Our research focuses on two topics:
- Redox regulatory mechanisms governing cell fate decisions
- Microbial hydrogen and sulfur metabolism in the human colon
1. Redox regulatory mechanisms governing cell fate decisions In simple terms, cancer can be viewed as a state in which the balance between cell proliferation and cell death aberrantly favors the former. We and others have discovered that the intracellular redox environment exerts a profound influence on the normal cellular processes that regulate this balance including DNA synthesis, enzyme activation, selective gene expression, cell cycle progression, proliferation, differentiation, and apoptosis. In fact, it could be argued that redox homeostasis is more central to the governance of cell fate than any other biochemical phenomenon. However, this is a difficult area of study and molecular mechanisms mediating redox sensitivity and regulation are poorly defined. Motivated by these limitations, we have created novel genetic constructs that enable real-time and extended assessment of alterations in intracellular redox without cellular disruption. We are using these genetically-encoded redox biosensors to study compartmentalization of the cellular redox environment and to address the crucial question of whether tumor cells have lost the ability to mount the apparent changes in intracellular redox potential that accompany normal cell growth or alternatively an ability to sense these changes.
2. Microbial hydrogen and sulfur metabolism in the human colon In fermentation - the anaerobic process by which most colonic microbes gain energy - nutrient substrates are incompletely oxidized and reduced fermentation products serve as terminal electron acceptors. In such cases, the amount of energy (ATP) that can be produced depends on the difference in redox potential between the substrate and the reduced end products. Another distinction is that in fermentation, the reduced pyridine (NADH) and flavin (FADH) nucleotides must be reoxidized to maintain redox balance, a reaction that is the primary source of H2 in the colon. Accordingly, the production of H2 by hydrogenogenic microbes is crucial to the efficiency of fermentation. However, H2 accumulation would rapidly lead to an H2 partial pressure that would thermodynamically restrict further fermentation. Such an outcome typically is prevented by the simultaneous oxidation of H2 by three groups of hydrogenotrophic (H2-utilizing) microbes that conserve energy through anaerobic respiration: reductive acetogens, methanogenic archaea, and sulfate-reducing bacteria (SRB). We are pursuing studies to determine the extent of individual variation in the abundance and diversity of the three groups of hydrogenotrophic microbes and how diet and genetic background influence interactions between hydrogenotrophic microbes and fermentative bacteria. A goal is to determine the extent to which dysbiosis in microbial hydrogen metabolism may be linked to colonic disorders.
Microbial sulfur metabolism in the human colon is likely more extensive than has been previously recognized. For example, H2S, the sulfated compound with the highest potential influence on digestive health, can be generated from cysteine degradation as well as via dissimilatory sulfate reduction. We are also examining the extent to which organic sulfur metabolism by resident microbes is operative in the human colon, how diet influences relative activities of the enzymatic pathways involved, and which microbial groups carry out these processes. A long-term goal is to understand how multifactorial interactions among host genotype, diet and epithelial responses to bacterial-derived sulfide may contribute to IBD-associated or sporadic colorectal cancer.