Irisin improves adiposity and exercise tolerance in a rat model of postmenopausal obesity through enhancing adipo-myocyte thermogenesis

Abstract

The prevalence of obesity and its associated metabolic disorders, along with their healthcare costs, is rising exponentially. Irisin, an adipomyokine, may serve as a critical cross-organ messenger, linking skeletal muscle with adipose tissue and the liver to integrate the energy homeostasis under diet-induced obesity. We aimed to explore the putative role of irisin in the protection against obesity in a postmenopausal rat model by modulating energy expenditure (EE). Bilateral ovariectomy (OVX) was performed. After 3 weeks of recovery, the OVX rats were classified according to their dietary protocol into rats maintained on normal diets (ND) (OVX) or high-fat diet (HFD) groups. The HFD-fed animals were equally divided into OVX/HFD, or irisin-treated OVX/HFD groups. Sham rats, maintained on ND, were selected as the control group. We evaluated anthropometric, EE, and molecular biomarkers of browning and thermogenesis in inguinal white adipose tissue and skeletal muscle, and the activity of the proteins related to mitochondrial long chain fatty acid transport, oxidation, and glycolysis. HFD of OVX further deteriorated the disturbed glucose homeostasis, lipid profile, and the reduced irisin, thermogenic parameters in adipose tissue and skeletal muscle, and EE. Irisin treatment improved the lipid profile and insulin resistance. That was associated with reduced hepatic gluconeogenic enzyme activities and restored hepatic glycogen content. Irisin reduced ectopic lipid infiltration. Irisin augmented EE by activating non-shivering thermogenesis in muscle and adipose tissues and decreasing metabolic efficiency. Our experimental evidence suggests irisin's use as a potential thermogenic agent, therapeutically targeting obesity in postmenopausal patients. Irisin modulates the non-shivering thermogenesis in skeletal muscle and adipose tissue in postmenopausal model.

Keywords: Irisin; Mitochondrial uncoupling protein-1; Postmenopausal obesity; Sarco-endoplasmic reticulum Ca2+-ATPase; Sarcolipin.

Fig. 1

Schematic representation of the time line and the experimental design utilized in the present study. In the first week, bilateral surgical ovariectomy was performed, followed by a 3-week recovery period. At the start of the fifth week, obesity was established by an 8-week dietary regimen for ovariectomized (OVX) rats. OVX rats were classified—according to their dietary protocol, into normal diet (ND, group II (OVX)) or high-fat diet (HFD). Each group was maintained on its diet regimen till the end of the study. The HFD-fed animals were equally divided into two weight-matched groups [group III (OVX/HFD) and group IV (irisin-treated OVX/HFD)], starting from the 13th week and continued for 8 weeks later. At the end of the experiment, all rats were subjected to exhaustive swimming exercise. N = 10 rats/group

Fig. 2

The effect of irisin on a) body weight gain and b) core body temperature, in an obese postmenopausal rat model. Body weight (a) and core body temperature (°C) (b) were measured weekly. Values are expressed as mean ± SD of 10 rats in each group. *Significance vs. control group (P < 0.05). ^#Significance vs. OVX group (P < 0.05). ^§Significance vs. OVX/HFD group (P < 0.05)

Fig. 3

The effect of irisin on the morphological remodeling of different fat depots (a–e) and the size of adipocytes (f) in an obese postmenopausal rat model. Hematoxylin/eosin staining of visceral white adipose tissue (WAT, a), inguinal WAT (b, c), and the interscapular (IS) brown adipose tissue (BAT) (d, e). Quantifications of adipocyte size (f) from WAT. Magnification at ×100 (interscapular BAT, d), ×200 (inguinal WAT (b) and interscapular BAT (e)), and ×400 (visceral WAT (a) and inguinal WAT (c)). *Significance vs. control group (P < 0.05). #Significance vs. OVX group (P < 0.05). §Significance vs. OVX/HFD group (P < 0.05). The figure shows WAT from a control group that is typically composed of uniform cells with unilocular lipid droplets, eccentric nuclei, and few dispersed blood vessels, giving WAT its peculiar white-yellow appearance. Surgical induction of menopause (OVX group) is associated with increased adipocyte size, compared to the control group. The OVX/HFD group revealed a further increase in adipocyte size. Meanwhile, in the irisin-treated OVX/HFD group, there is a reduced size of adipocytes, reflecting decreased fat storage. a and b demonstrate a heterogeneous appearance in groups I and II, with both unilocular and multilocular adipocytes coexisting with interstitial tissue. OVX/HFD shows the appearance of nests of brownlike or multilocular adipocytes, whereas irisin-treated OVX/HFD shows a significantly increased proportion of these nests with a brown phenotype, indicating that irisin has a significant beiging effect. c and d show brown adipocytes (interscapular depots) from the control group reveal smaller polygonal cells with multilocular lipid droplets and central nuclei. The BAT from OVX and OVX/HFD groups reveals larger lipid droplets, indicating thermogenically quiescent BAT, while the irisin-treated OVX/HFD group obviously exhibits browning

Fig. 4

The effect of irisin on hepatic steatosis (a), and quantification of the number of hepatic lipid droplets per unit area (b) in an obese postmenopausal rat model. Representative photomicrographs of H&E stained liver sections in all experimental groups: the control group (group I) displays normal liver structure with polygonal hepatocytes (HC) with acidophilic cytoplasm and rounded vesicular nuclei radiating from the central vein (CV). Narrow radiating blood sinusoids (s) in between hepatic cords and their lining endothelium, Kupffer cells (KC), are noticed. The OVX group (group II) shows a dilated central vein (dCV). Binucleated hepatocytes (BHCs) are seen (arrow). The accumulation of lipid droplets (asterisks) in HC can be observed. The OVX/HFD group (group III) displays massive lipid steatosis with marked fatty degeneration of HCs with typical macrovesicular lipid droplets. Dilated, congested central vein (CV), and HCs with vacuolated cytoplasm (V) and darkly stained nuclei (N) that appear separated by congested blood (S) are also noticed. The irisin-treated OVX/HFD group (group IV) shows nearly complete clearance of steatohepatitis with restored hepatic histological architecture that appears similar to the control. Chords of normal The HCs radiate from the dCV and are separated by slightly dilated blood (S). Magnification × 400. *Significance vs. control group (P < 0.05). ^#Significance vs. OVX group (P < 0.05). ^§Significance vs. OVX/HFD group (P < 0.05)

Fig. 5

The effect of irisin on the relative gene expressions of a) adipose UCP-1, muscle b) SERCA1, c) NRF-2, and d) TFAM in an obese postmenopausal rat model. Values are expressed as the mean ± SD of 6 rats in each group. *Significance vs. control group (P < 0.05). ^#Significance vs. OVX group (P < 0.05). ^§Significance vs. OVX/HFD group (P < 0.05). UCP1 was measured from inguinal fat depot. Other gene expressions were assayed from the extensor digitorum longus. NRF-2 nuclear respiratory factor erythroid 2‐related factor 2, OVX ovariectomized, OVX/HFD ovariectomized/high-fat diet fed group, SERCA1 sarcoplasmic reticulum Ca²⁺-ATPas, TFAM mitochondrial transcription factor A, UCP1 uncoupling protein

Fig. 6

The effect of irisin on skeletal muscle ultrastructure (a) and quantification of the number of intermyofibrillar mitochondria (b) in an obese postmenopausal rat model. Representative electron micrographs of longitudinal ultrathin sections of the tibialis anterior (TA) muscle, from all experimental groups. TA from group I (control group) displays a normal myofibril banding pattern with alternating dark (A) and light (I) bands. The M and Z lines appear in the middle of the A and I bands, respectively. Sarcomeres (S) are extended between two successive Z lines. Mitochondria (Mt) appear in between myofibrils. TA from group II (OVX rats) shows the appearance of the lipid droplets (asterisks) as well as the reduced number of degenerated mitochondria (Mt), focal areas of myofibril degeneration (arrow) in aged skeletal muscle. Group III (OVX/HFD group) displays a higher density of large-sized lipid droplets (asterisks) adjacent to the distorted mitochondria (Mt). Distorted myofibrils’ banding with focal areas of degeneration (arrow). Group IV (irisin-treated OVX/HFD group) reveals greatly preserved muscle structure with normally shaped mitochondria (Mt) distributed in between the organized sarcomeres (S), while small dispersed lipid droplets (asterisks) can be noticed in between some disorganized myofibrils. Magnifications: the control and OVX/HFD groups (× 5000), while OVX and irisin-treated OVX/HFD groups (× 3000). *Significance vs. control group (P < 0.05). ^#Significance vs. OVX group (P < 0.05). ^§Significance vs. OVX/HFD group (P < 0.05)

Fig. 7

The effect of irisin on the inguinal adipose tissue protein expression in an obese postmenopausal rat model. Peroxisome proliferator-activated receptor-gamma co-activator-1alpha (PGC-1α, a), uncoupling protein 1 (UCP-1, b), phosphorylated mitogen-activated protein kinases (P-MAPKs, c) and total MAPKs (t-MAPK, d), in the control (group I), OVX (II), OVX/HFD (III), and the irisin-treated OVX/HFD (IV). Data are presented as means ± SD. *Significance vs. control group (P < 0.05). ^#Significance vs. OVX group (P < 0.05). ^§Significance vs. OVX/HFD group (P < 0.05). OVX ovariectomized, OVX/HFD ovariectomized/high-fat diet fed group

Fig. 8

The effect of irisin on the muscle protein expression in an obese postmenopausal rat model. Sarcolipin (SLN, a), sarco/endoplasmic reticulum Ca²⁺-ATPase 1 (SERCA1, b), phosphorylated mitogen-activated protein kinases (P-MAPKs, c) and total MAPKs (t-MAPK, d) and peroxisome proliferator-activated receptor-gamma co-activator-1alpha (PGC-1α, e) in group I (control), II (OVX), III (OVX/HFD), and IV (irisin-treated OVX/HFD). Data is presented as a mean ± SD. *Significance vs. control group (P < 0.05). ^#Significance vs. OVX group (P < 0.05). ^§Significance vs. OVX/HFD group (P < 0.05). OVX ovariectomized, OVX/HFD ovariectomized/high-fat diet fed group