Bile acid modulation of macrophage phenotype in colorectal cancer risk

Colorectal cancer (CRC) remains the second leading cause of cancer-related mortality in the United States, with persistent racial disparities in incidence and outcomes. Decades of epidemiological evidence have linked high-fat, high-protein Western diets to CRC risk, implicating hydrophobic secondary bile acids, including deoxycholic acid (DCA) and lithocholic acid (LCA), as key pro-tumorigenic metabolites. However, emerging studies reveal that gut microbiota enzymatically transforms primary and secondary bile acids into structurally distinct derivatives, including allo-bile acids such as allo-lithocholic acid (ALCA) and isoallo-lithocholic acid (IALCA), with the potential to modulate intestinal immune homeostasis. Despite advances, the impact of these microbially derived bile acid isomers on macrophage polarization, lipid metabolism, and colorectal carcinogenesis remains incompletely defined. This dissertation integrates clinical population analyses, transcriptomic meta-analysis, and in vitro macrophage mechanistic studies to elucidate the role of secondary bile acid derivatives in immune modulation and CRC disparities. We hypothesized that microbially derived allo-bile acids influence macrophage phenotypic polarization and lipid remodeling, thereby shaping the tumor immune microenvironment and contributing to population-level differences in CRC risk. Within a clinical screening cohort, we quantified fecal and serum bile acid profiles from African American (AA) and non-Hispanic White (NHW) subjects. While total DCA and LCA concentrations were comparable between groups, a striking depletion of immunoregulatory bile acid derivatives—specifically ALCA and IALCA—was observed in AA individuals, particularly females. These bile acid shifts were associated with reduced gut microbial diversity, enrichment of bile acid 7α-dehydroxylating Clostridium species, and altered mucosal immune gene expression, including attenuated markers of macrophage activation and epithelial barrier function. Concurrent metagenomic analysis revealed differential enrichment of microbiota capable of indirect allo-bile acid biosynthesis pathways in NHWs, suggesting a population-level divergence in microbial metabolic capacity. To further delineate immune-metabolic consequences, we performed a metaanalysis of over 1,000 CRC tumors from The Cancer Genome Atlas (TCGA), focusing on bile acid metabolism and macrophage-associated gene expression. AA CRC tumors exhibited early suppression of M1-like macrophage markers, downregulation of bile acid receptor signaling genes, and progressive activation of lipid biosynthetic pathways compared to NHW tumors. Notably, lipid metabolism reprogramming signatures identified in AA tumors mirrored the lipid storage phenotype characteristic of alternatively activated macrophages, implicating bile acid-mediated immune remodeling in tumor progression. Limitations included the lack of sex-stratified metadata across datasets, emphasizing the need for more demographically detailed studies. Mechanistic experiments using RAW264.7 cells, bone marrow-derived macrophages (BMDMs), and peritoneal macrophages from C57BL/6 mice demonstrated that ALCA and IALCA reprogram macrophage phenotypes. Treatment with allo-bile acids promoted M2-like polarization, increased CD206 expression, enhanced phagocytosis, and suppressed pro-inflammatory cytokines including TNF-α, IL-6, and prostaglandin E₂ (PGE₂). Infrared imaging and lipidomic profiling revealed profound metabolic reprogramming, with allo-bile acid–treated macrophages displaying accumulation of triglycerides enriched in unsaturated acyl chains, consistent with a lipid storage–oriented M2 phenotype. Furthermore, bile acid treatment markedly impaired mitochondrial activity in M1-like macrophages while enhancing their phagocytic functional capacity, providing direct evidence of bile acid-driven shifts in innate immune metabolism. Collectively, these findings delineate a novel immunometabolic axis wherein microbially derived allo-bile acids modulate macrophage polarization, lipid metabolism, and inflammatory tone within the colonic microenvironment. By integrating clinical, transcriptomic, and mechanistic data, this dissertation demonstrates that bile acid profiles and microbial composition in human may contribute to CRC disparities through remodeling of macrophage function. While limitations such as cross-sectional clinical sampling and the need for in vivo validation remain, this work establishes foundational insight into the intersection of diet, microbiome metabolism, and innate immunity in CRC. Future studies leveraging longitudinal human cohorts, gnotobiotic mouse models, and receptor-targeted macrophage interventions will be critical for translating these findings into strategies to mitigate CRC risk and improve immunotherapy responsiveness, particularly among underserved populations.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Nutritional Sciences

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Dai, Hanchu

Contributors dc:contributor

• Gaskins, H. Rex
• Ridlon, Jason M
• Anakk, Sayeepriyadarshini
• Steelman, Andrew J
• Robben, Michael

• Bile Acids
• Macrophage
• Colorectal Cancer

Statement dc:rights

• © 2025 Hanchu Dai

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/129998

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F129998