NO: a key player in microbiome dynamics and cancer pathogenesis

The human microbiome refers to the genomic content of microorganisms inhabiting the human body, including the lungs, oral cavity, intestinal tract, esophagus, and other areas. The human oral microbiota is a diverse and complex ecosystem that includes bacteria, microeukaryotes, archaea, and viruses. These communities have a highly structured biogeography resulting from the various microenvironments in the oral cavity, shaping local metabolic exchange. Dietary nitrate (NO₃ ^-) is an ion naturally present in vegetables, especially leafy greens. When consumed, it leads to the production of nitric oxide (NO). This bioactive molecule benefits bodily functions like host defense and neuronal communication and improves vascular and metabolic health. Dietary NO₃ ^- is reduced to NO via the nitrate-nitrite-NO pathway, facilitated by nitrate-reducing bacteria inside the oral cavity. NO has a leading role in different types of diseases, including cancer, cardiovascular disease, and diabetes. The bioavailability of NO is greatly enhanced by the activity of bacteria residing in the mouth, which reduces NO₃ ^-to NO₂ ^- and increases the concentration of circulating NO₂ ^-. NO is the key to causing different malignancies, including gastrointestinal cancers. NO can cause cell death by inducing DNA damage and anti-apoptotic signaling pathways. Low to moderate levels of NO derived from tumors can activate angiogenesis and promote an invasive phenotype, while high levels of NO may have an anti-tumor effect in protecting against cancer. In this review, we intend to discuss the human microbiome, dietary NO₃ ^-consumption, the vital role of NO in the human body, types of cancers, and treatments based on it.