Ability of prebiotic polysaccharides to activate a HIF1α-antimicrobial peptide axis determines liver injury risk in zebrafish
The application of natural polysaccharides to improve obesity and attenuate hepatic steatosis may pose a risk to the body, lead to microbial dysbiosis and hepatic injury. And further revealed that the failure to activate HIF1α was the key factor of these risks.
Natural polysaccharides, such as polysaccharides from Ganoderma lucidum1, and Hirsutella sinensis2, have been proved to improve obesity and attenuate hepatic steatosis by microbiota modulation. However, apart from the well-reported benefits, dysregulated fermentation of some prebiotic polysaccharides (fibers) was reported to induce microbial dysbiosis and hepatic inflammation and even liver cancer3. Nevertheless, the underlying mechanisms determining the safety or risk of different polysaccharides when function as prebiotics are not clear.
In this study, we tested the function of exopolysaccharides (EPS) from two well used probiotic Lactobacillus strains L. rhamnosus GG (LGG) and L. casei BL23. We first found that oral administration of both LGG EPS and BL23 EPS ameliorated hepatic steatosis in HFD-fed zebrafish. However, the BL23 EPS, but not LGG EPS, induced liver inflammation and injury.
Further study showed that the liver injury effect by BL23 EPS was attributable to dysbiosis of the intestinal microbiota. BL23 EPS led to intestinal bacterial expansion, especially for Proteobacteria, while LGG EPS improved the intestinal homeostasis. We further observed that the differentiation of the BL23 EPS and LGG EPS-associated microbiotas was mainly due to that LGG EPS can directly induce intestinal HIF1a through TLR4ba, and the increased HIF1α boosts local antimicrobial peptides (AMP) to facilitate the microbial homeostasis (reduced abundance of Proteobacteria/Plesiomonas and increased abundance of Fusobacteria/Cetobacterium). In contrast, BL23 EPS do not have the ability to induce intestinal HIF1a, and therefore lack the driving force facilitating microbial homeostasis mediated by HIF1a-AMP axis. The fermentation of EPS by the gut bacteria in a HFD context leads to intestinal dysbiosis and Proteobacteria expansion in the absence of such counteracting effect mediated by HIF1a-AMP axis, which leads to liver inflammation and damage. Moreover, the LGG-EPS associated microbiota produces more butyrate, which can deplete O2 in the intestinal microenvironment and improve hypoxia. As hypoxia may stabilize HIF1α, the higher butyrate production by the LGG EPS associated microbiota may further contribute to the microbial homeostasis, while the low butyrate associated with BL23 EPS supports the dysbiotic state. Finally, we found that the liver injury risk was applicable to other commonly used natural polysaccharides, depending on their HIF1α activation efficiency. Together, these results support that HIF1α-AMP axis acts as a key physiological force maintaining the microbial homeostasis against the harmful fermentation associated with some risk-prone prebiotic polysaccharides in the context of high fat diet, and suggest that the HIF1α-AMP axis may be harnessed as a target to promote safe use of prebiotics.
1. Chang, C.J. et al. Ganoderma lucidum reduces obesity in mice by modulating the composition of the gut microbiota. Nat Commun 6, 7489 (2015).
2. Wu, T.R. et al. Gut commensal Parabacteroides goldsteinii plays a predominant role in the anti-obesity effects of polysaccharides isolated from Hirsutella sinensis. Gut 68, 248-262 (2019).
3. Singh, V. et al. Dysregulated microbial fermentation of soluble fiber induces cholestatic liver cancer. Cell 175, 679-694 (2018).