SIRT1-Mediated eNAMPT Secretion from Adipose Tissue Regulates Hypothalamic NAD+ and Function in Mice

Abstract

Nicotinamide phosphoribosyltransferase (NAMPT), the key NAD(+) biosynthetic enzyme, has two different forms, intra- and extracellular (iNAMPT and eNAMPT), in mammals. However, the significance of eNAMPT secretion remains unclear. Here we demonstrate that deacetylation of iNAMPT by the mammalian NAD(+)-dependent deacetylase SIRT1 predisposes the protein to secretion in adipocytes. NAMPT mutants reveal that SIRT1 deacetylates lysine 53 (K53) and enhances eNAMPT activity and secretion. Adipose tissue-specific Nampt knockout and knockin (ANKO and ANKI) mice show reciprocal changes in circulating eNAMPT, affecting hypothalamic NAD(+)/SIRT1 signaling and physical activity accordingly. The defect in physical activity observed in ANKO mice is ameliorated by nicotinamide mononucleotide (NMN). Furthermore, administration of a NAMPT-neutralizing antibody decreases hypothalamic NAD(+) production, and treating ex vivo hypothalamic explants with purified eNAMPT enhances NAD(+), SIRT1 activity, and neural activation. Thus, our findings indicate a critical role of adipose tissue as a modulator for the regulation of NAD(+) biosynthesis at a systemic level.

Figure 1

iNAMPT acetylation levels decrease in response to fasting in adipose tissue and are associated with eNAMPT secretion in differentiated adipocytes. (A) iNAMPT acetylation levels in brown and white adipose tissues (BAT and WAT) of fed and fasted wild-type female mice at 4–5 months of age. The right panel represents average values from two independent experiments. Relative iNAMPT acetylation levels are normalized to fed values. (B) iNAMPT acetylation levels in differentiated HIB-1B cells. Cells were treated with 5 μM Trichostatin A (T) and 5 mM nicotinamide (N) for 3 hr. M; mock treatment. (C) Reduction of eNAMPT secretion after Trichostatin A (T) and nicotinamide (N) treatment in differentiated HIB-1B cells. Levels of eNAMPT secretion were calculated as described in Materials and Methods. (D) Enhancement of eNAMPT secretion in response to low glucose in differentiated HIB-1B cells and 3T3-L1 cells. Cells were incubated with high glucose (H, 25 mM) or low glucose (L, 5 mM) media in the presence or absence of nicotinamide (N, 5 mM) for 3 hr. Bottom panels in C and D represent average values of three independent experiments. Each value is normalized to values from the mock (C) or the high glucose condition (D). Data were analyzed by the Student’s t test or one-way ANOVA with Fisher’s LSD posthoc test. All values are presented as mean ± SEM. *p ≤ 0.05; **p ≤ 0.01; ***p ≤ 0.001

Figure 2

Plasma eNAMPT levels increase in response to fasting in a SIRT1-dependent manner. (A) Plasma eNAMPT levels from fed or 48 hr-fasted Sirt1^+/+ and Sirt1^−/− male mice at 4–5 months of age (n= 7–12). The bottom panel shows average plasma eNAMPT levels normalized to the fed Sirt1^+/+ level in each condition. (B) iNAMPT and SIRT1 protein levels in white adipose tissue (WAT) and the liver of Sirt1^+/+ and Sirt1^−/− male mice at 3–4 months of age. iNAMPT levels are normalized to TUBULIN levels. The right panel shows average values normalized to the fed value in each tissue (n=3). (C) Nampt mRNA expression levels in WAT of Sirt1^+/+ and Sirt1^−/− male mice at 3–4 months of age (n= 4). (D) Plasma eNAMPT levels from fed or 48 hr-fasted AT-Sirt1^+/+ and AT-Sirt1^−/− female mice at 6 months of age (n=5). Data were analyzed by the Student’s t test or one-way ANOVA with the Fisher’s LSD post-hoc test. All values are presented as mean ± SEM. *p ≤ 0.05; ***p ≤ 0.001.

Figure 3

SIRT1 promotes eNAMPT secretion by deacetylating iNAMPT. (A and B) SIRT1 overexpression promotes eNAMPT secretion in differentiated HIB-1B cells (A) and HEK293 cells (B). Levels of eNAMPT secretion were calculated as described in Materials and Methods. The right panels represent average values from three independent experiments. Each value is normalized to those of GFP- or NAMPT-expressing cells. (C) SIRT1 overexpression decreases iNAMPT acetylation levels in HEK293 cells. Cells were incubated in the absence or presence of 5 μM Trichostatin A (T) and 5 mM nicotinamide (N) overnight prior to cell lysis. Acetylated iNAMPT levels are normalized to total iNAMPT levels. The right panel represents iNAMPT acetylation levels normalized to total iNAMPT levels in each condition (n=3). (D) Interaction between SIRT1 and iNAMPT in differentiated HIB-1B cells. (E) Subcellular localization of SIRT1 and iNAMPT in differentiated HIB-1B cells. GAPDH and TBP were examined as representative cytoplasmic and nuclear proteins, respectively. Cells were incubated with media containing high glucose (H, 25mM) or low glucose (L, 5 mM) overnight (D) or for 3 hr (E). Data were analyzed by the Student’s t test. All values are presented as mean ± SEM. *p ≤ 0.05; **p ≤ 0.01; ***p ≤ 0.001.

Figure 4

Deacetylation of lysine 53 on iNAMPT enhances eNAMPT secretion and enzymatic activity. (A) Acetyl-lysines detected on iNAMPT and eNAMPT prepared from cell lysate of HEK293 cells and culture supernatant of differentiated HIB-1B cells, respectively. HEK293 cells overexpressing C-terminally FLAG-tagged mouse NAMPT were treated with 5 μM Trichostatin A and 5 mM nicotinamide overnight prior to cell lysis. FLAG-tagged iNAMPT and eNAMPT were immunopurified and subjected to mass spectrometric analysis. Recombinant mouse NAMPT was also prepared from bacteria. (B) The location of K53 and K79 on the crystal structure of dimeric NAMPT (arrows). Each monomer is shown in orange or green, and NMN molecules bound to the catalytic sites (arrows) are shown in blue. (C) Changes in the acetylation status of wild-type (WT) or mutant (K53R, K79R, and K53/79R) iNAMPT by SIRT1 in HEK293 cells. Cells were treated with 5 μM Trichostatin A and 5 mM nicotinamide overnight prior to cell lysis. (D and F) The relative enzymatic activity of wild-type (WT) and mutant iNAMPT from differentiated HIB-1B cells. The enzymatic activity was determined as described in Materials and Methods. Each enzymatic activity is normalized to that of wild-type NAMPT (n=2–6). (E and G) Secretion of wild-type (WT) and mutant eNAMPT. Levels of eNAMPT secretion were calculated as described in Materials and Methods. Each value is normalized to the WT value. Each panel represents averages from three independent experiments. Data were analyzed by the Student’s t test or one-way ANOVA with the Fisher’s LSD posthoc test. All values are presented as mean ± SEM. *p ≤ 0.05; ***p ≤ 0.001.

Figure 5

ANKO mice exhibit reduced plasma eNAMPT levels and defects in NAD⁺ biosynthesis not only in adipose tissue but also in the hypothalamus. (A) iNAMPT levels in visceral white adipose tissue (WAT), subcutaneous WAT, brown adipose tissue (BAT), brain, liver, and muscle from 2-month-old female Nampt^flox/flox (f/f) and ANKO mice. (B) Tissue NAD⁺ levels in visceral WAT, subcutaneous WAT, BAT, liver, and skeletal muscle from 3–4-month-old female Nampt^flox/flox and ANKO mice (n=4–8). (C) Plasma eNAMPT levels in 5–6-month-old female Nampt^flox/flox and ANKO mice. Plasma was collected from mice fed or fasted for 48 hr. Bottom panels show average plasma eNAMPT levels normalized to those of Nampt^flox/flox mice (n=8–10). (D) Hypothalamic and hippocampal NAD⁺ levels in 3–4-month-old female Nampt^flox/flox and ANKO mice. (n=4–8) (E) Relative Ox2r mRNA expression levels in the hypothalami and the hippocampi from 3–4-month-old female Nampt^flox/flox and ANKO mice (n=4). Data in B–E were analyzed by the Student’s t test. All values are presented as mean ± SEM. *p ≤ 0.05; **p ≤ 0.01; ***p ≤ 0.001.

Figure 6

Physical activity is altered in loss- and gain-of-function mouse models of adipose NAMPT. (A) Wheel-running activity of ANKO and Nampt^flox/flox female mice at 5 months of age (n=3–6). (B) Wheel-running activity of NMN-injected ANKO female mice and PBS-injected ANKO and Nampt^flox/flox female mice at 5 months of age (n=3–6). NMN was injected at 500 mg/kg body weight at 5pm (indicated by an arrow). Activity counts per minute at each time point are shown as mean values ± SEM. Data in A and B were analyzed by Wilcoxon matched-pairs singled-ranks test. (C) Physical activity levels (total numbers of quadrants crossed) measured after 48-hr fasting in ANKO and Nampt^flox/flox female mice at 4–5 months of age (n=4). (D) Physical activity levels (total numbers of quadrants crossed) measured after 48-hr fasting in ANKI and control male and female mice at 4–6 months of age (n=4). (E) Hypothalamic and hippocampal NAD⁺ levels in 8–10 month-old ANKI and control male mice after 48-hr fasting (n=5). (F) Relative Ox2r mRNA expression levels in the hypothalami from 8–10 month-old ANKI and control male mice after 48-hr fasting (n=5). Data in C–F were analyzed by the Student’s t test. All values are presented as mean ± SEM. *p ≤ 0.05; **p ≤ 0.01; ***p ≤ 0.001.

Figure 7

eNAMPT regulates hypothalamic NAD⁺ levels, Ox2r expression, and neural activation. (A) NAD⁺ levels in the hypothalami and hippocampi isolated from B6 male mice after systemic injection of a NAMPT-neutralizing antibody. 20 μg of anti-NAMPT antibody (NAMPT Ab) and rabbit IgG (IgG) were injected into wild-type B6 male mice at 4–6 months of age. Hypothalamic NAD⁺ levels 1 hr after injection were measured by HPLC (n=10–12). (B) NAD⁺ levels (left) and cFos mRNA expression (right) in hypothalamic explants treated with eNAMPT-containing or control conditioned media. Hypothalamic explants prepared from female mice at 1–12 weeks of age were cultured in each conditioned medium for 1hr. For neural activity, ghrelin (10 μg/ml) and PBS were also added to eNAMPT-containing (+) or control- (−) conditioned media. NAD⁺ levels were measured by HPLC, and cFos expression levels were quantitated by qRT-PCR (n=4–6). (C) NAD⁺ levels and cFos and Ox2r mRNA expression in adult hypothalamic explants cultured in the media supplemented with the eNAMPT protein purified from HEK293 culture supernatants or bovine serum albumin (BSA) as a control for 3 hr (n=4–6). 1 μg of each protein was added to 50 μl of culture media containing a hypothalamic explant. Data were analyzed by the Student’s t test or one-way ANOVA with Fisher’s LSD posthoc test. All values are presented as mean ± SEM. *p ≤ 0.05; **p ≤ 0.01. (D) A model for the SIRT1-mediated regulation of eNAMPT secretion in adipose tissue. iNAMPT (a dimer in blue) is acetylated at lysine 53 (Ac), and SIRT1 specifically deacetylates this lysine, predisposing the protein to secretion. Acetyl-lysine 53 might provide a docking site to an unidentified factor (red) that prevents NAMPT from secretion. eNAMPT secreted from adipose tissue promotes NAD⁺ biosynthesis, SIRT1 activity, and neural activity in the hypothalamus.