In a paper published in the journal Frontiers in Cell and Developmental Biology, Okabe and colleagues at the University of Toyama, Japan, show that enhanced synthesis of the metabolic core molecule NAD+ is critical for the development of mature fat. Research reveals a new role for NAD+ biosynthesis in adipogenesis and suggests a new therapeutic approach to combat obesity!

What is the role of NAD+ in fat accumulation?

Adipogenesis -- the maturation of fat cells -- is a finely controlled process. These cells derive from precursors called preadipocytes, a process regulated by the fine control of gene regulatory proteins called transcription factors that coordinate the expression of hundreds of proteins. When preadipocyte differentiation is disrupted, adipocytes swell and ectopic lipid accumulation occurs in inflammation and metabolic dysfunction.

Metabolic changes accompany lipogenesis. Although several studies have been performed to determine the metabolic changes that occur during preadipocyte differentiation, little attention has been paid to the role of NAD+ metabolism.

Upregulation of NAD+ synthesis is required for preadipocyte differentiation

Okabe and colleagues sought to elucidate the metabolic changes that occur during preadipocyte differentiation. They found that levels of NAD+ and NMN, a NAD+ precursor, increased significantly during preadipocyte differentiation. This was accompanied by increased levels of enzymes that synthesize NAD+ and NMN. These data suggest that NAD+ synthesis is upregulated during preadipocyte differentiation.

Next, the researchers wondered whether increased levels of NAD+ were required for preadipocyte differentiation. To understand this, they blocked NAD+ production by inhibiting an enzyme that synthesizes NMN during preadipocyte differentiation. When they suppressed the elevation of NMN and NAD+ levels after induction of differentiation, the differentiation of preadipocytes was significantly reduced. Furthermore, NMN supplementation of differentiated preadipocytes completely restored NAD+ levels and preadipocyte differentiation. These data demonstrate that increased NAD+ production is required for preadipocyte differentiation.

NAD+ synthesis regulates the genetic program of adipocyte maturation

Okabe and colleagues next examined whether the adipogenesis program is affected by NAD+ levels. When NAD+ levels were reduced, the master regulator of adipogenesis, PPARγ, and target adipogenesis genes were significantly inhibited. These results suggest that NAD+ must be upregulated to induce PPARγ and that NAD+ metabolism is involved in the induction of PPARγ during adipogenesis.

Alpha-ketoglutarate regulates PPARγ activation in preadipocyte differentiation

To activate a gene, the PPARγ DNA sequence must be demethylated, removing molecular tags called methyl groups that bind to DNA. The researchers found that when demethylation was inhibited, levels of PPARγ activation and preadipocyte differentiation were inhibited.

In addition, the metabolite α-ketoglutarate is a known cofactor for demethylases. Interestingly, levels of α-ketoglutarate were significantly increased during preadipocyte differentiation, which correlated with increased levels of NAD+. However, the decrease in NAD+ levels after induction of differentiation abolished the increase in alpha-ketoglutarate levels.

The researchers then wanted to see if there was a link between NAD+, alpha-ketoglutarate, and lipogenesis. To do this, they supplemented the preadipocytes with alpha-ketoglutarate. This rescued PPARγ levels and preadipocyte differentiation, all of which were inhibited by inhibiting NAD+ synthesis. Taken together, these data demonstrate that increased alpha-ketoglutarate levels by NAD+ are required for the demethylation of the PPARgamma gene and lead to preadipocyte differentiation.

Alpha-ketoglutarate prevents diet-induced obesity by promoting proper lipogenesis

Finally, Okabe and colleagues investigated how NAD+ and alpha-ketoglutarate affect obesity. Mice fed an obesity-inducing high-fat, high-sucrose diet were orally administered alpha-ketoglutarate in drinking water. Surprisingly, alpha-ketoglutarate administration completely suppressed diet-induced obesity, similar to mice fed a normal diet.

The researchers also examined the physical characteristics of the mice's fat tissue. The obesity-inducing diet caused fat cells to swell, whereas adipocytes in the alpha-ketoglutarate-administered group were significantly smaller in size. Furthermore, the levels of genes in the adipogenesis program were impaired in mice fed an obesity-inducing diet, which were rescued by alpha-ketoglutarate administration.

Can NMN or alpha-ketoglutarate prevent obesity in humans?

Taken together, this study reveals new insights into how adipocytes mature, involving NAD+-dependent metabolic reprogramming. "Our results suggest that the NAD+-α-ketoglutarate axis is a potential therapeutic target for obesity-related metabolic disorders," said the researchers. Further research is needed to explore whether these effects can be reproduced in humans.