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Changes in circulating sirtuin 1 after bariatric surgery

  • Trine B. Opstad
    Correspondence
    Corresponding author. Center for Clinical Heart Research, Department of Cardiology Oslo, University Hospital Ullevål, Postbox 4950, Nydalen, N-0240, Oslo, Norway
    Affiliations
    Center for Clinical Heart Research, Department of Cardiology, Oslo University Hospital Ullevål, 0240 Oslo, Norway

    Faculty of Medicine, University of Oslo, 0315 Oslo, Norway
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  • Per G. Farup
    Affiliations
    Department of Research, Innlandet Hospital Trust, PB 104, N-2381 Brumunddal, Norway

    Department of Clinical and Molecular Medicine, Faculty of Medicine and Health Sciences, Norwegian University of Science and Technology, N-7491 Trondheim, Norway
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  • Helge Rootwelt
    Affiliations
    Department of Medical Biochemistry, Oslo University Hospital, 0372 Oslo, Norway
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  • Jan O. Aaseth
    Affiliations
    Department of Research, Innlandet Hospital Trust, PB 104, N-2381 Brumunddal, Norway

    Faculty of Health and Social Sciences, Inland Norway University of Applied Sciences, PB 400, N-2418 Elverum, Norway
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Open AccessPublished:September 22, 2022DOI:https://doi.org/10.1016/j.numecd.2022.09.009

      Highlights

      • Bariatric surgery reduced Sirtuin 1 levels in plasma.
      • Triglycerides and CRP, not BMI, were predictive of Sirtuin 1 levels.
      • Changes in Sirtuin 1 are hypothesized to be related to reduced metaflammation.

      Abstract

      Background and aims

      Obesity is associated with chronic inflammation and oxidative stress. Weight loss after bariatric surgery improves the inflammatory state and risk of cardiovascular disease. Improvement in metabolic dysfunction might be associated with changes in the activity of sirtuin 1 (SIRT1) and we aimed to investigate the effect of bariatric surgery on its circulating levels.

      Methods and results

      This is a sub-study of a prospective cohort study, including 110 subjects with morbid obesity. The surgical procedure was either laparoscopic Roux-en-Y gastric bypass (RYGB) or sleeve gastrectomy (SG). Blood was sampled at inclusion and six and 12 months after surgery. SIRT1 was measured in EDTA plasma with an enzyme-linked immunosorbent assay. The mean age in the population was 43 years, 80% were women and mean body mass index (BMI) was 38.8 kg/m2. RYGB and SG were performed in 89 and 21 subjects, respectively. SIRT1 concentration was significantly reduced from baseline to six and 12 months after surgery, with mean values (SD) 156.8 (82.6), 119.5 (65.6) and 94.9 (45.6) ng/mL, respectively, (p ≤ 0.002, all), accompanied by significant reductions in C-reactive protein (CRP), BMI and triglycerides from inclusion (p < 0.001, all). Type of surgery did not differently modify SIRT1 levels (p = 0.09). CRP and triglycerides were both positively predictive of SIRT1 levels (p ≤ 0.001, both).

      Conclusion

      SIRT1 concentration was significantly lower six and 12 months after bariatric surgery. CRP and triglycerides independently predicted SIRT1 levels, suggesting that reduction in SIRT1 levels might not intrinsically be related to weight reduction, but to improvement in metaflammation.

      Graphical abstract

      Keywords

      1. Introduction

      The prevalence of obesity, defined as body mass index (BMI) ≥ 30 kg/m2, has escalated worldwide [
      • Afshin A.
      • Forouzanfar M.H.
      • Reitsma M.B.
      • Sur P.
      • Estep K.
      • Lee A.
      • et al.
      Health effects of overweight and obesity in 195 countries over 25 years.
      ]. Obesity is characterized by a higher incidence of chronic inflammation and oxidative stress, with subsequent increase in metabolic complications including cardiovascular disease (CVD), diabetes type 2 (T2DM), the metabolic syndrome, hypertension, dyslipidaemia and several types of cancers [
      • Must A.
      • Spadano J.
      • Coakley E.H.
      • Field A.E.
      • Colditz G.
      • Dietz W.H.
      The disease burden associated with overweight and obesity.
      ,
      • Artham S.M.
      • Lavie C.J.
      • Milani R.V.
      • Ventura H.O.
      Obesity and hypertension, heart failure, and coronary heart disease-risk factor, paradox, and recommendations for weight loss.
      ,
      • Lauby-Secretan B.
      • Scoccianti C.
      • Loomis D.
      • Grosse Y.
      • Bianchini F.
      • Straif K.
      Body fatness and cancer--viewpoint of the IARC working group.
      ].
      Weight loss is associated with improvement in metabolic function, and bariatric surgery is considered a reliable treatment to achieve consistent weight loss. Laparoscopic Roux-en-Y gastric bypass (RYGB) and sleeve gastrectomy (SG) are both considered effective procedures. The large weight loss provided by bariatric surgery in subjects with morbid obesity affects both the amount and structure of the adipose tissue, and may reduce the chronic inflammatory state [
      • Illán Gómez F.
      • Gonzálvez Ortega M.
      • Aragón Alonso A.
      • Orea Soler S.
      • Alcaraz Tafalla M.
      • Pérez Paredes M.
      • et al.
      Obesity, endothelial function and inflammation: the effects of weight loss after bariatric surgery.
      ] and the risk of CVD [
      • Sjöström L.
      Bariatric surgery and reduction in morbidity and mortality: experiences from the SOS study.
      ].
      Sirtuin 1 (SIRT1) belongs to a family of nicotinamide adenine dinucleotide (NAD+)-dependent deacetylases, and is the most investigated sirtuin subtype in humans [
      • Haigis M.C.
      • Sinclair D.A.
      Mammalian sirtuins: biological insights and disease relevance.
      ]. Its activity has been associated with longevity and protection against metabolic and chronic degenerative diseases [
      • Saunders L.R.
      • Verdin E.
      Sirtuins: critical regulators at the crossroads between cancer and aging.
      ]. Recent studies have shown that SIRT1 induces both cellular and systemic protective effects in the cardiovascular system [
      • Conti V.
      • Forte M.
      • Corbi G.
      • Russomanno G.
      • Formisano L.
      • Landolfi A.
      • et al.
      Sirtuins: possible clinical implications in cardio and cerebrovascular diseases.
      ]. Experiments in genetically modified mice have also highlighted the role SIRT1 in metabolic disorders, and SIRT1 has been suggested to be a potent protector against age-associated pathologies [
      • Herranz D.
      • Serrano M.
      SIRT1: recent lessons from mouse models.
      ]. With its nuclear localization, SIRT1 regulates gene expression through its activity on histones and transcription factors [
      • Haigis M.C.
      • Sinclair D.A.
      Mammalian sirtuins: biological insights and disease relevance.
      ]. SIRT1 is also able to inhibit NF-κB signalling and thus suppress inflammation [
      • Kauppinen A.
      • Suuronen T.
      • Ojala J.
      • Kaarniranta K.
      • Salminen A.
      Antagonistic crosstalk between NF-κB and SIRT1 in the regulation of inflammation and metabolic disorders.
      ]. In contrast, being a multifaceted intracellular player, upregulation of SIRT1 has been reported in certain type of cancers, possibly reflecting a compensatory protective process or other not fully understood underlying mechanisms [
      • Alves-Fernandes D.K.
      • Jasiulionis M.G.
      The role of SIRT1 on DNA damage response and epigenetic alterations in cancer.
      ].
      Several studies have reported that caloric restriction induces SIRT1 activity [
      • Cohen H.Y.
      • Miller C.
      • Bitterman K.J.
      • Wall N.R.
      • Hekking B.
      • Kessler B.
      • et al.
      Calorie restriction promotes mammalian cell survival by inducing the SIRT1 deacetylase.
      ] also in clinical settings [
      • Lopez-Domenech S.
      • Abad-Jimenez Z.
      • Iannantuoni F.
      • de Maranon A.M.
      • Rovira-Llopis S.
      • Morillas C.
      • et al.
      Moderate weight loss attenuates chronic endoplasmic reticulum stress and mitochondrial dysfunction in human obesity.
      ,
      • Mansur A.P.
      • Roggerio A.
      • Goes M.F.S.
      • Avakian S.D.
      • Leal D.P.
      • Maranhao R.C.
      • Strunz C.M.C.
      Serum concentrations and gene expression of sirtuin 1 in healthy and slightly overweight subjects after caloric restriction or resveratrol supplementation: a randomized trial.
      ]. Whether bariatric surgery has the same beneficial effects on SIRT1 levels is less known. We have previously reported differentiated effects on circulating SIRT1 after caloric restriction according to sex and BMI [
      • Opstad T.B.
      • Sundfør T.
      • Tonstad S.
      • Seljeflot I.
      Effect of intermittent and continuous caloric restriction on Sirtuin1 concentration depends on sex and body mass index.
      ]. Obesity was previously reported to reduce SIRT1 expression in subcutaneous adipose tissue of obese women [
      • Pedersen S.B.
      • Ølholm J.
      • Paulsen S.K.
      • Bennetzen M.F.
      • Richelsen B.
      Low Sirt1 expression, which is upregulated by fasting, in human adipose tissue from obese women.
      ].
      The main aim of the present study was to assess alterations in SIRT1 plasma concentration six and 12 months after bariatric surgery in subjects with obesity. Secondary outcomes included potential differences in SIRT1 levels between the RYGB and SG procedures and any association of SIRT1 with age, sex, C-reactive protein (CRP), BMI, T2DM, and cardiovascular risk assessed by the lipid profile.

      2. Methods

      2.1 Study population

      The present investigation is a sub-study of a prospective cohort study performed at Innlandet Hospital Trust, Gjøvik, Norway, between 2012 and 2014 [
      • Aasbrenn M.
      • Lydersen S.
      • Farup P.G.
      A conservative weight loss intervention relieves bowel symptoms in morbidly obese subjects with irritable bowel syndrome: a prospective cohort study.
      ]. Subjects aged 18–60 years eligible for bariatric surgery, due to a BMI >40 kg/m2 or a BMI >35 kg/m2 with serious weight related comorbidities, such as T2DM and CVD, were consecutively recruited into the study. Exclusion criteria were previous surgery due to obesity, other major abdominal surgery, major psychiatric disorder and serious somatic pathological conditions not related to obesity, and alcohol or drug addiction.
      An initial dietetic counselling was given to all included subjects six months before surgery, including an eight week-course on life-style changes. Bariatric surgery was performed in accordance with current guidelines, either as standard RYGB [
      • Schauer P.R.
      • Ikramuddin S.
      • Hamad G.
      • Eid G.M.
      • Mattar S.
      • Cottam D.
      • et al.
      Laparoscopic gastric bypass surgery: current technique.
      ] or SG [
      • Roa P.E.
      • Kaidar-Person O.
      • Pinto D.
      • Cho M.
      • Szomstein S.
      • Rosenthal R.J.
      Laparoscopic sleeve gastrectomy as treatment for morbid obesity: technique and short-term outcome.
      ], the allocation to the operative method being done by the surgeons. Both techniques resulted in reduced size of the stomach followed by earlier satiety, and thus restricted food intake and less to absorb from the small intestine.
      Clinical data were registered, and blood sampled at baseline and at the follow-up visits at six and 12 months postoperatively. Subjects with comorbidity received standard medication, and the participants’ condition was provided on a case report form. The protocol was approved by Regional Committee for Medical and Health Research Ethics (REK), Region South-East, Norway, ref. Numbers 2012/966 and 2012/1394. The study conformed to the Declaration of Helsinki, and written informed consent was obtained from all subjects upon enrolment.

      2.2 Blood sampling and analysis

      Fasting whole blood samples were collected between 8:00 and 10:30 a.m. immediately before surgery and at six and 12 months postoperatively. Routine analyses were carried out by conventional methods. Pre-chilled ethylenediaminetetraacetic acid (EDTA) vials were centrifuged at 3000 g, for 10 min at +4 °C, and EDTA plasma was separated and stored at −80 °C until SIRT1 analysis. The Human SIRT1 ELISA kit from LSBio LifeSpan BioSciences (lnc, Seattle, USA) was used for the SIRT1analysis, performed at baseline before the surgery, and six and 12 months after surgery. Samples collected at different time-points from the same individual were analysed on the same ELISA plate to minimize the effect of assay variability between runs. SIRT1 was successfully measured in all available samples except for one. The inter-assay correlation of variation was 13%.

      2.3 Statistics

      Descriptive data were reported as mean with standard deviation (SD) and number (%). Comparisons between men and women at inclusion were analysed with t-test, unadjusted changes in the variables from before to after surgery with paired t-test, and correlations with Pearson's correlation test. The difference in SIRT1 levels after surgery was calculated from baseline to: 1) six months and 12 months, respectively, 2) the combined results after six and 12 months and 3) the change from six to 12 months. All multivariable analyses were performed with a linear mixed regression model for repeated measurements. Predictors of SIRT1 levels were adjusted for sex, age, type of surgery, time, and one by one of the other variables followed by stepwise forward regression adding one covariate at the time starting with the one with lowest p-value. The statistical analyses were reported as unstandardized coefficients (B-values), with 95% confidence intervals. P-values <0.05 were considered statistically significant. SPSS version 27 (SPSS Inc., IL, USA) was used for all statistical analyses.

      3. Results

      Out of 152 subjects included in the preoperative lifestyle intervention, 121 completed this initial conservative intervention period and were eligible for bariatric surgery. Subjects with at least one measurement of SIRT1 during the study period were included (n = 110). Of these, the numbers of available samples at baseline, and six and 12 months after surgery were 100, 97, and 88, respectively. Baseline characteristics of the 110 study participants are presented in Table 1, showing a dominance of women (80%), a mean age of 43 years and a mean BMI of 38.8 kg/m2. There was an overweight of subjects selected to RYGB (n = 89) compared to SG (n = 21). Mean values for lipids, fasting glucose, and glycated haemoglobin (HbA1c) were in the normal range (Table 1).
      Table 1Subject characteristics before bariatric surgery.
      CharacteristicsResultMale/Female
      p-values are given for statistically significant differences between male and female.
      Reference values
      Sex (male/female)22 (20)/88 (80)
      Age (years)42.9 (8.2)43.4 (7.6)/42.8 (8.4)
      BMI (kg/m2) (n = 95)38.8 (3.8)39.9 (3.6)/38.5 (3.8)
      Diabetes (n = 104)18 (17)5 (25)/13 (16)
      RYGB/GS89 (81)/21 (19)
      HbA1c in % (n = 100)5.47 (0.87)5.46 (0.57)/5.48 (0.93)4–6
      CRP (mg/L)4.44 (3.92)3.32 (2.63)/4.72 (4.15)<5
      Triglycerides (mmol/l) (n = 100)1.33 (0.52)1.39 (0.62)/1.31 (0.49)0.5–2.6
      Cholesterol (mmol/L) (n = 100)4.38 (0.87)4.33 (0.93)/4.40 (0.85)18–29 years: 2.9–6.2
      30–49 years: 3.3–6.9
      ≥50 years: 3.9–7.8
      HDL (mmol/L) (n = 100)1.11 (0.30)0.94 (0.21)/1.15 (0.31) p = 0.001Women: 1.0–2.7
      Men: 0.8–2.1
      LDL (mmol/L) (n = 100)2.80 (0.80)2.86 (0.91)/2.78 (0.78)18–29 years: 1.5–4.2
      30–49 years: 1.9–4.8
      50–79 years: 2.1–4.9)
      LDL/HDL ratio (n = 100)2.74 (1.16)3.31 (1.38)/2.60 (1.06) p = 0.013
      ApoA1 (g/L) (n = 99)1.14 (0.19)1.04 (0.12)/1.16 (0.19) p = 0.001Women: 1.1–2.3
      Men: 1.0–2.0
      ApoB (g/L) (n = 99)0.86 (0.21)0.88 (0.26)/0.85 (0.20)0.5–1.3
      Lp(a) (nmol/L) (n = 99)53.6 (63.0)68.1 (77.0)/50.0 (59.0)<75
      ApoB/ApoA1 ratio (n = 99)0.77 (0.23)0.87 (0.29)/0.75 (0.21) p = 0.047
      SIRT1 (ng/ML)156.5 (82.9)167.8(92.3)/153.7 (80.7)
      Values are presented as mean (SD) or number (%). Number is given if less than 110.
      BMI: body mass index, RYGB: Roux-en-Y gastric bypass, SG: sleeve gastrectomy, HbA1c: glycated haemoglobin, CRP: C-reactive protein, HDL; high-density lipoprotein, LDL; low-density lipoprotein, ApoA1: apolipoprotein A1, ApoB: apolipoprotein B, Lp(a): lipoprotein (a).
      a p-values are given for statistically significant differences between male and female.

      3.1 SIRT1 concentration

      SIRT1 concentration in plasma was significantly reduced from baseline to six and 12 months after surgery (Table 2), with mean values (SD) 156.8 (82.6), 119.5 (65.6) and 94.9 (45.6) ng/mL, respectively, (p ≤ 0.002, between all time-points), accompanied by reduction in BMI from 38.8 (3.8) to 30.2 (3.7) and 28.3 (3.9) kg/m2, CRP from 4.44 (3.92) to 1.90 (2.46) and 1.07 (1.33) mg/L, and triglycerides from 1.33 (0.52) to 1.03 (0.38) and 0.93 (0.33) mmol/L, respectively (p < 0.001, in changes from baseline).
      Table 2Changes in SIRT1 from before to six and 12 months after bariatric surgery.
      ChangesB; 95% CIp-value
      Change from before to combined six and 12 months after surgery
       All participants- 49.6 (−61.7; - 37.4)p < 0.001
       Change difference, RYGB compared with SG27.2 (- 4.3; 58.6)p = 0.090
       Change difference, men compared with women- 15.2 (- 45.6; 15.1),p = 0.323
      Change from before to six months after surgery
       All participants- 39.2 (- 52.9; - 25.8)p < 0.001
       Change difference, men compared with women- 18.5 (- 52.4; 15.5)p = 0.505
      Change from before to 12 months after surgery
       All participants- 61.2 (−75.2; - 47.2)p < 0.001
       Change difference, men compared with women- 11.8 (−46.9; 23.2)p = 0.284
      Change from six to 12 months after surgery
       All participants- 21.9 (- 35.8; 9.0)p = 0.002
       Change difference, men compared with women6.6 (- 41.4; 28.2)p = 0.708
      RYGB: Roux-en Y gastric bypass, SG: Sleeve gastrectomy.
      Boldface type indicate p-values ≤ 0.05.
      Mixed model linear regression analyses adjusted for age, gender, type of surgery and time. The results are reported as unstandardized coefficients (B-values) with 95% confidence intervals and p-values.
      The reductions in SIRT1 concentration (presented as delta values) were significant from baseline to six and 12 months after surgery, respectively, and also between SIRT1 at baseline and six and 12 months levels combined (p < 0.001, in all), and between six to 12 months levels (p = 0.002) (Table 2). The changes in SIRT1 levels between investigational time-points did not vary between gender (p > 0.25, all), and type of surgery techniques did not significantly modify SIRT1 levels (p = 0.09). In Fig. 1, the violin plot visualizes median and dispersal of SIRT1 levels before, six and 12 months after surgery, with comparisons between time-points (p ≤ 0.002, for all).
      Figure 1
      Figure 1SIRT1 concentration (ng/mL) before surgery, and six and 12 months after surgery. Violin plot describing the median value, the interquartile range and peaks of the data at baseline preoperatively (red colour), 6 months after surgery (blue colour) and 12 months after the surgery (green colour).
      Predictors of SIRT1 concentration are presented in Table 3, showing diabetes, CRP, triglycerides, HDL, and the LDL/HDL and ApoB/ApoA1 ratios to be predictive, when adjusting for age, sex, type of surgery and time (p ≤ 0.05, in all). Of the significantly related covariates, only HDL cholesterol was inversely associated to SIRT1. In stepwise forward regression, CRP and triglycerides remained statistically significant (p ≤ 0.001, for both, adjusted). Unadjusted analyses of the correlations (Pearson) between SIRT1 and CRP and triglycerides were r = 0.394, p < 0.001 and r = 0.346, p < 0.001, respectively.
      Table 3Mixed model linear regression analyses with SIRT1 as the dependent variable.
      IndependentUnadjusted regression coefficients (B)
      VariablesOne by one
      In the column “One by one”, the variables age, gender, type of surgery (RYGB) and time were included in all analyses, and in addition one by one of the variables from the line BMI to Lp(a).
      Stepwise forward
      In the column “Stepwise forward”, the variables age, gender, type of surgery and time were included in all analyses, and in addition a stepwise forward inclusion of the statistically significant variables in the column “One by one”, starting with the lowest p-value. The statistically significant associations are shown.
      B; (95% CI)p-valueDifference Male/female (p-value)B; (95% CI)p-value
      Sex (men)2.6 (- 25.8; 31.0)0.856
      Age (years)- 0.8 (- 2.2; 0.5)0.221
      Type of surgery (RYGB)9.6 (- 19.4; 38.6)0.513
      BMI (kg/m2)- 0.5 (−3.0; 2.1)0.7150.041
       Male1.7 (- 1.5; 4.9)0.296
       Female- 1.1 (- 3.7; 1.5)0.406
      Diabetes33.4 (2.5; 36.0)0.034
      HbA1c12.4 (- 1.1; 25.8)0.071
      CRP6.5 (3.8; 9.1)<0.0010.0155.5 (2.8; 8.2)<0.001
       Male14.6 (7.5; 21.6)<0.001
       Female5.7 (2.9; 8.4)<0.001
      Triglycerides40.6 (21.0; 60.1)<0.0010.24032.1 (12.6; 51.6)0.001
       Male33.9 (11.4; 56.4)0.003
       Female56.3 (23.5; 89.0)0.001
      Cholesterol5.8 (−4.7; 16.4)0.278
      HDL−32.2 (−64.3; −0.07)0.05
      LDL5.7 (−5.9; 17.3)0.337
      LDL/HDL ratio10.9 (0.7; 21.0)0.035
      ApoA1- 4.7 (- 49.2; 39.9)0.837
      ApoB40.1 (- 4.9; 85.0)0.080
      ApoB/ApoA1 ratio49.0 (2.3; 95.7)0.040
      Lp(a)- 0.01 (−0.19; 0.16)0.853
      RYGB: Roux-en-Y gastric bypass, BMI: body mass index, HbA1c: glycated haemoglobin, CRP: C-reactive protein, HDL; high-density lipoprotein, LDL; low-density lipoprotein, ApoA1: apolipoprotein A1, ApoB: apolipoprotein B, Lp(a): lipoprotein (a).
      a In the column “One by one”, the variables age, gender, type of surgery (RYGB) and time were included in all analyses, and in addition one by one of the variables from the line BMI to Lp(a).
      b In the column “Stepwise forward”, the variables age, gender, type of surgery and time were included in all analyses, and in addition a stepwise forward inclusion of the statistically significant variables in the column “One by one”, starting with the lowest p-value. The statistically significant associations are shown.

      4. Discussion

      The main finding in this prospective study of subjects with morbid obesity was that bariatric surgery significantly reduced plasma SIRT1 levels six and 12 months postoperatively. The SIRT1 reduction was independent of type of surgery. BMI was significantly reduced from the morbid obese classification to simple overweight, along with significant reductions in CRP and triglycerides. CRP and triglycerides were independently associated with SIRT1, with stronger impact of CRP in men vs. women.
      The beneficial health effects of increased intracellular SIRT1 activity are well known. Whether circulating SIRT1 reflects intracellular activity is not clear [
      • Mariani S.
      • di Giorgio M.R.
      • Martini P.
      • Persichetti A.
      • Barbaro G.
      • Basciani S.
      • et al.
      Inverse association of circulating SIRT1 and adiposity: a study on underweight, normal weight, and obese patients.
      ]. The observed reduction in SIRT1 plasma concentration was somewhat surprising, but the effect of bariatric surgery on SIRT1 has been insufficiently studied. Recently, a study performed only on women with morbid obesity reported an increase in adipose tissue SIRT1 expression after RYGB, with significant correlations between the delta SIRT1 mRNA level and BMI changes [
      • Ferraz-Bannitz R.
      • Welendorf C.R.
      • Coelho P.O.
      • Salgado Jr., W.
      • Nonino C.B.
      • Beraldo R.A.
      • et al.
      Bariatric surgery can acutely modulate ER-stress and inflammation on subcutaneous adipose tissue in non-diabetic patients with obesity.
      ]. We have previously reported differentiated effect of sex on circulating SIRT1 after caloric restriction, with reduction in SIRT1 serum levels in women that simultaneously reduced their BMI markedly [
      • Opstad T.B.
      • Sundfør T.
      • Tonstad S.
      • Seljeflot I.
      Effect of intermittent and continuous caloric restriction on Sirtuin1 concentration depends on sex and body mass index.
      ]. BMI was significantly reduced in the present investigation, but its impact on SIRT1 levels was not significant in the total cohort but differed significantly according to sex (Table 3). It might be speculated based on previous observations, that the BMI loss in women, constituting 80% of the investigated subjects, may indirectly have contributed to the observed reduced plasma SIRT1 values via metabolic changes. In accordance with this, it was recently reported that removal of visceral adipose tissue in rodents caused a decrease in serum SIRT1 concentration [
      • Braud L.
      • Pini M.
      • Stec D.F.
      • Manin S.
      • Derumeaux G.
      • Stec D.E.
      • et al.
      Increased Sirt1 secreted from visceral white adipose tissue is associated with improved glucose tolerance in obese Nrf2-deficient mice.
      ]. Interestingly, SIRT1 plasma levels were reported to be inversely and significantly associated with BMI and fat content in a cross-sectional study of underweight, normal weight and obese subjects [
      • Mariani S.
      • di Giorgio M.R.
      • Martini P.
      • Persichetti A.
      • Barbaro G.
      • Basciani S.
      • et al.
      Inverse association of circulating SIRT1 and adiposity: a study on underweight, normal weight, and obese patients.
      ]. Adipose tissue SIRT1 expression after RYGB has furthermore been reported increased in one small study (n = 13, mean age 37 years, 100% women, mean BMI 42.2 kg/m2) [
      • Ferraz-Bannitz R.
      • Welendorf C.R.
      • Coelho P.O.
      • Salgado Jr., W.
      • Nonino C.B.
      • Beraldo R.A.
      • et al.
      Bariatric surgery can acutely modulate ER-stress and inflammation on subcutaneous adipose tissue in non-diabetic patients with obesity.
      ], and both protein expression in the supernatant and mRNA levels in peripheral blood cells were observed increased after laparoscopic bariatric surgery in obese subjects with T2DM (n = 124, mean age 46 years, 63% women, mean BMI 32 kg/m2) [
      • Wang Y.
      • Wang D.S.
      • Cheng Y.S.
      • Jia B.L.
      • Yu G.
      • Yin X.Q.
      • Wang Y.
      Expression of MicroRNA-448 and SIRT1 and prognosis of obese type 2 diabetic mellitus patients after laparoscopic bariatric surgery.
      ]. Whether circulating SIRT1 inversely reflects its gene-expression and intracellular SIRT1 activity is still questionable, although previously indicated by our group [
      • Opstad T.B.
      • Berg T.J.
      • Holte K.B.
      • Arnesen H.
      • Solheim S.
      • Seljeflot I.
      Reduced leukocyte telomere lengths and sirtuin 1 gene expression in long-term survivors of type 1 diabetes: a Dialong substudy.
      ,
      • Opstad T.B.
      • Kalstad A.A.
      • Holte K.B.
      • Berg T.J.
      • Solheim S.
      • Arnesen H.
      • et al.
      Shorter leukocyte telomere lengths in healthy relatives of patients with coronary heart disease.
      ]. The contribution of different adipose tissue depots and other organismal sources on circulating SIRT1 in human is not clear. However, a significant inverse correlation was found between serum concentrations and epicardial adipose tissue thickness in subjects with obesity [
      • Mariani S.
      • Costantini D.
      • Lubrano C.
      • Basciani S.
      • Caldaroni C.
      • Barbaro G.
      • et al.
      Circulating SIRT1 inversely correlates with epicardial fat thickness in patients with obesity.
      ].
      The low-grade chronic inflammation accompanying obesity is often termed metaflammation, as it is orchestrated by dysmetabolic cell proliferation. Our population, with a massive amount of adipose tissue preoperatively, suffered from metabolic and hormonal dysfunctions, triggering a systemic inflammatory state. Postoperatively, we observed a significant reduction in CRP, predictive of SIRT1 levels, especially in men, with significant differences between sexes (Table 3). CRP has been shown to promote NF-κB activation [
      • Zhong Y.
      • Cheng C.F.
      • Luo Y.Z.
      • Tian C.W.
      • Yang H.
      • Liu B.R.
      • et al.
      C-reactive protein stimulates RAGE expression in human coronary artery endothelial cells in vitro via ROS generation and ERK/NF-κB activation.
      ], the main transcriptional regulator of genes related to inflammation. Recent studies have indicated that the regulation of innate immunity and energy metabolism is connected through an antagonistic crosstalk between NF-κB and SIRT1 signalling pathways [
      • Kauppinen A.
      • Suuronen T.
      • Ojala J.
      • Kaarniranta K.
      • Salminen A.
      Antagonistic crosstalk between NF-κB and SIRT1 in the regulation of inflammation and metabolic disorders.
      ]. SIRT1 inhibits NF-κB signalling by deacetylation of the p65 subunit of the NF-κB complex. SIRT1 can also stimulate oxidative energy production, via the activation of AMP-activated protein kinase (AMPK), peroxisome proliferator-activated receptor (PPAR)-ƴ and peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1α, which in turn inhibits NF-κB signalling and suppresses inflammation [
      • Kauppinen A.
      • Suuronen T.
      • Ojala J.
      • Kaarniranta K.
      • Salminen A.
      Antagonistic crosstalk between NF-κB and SIRT1 in the regulation of inflammation and metabolic disorders.
      ]. On the other hand, NF-κB signalling and the inflammatory response can downregulate SIRT1 activity through the expression of micro RNA (miR)-34a [
      • Yamakuchi M.
      • Ferlito M.
      • Lowenstein C.J.
      miR-34a repression of SIRT1 regulates apoptosis.
      ], interferon-ƴ [
      • Li P.
      • Zhao Y.
      • Wu X.
      • Xia M.
      • Fang M.
      • Iwasaki Y.
      • et al.
      Interferon gamma (IFN-γ) disrupts energy expenditure and metabolic homeostasis by suppressing SIRT1 transcription.
      ] and reactive oxygen species [
      • Kauppinen A.
      • Suuronen T.
      • Ojala J.
      • Kaarniranta K.
      • Salminen A.
      Antagonistic crosstalk between NF-κB and SIRT1 in the regulation of inflammation and metabolic disorders.
      ]. Accordongly, it was recently reported that endocrine system dysfunction induced an increase in SIRT1 expression due to decreased SIRT1 activity, in an attempt of decreasing the effect of pro-inflammatory cytokines via anti-oxidative mechanisms [
      • Elibol B.
      • Kilic U.
      High levels of SIRT1 expression as a protective mechanism against disease-related conditions.
      ]. The observed reduction in CRP may have downregulated NF-κB, subsequently restoring SIRT1 activity. In subjects affected by morbid obesity and related endocrine aberrations, an overexpression of SIRT1 as a compensatory or protective mechanism is a plausible explanation of high preoperative SIRT1 levels. As the health of the investigated subjects and their inflammatory status was improved postoperatively, SIRT mRNA expression might adaptively have been “normalized”, explaining the following reduction in plasma SIRT1 levels.
      Triglycerides, which also were significantly reduced postoperatively, showed a significant impact on SIRT1 plasma levels (Table 3). We have previously shown that triglycerides are strong predictors of SIRT1 mRNA expression in leukocytes [
      • Opstad T.B.
      • Berg T.J.
      • Holte K.B.
      • Arnesen H.
      • Solheim S.
      • Seljeflot I.
      Reduced leukocyte telomere lengths and sirtuin 1 gene expression in long-term survivors of type 1 diabetes: a Dialong substudy.
      ]. The change in lipid metabolism after surgery might thus have influenced SIRT1 expression, or, vice versa; SIRT1 might have regulated physiological processes including fat metabolism via certain nuclear receptors, such as PPARα and PCG-1α [
      • Sugden M.C.
      • Caton P.W.
      • Holness M.J.
      PPAR control: it's SIRTainly as easy as PGC.
      ]. Provided an initial overexpression of SIRT1 due to anti-inflammatory compensatory mechanisms, the cross–regulatory mechanisms and the subsequent fall in SIRT1 levels may have influenced and reduced triglycerides.

      4.1 Limitations

      The main limitation in our study is that measurements of body composition before and after surgery was not performed. Such analyses would probably have added more knowledge on potential sources of SIRT1 concentrations. Furthermore, as SIRT1 plasma levels were not measured before the dietetic and lifestyle counselling during the six months before surgery, we cannot exclude an initial rise in SIRT1 levels preoperatively, followed by a decrease during the year after surgery. A complete overview of medication status is lacking in our study. Both insulin and thyroxin have previously been shown to increase SIRT1 gene expression [
      • Engel N.
      • Mahlknecht U.
      Aging and anti-aging: unexpected side effects of everyday medication through sirtuin1 modulation.
      ], in addition to certain food additives (resveratrol and betacyanins, among others). However, as only one patient in our sample used thyroxin and none used insulin, this has not influenced our results. As the use of blood-pressure medication was not recorded, any potential influence on SIRT1 levels of medicinal use cannot be excluded. MiRNAs, especially miR34a and miR 448 [
      • Yamakuchi M.
      • Ferlito M.
      • Lowenstein C.J.
      miR-34a repression of SIRT1 regulates apoptosis.
      ,
      • Liu J.
      • Yang D.
      • Wang B.
      • Zeng Y.
      • Li W.
      The value of miRNAs in the prognosis of obese patients receiving bariatric surgery.
      ], seem to play an important role in weight reduction after bariatric surgery, thus, information on such regulation on SIRT1 might have strengthened our results. Likewise, the measurements of SIRT1 gene-expression, although the relationship between SIRT1 gene expression in different tissues and circulating SIRT1 are complex and poorly characterized. Direction of causality cannot be addressed from our study. Nevertheless, we presume that a primary drop in CRP and triglycerides after surgery has contributed to the reduced expression of SIRT1 and its plasma levels. Of note, a drop in plasma SIRT1 levels due to greater intracellular SIRT1 activity demands can be suggested as an additive or alternative mechanism. Finally, the effect of sex on the investigated markers after bariatric surgery needs further exploration.

      5. Conclusion

      In our population affected by morbid obesity, SIRT1 plasma levels were significantly reduced after bariatric surgery, suggesting a compensatory up-regulation of SIRT1 expression preoperatively due to the state of morbid obesity. The significant positive impact of postoperative reductions in CRP and triglycerides, but not of BMI, on SIRT1 plasma levels, implies that improvement in metaflammation might have stabilized SIRT1 gene expression through adaptive mechanisms. We suggest that loss of adipose tissue and/or potential changes in body composition, as well as reductions in CRP and triglycerides, might have contributed to the reduced SIRT1 concentrations after bariatric surgery.

      Funding

      This work was supported by Innlandet Hospital Trust, Brumunddal, Norway . The funding source was not involved in study design, analysis and interpretation of data, writing of the manuscript or in the decision to submit the article.

      Declaration of competing interest

      The authors have no conflict of interest.

      Acknowledgement

      No one is acknowledged in this study.

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