Nampt/PBEF/Visfatin regulates insulin secretion in beta cells as a systemic NAD biosynthetic enzyme

Abstract

Intracellular nicotinamide phosphoribosyltransferase (iNampt) is an essential enzyme in the NAD biosynthetic pathway. An extracellular form of this protein (eNampt) has been reported to act as a cytokine named PBEF or an insulin-mimetic hormone named visfatin, but its physiological relevance remains controversial. Here we show that eNampt does not exert insulin-mimetic effects in vitro or in vivo but rather exhibits robust NAD biosynthetic activity. Haplodeficiency and chemical inhibition of Nampt cause defects in NAD biosynthesis and glucose-stimulated insulin secretion in pancreatic islets in vivo and in vitro. These defects are corrected by administration of nicotinamide mononucleotide (NMN), a product of the Nampt reaction. A high concentration of NMN is present in mouse plasma, and plasma eNampt and NMN levels are reduced in Nampt heterozygous females. Our results demonstrate that Nampt-mediated systemic NAD biosynthesis is critical for beta cell function, suggesting a vital framework for the regulation of glucose homeostasis.

Figure 1

Tissue distribution of intracellular Nampt in mice and production of intra- and extracellular Nampt during adipocyte differentiation. A) Distribution of intracellular Nampt (iNampt) in mouse tissues. 22.5 μg of each tissue extract from a C57BL/6 mouse was analyzed by Western blotting with Nampt- and actin-specific antibodies. 5 μg of cell extract from a Nampt-overexpressing NIH3T3 cell line (Nampt1) was loaded as a reference. WAT, white adipose tissue; BAT, brown adipose tissue. B) Production of intra- and extracellular Nampt (iNampt and eNampt) during differentiation of HIB-1B brown preadipocytes. Upper panel: Confluent cultures of HIB-1B cells were differentiated, and cell extracts were prepared at the indicated days. 45 μg of each cell extract was analyzed. Lower panel: 20 μl of each culture supernatant collected at the indicated days was analyzed. C) Production of iNampt and eNampt during differentiation of 3T3-L1 white preadipocytes. Upper panel: 45 μg of each cell extract collected at the indicated days was analyzed. Middle panel: 20 μl of culture supernatant was analyzed. Lower panel: Concentrated culture supernatant was analyzed at day 8 to detect eNampt. 10 μg of the cell extract from Nampt-overexpressing fibroblasts (Nampt1) was loaded as a reference in each experiment. D) Production of iNampt and eNampt during differentiation of human SGBS white preadipocytes. Left and middle panels: 13 μg of each cell extract and 25 μl of each culture supernatant (10-fold concentrated) collected at the indicated points were analyzed. Adiponectin production is also shown as a positive control for adipocyte differentiation. Pre, undifferentiated preadipocytes; d4, differentiating adipocytes at day 4; d8, mature adipocytes; CM, control medium; SN, supernatant. Right panel: iNampt protein expression was analyzed in tissue extracts from human subcutaneous (sc) and visceral (visc) white adipose tissues (WAT).

Figure 2

eNampt produced by differentiated HIB-1B brown adipocytes has high Nampt enzymatic activity. A) eNampt-FLAG co-immunoprecipitates with untagged eNampt from culture supernatants. The eNampt-FLAG protein was immunoprecipitated with an anti-FLAG antibody from 8 ml of each culture supernatant and blotted with either the same antibody or an anti-Nampt antibody. The culture supernatants of the vector-transfected cells were used as a control. B) The eNampt-FLAG protein immunoprecipitated from culture supernatants of differentiated Nampt-FLAG HIB-1B cells has Nampt enzymatic activity. Results are presented as mean ± SE (n=3). C) The kcat values of the bacterially produced His-tagged recombinant Nampt and intra- and extracellular Nampt-FLAG from NIH3T3 and differentiated HIB-1B cells were calculated by measuring NMN synthesis and quantifying the amount of Nampt by Western blotting (data not shown). Results are presented as mean ± SEM (n=7 for His-tagged Nampt, 3 for iNampt from NIH3T3, 6 for iNampt from HIB-1B, and 4 for eNampt from HIB-1B), and all differences in pair-wise comparisons are statistically significant with the Student's t test (p < 0.05).

Figure 3

eNampt does not exert insulin-mimetic effects on adipogenesis, glucose uptake, and insulin signaling in cultured cells. A) Differentiation of human SGBS preadipocytes was induced over a period of 8 days in standard induction medium containing 20 nM insulin or 20 nM eNampt/visfatin. For negative control, cells were treated in induction medium without insulin. Mature adipocytes were stained with Sudan III to visualize lipid accumulation at day 8. B) mRNA expression levels of two adipocyte differentiation markers, adiponectin and PPARγ, were analyzed with real-time quantitative RT-PCR. SGBS preadipocytes were differentiated in standard induction medium with 20nM insulin (3FC) or eNampt/visfatin proteins produced in bacteria (P) or in mammalian cells (E) at concentrations indicated. C) Differentiated SGBS and 3T3-L1 adipocytes were incubated with increasing concentrations of insulin or 100 nM eNampt/visfatin produced in mammalian cells. Glucose uptake was determined using [¹⁴C]-2-deoxyglucose at 0.5 μCi/ml for 5 min. Experiments were performed in quadriplicates, and glucose uptake was normalized to the amount of protein. Basal glucose uptake in non-stimulated cells is assigned as 1. D) The effect of eNampt/visfatin on phosphorylation of the insulin receptor (InsR, upper panels) and Akt/PKB kinase (Akt, lower panels) was examined in undifferentiated and differentiated human SGBS and mouse 3T3-L1 cells. Cells were starved overnight and then exposed to serum free medium (SFM), 10 nM insulin (Ins), or 10 nM eNampt/visfatin produced in bacteria (P) or in mammalian cells (E). Signals of phosphorylated proteins are normalized to those of non-phosphorylated proteins, and values are shown relative to the signal in serum free medium (n ≥ 3). E) Insulin receptor phosphorylation was examined in R-IR cells treated with different concentrations of insulin and eNampt/visfatin. Cells were treated with the indicated concentrations of insulin, eNampt/visfatin produced in mammalian cells or both (10 nM each). The insulin receptor protein (InsR) was immunoprecipitated with an anti-InsR antibody 29B4 and blotted with an anti-pan-phosphotyrosine antibody (upper panel). The membrane was re-probed with another anti-InsR antibody C19 (lower panel). All results are expressed as mean ± SEM.

Figure 4

Nampt^+/− mice show moderately impaired glucose tolerance and reduced glucose-stimulated insulin secretion. A) Body weights of Nampt^+/− and wild-type littermates at 8 weeks of age (n=7-15). B) Fed and fasted glucose levels in males (M) and females (F) of Nampt^+/− and control mice (n=10-18). C) Intraperitoneal glucose tolerance tests (IPGTTs). Nampt^+/− (n=13) and control (n=18) females were injected with PBS and fasted for 12-14 hrs. Dextrose (3 g/kg body weight) was injected intraperitoneally, and blood glucose levels were measured. D) Plasma insulin levels in Nampt^+/− and control female littermates at 0, 15, and 30 min time points in IPGTTs. Nampt^+/− mice (n=14), control mice (n=17) for 0 and 30 min time points; Nampt^+/− mice (n=7), control mice (n=11) for 15 min time point. E) Insulin tolerance tests (ITTs). Nampt^+/− (n=9) and control (n=13) females were injected with human insulin (0.75 U/kg body weight) after fasting for 4 hrs, and blood glucose levels were measured. F) NAD levels (pmole) in primary islets isolated from Nampt^+/− and control female mice. NAD levels were measured by HPLC in duplicates of primary islets pooled from three individual mice of each genotype. G) Insulin secreted (ng/ml/hr) from Nampt^+/− and control islets at the indicated glucose concentrations (n=4 mice for each genotype). Isolated primary islets were cultured overnight in the RPMI media containing 1 μM nicotinamide prior to insulin secretion experiments. All results are expressed as mean ± SEM. *p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001.

Figure 5

NMN administration ameliorates the defects in Nampt^+/− mice and islets. A) IPGTTs after NMN administration. The same Nampt^+/− (n=18) and control (n=19) cohorts that were used for the IPGTTs shown in Figure 4C were injected with NMN (500 mg/kg body weight) ∼14 hrs prior to IPGTTs. B) Plasma insulin levels in Nampt^+/− and control female littermates at 0, 15, and 30 min time points in IPGTTs. Nampt^+/− mice (n=16), control mice (n=17) for 0 and 30 min time points; Nampt^+/− mice (n=10), control mice (n=8) for 15 min time point. C) Insulin secreted (ng/ml/hr) from NMN-treated Nampt^+/− and control islets at the indicated glucose concentrations (n=4 mice for each genotype). Isolated primary islets were cultured overnight in the RPMI media containing 1 μM nicotinamide plus 50 μM NMN prior to insulin secretion experiments. D) NAD levels (pmole) in wild-type primary islets treated overnight with NMN, FK866, or the combination of these compounds at the indicated concentrations. NAD levels were measured by HPLC in triplicates of primary islets pooled from four individual wild-type mice. E) Insulin secreted (ng/ml/hr) at the indicated glucose concentrations from control, FK866-treated, and FK866 plus NMN-treated wild-type primary islets. The experiments were conducted in triplicates of primary islets pooled from four individual mice cultured for 48h in the RPMI media containing 1 μM nicotinamide and indicated compounds (10 nM for FK866 and 100 μM for NMN) prior to insulin secretion experiments. All results are expressed as mean ± SEM. *p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001.

Figure 6

NMN circulates systemically in mouse blood. A) HPLC chromatograms of mouse plasma extracts and standard NMN. The extract that corresponded to 5 μl of plasma and standard NMN were run in an isocratic condition at a flow rate of 1 ml/min. The peak indicated with an arrow showed the same elution time as that of standard NMN. B) HPLC chromatograms of mouse plasma extracts with or without a spike of 100 pmole of NMN. Extracts that corresponded to 2.5 μl of plasma were run at a flow rate of 0.7 ml/min. The peak indicated with arrows showed a significant increase after spiking NMN. C) HPLC chromatograms of mouse plasma extracts at 0 and 15 min time points after NMN injection (500 mg/kg body weight). The peaks indicated by arrows showed an increase at 15 min time point. D) Ion trap tandem mass spectrometry analysis for the fractions that contain the peaks indicated by arrows in HPLC chromatograms. Upper panel, standard NMN; lower panel, the peak fraction of a mouse plasma extract. The signature m/z of 123 (a) and 335 (b) for NMN were detected in the peak fraction. E) Plasma NMN concentrations in control and Nampt^+/− male and female mice. The extracts corresponding to 2.5 μl of plasma were analyzed by HPLC, and the NMN peaks were quantitated for control (n=3) and Nampt^+/− (n=4) mice at ∼8 months of age. Results are expressed as mean ± SEM.

Figure 7

A model for the regulation of insulin secretion by Nampt-mediated systemic NAD biosynthesis in pancreatic β cells. See texts for details. eNampt, extracellular Nampt; iNampt, intracellular Nampt; NMN, nicotinamide mononucleotide; Nmnat, NMN adenylyltransferase.