The Oxidative Stress and Inflammatory Metabolic Pathways of Some Environmental Toxicants Inflicting Human Disorders

Numerous types of environmental toxicants alter human metabolomics, directly or indirectly, through gut microbial dysbiosis, upsetting immune homeostasis. Environmental toxicants capable of these direct and indirect effects have included heavy metals, pesticides, herbicides, polychlorinated biphenyl (PCB) congeners, and endocrine-disrupting chemicals. The cell and molecular events induced by some diverse toxicants are reviewed, along with their potential additive, synergistic, and antagonistic effects on immune homeostasis (increasing proinflammatory immune cell activation and suppressing immunoregulation), which leads to systemic oxidative stress (OS). As people are exposed in varying degrees to countless chemicals and environmental factors over a lifetime, it is challenging to correlate specific diagnoses to any single toxicant or exposure, which is often a key challenge in linking environmental exposure to health outcomes. However, many toxicants have the common mechanistic effect of OS. The effects of toxicants are more pronounced with aging due to cumulative exposures and immunoaging (immunosenescence), with chronic low-grade inflammation referred to as "inflammaging". The cell and molecular mechanisms of toxicants include altered calcium flux, mitochondrial dysfunction, and increased levels of damage-associated molecular patterns (alarmins) that trigger an inflammatory response and possibly promote autoimmune and neurological disorders. OS skews type-1 immunity for defenses against pathogens and cancers more toward type-2 immune responses to self-antigens (autoimmunity). The toxicants may directly affect the innate and adaptive immune cells inducing this skewing, or they may modify portions of gut microbial species and strains and their production of metabolites that indirectly affect systemic immunity. These latter toxicant influences require metabolomic analysis of the differential structures and activities of the microbial metabolites. The damaging effects of OS and inflammation disrupting immune homeostasis and leading to disorders are reviewed and discussed. The need for well-designed studies that allow for standardized comparison of exposures and related effects are emphasized, and their real-world limitations noted.