The Focus on Core Genetic Factors That Regulate Hepatic Injury in Cattle Seems to Be Important for the Dairy Sector’s Long-Term Development

Abstract

The cattle during the perinatal period, as well as malnutrition, generate oxidative stress which leads to high culling rates of calves after calving across the world. Although metabolic diseases have such a negative impact on the welfare and economic value of dairy cattle, that becomes a serious industrial concern across the world. According to research, genetic factors have a role or controlling fat deposition in the liver by influencing the biological processes of hepatic lipid metabolism, insulin resistance, gluconeogenesis, oxidative stress, endoplasmic reticulum stress, and inflammation, all of which contribute to hepatic damage. This review focuses on the critical regulatory mechanisms of VEGF, mTOR/AKT/p53, TNF-alpha, Nf-kb, interleukin, and antioxidants that regulate lipid peroxidation in the liver via direct or indirect pathways, suggesting that they could be a potential critical therapeutic target for hepatic disease.

1. Introduction

The “oxygen paradox,” which happens when free radicals (RL) are produced during mitochondrial respiration, is supported by a huge body of research that shows that, despite the necessity of oxygen for life, it also has a damaging effect on the body [1]. Now it is interesting to learn what changes oxidation conditions in cattle or bovine liver. Since oxidative stress is the root cause of several illnesses in cattle, such as sepsis, mastitis, enteritis, pneumonia, and respiratory and joint issues, its effects on food are well-known, but they are also, gradually, recognized to have impacts on the organism “in vivo” [2]. Numerous studies have demonstrated the significance of providing antioxidants for animal nutrition and their connection to oxidative stress, taking into consideration the significance at each stage [3, 4]. Consequently, we can state that the antioxidant impact will not only improve the health of the animals but also raise the quality of the finished product (meat and milk). The large amount of non-esterified fatty acids absorbed by the liver exceeds its ability for oxidation and, as a result, encourages liver-related illnesses including fatty liver and ketosis. Additionally, early breastfeeding cows’ livers experience metabolic stress due to high rates of hepatic gluconeogenesis, which produces glucose for lactose in the mammary gland [5, 6]. In addition to this metabolic stress, early-lactating cows experience a variety of inflammatory challenges, including microbial components (lipopolysaccharides, or LPS), pro-inflammatory cytokines (tumor necrosis factor a (TNF-α), interleukin (IL)-1b, and IL-6), and reactive oxygen species (ROS), as a result of infectious diseases like mastitis and endometritis, as well as subacute [7]. As a result, transition dairy cows experience a state similar to inflammation, which is demonstrated by the production of an acute phase response (APR).The production of positive acute phase proteins (APPs), such as serum amyloid A (SAA), haptoglobin (HP), or C-reactive protein (CRP), which compete with the production of negative APPs, or essential liver proteins, such as albumins, enzymes, lipoproteins, transferring or carriers of vitamins (such as retinol-binding protein), and hormones, is a hallmark of the APR [8]. Thus, the creation of an inflammatory process in the liver exacerbates the biologically existing metabolic load in the liver of early nursing dairy cows, which impairs liver function. Decreased milk production and lower reproductive efficiency in dairy cows are both linked to low blood levels of negative APPs, which are a sign of a severe inflammatory reaction in the liver. Additionally, it has been noted that low-level intravenous TNF-alpha injection causes triacylglycerol build-up and hepatic inflammation in dairy cows [9]. In early lactation, cows with a low “liver functioning index” (LFI) have a significant inflammatory response that is characterized by an obvious increase in positive APPs, a notable drop in negative APPs, poor immunological function, and increased clinical issues [10]. Alternatively, cows with a high LFI display a reduced inflammatory response, better liver function, a slower increase in positive APPs, a slower decline in negative APPs, and fewer clinical issues during this phase [11]. This suggests that these cows have a greater ability to handle the inflammatory challenges that arise during the periparturient phase and are less likely to develop liver-related diseases. There is a lack of knowledge on the molecular underpinnings of liver-related disorders and the variations in susceptibility that exist across individuals. A better understanding of the molecular mechanisms underlying liver-associated diseases in transition dairy cows, however, may help to develop ways to avoid the occurrence of liver-associated disorders and increase production in dairy cows given that the occurrence of liver-associated diseases in dairy cows is critical because it impairs the metabolic function of the liver, overall health status, and productive and reproductive performance.

2. Pathogenesis liver disease in cattle

The build-up of excessive levels of free fatty acids (FFA) in blood or triglycerides (TAG) deposited in the liver are the main contributors to the pathogenesis of fatty liver in newborn dairy cows. In animals, the liver, which is an essential organ, controls the metabolic balance of protein, fat, and carbohydrates. Dairy cows’ food consumption continues to decline after birthing, but their lactation gradually increases. As a result, the cow’s body might quickly experience an inadequate supply of sugar due to the absorption of lactose, which encourages the liver to mobilize fat. The liver serves as the primary location for the metabolism of both substances and energy [12]. A negative nutritional balance is improved by the rising fat mobilization, which also encourages gluconeogenesis, raises blood sugar levels, and increases blood sugar concentration. The liver’s production of non-esterified fatty acids (NEFA), which are partially re-esterified to create triglycerides (TAG), a kind of very-low-density lipoprotein (VLDL) that is seldom ever transported beyond the liver, increases dramatically as a result of the increased fat mobilization. Because dairy cattle lack esterase, TAG accumulates abundantly, making them more susceptible to fatty liver disease [13, 14]. Non-alcoholic fatty liver disease (NAFLD) in people is characterized by aberrant lipid build-up in the liver, raised fasting aminotransferase (AST/ALT), and/or triglycerides (TAG) levels, increased plasma insulin and fatty acid concentration, and metabolic disorder syndromes [15]. Histological evidence of hepatic inflammation brought on by acute inflammation and subacute inflammation is also one of the most significant risk factors. Dairy cows with fatty liver disease are a classic animal model for NAFLD, useful for illuminating its pathology and etiology [16]. A “two-hit” idea has been put out by researchers recently to explain the pathogenic processes of NAFLD. (1) Insulin resistance was the “first hit’s” cause (IR). In addition to causing hyperinsulinemia, IR can intensify the lipolysis of nearby tissues. Adipose tissue lipolysis causes the liver’s production of TAG and FFA to rise [17, 18]. The FFA is harmful to hepatocellular function, increasing cell membrane permeability and impairing mitochondrial activity by inhibiting associated enzymes [19]. The imbalance between the coexisting oxidation and anti-oxidation processes in the liver was what led to the “second hit.” Reactive oxygen species (ROS) are produced persistently as a result of increased lipid peroxidation. Other new or additional variables, such as inflammatory cytokines, adipokines, endotoxins, and mitochondrial inactivation, might boost lipid peroxidation for a second blow to the liver in addition to the pre-existing components associated with the increased oxygen stress. The second hit will eventually result in the advancement of NASH (non-alcoholic steatohepatitis), which promotes oxidative stress, inflammation, cell death, and fibrosis beyond hepatic steatosis [20]. Particularly, the inflammation inhibits lipase activity, prevents the transit of lipids and/or lipoproteins, and results in lipid build-up, which is negatively connected with lipolysis and positively correlated with liver damage [21]. Alternatively, it causes cell apoptosis, IR, and lipid peroxidation, exacerbating the pathophysiology of NAFLD. (3) Hepatocyte cell death and irreversible cell repair constitute a “third hit,” in fact. (4) In addition, endoplasmic reticulum (ER) stress is another significant “hit” in the pathophysiology of NAFLD. Obesity and diabetes, two metabolic illnesses, can result in ER stress, which impairs the physiological processes of liver cells by causing an accumulation of improperly folded proteins (unfolded protein response, UPR) [22]. It is important to note that ER stress can cause SREBP (sterol-regulatory element binding protein) to become active. This promotes the transcription of acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS), which leads to an increase in the production of TAG and fatty acids in the liver [23]. Furthermore, the production of ROS by liver cells under oxidative stress can cause ER stress, which can result in improper protein folding and/or protein modification. Oxidative stress can also be brought on by ER stress. Through many mechanisms, the biological processes of ER stress and oxidative stress interact with one another, causing IR and exacerbating NAFLD. The pathophysiology of NAFLD is largely unknown, though [24, 25]. It was believed that the key route and/or the primary risk factors implicated in the etiology of NAFLD are abnormalities in lipid and lipoprotein metabolism coupled with chronic inflammation and oxidative stress. The prevalence of fatty liver is widely thought to be linked to biological processes such as disordered glycometabolism, oxidative stress, and intracellular inflammatory response in addition to being directly tied to insulin resistance and fat metabolism issue. Additionally, these processes are connected to and/or coordinated with one another, which accelerates the development of NAFLD [26, 27].

NAFLD’s pathophysiology is largely unknown, though. It was believed that the key route and/or the primary risk factors implicated in the etiology of NAFLD are abnormalities in lipid and lipoprotein metabolism coupled with chronic inflammation and oxidative stress. Additionally, these processes are connected to and/or coordinated with one another, which quickens the development of NAFLD. The pathophysiology of NAFLD is influenced by a wide range of variables. In the hepatic lipid metabolism, lipid transport, and secretion, for instance, PPAR (peroxisome proliferator-activated receptor) and/or PPAR, microsomal triglyceride transport protein (MTP), and apolipoprotein (apo B) play crucial roles [28, 29, 30]. Tumor necrosis factor, leptin, and adiponectin are examples of adipocytokines. Cytokines include interleukin-6 (IL-6), glucagon-like peptide-1 (GLP-1), fibroblast growth factor 19 (FGF-19), and fibroblast growth factor 21 (FGF-21), growth hormone-releasing hormone (GHRH), and others [31, 32, 33]. Toll-like receptors (TLRs) also play a role in insulin resistance. It has been suggested that microRNAs (such as mir-107 and miR-103) control insulin resistance [34, 35, 36]. The antagonist against microRNA-103/107, RG-125 (also known as AZD4076), just started a phase I clinical trial to treat NASH (non-alcoholic steatohepatitis) [37]. As a result, aberrant hepatic lipid metabolism, gluconeogenesis, oxidative stress, and inflammation are frequently linked to the etiology of NAFLD. These biological processes’ causal connections and underlying molecular mechanisms are yet unknown. However, it would be beneficial to understand the molecular etiology of NAFLD if some significant regulatory factors or genes that control all these biological processes were discovered.

3. Vascular endothelial growth factor (VEGF)

Native VEGF is a homodimeric glycoprotein of 45 kDa that may bind to heparin and stimulate the proliferation of vascular endothelial cells generated from arteries, veins, and lymphatics [38, 39]. It is expressed by a single gene. Even though VEGF primarily targets endothelial cells, studies have shown that several non-endothelial cell types, such as retinal pigment epithelial cells, pancreatic duct cells, Schwann cells, and maybe placental cell functions, are also subject to mitogenic effects. Five distinct VEGF isoforms, with respective amino acid compositions of 120, 145, 165, 188, and 205, are produced [40] as a result of alternative exon splicing. VEGFR-1/Flt-1, or Fms-like tyrosine kinase 1, and VEGFR-2/KDR, or kinase insert domain-containing region, are the two tyrosine kinase receptors that mediate the biological functions of VEGF. Both VEGFR-1 and VEGFR-2 have a single transmembrane region, seven Ig-like structures in the extracellular domain, and a tyrosine kinase sequence that is broken by a kinase-insert domain [41, 42]. Another RTK in the same family, VEGFR-3 or Flt-4, has affinities for VEGF-C and VEGF-D. In addition to RTKs, VEGF also interacts with the neuropilin family of coreceptors [43]. The two different domains of the VEGF molecule, which are found on the opposing terminal of the VEGF monomer, allow VEGF to interact with its receptors, Flt-1 and KDR. While the 120, 145, 165, and 188 isoforms of VEGF activate KDR, VEGF165 and VEGF188 activate Flt-1 [40]. The VEGF system was found in the uterine epithelium, trophoblast, vascular tissue, and uterine glands of placentomes [44]. Bovine VEGF may have the following traditional roles in angiogenesis and vascular permeability, as well as chemotactic activity in endothelial capillaries, autocrine influence on the migration of giant trophoblastic cells, which promotes maternal-fetal interchange, and modulatory action in trophoblastic function, specifically in steroidogenesis [45, 46, 47]. These functions are all suggested by the presence of the VEGF system in the maternal-fetal interface and the vascular system. The improvement of reproductive function and productive efficiency is crucial while growing dairy and meat cattle. Numerous studies have shown that cows, especially those with high milk output, gradually exhibit a rise in reproductive issues, which are reportedly caused by several factors, including poor energy balance and unequal gene expression. In addition to facilitating the transmission of physiological information between the mother and the fetus, the placenta produces and secretes steroid hormones, such as progesterone and estrogens, which are in charge of optimizing the environment for fetal growth. Therefore, the success of gestation depends on its efficient operation. The location of these growth factors and their corresponding receptors in non-endothelial cells, however, suggests that they are involved in other physiological processes, such as the stimulation of hormone synthesis in steroidogenic tissues. NAFLD has been studied using a variety of rat models, each of which mimics one or more characteristics of human NAFLD, including steatosis, NASH, fibrosis, and HCC [48]. Numerous studies have documented how angiogenesis manifests in various models. First, it was demonstrated that mice who were given a high-fat diet (HFD) had higher levels of VEGFR-2 expression and CD31 expression, the most widely used marker of endothelial cells, in their livers [49]. In the livers of rats given a CDAA diet and mice fed an MCD diet, and cattle fed a high-fat artificial diet can cause an increase in VEGF protein was observed. Interestingly, VEGF mRNA levels remained the same in the latter scenario, pointing to posttranscriptional regulatory mechanisms [48]. The global vasculature of the cattle liver with NAFLD was examined using specific imaging methods. NAFLD was linked to an international alteration of the hepatic vascular architecture, which included not only an increase in the number of vessels but also in a visibly different phenotype of vessels, which displayed an enlarged diameter and a disrupted organization, according to research using scanned electronic microscopy of vascular corrosion casts of the liver [50].

4. Practices for consuming more antioxidants

Antioxidants may be produced by the body, obtained through food, or given orally. Some vitamins, such as vitamin K, may be produced by the ruminal and intestinal flora while vitamin D can be produced by UV radiation on the skin in ruminants [51]. Several natural feed ingredients are also high in antioxidants, such as vitamin E or precursors to vitamin A. But, due to the wide range in vitamin concentrations in feeds and exposure to sunshine, depending only on these naturally occurring quantities might put the animals in danger of deficient disorders. Additionally, many dairy farms confine their animals indoors, where they have little exposure to sunlight and fresh fodder and the majority of natural vitamins quickly deteriorate after ensilage. As a result, these animals need to be supplemented with vitamins and trace elements, albeit the needs of grazing cattle may be different from those of cattle-fed preserved forages. Additionally, it is advised to administer an additional supplementation during times of increased need, like right before calving. The addition of vitamins and minerals to the animals’ diets is arguably the approach to antioxidant supplementation that is utilized the most frequently in industrial dairy farms, particularly when premixes are added to the overall mixed ration. The injection of vitamins and trace minerals to these animals, however, facilitates the supplementation without necessitating the creation of a specific management group of cows during this period because the needs for antioxidants are increased in moments of augmented metabolisms, such as the transition period, in farms with several animals not large enough to practically implement a specific diet for similar dry cows [52]. Numerous solutions, either for individual vitamins and trace elements or for a mix of them, are offered on the market for this purpose. The two antioxidants that are most frequently found in dairy cattle diets, either separately or together, are vitamin E and selenium. As a result, the majority of study has concentrated on these compounds’ effects. Chain-breaking antioxidant vitamin E, which is highly lipid-soluble, stops the spread of free radicals in plasma lipoproteins and membranes. The effects of this trace element on selenoproteins, such as GSH-Px, are attributed to its function as a cofactor [52, 53]. However, a recent study has emphasized the direct function of selenium (Se) in preventing oxidative stress (OS) and controlling immunity in dairy calves around the time of calving. At the start of lactation, cows frequently have decreased plasma levels of vitamin E [53, 54]. It is unclear why the content of vitamin E a-tocopherol drops near the end of pregnancy. A reduction in the development of various vitamin carrier proteins in the liver of dairy cows during the transition phase causes reduced plasma levels of vitamin E. This may be partly attributed to the usage of antioxidants for colostrum formation. To avoid the reduction in plasma a-tocopherol concentrations around parturition, supplementation with relatively high vitamin E levels is required [55].

5. Transport stress

Feeder’s calves are more likely to acquire bovine respiratory disease (BRD) due to transportation stress, which is frequently exacerbated by the stress of weaning. The direct cost of treating BRD was $23.60 per case, according to the USDA (2013) [56], and at some time during the feeding period, 16.2% of feedlot cattle showed indications of respiratory illness [56, 57]. Bruising and dark-cutting carcasses are two additional beef quality issues related to shipping. Cattle can become bruised when they collide with one another during transport or when they come in touch with trailer parts, especially while loading and unloading. 68.2% of the 9860 corpses that were seen at three separate slaughterhouses in the United States had bruises, and 53.5% of those injuries were along the dorsal midline, the area of the carcass with the highest economic value depends [58]. Before slaughter, prolonged stress depletes muscle glycogen reserves, resulting in a condition called dark cutting carcasses. Long-term stress before slaughter depletes muscle glycogen levels, causing a characteristic called “dark cutting carcasses.” The possibility that transportation might have a detrimental impact on carcass and meat qualities has sparked interest in transportation beef quality assurance, a program that informs cattle transporters of optimum management practices [59]. A change in the prooxidant-antioxidant equilibrium favouring the former was the original definition of oxidative stress. This condensed definition was eventually expanded to mean an imbalance favouring oxidants over antioxidants, which disrupts redox signalling and causes molecular damage in addition to impairing physiological performance. Under typical biological circumstances, the antioxidant defense mechanism of the cell balances the quantity of oxidants, causing it to vary within a specific range known as basal oxidative stress or oxidative eustress. Cell death by necrosis, apoptosis, or both may occur when oxidative stress reaches a high intensity, known as oxidative distress. When determining the level of oxidative stress in biological samples, it is crucial to take into account both sides of the equation since oxidative stress is defined in terms of the interaction between oxidants and antioxidants. Direct measurements of reactive oxygen species (ROS), oxidative changes to proteins, lipids, and nucleic acids, antioxidant enzyme activities, antioxidant concentrations, and ROS levels are a few of the indicators that are frequently evaluated.

6. Antioxidant and lipid peroxidation

Endogenous nonenzymatic biological sources and antioxidants received exogenously make up the cellular antioxidant defense system [60]. Since several antioxidants can work together to combat the oxidative offense and a lack of one specific antioxidant does not always signal that the sample’s overall neutralizing ability is compromised, quantifying antioxidant components individually does not give a fair picture of the sample’s antioxidant capacity [61]. As a result, several techniques have been created to determine the overall antioxidant capacity. This takes into account the overall antioxidant activity of every sample that was analyzed rather than just the total amount of detectable antioxidants [62, 63]. The following are some of the most popular analytical techniques for determining the status of all antioxidants: the overall antioxidant capacity to capture radicals comparable antioxidant power to Trolox capability for absorbing oxygen radicals the sample’s ability to reduce iron or the pool of antioxidants that would be severely oxidized by a large dose of hypochlorous acid. It is not unexpected that several endogenous regulatory processes are affected by an external antioxidant supply as a result of this interaction between antioxidant compounds. Recent studies show that these processes may be responsible for some of the contentious conclusions regarding antioxidant supplementation, even if further investigation is necessary to completely understand all the regulatory pathways in dairy cattle: The transcription of genes encoding different antioxidative and cytoprotective proteins is regulated by the nuclear factor E2-related factor 2 (Nrf2), making Nrf2 necessary for the transcription of GSH-Px 2 (and likely GSH-Px 1 as well). Found that a significant increase of Nrf2 target genes with anti-oxidative capabilities occurs throughout the transition from late pregnancy to the start of breastfeeding [64, 65]. Additionally, at this time, the unfolded protein response is triggered in dairy cow livers, activating Nrf2 via the PERK pathway and enhancing the production of antioxidant enzymes and antioxidant capacity [66, 67]. These systems may be physiological defenses against tissue damage brought on by inflammation and ROS generation [68]. They, therefore, serve as crucial benchmarks for ensuring successful adaptation during the period of transition. Additionally, the fact that excessive antioxidant supplementation reduces antioxidant capacity may be explained by the endogenous regulation of antioxidant molecules, as high antioxidant doses may reduce antioxidant capacity by suppressing Nrf2 due to lower ROS levels resulting in decreased expression of antioxidant enzymes [69]. Mastitis, which frequently originates from microbial infection of the mammary gland, is the most expensive inflammatory condition in dairy cows. LPS produced from the bacterial outer membrane is the major pathogen component starting inflammatory reactions if the infection is brought on by gram-negative bacteria.

7. Oxidative stress

Saturated fatty acid excess in the mitochondria causes redox equilibrium to break down and speeds up the production of oxygen radicals. Insulin resistance and non-alcoholic FLD in non-ruminants have been linked to the lipotoxic syndrome known as oxidative stress. As discussed by many authors, the ongoing generation of ROS triggers serine/threonine kinase signalling cascades that prevent the induction of insulin-stimulated insulin receptor substrate [70, 71]. In response, simple steatosis may be promoted by increased intrahepatic lipid accumulation caused by insulin resistance (i.e. non-inflammatory phenotype). The activation of Kupffer cells, which in turn activate redox-sensitive transcription factors like nuclear factor-B and upregulate pro-inflammatory TNF-α, may also be facilitated by oxidative stress. Unfortunately, NADPH oxidase and cytochrome P450 (family 2, subfamily E, polypeptide 1; commonly known as CYP2E1) are also upregulated in inflammatory steatohepatitis, which further reduces antioxidant capacity and promotes hepatocellular damage [72]. Depletion of n-3 long-chain polyunsaturated fatty acids (FA) due to poor fatty acid desaturation and increased peroxidation in the liver are additional effects of excessive ROS formation. Furthermore, polyunsaturated fatty acids (FA) that have been exposed to enzymatic (through cyclooxygenase, lipoxygenase, and CYP2E1) or non-enzymatic (by ROS) oxidation can produce oxylipins with a variety of inflammatory activities [73, 74]. For instance, proinflammatory hydroxyl-octadecadienoic acid and hydroxyl-eicosatetraenoic acid are both oxylipins produced from n-6 arachidonic acid and linoleic acid, respectively [75]. The widespread mitochondrial FA oxidation, albeit incomplete breakdown, that occurs in the transition cow with diminished antioxidant capability is probably a contributing factor in the development of ROS build-up [76]. Adipose tissue lipolysis may be boosted by oxidative stress, which will worsen the oxidant state. Unsaturated FA (fatty acid) produced from lipolysis can undergo ROS oxidation, which produces isoprostane and lipid hydroperoxide [77]. Adipose tissue releases fatty acids that are also utilized by the liver to produce hepatic -hydroxybutyrate, which may trigger p38 mitogen-activated protein kinase activity and increase hepatocyte death. Additionally, nuclear factor B may be induced by lipolytic FA in hepatocytes through ROS-dependent processes that cause inflammation [78, 79]. These results imply that FLD pathogenesis involves oxidative stress. As previously discussed, the buildup of ROS or oxylipids probably affects the immunological responses of cattle. For instance, in peripheral blood mononuclear cells from healthy transition cows, plasma oxylipin levels are associated with the production of interleukin-12 and inducible nitric oxide synthase-2 [80]. Elevations in the arachidonic acid metabolite 15-hydroxy-peroxyeicosatetraenoic acid occur in endothelial cells along with death, caspase-3 activation, leukocyte migration, and the production of inflammatory cytokines [81]. Oxylipids generated from cytochrome P450 and lipoxygenase accumulate in the plasma and adipose tissue of postpartum cows, where they may have an impact on immune cell trafficking and inflammation. Examples of such oxylipids include 5-hydroxy-eicosatetraenoic acid [82]. The significance of oxidized lipids in the development of inflammatory illnesses such as mastitis and metritis has therefore been highlighted. The significance of oxidized lipids has been studied since one of the main characteristics of inflammatory illnesses, such as mastitis and metritis, is inflammatory dysfunction [82]. For instance, hydroxyl-octadecadienoic acid accumulation and breast inflammation are two features of Streptococcus uberis mastitis. Oxylipid production and associated health consequences may be influenced by dietary antioxidant intake, trace mineral intake that aids in antioxidant defense mechanisms, and the kind and quantity of FA supplied to transition cows [83]. Uncontrolled or chronic inflammatory states can be harmful, even though they often result in the recovery from infection after a controlled inflammatory phase. Therefore, after the removal of the infectious agents, a speedy resolution phase is required for a perfect acute inflammatory response. Anti-inflammatory cytokines like IL-10 and n-3 (omega-3) fatty acid derivatives like resolving and protectors are important resolving signals.

8. Cytokines

Gene activity TNF-alpha is one of the most significant cytokines involved in starting and growing the acute-phase response [84]. TNF-α is necessary for healthy liver regeneration and increases hepatic DNA synthesis by activating NF-B [85]. Numerous cell types, particularly macrophages and mast cells, generate cytokines such tumor necrosis factor (TNF), interleukin (IL)-1, and interleukin (IL)-6. By stimulating the acute phase response and activating leukocytes and endothelial cells, they play significant roles in the inflammatory response. The liver of rats that have experienced chronic starvation maintains its acute-phase reactivity. According to the new research, although the reported rise in TNFA expression between days 14 and there was small, it may have been the result of a systemic inflammatory response that was triggered within the uterus as parturition neared (i.e., higher IL-1 and IL-8 production) [86]. The cows’ metabolisms were put under additional stress due to a decreased energy balance brought on by the decrease in energy intake. It is generally known that IL-1 suppresses appetite in humans, but plasma concentrations of TNF- are linked to higher energy expenditure [87, 88]. Early lactation cows were given recombinant bovine TNF- alpha, which raised blood haptoglobin, NEFA, cortisol, growth hormone, and nitric oxide while decreasing feed intake. Some of these symptoms closely reflect periparturient-period reactions that we and others have seen, as well as reactions to endotoxin treatment [89, 90]. Since proinflammatory and signaling genes are upregulated in the liver of mice that have been induced to develop fatty liver and insulin resistance by high-fat diets, TNFA upregulation in liver macrophages may act in a paracrine manner and cause potent upregulation of SAA1 in hepatocytes [91, 92]. Increased inflammation during the formation of fatty livers in transition dairy cows may be caused by NF-B-mediated proinflammatory signals [93]. Bovine recombinant When given to breastfeeding cattle, TNF- raised their blood levels of haptoglobin, NEFA, cortisol, growth hormone, and nitric oxide while decreasing their appetite for food [94]. SAA1’s expression has increased by over sixfold, which is partially explained by the fact that TNF- and IL-1 boost the manufacture of positive acute phase proteins like SAA1. The acute-phase reaction causes a 1000-fold increase in SAA1 production in the liver [95]. Furthermore, it has been demonstrated in several reports that pro-inflammatory and signalling genes are upregulated in the liver of mice induced to develop fatty liver and insulin resistance by high-fat diets. This suggests that the upregulation of TNFA in liver macrophages may act in a paracrine manner and result in a potent upregulation of SAA1 in hepatocytes.

In addition to having a deleterious impact on the neurophysiological systems controlling feed intake, IL-8 and IL-1 released from the placenta directly upregulate the expression of SAA1 and TNFA in the liver [96]. Negative energy balance, hyperinsulinemia, increased adipose tissue lipolysis, and decreased feed intake all have an impact on the liver’s ability to access nutrients. Blood NEFA and -hydroxybutyrate levels may be considerably elevated and lipolysis may be further stimulated by cytokines from the liver and/or placenta (BHBA) [97]. Circulating NEFA most likely operate as endogenous ligands for PPARA and HNF4A, upregulating them and activating downstream genes involved in fatty acid oxidation, ketogenesis, and gluconeogenesis (ACSL1, ACOX1, carnitine palmitoyl transferase 1A (CPT1A), ACADVL) (PCK1) [98, 99]. The outcome of metabolic processes that were partially sparked by the overexpression of PPARA and HNF4A is net hepatic glucose synthesis and glucose and amino acids for milk synthesis [100]. Through the direct overexpression of GPAM, fatty acid synthase (FASN), ATP-citrate lyase (ACLY), and Spot 14, activation of PPARA can reduce the expression of the gene strongly connected with lipid synthesis, sterol regulatory element binding transcription factor 1 (SREBF1) (S14) [101]. Higher levels of liver triacylglycerol are correlated with the upregulation of SREBF1 via cytokines or fatty acids and GPAM [102]. Limitations in insulin and amino acid delivery to the liver may suppress IGFBP3, EIF4B, 3-phosphoinositide dependent protein kinase-1 (PDPK1), proteasome (prosome, macropain) 26S subunit, ATPase 2 (PSMC2), and/or PDPK1-dependent protein kinase, resulting in a reduction in hepatic protein synthesis, circulating blood IGF-I, and liver glycogen [103]. Increased lipid peroxidation in the liver may result from the downregulation of GSTM5 expression. The danger for hepatic periparturient health disorder is increased by both less ability to detoxify lipid peroxidation products and larger triacylglycerol build-up in the liver [104].

9. Endoplasmic reticulum stress triggers cytoprotective pathways

It’s interesting to note that cryoprotective pathways, including the nuclear factor E2-related factor 2 (Nrf2) pathway, are activated by ER stress brought on by ROS or pro-inflammatory cytokines [105]. This activation is PERK-dependent. Various antioxidative and cryoprotective proteins are controlled by the redox-sensitive transcription factor Nrf2. ER stress causes Keap1 to become disassociated from Nrf2, allowing Nrf2 to move into the nucleus and activate antioxidant and cytoprotective genes by binding to antioxidant response elements in the promoter regions of its target genes. In the absence of ER stress-inducing stimuli, Nrf2 is inactive and retained in the cytoplasm by interaction with Kelch-like ECH-associated protein 1 (Keap1) [106]. The activation of Nrf2 also lowers pro-inflammatory signalling, attenuates inflammatory damage, and neutralizes ROS generated under pro-inflammatory situations, which decreases the vulnerability of tissues to oxidative damage and cytotoxicity [107]. Thus, it has been proposed that ER stress-induced activation of Nrf2 is a method for reducing oxidative damage that is brought on by ER stress.Following the presence of enhanced ROS and pro-inflammatory cytokines in the liver of transition dairy cows, we have recently noticed Nrf2 activation as demonstrated by overexpression of Nrf2 target genes, including catalase, glutathione peroxidase 3, microsomal glutathione S-transferase 3, haem oxygenase 2, metallothionein 2A, NAD(P)H dehydrogenase, quinone 1 [108]. Furthermore, it’s been proposed that Nrf2 is a physiological target to preserve liver function and enhance overall health in high-yielding dairy cows. It is not unlikely that interindividual variations in the effectiveness of activating the Nrf2 pathway in response to ROS or inflammatory stimuli explain the molecular level for variations in the susceptibility to develop liver-associated diseases between early-lactating dairy cows with similar NEB and milk yield [109]. This is because Nrf2 plays a crucial role in preventing liver damage.

10. Tnf-α and NF-kb

The pattern-recognition receptor toll-like receptor 4 (TLR4), which recognizes endogenous ligands and external pathogen-associated molecular patterns, is crucial in the development of the inflammatory response [110]. The production of pro-inflammatory cytokines and the activation of nuclear factor kappa B (NF-B) signalling pathways in a variety of cell types are both correlated with TLR4 activation. In rat models of cardiac ischemia-reperfusion, it has been found that TLR4 expression positively correlates with the levels of tumor necrosis factor (TNF) and interleukin-6 (IL-6) [111]. In dairy cattle, disease -affected cows’ milk and intra-mammary epithelial cells showed elevated NF-B activity. Lymphocyte antigen-96, also known as MD-2, and CD14 form a complex when lipopolysaccharide (LPS) binds to TLR4 [112]. This complex then triggers TIR (Toll/IL1 Receptor domain) intracellular signalling through adaptor molecules, primarily myeloid differentiation actor 88. (MyD88) [113]. This TLR4 and damage signalling causes downstream kinases to become active, which in turn causes IKB to degrade, releasing NF-B to go to the nucleus [114, 115]. In the promoter region of genes encoding pro-inflammatory cytokines, such as IL-1B and IL-6, it binds B sites [116]. Bovine mammary epithelial cells (bMEC) react differently to diverse pathogenic stressors, according to some researchers. While the reaction to Staphylococcus aureus culture supernatant (SaS) was linked to an AP-1 and IL-17A signalling route, crude LPS from Escherichia coli was linked to an NF-B and Fas signalling network [117]. The impact of intra-mammary cephapirin therapy, either alone or in combination with prednisolone, on gene expression patterns in experimentally induced mastitis in Holstein Friesian cows was examined [118]. In comparison to challenged, untreated areas, they discovered that both treatments led to a down-regulation of gene transcripts implicated in chemokine and TLR-signaling pathways. It is widely known that TLR4 is a key cell surface receptor for the inflammatory response because it recognizes LPS from the cell wall of gram-negative bacteria and starts the MyD88-IKKNF-B pathway response [119]. The MyD88-dependent pathway is activated by TLR4 regulation of LPS (Mediated by TLR-IL-1 receptor domain containing adapter protein/TIRAP), which triggers the immediate activation of NF-B and the subsequent induction of a number of pro-inflammatory cytokines [120, 121]. Additionally, it was shown that thymol may suppress NF-B activation and down-regulate the mRNA production of tracheal antimicrobial peptide and -defensin, hence reducing the internalization of S. aureus into bMEC (BNBD5) [122].

11. AKT and mTOR

A subset of genes involved in lysosomal biogenesis and function, as well as those involved in the creation of autophagosomes, are controlled by transcription factor EB (TFEB), a master transcription regulator [123]. Lysosome-associated membrane protein (LAMP1) and the V0 domain of the vacuolar ATPase (ATP6V0A1) are two examples of the hepatic lysosome-regulated genes that were abundantly expressed in mice after TFEB activation or overexpression. Furthermore, TFEB overexpression in HeLa cells increased the expression of autophagy-related genes including genes such sequestosome-1 (SQSTM1) and microtubule-associated protein 1 light chain 3 (MAP1LC3) (ATG12) [124]. It’s important to note that studies have shown that the livers of mice and people with non-alcoholic fatty liver disease had decreased TFEB transcriptional activity and lysosomal function [125, 126]. Furthermore, low levels of the genes MAP1LC3, SQSTM1, ATG7, and ATG12 in the liver of ketotic dairy cattle propose a reduction in the formation of autophagosomes, suggesting that impaired TFEB transcriptional activity may exist in the liver of dairy cows with ketosis and result in elevated aminotransferase enzyme levels [127]. Mechanistic target of rapamycin kinase complex 1 (mTORC1) phosphorylates the transcription factor TFEB at Ser 211 in nonruminants to prevent its subcellular localization and activity [128]. Other kinases, such as protein kinase B (Akt), glycogen synthase kinase-3 (GSK3), and extracellular signal-regulated kinase (ERK1/2), which phosphorylate TFEB at Ser 467, Ser 138, and Ser 211, respectively, also influence TFEB nuclear localization [129]. These other kinases work in conjunction with mTORC1 to affect TFEB nuclear localization. In the livers of dairy cattle in ketosis, changes in the activity of Akt, GSK3, and ERK1/2 have been observed [130, 131]. The upstream substrates of mTORC1 that are activated support anabolic pathways while blocking catabolic ones. RPS6KB, EIF4EBP1, and TFEB were more heavily phosphorylated in the current research, which is suggestive of an overactive hepatic mTORC1 state during ketosis [132]. It’s probable that a similar process operates in dairy cows with ketosis as hepatic overactivation of mTORC1 lowered TFEB transcriptional activity and compromised lysosomal function in mice with fatty liver. Therefore, the decrease in molecules related to lysosomal function that researcher found in ketotic cattle may have been brought on by an overactive mTORC1 that inhibits the transcription of TFEB. It has been observed that Akt, GSK3 and ERK1/2 cause TFEB to become more phosphorylated and less likely to go into the nucleus [133]. Accordingly, in line with earlier research, phosphorylated Akt, GSK3, and ERK1/2 were reduced in the liver of dairy cows in ketosis, which decreased their impacts on TFEB transcriptional activity regulation [134].

12. Conclusion

The environment and dietary habits have a substantial influence on the health of cattle and their hepatic condition in the future. Climate change may encourage the formation of poisons or poisonous spores, which cattle are exposed to when grazing diseased grass. Using mechanical or natural methods, senescent rough dog’s tail grass contaminants might be moved to a neighbouring palatable pasture. The specific toxin(s) and their source, however, are still speculative, and it is uncertain if they are stable in the environment. The analysis of tissues from dead animals may thus be able to shed some light on this scenario, even if the concentration of the toxic substances that cause disease would be higher in feed source materials. Any unusual components may reveal information about the type of poison, according to a theory (s). Additionally, it is suspicious that there were many types of insects and regularly contaminated food present during an outbreak of liver injury. As a result, it is suggested that the source of the relevant toxin(s) may be one of numerous factors linked to increased liver damage in the cattle.

Acknowledgments

The authors thank the editors and anonymous publisher for their helpful feedback for this manuscript.

Declaration of interest

The authors declare that there is no conflict of interest for this manuscript.

Informed consent statement

Not applicable.