Hepatic patatin-like phospholipase domain-containing protein 3 sequence, single nucleotide polymorphism presence, protein confirmation, and responsiveness to energy balance in dairy cows

Responsiveness of PNPLA3 and lipid-related transcription factors is dependent upon fatty acid profile in primary bovine hepatocytes

Knockdown of patatin-like phospholipase domain-containing protein 3 (PNPLA3) increased triglycerides (TG) in primary bovine hepatocytes, suggesting that PNPLA3 plays a causal role in hepatic TG clearing. In vivo, PNPLA3 abundance across the periparturient period is inversely related to hepatic TG accumulation and circulating fatty acid (FA) concentrations. The purpose of this research was to determine if PNPLA3, as well as other lipases, transcription factors, or FA-mediated genes, are regulated by FA mimicking liver lipid accumulation (ACCUM) and liver lipid clearing (RECOV) or singular FA physiologically found in dairy cows at 0.5 mM of circulating RECOV (iRECOV). Abundance of PNPLA3 tended to decrease with ACCUM and increased quadratically with RECOV (P ≤ 0.10), differing from PNPLA3 expression, but consistent with previous in vivo research. Adipose TG lipase abundance, but not other lipase abundances, was quadratically responsive to both ACCUM and RECOV (P ≤ 0.005). Abundance of PNPLA3 and SREBP1c and expression of LXRA responded similarly to iRECOV, with C18:0 tending to decrease abundance (P ≤ 0.07). Results indicate that bovine PNPLA3 is translationally regulated by FA and although a LXRA-SREBP1c pathway mediation is possible, the mechanism warrants further investigation.

Greater liver PNPLA3 protein abundance in vivo and in vitro supports lower triglyceride accumulation in dairy cows

Fatty liver syndrome is a prevalent metabolic disorder in peripartum dairy cows that unfavorably impacts lactation performance and health. Patatin-like phospholipase domain-containing protein 3 (PNPLA3) is a lipase that plays a central role in human non-alcoholic fatty liver disease etiology but has received limited attention in bovine fatty liver research. Thus, we investigated the relationship between tissue PNPLA3 expression and liver triglyceride accumulation in vivo via a ketosis induction protocol in multiparous dairy cows peripartum, as well as in vitro via small interfering RNA knockdown of PNPLA3 mRNA expression in bovine primary hepatocytes. Results demonstrated a negative association (P = 0.04) between liver PNPLA3 protein abundance and liver triglyceride content in peripartum dairy cows, while adipose PNPLA3 protein abundance was not associated with liver triglyceride content or blood fatty acid concentration. Knockdown of PNPLA3 mRNA resulted in reduced PNPLA3 protein abundance (P < 0.01) and greater liver triglyceride content (P < 0.01). Together, these results suggest greater liver PNPLA3 protein abundance may directly limit liver triglyceride accumulation peripartum, potentially preventing bovine fatty liver or accelerating recovery from fatty liver syndrome.

ADSA Foundation Scholar Award: Influencing hepatic metabolism: Can nutrient partitioning be modulated to optimize metabolic health in the transition dairy cow?

Hepatic de novo production of glucose and oxidation of fatty acids are critical in supporting milk production during the transition to lactation period. During this period of metabolic challenge, there is an increase in fatty acids taken up by the liver. Although the primary fate for these fatty acids is complete oxidation, alternative fates include incomplete oxidation via ketogenesis, storage within the liver as triglycerides (TG), and secretion of TG within very low density lipoproteins. Influencing the relative capacity of these pathways, and thus shifting nutrient partitioning, may allow for improved hepatic efficiency and metabolic health. Hepatic nutrient partitioning reflects complex regulation of key metabolic pathways by factors such as fatty acids and other substrates. Relative flux of fatty acid through oxidation or re-esterification to TG leads to the onset of metabolic disorders that are associated with negative production outcomes, such as hyperketonemia and fatty liver. Although recent work has focused on understanding how stored TG are lipolyzed for subsequent oxidation, the mechanism and regulation of this remains unclear. The source of mobilized fatty acids is similarly important, both in terms of amount and profile of fatty acids mobilized. There is likely a complex, coordinated whole-body response, given that fatty acids mobilized from adipose tissue affect hepatic regulation. Fatty acids mobilized from adipose tissue have regulatory effects on genes such as pyruvate carboxylase; however, in vivo work suggests there may also be other influences resulting in differential regulation between cows that subsequently develop sub-clinical ketosis and those that do not. Optimizing nutrient partitioning between critical metabolic pathways may allow for nutritional opportunities to reduce incidence of metabolic challenges and improve feed efficiency. Although further research is needed to continue refining our understanding of the intricate balance regulating hepatic metabolism, shifting nutrient partitioning may be key in supporting both efficiency and metabolic health.