Effects of viable Lactobacillus rhamnosus GG for the prevention of diet-induced obesity, liver steatosis and insulin resistance in rats: A comparison with its heat-inactivated postbiotic

• Autores: Laura Arellano García
• Directores de la Tesis: Iñaki Milton Laskibar (codir. tes.), María Puy Portillo Baquedano (codir. tes.)
• Lectura: En la Universidad del País Vasco - Euskal Herriko Unibertsitatea ( España ) en 2025
• Idioma: inglés
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• Resumen

  • The prevalence of obesity and its associated comorbidities, namely NAFLD and IR, has been continuously increasing over the last decades. More importantly, projections of the prevalence growth indicate no signals of decline among the adult population. Although various factors contribute to the onset of these metabolic alterations, inadequate dietary patterns and a sedentary lifestyle have emerged as major contributors. Furthermore, gut microbiota dysbiosis, defined as an imbalance in microbial composition and intestinal function, has shown to be involved in the pathogenesis not only of obesity but also of NAFLD and IR. Given the association between dysbiosis and the aforementioned metabolic impairments, and considering the poor adherence of patients to traditional lifestyle modification-based interventions, the scientific community has been actively seeking additional therapies aimed at modulating the gut microbiota composition itself. In this regard, the use of different strains of probiotic bacteria, commonly prescribed for the management of gastrointestinal impairments, has also shown beneficial effects in a range of metabolic alterations. However, viable bacteria administration may pose risks in vulnerable individuals. In this respect, emerging evidence indicates that the benefits of probiotics may not be exclusive of viable bacteria, but also may be related to their metabolites or bacterial components, which have been defined as postbiotics. In this context, the present Doctoral Thesis aims to evaluate the potential benefits of viable and heat-inactivated Lactobacillus rhamnosus GG (LGG) administration in the prevention of diet-induced obesity, NAFLD and IR in rats. Additionally, this project aims to compare the potential effects exerted by both treatments, in order to determine whether bacterial viability is required, as well as to identify and characterise the involved mechanisms of action. For this purpose, 34 male Wistar rats were allocated in four experimental groups, and fed either a standard laboratory diet (C group), a high-fat high-fructose diet alone (HFHF group), or supplemented with a probiotic (1x109 CFU/day LGG; PRO group) or with a postbiotic (heat-inactivated LGG, same dose; POST group) for six weeks. In the evaluation of the tested treatments as potential therapeutic approaches to prevent diet-induced obesity, both viable and heat-inactivated LGG similarly prevented the diet-induced body-weight gain and body fat mass increase. However, the probiotic regulated a broader range of metabolic processes. More specifically, while viable LGG administration reduced FA uptake (LPL activity) and de novo lipogenesis (FAS activity) in pWAT, and enhanced lipolysis (ATGL protein expression), the postbiotic mainly acted decreasing de novo lipogenesis (FAS activity) in the same tissue. As regards the effects of the tested treatments in iBAT, a tissue well-known for its role in energy expenditure, both interventions enhanced thermogenesis (UCP1, CIDEA and DIO2 protein expression) and lipid oxidation (CPT-1b activity) in this tissue to the same extent. However, bacterial viability appeared essential for regulating mitochondriogenesis and for the maintenance of mitochondrial function, as the probiotic regulated more processes (enhanced p38 MAPK protein activation, NRF1 protein expression and CS activity) compared to the postbiotic (higher p38 MAPK protein activation, TFAM and NRF1 protein expression). As for the effects of the tested treatments in the prevention of diet-induced NAFLD, probiotic administration reduced the steatosis grade by decreasing liver lipid uptake (FATP2 protein expression) and de novo lipogenesis (FAS enzymatic activity and SREBP-1c protein expression), and enhancing liver lipid export (MTTP activity). By contrast, the postbiotic only reduced de novo lipogenesis (FAS enzymatic activity and SREBP-1c protein expression), thus showing milder effects on steatosis prevention compared to its viable counterpart. Conversely, the effects of heat-inactivated LGG in preventing diet-induced hepatic oxidative stress and inflammation were stronger compared to those exerted by its viable counterpart, enhancing GPx activity, nonenzymatic GSH levels and total antioxidant capacity and reducing the levels of the pro-inflammatory marker TNF-¿. Finally, and regarding the effects of the tested treatments in the prevention of diet-induced IR, although both viable and heat-inactivated LGG reduced serum glucose and insulin levels compared to the non-treated animals, the effects of the postbiotic on the latter were more pronounced. Despite this difference, glycaemic control indexes (HOMA-IR and TyG index) were similar in both treated groups. Additionally, while probiotic administration increased skeletal muscle lipid uptake (LPL activity) and oxidation (CPT-1b activity) without modifying the lipid content in this tissue, the postbiotic promoted lipid oxidation (CPT-1b activity and SIRT3 protein expression), reducing skeletal muscle TG content. Notably, both treatments prevented diet-induced impairments in the insulin-signalling cascade to a different extent. While the probiotic increased GLUT4 translocation to the cell membrane (AS160 protein activation), the postbiotic enhanced GLUT4 protein expression. In summary, these results suggest that although LGG viability is not required to prevent obesity, NAFLD and IR under these experimental conditions in rats, it significantly affects the underlying mechanisms of action, underscoring the importance of treatment individualization depending on the present metabolic alteration. Keywords: obesity; NAFLD; IR; Lactobacillus rhamnosus GG; probiotic; postbiotic