improved High-Fat Diet-Induced Glucose Intolerance in Mice

Although pancreatic endoplasmic reticulum kinase (PERK) is indispensable to beta cells, low-dose PERK inhibitor improved glucose- stimulated insulin secretion (GSIS) and hyperglycemia in diabetic mice. Current study examined if partial deletion of Perk (Perk+/-) recapitulated the effects of PERK inhibitor, on the contrary to the complete deletion. Perk+/- mice and wild-type controls were fed with a high-fat diet (HFD) for 23 weeks. Glucose tolerance was evaluated along with serum insulin levels and islet morphology. Perk+/- mice on normal chow were comparable to wild-type mice in various metabolic features. HFD-induced obesity was not influenced by Perk reduction; however, HFD-induced glucose intolerance was significantly improved since 15-week HFD. HFD-induced compromises in GSIS were relieved by Perk reduction, accompanied by reductions in phosphorylated PERK and activating transcription factor 4 (ATF4) in the islets. Meanwhile, HFD-induced islet expansion was not significantly affected. In summary, partial deletion of Perk improved glucose tolerance and GSIS impaired by diet-induced obesity, without changes in body weights or islet mass.

Inhibition of cluster antigen 36 protects against fatty acid-induced lipid accumulation, oxidative stress, and inflammation in bovine hepatocytes

When ketosis occurs, supraphysiological concentrations of nonesterified fatty acids (NEFA) display lipotoxicity and are closely related to the occurrence of hepatic lipid accumulation, oxidative stress, and inflammation, resulting in hepatic damage and exacerbating the progression of ketosis. However, the mechanism of these lipotoxic effects caused by high concentrations of NEFA in ketosis is still unclear. Cluster antigen 36 (CD36), a fatty acid transporter, plays a vital role in the development of hepatic pathological injury in nonruminants. Thus, the aim of this study was to investigate whether CD36 plays a role in NEFA-induced hepatic lipotoxicity in dairy cows with clinical ketosis. Liver tissue and blood samples were collected from healthy (n = 10) and clinically ketotic (n = 10) cows at 3 to 15 d in milk. In addition, hepatocytes isolated from healthy calves were treated with 0, 0.6, 1.2, or 2.4 mM NEFA for 12 h; or infected with CD36 expressing adenovirus or CD36 silencing small interfering RNA for 48 h and then treated with 1.2 mM NEFA for 12 h. Compared with healthy cows, clinically ketotic cows had greater concentrations of serum NEFA and β-hydroxybutyrate and activities of aspartate aminotransferase and alanine aminotransferase but lower serum glucose. In addition, dairy cows with clinical ketosis displayed excessive hepatic lipid accumulation. More importantly, these alterations were accompanied by an increased abundance of hepatic CD36. In the cell culture model, exogenous NEFA (0, 0.6, 1.2, or 2.4 mM) treatment could dose-dependently increase the abundance of CD36. Meanwhile, NEFA (1.2 mM) increased the content of triacylglycerol, reactive oxygen species and malondialdehyde, and decreased the activities of glutathione peroxidase and superoxide dismutase. Moreover, NEFA upregulated phosphorylation levels of nuclear factor κB (NF-κB) and the inhibitor of NF-κB (IκB) α, along with the upregulation of protein abundance of NLR family pyrin domain containing 3 (NLRP3) and caspase-1, and mRNA abundance of IL1B, IL6, and tumor necrosis factor α (TNFA). These alterations induced by NEFA in bovine hepatocytes were associated with increased lipid accumulation, oxidative stress and inflammation, which could be further aggravated by CD36 overexpression. Conversely, silencing CD36 attenuated these NEFA-induced detriments. Overall, these data suggest that CD36 may be a potential therapeutic target for NEFA-induced hepatic lipid accumulation, oxidative stress, and inflammation in dairy cows.

AdipoRon alleviates fatty acid-induced lipid accumulation and mitochondrial dysfunction in bovine hepatocytes by promoting autophagy

During the transition period in dairy cows, high circulating concentrations of nonesterified fatty acids (NEFA) increase hepatic lipid deposits and are considered a major pathological factor for liver damage. We investigated whether AdipoRon, a synthetic small-molecule agonist of adiponectin receptors 1 and 2 shown to prevent liver lipid accumulation in nonruminants, could alleviate NEFA-induced lipid accumulation and mitochondrial dysfunction. Bovine hepatocytes were isolated from 5 healthy Holstein female newborn calves (1 d of age, 30-40 kg, fasting), and independently isolated hepatocytes from at least 3 different calves were used for each subsequent experiment. The composition and concentration of NEFA used in this study were selected according to hematological criteria of dairy cows with fatty liver or ketosis. First, hepatocytes were cultured with various concentrations of NEFA (0, 0.6, 1.2, or 2.4 mM) for 12 h. In a second experiment, hepatocytes were treated with AdipoRon at different concentrations (0, 5, 25, or 50 μM for 12 h) and times (25 μM for 0, 6, 12, or 24 h) with or without NEFA (1.2 mM) treatment. In the last experiment, hepatocytes were treated with AdipoRon (25 μM), NEFA (1.2 mM), or both for 12 h after treatment with or without the autophagy inhibitor chloroquine. Hepatocytes treated with NEFA had increased protein abundance of sterol regulatory element-binding protein 1c (SREBP-1c) and mRNA abundance of acetyl-CoA carboxylase 1 (ACACA), and decreased protein abundance of peroxisome proliferator-activated receptor α (PPARA), proliferator-activated receptor gamma coactivator-1 α (PGC-1α), mitofusin 2 (MFN2), cytochrome c oxidase subunit IV (COX IV), and mRNA abundance of carnitine palmitoyltransferase 1A (CPT1A), along with lower ATP concentrations. AdipoRon treatment reversed these effects, suggesting this compound had a positive effect on lipid metabolism and mitochondrial dysfunction during the NEFA challenge. In addition, upregulated expression of microtubule-associated protein 1 light chain 3-II (LC3-II, encoded by MAP1LC3) and downregulated expression of sequestosome-1 (SQSTM1, also called p62) indicated that AdipoRon enhanced autophagic activity in hepatocytes. The fact that chloroquine impeded the beneficial effects of AdipoRon on lipid accumulation and mitochondrial dysfunction suggested a direct role for autophagy during NEFA challenge. Our results suggest that autophagy is an important cellular mechanism to prevent NEFA-induced lipid accumulation and mitochondrial dysfunction in bovine hepatocytes, which is consistent with other studies. Overall, AdipoRon may represent a promising therapeutic agent to maintain hepatic lipid homeostasis and mitochondrial function in dairy cows during the transition period.