Invited review: Fueling milk production carbon by carbon: Regulation of hepatic glucose production in dairy cattle.

Milk production depends on glucose supply to the mammary gland, which in ruminants is driven by the liver for gluconeogenesis (GNG). Dairy cattle rely on endogenous glucose production (EGP) via GNG for 90% of glucose needs, and the capacity for EGP depends on substrate supply, activity of GNG enzymes, and rate of flux of substrates into and through metabolic pathways like the tricarboxylic acid (TCA) cycle. Propionate serves as the primary EGP precursor, but also directly affects the activity of the rate-limiting GNG enzymes, pyruvate carboxylase (PC), and phosphoenolpyruvate carboxykinase (PEPCK). Short-chain and long-chain fatty acids, hormones, and feed restriction also regulate expression levels of PC and cytosolic PEPCK (PCK1). Herein, we review the literature regarding molecular regulation of GNG in dairy cattle and discuss the consequence of the dramatic shifts in these factors that occur around the time of calving, which mediate changes in PC and PCK1 expression, with implications for the health and nutritional management of dairy cattle. Discussion of data from studies that applied fluxomic tools to map carbon flow of stable isotope-labeled substrates through the TCA cycle and GNG pathways combined with proteome or transcriptome data in response to alterations in PC, fatty acid supply, and circadian disruption is included to tie control mechanisms with pathway flux outcomes. In the end, we advocate for greater understanding of the nuances of bovine metabolic control that are achieved through the application of multi-omic tools to deepen our understanding of the regulatory processes of hepatic GNG and to identify targets for interventions that support EGP to optimize milk production and cow health.