Palmitate‐mediated induction of neuropeptide Y expression occurs through intracellular metabolites and not direct exposure to proinflammatory cytokines

Abstract

A contributing factor to the development of obesity is the consumption of a diet high in saturated fatty acids, such as palmitate. These fats induce hypothalamic neuroinflammation, which dysregulates neuronal function and induces orexigenic neuropeptide Y (Npy) to promote food intake. An inflammatory cytokine array identified multiple candidates that could mediate palmitate-induced up-regulation of Npy mRNA levels. Of these, visfatin or nicotinamide phosphoribosyltransferase (NAMPT), macrophage migratory inhibitory factor (MIF), and IL-17F were chosen for further study. Direct treatment of the neuropeptide Y/agouti-related peptide (NPY/AgRP)-expressing mHypoE-46 neuronal cell line with the aforementioned cytokines demonstrated that visfatin could directly induce Npy mRNA expression. Preventing the intracellular metabolism of palmitate through long-chain acyl-CoA synthetase (ACSL) inhibition was sufficient to block the palmitate-mediated increase in Npy gene expression.Furthermore, thin-layer chromatography revealed that in neurons, palmitate is readily incorporated into ceramides and defined species of phospholipids. Exogenous C16 ceramide, dipalmitoyl-phosphatidylcholine, and dipalmitoyl-phosphatidylethanolamine were sufficient to significantly induce Npy expression. This study suggests that the intracellular metabolism of palmitate and elevation of metabolites, including ceramide and phospholipids, are responsible for the palmitate-mediated induction of the potent orexigen Npy. Furthermore, this suggests that the regulation of Npy expression is less reliant on inflammatory cytokines per se than palmitate metabolites in a model of NPY/AgRP neurons. These lipid species likely induce detrimental downstream cellular signaling events ultimately causing an increase in feeding, resulting in an overweight phenotype and/or obesity.

Central Regulation of Brown Fat Thermogenesis in Response to Saturated or Unsaturated Long-Chain Fatty Acids

“…The obesogenic action of ICV saturated FA has been related to the promotion of inflammation [ 40 , 41 ], as observed in our study in terms of microglia activation by ICV palmitate infusion ( Supplementary Figure S3 ). The central inflammation led by saturated FA along with changes in the levels of specific intracellular metabolites (i.e., ceramides and phospholipids) in hypothalamic neurons could ultimately interfere in BAT activation and finally overweight [ 42 ].…”

Mechanisms Driving Palmitate-Mediated Neuronal Dysregulation in the Hypothalamus

“…Excessive exposure to saturated fatty acids such as palmitate, which are commonly found in HFDs, can disturb hypothalamic neurons resulting in resistance to insulin and leptin, dysregulation of feeding and reproductive neuropeptides, and disruptions of circadian rhythms that maintain energy homeostasis, and in turn reproduction. The effects of palmitate have been primarily linked to the induction of cellular stress, including neuroinflammation and ER stress, but avenues remain yet to be fully explored, including the actions of palmitate metabolites [110]. miRNAs play a key role in hypothalamic function and are dysregulated in disease states, and as a result, have become a novel and exciting area of study.…”

“…The development of chronic, low-grade neuroinflammation in vivo has been suggested to play a significant role in altering feeding neuropeptide expression [106,109]. Studies in the mHypoE-46 cell line demonstrate that exogenous administration of TNFα and visfatin, proinflammatory cytokines induced by palmitate, induce Npy expression [106,110]. However, the exogenous administration of other palmitate-induced pro-inflammatory cytokines, Macrophage migration inhibitory factor (MIF) and IL-17F, were unable to affect Npy expression in the mHypoE-46 cell line [110].…”

“…Studies in the mHypoE-46 cell line demonstrate that exogenous administration of TNFα and visfatin, proinflammatory cytokines induced by palmitate, induce Npy expression [106,110]. However, the exogenous administration of other palmitate-induced pro-inflammatory cytokines, Macrophage migration inhibitory factor (MIF) and IL-17F, were unable to affect Npy expression in the mHypoE-46 cell line [110]. Whereas PS1145 was only able to partially block the induction of Npy by palmitate [106], inhibition of acyl-coA synthetase completely blocks the increase in the mHypoE-46 cells [110], suggesting that the palmitate-mediated effects of Npy occur primarily through the metabolism of palmitate to ceramides and certain phospholipid species and is only partially dependent on neuroinflammation [110].…”

“…However, the exogenous administration of other palmitate-induced pro-inflammatory cytokines, Macrophage migration inhibitory factor (MIF) and IL-17F, were unable to affect Npy expression in the mHypoE-46 cell line [110]. Whereas PS1145 was only able to partially block the induction of Npy by palmitate [106], inhibition of acyl-coA synthetase completely blocks the increase in the mHypoE-46 cells [110], suggesting that the palmitate-mediated effects of Npy occur primarily through the metabolism of palmitate to ceramides and certain phospholipid species and is only partially dependent on neuroinflammation [110]. Furthermore, Npy dysregulation by palmitate in the mHypoE-44 cell line coincides with disruptions in the cyclic expression of circadian rhythm genes [108], including brain and muscle ARNT-like 1 (BMAL1), an essential mediator of palmitate-induced Npy upregulation [111].…”