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Small for gestational age (SGA) is linked to increased risk of obesity and metabolic disorders in adulthood. Amino acids and insulin stimulate protein synthesis through mammalian target of rapamycin (mTOR) signaling pathway. Phosphorylation of ribosomal protein S6 kinase (S6K) is the effector of mTOR. They are involved in mRNA translation and regulation of protein synthesis in skeletal muscle. This study aimed to investigate the effects of mTOR/S6K activity on the body composition of SGA.
Twelve SGA rats were developed by dietary restriction in pregnant rats. Twelve appropriate for gestational age (AGA) rats served as the control group. Only male rats were used in the study. The body composition, adipocyte size, skeletal muscle fiber size, serum adiponectin level, expression of mTOR, phospho-mTOR, S6K and phospho-S6K in skeletal muscle were measured at 12 weeks of age. Percentage of fat mass and lean mass (FM % and LM %) were performed by dual-energy X-ray absorptiometry and analyzed with the small animal software. The weight of bilateral perirenal fat and epididymal fat pads was indicated as visceral fat mass (VFM). The percentage of visceral fat mass body weight (VFM %) was calculated. Photomicrographs were taken by light microscope and analyzed with computer-assisted system. Enzyme-linked immunosorbent assay was used to assess serum adiponectin level. Western blot assay was used to detect the expression of mTOR, phospho-mTOR, S6K and phospho-S6K in skeletal muscle.
Compared to AGA adult rats, SGA adult rats showed increased FM % [(18.80±2.01) % vs (14.48±1.67) %, P < 0.01] and adipocyte area of visceral fat mass [(6834.34±665.39) μm2 vs (5763.71±468.59) μm2, P < 0.01] and decreased LM% [(78.72±1.84) % vs (83.44±1.61) %, P < 0.01] and cross sectional area of skeletal muscle fibers [(2658.17±247.85) μm2 vs (3385.11±273.39) μm2, P < 0.01]. The VFW [(15.36±1.50) g vs (7.69±0.94) g, P < 0.01] and VFW% [(3.58±0.33) % vs (1.80±0.18) %, P < 0.01] were significantly higher in the SGA adult rats than in the AGA adult rats. Compared to AGA adult rats, SGA adult rats showed decrease in serum adiponectin level [(4.74±0.61) μg/ml vs (5.84±0.59) μg/ml, P < 0.01], expression of phospho-mTOR (0.81±0.09 vs 1.00±0.11, P < 0.01) and phospho-S6K (0.82±0.08 vs 1.000±0.12, P < 0.01) in skeletal muscle.
Decrease lean mass and increase fat mass of SGA adult rats associated with down-regulation of the mTOR/S6K pathway in skeletal muscle.