Aim & background: Increasing evidence indicates that commensal bifidobacteria of the natural intestinal microbiota play a critical role in energy metabolism in vivo. However, the precise molecular basis and biochemical causes are not well understood. It has been shown that there is a reciprocal regulation of the TLR4/NF-kB and the Wnt/β-catenin pathway in vivo. We, therefore, aimed at investigating the possible role of this system of reciprocal regulation in the modulation of cellular energy metabolism by Bifidobacterium breve (B. breve) and Lactobacillus rhamnosus GG (LGG) in three different cell types-based 2D and 3D co-culture models.

Results: Bifidobacteria inhibited LPS- and fructose-induced expression of inflammatory cytokines (IL-1β, IL-6, TNF-α) in luminal cell milieu of co-cultured cells, concomitant with decreases in cellular TG, Fetuin-A, and Hepcidin in 3D HepG2 cells. Exposure of co-cultured cells to bifidobacteria inhibited the LPS- and fructosereduced GSK-3β and Akt phosphorylation, and blocked the NF-kB activity through inhibition of IκBα, ASK1, GS, p38 MAPK and NF-kB 65 subunit phosphorylation. The loss of NF-κB activity was also associated with a significant increase of total β-catenin protein levels, as reflected by a decrease in phosphorylation at the serine 33/ 37 residues. Quantitative RT- PCR and western blot analysis revealed that Bifidobacteria inhibited LPS- and/or fructose-induced mRNA and protein expression of wnt5a (as known ligand for TLR4). However, the pattern of wnt3a mRNA and protein expression was quite the opposite of wnt5a.

 Conclusion: We describe a regulatory mechanism in which commensal bifidobacteria prevent LPS- and/or fructose-mediated transcriptional activation of metabolic disorder-causing factors (IL-1β, IL-6, TNF-α) and disruption of normal cellular energy metabolism by attenuating NF-kB activity and interfering with the Wnt/β- catenin pathway. The latter positively regulates NF-κB activity, thereby simultaneously reducing cellular triglyceride concentrations and enhancing cellular glucose consumption. This may be a molecular basis by which commensal bifidobacteria enhance intrinsic cellular tolerance against excess consumption of energy – yielding substrates and participate in homeostatic regulation of metabolic processes in vivo.