The Roles of White Adipose Tissue and Liver NADPH in Dietary Restriction-Induced Longevity

Abstract

Dietary restriction (DR) protocols frequently employ intermittent fasting. Following a period of fasting, meal consumption increases lipogenic gene expression, including that of NADPH-generating enzymes that fuel lipogenesis in white adipose tissue (WAT) through the induction of transcriptional regulators SREBP-1c and CHREBP. SREBP-1c knockout mice, unlike controls, did not show an extended lifespan on the DR diet. WAT cytoplasmic NADPH is generated by both malic enzyme 1 (ME1) and the pentose phosphate pathway (PPP), while liver cytoplasmic NADPH is primarily synthesized by folate cycle enzymes provided one-carbon units through serine catabolism. During the daily fasting period of the DR diet, fatty acids are released from WAT and are transported to peripheral tissues, where they are used for beta-oxidation and for phospholipid and lipid droplet synthesis, where monounsaturated fatty acids (MUFAs) may activate Nrf1 and inhibit ferroptosis to promote longevity. Decreased WAT NADPH from PPP gene knockout stimulated the browning of WAT and protected from a high-fat diet, while high levels of NADPH-generating enzymes in WAT and macrophages are linked to obesity. But oscillations in WAT [NADPH]/[NADP⁺] from feeding and fasting cycles may play an important role in maintaining metabolic plasticity to drive longevity. Studies measuring the WAT malate/pyruvate as a proxy for the cytoplasmic [NADPH]/[NADP⁺], as well as studies using fluorescent biosensors expressed in the WAT of animal models to monitor the changes in cytoplasmic [NADPH]/[NADP⁺], are needed during ad libitum and DR diets to determine the changes that are associated with longevity.

Keywords: NADPH; aging; dietary restriction; lifespan; metabolism; white adipose.

Figure 1

The citratrate-α-ketoglutarate shuttle and the citrate–pyruvate shuttle compete for cytoplasmic citrate. Both can generate cytoplasmic NADPH, but the citrate–pyruvate shuttle also synthesizes cytoplasmic acetyl-CoA and transfers cytoplasmic NADH-reducing equivalents into the mitochondrial matrix. The citrate–pyruvate shuttle relies upon pyruvate carboxylase (PC) to regenerate oxaloacetate to react with the pyruvate-derived acetyl-CoA. In the liver, both shuttles may operate simultaneously as the citrate–pyruvate shuttle can only provide roughly half of the NADPH required for fatty acid synthesis. In WAT, the remainder of the NADPH for fatty acid synthesis not provided by the citrate–pyruvate shuttle is likely synthesized by the PPP. Transport reactions and unnamed Krebs cycle reactions are shown as dashed arrows, while other chemical reactions are shown as solid arrows. Enzyme names are shown in blue font, metabolite names are shown in black font, coenzyme names are shown in maroon font, and transporter names and gaseous co-reactants and co-products are shown in gray font.

Figure 2

Serine biosynthesis pathway, the folate cycle, the methionine cycle, and the transsulfuration pathway. Serine is synthesized in a 3-enzyme pathway from the glycolytic intermediate 3-phosphoglycerate, as shown shaded in gray. In most tissues, with the known exception of liver, serine is imported into mitochondria and metabolized by SHMT2 to drive the folate cycle, shown as the circle of yellow arrows in the figure, in the clockwise direction. Formate, a one-carbon intermediate, and tetrahydrofolate are exported from mitochondria and react in the cytoplasm to from 10-formyltetrahydrofolate (10-formylTHF). Cytoplasmic one-carbon units can either be used for NADPH generation with the release of CO₂ or in metabolism, where they are commonly used for methylation reactions or nucleotide synthesis. NADPH oxidation is used to regenerate cytoplasmic tetrahydrofolate (THF) from dihydrofolate. To be used for methylation reactions, the one-carbon unit is funneled into the methionine cycle, shown shaded as a light tan or cream color in the figure. Homocysteine can either be methylated to methionine and re-enter the methionine cycle or react with serine to enter the transsulfuration pathway, as shown shaded in light green in the figure, leading to cysteine and α-ketobutyrate synthesis. Different forms of folate are shown with a white font and blue background. Amino acid names are shown in red font. Other metabolite names are shown in green font, with the exception of coenzyme names that are shown in a purple font. Gaseous products are shown in light blue font. Enzyme names are shown in black font. Single chemical reactions are shown as solid arrows, while multiple reactions are shown as a dashed arrow. Metabolites that are transported into or out of the mitochondrial matrix are boxed.

Figure 3

Important transcriptional regulators that play a role in WAT NADPH and lipid metabolism that appear to drive DR-mediated longevity and whether these transcriptional regulators are activated during the feeding or fasting portion of the DR diet. After consuming a meal on the DR diet, the transcriptional regulators SREBP-1c, CHREBP, and PPAR-γ are induced in WAT, which leads to the expression of FGF21, PGC-1α, and lipogenic genes, including cytoplasmic NADPH-generating enzymes. Together, this leads to increased NADPH levels, HDAC3 inhibition, increased mitochondrial biogenesis, the browning of WAT, and fatty acid cycling. For further details, see the following references [145,353]. During the fasting portion of the DR diet, decreased insulin signaling leads to the transcriptional activation of FOXO3, while decreased amino acid levels lead to ISR and ATF4 activation that leads to increased expression of ATF3, which contributes to the browning of WAT, with each contributing to longevity. Dotted arrows represent causation, while solid arrows represent increased or decreased levels and/or activity.