Implications

Metabolic stress may compromise the ability of the dairy cow to successfully adapt to a productive lactation and remain fertile and healthy. This review discusses changes in lipid metabolism that define maternal adaptation and metabolic stress in the cow. The narrative describes the roles of fatty acids, mitochondria and acylcarnitines, lipid mediators of insulin antagonism, including ceramide, glycerophospholipids and triglyceride secretion, and oxylipids during oxidative imbalance. The influence of leptin, adiponectin and fibroblast growth factors is considered. Defining the peripartal lipidome is important to refine existing or identify new therapeutic strategies to improve cow health and production efficiency.

Fatty acids, and insulin secretion and sensitivity

In the transition cow, negative energy balance develops with increased concentrations of palmitic, stearic, oleic and linoleic acids in serum (Rukkwamsuk et al., Reference Rukkwamsuk, Geelen, Kruip and Wensing2000; Douglas et al., Reference Douglas, Rehage, Beaulieu, Bahaa and Drackley2007). These changes reflect the FA profile of adipose tissue. As the proportion of saturated FA increase in plasma, the proportion of long-chain polyunsaturated FA decreases (e.g. arachidonic acid; Douglas et al., Reference Douglas, Rehage, Beaulieu, Bahaa and Drackley2007). In parallel with increased lipolysis, the postpartum liver of the cow accumulates greater amounts of palmitic, oleic and linoleic acids, but not stearic acid (Rukkwamsuk et al., Reference Rukkwamsuk, Geelen, Kruip and Wensing2000). Enrichment of palmitic acid, oleic acid and linoleic acid is observed in hepatic triglycerides and phospholipids, whereas polyunsaturated FA (i.e. arachidonic and eicosapentaenoic acids) depletion is evident (Douglas et al., Reference Douglas, Rehage, Beaulieu, Bahaa and Drackley2007). Although not completely understood, the modulation of plasma FA status has implications for insulin secretion and sensitivity as well as liver health in the dairy cow.

During early lactation, insulin secretion in response to insulinotropic agents (i.e. glucose and propionate) is suppressed (Lomax et al., Reference Lomax, Baird, Mallinson and Symonds1979). This action advantageously removes the inhibitory signal on lipolysis, and thus circulating FA concentrations increase for oxidation and milk fat production. Therefore, it would be counterintuitive for FA to promote insulin secretion and lipogenesis during this stage of lactation. However, this concept deserves consideration especially in the mid- or late-lactation cow that is more responsive to insulinotropic compounds. The reason is that FA promote insulin secretion in non-ruminants (Poitout et al., Reference Poitout, Hagman, Artner, Stein, Harmon and Robertson2006). Evidence also suggests that these responses are uniquely influenced by the type of FA studied. In rats, glucose-stimulated insulin secretion from perfused pancreas is higher in response to co-stimulation with saturated palmitic or stearic acids, relative to oleic or linoleic acids (Stein et al., Reference Stein, Stevenson, Chester, Basit, Daniels, Turley and McGarry1997). The ability of saturated FA supplementation to increase circulating insulin concentrations has been observed in peak-lactation dairy cows (Harvatine and Allen, Reference Harvatine and Allen2006). One notable exception is that chronic excess of palmitic acid reduces insulin synthesis and secretion in human pancreatic islets via mechanisms that may depend on ceramide-mediated inhibition of insulin gene expression as well as reduced β-cell turnover and apoptotic activation (Maedler et al., Reference Maedler, Oberholzer, Bucher, Spinas and Donath2003). Considering that circulating palmitic acid and ceramide is elevated when plasma insulin is low during early lactation, might palmitic acid-induced lipotoxicity and ceramide-dependent mechanisms explain suppressed insulin secretion during early lactation? Although this question remains unanswered, it should be mentioned that monounsaturated FA (i.e. palmitoleic or oleic acids) counteract the harmful effects of palmitic acid on human pancreatic β-cell turnover and function (Maedler et al., Reference Maedler, Oberholzer, Bucher, Spinas and Donath2003). Of potential relevance in the cow, feeding a blend of palmitic and oleic acids increased circulating insulin concentrations in mid-lactation Holstein cows, relative to those fed high palmitic acid or a blend of palmitic and stearic acids (de Souza et al., Reference de Souza, Preseault and Lock2018).

In the cow, the alteration of peripheral insulin sensitivity by FA may influence lipolysis and the mammary utilization of glucose for milk synthesis. Specifically, diet- or lipolytic-derived FA may control insulin signaling, although regulation likely depends on the degree of FA saturation, chain length and concentration, and whether the cow is under homeorhetic control. In non-ruminants, it is hypothesized that saturated FA inhibit insulin sensitivity by promoting lipid mediator synthesis (e.g. ceramide or diacylglycerol (DAG)), inflammation via toll-like receptor-4 signaling, oxidative stress or endoplasmic reticulum stress (Boden, Reference Boden2011). In contrast, n-3 FA such as docosahexaenoic acid (DHA) appear to counteract insulin resistance development by stimulating mitochondrial function, reducing reactive oxygen species (ROS) production and preventing inflammation in non-ruminants, as reviewed by Lepretti et al. (Reference Lepretti, Martucciello, Burgos Aceves, Putti and Lionetti2018). Similar benefits are observed when studying monounsaturated FA (i.e. oleic acid; Palomer et al., Reference Palomer, Pizarro-Delgado, Barroso and Vázquez-Carrera2018). In dairy cattle, an intravenous infusion of saturated tallow caused insulin resistance (Pires et al., Reference Pires, Souza and Grummer2007). Evidence suggests that palmitic acid partitions nutrients to the mammary gland, whereas oleic acid shifts energy away from milk production and toward body fat accretion (de Souza et al., Reference de Souza, Preseault and Lock2018). In addition, abomasal infusion of linseed oil rich in polyunsaturated linolenic acid appeared to enhance the anti-lipolytic effects of insulin in feed-restricted cows (Pires et al., Reference Pires, Pescara, Brickner, Silva del Rio, Cunha and Grummer2008). In growing steers, long-chain eicosapentaenoic and DHA increases insulin sensitivity, relative to animals unsupplemented with n-3 FA (Cartiff et al., Reference Cartiff, Fellner and Eisemann2013). Although a better mechanistic understanding is required, the current literature suggests that saturated FA reduce insulin sensitivity in dairy cattle, whereas unsaturated FA (particularly n-3 FA) enhance insulin action. Future research should determine whether and how saturated and unsaturated FA influence insulin signaling and inflammation in early-lactation cow, and whether these changes are associated with the advancement of FLD, the incidence of metabolic disorders and long-term lactation and reproductive success of the cow.

Acylcarnitines and mitochondrial β-oxidation

A key step in the β-oxidation of FA is the formation of acylcarnitines as a means to shuttle fatty acyl-CoA from the cytosol into the mitochondrion. This process is facilitated by carnitine palmitoyltransferase-1, which is inhibited by malonyl-CoA generated by the lipogenic enzyme acetyl-CoA carboxylase. In the study of obesity and type 2 diabetes, acylcarnitines have received attention as biomarkers of lipid-induced mitochondrial dysfunction (Schooneman et al., Reference Schooneman, Vaz, Houten and Soeters2013). The accumulation of acylcarnitines reflects a reduction in FA oxidation capacity and the partitioning of FA toward the synthesis of triglyceride and lipid mediators of insulin antagonism, including ceramide and diacylglycerol. Types of acylcarnitines include ketone-derived (e.g. the carnitine ester of β-hydroxybutyrate), branched-chain amino acid-derived (e.g. C4-dicarboxylic-carnitine), and acylcarnitines varying in FA chain length (e.g. acetyl-, myristoyl- or palmitoyl-carnitine, respectively). Focus has centered on long-chain acylcarnitines because they are linked to insulin resistance (Schooneman et al., Reference Schooneman, Vaz, Houten and Soeters2013). In the study of Koves et al. (Reference Koves, Ussher, Noland, Slentz, Mosedale, Ilkayeva, Bain, Stevens, Dyck and Newgard2008), chronic high-fat feeding increased postprandial serum total FA as well as C8:1-, C10:3-, C16:0-, C18:0- and C18:1-carnitine concentrations in rats. In the same study, acylcarnitine levels were elevated in gastrocnemius muscles in ad libitum high-fat fed obese Zucker diabetic fatty rats experiencing elevations in FA oxidation to acid-soluble metabolites (a measure of incomplete FA catabolism), relative to lean animals. More recent data suggest that acylcarnitines may directly modulate insulin signaling. In differentiated myotubes of non-ruminant origin, C16:0-carnitine treatment inhibited insulin-stimulated protein kinase B (AKT) activation and glucose uptake (Aguer et al., Reference Aguer, McCoin, Knotts, Thrush, Ono-Moore, McPherson, Dent, Hwang, Adams and Harper2015). In addition, lowering of acetyl-carnitine concentrations by mildronate (an inhibitor of acylcarnitine transferase) prevents palmitate-induced reductions in insulin-stimulated glucose utilization by human primary myotubes derived from lean subjects (Aguer et al., Reference Aguer, McCoin, Knotts, Thrush, Ono-Moore, McPherson, Dent, Hwang, Adams and Harper2015). Interestingly, mildronate also prevented palmitic acid-induced accumulation of ROS, which suggests that acylcarnitines may promote oxidative stress concomitantly with impaired insulin sensitivity.

Acylcarnitine metabolism has also been implicated in the development of FLD. Specifically, hepatic acylcarnitine levels are elevated in humans with non-alcoholic FLD (e.g. C16:0- and C12:0-carnitines in steatosis and steatohepatitis, respectively; Lake et al., Reference Lake, Novak, Shipkova, Aranibar, Robertson, Reily, Lehman-McKeeman, Vaillancourt and Cherrington2015). Albeit de novo lipogenesis plays a role in non-ruminant FLD, mitochondrial dysfunction and FA excess also provides FA substrate for excess triglyceride storage, which favors steatosis. The advancement of hepatic injury in the form of inflammation (steatosis to steatohepatitis) may be provoked by acylcarnitines. In support, C14:0-carnitine supplementation stimulated the expression and secretion of pro-inflammatory tumor necrosis factor-α (TNF-α) from RAW 264.7 macrophage cells (Rutkowsky et al., Reference Rutkowsky, Knotts, Ono-Moore, McCoin, Huang, Schneider, Singh, Adams and Hwang2014). This is of potential significance considering that TNF-α is associated with FLD in humans and dairy cows (Ohtsuka et al., Reference Ohtsuka, Koiwa, Hatsugaya, Kudo, Hoshi, Itoh, Yokota, Okada and Kawamura2001; Marcellini et al., Reference Marcellini, Nobili, Manco and Giannone2007). Interestingly, hepatic branched-chain amino acid concentrations (i.e. leucine, isoleucine and valine) are elevated with amplified acylcarnitine status during advanced FLD (Lake et al., Reference Lake, Novak, Shipkova, Aranibar, Robertson, Reily, Lehman-McKeeman, Vaillancourt and Cherrington2015). Moreover, elevations in branched-chain amino acid concentrations in the presence of high saturated fat intake may overload mitochondrial fuel oxidation via anaplerosis to form incompletely oxidized lipid-derived metabolites such as acylcarnitines and exacerbate insulin resistance (Newgard et al., Reference Newgard, An, Bain, Muehlbauer, Stevens, Lien, Haqq, Shah, Arlotto, Slentz, Rochon, Gallup, Ilkayeva, Wenner, Yancy, Eisenson, Musante, Surwit, Millington, Butler and Svetkey2009). Lastly, although the accrual of hepatic acylcarnitines promotes their accumulation in circulation via mechanisms that may involve organic cation/carnitine transporter-2, it deserves mentioning that the plasma acylcarnitine profile may inadequately reflect hepatic acylcarnitine metabolism in mammals (i.e. plasma levels do not reflect tissue levels; Schooneman et al., Reference Schooneman, Achterkamp, Argmann, Soeters and Houten2014).

In dairy cattle transitioning from gestation to lactation, the capacity to completely oxidize FA to CO₂ is lowered, whereas incomplete oxidation to acid-soluble metabolites is elevated (Litherland et al., Reference Litherland, Dann and Drackley2011). In the muscle of transition cow, long-chain acylcarnitines accumulate with short- and medium-chain acylcarnitines during the peripartum (Yang et al., Reference Yang, Sadri, Prehn, Adamski, Rehage, Dänicke, Saremi and Sauerwein2019), suggesting that β-oxidation to acetyl-CoA increased but the tricarboxylic acid cycle was downregulated. In sick transition cows (i.e. those with mastitis, metritis, retained placenta and/or laminitis; n = 6), plasma short-chain propionyl-carnitine concentrations were elevated, relative to six healthy controls (Hailemariam et al., Reference Hailemariam, Mandal, Saleem, Dunn, Wishart and Ametaj2014). In over-conditioned transition cows that mobilize more FA postpartum, plasma C14:0-, C16:0-, C18:0- and C20:0-carnitine levels are elevated, relative to lean cows (Rico et al., Reference Rico, Zang, Haughey, Rius and McFadden2018b). In the same study, plasma total acylcarnitine levels were positively correlated with circulating total FA concentrations, as well as total and C24:0-linked ceramide. Although this study supports that long-chain acylcarnitines were formed and exported out of the mitochondrion, the work did not assess FA oxidation or functionality of the TCA cycle. Regardless, hepatic FA that are not oxidized can serve as substrate for triglyceride esterification and are potentially partitioned toward ceramide synthesis in over-conditioned cows prone to accelerated lipolysis and FLD.

Our understanding of acylcarnitine status within the context of productivity and health requires further study. The reason is because elevations in serum carnitine or long-chain acylcarnitine (e.g. C16:0- or 18:2-carnitine) levels during the transition period or peak lactation have been observed in cows that remain clinically healthy beyond 100 days in milk, relative to those culled beyond peak milk production (Huber et al., Reference Huber, Dänicke, Rehage, Sauerwein, Otto, Rolle-Kampczyk and Von Bergen2016). It remains to be determined whether elevations in acylcarnitines during transition are predictive of metabolic disorders (e.g. ketosis) during early lactation. It is conceivable that fully functional mitochondria that manage FA surfeit during early lactation would protect the cow from lipotoxicity and FLD to enhance performance. In support, dietary carnitine supplementation has been shown to stimulate palmitate β-oxidation and decrease liver triglyceride concentrations in postpartum cows (Carlson et al., Reference Carlson, McFadden, D’Angelo, Woodworth and Drackley2007). Cows with a single nucleotide polymorphism in mitochondrial transcription factor A, an autosomal gene essential for transcription and replication of mitochondrial DNA, are less fertile and more likely to be culled, albeit they produce more milk (Clempson et al., Reference Clempson, Pollott, Brickell, Bourne, Munce and Wathes2011). Moreover, evidence in growing steers suggests that increased mitochondrial function may contribute to enhanced feed efficiency during compensatory growth that follows feed restriction (Connor et al., Reference Connor, Kahl, Elsasser, Parker, Li, Van Tassell, Baldwin and Barao2010).

Lipid mediators, insulin sensitivity and nutrient partitioning

Phosphatidylcholines and hepatic triglyceride disposal

In humans, non-alcoholic FLD is attributed to enhanced de novo lipogenesis involving the induction of sterol regulatory element-binding protein (Kohjima et al., Reference Kohjima, Higuchi, Kato, Kotoh, Yoshimoto, Fujino, Yada, Yada, Harada and Enjoji2008). In contrast, hepatic triglyceride deposition in peripartal cows with FLD is due to enhanced esterification of lipolytic-derived FA. This process is likely exacerbated by the aforementioned inadequate mitochondrial β-oxidation of FA in the liver as well as adipose insulin antagonism, which would enhance FA supply for hepatic uptake. Additionally, the pathophysiology of FLD in cows likely involves a limited capacity to secrete triglyceride within VLDL. Indeed, triglyceride secretion is limited in ruminants, relative to non-ruminants (Pullen et al., Reference Pullen, Liesman and Emery1990), and the rate of hepatic VLDL secretion plateaus as intrahepatic fat content becomes severe in humans with non-alcoholic FLD (Fabbrini et al., Reference Fabbrini, Mohammed, Magkos, Korenblat, Patterson and Klein2008). In transition cows, a downregulation of hepatic apolipoprotein B₁₀₀ mRNA expression and protein abundance may limit VLDL assembly and export to promote FLD (Bernabucci et al., Reference Bernabucci, Basiricò, Pirazzi, Rueca, Lacetera, Lepri and Nardone2009). An alternative hypothesis focuses on phosphatidylcholine (PC), a glycerophospholipid and principal component of the VLDL monolayer. Studies employing rodent models and choline-deficient diets have demonstrated the essential requirement of PC synthesis for VLDL secretion as a means to prevent steatosis (Fast and Vance, Reference Fast and Vance1995). Although the rapid ruminal degradation of unprotected choline and incorporation of choline into milk phospholipids may partially explain why plasma total choline and PC concentrations are low in postpartum cows (Artegoitia et al., Reference Artegoitia, Middleton, Harte, Campagna and de Veth2014), reduced choline availability has a potential to limit hepatic PC synthesis and VLDL secretion to favor the advancement of steatosis in the ruminant.

Hepatic synthesis of PC involves the cytidine diphosphate (CDP)–choline pathway (i.e. the Kennedy pathway) and the phosphatidylethanolamine N-methyltransferase (PEMT) pathway. Initiating the CDP–choline pathway, choline kinase utilizes free choline and ATP to form the PC-precursor phosphocholine. As observed in rat hepatocytes, choline kinase serves a critical function considering that ~70% of total PC are produced by the CDP–choline pathway (DeLong et al., Reference DeLong, Shen, Thomas and Cui1999). In liver, the PEMT pathway is recognized as a compensatory pathway that relies on coupled folate and transmethylation cycles, which fundamentally define one-carbon metabolism. Methyl donors, including methionine, choline and betaine, and one-carbon donors glycine and serine are utilized to generate S-adenosylmethionine for the transformation of phosphatidylethanolamine to PC by PEMT. Although undefined in cows, the CDP–choline pathway prefers DAG enriched in saturated and monounsaturated FA (i.e. palmitic and oleic acids, respectively). Instead, PEMT prefers phosphatidylethanolamine enriched in long- and very-long-chain polyunsaturated FA, including eicosatetraenoic acid and DHA (DeLong et al., Reference DeLong, Shen, Thomas and Cui1999). Indeed, circulating PC containing DHA has been recognized as a biomarker for PEMT activation in humans (da Costa et al., Reference da Costa, Sanders, Fischer and Zeisel2011). In the transition cow, lipolytic-derived FA (i.e. palmitic and oleic acids) may support CDP–choline pathway activation; however, it is hypothesized that hepatic polyunsaturated FA depletion may prevent PEMT activation (Myers et al., Reference Myers, Rico, Davis, Fontoura, Dineen, Tate and McFadden2019).

Associative relationships between circulating PC and the severity of hepatic steatosis have been repeatedly observed in dairy cows. For example, the levels of serum PC with short-chain FA moieties are low in cows with severe hepatic lipidosis, relative to clinically healthy cows (Imhasly et al., Reference Imhasly, Naegeli, Baumann, von Bergen, Luch, Jungnickel, Potratz and Gerspach2014). Moreover, suppressed hepatic levels of highly unsaturated PC have been observed in Holstein cows with moderate FLD (Saed Samii et al., Reference Saed Samii, Zang, Myers, Grilli and McFadden2018). With the intent to increase hepatic PC synthesis, feeding strategies that emphasize rumen-protected choline or methionine supplementation are routinely studied and applied as a potential means to prevent FLD in transition cows. For example, peripartal dietary rumen-protected choline supplementation has been shown to reduce hepatic triglyceride concentrations in postpartum cows (Zom et al., Reference Zom, Van Baal, Goselink, Bakker, De Veth and Van Vuuren2011). Recent in vitro work has aimed to delineate how choline and methionine are utilized by the CDP–choline and PEMT pathways (Zhou et al., Reference Zhou, Zhou, Batistel, Martinez-Cortés, Pate, Luchini and Loor2018). The potential of methyl donor feeding to enhance hepatic PC synthesis and lipid disposal may also have implications for improving inflammatory and immune status in cows (Zhou et al., Reference Zhou, Bulgari, Vailati-Riboni, Trevisi, Ballou, Cardoso, Luchini and Loor2016). Moving forward, the interplay between dietary FA and methyl donor efficacy to enhance hepatic PC synthesis and VLDL secretion should be examined. In support, Myers et al. (Reference Myers, Rico, Davis, Fontoura, Dineen, Tate and McFadden2019) hypothesize that DHA may selectively induce PEMT activation in cows.

Oxylipids and oxidative stress

Mitochondrial overload of saturated FA triggers the loss of redox homeostasis and accelerates the formation of oxygen radicals (i.e. ROS). This lipotoxic condition called oxidative stress has been implicated in the development of insulin resistance as well as non-alcoholic FLD in non-ruminants (Videla et al., Reference Videla, Rodrigo, Araya and Poniachik2006). As reviewed by Videla et al. (Reference Videla, Rodrigo, Araya and Poniachik2006), persistent ROS production activates serine/threonine kinase signaling cascades that inhibit insulin-stimulated insulin receptor substrate induction. In turn, insulin resistance may enhance intrahepatic lipid content to promote simple steatosis (i.e. non-inflammatory phenotype). However, oxidative stress may also promote Kupffer cell activation to activate redox-sensitive transcription factors, including nuclear factor-κB, and upregulate pro-inflammatory TNF-α. Unfortunately, inflammatory steatohepatitis is also characterized by the upregulation of NADPH oxidase and cytochrome P450 (family 2, subfamily E, polypeptide 1; otherwise known as CYP2E1), which further reduce antioxidant capacity and promote hepatocellular damage. Another consequence of rampant ROS generation includes n-3 long-chain polyunsaturated FA depletion caused by defective FA desaturation and enhanced peroxidation in the liver (Videla et al., Reference Videla, Rodrigo, Araya and Poniachik2004). In addition, polyunsaturated FA subjected to enzymatic (via cyclooxygenase, lipoxygenase and CYP2E1) or non-enzymatic (via ROS) oxidation form oxylipids with diverse inflammatory functions. For instance, oxylipids derived from n-6 arachidonic acid and linoleic acid include pro-inflammatory hydroxyl-octadecadienoic acid and hydroxyl-eicosatetraenoic acid, respectively.

In the transition cow experiencing reduced antioxidant potential (Sordillo and Aitken, Reference Sordillo and Aitken2009), ROS accumulation likely develops in part because of rampant mitochondrial FA oxidation, albeit incomplete breakdown. In turn, oxidative stress may enhance adipose tissue lipolysis to exacerbate oxidant status (Krawczyk et al., Reference Krawczyk, Haller, Ferrante, Zoeller and Corkey2012). One fate of lipolytic-derived unsaturated FA is ROS oxidation, which results in lipid hydroperoxide and isoprostane formation. Fatty acids released by adipose tissue are also used for hepatic β-hydroxybutyrate generation, which may promote hepatocyte apoptosis via ROS-mediated p38 mitogen-activated protein kinase activation (Song et al., Reference Song, Li, Gu, Fu, Peng, Zhao, Zhang, Li, Wang, Li and Liu2016). In addition, lipolytic FA may induce nuclear factor-κB in hepatocytes via ROS-dependent mechanisms that trigger inflammation (Li et al., Reference Li, Huang, Gu, Du, Lei, Yuan, Sun, Wang, Li and Liu2015). These findings suggest that oxidative stress is involved in FLD pathology.

As previously reviewed by Sordillo and Aitken (Reference Sordillo and Aitken2009), the accumulation of ROS or oxylipids likely influences bovine immune responses. For instance, plasma oxylipid levels are correlated with the expression of interleukin-12β and inducible nitric oxide synthase-2 in peripheral blood mononuclear cells derived from healthy transition cows (Raphael et al., Reference Raphael, Halbert, Contreras and Sordillo2014). In endothelial cells, elevations in the arachidonic acid metabolite 15-hydroxy-peroxyeicosatetraenoic acid develops with apoptosis and caspase-3 activation (Sordillo et al., Reference Sordillo, Weaver, Cao, Corl, Sylte and Mullarky2005), and leukocyte recruitment and inflammatory cytokine release (Whatling et al., Reference Whatling, McPheat and Herslof2007). Cytochrome P450- and lipoxygenase-derived oxylipids accumulate in plasma (e.g. 11,12-epoxyoctadecanoic acid) and adipose tissue (e.g. 5-hydroxy-eicosatetraenoic acid) of postpartum cows, respectively, and may act as positive or negative modulators of inflammation and immune cell trafficking during intense lipolysis (Contreras et al., Reference Contreras, Strieder-Barboza, De Souza, Gandy, Mavangira, Lock and Sordillo2017). Inflammatory dysfunction is a key attribute of inflammatory disorders including mastitis and metritis; therefore, the role of oxidized lipids has been considered. For instance, Streptococcus uberis mastitis is characterized by mammary inflammation and the accrual of oxylipids, including hydroxyl-octadecadienoic acid (Ryman et al., Reference Ryman, Pighetti, Lippolis, Gandy, Applegate and Sordillo2015). The amount of dietary antioxidants (i.e. vitamin E and β-carotene) and trace minerals (i.e. copper or zinc) that play a role in antioxidant defense mechanisms, and the type and amount of FA fed to transition cows, may influence oxylipid formation and affiliated health outcomes.

Leptin, adiponectin and fibroblast growth factor-21

Although the role of insulin within the framework of glucose utilization and lipolysis is described, adipokines and hepatokines deserve consideration to understand lipid metabolism in the transition cow. First, the adipokine leptin is produced by adipocytes and mediates its action through the Janus kinase signal transducer and activator of transcription pathway (Yadav et al., Reference Yadav, Kataria, Saini and Yadav2013). The hormone regulates energy metabolism, insulin sensitivity, thyroid hormone secretion, immunity and appetite. In a demonstration of function, insulin resistance and inflammatory non-alcoholic FLD develops in obese leptin-deficient ob/ob mice (Perfield et al., Reference Perfield, Ortinau, Pickering, Ruebel, Meers and Rector2013). Moreover, ob/ob mice experience lipotoxicity including muscle ceramide and DAG accrual (Turpin et al., Reference Turpin, Ryall, Southgate, Darby, Hevener, Febbraio, Kemp, Lynch and Watt2009); however, chronic treatment of ob/ob mice with myriocin decreases circulating ceramides, steatosis and improves glucose homeostasis (Yang et al., Reference Yang, Badeanlou, Bielawski, Roberts, Hannun and Samad2009). In hyperlipidemic animals, leptin acts by preventing the accumulation of ceramide and DAG to protect against insulin resistance (Dube et al., Reference Dube, Bhatt, Dedousis, Bonen and O’Doherty2007). Such results may be explained by a stimulatory effect of leptin on FA breakdown (Shimabukuro et al., Reference Shimabukuro, Koyama, Chen, Wang, Trieu, Lee, Newgard and Unger1997). However, the lipid mediator-lowering effect of leptin is not always observed. In hyperleptinemic lean animals, for example, soleus and superficial vastus muscle concentrations of ceramide are increased, DAG unchanged, and triglyceride decreased (Dube et al., Reference Dube, Bhatt, Dedousis, Bonen and O’Doherty2007). These findings suggest that mechanisms by which leptin modulate insulin action likely vary by adiposity phenotype.

In the late-gestating cow, fatter cows exhibit higher plasma leptin concentrations, relative to lean cows (Kokkonen et al., Reference Kokkonen, Taponen, Anttila, Syrjälä-Qvist, Delavaud, Chilliard, Tuori and Tesfa2005), whereas all cows experience a decline in circulating leptin and white adipose tissue leptin mRNA expression at the onset of lactation (Block et al., Reference Block, Butler, Ehrhardt, Bell, Van Amburgh and Boisclair2001). With this physiology in mind, it can be argued that the observed elevations in muscle FA oxidation occur independent of leptin signaling (Schäff et al., Reference Schäff, Börner, Hacke, Kautzsch, Sauerwein, Spachmann, Schweigel-Röntgen, Hammon and Kuhla2013). In ruminants, leptin may only be responsive during positive energy balance. In support, an intravenous infusion of a triglyceride emulsion increases circulating FA and leptin concentrations during late gestation but not early lactation (Chelikani et al., Reference Chelikani, Keisler and Kennelly2003). The authors suggest that this effect may be due to elevations in insulin-like growth factor-1, which is known to regulate leptin secretion. Although current evidence suggests a limited role of leptin in postpartum dairy cattle, the administration of human leptin to early-lactation cows decreases liver triglyceride content (Ehrhardt et al., Reference Ehrhardt, Foskolos, Giesy, Wesolowski, Krumm, Butler, Quirk, Waldron and Boisclair2016); therefore, strategies that elevate leptin status during the periparturient period may be therapeutic.

Adiponectin is predominantly expressed in adipose tissue, and the expression of adiponectin decreases with increasing adiposity (Yadav et al., Reference Yadav, Kataria, Saini and Yadav2013). This adipokine increases insulin sensitivity, reduces circulating glucose, enhances FA catabolism and prevents inflammation (Yadav et al., Reference Yadav, Kataria, Saini and Yadav2013). The mechanisms of adiponectin action involve the activation of peroxisome proliferator activator receptor-α and AMP-activated protein kinase. In the muscle, downstream outcomes include enhanced FA transport and oxidation, reduced intracellular triglyceride content and enhanced insulin-stimulated translocation of glucose transporters to the plasma membrane.

In multiparous dairy cow transitioning to lactation, circulating adiponectin levels are lowest at parturition (Giesy et al., Reference Giesy, Yoon, Currie, Kim and Boisclair2012). Suppressed adiponectin secretion in the multiparous transition cow could limit FA oxidation and glucose utilization in response to insulin. In support, enhanced plasma adiponectin levels are associated with reduced plasma concentrations of long-chain acylcarnitines (Rodríguez-Gutiérrez et al., Reference Rodríguez-Gutiérrez, Lavalle-González, Martínez-Garza, Landeros-Olvera, López-Alvarenga, Torres-Sepúlveda, González-González, Mancillas-Adame, Salazar-Gonzalez and Villarreal-Pérez2012), and adiponectin decreases ceramide accumulation in obese mice (Holland et al., Reference Holland, Adams, Brozinick, Bui, Miyauchi, Kusminski, Bauer, Wade, Singhal, Cheng, Volk, Kuo, Gordillo, Kharitonenkov and Scherer2013). It deserves to be emphasized that serum adiponectin concentrations appear to increase after parturition in primiparous cows (Urh et al., Reference Urh, Denißen, Harder, Koch, Gerster, Ettle, Kraus, Schmitz, Kuhla, Stamer, Spiekers and Sauerwein2019). It has been hypothesized that greater adiponectin concentrations in primiparous cows may enhance peripheral insulin sensitivity and nutrient partitioning toward growth (Koster et al., Reference De Koster, Urh, Hostens, Van den Broeck, Sauerwein and Opsomer2017; Urh et al., Reference Urh, Denißen, Harder, Koch, Gerster, Ettle, Kraus, Schmitz, Kuhla, Stamer, Spiekers and Sauerwein2019). If this were observed, it can be further hypothesized that circulating acylcarnitines and ceramides would be lower in primiparous cows, relative to multiparous cows.

Studies have also focused on the expression of adiponectin receptor-1 and -2 in bovine tissues during the transition period. Giesy et al. (Reference Giesy, Yoon, Currie, Kim and Boisclair2012) observed elevated mRNA expression of adiponectin receptor-1 and -2 during early lactation in the muscle and liver, respectively, relative to late pregnancy. Similar changes in hepatic adiponectin receptor-2 expression have been confirmed (Saremi et al., Reference Saremi, Winand, Friedrichs, Kinoshita, Rehage, Dänicke, Häussler, Breves, Mielenz and Sauerwein2014). In adipose tissue, Giesy et al. (Reference Giesy, Yoon, Currie, Kim and Boisclair2012) did not observe a change in adiponectin receptor expression, which contrasts with other work demonstrating reduced adiponectin receptor-1 and -2 expression in adipose tissue in early lactation, relative to late gestation (Lemor et al., Reference Lemor, Hosseini, Sauerwein and Mielenz2009; Saremi et al., Reference Saremi, Winand, Friedrichs, Kinoshita, Rehage, Dänicke, Häussler, Breves, Mielenz and Sauerwein2014). Future work should define how changes in adiponectin receptor mRNA expression influence adiponectin signaling.

Fibroblast growth factor-21 (FGF21) lowers blood glucose, insulin and triglyceride concentrations, and reduces body weight in non-ruminants (Holland et al., Reference Holland, Adams, Brozinick, Bui, Miyauchi, Kusminski, Bauer, Wade, Singhal, Cheng, Volk, Kuo, Gordillo, Kharitonenkov and Scherer2013). Interestingly, FGF21 stimulates adiponectin secretion, and adiponectin mediates FGF21-induced increases in energy expenditure (Holland et al., Reference Holland, Adams, Brozinick, Bui, Miyauchi, Kusminski, Bauer, Wade, Singhal, Cheng, Volk, Kuo, Gordillo, Kharitonenkov and Scherer2013). In peripartal cow, plasma FGF21 concentrations peak at parturition. Moreover, plasma FGF21 levels and hepatic FGF21 mRNA are increased by feed restriction in non-pregnant, late-lactating dairy cows (Schoenberg et al., Reference Schoenberg, Waldron, Giesy, Boisclair, Harvatine, Kharitonenkov and Cheng2011); however, elevations in circulating FGF21 does not appear to prevent FA-induced increases in liver triglyceride in non-pregnant, non-lactating dairy cows (Caixeta et al., Reference Caixeta, Giesy, Krumm, Perfield, Butterfield, Schoenberg, Beitz and Boisclair2017). Because plasma FGF21 and ceramide concentrations are elevated and adiponectin secretion is suppressed in transition cows, it would appear that the FGF21–adiponectin–ceramide axis observed in non-ruminants (Holland et al., Reference Holland, Adams, Brozinick, Bui, Miyauchi, Kusminski, Bauer, Wade, Singhal, Cheng, Volk, Kuo, Gordillo, Kharitonenkov and Scherer2013) is not intact in peripartal ruminants but may explain metabolic dysfunction (i.e. FLD and impaired insulin antagonism).